natsci

VOLCANO

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface.

Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust in the interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.

Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. Volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Historically, so-called volcanic winters have caused catastrophic famines.

Etymology The word volcano is derived from the name

of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn originates from Vulcan, the name of a god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology.

Plate tectonic

Divergent plate boundariesAt the mid-oceanic ridges, two tectonic

plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

Convergent plate boundariesSubduction zones are places where two plates,

usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be

very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire."Hotspots"

"Hotspots" is the name given to volcanic provinces postulated to be formed by mantle plumes. These are postulated to comprise columns of hot material that rise from the core-mantle boundary. They are suggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonic plates move across them, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the postulated plume. The Hawaiian Islands have been suggested to have been formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot. This theory is currently under criticism, however.

Volcanic FeaturesThe most common perception of a volcano is of

a conical mountain, spewing lava and poisonous gases from a crater at its summit. This describes just one of many types of volcano, and the features of volcanoes are much more complicated. The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform. Many of these vents give rise to smaller cones such as Puʻu   ʻŌʻō  on a flank of Hawaii's Kīlauea. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes, except when a mud volcano is actually a vent of an igneous volcano.

Fissure ventsVolcanic fissure vents are flat, linear cracks

through which lava emerges.

Shield volcanoesShield volcanoes, so named for their broad,

shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.

Lava domesLava The word volcano is derived from the name

of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn originates from Vulcan, the name of a god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology.

At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and

solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.

"Hotspots" is the name given to volcanic provinces postulated to be formed by mantle plumes. These are postulated to comprise columns of hot material that rise from the core-mantle boundary. They are suggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonic plates move across them, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the postulated plume. The Hawaiian Islands have been suggested to have been formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot. This theory is currently under criticism, however.

The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit. This describes just one of many types of volcano, and the features of volcanoes are much more complicated. The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform. Many of these vents give rise to smaller cones such as Puʻu   ʻŌʻō  on a flank of Hawaii's Kīlauea. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes, except when a mud volcano is actually a vent of an igneous volcano.

Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.

Domes are built by slow eruptions of highly viscous lavas. They are sometimes formed within the crater of a previous volcanic eruption (as in Mount Saint Helens), but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lavas generally do not flow far from the originating vent.

CryptodomesCryptodomes are formed when viscous lava

forces its way up and causes a bulge. The 1980 eruption of Mount St. Helens was an example. Lava was under great pressure and forced a bulge in the mountain, which was unstable and slid down the north side.

Volcanic cones (cinder cones)Volcanic cones or cinder cones result from

eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico, Caja del Rio is a volcanic field of over 60 cinder cones.

Stratovolcanoes (composite volcanoes)

Stratovolcanoes or composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes, created from several structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process begins again. Classic examples include Mt. Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.

Throughout recorded history, ash produced by the explosive eruption of stratovolcanoes has posed the greatest hazard to civilizations as compared to other types of volcanoes. No supervocano has erupted in human history. Shield volcanos have smaller pressure buildup

Cross-section through a stratovolcano (vertical scale is exaggerated):1. Large magma chamber2. Bedrock3. Conduit (pipe)4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank

9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15. Ash cloud

from the underlying lava flow as compared to stratovolcanoes. Fissure vents and monogenetic volcanic fields(volcanic cones) have less powerful eruptions, as they are many times underextension. Stratovolcanoes have been a greater historical threat because they are steeper than shield volcanos, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars.

SupervolcanoesA supervolcano is a large volcano that usually

has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States), Lake Taupo in New Zealand, Lake Toba in Sumatra, Indonesia and Ngorogoro Crater in Tanzania, Krakatoa near Java and Sumatra, Indonesia. Supervolcanoes are hard to identify centuries later, given the enormous areas they cover. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted, but are non-explosive.

Submarine volcanoesSubmarine volcanoes are common features on

the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases, although they can be detected by hydrophones and discoloration of water because of volcanic gases. Pumice rafts may also appear. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanoes. They may become so large that they break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals.

Subglacial volcanoesSubglacial volcanoes develop

underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapsed leaving a flat-topped mountain. These volcanoes are also called Table Mountains, tuyas or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland; however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory.

Mud volcanoesMud volcanoes or mud domes are formations

created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.

Erupted Materials

Lava compositionAnother way of classifying volcanoes is by

the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions (Cas & Wright, 1987):

If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic.

Felsic lavas (dacites or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome.

Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaska's Valley of Ten Thousand Smokes, formed by the eruption of Novaruptanear Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash that is light enough to be erupted high into the Earth's atmosphere may travel many kilometers before it falls back to ground as a tuff.

If the erupted magma contains 52–63% silica, the lava is of intermediate composition.

These "andesitic" volcanoes generally only occur above subduction zones (e.g. Mount in Indonesia).

Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes:

Hydration melting of peridotite and fractional crystallization

Melting of subducted slab containing sediments

Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow.

If the erupted magma contains <52% and >45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe))

or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide range of settings:

At mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap;

Shield volcanoes  (e.g. the Hawaiian Islands, including Mauna Loa and Kilauea), onboth oceanic and continental crust;

As continental flood basalts.

Some erupted magmas contain <=45% silica and produce ultramafic lava. Ultramafic flows, also known as komatiites, are very rare; indeed, very few have been erupted at the Earth's surface since the Proterozoic, when the planet's heat flow was higher. They are (or were) the hottest lavas, and probably more fluid than common mafic lavas.

Lava textureTwo types of lava are named according to the

surface texture: ʻAʻa (pronounced [ˈʔaʔa]) and pāhoehoe ([paːˈho.eˈho.e]), both Hawaiianwords. ʻAʻa is characterized by a rough, clinkery surface and is the typical texture of viscous lava flows. However, even basaltic or mafic flows can be erupted as ʻaʻa flows, particularly if the eruption rate is high and the slope is steep.

Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Usually, only mafic flows will erupt as pāhoehoe, since they often erupt at higher temperatures or have the proper chemical make-up to allow them to flow with greater fluidity.Volcanic Activity

Popular classification of volcanoesA popular way of classifying magmatic volcanoes

is by their frequency of eruption, with those that erupt regularly are called active, those that have erupted in historical times but are now quiet are called dormant or inactive, and those that have not erupted in historical times are called extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.Active

There is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By human lifespans, however, they are not.

Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake

activity or significant new gas emissions. Most scientists consider a volcano active if it has erupted in the last 10,000 years (Holocene times) – the Smithsonian Global Volcanism Program uses this definition of active. There are about 1500 active volcanoes in the world – the majority along the Pacific Ring of Fire – and around 50 of these erupt each year. An estimated 500 million people live near active volcanoes.

Historical times (that is, in recorded history) is another timeframe for active. The Catalogue of the Active Volcanoes of the World, published by the International Association of Volcanology, uses this definition, by which there are more than 500 active volcanoes. However the span of recorded history differs from region to region. In China and the Mediterranean, it reaches back nearly 3,000 years, but in the Pacific Northwest of the United States and Canada, it reaches back less than 300 years, and in Hawaii and New Zealand, only around 200 years.

ExtinctExtinct volcanoes are those that scientists

consider unlikely to erupt again, because the volcano no longer has a magma supply. Examples of extinct volcanoes are many volcanoes on the Hawaiian – Emperor seamount chain in the Pacific Ocean, Hohentwiel, Shiprock and the Zuidwal volcano in the Netherlands. Edinburgh Castle in Scotland is famously located atop an extinct volcano. Otherwise, whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Some volcanologists refer to extinct volcanoes as inactive, though the term is now more commonly used for dormant volcanoes once thought to be extinct.

DormantIt is difficult to distinguish an extinct volcano from

a dormant (inactive) one. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless, volcanoes may remain dormant for a long period of time. For example, Yellowstone has a repose/recharge period of around 700 ka, and Toba of around 380 ka. Vesuvius was described by Roman writers as having been covered with gardens and vineyards before its famous eruption of AD 79, which destroyed the towns of Herculaneum and Pompeii. Before its catastrophic eruption of 1991, Pinatubo was an inconspicuous volcano, unknown to most people in the surrounding areas. Two other examples are the long-dormant Soufrière Hills volcano on the island of Montserrat, thought to be extinct before activity resumed in 1995 and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BC and had long been thought to be extinct.

Technical classification of volcanoesThe three common popular classifications of

volcanoes can be subjective and some volcanoes thought to have been extinct have erupted again. To help prevent people from falsely believing they are not at risk when living on or near a volcano, countries have adopted new classifications to describe the various levels and stages of volcanic activity. Some alert systems use different numbers or colors to designate the different stages. Other

systems use colors and words. Some systems use a combination of both.

Effects of VolcanoesThere are many different types of volcanic

eruptions and associated activity: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite), effusive eruption of low-silica lava (e.g., basalt),pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to humans. Earthquakes, hot springs,fumaroles, mud pots and geysers often accompany volcanic activity.

The concentrations of different volcanic gases can vary considerably from one volcano to the next.Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other principal volcanic gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen, carbon monoxide, halocarbons, organic compounds, and volatile metal chlorides.

Large, explosive volcanic eruptions inject water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock andpumice) into the stratosphere to heights of 16–32 kilometres (10–20 mi) above the Earth's surface. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the Earth's albedo—its reflection of radiation from the Sun back into space – and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming thestratosphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years – sulfur dioxide from the eruption of Huaynaputina probably caused theRussian famine of 1601–1603.

One proposed volcanic winter happened c. 70,000 years ago following the supereruption of Lake Toba on Sumatra Island in Indonesia.  According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences, killing most humans then alive and creating a population bottleneck that affected the genetic inheritance of all humans today. The 1815 eruption of Mount Tambora created global climate anomalies that became known as the "Year Without a Summer" because of the effect on North American and European weather. Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in one of the worst famines of the 19th century. The freezing winter of 1740–41, which led to widespread famine in northern Europe, may also owe its origins to a volcanic eruption.

It has been suggested that volcanic activity caused or contributed to the End-Ordovician, Permian-Triassic, Late Devonian mass extinctions, and possibly others. The massive eruptive event which formed the Siberian Traps, one of the largest known volcanic events of the last 500 million years of Earth's geological history, continued for a million years and is considered to be the likely cause of the "Great Dying" about 250 million years ago, which is estimated to have killed 90% of species existing at the time.

The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates chlorine monoxide (ClO), which destroys ozone (O3). As the aerosols grow and coagulate, they settle down into the upper troposphere where they serve as nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) are dissolved in water droplets in the eruption cloud and quickly fall to the ground as acid rain. The injected ash also falls rapidly from the stratosphere; most of it is removed within several days to a few weeks. Finally, explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of carbon for biogeochemical cycles.

Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic activity releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year. Volcanic eruptions may inject aerosols into the Earth's atmosphere. Large injections may cause visual effects such as unusually colorful sunsets and affect global climate mainly by cooling it. Volcanic eruptions also provide the benefit of adding nutrients to soil through the weathering process of volcanic rocks. These fertile soils assist the growth of plants and various crops. Volcanic eruptions can also create new islands, as the magma cools and solidifies upon contact with the water.

Ash thrown into the air by eruptions can present a hazard to aircraft, especially jet aircraft where the particles can be melted by the high operating temperature. Dangerous encounters in 1982 after the eruption of Galunggung in Indonesia, and 1989 after the eruption of Mount Redoubt in Alaska raised awareness of this phenomenon. Nine Volcanic Ash Advisory Centers were established by the International Civil Aviation Organization to monitor ash clouds and advise pilots accordingly. The 2010 eruptions of Eyjafjallajökull caused major disruptions to air travel in Europe.

Volcanoes on other Planetary BodiesThe Earth's Moon has no large volcanoes and no

current volcanic activity, although recent evidence suggests it may still possess a partially molten core. However, the Moon does have many volcanic features such as maria (the darker patches seen on the moon), rilles and domes.

The planet Venus has a surface that is 90% basalt, indicating that volcanism played a major role in shaping its surface. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Changes in the planet's atmosphere and observations of lightning have been attributed to ongoing volcanic eruptions, although there is no confirmation of whether or not Venus is still volcanically active. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank.

There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth. They include Arsia Mons, Ascraeus Mons, Hecates

Tholus, Olympus Mons, and Pavonis Mons. These volcanoes have been extinct for many millions of years, but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well.

Jupiter's moon Io is the most volcanically active object in the solar system because of tidal interaction with Jupiter. It is covered with volcanoes that erupt sulfur, sulfur dioxide and silicate rock, and as a result, Io is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1,800 K (1,500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io. Europa, the smallest of Jupiter's Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into ice on the frigid surface. This process is known as cryovolcanism, and is apparently most common on the moons of the outer planets of the solar system.

In 1989 the Voyager 2 spacecraft observed cryovolcanoes (ice volcanoes) on Triton, a moon of Neptune, and in 2005 the Cassini–Huygens probe photographed fountains of frozen particles erupting from Enceladus, a moon of Saturn. The ejecta may be composed of water, liquid nitrogen, dust, or methane compounds. Cassini–Huygens also found evidence of a methane-spewing cryovolcano on theSaturnian moon Titan, which is believed to be a significant source of the methane found in its atmosphere. It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.

A 2010 study of the exoplanet COROT-7b, which was detected by transit in 2009, studied that tidal heating from the host star very close to the planet and neighboring planets could generate intense volcanic activity similar to Io.

Theories about Earth

Tidal TheoryThe tidal theory, proposed by James Jeans and

Harold Jeffreys in 1918, is a variation of the planetesimal concept: it suggests that a huge tidal wave, raised on the

sun by a passing star, was drawn into a long filament and became detached from the principal mass. As the stream of gaseous material condensed, it separated into masses of various sizes, which, by further condensation, took the form of the planets.

Nebular HypothesisIn cosmogony, the nebular hypothesis is the most

widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation is now thought to be at work throughout the universe.[3] The widely accepted modern variant of the nebular hypothesis is Solar Nebular Disk Model (SNDM) or simply Solar Nebular Model.

According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.

The protoplanetary disk is an accretion disk which continues to feed the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ices is possible. The grains may eventually coagulate into kilometer-sized planetesimals. If the disk is massive enough the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes around 100 million to a billion years.

The formation of giant planets is a more complicated process. It is thought to occur beyond the so-called snow line, where planetary embryos are mainly made of various ices. As a result they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. However, some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses it accelerates and proceeds in a runaway manner. The Jupiter and Saturn–like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. The ice giants like Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.

Dynamic Encounter TheoryThe dynamic encounter theory is the theory for

the historical origin of the planets as a result of a near collision of the sun and another star.

This theory was first proposed by Georges Leclerc, Comte de Buffon (1707-1788), director of the royal botanical collection in Paris. Later Buffon disassociated himself from his own views, affirming (in an echo of Galileo) that he held to the literal Mosaic account and would give up his theory of the formation of the planets, which had been only "a purely philosophical supposition" (i.e., a thought experiment). These statements were probably ironic, since Buffon's later efforts continued his former work.

Solar Disruption TheorySolar disruption theory was one of several

theories that emerged before the 18th century concerning the formation of the solar system. Solar disruption theory states that the collision of the sun with another stars caused debris to be ejected from its mass and these debris eventually became the planets. This theory was later discarded for the nebula theory of solar system formation. However there are some scientists that propose that it has some merit.The big question up until the 18th century was how the solar system was born. There were many explanations for why this happen but many were really only conjecture given the tools available to astronomers at the time. The real question was what would be a probable origin under the known laws of physics. The advent of classical mechanics came to prove the nebular theory as the likely theory for the creation of the solar system. The reason was that most other theories could not explain how the planets formed without giving in to the Sun’s gravity and falling in.

A new argument has emerged for a different form of solar disruption theory in this version it answers the idea in a more roundabout way that answers an interesting question. We know that the formation of the solar system itself was volatile but did the Sun and its planets really form in relative isolation from other star emerging in the Nebula? This new theory that emerged in 2004 supposed proposed that the influence of other stars may have influenced the formation of planets in the solar system.

In the meanwhile the main theory stands. We know in the nebular theory that stars are formed from spinning nebulas of gases and cosmic dust. Over time the masses clump together to the point where the mass reaches the level needed for gravity to initiate fusion. The planets are formed from the clumps of debris in the nebular disk that did not fall into the Sun and that they eventually ended up colliding with each other forming planets. Any theory that suggests interference from the gravity fields of other star systems has not been tested yet. It may have merit but we don’t have the technology to test theories on such large scales.

Planetesimal TheoryA widely accepted theory of planet formation, the

so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals can form in a very dense

layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta [1] and 90 Antiope,[2] but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.

It is generally believed that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the Solar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the Jovian planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of Mars), and many of the small high-inclination moons of the Jovian planets.

Planetesimals that have survived to the current day are valuable to scientists because they contain information about the formation of the Solar System. Although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a 'time capsule', and their composition can tell us of the conditions in the Solar Nebula from which our planetary system was formed (see also meteorites and comets).

Condensation TheoryThe condensation theory of the solar system

explains why the planets are arranged in a circular, flat orbit around the sun, why they all orbit in the same direction around the sun, and why some planets are made up primarily of rock with relatively thin atmospheres. Terrestrial planets such as Earth are one type of planet while gas giants -- Jovian planets such as Jupiter -- are another type of planet.

This theory proposes that the Moon and the Earth condensed individually from the nebula that formed the solar system, with the Moon formed in orbit around the Earth. However, if the Moon formed in the vicinity of the Earth it should have nearly the same composition. Specifically, it should possess a significant iron core, and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.

Big Bang TheoryAccording to the Big Bang model, the Universe

expanded from an extremely dense and hot state and continues to expand today. A common analogy explains that space itself is expanding, carrying galaxies with it, like spots on an inflating balloon. The graphic scheme above is an artist's concept illustrating the expansion of a portion of a flat universe.

The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, the Big Bang occurred approximately 13.75 billion years ago, which is thus considered the age of the Universe. After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various

subatomic particles, including protons, neutrons, and electrons. While protons and neutrons combined to form the first atomic nuclei only a few minutes after the Big Bang, it would take thousands of years for electrons to combine with them and create electrically neutral atoms. The first element produced was hydrogen, along with traces of helium and lithium. Giant clouds of these primordial elements would coalesce through gravity to form stars and galaxies, and the heavier elements would be synthesized either within stars or during supernovae.

The Big Bang is a well-tested scientific theory and is widely accepted within the scientific community. It offers a comprehensive explanation for a broad range of observed phenomena. Since its conception, abundant evidence has been uncovered in support of the model. The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, and the formation of galaxies—are derived from many observations that are independent from any cosmological model; these include the abundance of light elements, the cosmic microwave background, large scale structure, and the Hubble diagram for Type Ia supernovae. As the distance between galaxy clusters is increasing today, it can be inferred that everything was closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment in such conditions, resulting in further development of the model. On the other hand, these accelerators have limited capabilities to probe into such high energy regimes. There is little evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the universe going forward from that point on.

Georges Lemaître first proposed what would become the Big Bang theory in what he called his "hypothesis of the primeval atom." Over time, scientists would build on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The governing equations had been formulated by Alexander Friedmann. In 1929, Edwin Hubble discovered that the distances to far away galaxies were generally proportional to their redshifts—an idea originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.

While the scientific community was once divided between supporters of the Big Bang and those of alternative cosmological models, most scientists became convinced that some version of the Big Bang scenario best fit observations after the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body. Since then, astrophysicists have formulated further hypotheses to account for some discrepancies that have arisen within the model.

Divene TheoryThe Judeo-Christian creation story occurs at the

very beginning of the Bible. According to this story the universe was created by God's command. These verses from the King James Version of the Bible also reflect other

important Judeo-Christian traditions, including the sanctity of the Sabbath and the idea that God created the human race “in his own image.”

Genesis 1:1-31; 2:1-3Genesis 11 In the beginning God created the heaven and the earth.2 And the earth was without form, and void; and darkness

was upon the face of the deep. And the Spirit of God moved upon the face of the waters.

3 And God said, Let there be light: and there was light.4 And God saw the light, that it was good: and God

divided the light from the darkness.5 And God called the light Day, and the darkness he called

Night. And the evening and the morning were the first day.

6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.

7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.

8 And God called the firmament Heaven. And the evening and the morning were the second day.

9 And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so.

10 And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good.

11 And God said, Let the earth bring forth grass, the herb yielding seed, and the fruit tree yielding fruit after his kind, whose seed is in itself, upon the earth: and it was so.

12 And the earth brought forth grass, and herb yielding seed after his kind, and the tree yielding fruit, whose seed was in itself, after his kind: and God saw that it was good.

13 And the evening and the morning were the third day.14 And God said, Let there be lights in the firmament of

the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years.

15 And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.

16 And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also.

17 And God set them in the firmament of the heaven to give light upon the earth.

18 And to rule over the day and over the night, and to divide the light from the darkness: and God saw that it was good.

19 And the evening and the morning were the fourth day.20 And God said, Let the waters bring forth abundantly

the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.

21 And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good.

22 And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.

23 And the evening and the morning were the fifth day.24 And God said, Let the earth bring forth the living

creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.

25 And God made the beast of the earth after his kind, and cattle after their kind, and every thing that creepeth upon the earth after his kind: and God saw that it was good.

26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.

27 So God created man in his own image, in the image of God created he him; male and female created he them.

28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.

29 And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree yielding seed; to you it shall be for meat.

30 And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein there is life, I have given every green herb for meat: and it was so.

31 And God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day.

Genesis 2 1 Thus the heavens and the earth were finished, and all

the host of them.2 And on the seventh day God ended his work which he

had made; and he rested on the seventh day from all his work which he had made.

3 And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.

The most contentious debates over evolution have involved religion. From Darwin’s day to the present, members of some religious faiths have perceived the scientific theory of evolution to be in direct and objectionable conflict with religious doctrine regarding the creation of the world. Most religious denominations, however, see no conflict between the scientific study of evolution and religious teachings about creation. Christian Fundamentalists and others who believe literally in the biblical story of creation choose to reject evolutionary theory because it contradicts the book of Genesis, which describes how God created the world and all its plant and animal life in six days. Many such people maintain that the Earth is relatively young—perhaps 6,000 to 8,000 years old—and that humans and all the world’s species have remained unchanged since their recent creation by a divine hand.

Oceans and Seas

The following table lists the world's oceans and seas, according to area and average depth, including the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean, Mediterranean Sea, Arctic Ocean, Caribbean Sea, Bering Sea, and more.

Oceans cover more than 70% of Earth’s surface. About 97% of Earth’s water is contained in oceans and seas. The five oceans—the Pacific, Atlantic, Indian, Southern, and Arctic—are all connected and form an enormous mass of water. Seas are smaller bodies of salty water within the oceans.The oceans are salty, which makes them unsuitable for drinking. Most of the salt comes from minerals from rocks and soil that have been washed from the land and carried into the oceans by rivers. The minerals are mostly chloride and sodium, which combine to make salt. Most of our table salt comes from the oceans.The salinity, the amount of salt, varies in the oceans. Oceans in warm, dry areas are more salty than the oceans in cold climates, such as near the North and South Poles. That’s because ocean water in warmer areas evaporates quicker, leaving more salt behind. The saltiest water in the world is in the Red Sea, which is almost entirely surrounded by deserts.

Tides

Tides, waves, and currents cause the oceans to move constantly. A tide is the regular rise and fall of the sea level in oceans and other bodies of water. It is caused by the effect of the Moon’s gravity on Earth. Water on the side of Earth that is closest to the Moon is pulled in the direction of the Moon, creating a bulge in the ocean. On the other side of Earth, the Moon’s gravity pulls Earth away from the water, producing a matching bulge. These bulges form high tides. Because the Earth rotates on its axis, every point on Earth (except the poles) travels through the two high tide zones and the two low tides zones each day. This is why there are two high tides and two low tides each day.Spring tides, which are especially strong, occur during new and full moons—every two weeks—when Earth, the

Moon, and the Sun are in alignment. Neap tides, which are weaker, occur during first and last quarter moons, when the Sun, the Earth, and the Moon form a right angle.

Waves

Wind causes waves in the ocean. As the wind blows over the surface of the ocean, it pushes on the water and transfers some of its energy to the water. The water gets energy from the wind because of the friction between air molecules and water molecules. It may seem that waves move forward or horizontally, but they do not. They move up and down. You can see this by watching a buoy in the water. It bobs up and down with waves rather than moving from side to side.

When the Earth formed about 4.5 billion years ago...

The ocean is not just where the land happens to be covered by water. The sea floor is geologically distinct from the continents. It is locked in a perpetual cycle of birth and destruction that shapes the ocean and controls much of the geology and geological history of the continents. Geological processes that occur beneath the waters of the sea affect not only marine life, but dry land as well. The processes that mold ocean basins occur slowly, over tens and hundreds of millions of years. On this timescale, where a human lifetime is but the blink of an eye, solid rocks flow like liquid, entire continents move across the face of the earth and mountains grow from flat plains. To understand the sea floor, we must learn to adopt the unfamiliar point of view of geological time. Geology is very important to marine biology. Habitats, or the places where organisms live, are directly shaped by geological processes. The form of coastlines; the depth of the water; whether the bottom is muddy, sandy, or rocky; and many other features of a marine habitat are determined by this geology. The geologic history of life is also called Palentology   .

The presence of large amounts of liquid water makes our planet unique. Most other planets have very little water, and on those that do, the water exists only as perpetually frozen ice or as vapor in the atmosphere. The earth, on the other hand, is very much a water planet. The ocean covers most of the globe and plays a crucial role in regulating our climate and atmosphere. Without water, life itself would be impossible.

Our ocean covers 72% of the earth's surface. It is not distributed equally with respect to the Equator. About two-thirds of the earth's land area is found in the Northern Hemisphere, which is only 61% ocean. About 80% of the Southern Hemisphere is ocean.

The ocean is traditionally classified into four large basins. The Pacific is the deepest and largest, almost as large as all the others combined. The Atlantic "Ocean" is a little larger than the Indian "Ocean", but the two are similar in average depth. The Arctic is the smallest and shallowest. Connected or marginal to the main ocean basins are various shallow seas, such as the Mediterranean Sea, the Gulf of Mexico and the South China Sea.Though we usually treat the oceans as four separate entities, they are actually interconnected. This can be seen most easily by looking at a map of the world as seen from the South Pole. From this view it is clear that the

Pacific, the Atlantic and Indian oceans are large branches of one vast ocean system. The connections among the major basins allow seawater, materials, and some organisms to move from one "ocean" to another. Because the "oceans" are actually one great interconnected system, oceanographers often speak of a single world ocean. They also refer to the continuous body of water that surrounds Antarctic as the Southern Ocean.

The earth and the rest of the solar system are thought to have originated about 4.5 billion years ago from a cloud or clouds of dust. This dust was debris remaining from a huge cosmic explosion called the big bang, which astrophysicists estimate occurred about 15 billion years ago. The dust particles collided with each other, merging into larger particles. These larger particles collided in turn, joining into pebble-sized rocks that collided to form larger rocks, and so on. The process continued, eventually building up the earth and other planets.

So much heat was produced as the early earth formed that the planet was probably molten. This allowed materials to settle within the planet according to their density. Density is the weight, or more correctly, the mass, of a given volume of a substance. Obviously, a pound of styrofoam weighs more than an ounce of lead, but most people think of lead as "heavier" than styrofoam. This is because lead weighs more than styrofoam if equal volumes of the two are compared. In other words, lead is denser than styrofoam. The density of a substance is calculated by dividing its mass by its volume. If two substances are mixed, the denser material will tend to sink and the less dense will float.During the time that the young earth was molten, the densest material tended to flow toward the center of the planet, while lighter materials floated toward the surface. The light surface material cooled to make a thin crust. Eventually, the atmosphere and oceans began to form. If the earth had settled into orbit only slightly closer to the sun, the planet would have been so hot that all the water would have evaporated into the atmosphere. With an orbit only slightly farther from the sun, all the water would be perpetually frozen. Fortunately for us, our planet orbits the sun in a narrow zone in which liquid water can exist. Without liquid water, there would be no life on earth.

The earth is composed of three main layers: the iron-rich core, the semiplastic mantle and the thin outer crust. The crust is the most familiar layer of earth. Compared to the deeper layers it is extremely thin, like a rigid skin floating on top of the mantle. The composition and characteristics of the crust differs greatly between the oceans and the continents.

The geological distinction between ocean and continents is caused by the physical and chemical differences in the rocks themselves, rather than whether or not the rocks happen to be covered with water. The part of earth covered with water, the ocean, is covered because of the nature of the underlying rock.Oceanic crustal rocks, which make up the sea floor, consists of minerals collectively called basalt that have a dark color. Most continental rocks are of general type called granite, which has a different mineral composition than basalt and is generally lighter in color. Ocean crust is denser than continental crust, though both are less dense than the underlying mantle. The continents can be thought of as thick blocks of crust "floating" on the mantle, much as icebergs float on water. Oceanic crust floats on the mantle too, but because it is denser it doesn't float as high as continental crust. This is why the

continents lie high and dry above sea level and oceanic crust lies below sea level and is covered by water. Oceanic crust and continental crust also differ in geological age. The oldest oceanic rocks are less 200 million years old, quite young by geological standards. Continental rocks, on the other hand, can be very old, as old as 3.8 billion years...!

In the years after World War II, sonar allowed the first detailed surveys of large areas of the sea floor. These surveys resulted in the discovery of the mid-oceanic ridge system, a 40,000 mile continuous chain of volcanic submarine mountains and valleys that encircle the globe like the seams of a baseball. The mid-oceanic ridge system is the largest geological feature on the planet. At regular intervals the mid-ocean ridge is displaced to one side or the other by cracks in the earth's crust known as transform faults. Occasionally the submarine mountains of the ridge rise so high that they break the surface to form islands, such as Iceland and the Azores.

The portion of the mid-ocean ridge in the Atlantic, known as the Mid-Atlantic Ridge, runs right down the center of the Atlantic Ocean, closely following the curves of the opposing coastlines. The ridge forms an inverted Y in the Indian Ocean and runs up the eastern side of the Pacific. The main section of ridge in the eastern Pacific is called the East Pacific Rise. Surveys also revealed the existence of a system of deep depressions in the sea floor called trenches. Trenches are especially common in the Pacific.

When the mid-ocean ridge system and trenches were discovered, geologists wanted to know how they were formed and began intensively studying them. They found that there's a great deal of geological activity around these features. Earthquakes are clustered at the ridges, for example, and volcanos are especially common near trenches. The characteristics of sea floor rocks are also related to the mid-oceanic ridges. Beginning in 1968, a deep-sea drilling ship, the Glomar Challenger   , obtained samples of the actual sea floor rock. It was found that the farther rocks are from the ridge crest the older they are. One of the most important findings came from the studying the magnetism of rocks on the sea floor. Bands of rock alternating between normal and reversed magnetism parallel the ridge.

It was the discovery of the magnetic anomalies on the sea floor, together with other evidence, that finally led to an understanding of plate tectonics. The earth surface is broken up into a number of plates. These plates, composed of the crust and the top parts of the mantle, make up the lithosphere. The plates are about 100    km thick. As new lithosphere is created, old lithosphere is destroyed somewhere else. Otherwise, the earth would have to constantly expand to make room for the new lithosphere. Lithosphere is destroyed at the trenches. A trench is formed when two plates collide and one plate dips below the other and slides back down in to the mantle. This downwards movement of the plate into the mantle is called subduction. Because subduction occurs at the trenches, trenches are often called subduction zones. Subduction is the process that produces earthquakes and volcanoes, also underwater. The volcanoes may rise from the sea floor to create chains of volcanic islands.We now realize that the earth's surface has undergone dramatic alterations. The continents have been carried long distances by the moving sea floor, and the ocean basins have changed in size and shape. In fact, new oceans have been born. Knowledge of the process of plate tectonics has allowed scientists to reconstruct much of

the history of these changes. Scientists have discovered, for example, that the continents were once united in a single supercontinent called Pangaea    that began to break up about 180 million years ago. The continents have since moved to their present position.

Seawater

The characteristics of seawater are due both to the nature of pure water and to the materials dissolved in it. The solids dissolved in seawater come from two main sources. Some are produced by the chemical weathering of rocks on land and are carried to sea by rivers. Other materials come from the earth's interior. Most of these are released into the ocean at hydrothermal vents. Some are released into the atmosphere from volcanoes and enter the ocean in rain and snow. Seawater contains at least a little of almost everything, but most of the solutes or dissolved materials, are made up of a surprisingly small group of ions. In fact, only six ions compose over 98% of the solids in seawater. Sodium and chloride account for about 85% of the solids, which is why seawater tastes like table salt. The salinity of the water strongly affects the organisms that live in it. Most marine organisms, for instance, will die in fresh water. Even slight changes in salinity will harm some organisms.

History of Oceanography

Oceanography may be one of the newest fields of science, but its roots extend back several tens of thousands of years when people began to venture from their coastlines in rafts. These first seafaring explorers, navigators and oceanographers began to pay attention to the ocean in many ways. They observed waves, storms, tides, and currents that carried their rafts in certain directions at different times. They sought fish for food. They realized that although ocean water didn’t look different from river water, it was salty and undrinkable. Their experiences and understanding of the oceans were passed down over thousands of years from generation to generation in myths and legends. 

But it wasn’t until about 2,850 years ago (850 BC) that early naturalists and philosophers started trying to make sense of the enormous bodies of water they saw from land. Because people could see only endless ocean from the shoreline, they believed the world was flat. That didn’t keep Columbus and others exploring the oceans in the late 1400s and early 1500s and finally discovering that the world is not flat, but round- a sphere whose surface is nearly 3/4-covered by oceans.

Modern oceanography began as a field of science only a little less than 130 years ago, in the late 19th century, after Americans, British and Europeans launched a few expeditions to explore ocean currents, ocean life, and the seafloor off their coastlines. The first scientific expedition to explore the world’s oceans and seafloor was the Challenger Expedition, from 1872 to 1876, on board the British three-masted warship HMS Challenger. 

But modern oceanography really took off less than 60 years ago, during World War II, when the U.S. Navy wanted to learn more about the oceans to gain fighting advantages, especially in submarine warfare. This section of Deeper Discovery will give you some background and history on the science of oceanography. It will show you

how important early studies were and how far we have come since then in understanding the oceans and seafloor -- Earth’s inner space.

Polynesian SeafarersMasters of Ocean CurrentsAbout 30,000 years ago, human cultures along the western coastline of the Pacific Ocean -- in the area between what is now Australia and China -- started to migrate eastward across the great expanse of the Pacific Ocean. We are not sure exactly why the migrations started, but tribal wars, disease epidemics, the search for food, or natural disasters such as large volcanic eruptions and earthquakes, may have been factors. 

Over about 25,000 years, these people, called the Polynesians, eventually colonized the islands of the south and western Pacific, from New Guinea in the west to Fiji and Samoa in the middle. Then they moved onward to Tahiti and finally Easter Island in the eastern south Pacific. The Polynesians colonized the Hawaiian Islands about 500 years ago. The Hawaiian Islands are among the world’s most remote island groups and were one of the last major island groups to be colonized by native cultures. How did the Polynesians manage to travel across thousands of miles of ocean without compasses, sextants, clocks, or other tools of modern navigation? Their migration was truly one of the great achievements of early seafaring cultures, and it marks the start of oceanographic observations by people who lived in harmony with the ocean.

The Polynesians were very observant. They noted the directions that waves came from and how they affected or rocked their canoes. They had a keen sense of ocean currents and variations in bird and sea life in different places in the Pacific. They also were among the first people to use astronomical observations of the stars to help them navigate across the ocean.They made the earliest form of navigational or oceanographic map, called stick charts. These were made of pieces of bamboo or other wood that were tied together. The locations of islands were often marked with shells or knots, and curved pieces of wood represented the bending of ocean waves around the islands and the way waves rocked their canoes. Polynesians handed down their lore of the sea in both the oral and stick map traditions.

The Mediterranean SeaAncient Myths About the OceansThe people who lived around the Mediterranean Sea began exploring this nearly landlocked sea several thousand years ago. Sailors from Egypt, Phoenicia and Crete mapped the regional coastlines to establish some of the earliest trading routes. Early Mediterranean civilizations, including the Greeks, have passed down many myths that include gods and goddesses who ruled over nature, such as Poseiden with his triton. Many Mediterranean legends, such as Jason and the Argonauts, also involved adventures on large and dangerous seas. 

Many of our earliest maps of the oceans and coastlines come from this region. These early mapmakers, or cartographers, were probably Mediterranean traders who made the maps to help them get back and forth to different cities on the Mediterranean coast. 

About 2,900 years ago, the Greeks began to venture outside the Mediterranean, past the Straits of Gibraltar at the western end of the Mediterranean Sea.

This narrow channel separates Europe from Africa, and the Mediterranean from the Atlantic Ocean. Just outside of the Straits of Gibraltar, early Greek sailors noticed a strong current running from north to south. Because the sailors had only seen currents in rivers, they thought this great body of water on the other side of the Straits was a very big river. The Greek word for river was okeano, which is the root of our word for ocean.

Voyages of Exploration and ScienceThe Age of DiscoveryAbout 650 years ago, European explorers turned to the sea to find faster trade routes to cities in Asia and Europe. Prince Henry the Navigator of Portugal recognized the oceans’ importance to trade and commerce and he established a center of learning for the marine sciences. You could think of it as the first oceanographic institution. Mariners came to the center in Sagres, Portugal, to learn about the oceans and currents and how to make maps. These early maps provided the basis for important expeditions. In the late 1400s, Cristopher Columbus became the first European to sail westward across the Atlantic Ocean and return home. In the early 1500s Ferdinand Magellan sailed all the way around, or circumnavigated, the globe. 

In the early 1700s, several European countries (mainly Spain, France and Britain) sought to expand their empires and discover new lands for raw materials, colonies or trade, and for spices from the East Indies, which they believed would help cure the Plague. They launched expeditions to survey faraway lands across the Atlantic, Pacific and Indian Oceans, and in doing so also explored the Arctic and Antarctic Oceans.

One of the most famous voyages of discovery of this time began in 1768 when the HMS Endeavour left Portsmouth, England, under the command of Captain James Cook. Over 10 years Cook led three world-encircling expeditions and mapped many countries, including Australia, New Zealand and the Hawaiian Islands. He was an expert seaman, navigator and scientist who made keen observations wherever he went. He was also one of the first ship captains to recognize that a lack of Vitamin C in sailors’ diets (due mostly to a lack of fresh fruit) caused scurvy, a serious disease that killed many sailors in those times. Cook always sailed with lots of pickled cabbage, which he insisted that the sailors eat. Scurvy was never a problem on his ships because the cabbage contained lots of Vitamin C.

In 1728, John Harrison, a British cabinetmaker and inventor, started working on an important instrument to aid seafarers navigating across large areas of ocean, far away from land or coastlines. At the time, pendulum clocks kept time. Obviously, these clocks did not work well on a ship on the rolling ocean! In 1736, after years of work, Harrison invented a clock that used a spring instead of a pendulum. It was the first marine chronometer, an instrument that could give accurate time on a rolling ship. With it, sailors could figure out how far east or west they had gone from 0° Longitude, or the prime meridian, and what longitude they were sailing past. By 1761, Harrison had built four clocks, each better than the one before. The last clock was tested on a voyage between England and Jamaica, and it kept excellent time. It ran only about 5 seconds slow per day, and the ship steered a clear course to Jamaica, a true feat in those days.

Benjamin Franklin

Discovering the Gulf StreamBesides being a famous statesman and diplomat, Benjamin Franklin was a well-known American scientist. He contributed to oceanography in the mid- to late 1700s by making and compiling good observations of ocean currents off the US East Coast. He was particularly interested in the Gulf Stream, a fast-moving current of warm surface water that sweeps up from Florida, along the continental slope off the US East Coast, and then bends eastward across the North Atlantic all the way to Europe. Franklin was the first to refer to the Gulf Stream as a “river in the ocean.” As Deputy Postmaster General of the American colonies, Franklin promoted using the Gulf Stream to speed up delivery of mail from America to Europe, as well as to improve other commercial shipping. 

The Gulf Stream is not really a “river in the ocean” as Franklin thought. But the waters that make up the Gulf Stream are “channeled” into a certain direction and speed by many factors-including prevailing winds, the rotation of the planet, and colder currents around and below the Gulf Stream.

This map of the Gulf Stream appears in the book by Benjamin Franklin and dates from 1769. The Gulf Stream is depicted as the dark gray swath that runs along the east coast of what is now the United States.

The amount of water carried in the Gulf Stream is equal to almost 100 million cubic meters per second, which is nearly 100 times the combined flow of all the rivers on Earth! The speed of the Gulf Stream can be as high as 5 knots. Now you can see why ships heading north and eastward across the North Atlantic tried to stay in the current. It would nearly double their speed, so they could complete their voyages more quickly.

OCEAN GEOGRAPHY

There are 328,000,000 cubic miles of seawater on earth, covering approximately 71 percent of earth's surface.

By volume, the ocean makes up 99 percent of the planet's living space- the largest space in our universe known to be inhabited by living organisms.

About 97 percent of all water on earth is in our oceans, 2 percent is frozen in our ice caps and glaciers, less than 0.3 percent is carried in the atmosphere in the form of clouds, rain, and snow. All of our inland seas, lakes and channels combined add up to only 0.02 percent of earth's water.

The Antarctic Ice Sheet is almost twice the size of the United States.

Earth's ocean is made up of more than 20 seas and four oceans: Atlantic, Indian, Arctic, and Pacific, the oldest and the largest.

The ocean accounts for 0.022 percent of the total weight of earth, weighing an estimated 1,450,000,000,000,000,000 short tons (1 short ton = 2,000lbs).

The average worldwide ocean depth is about 12,460 feet (3,798 meters), with the deepest

point of 36,198 feet (11,033 meters) which is located in the Mariana Trench in the Pacific Ocean; the tallest mountain, Mount Everest, measures 29,022 feet (8,846 meters). If Mount Everest were to be placed into the Mariana Trench it would be covered with sea water more than a mile (1.5 km ) deep.

Although Mount Everest is often called the tallest mountain on Earth, Mauna Kea, an inactive volcano on the island of Hawaii, is actually taller. Only 13,796 feet of Mauna Kea stands above sea level, yet it is 33,465 feet tall if measured from the ocean floor to its summit

A slow cascade of water beneath the Denmark Strait sinks 2.2 miles; more than 3.5 times farther than Venezuela's Angel Falls, the tallest waterfall on land.

Earth's largest continuous mountain chain is the Mid-Ocean Ridge, stretching for 40,000 miles, rising above the surface of the water in a few places, such as Iceland. It is four times longer than the Andes, Rocky Mountains, and Himalayas combined.

Ninety percent of all volcanic activity occurs in the oceans. In 1993, scientists located the largest known concentration of active volcanoes on the sea floor in the South Pacific. This area, the size of New York State, hosts 1,133 volcanic cones and seamounts. Two or three could erupt at any moment.

The highest tides in the world are at the Bay of Fundy, which separates New Brunswick from Nova Scotia. At some times of the year the difference between high and low tide is 53 feet 6 inches, the equivalent of a five-story building.

Canada has the longest coastline of any country, at 56,453 miles or around 15 percent of the world's 372,384 miles of coastlines.

In 1958, the United States Coast Guard icebreaker East Wind measured the world's tallest known iceberg off western Greenland. At 550 feet it was only 5 feet 6 inches shorter than the Washington Monument in Washington, D.C.

The volume of the Earth's moon is the same as the volume of the Pacific Ocean.

Ninety percent of all volcanic activity occurs in the oceans. In 1993, scientists located the largest known concentration of active volcanoes on the sea floor in the South Pacific. This area, the size of New York state, hosts 1,133 volcanic cones and sea mounts. Two or three could erupt at any moment.

The highest tides in the world are at the Bay of Fundy, which separates New Brunswick from Nova Scotia. At some times of the year the difference between high and low tide is 53 feet 6 inches, the equivalent of a three-story building.

The oceans cover 71 percent of the Earth's surface and contain 97 percent of the Earth's water. Less than 1 percent is fresh water, and 2-3 percent is contained in glaciers and ice caps.

Earth's longest mountain range is the Mid-Ocean Ridge, which winds around the globe from the Arctic Ocean to the Atlantic, skirting Africa, Asia and Australia, and crossing the Pacific to the west coast of North America. It is four times longer than the Andes, Rockies, and Himalayas combined.

Canada has the longest coastline of any country, at 56,453 miles or around 15 percent of the world's 372,384 miles of coastlines.

A slow cascade of water beneath the Denmark Strait sinks 2.2 miles, more than 3.5 times farther than Venezuela's Angel Falls, the tallest waterfall on land.

El Niño, a periodic shift of warm waters from the western to eastern Pacific Ocean, has dramatic effects on climate worldwide. In 1982-1983, the most severe El Niño of the century created droughts, crop failures, fires, torrential rains, floods, landslides--total damages were estimated at more than $8 billion.

At the deepest point in the ocean the pressure is more than 8 tons per square inch, or the equivalent of one person trying to support 50 jumbo jets.

At 39 degrees Fahrenheit, the temperature of almost all of the deep ocean is only a few degrees above freezing.

If mined, all the gold suspended in the world's seawater would give each person on Earth 9 pounds.

In 1958, the United States Coast Guard icebreaker East Wind measured the world's tallest known iceberg off western Greenland. At 550 feet it was only 5 feet 6 inches shorter than the Washington Monument in Washington, D.C.

Although Mount Everest, at 29,028 feet, is often called the tallest mountain on Earth, Mauna Kea, an inactive volcano on the island of Hawaii, is actually taller. Only 13,796 feet of Mauna Kea stands above sea level, yet it is 33,465 feet tall if measured from the ocean floor to its summit.

If the ocean's total salt content were dried, it would cover the continents to a depth of 5 feet.

Undersea earthquakes and other disturbances cause tsunamis, or great waves. The largest recorded tsunami measured 210 feet above sea level when it reached Siberia's Kamchatka Peninsula in 1737.

The Antarctic Ice Sheet is almost twice the size of the United States.

THE WEATHER MAKER

The ocean determines climate and plays a critical role in Earth's habitability. Most of the solar energy that reaches the Earth is stored in the ocean and helps power oceanic and atmospheric circulation. In this manner, the ocean plays an important role in influencing the weather and climatic patterns of the Earth.

Two hundred million years of recorded geologic and biologic history of the Earth are found in the ocean's floor. By studying ocean sediments, scientists can learn about ancient climate, how it changed, and how better to predict our own climate.

The top 10 feet of the ocean hold the same amount of thermal energy as exists in the entire atmosphere.

El NiZo, a periodic shift of warm waters from the western to eastern Pacific Ocean, has dramatic effects on climate worldwide. In 1997-1998, the most severe El NiZo of the century created droughts, crop failures, fires, torrential rains, floods, landslides--total damages were estimated at more than $90 billion (United Nations)

Undersea earthquakes and other disturbances cause tsunamis, or great waves. The largest recorded tsunami measured 210 feet above sea level when it reached Siberia's Kamchatka Peninsula in 1737.

OUR USE OF THE OCEAN

Substances from marine plants and animals are used in scores of products, including medicine, ice cream, toothpaste, fertilizers, gasoline, cosmetics, and livestock feed.

Examine the foods in your own kitchen and you may find the terms "alginate" and "carrageenan" on the labels. Carrageenans are compounds extracted from red algae that are used to stabilize and jell foods and pharmaceuticals. Brown algae contain alginates that make foods thicker and creamier and add to shelf life. They are used to prevent ice crystals from forming in ice cream. Alginates and carrageenans are often used in puddings, milkshakes, and ice cream. The commonly used color additive beta-carotene often comes from green algae as well as many vegetables, including carrots.  Many people don't realize that kelp is harvested like wheat; a substance called algin is extracted and is used in lipstick, toothpaste and ice cream. You might be wearing kelp right now, since it is used in the dyes that color our clothes.

Oils from the orange roughy, Hoplostethus atlanticus, a deep-sea fish from New Zealand, are used in making shampoo.

The remains of diatoms, algae with hard shells, are used in making pet litter, cosmetics, pool filters and tooth polish.

The ocean holds immense quantities of protein. The total annual commercial harvest from the seas exceeds 85 million metric tons. Fish is the biggest source of wild or domestic protein in the world.

Since the architecture and chemistry of coral are very close to human bone, coral has been used to replace bone grafts in helping human bones to heal quickly and cleanly.

Horseshoe crabs have existed in essentially the same form for the past 135 million years. Their blood provides a valuable test for the toxins that cause septic shock, which previously led to half of all hospital-acquired infections and one-fifth of all hospital deaths.

Over 90 percent of trade among countries is carried by ships. 

The ocean is a source of mineral deposits, including oil and gas.

About half the communications between nations are via underwater cables. 

Many nations' battles have been fought on or under the water. 

Knowing oceanography can enhance the conditions for trade, communications, and defense.

OUR MISUSE OF THE OCEAN

In 1993, United States beaches were closed or swimmers advised not to get in the water over 2,400 times because of sewage contamination. The problem is even worse than the numbers indicate: there are no federal requirements for notifying the public when water-quality standards are violated, and some coastal states don't monitor water at beaches.

The largest amount of oil entering the ocean through human activity is the 363 million gallons that come from industrial waste and automobiles. When people pour their used motor oil into the ground or into a septic system, it eventually seeps into the groundwater. Coupled with industrial waste discharged into rivers, oil becomes part of the run-off from waterways that empty into the ocean. All of this oil impacts ocean ecosystems.

The Coast Guard estimates that for United States waters, sewage treatment plants discharge twice as much oil each year as tanker spills.

Animals may perish when the oil slicks their fur or downy feathers, decreasing the surface area so they are no longer insulated from the cold water. Or the animals may ingest the oil, then become sick or unable to reproduce properly.

Each year industrial, household cleaning, gardening, and automotive products are added as water pollutants. About 65,000 chemicals are

used commercially in the United States today, with about 1,000 new ones added each year. Only about 300 have been extensively tested for toxicity.

It is estimated that medical waste that washed up onto Long Island and New Jersey beaches in the summer of 1988 cost as much as $3 billion in lost revenue from tourism and recreation.

The most frequently found item in beach cleanups are pieces of plastic. The next four items are plastic foam, plastic utensils, pieces of glass and cigarette butts.

Lost or discarded fishing nets keep on fishing. Called "ghost nets," this gear entangles fish, marine mammals, and sea birds, preventing them from feeding or causing them to drown. As many as 20,000 northern fur seals may die each year from becoming entangled in netting.

The Mississippi River drains more than 40 percent of the continental United States, carrying excess nutrients into the Gulf of Mexico. Decay of the resulting algae blooms consumes oxygen, kills shellfish and displaces fish in a 4,000 square mile bottom area off the coast of Louisiana and Texas, called the "dead zone."

The zebra mussel is the most famous unwanted ship stowaway, but the animals and plants being transported to new areas through ship ballast water is a problem around the world. Poisonous algae, cholera, and countless plants and animals have invaded harbor waters and disrupted ecological balance.

There are 109 countries with coral reefs. Reefs in 90 of them are being damaged by cruise ship anchors and sewage, by tourists breaking off chunks of coral, and by commercial harvesting for sale to tourists.

One study of a cruise ship anchor dropped in a coral reef for one day found an area about half the size of a football field completely destroyed, and half again as much covered by rubble that died later. It was estimated that coral recovery would take fifty years.

Egypt's High Aswan Dam, built in the 1960s to provide electricity and irrigation water, diverts up to 95 percent of the Nile River's normal flow. It has since trapped more than one million tons of nutrient rich silt and caused a sharp decline in Mediterranean sardine and shrimp fisheries.

The United Nations Food and Agriculture Organization estimates that of the seventeen major fisheries areas in the world, four are depleted and the other thirteen are either fished to capacity or overfished.

Commercial marine fisheries in the United States discard up to 20 billion pounds of non-target fish each year-- twice the catch of desired commercial and recreational fishing combined. Worldwide this

adds up to a staggering 60 billion pounds each year!!

With only 4.3 percent of the world population, Americans use about one-third of the world's processed mineral resources and about one-fourth of the world's non-renewable energy sources, like oil and coal.

FACTS ABOUT OCEAN LIFE

Life began in the seas 3.1 billion to 3.4 billion years ago. Land dwellers appeared 400 million years ago; a relatively recent point in the geologic time line.

The blue whale, Balaenoptera musculus, is the largest known animal ever to have lived on sea or land. They can reach over 110 feet and weigh almost 200 tons (more than the combined weight of 50 adult elephants). The blue whale's blood vessels are so broad that a full-grown trout could swim through them, and the heart is the size of a small car.

The oarfish, Regalecus glesne, is the longest bony fish in the world. With its snakelike body, sporting a magnificent red fin along its 50-foot length horselike face and blue gills, it accounts for many sea-serpent sightings

Green turtles can migrate more than 1,400 miles to lay their eggs.

Bluefin tuna, Thunnus thynnus, are among the largest and fastest marine fish. An adult may weigh 1,500 pounds and swim up to 55 miles per hour.

Penguins "fly" underwater at up to 55 miles per hour.

A group of herring is called a seige. A group of jelly fish is called a smack.

Many fish can change sex during the course of their lives. Others, especially rare deep-sea fish, have both male and female sex organs.

Giant kelp, the fastest growing plant in the ocean, can grow up to 2 feet per day. Under optimal conditions, giant kelp can grow to a length of more than 100 feet in little more than a year and can grow to a maximum of 200 feet. 

Hydrothermal vents, fractures in the sea floor that spew sulphur compounds, support the only complex ecosystem known to run on chemicals, rather than energy from the sun.

OCEAN PHYSICS AND CHEMISTRY

At the deepest point in the ocean the pressure is more than 8 tons per square inch, or the equivalent of one person trying to hold-up 50 jumbo jets against the force of gravity.

The major ions in seawater are Na+, Mg2+, Ca2+, K+, Sr2+, Cl-, SO4

2- (sulfate), HCO3-(bicarbonate), Br-,

B(OH)3 (boric acid), and F-. Together, they account for almost all of the salt in seawater.

At 39 degrees Fahrenheit (3.89 degrees Celsius), the temperature of almost all of the deep ocean is only a few degrees above freezing.

If extracted, it is estimated that all the gold suspended in the world's seawater would give each person on Earth 9 pounds.

If the ocean's total salt content were dried, it would cover the continents to a depth of 500 feet.

When nitrogen and phosphorus from sources such as fertilizer, sewage and detergents enter coastal waters, oxygen depletion occurs. One gram of nitrogen can cause enough organic growth to require 15 grams of oxygen to decompose the resulting vegetation. A single gram of phosphorus will deplete about one hundred grams of oxygen.

Sea

A sea is a large body of saline water that may be connected with an ocean or may be a large saline lake that, like the Caspian Sea, lacks a natural outlet. Sometimes the terms sea and ocean are used synonymously. The Arctic (belonging to the Arctic Ocean) and Antarctic (Southern Ocean) seas are two seas that freeze in winter (this occurs below the freezing point of pure water—at about −1.8 °C (28.8 °F). Frozen salt water is transformed into "sea ice".Humans have navigated the seas since antiquity. The Ancient Egyptians and Phoenicians navigated the Mediterranean Sea and the Red Sea, while Hannu was the first sea explorer for whom substantial information exists in the modern era. Hannu sailed along the Red Sea, eventually reaching theArabian Peninsula and the African Coast around 2750 BC.[3] In the 1st millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean, including outlets like the Black Sea. The seas along the eastern and southern Asian coast were used by the Arabs and Chinese for navigation, while the North Sea and the Baltic Sea were known to Europeans during Roman times. Other seas were not used for navigation in the ancient era, as they had yet to be discovered.The White Sea was known to Novgorodians and used for navigation since not later that the 13th century.[4] Pomors, living at the White Sea coast, also sailed to Svalbard, but the Barents Sea got its name later, due to the 16th century Dutch expedition headed by Willem Barentsz. Other seas in Arctic Russia were explored in connection with the search of the Northern Sea Route. In the first half of the 17th century the Kara Sea was already used on a regular basis for navigation between the city of Arkhangelsk and the mouth of the Ob River and upstream to the city of Mangazeya (Mangazeya Trade Route) and to the mouth of the Yenisei River (Yenisey Trade Route).[5] In 1648, Semyon Dezhnev led an expedition down the Kolyma River, around the cape now known as Cape Dezhnev, and to the mouth of the Anadyr

River.[6] By the end of the 17th century, the seas along what is now the Arctic and Pacific coasts of Russia were already discovered, although the systematic description and reliable mapping of the coast line only began in the 18th century, and the geographical locations of all islands were not established until the first half of the 20th century,  when aviation was employed.

List of the Ocean and Seas

Atlantic Ocean

Baltic Sea

Archipelago SeaBay of BothniaBothnian Sea

Central Baltic SeaGulf of BothniaGulf of Finland

Gulf of RigaOresund StraitSea of Åland

Mediterranean Sea

Aegean SeaMirtoon SeaSea of CreteThracian SeaAdriatic SeaAlboran Sea

Balearic SeaCatalan SeaCilician SeaGulf of SidraIonian SeaLevantine Sea

Libyan SeaLigurian SeaSea of SardiniaSea of SicilyTyrrhenian Sea

Others

Argentine SeaBay of BiscayBay of CampecheBay of FundyBlack SeaCaribbean SeaCeltic SeaChesapeake BayDavis Strait

Denmark StraitEnglish ChannelGulf of GuineaGulf of MaineGulf of MexicoGulf of St. LawrenceGulf of VenezuelaIrish Sea

Labrador SeaMarmara SeaNorth SeaNorwegian SeaSargasso SeaSea of AzovSea of the HebridesWadden Sea

Arctic Ocean

Amundsen GulfBaffin BayBarents SeaBeaufort SeaChukchi SeaEast Siberian Sea

Greenland SeaHudson BayJames BayKara SeaKara Strait

Laptev SeaLincoln SeaPrince Gustav Adolf SeaPechora SeaWhite Sea

Southern Ocean

Amundsen Sea D'Urville Sea Mawson Sea

Bass StraitBellingshausen SeaCooperation SeaCosmonauts SeaDavis Sea

Drake PassageGreat Australian BightGulf St VincentKing Haakon VII SeaLazarev Sea

Riiser-Larsen SeaRoss SeaScotia SeaSomov SeaSpencer GulfWeddell Sea

Indian Ocean

Andaman SeaArabian SeaBay of BengalGulf of Aden

Gulf of OmanLaccadive SeaMozambique Channel

Persian GulfRed SeaTimor Sea

Pacific Ocean

Arafura SeaBanda SeaBering SeaBismarck SeaBohai SeaBohol Sea (also known as the Mindanao Sea)Camotes SeaCelebes SeaCeram SeaChilean SeaSea of ChiloéCoral Sea

East China SeaFlores SeaGulf of AlaskaGulf of California (also known as the Sea of Cortéz)Gulf of CarpentariaGulf of ThailandHalmahera SeaJava SeaKoro SeaMolucca SeaPhilippine SeaSalish Sea

Savu SeaSea of JapanSea of OkhotskSeto Inland SeaSibuyan SeaSolomon SeaSouth China SeaSulu SeaTasman SeaVisayan SeaYellow Sea

Landlocked seas

Some large inland lakes, usually brackish, are called "seas".

Aral SeaCaspian SeaDead Sea

Great Salt LakeSalton Sea

Galaxy

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component called dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way galaxy. Examples of galaxies range from dwarfs with as few as ten million (107) stars to giants with a hundred trillion (1014) stars, each orbiting their galaxy's own center of mass.

Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust,

and cosmic rays. Dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.

Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merging, sometimes induce significantly increased incidents of star formation leading to starburst galaxies. Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.

There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe. Most are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.

NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years away from Earth.

Etymology

The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας, "milky one"), or kyklos

("circle") galaktikos ("milky")[12] for its appearance in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine

milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our galaxy, the Milky Way, to distinguish it from the billions of other galaxies. The English term Milky Way can be traced back to a story by Chaucer:

"See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt."

—Geoffrey Chaucer. The House of Fame, c. 1380.

When William Herschel constructed his catalog of deep sky objects in 1786, he used the name spiral nebula for certain objects such as M31. These would later be recognized as immense conglomerations of stars, when the true distance to these objects began to be appreciated, and they would be termed island universes. However, the word Universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.

Observation history

Milky Way

Galactic Center of the Milky Way

The Greek philosopher Democritus (450–370 BC) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[16] Aristotle (384–322 BC), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 AD)

was scientifically critical of this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial.

This idea would be influential later in the Islamic world.According to Mohani Mohamed, the Arabian

astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it was very remote from the Earth and did not belong to the atmosphere." The Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." The Andalusian astronomer Ibn Bajjah ("Avempace", d. 1138) proposed that the Milky Way was made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near. In the 14th century, the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars".

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. In 1750 the English astronomer Thomas Wright, in

his An original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.

The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the solar system was assumed to be near the center.

The first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.

Distinction from other nebulae

In the 10th century, the Persian astronomer, Azophi, made the earliest recorded observation of the Andromeda Galaxy, describing it as a "small cloud". The Andromeda Galaxy was independently rediscovered by Simon Marius in 1612. Al-Sufi also identified the Large Magellanic Cloud, which is visible from Yemen, though not from Isfahan; it was not seen by Europeans until Magellan's voyage in the 16th century. These were the first galaxies other than the Milky Way to be observed from Earth. Al-Sufi published his findings in his Book of Fixed Stars in 964.

In 1750 Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. In 1755, Immanuel Kant introduced the term "island universe" for these distant nebulae.Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.

Sketch of Messier 51 by Lord Rosse in 1845, later known as the Whirlpool Galaxy

In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine if they were made from chemicals that would be expected in a planetary system. However, Slipher discovered that the spiral nebulae had high red shifts, indicating that they were moving away at rate higher than the Milky Way's escape velocity. Thus they were not gravitationally bound to the Milky Way, and were unlikely to be a part of the galaxy.

In 1917, Heber Curtis had observed a nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.

In 1920 the so-called Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.The matter was conclusively settled in the early 1920s. In 1922, the Estonian astronomer Ernst Öpik gave a distance determination which supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100 inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

Modern research

In 1944, Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm resulting from interstellar atomic hydrogen gas; this radiation was observed in 1951. The radiation allowed for much improved study of the Milky Way Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structure in the center of the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies.In the 1970s it was discovered in Vera Rubin's study of the rotation speed of gas in galaxies that the total visible mass (from the stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. The Hubble Deep Field, an extremely long exposure of a

relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the universe. Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked by the Milky Way) have revealed a number of new galaxies.

Types and morphology

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) and activity in the core (in active galaxies).

Types of galaxies according to the Hubble classification scheme. An E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy.

Ellipticals

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters.

The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical

galaxies are often found near the core of large galaxy clusters. Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.

Spirals

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves". As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.

A majority of spiral galaxies have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) that indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011) stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.

Other morphologies

Hoag's Object, an example of a ring galaxy

Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy. Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars. (Barred lenticular galaxies receive Hubble classification SB0.)

In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme. Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted. Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.

NGC 5866, an example of a lenticular galaxy

Dwarfs

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a

few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether the galaxy has thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.

Interacting

The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter. Hence interactions between these galaxies are relatively frequent, and play an important role in their evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.

The Antennae Galaxies are undergoing a collision that will result in their eventual merger.

Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars within these interacting galaxies will typically pass straight through without colliding. However, the gas and dust within the two forms will interact. This can trigger bursts of star formation as the interstellar medium becomes disrupted and compressed. A collision can severely distort the shape of one or both galaxies, forming bars, rings or tail-like structures.At the extreme of interactions are galactic mergers. In this case the relative momentum of the two galaxies is insufficient to allow the galaxies to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. In the case where one of the galaxies is much more massive, however, the result is known as cannibalism. In this case the larger galaxy will remain relatively undisturbed by the merger, while the smaller galaxy is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.

Starburst

M82, the archetype starburst galaxy, has experienced a 10-fold increase[69] in star formation rate as compared to a "normal" galaxy.

Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, known as a starburst. Should they continue to do so, however, they would consume their reserve of gas in a time frame lower than the lifespan of the galaxy. Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the universe, and, at present, still contribute an estimated 15% to the total star production rate.

Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions. These massive stars produce supernova explosions, resulting in expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the starburst activity come to an end.Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.

Active nucleus

A portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energy output from the galaxy is emitted by a source other than the stars, dust and interstellar medium.

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc. In about 10% of these objects, a diametrically opposed pair of energetic jets ejects particles from the core at velocities close to the speed of light. The mechanism for producing these jets is still not well understood.

Active galaxies that emit high-energy radiation in the form of x-rays are classified as Seyfert galaxies or quasars, depending on the luminosity. Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of the Earth. A radio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer.

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. Approximately one-third of nearby galaxies are classified as containing LINER nuclei.

Formation and evolution

The study of galactic formation and evolution attempts to answer questions regarding how galaxies formed and their evolutionary path over the history of the universe.

Some theories in this field have now become widely accepted, but it is still an active area in astrophysics.

Formation

Current cosmological models of the early Universe are based on the Big Bang theory. About 300,000 years

after this event, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result this period has been called the "Dark Ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures would eventually become the galaxies we see today.Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and primordial galaxy yet seen. While some scientists have claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in

an earlier stage of the Universe's evolution), IOK-1's age and composition have been more reliably established. The existence of such early protogalaxies suggests that they must have grown in the so-called "Dark Ages".

Artist's impression of a young galaxy accreting material. Credit ESO/L. Calçada

The detailed process by which such early galaxy formation occurred is a major open question in astronomy. Theories could be divided into two categories: top-down and bottom-up. In top-down theories (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.

Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. These were composed almost entirely of hydrogen and helium, and may have been massive. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium. This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.

Evolution

Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added. During this early epoch, galaxies undergo a major burst of star formation.

During the following two billion years, the accumulated matter settles into a galactic disc. A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life. This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.

Hubble eXtreme Deep Field (XDF)The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology. Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676 or the Antennae Galaxies.

As an example of such an interaction, the Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two may collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda

before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked approximately ten billion years ago.

Future trends

At present, most star formation occurs in smaller galaxies where cool gas is not so depleted. Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms. Elliptical galaxies are already largely devoid of this gas, and so form no new stars. The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.

Larger-scale structures

Seyfert's Sextet is an example of a compact galaxy group.

Deep sky surveys show that galaxies are often found in relatively close association with other galaxies. Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, these isolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller, satellite galaxies. Isolated galaxies can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early in the universe, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This on-going merger process (as well as an influx of infalling gas) heats the inter-galactic gas within a cluster to very high temperatures, reaching 30–100 megakelvins. About 70–80% of the mass in a cluster is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent of the matter in the form of galaxies.

Most galaxies in the universe are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchy of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster, and these formations contain a majority of the galaxies (as well as most of the baryonic mass) in the universe. To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see Virial theorem). If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.

Larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time, tidally destroys its satellite galaxies and adds their mass to its own.

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. Above this scale, the universe appears to be isotropic and homogeneous.

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two galaxies. The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster.

Multi-wavelength observation

A visual light image of Andromeda Galaxy shows the emission of ordinary stars and the light reflected by dust.

After galaxies external to the Milky Way were found to exist, initial observations were made mostly using visible light. The peak radiation of most stars lies here, so the observation of the stars that form galaxies has been a major component of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions, and for examining the distribution of dusty arms.

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier in the history of the universe. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.) Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via 21   cm radiation ), including, potentially, the non-

ionized matter in the early universe that later collapsed to form galaxies.

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. An ultraviolet flare was observed when a star in a distant galaxy was torn apart from the tidal forces of a black hole. The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of super-massive black holes at the cores of galaxies was confirmed through X-ray astronomy.

This ultraviolet image of Andromeda shows blue regions containing young, massive stars.

List of galaxies

Galaxy Constellation Notes

M82 Ursa Major Also called the Cigar Galaxy. This is the prototype starburst galaxy.

M87 VirgoThis is the central galaxy of the Virgo Cluster, the central cluster of the Local Supercluster [1]

M102 Draco (Ursa Major)

This galaxy cannot be definitively identified, with the most likely candidate being NGC 5866, and a good chance of it being a misidentification of M101. Other candidates have also been suggested.

NGC 2770 Lynx

NGC 2770 is referred to as the Supernova Factory due to three recent supernovae occurring within it.

NGC 3314

NGC 3314aNGC 3314b

Hydra

This is a pair of spiral galaxies, one superimposed on another, at two separate and distinct ranges, and unrelated to each other. It is a rare chance visual alignment.

ESO 137-001 Triangulum Australe

Lying in the galaxy cluster Abell 3627, this galaxy is being stripped of its gas by the pressure of the intracluster medium (ICM), due to its high speed traversal through the cluster, and is leaving a high density tail with large amounts of star formation. The tail features the largest amount of star formation outside of a galaxy seen so far. The galaxy has the appearance of a comet, with the head being the galaxy, and a tail of gas and stars.

Comet Galaxy Sculptor

Lying in galaxy cluster Abell 2667, this spiral galaxy is being tidally stripped of stars and gas through its high speed traversal through the cluster, having the appearance of a comet.

List of named galaxies

This is a list of galaxies that are well known by something other than an entry in a catalog or list, or a set of coordinates, or a systematic designation.

Galaxy Constellation Origin of name Notes

Milky Way Galaxy

Sagittarius (centre)

This is the galaxy that contains Earth, it is named after the nebulosity in the night sky that marks the densest concentration of stars of our galaxy in the sky, which appears to blur together into a faint glow, called the Milky Way.

Andromeda Andromeda

Commonly just Andromeda, this, called the Andromeda Galaxy, Andromeda Nebula, Great Andromeda Nebula, Andromeda Spiral Nebula, and such, has been traditionally called Andromeda, after the constellation in which it lies.

Bode's Galaxy Ursa Major

Named for Johann Elert Bode who discovered this galaxy in 1774.

Cartwheel Galaxy Sculptor

Its visual appearance is similar to that of a spoked cartwheel.

Cigar Galaxy Ursa Major Appears similar in

shape to a cigar.

Comet Galaxy Sculptor

This galaxy is named after its unusual appearance, looking like a comet.

The comet effect is caused by tidal stripping by its galaxy cluster, Abell 2667.

Hoag's Object

Serpens Caput

This is named after Art Hoag, who discovered this ring galaxy.

It is of the subtype Hoag-type galaxy, and may in fact be a polar-ring galaxy with the ring in the plane of rotation of the central object.

Large Magellanic Cloud

Dorado/Mensa

Named after Ferdinand Magellan

This is the fourth largest galaxy in the Local Group, and forms a pair with the SMC, and from recent research, may not be part of the Milky Way system of satellites at all.

Small Magellanic Cloud

Tucana Named after Ferdinand Magellan

This forms a pair with the LMC, and from recent research, may not be part of the Milky Way system of satellites at all.

Mayall's Object

Ursa Major This is named after Nicholas U. Mayall, of the Lick Observatory, who discovered it.[6][7]

Also called VV 32 and Arp 148, this is a very peculiar looking object,

[8]

and is likely to be not one galaxy, but two galaxies undergoing a collision. Event in images is a spindle shape and a ring shape.

Pinwheel Galaxy Ursa Major Similar in appearance

to a pinwheel (toy).

Sombrero Galaxy Virgo Similar in appearance

to a sombrero.

Sunflower Galaxy

Canes Venatici

Tadpole Galaxy Draco

The name comes from the resemblance of the galaxy to a tadpole.

This shape resulted from tidal interaction that drew out a long tidal tail.

Whirlpool Galaxy

Canes Venatici

From the whirlpool appearance this gravitationally disturbed galaxy exhibits.

List of naked-eye galaxies

This is a list of galaxies that are visible to the naked-eye, for at the very least, keen-eyed observers in a very dark-sky environment that is high in altitude, during clear and stable weather.

Naked-eye Galaxies

GalaxyApparent Magnitude

Distance Constellation Notes

Milky Way Galaxy

-26.74 (the Sun) 0 Sagittarius

(centre)

This is our galaxy, most things visible to the naked-eye in the sky are part of it, including the Milky Way composing the Zone of Avoidance.

Large Magellanic Cloud

0.9 160 kly (50 kpc)

Dorado/Mensa

Visible only from the southern hemisphere. It is also the brightest patch of nebulosity in the sky.[9][10][11]

Small Magellanic Cloud (NGC292)

2.7 200 kly (60 kpc) Tucana

Visible only from the southern hemisphere.

Andromeda Galaxy (M31, NGC224)

3.4 2.5 Mly (780 kpc) Andromeda

Once called the Great Andromeda Nebula, it is situated in the Andromeda constellation.

Omega Centauri (NGC5139)

3.7 18 kly (5.5 kpc)

Centaurus Once thought to be a star and later a globular

cluster, Omega Centauri was confirmed as having a black hole at its center and thus its status has been changed to being a dwarf galaxy as of April 2010.

Triangulum Galaxy (M33, NGC598)

5.7 2.9 Mly (900 kpc) Triangulum

Being a diffuse object, its visibility is strongly affected by even small amounts of light pollution, ranging from easily visible in direct vision in truly dark skies to a difficult averted vision object in rural/suburban skies.

Centaurus A (NGC 5128)

7.8

13.7 ± 0.9 Mly (4.2 ± 0.3 Mpc)

Centaurus

Centaurus A has been spotted with the naked eye by Stephen James O'Meara

Bode's Galaxy (M81, NGC3031)

7.89 12 Mly (3.6 Mpc) Ursa Major

Highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions.

Sculptor Galaxy (NGC 253)

8.0

11.4 ± 0.7 Mly (3.5 ± 0.2 Mpc)

Sculptor

According to Brian A. Skiff, the naked-eye visibility of this galaxy is discussed in an old Sky & Telescope letter or note from the late 1960s or early 1970s.

Messier 83 (NGC 5236)

8.2 14.7 Mly (4.5 Mpc) Hydra

M83 has reportedly been seen with the naked eye.

Sagittarius Dwarf Elliptical Galaxy is not listed, because it is not discernible as being a separate galaxy in the sky.

Firsts

Galactic Firsts

First Galaxy Constellation Date Notes

First galaxy Milky Way Galaxy & Andromeda Galaxy

Sagittarius (centre) & Andromeda

1923 Edwin Hubble determined the distance to the Andromeda Nebula, and found that it could not be part of the Milky Way, so defining that Milky

Way was not the entire universe, and making the two separate objects, and two galaxies. However, the first galaxies seen would be all of the naked-eye galaxies, but they were not identified as such until the 20th century.

First radio galaxy Cygnus A Cygnus 1952

Of several items, then called radio stars, Cygnus A was identified with a distant galaxy, being the first of many radio stars to become a radio galaxy.

First quasar 3C2733C48

VirgoTriangulum

19621960

3C273 was the first quasar with its redshift determined, and by some considered the first quasar. 3C48 was the first "radio-star" with an unreadable spectrum, and by others considered the first quasar.

First Seyfert galaxy

NGC 1068 (M77) Cetus 1908

The characteristics of Seyfert galaxies were first observed in M77 in 1908, however, Seyferts were defined as a class in 1943.

First low surface brightness galaxy

Malin 1 Coma Berenices 1986

Malin 1 was the first verified LSB galaxy. LSB galaxies had been first theorized in 1976.

First radio galaxy Cygnus A Cygnus 1951

First discovered object, later identified to be a cannibalized galaxy

Omega Centauri Centaurus

Omega Centauri is considered the core of a disrupted dwarf spheroidal galaxy cannibalized by the Milky Way, and was originally catalogued in 1677 as a nebula. It is currently catalogued as a globular cluster.

First superluminal galactic jet

3C279 Virgo 1971 The jet is emitted by a quasar

First superluminal jet from a Seyfert

III Zw 2 Pisces 2000

First spiral galaxy

Whirlpool Galaxy

Canes Venatici

1845 Lord William Parsons, Earl of Rosse discovered the first spiral nebula from

observing the M51 white nebula.[28]

Prototypes

This is a list of galaxies that became prototypes for a class of galaxies.

Prototype Galaxies

Class Galaxy Constellation Date Notes

BL Lac object

BL Lacertae (BL Lac)

Lacerta

This AGN was originally catalogued as a variable star, and "stars" of its type are considered BL Lac objects.

Hoag-type Galaxy

Hoag's Object

Serpens Caput

This is the prototype Hoag-type Ring Galaxy

Giant LSB galaxy Malin 1 Coma

Berenices 1986

FR II radio galaxy(double-lobed radio galaxy)

Cygnus A Cygnus 1951

Extremes

Title Galaxy Data Constellation Notes

Least separation between binary central black holes

4C 37.11

24 ly (7.3 pc) Perseus

OJ 287 has an inferred pair with a 12 year orbital period, and thus would be much closer than 4C 37.11's pair.

Distances

Title Galaxy Constellation

Distance Notes

Closest neighbouring galaxy

Canis Major Dwarf Canis Major 0.025 Ml

y

Discovered in 2003, a satellite of the Milky Way, slowly being cannibalized by it.

Most distant galaxy

UDFj-39546284

Fornax z≃10.3 With an estimated distance of about 13.2

billion light-years (comoving distance), it is announced as the oldest and farthest astronomical object known.

Closest quasar 3C 273 Virgo z=0.158

First identified quasar, this is the most commonly accepted nearest quasar.

Most distant quasar

CFHQS J2329-0301 Pisces z=6.43 Discovered in

2007.

Closest radio galaxy

Centaurus A (NGC 5128, PKS 1322-427)

Centaurus 13.7 Mly

Most distant radio galaxy

TN J0924-2201 Hydra z=5.2

Closest Seyfert galaxy

Circinus Galaxy Circinus 13 Mly

This is also the closest Seyfert 2 galaxy. The closest Seyfert 1 galaxy is NGC 4151.

Most distant Seyfert galaxy

z=

Closest blazar

Markarian 421 (Mrk 421, Mkn 421, PKS 1101+384, LEDA 33452)

Ursa Major z=0.030 This is a BL Lac object.

Most distant blazar Q0906+6930 Ursa Major z=5.47

This is a flat spectrum radio-loud quasar type blazar.

Closest BL Lac object

Markarian 421 (Mkn 421, Mrk 421, PKS 1101+384, LEDA 33452)

Ursa Major z=0.030

Most distant BL Lac object

z=

Closest LINER

Most distant LINER z=

Closest LIRG

Most distant LIRG z=

Closest ULIRG

IC 1127 (Arp 220, APG 220)

Serpens Caput z=0.018

Most distant ULIRG z=

Closest starburst galaxy

Cigar Galaxy (M82, Arp 337/APG 337,

Ursa Major 3.2 Mpc

3C 231, Ursa Major A)

Most distant starburst galaxy

z=

[edit] Brightness and power

Title Galaxy Data Notes

Apparently brightest galaxy

Baby Boom Galaxy

Starburst galaxy located in the very distant universe.

Apparently faintest galaxy

Apparent magnitude

Intrinsically brightest galaxy

Absolute magnitude

Markarian 231 is the most luminous nearby galaxy (~590 Mly; apmag 13.8).

Intrinsically faintest galaxy

Boötes Dwarf Galaxy (Boo dSph)

Absolute magnitude -6.75

This does not include dark galaxies.

Highest surface brightness galaxy

Lowest surface brightness galaxy

Andromeda IX

Visually brightest galaxy

Large Magellanic Cloud

Apparent magnitude 0.6

This galaxy has high surface brightness combined with high apparent brightness.

Visually faintest galaxy

This galaxy has low surface brightness combined with low apparent brightness.

Mass

Title Galaxy Mass Notes

Least massive galaxy Willman 1 ~500,000

MSun

Most massive galaxy

Messier 87 (M87, NGC 4486, Virgo A)

6×1012

MSun

Most massive spiral galaxy ISOHDFS 27 1.04×1012

MSun

The preceding most massive spiral was UGC 12591

Least massive galaxy with globular cluster(s)

Andromeda I

Dimension

Title Galaxy Size Notes

Most expansive galaxy

IC 1101 5-6 million light-

years

Least expansive galaxy

Visually largest galaxy

Large Magellanic Cloud

650 × 550 arcmin

The LMC takes up more of the sky than any other galaxy, due to its nearness to us.

NOTE: The Milky Way Galaxy, our galaxy, cannot be measured, as we reside inside it. However, if only counting the Milky Way, that bright path in the sky, it would be by far the largest.

Visually smallest galaxy

Many distant galaxies are unresolvable, and cannot have their angular size determined.

Closest galaxies

5 Closest Galaxies

Rank Galaxy Distance Notes

1 Milky Way Galaxy 0 This is our galaxy, we are part of it.

2 Omega Centauri 0.0183 Mly

3 Canis Major Dwarf 0.025 Mly

4 Virgo Stellar Stream 0.030 Mly

5 Sagittarius Dwarf Elliptical Galaxy 0.081 Mly

6 Large Magellanic Cloud 0.163 Mly

Mly represents millions of light-years, a measure of distance.

Distances are measured from Earth, with Earth being at zero.

Nearest Galaxies by Type

Title Galaxy Date Distance Notes

Nearest galaxy Milky Way always 0 This is our galaxy

Nearest galaxy to our own

Canis Major Dwarf 2003 0.025 Ml

y

Nearest dwarf galaxy

Canis Major Dwarf 2003 0.025 Ml

y

Nearest large galaxy to our own

Andromeda Galaxy

always 2.54 Mly

First identified as a separate galaxy in 1923

Nearest giant galaxy Centaurus A 12 Mly

Nearest Neighbouring Galaxy Title-holder

Galaxy Date Distance Notes

Canis Major Dwarf 2003 - 0.025 Ml

y

Sagittarius Dwarf Elliptical Galaxy

1994 − 2003 0.081 Mly

Large Magellanic Cloud

antiquity − 1994

0.163 Mly

This is the upper bound, as it is nearest galaxy observable with the naked-eye.

Small Magellanic Cloud

1913–1914

This was the first intergalactic distance measured. In 1913, Ejnar Hertzsprung measures the distance to SMC using Cepheid variables. In 1914, he did it for LMC.

Andromeda Galaxy 1923

This was the first galaxy determined to be not part of the Milky Way.

Mly represents millions of light-years, a measure of distance.

Distances are measured from Earth, with Earth being at zero.

Omega Centauri does not appear on this list because is not currently considered a galaxy, per se, it is considered a former galaxy, and all that remains of one that was cannibalized by the Milky Way.

Farthest galaxies

Most Remote Galaxies by Type

Title Galaxy Date Distance Notes

Most remote galaxy

UDFj-39546284 2011 z=10.3

Most remote normal galaxy

UDFy-38135539 2010 z=8.55

Most remote quasar

ULAS J1120+0641 2011 z=7.085

This is the undisputed most remote quasar of any type, and the first with a redshift beyond 7.

Further information: List of quasars

Most distant non-quasar SMG

Baby Boom Galaxy (EQ J100054+023435)

2008 z=4.547

grand-design spiral galaxy

Q2343-BX442 2012 z=2.18

z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion

Most Remote Galaxy Record-holders

Galaxy Date Distance Notes

UDFj-39546284 2011 - z=10.3

This was the remotest object known at time of discovery.

UDFy-38135539 2010 − 2011 z=8.55

This was the remotest object known at time of discovery. It exceeded the distance of IOK-1 and GRB 090423

IOK-1 2006 − 2010 z=6.96

This was the remotest object known at time of discovery. In 2009, gamma ray burst GRB 090423 was discovered at z=8.2, taking the title of most distant object. The next galaxy to hold the title also succeeded GRB 090423, that being UDFy-38135539.

SDF   J132522.3+27 3520 2005 − 2006 z=6.597

This was the remotest object known at time of discovery.

SDF   J132418.3+27 1455 2003 − 2005 z=6.578

This was the remotest object known at time of discovery.

HCM-6A 2002 − 2003 z=6.56

This was the remotest object known at time of discovery. The galaxy is lensed by galaxy cluster Abell 370. This was the first galaxy, as opposed to quasar, found to exceed redshift 6. It exceeded the redshift of quasar SDSSp J103027.10+052455.0 of z=6.28

SSA22−HCM1 1999 − 2002 z=5.74 This was the remotest object known at time of discovery. In 2000, the quasar SDSSp J104433.04-012502.2 was discovered at z=5.82, becoming the most remote object in the universe known. This was followed by another quasar, SDSSp J103027.10+052455.0 in 2001, the first object

exceeding redshift 6, at z=6.28

HDF 4-473.0 1998 − 1999 z=5.60

This was the remotest object known at the time of discovery.

RD1 (0140+326   RD1 ) 1998 z=5.34

This was the remotest object known at time of discovery. This was the first object found beyond redshift 5.

CL   1358+62   G1 & CL   1358+62   G2 1997 − 1998 z=4.92

These were the remotest objects known at the time of discovery. The pair of galaxies were found lensed by galaxy cluster CL1358+62 (z=0.33). This was the first time since 1964 that something other than a quasar held the record for being the most distant object in the universe. It exceeded the mark set by quasar PC 1247-3406 at z=4.897

From 1964 to 1997, the title of most distant object in the universe were held by a succession of quasars.[66] That list is available at list of quasars.

8C 1435+63 1994 − 1997 z=4.25

This is a radio galaxy. At the time of its discovery, quasar PC 1247-3406 at z=4.73, discovered in 1991 was the most remote object known. This was the last radio galaxy to hold the title of most distant galaxy. This was the first galaxy, as opposed to quasar, that was found beyond redshift 4.

4C 41.17 1990 − 1994 z=3.792 This is a radio galaxy. At the time of its discovery, quasar PC 1158+4635, discovered in 1989, was the most remote

object known, at z=4.73 In 1991, quasar PC 1247-3406, became the most remote object known, at z=4.897

1 Jy 0902+343 (GB6 B0902+3419, B2 0902+34)

1988 − 1990 z=3.395

This is a radio galaxy. At the time of discovery, quasar Q0051-279 at z=4.43, discovered in 1987, was the most remote object known. In 1989, quasar PC 1158+4635 was discovered at z=4.73, making it the most remote object known. This was the first galaxy discovered above redshift 3. It was also the first galaxy found above redshift 2.

3C 256 1984 − 1988 z=1.819

This is a radio galaxy. At the time, the most remote object was quasar PKS 2000-330, at z=3.78, found in 1982.

3C 241 1984 z=1.617

This is a radio galaxy. At the time, the most remote object was quasar PKS 2000-330, at z=3.78, found in 1982.

3C 324 1983 − 1984 z=1.206

This is a radio galaxy. At the time, the most remote object was quasar PKS 2000-330, at z=3.78, found in 1982.

3C 65 1982 − 1983 z=1.176

This is a radio galaxy. At the time, the most remote object was quasar OQ172, at z=3.53, found in 1974. In 1982, quasar PKS 2000-330 at z=3.78 became the most remote object.

3C 368 1982 z=1.132 This is a radio galaxy. At the time, the most remote object was quasar OQ172, at z=3.53, found in

1974.

3C 252 1981 − 1982 z=1.105

This is a radio galaxy. At the time, the most remote object was quasar OQ172, at z=3.53, found in 1974.

3C 6.1 1979 - z=0.840

This is a radio galaxy. At the time, the most remote object was quasar OQ172, at z=3.53, found in 1974.

3C 318 1976 - 0.752

This is a radio galaxy. At the time, the most remote object was quasar OQ172, at z=3.53, found in 1974.

3C 411 1975 - 0.469

This is a radio galaxy. At the time, the most remote object was quasar OQ172, at z=3.53, found in 1974.

From 1964 to 1997, the title of most distant object in the universe were held by a succession of quasars. That list is available at list of quasars.

3C 295 1960 - z=0.461

This is a radio galaxy. This was the remotest object known at time of discovery of its redshift. This was the last non-quasar to hold the title of most distant object known until 1997. In 1964, quasar 3C 147 became the most distant object in the universe known.

LEDA 25177 (MCG+01-23-008) 1951 − 1960

z=0.2(V=61000 km/s)

This galaxy lies in the Hydra Supercluster. It is located at B1950.0 08h 55m 4s

+03° 21′ and is the BCG of the fainter Hydra Cluster Cl 0855+0321 (ACO 732).

LEDA 51975 (MCG+05-34-069)

1936 - z=0.13(V=39000 km/s)

The brightest cluster galaxy of the Bootes cluster (ACO 1930), an

elliptical galaxy at B1950.0 14h 30m 6s

+31° 46′ apparent magnitude 17.8, was found by Milton L. Humason in 1936 to have a 40,000 km/s recessional redshift velocity.

LEDA 20221 (MCG+06-16-021) 1932 -

z=0.075(V=23000 km/s)

This is the BCG of the Gemini Cluster (ACO 568) and was located at B1950.0 07h 05m 0s

+35° 04′

BCG of WMH Christie's Leo Cluster

1931 − 1932z=(V=19700 km/s)

BCG of Baede's Ursa Major Cluster 1930 − 1931

z=(V=11700 km/s)

NGC 4860 1929 − 1930z=0.026(V=7800 km/s)

NGC 7619 1929z=0.012(V=3779 km/s)

Using redshift measurements, NGC 7619 was the highest at the time of measurement. At the time of announcement, it was not yet accepted as a general guide to distance, however, later in the year, Edwin Hubble described redshift in relation to distance, leading to a seachange, and having this being accepted as an inferred distance.

NGC 584 (Dreyer nebula 584) 1921 − 1929

z=0.006(V=1800 km/s)

At the time, nebula had yet to be accepted as independent galaxies. However, in 1923, galaxies were generally recognized as external to the Milky Way.

M104 (NGC 4594) 1913 − 1921 z=0.004(V=1180 km/s)

This was the second galaxy whose redshift was determined; the first being Andromeda - which is approaching us

and thus cannot have its redshift used to infer distance. Both were measured by Vesto Melvin Slipher. At this time, nebula had yet to be accepted as independent galaxies. NGC 4594 was originally measured as 1000 km/s, then refined to 1100, and then to 1180 in 1916.

M81

antiquity - 20th century

antiquity - 1913 (based on redshift)antiquity - 1930 (based on Cepheids)

11.8 Mly (z=-0.10)

This is the lower bound, as it is remotest galaxy observable with the naked-eye. It is 12 million light-years away. Redshift cannot be used to infer distance, because it's moving toward us faster than cosmological expansion.

Messier 101 1930 -

Using the pre-1950s Cepheid measurements, M101 was one of the most distant so measured.

Triangulum Galaxy 1924–1930

In 1924, Edwin Hubble announced the distance to M33 Triangulum.

Andromeda Galaxy 1923–1924

In 1923, Edwin Hubble measured the distance to Andromeda, and settled the question whether there were galaxies, or was everything in the Milky Way.

Small Magellanic Cloud 1913–1923

This was the first intergalactic distance measured. In 1913, Ejnar Hertzsprung measures the distance to SMC using Cepheid variables.

z represents redshift, a measure of recessional velocity and inferred distance due to cosmological expansion

quasars and other AGN are not included on this list, since they are only galactic cores, unless the host galaxy was

observed when it was most distant

A1689-zD1 , discovered in 2008, with z=7.6, does not appear on this list because it has not been confirmed with a spectroscopic redshift.

Abell 68 c1 and Abell 2219 c1, discovered in 2007, with z=9, do not appear on this list because they have not been confirmed.

IOK4 and IOK5, discovered in 2007, with z=7, do not appear on this list because they have not been confirmed with a spectroscopic redshift.

Abell 1835 IR1916 , discovered in 2004, with z=10.0, does not appear on this list because its claimed redshift is disputed. Some follow-up observations have failed to find the object at all.

STIS 123627+621755 , discovered in 1999, with z=6.68, does not appear on this list because its redshift was based on an erroneous interpretation of an oxygen emission line as a hydrogen emission line.

BR1202-0725 LAE , discovered in 1998 at z=5.64 does not appear on the list because it was not definitively pinned. BR1202-0725 (QSO 1202-07) refers to a quasar that the Lyman alpha emitting galaxy is near. The quasar itself lies at z=4.6947

BR2237-0607 LA1 and BR2237-0607 LA2 were found at z=4.55 while investigating around the quasar BR2237-0607 in 1996. Neither of these appear on the list because they were not definitively pinned down at the time. The quasar itself lies at z=4.558

Two absorption dropouts in the spectrum of quasar BR 1202-07 (QSO 1202-0725, BRI 1202-0725, BRI1202-07) were found, one in early 1996, another later in 1996. Neither of these appear on the list because they were not definitively pinned down at the time. The early one was at z=4.38, the later one at z=4.687, the quasar itself lies at z=4.695

In 1986, a gravitationally lensed galaxy forming a blue arc was found lensed by galaxy cluster CL 2224-02 (C12224 in some references). However, its redshift was only determined in 1991, at z=2.237, by which time, it would no longer be the most distant galaxy.

An absorption drop was discovered in 1985 in the light spectrum of quasar PKS 1614+051 at z=3.21 This does not appear on the list because it was not definitively fixed down. At the time, it was claimed to be the first non-QSO galaxy found beyond redshift 3. The quasar itself is at z=3.197

In 1975, 3C 123 was incorrectly determined to lie at z=0.637 (actually z=0.218)

From 1964 to 1997, the title of most distant object in the universe were held by a succession of quasars.[66] That list is available at list of quasars.

In 1958, cluster Cl 0024+1654 and Cl 1447+2619 were estimated to have redshifts of z=0.29 and z=0.35 respectively. However, no galaxy was spectroscopically determined.

Field galaxies

Galaxy Data Notes

NGC 4555

SDSS J1021+1312

Interacting galaxies

List of galaxies in tidal interaction

Galaxies Data Notes

Milky Way Galaxy

Large Magellanic Cloud

Small Magellanic Cloud

The Magellanic Clouds are being tidally disrupted by the Milky Way Galaxy, resulting in the Magellanic Stream drawing a tidal tail away from the LMC and SMC, and the Magellanic Bridge drawing material from the clouds to our galaxy.

Messier 51 (Arp 85)

o Whirlpool Galaxy (NGC 5194, M51a)

o NGC 5195 (M51b)

The smaller galaxy NGC 5195 is tidally interacting with the larger Whirlpool Galaxy, creating its grand design spiral galaxy architecture.

M81

M82

NGC 3077

These three galaxies interact with each other and draw out tidal tails, which are dense enough to form star clusters. The bridge of gas between these galaxies is known as Arp's Loop.

NGC 6872 and IC 4970

o NGC 6872

o IC 4970

NGC 6872 is a barred spiral galaxy with a grand design spiral nucleus, and distinct well-formed outer

barred-spiral architecture, caused by tidal interaction with satellite galaxy IC 4970.

Tadpole Galaxy

The Tadpole Galaxy tidally interacted with another galaxy in a close encounter, and remains slightly disrupted, with a long tidal tail.

List of galaxies in non-merger significant collision

Galaxies Data Notes

Arp 299 (NGC 3690 & IC 694)

These two galaxies have recently collided and are now both barred irregular galaxies.

List of galaxies disrupted post significant non-merger collisions

Galaxies Data Notes

Mayall's Object

This is a pair of galaxies, one which punched through the other, resulting in a ring galaxy.

Galaxy mergers

List of galaxies undergoing near-equal merger

Galaxies Data Notes

Antennae Galaxies (Ringtail Galaxy, NGC 4038 & NGC 4039, Arp 244)

2 galaxies

Two spiral galaxies currently starting a collision, tidally interacting, and in the process of merger.

Butterfly Galaxies (Siamese Twins Galaxies, NGC 4567 & NGC 4568)

2 galaxies

Two spiral galaxies in the process of starting to merge.

Mice Galaxies (NGC 4676, NGC 4676A & NGC 4676B, IC 819 & IC 820, Arp 242)

2 galaxies

Two spiral galaxies currently tidally interacting and in the process of merger.

NGC 5202 galaxies

Two spiral galaxies undergoing collision, in the process of merger.

NGC 2207 and IC 2163 (NGC 2207 & IC 2163)

2 galaxies

These are two spiral galaxies starting to collide, in the process of merger.

NGC 5090 and NGC 5091 (NGC 5090 & NGC 5091)

2 galaxies

These two galaxies are in the process of colliding and merging.

NGC 7318 (Arp 319, NGC 7318A & NGC 7318B)

2 galaxies

These are two starting to collide

Four galaxies in CL0958+4702

4 galaxies

These four near-equals at the core of galaxy cluster CL 0958+4702 are in the process of merging.

Galaxy protocluster LBG-2377

z=3.03 This was announced as the most distant galaxy merger ever discovered. It is expected that this proto-cluster of galaxies will merge to form a brightest cluster galaxy, and become the core of a larger

galaxy cluster.[122][123]

List of recently merged galaxies of near-equals

Galaxy Data Notes

Starfish Galaxy (NGC 6240, IC 4625)

This recently coalesced galaxy still has two prominent nuclei.

List of galaxies undergoing disintegration by cannibalization

Disintegrating Galaxy

Consuming Galaxy Notes

Canis Major Dwarf Galaxy

Milky Way Galaxy

The Monoceros Ring is thought to be the tidal tail of the disrupted CMa dg.

Virgo Stellar Stream Milky Way Galaxy

This is thought to be a completely disrupted dwarf galaxy.

Sagittarius Dwarf Elliptical Galaxy

Milky Way Galaxy

M54 is thought to the be core of this dwarf galaxy.

List of objects considered destroyed galaxies

Defunct Galaxy Galaxy Notes

Omega Centauri

Milky Way Galaxy

This is now categorized a globular cluster of the Milky Way. However, it is considered the core of a dwarf galaxy that the Milky Way cannibalized.[14]

Mayall II Andromeda Galaxy

This is now categorized a globular cluster of Andromeda. However, it is considered the core of a dwarf galaxy that Andromeda cannibalized.

List of objects mistakenly identified as galaxies

"Galaxy" Object Data Notes

G350.1-0.3

Supernova remnant

Due to its unusual shape, it was originally misidentified as a galaxy.

Future of the EarthThe biological and geological future of the Earth can be extrapolated based upon the estimated effects of several long-term influences. These include the chemistry at the Earth's surface, the rate of cooling of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as geo-engineering which could cause significant changes to the planet. The current biotic crisis is being caused by technology and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slow evolution pace only resulting from long term natural processes.

Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, seven billion years from now

Over time intervals of hundreds of millions of years, random celestial events pose a global risk to the biosphere, which can result in mass extinctions. These include impacts by comets or asteroids with diameters of 5–10 km or more, and the possibility of a massive stellar explosion, called a supernova, within a 100 light year radius. Other large-scale geological events are more predictable. If the long-term effects of global warming are disregarded, Milankovitch theory predicts that the planet will continue to undergo glacial periods at least until the quaternary glaciation comes to an end. These periods are caused by eccentricity, axial tilt, and precession of the Earth's orbit. As part of the ongoing supercontinent cycle, plate tectonics will probably result in a supercontinent in 250–350 million years. Sometime in the next 1.5–4.5 billion years, the axial tilt of the Earth may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.

During the next four billion years, the luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will cause a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years, the level of CO2 will fall below the level needed to sustain the C3 method of photosynthesis used by trees. Some plants use the C4 method, allowing them to persist at CO2 concentrations as low as 10 parts per million. However, the long term trend is for plant life to die off altogether. The resulting loss of oxygen replenishment will cause the extinction of animal life a few million years later.

In about 1.1 billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end. Following this event, the planet's magnetic dynamo may come to an end, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in the Earth's surface temperature will cause a runaway greenhouse effect. By that point, most if not all the life on the surface will be extinct.The most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded to cross the planet's current orbit.

Human influence

Humans now play a key role in the biosphere, with the large human population dominating many of Earth's ecosystems. This has resulted in a widespread, ongoing extinction of other species during the present geological epoch, now known as the Holocene extinction. The large scale loss of species caused by human influence since the 1950s has been called a biotic crisis, with an estimated 10% of the total species lost as of 2007. At current rates, about 30% of species are at risk of extinction in the next hundred years. The Holocene extinction event is the result of habitat destruction, the widespread distribution of invasive species, hunting, and climate change. In the present day, human activity has had a significant impact on the surface of the planet. More than a third of the land surface has been modified by human actions, and humans use about 20% of

global primary production. The concentration of carbon dioxide in the atmosphere has increased by close to 30% since the start of the Industrial Revolution.

The consequences of a persistent biotic crisis have been predicted to last for at least five million years. It could result in a decline in biodiversity and homogenization of biota, accompanied by a proliferation of species that are opportunistic, such as pests and weeds. Novel species may also emerge; in particular taxa that prosper in human-dominated ecosystems may rapidly diversify into many new species. Microbes are likely to benefit from the increase in nutrient-enriched environmental niches. However, no new species of existing large vertebrates are likely to arise and food chains will probably be shortened. There are multiple scenarios for known risks that can have a global impact on the planet. From the perspective of humanity, these can be subdivided into survivable risks and terminal risks. Risks that humanity pose to itself include the misuse of nanotechnology, a nuclear holocaust, warfare with a programmed super intelligence, a genetically engineered disease, or perhaps a disaster caused by a physics experiment. Similarly, several natural events may pose a doomsday threat, including a highly virulent disease, the impact of an asteroid or comet, runaway greenhouse effect, and resource depletion. There may also be the possibility of an infestation by an extraterrestrial life form. The actual odds of these scenarios are difficult if not impossible to deduce.

Should the human race become extinct, then the various features assembled by humanity will begin to decay. The largest structures have an estimated decay half-life of about 1,000 years. The last surviving structures would most likely be open pit mines, large landfills, major highways, wide canal cuts, and earth-fill flank dams. A few massive stone monuments like the pyramids at the Giza Necropolis or the sculptures at Mount Rushmore may still survive in some form after a million years.

Random events

As the Sun orbits the Milky Way, randomly-moving stars may approach close enough to have a disruptive influence on the Solar System. A close stellar encounter may cause a significant reduction in the perihelion distances of comets in the Ort cloud—a spherical region of icy bodies orbiting within half a light year of the Sun. Such an encounter can trigger a 40-fold increase in the number of comets reaching the inner Solar System. Impacts from these comets can trigger a mass extinction of life on Earth. These disruptive encounters occur at an average of once every 45 million years. The mean time for the Sun to collide with another star in the solar neighborhood is approximately 3 × 1013 years, which is much longer than the estimated age of the Milky Way galaxy, at 1–2 × 1010 years. This can be taken as an indication of the low likelihood of such an event occurring

during the lifetime of the Earth.

The energy release from the impact of an asteroid or comet with a diameter of The Barringer Meteorite Crater in

Flagstaff, Arizona, showing evidence of the impact of celestial objects upon the Earth

5–10 km or larger is sufficient to create a global environmental disaster and cause a statistically significant increase in the number of species extinctions. Among the deleterious effects resulting from a major impact event is a cloud of fine dust eject a blanketing the planet, which lowers land temperatures by about 15°C within a week and halts photosynthesis for several months. The mean time between major impacts is estimated to be at least 100 million years. During the last 540 million years, simulations demonstrated that such an impact rate is sufficient to cause 5–6 mass extinctions and 20–30 lower severity events. This matches the geologic record of significant extinctions during the Phanerozoic era. Such events can be expected to continue into the future.

A supernova is a cataclysmic explosion of a star. Within the Milky Way galaxy, supernova explosions occur on average once every 30 years. During the history of the Earth, multiple such events have likely occurred within a distance of 100 light years. Explosions inside this distance can contaminate the planet with radioisotopes and possibly impact the biosphere. Gamma rays emitted by a supernova react with nitrogen in the atmosphere, producing nitrous oxides. These molecules cause a depletion of the ozone layer that protects the surface from ultraviolet radiation from the Sun. An increase in UV-B radiation of only 10–30% is sufficient to cause a significant impact to life; particularly to the phytoplankton that form the base of the oceanic food chain. A supernova explosion at a distance of 26   light years will reduce the ozone column density by half. On average, a supernova explosion occurs within 32 light years once every few hundred million years, resulting in a depletion of the ozone layer lasting several centuries. Over the next two billion years, there will be about 20 supernova explosions and one gamma ray burst that will have a significant impact on the planet's biosphere.

The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods. This does not significantly affect the stability of the Solar System over intervals of a few millions years or less, but over billions of years the orbits of the planets become unpredictable. Computer simulations of the Solar System's evolution over the next five billion years suggest that there is a small (less than 1%) chance that a collision could occur between Earth and either Mercury, Venus, or Mars. During the same interval, the odds that the Earth will be scattered out of the Solar System by a passing star are on the order of one part in 105. In such a scenario, the oceans would freeze solid within a several million years, leaving only a few pockets of liquid water about 14 km underground. There is a remote chance that the Earth will instead be captured by a passing binary star system, allowing the planet's biosphere to remain intact. The odds of this happening are about one chance in three million.

Orbit and rotation

The gravitational perturbations of the other planets in the Solar System combine to modify the orbit of the Earth and the orientation of its spin axis. These changes can influence the planetary climate. GlaciationHistorically, there have been cyclical ice ages in which glacial sheets periodically covered the higher latitudes of the continents. Ice ages may occur because of changes in

ocean circulation and continentality induced by plate tectonics. The Milankovitch theory predicts that glacial periods occur during ice ages because of astronomical factors in combination with climate feedback mechanisms. The primary astronomical drivers are a higher than normal orbital eccentricity, a low axial tilt (or obliquity), and the alignment of summer solstice with the aphelion. Each of these effects occur cyclically. For example, the eccentricity changes over time cycles of about 100,000 and 400,000 years, with the value ranging from less than 0.01 up to 0.05. This is equivalent to a change of the semi minor axis of the planet's orbit from 99.95% of the semi major axis to 99.88%, respectively.

The Earth is passing through an ice age known as the quaternary glaciation, and is presently in the Holocene interglacial period. This period would normally be expected to end in about 25,000 years. However, the increased rate of carbon dioxide release into the atmosphere by humans may delay the onset of the next glacial period until at least 50,000–130,000 years from now. However, a global warming period of finite duration (based on the assumption that fossil fuel use will cease by the year 2200) will probably only impact the glacial period for about 5,000 years. Thus, a brief period of global warming induced through a few centuries worth of greenhouse gas emission would only have a limited impact in the long term.

ObliquityThe tidal acceleration of the Moon slows the rotation rate of the Earth and increases the Earth-Moon distance. Friction effects—between the core and mantle and between the atmosphere and surface—can dissipate the Earth's rotational energy. These combined effects are expected to increase the length of the day by more than 1.5 hours over the next 250 million years and to increase the obliquity by about a half degree. The distance to the Moon will increase by about 1.5 Earth radii during the same period.

Based on computer models, the presence of the Moon appears to stabilize the obliquity of the Earth, which may help the planet to avoid dramatic climate changes. This stability is achieved because the Moon increases the precession rate of the Earth's spin axis, thereby avoiding resonances between the precession of the spin and precession frequencies of the ascending node of the planet's orbit. (That is, the precession motion of the ecliptic.) However, as the semi major axis of the Moon's orbit continues to increase in the future, this stabilizing effect will diminish. At some point perturbation effects will probably cause chaotic variations in the obliquity of the Earth, and the axial tilt may change by angles as high as 90° from the plane of the orbit. This is expected to occur within about 1.5–4.5 billion years, although the exact time is unknown.

A high obliquity would probably result in dramatic changes in the climate and may destroy the planet's habitability. When the axial tilt of the Earth reaches 54°, the equator will receive

less radiation from the Sun than the poles. The planet could remain at an obliquity of 60° to 90° for periods as long as 10 million years. Geodynamics

Tectonics-based events will continue to occur well into the future and the surface will be steadily reshaped by tectonic uplift, extrusions, and erosion. Mount Vesuvius can be expected to erupt about 40 times over the next 1,000 years. During the same period, about five to seven

earthquakes of magnitude 8 or greater should occur along the San Andreas Fault, while about 50 magnitude 9 events may be expected worldwide. Mauna Loa should experience about 200 eruptions over the next 1,000 years, and the Old Faithful Geyser will likely cease to operate. The Niagara Falls will continue to retreat upstream, reaching Buffalo in about 30,000–50,000 years.

In 10,000 years, the post-glacial rebound of the Baltic Sea will have reduced the depth by about 90 m. The Hudson Bay will decrease in depth by 100 m over the same period. After 100,000 years, the island of Hawaii will have shifted about 9 km to the northwest. The planet may be entering another glacial period by this time.

Continental driftThe theory of plate tectonics demonstrates that the continents of the Earth are moving across the surface at the rate of a few centimeters per year. This is expected to continue, causing the plates to relocate and collide. Continental drift is facilitated by two factors: the energy generation within the planet and the presence of a hydrosphere. With the loss of either of these, continental drift will come to a halt. The production of

heat through radiogenic processes is sufficient to maintain mantle convection and plate subduction for at least the next 1.1 billion years.

At present, the continents of North and South America are moving westward from Africa and Europe. Researchers have produced several scenarios about how this will continue in the future. These geodynamic models can be distinguished by the subduction flux, whereby the oceanic crust moves under a continent. In the introversion model, the younger, interior, Atlantic ocean becomes preferentially subducted and the current migration of North and South America is reversed. In the extroversion model, the older, exterior, Pacific ocean remains preferentially subducted and North and South America migrate toward eastern Asia.

As the understanding of geodynamics improves, these models will be subject to revision. In 2008, for example, a computer simulation was used to predict that a reorganization of the mantle convection will occur, causing a supercontinent to form around Antarctica.

Regardless of the outcome of the continental migration, the continued subduction process causes water to be transported to the mantle. After a billion years from the present, a geophysical model gives an estimate that 27% of the current ocean mass will have been subducted. If this process were to continue unmodified into the future, the subduction and release would reach a point of stability after 65% of the current ocean mass has been subducted.

IntroversionChristopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of the Paleo map Project. In their scenario, 50 million years from now the Mediterranean Sea may vanish and the collision between Europe and Africa will create a long mountain range extending to the current location of the Persian Gulf. Australia will merge with Indonesia, and Baja California will slide northward along the coast. New subduction zones may appear off the eastern coast of North and South America, and mountain chains will form along those coastlines. To the south, the migration of Antarctica to the north will cause all of its ice sheets to melt. This, along with the melting of the Greenland ice sheets, will raise the average ocean level by 90 meters (300 ft). The inland flooding of the continents will result in climate changes.

As this scenario continues, by 100 million years from the present the continental spreading will have reached its maximum extent and the continents will then begin to coalesce. In 250 million years, North America will collide with Africa while South America will wrap around the southern tip of Africa. The result will be the formation of a new supercontinent (sometimes called Pangaea Ultima), with the Pacific Ocean stretching across half the planet. The continent of Antarctica will reverse direction and return to the South Pole, building up a new ice cap.

ExtroversionThe first scientist to extrapolate the current motions of the continents was Canadian geologist Paul F. Hoffman of Harvard University. In 1992, Hoffman predicted that the continents of North and South America would continue to advance across the Pacific Ocean, pivoting about Siberia until they begin to merge with Asia. He dubbed the resulting supercontinent, Amasia. Later, in the 1990s, Roy Livermore calculated a similar scenario. He predicted that Antarctica would start to migrate

The rotational offset of the tidal bulge exerts a net torque on the Moon, boosting it while slowing the Earth's rotation. This image is not to scale.

Pangaea was the last supercontinent to form before the present.

northward, and east Africa and Madagascar would move across the Indian Ocean to collide with Asia.

In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years. This marks the completion of the current supercontinent cycle, wherein the continents split apart and then rejoin each other about every 400–500 million years. Once the supercontinent is built, plate tectonics may enter a period of inactivity as the rate of subduction drops by an order of magnitude. This period of stability could cause an increase in the mantle temperature at the rate of 30–100 K every 100 million years, which is the minimum lifetime of past supercontinents. As a consequence, volcanic activity may increase.

SupercontinentThe formation of a supercontinent can dramatically affect the environment. The collision of plates will result in mountain building, thereby shifting weather patterns. Sea levels may fall because of increased glaciation. The rate of surface weathering can rise, resulting in an increase in the rate that organic material is buried. Supercontinents can cause a drop in global temperatures and an increase in atmospheric oxygen. These changes can result in more rapid biological evolution as new niches emerge. This, in turn, can affect the climate, further lowering temperatures.

The formation of a supercontinent insulates the mantle. The flow of heat will be concentrated, resulting in volcanism and the flooding of large areas with basalt. Rifts will form and the supercontinent will split up once more. The planet may then experience a warming period, as occurred during the Cretaceous period.

Solidification of the outer coreThe iron-rich core region of the Earth is divided into a 1,220 km radius solid inner core and a 3,480 km radius liquid outer core. The rotation of the Earth creates convective eddies in the outer core region that cause it to function as a dynamo. This generates a magnetosphere about the Earth that deflects particles from the solar wind, which prevents significant erosion of the atmosphere from sputtering. As heat from the core is transferred outward toward the mantle, the net trend is for the inner boundary of the liquid outer core region to freeze, thereby releasing thermal energy and causing the solid inner core to grow. This iron crystallization process has been ongoing for about a billion years. In the modern era, the radius of the inner core is expanding at an average rate of roughly 0.5 mm per year, at the expense of the outer core. Nearly all of the energy needed to power the dynamo is being supplied by this process of inner core formation.

The growth of the inner core may be expected to consume most of the outer core by some 3–4 billion years from now, resulting in a nearly solid core composed of iron and other heavy elements. The surviving liquid envelope will mainly consist of lighter elements that will undergo less mixing. Alternatively, if at some point plate tectonics comes to an end, the interior will cool less efficiently, which may end the growth of the inner core. In either case, this can result in the loss of the magnetic dynamo. Without a functioning dynamo, the magnetic field of the Earth will decay in a geologically short time period of roughly 10,000 years. The loss of the magnetosphere will cause an increase in erosion of

light elements, particularly hydrogen, from the Earth's outer atmosphere into space, resulting in less favorable conditions for life.

Solar evolution

The energy generation of the Sun is based upon thermonuclear fusion of hydrogen into helium. This occurs in the core region of the star using the proton–proton chain reaction process. Because there is no convection in the solar core, the helium concentration builds up in that region without being distributed throughout the star. The temperature at the core of the Sun is too low for nuclear fusion of helium atoms through the triple-alpha process, so these atoms do not contribute to the net energy generation that is needed to maintain hydrostatic equilibrium of the Sun. At present, nearly half the hydrogen at the core has been consumed, with the remainder of the atoms consisting primarily of helium. As the number of hydrogen atoms per unit mass decrease, their energy output provided through nuclear fusion also decreases. This results in a decrease in pressure support, which causes the core to contract until the increased density and temperature bring the core pressure in to equilibrium with the layers above. The higher temperature causes the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium.

The result of this process has been a steady increase in the energy output of the Sun. When the Sun first became a main sequence star, it radiated only 70% of the current luminosity. The luminosity has increased in a nearly linear fashion to the present, rising by 1% every 110 million years.[68] Likewise, in three billion years the Sun is expected to be 33% more luminous. The hydrogen fuel at the core will finally be exhausted in 4.8 billion years, when the Sun will be 67% more luminous than at present. Thereafter the Sun will continue to burn hydrogen in a shell surrounding its core, until the increase in luminosity reaches 121% of the present value. This marks the end of the Sun's main sequence lifetime, and thereafter it will pass through the sub-giant stage and evolve into a red giant.

Climate impactWith the increased surface area of the sun, the amount of energy emitted will increase. The global temperature of

Evolution of the Sun's luminosity, radius and effective temperature compared to the present Sun. After Ribas (2010)

the Earth will climb because of the rising luminosity of the Sun, the rate of weathering of silicate minerals will increase. This in turn will decrease the level of carbon dioxide in the atmosphere. Within the next 600 million years from the present, the concentration of CO2 will fall below the critical threshold needed to sustain C3  photosynthesis : about 50 parts per million. At this point, trees and forests in their current forms will no longer be able to survive. However, C4  carbon fixation  can continue at much lower concentrations, down to above 10 parts per million. Thus plants using C4 photosynthesis may be able to survive for at least 0.8 billion years and possibly as long as 1.2 billion years from now, after which rising temperatures will make the biosphere unsustainable. Currently, C4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species. For example, about 50% of all grass species Poaceae) use the C4 photosynthetic pathway, as do many species in the herbaceous family Amaranthaceae.

When the levels of carbon dioxide fall to the limit where photosynthesis is barely sustainable, the proportion of carbon dioxide in the atmosphere is expected to oscillate up and down. This will allow land vegetation to flourish each time the level of carbon dioxide rises due to tectonic activity and animal life. However, the long term trend is for the plant life on land to die off altogether as most of the remaining carbon in the atmosphere becomes sequestered in the Earth. Some microbes are capable of photosynthesis at concentrations of CO2 of a few parts per million, so these life forms would probably disappear only because of rising temperatures and the loss of the biosphere. The loss of plant life will also result in the eventual loss of oxygen.

In their work The Life and Death of Planet Earth, authors Peter D. Ward   and   Donald Brownlee  have argued that some form of animal life may continue even after most of the Earth's plant life has disappeared. Initially, they expect that some insects, lizards, birds and small mammals may persist, along with sea life. Without oxygen replenishment by plant life, however, they believe that the animals would probably die off from asphyxiation within a few million years. Even if sufficient oxygen were to remain in the atmosphere through the persistence of some form of photosynthesis, the steady rise in global temperature would result in a gradual loss of biodiversity. As temperatures continue to rise, the last animal life will inevitably be driven back toward the poles, terrestrial food chains will become fungus-based, and many of these animals will become simpler but tougher in body structure. Much of the surface would become a barren desert and life would primarily be found in the oceans. As a result of these processes, multi-cellular life forms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years from now, leaving only the prokaryotes.

Ocean-free eraBy one billion years from now, about 27% of the modern ocean will have been subducted into the mantle. If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the current surface reservoir would remain at the surface. Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C). The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans. At this point, models of the Earth's future environment demonstrate that the stratosphere would contain increasing levels of water. These water molecules

will be broken down through photo dissociation by solar ultraviolet radiation, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's sea water by about 1.1 billion years from the present.

In this ocean-free era, there will continue to be reservoirs at the surface as water is steadily released from the deep crust and mantle. Some water may be retained at the poles and there may be occasional rainstorms, but for the most part the planet would be a dry desert. Even in these arid conditions, the planet may retain some microbial and possibly even multi-cellular life. Most of these microbes will be halophiles. However, the increasingly extreme conditions will likely lead to the extinction of the procaryotes some 1.6 billion years from now. What happens next depends on the level of tectonic activity. A steady release of carbon dioxide by volcanic eruption could eventually cause the atmosphere to enter a "super greenhouse" state like that of the planet Venus. But without surface water, plate tectonics would probably come to a halt and most of the carbonates would remain securely buried.

The loss of the oceans could be delayed until two billion years in the future if the total atmospheric pressure were to decline. A lower atmospheric pressure would reduce the greenhouse effect, thereby lowering the surface temperature. This could occur if natural processes were to remove the nitrogen from the atmosphere. Studies of organic sediments has shown that at least 100 kilopascals (1 bar) of nitrogen has been removed from the atmosphere over the past four billion years; enough to effectively double the current atmospheric pressure if it were to be released. This rate of removal would be sufficient to counter the effects of increasing solar luminosity for the next two billion years. However, beyond that point, the amount of water in the lower atmosphere will have risen to 40% and the runaway moist greenhouse will commence.

If it has not already, a runaway greenhouse effect will take place when the luminosity from the Sun reaches 35–40% more than its current value, 3–4 billion years from now. The atmosphere will heat up and the surface temperature will rise sufficiently to melt surface rock. However, most of the atmosphere will be retained until the Sun has entered the red giant stage.

Red giant stageOnce the Sun changes from burning hydrogen at its core to burning hydrogen around its shell, the core will start to contract and the outer envelope will expand. The total luminosity will steadily increase over the following billion years until it reaches 2,730 times the Sun's current luminosity at the age of 12.167 billion years.

The size of the current Sun (now in the main sequence) compared to its estimated size during its red giant phase

During this phase the Sun will experience more rapid mass loss, with about 33% of its total mass shed with the solar wind. The loss of mass will mean that the orbits of the planets will expand. The orbital distance of the Earth will increase to at most 150% of its current value.

The most rapid part of the Sun's expansion into a red giant will occur during the final stages, when the Sun will be about 12 billion years old. It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 astronomical units(180,000,000 km). The Earth will interact tidally with the Sun's outer atmosphere, which would serve to decrease Earth's orbital radius.

Drag from the chromosphere of the Sun would also reduce the Earth's orbit. These effects will act to counterbalance the effect of mass loss by the Sun, and the Earth will most likely be engulfed by the Sun. Theablation and vaporization caused by its fall on a spiral trajectory towards the Sun will remove Earth's crust and mantle, then finally destroy it after at most 200 years. Earth's sole legacy will be a very slight increase (0.01%) of the solar metallicity. The drag from the solar atmosphere may cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km, it will cross the Earth's Roche limit. Tidal interaction with the Earth would then break apart the Moon, turning it into a ring system. Most of the orbiting ring will then begin to decay, and the debris will impact the Earth. Hence, even if the Earth is not swallowed up by the Sun, the planet may be left moonless.

EARTH

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world, the Blue Planet, or by its Latin name, Terra.

Earth formed approximately 4.54 billion years ago by accretion from the solar nebula, and life appeared on its surface within one billion years. The planet is home to millions of species, including humans. Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocks harmful solar radiation, thus permitting formerly ocean-confined life to move safely to land.  The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will to be able to continue to support life range from 500 million years, to as long as 2.3 billion years.

Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the Polar ice packs. The planet's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in space, especially the Sun and the Moon. During one orbit around the sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year. The Earth's axis of rotation is tilted23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). The Moon is Earth's only natural satellite. It began orbit about 4.53 billion years ago. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation. Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the Moon's greater surface environment.

Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.Name and etymology

The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), ultimately deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, German Erde, and Swedish, Norwegian, and Danish jord. The Earth is personified as a goddess in Germanic paganism (appearing as Jörð in Norse mythology, mother of the god Thor).

In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. "Earth", "the Earth", "earth", or "the earth"). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth).ChronologyFormation

The atmosphere of Venus is in a "super greenhouse" state.

The earliest material found in the Solar System is dated to 4.5666-4.5678 bya (billion years ago); therefore, it is inferred that the Earth, must have formed around this time. By 4.50-4.58 bya, the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory, planetesimal commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr (million years). The Moon formed shortly thereafter, 4.53 bya.

The Moon's formation remains a mystery. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object, dubbed Theia, had a giant impact with Earth, but the model is not self-consistent. In this scenario the mass of Theia is 10% of the Earth's mass, it impacts with the Earth in a glancing blow, and some of its mass merges with the Earth.

Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity. By3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.

A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models that explain land mass propose either a steady growth to the present-day forms or, more likely, a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya(million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.

Evolution of lifeHighly energetic chemistry is believed to have

produced a self-replicating molecule around 4 bya (billion years ago) and half a billion years later the last common ancestor of all life existed. The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. True multi-cellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya (million years ago), during theNeoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.

Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions. The most recent such event was 65 mya, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago. Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation, and is probably the only one with active plate tectonics. Shape

The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of the Earth, and causes the diameter at the equator to be43 km (kilometer) larger than the pole-to-pole diameter. For this reason the furthest point on the surface from the Earth's center of mass is the Chimborazo volcano in Ecuador. The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.

Local topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls. The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru. Chemical composition

The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%),magnesium (13.9%), sulfur (2.9%), nickel  (1.8%), calcium (1.5%), and aluminum (1.4%);

with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and

fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.Internal structure

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km (kilometers) under the oceans and 30-50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year. Heat

Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the

center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa. Because much of the heat is provided by radioactive decay,

scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3 byr (billion years ago), would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.

The mean heat loss from the Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013 W.  A portion of the core's thermal energy is transported toward the crust by mantle plumes; a form of convection consisting of upwelling of higher-temperature rock. These plumes can produce hotspots and flood basalts. More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs in the oceans because the crust there is much thinner than that of the continents. Tectonic plates

The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries. The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates, and their motion is strongly coupled with convection patterns inside the Earth's mantle.

Chemical composition of the crust

Compound Formula CompositionContinental Oceanic

silica SiO2 60.2% 48.6%alumina Al2O3 15.2% 16.5%lime CaO 5.5% 12.3%magnesia MgO 3.1% 6.8%iron(II) oxide FeO 3.8% 6.2%sodium oxide Na2O 3.0% 2.6%potassium oxide K2O 2.8% 0.4%iron(III) oxide Fe2O3 2.5% 2.3%water H2O 1.4% 1.1%carbon dioxide CO2 1.2% 1.4%titanium dioxide TiO2 0.7% 1.4%phosphorus pentoxide

P2O5 0.2% 0.3%

Total 99.6% 99.9%

Geologic layers of the EarthDepthkm

Component Layer Densityg/cm3

0–60 Lithosphere —0–35 Crust 2.2–2.935–60 Upper mantle 3.4–4.435–2890 Mantle 3.4–5.6100–700 Asthenosphere —2890–5100 Outer core 9.9–12.25100–6378 Inner core 12.8–13.1

As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Because of this recycling, most of the ocean floor is less than100 myr (million years old) in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 myr. By comparison, the oldest dated continental crust is 4,030 myr.

The seven major plates are the Pacific, North, American,  Eurasian,  African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/year. Surface

The Earth's terrain varies greatly from place to place. About 70.8%of the surface is covered by water, with much of the continental shelf below sea level. This equates to 148.94 million km2 (57.51 million sq mi). The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods because of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts also act to reshape the landscape.

The continental crust consists of lower density material such as the igneous rocks granite and andesite.

Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors. Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine. Common carbonate minerals include calcite (found in limestone) and dolomite..

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated

1.3×107 km2 of cropland and 3.4×107 km2 of pastureland. The elevation of the land surface of the Earth

varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m. Hydrosphere

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m. The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3. If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km. About 97.5% of the water is saline,

Earth's main plates

Plate name Area106 km2

  Pacific Plate 103.3  African Plate 78.0  North American Plate 75.9  Eurasian Plate 67.8  Antarctic Plate 60.9  Indo-Australian Plate 47.2  South American Plate 43.6

while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.

The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35%). Most of this salt was released from volcanic activity or extracted from cool, igneous rocks. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir. Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation. Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 bya (billion years ago), forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature. This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be −18 °C, in contrast to the current +15 °C, and life would likely not exist. Weather and climate

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the western lies in the mid-latitudes between 30° and 60°. Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the Polar Regions.

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation. Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface

features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.

The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at a lower angles and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C per per degree of latitude away from the equator.[120] The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the Polar Regions, these are the tropical (or equatorial), subtropical, temperate and polar climates. Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes. Upper atmosphere

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind. Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.

Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses. The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere. Hence, the ability of hydrogen to

Schematic of Earth's magnetosphere.

Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit

escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet. In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere. Magnetic Field

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. At the equator of the magnetic field, the magnetic field strength at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora. Orbit and rotationRotation

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds). As the Earth's solar day is now slightly longer than it was during the 19th century because of tidal acceleration, each day varies between 0 and 2 SI ms longer.

Earth's rotation period relative to the fixed stars, called its stellar  bythe International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691

seconds of mean solar time (UT1), or 23h 56m 4.098903691s. Earth's rotation period relative to the precessing or moving mean vernal equinox,

misnamed its sidereal day, is86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s). Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[135] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005.

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or Moon every two minutes; from the planet's surface, the apparent sizes of the Sun and the Moon are approximately the same.

OrbitEarth orbits the Sun at an average distance of

about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter, every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial North Pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius. This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galaxy's equatorial plane in the Orion spiral arm. Axial tilt and seasons

Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter.

Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.

By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.

The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasi-periodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9% in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism. The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climatic conditions at the surface.[158]

BiosphereThe planet's life forms are sometimes said to form

a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 bya (billion years ago). The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high

altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes. Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.

The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land. Humans also live on the land by using building materials to   construct shelters. In 1993, human use of land is approximately:

Land use Arable land

Permanent crops

Permanent pastures

Forests and woodland

Urban areas

Other

Percentage

13.13%

4.71% 26% 32% 1.5% 30%

The estimated amount of irrigated land in 1993 was 2,481,250 km2. Natural and environmental hazards

Large areas of the Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980–2000, these events caused an average of 11,800 deaths per year. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes,  sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation,  desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

According to the United Nations, a scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels. Human geography

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has reached approximately 7,000,000,000 human inhabitants as of October 31, 2011. Projections indicate that the world's human population will reach 9.2 billion in 2050. Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.

It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),high mountains (27%),or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada. (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at

the South Pole (90°S).Other Important Facts

Alternative names Terra, Gaia

Orbital characteristicsEpoch J2000.0Aphelion 152,098,232 km

1.01671388 AUPerihelion 147,098,290 km

0.98329134 AU Semi-major axis 149,598,261 km

1.00000261 AUEccentricity 0.01671123Orbital period 365.256363004 days

1.000017421 yrAverage orbital speed 29.78 km/s

107,200 km/hMean anomaly 357.51716°Inclination 7.155°to Sun's equator

1.57869°[4] to invariable planeLongitude of ascending node 348.73936°Argument of perihelion 114.20783° Satellites 1 natural (the Moon)

8,300+ artificial (as of 1 March 2001) Physical characteristicsMean radius 6,371.0 kmEquatorial radius 6,378.1 km Polar radius 6,356.8 kmFlattening 0.0033528Circumference 40,075.017 km (equatorial)

40,007.86 km (meridional) Surface area 510,072,000 km2 148,940,000 km2 land (29.2 %)

361,132,000 km2 water (70.8 %)Volume 1.08321×10 12  km3

Mass 5.9736×1024 kg

Mean density 5.515 g/cm3

Equatorial surface gravity 9.780327 m/s 2 0.99732 g

Escape velocity 11.186 km/sSidereal rotation period 0.99726968 d

23h 56m 4.100s

Equatorial rotation velocity 1,674.4 km/h (465.1 m/s)

Axial tilt 23°26'21".4119

Albedo 0.367 (geometric)

0.306 (Bond) Surface temp.  Kelvin Celsius

Min mean Max

184 K 287.2 K 331 K

−89.2 °C 14 °C 57.8 °C

AtmosphereSurface pressure 101.325 kPa (MSL)Composition 78.08% nitrogen (N2)(dry air)

20.95% oxygen (O2)0.93% argon0.038% carbon dioxideAbout 1% water vapor (varies with climate)

Name

AreaAverage

depthGreatest known

depth Place ofgreatest known

depthsq. mi. sq. km ft. m ft. m

Pacific Ocean 60,060,700 155,557,000 13,215 4,028 36,198 11,033 Mariana Trench

Atlantic Ocean 29,637,900 76,762,000 12,880 3,926 30,246 9,219 Puerto Rico Trench

Indian Ocean 26,469,500 68,556,000 13,002 3,963 24,460 7,455 Sunda Trench

Southern Ocean1 7,848,300 20,327,000 13,100–16,400

4,000–5,000

23,736 7,235 South Sandwich Trench

Arctic Ocean 5,427,000 14,056,000 3,953 1,205 18,456 5,625 77°45'N; 175°W

Mediterranean Sea2

1,144,800 2,965,800 4,688 1,429 15,197 4,632 Off Cape Matapan, Greece

Caribbean Sea 1,049,500 2,718,200 8,685 2,647 22,788 6,946 Off Cayman Islands

South China Sea 895,400 2,319,000 5,419 1,652 16,456 5,016 West of Luzon

Bering Sea 884,900 2,291,900 5,075 1,547 15,659 4,773 Off Buldir Island

Gulf of Mexico 615,000 1,592,800 4,874 1,486 12,425 3,787 Sigsbee Deep

Okhotsk Sea 613,800 1,589,700 2,749 838 12,001 3,658 146°10'E; 46°50'N

East China Sea 482,300 1,249,200 617 188 9,126 2,782 25°16'N; 125°E

Hudson Bay 475,800 1,232,300 420 128 600 183 Near entrance

Japan Sea 389,100 1,007,800 4,429 1,350 12,276 3,742 Central Basin

Andaman Sea 308,000 797,700 2,854 870 12,392 3,777 Off Car Nicobar Island

North Sea 222,100 575,200 308 94 2,165 660 Skagerrak

Red Sea 169,100 438,000 1,611 491 7,254 2,211 Off Port Sudan

Baltic Sea 163,000 422,200 180 55 1,380 421 Off Gotland

Tribes in the Philippines

Philippines, basically an archipelago, has a variety of ethnic groups inhabiting its land. The languages spoken by these ethnic groups are Austronesian in origin.The culture of Philippines has been influenced by many different cultures of the world, including the Spanish and American. Today, Philippines is roughly divided into upland ethnic groups known as the 'Igorot' and the lowland Filipinos, who embraced the modern lifestyle. Quite a large number of indigenous tribes in the Philippines have accepted Islam as their religion. These people are known as Moros.

Different Tribes of PhilippinesLet's try to understand the culture and traditions of some of

the numerous tribes of the Philippines. B'laan: One of the many tribes, the B'laan people come from the

Saranganai region, the southeastern part of Davao. People of this tribe are also identified by names such as Bira-an, Baraan, Vilanes and B'laan. These people are known for the beadwork, n'talak weave and brasswork. They wear embroidered costumes and jewelry made from brass.

Bontocs: The tribes living in the Mountain province of Philippines and falling in today's Bontoc municipality are known as Bontocs. The Bontoc region today, is divided into 16 subdivisions called barangays and its total population is 24,798.

Ibaloi: The Ibaloi is an agricultural community, which cultivates rice in terraced fields. One of the communities living in the mountains of the Cordillera Central, the Ibaloi people are 55,000 in number today. The Ibaloi people practiced mummification in the olden days. The process of mummification involved dehydrating the dead body completely, with the help of smoke. The Ibaloi language they speak comes under the family of Austronesian languages.

Lumad: These people come from southern Philippines. In the Cebuano language the word Lumad means 'native'. Lumads are further subdivided in 18 ethnolinguistic groups. There are 17 Lumad ethnolinguistic groups namely, Atta, Bagobo, Banwaon, B’laan, Bukidnon, Dibabawon, Higaonon, Mamanwa, Mandaya, Manguwangan, Manobo, Mansaka, Tagakaolo, Tasaday, Tboli, Teduray, and Ubo.According to the Lumad Development Center Inc., there are about eighteen Lumad groups in 19 provinces across the country. They comprise 12 to 13 million or 18% of the Philippine population and can be divided into 110 ethno-linguistic groups. Considered as "vulnerable groups", they live in hinterlands, forests, lowlands and coastal areas.[1]

Katawhang Lumad are the un-Islamized and un-Christianized Austronesian peoples of Mindanao, namely Erumanen ne Menuvu`, Matidsalug Manobo, Agusanon Manobo, Dulangan Manobo, Dabaw Manobo,Ata Manobo, B'laan, Kaulo, Banwaon, Teduray, Lambangian, Higaunon, Dibabawon, Mangguwangan, Mansaka, Mandaya, K'lagan, T'boli, Mamanuwa, Talaandig, Tagabawa, and Ubu`, Tinenanen, Kuwemanen, K'lata and

Diyangan. There are about twenty general hilltribes of Mindanao, all of which are Austronesian.The term Lumad excludes the Butuanons and Surigaonons--even the said two ethnic groups are native to Mindanao and the word tells it so—because those two are Visayans and Lumad are not ethnically related to them, which creates a contradiction because the word lumad literally means "native" in Visayan.Mangyan: It is a common name used to refer to eight ethnic tribes in Philippines. The Mangyan people come from the Mindoro islands and their population is around 282,593. The Mangyan people practice subsistence agriculture and they cultivate a number of varieties of the sweet potato along with taro and rice. They follow a religion called Animism.

Maranao: Primarily known for their sophisticated weaving and artwork, the Maranaos come from the island of Mindanao. They are also known as the 'People of the Lake', since, they inhabited the region around the Lake Lanao. Maranaos come under a bigger group of Filipinos, the 'Moros' who follow the Muslim religion. To put it in a nutshell, all Muslims in Philippines are Moros. An ancient form of instrumental music, the 'kulintang' holds great importance in Maranao culture. The life of the Maranaos is centered on Lake Lanao, the largest in Mindanao, and the second largest and deepest lake in the Philippines. This breathtakingly beautiful lake is surrounded with myths and legends, it is the main source of fisheries, and the main source of a hydroelectric plant installed on it; and the Agus River system that generates 70% of the electricity used by the people of Mindanao. A commanding view of the lake is offered by Marawi City, the provincial capital.Negrito: The term Negrito is a Spanish word, a diminutive of the word Negro. In this case, Negritos refers to a large group of indigenous tribes in Philippines. It includes the subgroups called the Agta, Aeta, Ati, Ayta, Dumagat and 25 more tribes from the Philippines. Although the Negritos of the Philippines possess some physical similarities with the pygmies of Africa, they are completely unrelated in terms of genetics.Tagbanua: Inhabiting the northern and central region of Palawan, Tagbanua is one of the oldest tribes in Philippines. Tagbanuas live in tiny villages that are compact with only 45 to 500 people living in a single village. They speak the Palawano language and worship four deities, known as 'Nagabacaban' or 'Mangindusa', 'Polo', 'Sedumunadoc' and 'Tabiacoud'. The family structure of Tagbanuas is a 'nuclear' one. They live in houses made of bamboo. The Tagbanwa or Tagbanua, one of the oldest ethnic groups in the Philippines, can be mainly found in the central and northern Palawan. Research has shown that the Tagbanwa are possible descendants of the Tabon Man; thus, making them one of the original inhabitants of the Philippines.[1] They are brown-skinned, slim and straight-haired ethnic group.[2]

There are two major classifications based on the geographical location where they can be found. Central Tagbanwas are found in the western and eastern coastal areas of central Palawan. They are concentrated in the municipalities of Aborlan, Quezon, and Puerto Princesa. Calamian Tagbanwa, on the other hand, are found in Baras coast, Busuanga Island, Coron Island and in some

parts of El Nido.[3] These two Tagbanwa sub-groups speak different languages and do not exactly have the same custom.[1][4]

Tagbanwa live in compact villages of 45 to 500 individuals.[5] In 1987, there are 129,691 Tagbanwas living in Palawan. [4] At present, Tagbanwa tribe has an estimated population of over 10,000.[1] 1,800 of these are in the Calamianes.

Tausug: One of the indigenous tribes in the Philippines falling under a larger ethnic group, Moro, the Tausug community comes from the Sulu Archipelago. The Sulu Archipelago is a group of islands in the southwestern parts of Philippines. Earlier, the Tausug people governed a bigger kingdom known as the 'Sulu

Sultanate', which covered the modern-day provinces of Palawan, Basilan, Sulu and Tawi-Tawi.

The first humans to have inhabited the Philippines were the 'Tabon Man'. Negritos who appear similar to the tribals of Andaman islands followed later and settled in Philippines. There are a large number of tribes in Philippines. In spite of enduring years of colonization, the descendants of the original inhabitants have been able to preserve the culture of Philippines. The great diversity in language, arts, music and traditions provide us with an idea about the richness of this archipelago, the Philippi