General Science Notes

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The Earth’s Spheres

Our planet formed 4.5 billion years ago. Since then, it has developed and modified four main physical environments that interact strongly with one another.

1. Atmosphere: The layer of gases that surrounds the Earth. The atmosphere protects us from the sun’s intense heat and radiation, provides the air we breathe, and produces weather.

2. Hydrosphere: The Earth’s water. The hydrosphere includes all the liquid and frozen water of the Earth’s oceans and land (groundwater), as well as water vapor in the atmosphere.

3. Biosphere: All organisms living on and inside the Earth’s surface.

4. Lithosphere: The rigid, relatively cool rocky zone immediately under the Earth’s surface. The lithosphere includes the Earth’s crust and part of the upper mantle. The asthenosphere is the region in the upper mantle (beneath the lithosphere) where rocks melt to form magma (molten rock). The asthenosphere is less rigid than the lithosphere and is able to flow. Movement of the lithosphere is directly connected to flow within the asthenosphere.

The Earth’s Interior

The Earth’s interior is divided as follows:

1. Crust (5–40 km thick): The thin outer skin of the planet. 2. Mantle (2,885 km thick): The origin of most magma. 3. Core (3,486 km thick): A dense, metal-rich ball inside

the Earth. The core is composed of the liquid outer core and solid inner core.

Plate Tectonics

Plates are the slabs of the Earth’s crust that make up the lithosphere.

Plates and the Earth’s Crust

The Earth’s crust is composed of the continental crust (30–100 km thick; forms the continents) and the oceanic crust (about 10 km thick; denser than continental crust; mostly covered by oceans).

Plate Tectonic Theory

Geologists developed plate tectonic theory as a model of movement on Earth’s crust on the surface of our planet. Observations and measurements of the processes that lead to and result from this movement support the plate tectonic model.

Continental drift: In the early 1900s, scientists noticed that, based on the continents’ shapes, it looked like the continents could fit snugly together. Geologists proposed that the continents gradually float around on the surface of the planet, bumping into each other and pulling apart.

Wilson cycle: In the 1960s, J. Tuzo Wilson proposed that landmasses, over time, repeatedly join to form a supercontinent—an amalgamation of all the continents into one big mass—and subsequently split apart.

Isostasy: The concept that the crust ―floats‖ on the heavier mantle in gravitational balance, like a block of ice in water.

Mountains have ―roots‖ that enable them to stay in balance; bigger mountains have bigger roots.

When a great load is removed from Earth’s surface (like when a glacier melts), the crust rebounds, or gently rises, to maintain isostatic equilibrium.

Plate Boundaries

The plates meet at plate boundaries, which are the sites of most earthquakes, volcanoes, and mountain formation. There are three types of plate boundaries:

1. Convergent boundary: The margin between two plates that are moving toward each other. Plate convergence leads to ocean-ocean, ocean-continent, or continent-continent collision.

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1. Subduction: Dense oceanic crust sinks beneath less dense continental crust at a convergent boundary. In this setting, a deep oceanic trench forms along the coast above the subduction zone, and volcanoes arise on the continental plate. An example of ocean continent convergence is seen today in the Aleutian Arc of Alaska.

2. This convergence eventually leads to continent-continent collision and mountain formation as two landmasses crumple into each other. A classic example of this mountain formation is the convergence between India and Asia, which continues to build the Himalayan chain and the tallest mountain in the world, Mt. Everest.

3. “Ring of Fire”: The circumference of the Pacific Ocean, bounded by subduction zones at the edges of the Pacific plate, that is the site of many volcanoes.

2. Divergent boundary (spreading center): The margin between two plates, usually both oceanic, that are moving away from each other. Plates grow at spreading centers, which are often coincident with mid-ocean ridges like the Mid-Atlantic Ridge. At a mid-ocean ridge, magma rises from the asthenosphere, pushing the plates apart and accreting, or sticking onto, the sides of the plates. The plates widen in parallel strips as they diverge from each other. This is also the source of magnetic striping on the sea floor (see Magnetic polarity reversals).

3. Transform boundary: The margin between two plates that are sliding past each other. Transform boundaries are prominent features on sea floors, where they connect offset mid ocean ridge segments. The most famous transform boundary is along the San Andreas Fault in California, where the Pacific and North American plates slide past each other.

Deformation

Rock layers crumple when the Earth’s crust is subject to stresses. These stresses may result in folds (warping or bending of rock layers, such as in the diagram below) or faults (fractures in the crust).

Earthquakes and Seismology

Faults

Fault: A fracture in the Earth’s crust caused by stress. There are several different types of faults:

1. Normal fault: A fault in which the hanging wall (the block of crust above the fault) moves down relative to the footwall (the block of crust below the fault) as a result of extension.

2. Reverse fault: A fault in which the hanging wall moves up relative to the footwall as a result of compression.

3. Strike-slip fault: A fault in which two blocks of crust slide past each other on the same plane. The San Andreas Fault is a strike-slip fault.

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Earthquakes

Earthquake: A vibration of the Earth caused by slippage along a fault.

1. Hypocenter (focus): The exact location of an earthquake (often far below the surface).

2. Epicenter: The point on the Earth’s surface directly above the hypocenter.

3. Foreshocks: Small earthquakes that commonly precede a major earthquake.

4. Aftershocks: Small earthquakes that commonly occur after a major earthquake.

Seismic Waves

Energy travels away from an earthquake’s focus in waves, both through the Earth and along its surface. Different types of seismic waves include:

1. Surface waves: Seismic waves that travel along the Earth’s surface.

2. Body waves: Seismic waves that travel through the Earth’s interior. There are two types:

1. Primary waves (P waves): Body waves that compress and expand rock in the direction the waves travel (like a slinky).

2. Secondary waves (S waves): Body waves that shake material at right angles to the direction the waves travel (like shaking a rope). Solid rock transmits S waves, but gases and liquids do not.

Measuring Earthquakes

Geologists use the Richter scale to assign magnitude to earthquakes by assessing the amplitude (height) of the largest seismic wave each earthquake creates. Each additional unit of magnitude denotes a tenfold increase in the power of the earthquake (e.g., a magnitude 7.0 earthquake is ten times more powerful than a magnitude 6.0).

Locating Earthquakes

The exact location of an earthquake’s epicenter is determined through triangulation, which requires several seismometers (instruments that record seismic waves) stationed around the world.

1. Seismometers record P wave arrival first, followed by S wave arrival.

1. The time difference in arrival is used to calculate the distance from the seismometer to the earthquake epicenter.

2. However, this measurement tells only the distance to the earthquake, not the direction in which it lies.

2. To determine location, each of three stations draws a circle around their station location with the radius of the distance it calculated. The epicenter is at the intersection of the three circles.

Earthquake Aftermath

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In addition to causing great destruction at the epicenter, an earthquake sometimes triggers other natural disasters.

1. Tsunami: A massive wave created when an earthquake shakes coastal or undersea land. Tsunamis have a short height but a long length (see Shorelines), causing amplification of tides.

1. Tsunamis are especially dangerous because they cause low tides to be very low. While people are walking the freshly exposed beach, the high tide comes in quickly and much higher than normal.

2. A tsunami can move thousands of miles across the ocean at hundreds of miles per hour. An earthquake in Japan, for instance, can send a tsunami all the way to Hawaii.

2. Landslide: A fast-moving wall of dirt and mud that an earthquake shakes loose. Landslides are primarily a problem in hilly, populated regions like Southern California.

Seismic Imaging

The study of seismic waves has revealed much about the structure and composition of the Earth’s interior.

1. Seismic waves travel at different speeds through different materials. As seismic waves travel through Earth, their velocity increases abruptly below the crust (at a compositional break called the Moho), decreases beneath the lithosphere, and changes abruptly again at the mantle/core boundary.

2. Geologists have thus been able to ―see‖ Earth’s interior. The only other evidence we have of its makeup is from volcanic material.

Minerals

Minerals are earth materials that have four main characteristics: they are solid, inorganic, naturally occurring, and have a definite chemical structure.

Mineral Properties

Minerals are identifiable based on a number of specific properties:

1. Crystal form: The outward expression of a mineral’s chemical structure. For example, quartz has a hexagonal, or 6-sided, crystal form.

2. Cleavage: Planes of weakness in the mineral’s crystal lattice along which the mineral tends to break. Cleavage faces are usually flat surfaces.

3. Fracture: If a mineral lacks cleavage, it fractures in an irregular, jagged manner.

4. Hardness: The resistance of a mineral to being scratched. Geologists use the Mohs scale to assign each mineral a hardness between 1 (softest) and 10 (hardest).

The Mohs hardness scale

Hardness Mineral

10 diamond

9 corundum

8 topaz

7 quartz

6 feldspar

5 apatite

4 fluorite

3 calcite

2 gypsum

1 talc

5. Streak: The color a mineral leaves when rubbed across a piece of unglazed porcelain.

o A mineral’s visible color is not a reliable diagnostic property. A single mineral may vary in color from sample to sample, but its streak color does not. For example, quartz may be clear, gray, purple, or pink, but its streak is always colorless.

6. Luster: The way light reflects off a mineral’s surface. Luster may be described as vitreous (glassy), metallic, pearly, silky, or dull.

7. Specific gravity: The comparison of a mineral’s weight to the weight of an equal volume of water (water’s specific gravity is 1). The greater a mineral’s specific gravity, the greater its density.

8. Other diagnostic properties: Some minerals are magnetic, some taste salty, and some fizz when hydrochloric acid is dropped on them.

Mineral Groups

1. Silicates: The most common mineral group. Silicates have a framework of silicon (Si) and oxygen (O), the two most common elements in the Earth’s crust.

1. Silicon-oxygen tetrahedron: The basic silicate structure, which consists of four oxygen atoms around a central silicon atom.

2. Silicate minerals can form from: 1. A single tetrahedron (e.g.,

olivine) 2. Single chains (pyroxenes, e.g.,

augite) 3. Double chains (amphiboles, e.g.

hornblende)

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4. Sheets (micas, e.g., muscovite, biotite)

5. Three-dimensional networks (e.g., feldspar, quartz)

2. Nonsilicates: Less common but also important rock-forming minerals.

1. Carbonates: Contain carbon and oxygen in a carbonate group (CO3). Calcite (CaCO3), which forms limestone and marble, is a common carbonate.

2. Oxides: Usually consist of oxygen and another element. Common oxides include ice (H2O) and magnetite (Fe3O4).

3. Sulfides: Contain sulfur ions. Pyrite, or ―fool’s gold,‖ is a common sulfide.

4. Sulfates: Contain sulfur and oxygen in a sulfate group (SO4). Gypsum, a material used in buildings, is a common sulfate.

5. Halides: Contain a ―salt‖ ion such as Na, Cl, or F. Halite, or common table salt (NaCl), is a halide.

6. Native elements: Minerals that exist in pure elemental form. Native elements include gold (Au), silver (Ag), and copper (Cu).

Common Rock-Forming Materials

1. Felsic minerals: Comprise over 50% of the Earth’s crust. Felsic minerals are silicates that are light in color, contain little iron and magnesium, and have abundant silica.

1. Quartz (SiO2): Has vitreous luster; lacks cleavage but has conchoidal fracture (smooth, curved fracture like that of glass); lacks streak; and is usually gray in color but can be pink, purple, or black.

2. Feldspars:Potassium feldspar (KAlSi3O8) and plagioclase ((Ca,Na)AlSi3O8) both have distinct cleavage planes that meet at about a 90° angle. Potassium feldspar usually is cream or pink in color, whereas plagioclase usually is in a range between white and light gray.

3. Mica: A family of sheet silicates, including silvery muscovite and black biotite. Micas are important minerals and often give rocks a sparkly appearance.

2. Mafic minerals: Contain iron and/or magnesium, making them dark.

1. Olivine ((Fe, Mg)2SiO4): Has glassy luster, conchoidal fracture, and is usually dark green. Olivine is a major component of the upper mantle.

2. Pyroxenes: Usually dark green to black, with distinctive cleavage planes that meet at right angles. Pyroxenes form a group of chemically complex minerals, the most common of which is augite, which are common in oceanic crust.

3. Amphiboles: A complex group, distinguished from pyroxenes on sight by

their cleavage planes, which meet at 60° and 120°. The most common is hornblende.

Rocks and Their Environments

Rocks are aggregates of minerals.

Igneous Rocks

1. As magma (molten material) cools, ions arrange themselves into orderly patterns during crystallization. There are two types of crystallization:

1. Volcanic (extrusive): Magma crystallizes quickly at spreading centers and from volcanic eruptions.

2. Plutonic (intrusive): Magma crystallizes slowly deep below the Earth’s surface.

2. Magma’s rate of cooling affects crystal size and mineral composition. Fast cooling results in smaller crystals, more mafic; slow cooling results in larger crystals, more felsic.

1. Glass: No crystals. Forms when magma cools too rapidly to form crystals.

2. Fine-grained (aphanitic): Crystals too small to distinguish individual minerals with the unaided eye. Gas bubbles leave openings or vesicles. Aphanitic rocks form quickly at Earth’s surface or in the upper crust (volcanic).

3. Coarse-grained (phaneritic): Crystals large enough to distinguish minerals with the naked eye. Phaneritic rocks form in a slowly cooling magma chamber deep in the crust (plutonic).

4. Porphyritic: Large crystals in a matrix of smaller crystals. Porphyritic rocks form when magma crystallizes rapidly, forming a fine-grained matrix, but then moves to a slower-cooling environment before all the melt has crystallized. The remaining melt forms large crystals.

3. Bowen’s reaction series: The geologist N. L. Bowen (1887–1956) created a chart showing the series in which different minerals crystallize from cooling magma:

o On the left side: Mafic minerals begin to crystallize. After each mineral crystallizes, it reacts with the remaining magma to form the next mineral in the series.

o On the right side: Felsic, calcium-rich minerals crystallize to form early feldspars, which then react with sodium in the remaining magma to form more sodium-rich feldspars.

o At the bottom of the series: When magma crystallization is nearly complete, the

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remaining magma is mostly SiO2, and quartz forms.

Volcanoes

1. Volcanoes form where magma burns through the crust, at subduction zones, at spreading centers, or at ―hot spots‖ like Hawaii.

1. Successive eruptions build a cone of hardened lava. Eruptions are explosive (pyroclastic) if the magma is gas-rich and felsic, slow if the magma is gas-poor and mafic.

2. Although volcanoes typically form at subduction zones or spreading centers, they also may form within a plate, as in the Yellowstone region of Wyoming.

2. Volcano morphology 1. Crater: The pit inside a volcano. A crater

more than 1 km wide is called a caldera. 2. Vent: A pipelike structure connecting the

underground magma chamber to the crater.

3. Types of volcanoes 1. Shield volcano: A broad, slightly domed

structure typically built of liquid basalt. The Hawaiian volcanoes are shield volcanoes.

2. Composite cone (stratovolcano): A large, nearly symmetrical cone made of alternating lava flows and pyroclastic volcanic debris.

3. Cinder cone: A generally small volcano with steep sides, built from ejected lava fragments and often in groups near larger volcanoes.

4. Volcanic rocks 1. Basalt: Dark green to black, fine-grained,

mostly pyroxene and plagioclase feldspar, with some olivine. The ocean floor is mostly basalt.

2. Tuff: Hardened ash from an explosive volcano.

5. Plutons are the site of plutonic rock formation. Most magma in the Earth is deep underground, in chambers that cool slowly or rise slowly to intrude into preexisting rock.

1. Plutonic rocks 1. Gabbro: Has a basaltic

composition (mafic) but large grain size.

2. Granite: A phaneritic igneous rock with 25–35% quartz and more than 50% feldspar, with hornblende, muscovite and biotite.

2. Pluton forms 1. Batholith: A large expanse of

granitic rock (more than 100 km2). Batholiths frequently form the cores of mountains, exposed only after much of the ground surface erodes.

2. Sill: A lateral layer of igneous rock formed when fluid basaltic magma rises from a magma

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chamber and squeezes into horizontal strata.

3. Dike: A vertical or angled layer of igneous rock that cuts across other rock layers, usually by injection into fractures.

Metamorphic Rocks

1. High temperature, high pressure, or variable chemical conditions can change country (preexisting) rocks through the process of metamorphism. Rocks remain solid during the process.

1. Regional metamorphism: An extensive volume of the crust is metamorphosed, usually by intensive compression at convergent boundaries.

2. Contact metamorphism: Intruding magma heats cold country rock nearby and causes it to recrystallize.

3. Metasomatism: Hot fluids dissolve original minerals, and then chemical reactions cause new minerals to grow.

2. Rocks undergo both mineral and textural changes during metamorphism.

1. Mineral changes: During metamorphism, two minerals can react, and their ions can diffuse across grain boundaries, resulting in a new mineral. Alternatively, complex minerals may break down into simpler ones.

2. Textural changes: Rocks gain foliation (alignment) as minerals align into bands. With increasing temperature and pressure, grain size increases and texture coarsens.

3. Classification: Metamorphic rocks are classified by strength of metamorphism. The following are listed in order from weak to strong metamorphism:

1. Foliated rocks: 1. Slate: A fine-grained rock,

usually made of metamorphosed fine sediments.

2. Phyllite: Similar to slate but slightly coarser-grained, and shiny due to high mica content.

3. Schist: A coarse-textured metamorphic rock, with minerals aligned in parallel bands, containing more than 50% platy minerals (minerals with a planar, layered structure) like mica.

4. Gneiss: Bands of abundant coarse grains, mostly feldspar and quartz, alternated with bands of flaky minerals.

2. Nonfoliated rocks: 1. Marble: Metamorphosed

limestone with a sugary texture.

Marble is composed of interlocking calcite grains.

2. Quartzite: Metamorphosed quartz sandstone. Quartzite is very hard and is composed of interlocking quartz grains.

3. Hornfels: Fine-grained rock altered in contact zones around igneous intrusions.

Sedimentary Rocks

1. When weather and other forces of erosion wear away rocks, sediments form. Those sediments can be compacted, through lithification, to form sedimentary rocks.

1. Erosion: The transport of material around Earth’s surface by a mobile agent like water or wind. Erosion and weathering form sediments and soil.

1. Mechanical (physical) weathering: Rocks break into smaller pieces, with each piece retaining the original mineral composition.

Frost wedging: Water freezes and expands in a rock, breaking off fragments.

Unloading: Erosion removes material from above buried rock. Pieces pop off in response to the lowered pressure.

Biological activity: Roots wedge into and widen rock fractures, or animals burrow into soil and expose rock to the surface.

2. Chemical weathering: Rocks break down chemically, and their constituent minerals alter during the process.

Oxidation: Water (H2O) is the strongest chemical weathering agent. It causes iron-rich rocks to oxidize, or rust.

Ionization: CO2 + H2O → carbonic acid, which breaks granite down into clay minerals.

2. Lithification: After erosion and weathering, sediments cement to form sedimentary rocks.

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2. Sedimentary settings: Sedimentary rocks can form anywhere on or just below the Earth’s surface, in dry or wet environments.

3. Classification: Sedimentary rocks may be classified in several different ways:

1. Based on origin: 1. Detrital sediments: Sediments

that are fragments of broken-down rock. They are listed here in order of decreasing grain size:

Breccia: Lithified angular blocks of rock.

Conglomerate: Lithified round rock fragments, pebble-sized and larger.

Sandstone: Cemented sand.

Shale: Compacted clay, mud, or silt.

2. Chemical and biochemical sediments: Sediments that form from minerals that precipitate from water, either physically or biologically (as organisms pull elements out of water to make their skeletons):

Limestone (CaCO3): Formed from cemented fragments of any size of shell.

Chert: Cemented shells made of silica.

3. Evaporites: Sediments that form as water evaporates from a closed basin and the solution becomes supersaturated with certain elements, which then precipitate out as minerals like halite.

4. Coal: An organic material that nonetheless is considered a sedimentary rock because it consists of compacted plant matter.

2. Based on grain size and sorting: 1. Grain size: The physical size of

individual grains that make up sedimentary rock.

Gravel (>2 mm) forms conglomerate, breccia

Sand (1/16–2 mm) forms sandstone, greywacke

Mud (<1/16 mm) forms shale, mudstone

2. Sorting: The degree of variety of grain size and composition within a sedimentary rock.

Well sorted: One grain size and type dominates the rock’s composition.

Poorly sorted: The rock is composed of grains of many sizes and compositions.

3. Based on sedimentary structure: 1. The method of sediment

deposition often imparts a distinctive pattern on a package of sedimentary rocks.

2. Bedding planes: The planes that separate strata (layers) of sedimentary rock. Often, these planes are the planes along which the rock breaks.

Cross-bedding: Wind or waves deposit sediments in an upsweeping pattern.

Graded bedding: Grain size becomes coarser or finer from the bottom to the top of a layer.

The Rock Cycle

1. Heat, pressure, erosion, and other forces are always at work on the Earth, changing the composition of rocks: from igneous to sedimentary, sedimentary to metamorphic, igneous to metamorphic, and so on.

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2. These continual conversions from one type of rock to another are collectively termed the rock cycle.

1. All rocks exposed at Earth’s surface are subject to weathering, which leads to the formation of sediments.

2. All sediments are subject to burial, after which they are undergo lithification (cementation and compaction) to form sedimentary rocks.

3. All rocks can be exposed to heat and pressure, which often leads to formation of metamorphic rock.

4. When metamorphic rocks melt into magma, the magma sometimes cools and crystallizes into igneous rock.

Continental Edges and the Ocean Floor

The edge of each continent gradually slopes downward underwater for a number of miles offshore. After this gradual sloping, there is a sudden drop onto the deep sea floor.

1. Continental margin: The stretch of crust from the shoreline to the deep sea. The continental margin includes the:

1. Continental shelf: The zone of gently sloping underwater ground where the water gradually deepens.

2. Shelf break: The sudden drop at end of the continental shelf.

3. Continental slope: The steep underwater cliff after the shelf break.

4. Continental rise: The gentle slope at the base of the break. The continental rise is at the end of the continental margin and leads to the abyssal plain.

5. Abyssal plain: The very flat, deep-ocean floor.

2. Undersea features: Water and sediment eroding from the continents leave imprints on the continental margin. The continental slope can be unstable.

1. Submarine canyons: Deep, steep-sided underwater valleys, possibly former river channels.

2. Turbidity currents: Sediment-laden water at the continental slope. Turbidity currents often flow downward, eroding the slope and entraining (drawing along and transporting) more sediment.

3. Turbidites (deep-sea fans): Sediments deposited by turbidity currents on the continental rise. Turbidites typically are composed of sequences of sediments that are coarse grained at the bottom and fine-grained at the top.

Shorelines

1. Tides: Daily changes in ocean surface height caused by the gravitational attraction between the Earth and the moon. This gravitational attraction leads to rhythmic rising and falling of the waterline.

2. Currents: Continuous flows of water in one direction. 3. Waves: Water surface ripples generated by wind.

1. Wave crest: The top peak of a wave, vs. trough, the lowest point between waves.

2. Wave height: The vertical distance from trough to peak.

3. Wavelength: The horizontal distance from one wave peak to the next.

4. Waves begin to slow when water depth is 1/2 the wavelength. When wave height is 1/7 of the wavelength, the wave breaks, or collapses. Breaking waves carve the shoreline.

4. Beach: The sloping shoreline made of sediments moved by waves, tides, and currents.

1. Offshore: The region below the low tide line.

2. Foreshore: The region between the low tide line and above the flat beach.

3. Backshore: The region landward of the beach face.

4. Berm: A small hill of sediments just above the flat beach area. Gentle waves build the berm up in summer, and storms carve the berm away in winter.

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Deserts and Wind

1. Deserts form in land areas with low precipitation (typically less than 25 cm of rain per year).

2. Wind: The movement of air on the Earth’s surface stems from the uneven distribution of solar heat. Hot air rises over the equator, drops out moisture, and descends as cool, dry air over latitudes 30 N and 30 S. Deserts are found at these latitudes.

Wind-Created Features

Wind is a strong sculpting agent. It carves away rocks and sediments and deposits sediments elsewhere.

1. Bed load: The sand grains and other particles that wind (or water) carries on or just above the ground.

2. Suspended load: The fine particles that wind (or water) keeps aloft.

3. Saltation: The ―jumping‖ of sand grains due to strong wind. Wind blowing perpendicular to a surface decreases the pressure on that surface. When the inertia of a sand grain is overcome, it begins to roll. When it hits other grains, they bounce into the air, where they are carried forward until gravity pulls them back down.

4. Deflation: A process by which wind carries fine particles away and leaves a compact surface of larger pebbles.

5. Dunes: Sand mounds or ridges that the wind creates. Dunes have a steep side called a slip face. Types of dunes include:

1. Barchan dune: A solitary dune shaped like a horseshoe, with its tips pointing away from the wind. Barchan dunes form on flat surfaces where sand supply is low.

2. Transverse dune: A long ridge of sand oriented perpendicular to the direction of the wind. Transverse dunes form where wind is steady and sand is plentiful.

3. Longitudinal dune: A dune that forms parallel to wind direction, in places where sand supply is limited.

4. Parabolic dune: A dune shaped like a barchan dune but with its tips pointing into the wind. Parabolic dunes form on beaches with abundant sand and are partly covered by vegetation.

Glaciers and Glaciation

Glacier: A large mass of ice, formed on land by compaction of snow, that flows downhill from snow accumulated at the head. Glaciers survive by accumulating more snow each year than they lose to snowmelt.

1. Alpine glacier (valley glacier): A glacier in mountains that flows down channels previously eroded by streams.

2. Piedmont glacier: A glacier that forms when several valley glaciers flow out onto land at the front of a mountain range and merge with one another.

3. Ice sheet (continental glacier): A large expanse of ice that flows in all directions.

Locations

Most expanses of ice on Earth are at the extreme latitudes, where the weather remains consistently cold. Greenland, in the north, houses one huge ice sheet, and Antarctica, in the south, is the site of the other. Together, these ice sheets cover 10% of the Earth’s land surface. Smaller ice sheets are found at high altitudes, in places like Alaska, Canada, and the Alps.

Formation and Morphology

1. Snow line: The line above which more snow falls than melts in a year, creating permanent snow cover.

2. Firn: Packed snow that turns into ice over time. Under pressure, the boundaries of ice grains melt, and the grains refreeze together, forming the interlocking ice crystals that comprise a glacier.

3. Glacial terminus: The front toe of a glacier. 4. Ablation: The direct conversion of ice into vapor.

Glaciers lose mass by this process. 5. Zone of fracture: The upper 35 meters of glacial ice,

where ice responds rigidly to stress by cracking. 6. Zone of plastic flow: The area of glacial ice 35

meters below the surface and deeper. In this zone, glaciers deform under their own weight. Squeezing and flow therefore are greatest where the glacial ice is thickest.

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Erosion and Landforms

Glaciers carve the landscape as they flow and leave deposits in their wakes.

1. Glaciers cause erosion in several ways: 1. Plucking: Glacial melt water runs into rock

crevices, freezes, expands, and causes fragments to break off.

2. Abrasion: Rocks entrained in the bottom of a glacier grind against the surface over which the glacier flows. Abrasion causes glacial striations, parallel grooves worn into bedrock.

2. Glacial movements can create many different landforms:

1. Cirque: An amphitheater-shaped scoop out of bedrock, caused by plucking.

2. Arête: A knife-edged rock ridge between adjoining cirques.

3. Glacial trough: A valley with a U-shaped bottom, carved by a glacier.

4. Fjord: A glacial trough that extends into the sea.

5. Till: Unlayered sediments deposited by a glacier. Till often includes boulders, gravel, sand, and clay together.

6. Moraine: A ridge of till left behind by a retreating (melting) glacier. Types of moraines include:

1. Terminal moraine: The ridge left behind at the farthest point a glacier reaches (the line of maximum advance).

2. Lateral moraine: A long, narrow mound of till that forms parallel to the glacier’s direction of motion (as a line within the glacier).

7. Glacial erratic: A large boulder that a glacier carries from its place of origin and drops in a different place.

8. Drumlin: A low, rounded hill of till, with a tapered end pointed in the direction the glacier flowed. The other end has a steep, squared face.

9. Outwash plain: A large sheet of stratified sediment deposited by melt water streaming out of the toe of a glacier.

10. Esker: A winding ridge of sediment left behind when a glacier melts.

11. Kettle: A scoured depression in an outwash plain, formed when a block of glacial ice is left behind, buried, and melts.

Ice Ages

Over geologic time, the Earth has undergone periods of extreme cold during which ice sheets expanded, and periods during which glacial ice melted (see Climate Change).

Other Erosive Forces

Mass Wasting

Mass wasting is movement of rocks and soil caused by the loosening effects of water and the downward force of gravity. There are several types of mass wasting.

1. Creep: Slow earth movement that has effects seen only after some time. Creep usually occurs where soil freezes, expands, thaws, and settles. Evidence of creep is seen in tilted telephone poles and cemetery headstones.

2. Fall: The unrestrained fall of rock fragments off a cliff. A rock fall creates talus—fields of rock fragments that collect at the bottom of a cliff.

3. Slide: The breaking off of rocks or soil from a plane of weakness and their subsequent slide down the face of a steep slope.

4. Slump: A type of slide that involves intact blocks of rock sliding down a concave surface.

5. Flow: The quick movement of water-soaked sediments down a slope in one large mass. Flow occurs when these water-soaked sediments are shaken.

6. Solifluction: The flow of watery soil due to repeated thawing and freezing, as happens to a dirt road after winter.

The Water Cycle

Hydrologic Cycle

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The Earth’s water supply is always in motion, going through an unending cycle of water running from the land to the sea, precipitating, evaporating, and reprecipitating. Water is always recycled or moved from place to place—never completely destroyed or created anew.

1. Runoff: Water that flows off the land surface into rivers, lakes, streams, and oceans.

2. Transpiration: The process by which plants release the water they absorb into the atmosphere.

3. Water budget: Most of the Earth’s water is contained in the oceans, but other reservoirs hold significant water as well. Geologists measure the Earth’s water supply in terms of volume (measured in km3):

1. Oceans: 1,350,000,000 2. Glaciers: 27,500,000 3. Groundwater: 8,200,000 4. Lakes: 205,000 5. Atmosphere: 13,000 6. Streams: 1,700

Streams

Streams are channeled flows of any amount of water. Although streams hold only a small percentage of the Earth’s water at any given time, the energy of streams has done much to sculpt the landscape. Stream energy is controlled by channel size and slope.

1. Gradient: The steepness of land over which a stream flows. As a stream flows down a slope, its potential energy converts to kinetic energy. The steeper the gradient, the faster the stream flows.

2. Base level: The lowest level to which a stream can erode its channel. Oceans are considered the ultimate base level because they are the final destination of streams. More often, local base levels like lakes, dams, or stream junctions control stream flow.

3. Cross section: The area of water in a cross-sectional slice of a stream. For a flat stream, cross section is calculated by multiplying depth by width. For a semi-circular stream, it is calculated using stream radius: (1/2)πr2.

4. Discharge: The volume of water that flows past a certain point in a stream over a measured time interval. Discharge is calculated by multiplying the cross section of the stream by the velocity of the stream.

Stream Flow and Transport

1. Water can flow within a stream in two ways: 1. Laminar flow: In slow-moving streams,

water flows in parallel paths. 2. Turbulent flow: In fast-moving streams

and in rough stream channels, water swirls around as it moves down a gradient.

2. Capacity: The amount of sediment a stream can carry past a certain point in a given time.

3. Competence: A measure of how strong a stream is, based on the biggest size of an object the stream can move.

4. Saltation: Skipping and bouncing of particles on the bottom of a stream caused by water flow pushing the particles.

5. Load: The material a stream carries. There are several types:

1. Bed load: Heavy objects dragged along a stream bottom.

2. Suspended load: Fine particles carried suspended in a stream’s moving water.

3. Dissolved load: Material (salt, carbonate, or other ions) dissolved in the stream water.

6. Graded stream: A stream with a slope and channel that have adjusted enough over time so that the stream has just enough energy to carry its load, but no excess energy so that it erodes its banks.

Stream Settings

1. Alluvial fan: A gently sloping blanket of alluvium, or sediment deposited by a stream, where it exits a gully onto a flatter surface

2. Flood plain: A plain surrounding a stream. Streams periodically overflow their banks or move laterally across surrounding flood plains, leaving layers of sediments in their wake.

3. Delta: The mouth of a stream, where the stream slows due to a gentler gradient and deposits much of its sediments as it moves to base level.

4. Tributary: A small stream that flows into a larger stream.

5. Drainage system: All the land area that contributes to a stream system. A continental divide is a ridge that separates streams flowing in opposite directions on either side. For example, the Great Divide in the United States follows the Rocky Mountains: all streams east of the divide flow east to the Atlantic, whereas all streams west of the divide flow west to the Pacific.

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Stream Shapes and Patterns

There are several types of streams and drainage patterns, which are dictated by landforms and also shape those landforms. Whereas glaciers carve flat-bottomed, U-shaped valleys, streams carve sharp canyons, or V-shaped valleys.

1. Braided stream: A stream that divides into smaller streams. When a stream gradient decreases, its flow slows, causing the stream to branch into smaller subchannels. Braided streams are common on alluvial fans and glacial outwash plains.

2. Meandering stream: A stream that carves a path sideways and forms wide loops, called meanders, as it flows downstream. Often, when water in a stream flows over a bump, ripples are created that deflect water toward one side of the stream and carve into the side. This sideways flow creates a bend in the channel, and water flowing out of this bend then deflects toward the opposite side of the stream, carving a bend there.

1. Point bar: Sediment deposited in the inner curves of a meandering stream. The stream moves slowest in these inner curves, so the stream drops sediment here.

2. Oxbow lake: A lake that splits off from a meandering stream when erosion carves a straight channel that cuts off the flow into one of the stream’s meanders.

3. Streams can follow several different drainage patterns: 1. Dendritic drainage: Several substreams

branch out from a main stream in a treelike pattern.

2. Radial drainage: Streams run in all directions from a central high point.

3. Rectangular drainage: Streams make right-angled turns, following rectangular fracture patterns in the bedrock over which they flow.

4. Trellis drainage: Tributaries flow perpendicular to the main channel, following parallel beds of weak strata. Trellis drainage often occurs in tilted or folded rocks.

Groundwater

Groundwater is surface water that seeps into the ground. It constitutes 95% of the Earth’s supply of fresh water (outside of glaciers) and feeds not only humans and crops but also streams and lakes.

Groundwater Distribution and Movement

1. The ground under the surface of the Earth’s landmasses is divided into two zones based on the presence or absence of groundwater:

1. Zone of aeration: The area just below ground in which spaces between rocks and soil are filled with air.

2. Zone of saturation: The area below the zone of aeration in which the spaces between particles are filled with water. The top level of the zone of saturation is called the water table. The water in the zone of saturation is groundwater.

2. The porosity and permeability of soil and rock dictate the accumulation and movement of groundwater.

1. Porosity: The ratio of open spaces to volume of material. Porosity is quantified in percentages.

2. Permeability: A measure of the ease with which sediments transport water. Permeability is calculated as the volume of water that can move through a cross-section of sediments in a given time. Permeability is classified on a scale from very low to excellent.

Sand typically has 20% porosity and excellent permeability.

Clay has a 50% porosity but poor permeability.

3. The water table roughly follows topography, rising slightly beneath hills and depressed beneath stream channels. Where the land surface cuts low, the water table intersects the land, typically at stream channels.

1. Effluent stream: A stream that gains water from the zone of saturation, typically in wet environments.

2. Influent stream: A stream that loses water to the water table, typically in dry environments.

3. Hydraulic gradient: The slope of the water table. The hydraulic gradient, along with the

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permeability of material through which water flows, influences the speed of groundwater flow.

4. Recharge area: An area (usually higher elevation) that receives precipitation that soaks into the zone of saturation.

5. Discharge area: An area (usually near a stream) that receives groundwater from the zone of saturation and carries it away.

6. When recharge and discharge are in balance, the water table remains steady.

4. Groundwater availability is affected by the types of rock underground and the flux of water on the Earth’s surface.

1. Aquifer: An underground body of permeable rock or sediment that conducts water. Aquifers typically are composed of sand or gravel.

2. Confining bed: A laterally continuous sheet of rock that is impermeable to water and prevents the escape of water from aquifers. Typically, a confining bed is composed of shale (see Sedimentary Rocks).

1. Confined aquifer: An aquifer between two confining beds.

2. Unconfined aquifer: An aquifer above a confining bed.

Artesian well: A well drilled into a confined aquifer. 0. Confined aquifers cannot receive

precipitation from directly above; instead, water seeps in from far away on the sides, where the top confining bed thins to nothing.

1. As a result, a confined aquifer often has high water pressure. When a well is drilled into the top confining bed, water gushes upward out of the confined aquifer.

Perched water table: A pocket of groundwater stranded above the main water table by a confining bed beneath it

Geology and Groundwater

Groundwater can move in confined channels underground, carving out spaces like caves.

1. Cave: A crevice in a rock large enough for a person to enter. Caves usually form when acidic water flows through limestone formations and dissolves the calcium carbonate. As this carbonate-rich water drips off a cave ceiling, it forms:

1. Stalactites: Calcite deposits that hang from a cave ceiling.

2. Stalagmites: Calcite spears that point up from a cave floor.

2. When groundwater dissolves underground limestone, strange topographical features may result:

1. Sinkhole: A depression that forms on the surface when the roof of an underground cavern collapses.

2. Karst topography: Irregular topography on the surface that results when groundwater below flows through an extensive area of limestone, carving underground channels and caverns until surface water flows only underground. The land above these areas takes on irregular patterns as it sinks into various holes and grooves.

3. Geysers and hot springs: Features that form when magma exists near the surface of the Earth (e.g., near volcanoes) and heats the groundwater. Some water turns to steam, expands, and erupts out of holes in the ground in geysers. In other places, hot water trickles out of springs.

Climate Change

The Earth’s climate has changed considerably over the planet’s history. Scientists have determined that these changes occur in cycles driven by a number of factors.

Climate Cycles

Evidence suggests that the Earth has experienced climate cycles—alternating periods of extreme warmth and cold—throughout geologic time.

1. Periods of glacial climate, which have fostered the growth of glaciers, have alternated with interglacial periods, during which temperatures are so warm that glaciers melt.

2. During the Ice Age around 2–3 million years ago, ice sheets spread over much of the Earth’s land surface. About 55 million years ago, however, air and sea temperatures were so warm that geologists think glaciers melted away completely.

Driven by the Earth’s Orbit

Most geologists believe that these dramatic temperature changes result from variations in the Earth’s orbit

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1. In the 1920s, Serbian astrophysicist Milutin Milankovitch formulated a model of climate cycles based on three properties of the Earth’s orbit:

1. Eccentricity: Changes in the shape of the ellipse that the Earth traces as it orbits the sun. The ellipse is at its longest once every 100,000 years.

2. Obliquity: The tilt of the Earth toward the sun on its axis of rotation. The Earth’s obliquity shifts between 21.5 and 24.5 every 40,000 years. When the tilt is greatest, polar regions receive more summer sunlight and less winter sunlight.

3. Precession: The wobble of the Earth on its axis. Precession completes a full cycle every 26,000 years and affects the intensity of sunlight that reaches the Earth’s polar regions.

2. These three cycles regularly reinforce each other. At certain times, they combine to maximize the input of solar radiation to the Earth, which leads to warming of the Northern Hemisphere and glacial retreat. At other times, they combine to minimize the heat that the Northern Hemisphere receives, leading to glacial advance.

Driven by Tectonics

1. When the Earth’s tectonic plates form a supercontinent at high latitudes (see Plate Tectonics), ice growth is encouraged. This convergence of the continents is a rare event in Earth history, however.

2. More frequently, extensive volcanism leads to outpouring of CO2 into the atmosphere, which traps heat and leads to a greenhouse effect.

3. Glaciers record climate change. Geologists are able to drill cores out of glacial ice to measure the CO2 content of the atmosphere at different times in the past. They have determined that the CO2 content of the Earth’s atmosphere has fluctuated over time and that these fluctuations correspond to rising and falling temperatures.

Driven by Humans

Today, as we burn fossil fuels that release CO2 into the atmosphere, we are experiencing human-induced climate change.