Plate Tectonics

Plate Tectonics

By: Lauren Brown

Continental Drift• Wegener's Theory: From 1912, Wegener introduced the theory of “ continental drift ,” arguing that all

continents were once joined together in a single landmass and have drifted apart. He claimed that the cause of this might be the centrifugal force in the earth’s rotation, or

precession. o Precession is the change in the orientation of the rotational axis of a rotating body.• He named his theory “Pangaea,” meaning “all lands” in the Greek language

• Evidence:• In the early 1950’s , Cambridge University and Imperial College was producing data in favor

of Alfred Wegener’s theory on Plate Tectonics.• Samples were taken from India showing that the country had previously been in the

Southern Hemisphere as predicted by Alfred Wegener.• Problems with Theory:• Wegener had problems convincing others how the continents might move • A flaw in Wegener’s original data caused h m to make some incorrect and irrational decisions

Supercontinent Theory

• The supercontinent cycle is a geologic cycle where the Earth's continents alternatively merge into a single supercontinent, then split apart into numerous continents, then merge again. This cycle is estimated to be 300 - 500 million years long.

• Pangaea is when all of the continents were together, then they separated. • Panthalassa is when all the oceans were together and all connected before the continents

were apart.• Accretion is the process where something grows through the gradual addition of smaller

items. In geology for example, when a tectonic plate becomes larger because material is added from a convergent boundary.

• Rifting is the process that pulls away continents. • Plate tectonics affect climate by dividing the regions. When they were all together, they were

the same climate. When they diverged they were put into different regions with varied climate adapting to their region. -Rift Valley

Theory of Plate Tectonics • A lithosphere is the solid, rocky layer covering the entire surface of the planet,

composed of the crust and the hard uppermost mantle, and reacts to stresses as a brittle solid.

• The asthenosphere is part of earths crust, and can be found below the lithosphere. It is considered the upper mantle. It is considered putty because the temperature is 1600 degrees Celsius.

• Plates move because of the intense heat in the Earth’s core that causes molten rock in the mantle layer to move. It moves in a pattern called a convection cell that forms when warm material rises, cools, and eventually sink down. As the cooled material sinks down, it is warmed and rises again.

• Divergent, convergent, and transform are the 3 types of plate boundaries.• Isostasy refers to the flotation of the Earth's tectonic plate. It is the gravitational

equilibrium created by the surface of the Earth exerting pressure on the Earth's mantle.• The crust of the Earth is buoyant on the mantle because it is less dense. When the

volume of the crust changes due to mountain building or erosion, it has to sink or rebound accordingly. That is the isostatic adjustments.

• When two tectonic plates collide, it is called compressional stress at convergent boundaries.

Sea-Floor Spreading • Mid-Ocean Ridges: Term for an underwater mountain system that consists of

various mountain ,typically having a valley known as a rift running along its spine, formed by plate tectonics.

Examples : The Gakkel Ridge (also known as Nansen Ridge) is a mid-oceanic ridge located in the Arctic Ocean between Greenland and Siberia with a length of about 1800 kilometers.

The Mid-Atlantic Ridge, which splits nearly the entire Atlantic Ocean north to south, is probably the best-known and most-studied example of a divergent-plate boundary.

Paleomagnetism• Magnetic reversal: “flip,” means the North pole is transformed into a

South pole and the South pole becomes a North pole.• Magnetic Symmetry: when the sea floor spreads the magnetic orientation

in the rocks as they cooled is preserved. As the earth's magnetic field changes then a distinct pattern is imprinted in the rocks. If sea floor spreading is true then this unique pattern should be the same on both sides from the spreading point. Measurements of sea bottom rocks verify this symmetry is true. Therefore there is magnetic symmetry.

Normal Faults• Crystal Springs Reservoir fills the linear valley of the San

Andreas Fault in San Mateo County.

• A small scarp deforms the fairway of this golf course along the Hayward Fault in Contra Costa County

Volcanoes• 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 the magma chamber below the surface.

• Magma is a mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth.

• 3 conditions in which magma forms: temperature, composition , and pressure.

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling.

Volcanoes• 1. Mount St. Helens – Washington State• 2. Mount Vesuvius - Italy• 3. Mt. Fuji - Japan. Currently Dormant• 4. Mt. Kilauea & Mauna Loa - Hawaii• 5. Mount Kilimanjaro - Tanzania. Currently dormant.• 6. Popocatepetl - Mexico• 7. Mount Etna - Greece• 8. Krakatoa - Indonesia• 9. Mount Pelee - Martinique• 10. Alaska Volcanoes - Shishaldin Volcano, Mount Redoubt, Mount Augustine,

• The Ring of Fire, coinciding with the edges of one of the world's main tectonic plates, contains over 450 volcanoes and is home to approximately 75% of the world's active volcanoes.

Volcanoes• Pyroclastic material is another name for a cloud of ash, lava fragments

carried through the air, and vapor.• Viscosity is a measure of the resistance of a fluid which is being deformed

by either shear stress or tensile stress.• An eruption is driven by gas accumulating under great pressure. By hot

rising magma, it interacts with ground water until the pressure builds to the point until it bursts violently through the mantle of the rock. In many cases the magma will contain dissolved gas which can explode.

Types of Volcanoes

• Composite Volcanoes • Shield Volcanoes• Cinder Cones • Spatter Cones• Complex Volcanoes

Types of volcanoes

• Shield volcanoes are huge in size. They are built by many layers of lava flows. Lava spills out of a central , or group of vents. A broad shaped, gently sloping cone is formed. This is caused by the fluid, lava which can't be piled up into steep mounds.

• Mauna Loa volcano

Types of volcanoes

• A cinder cone is a steep cone hill formed above a vent. Cinder cones are most common volcanic landforms found in the world. They aren't famous as their eruptions usually don't cause any death.

Types of volcanoes• Composite volcanoes usually erupt in an explosive way. This is

usually caused by viscous magma. When very viscous magma rises to the surface, it usually clogs the craterpipe, and gas in the craterpipe gets locked up. Therefore, the pressure will increase resulting in an explosive eruption.

• New Zealand

Types of Volcanoes

• Caldera Volcanoes form when so much lava is erupted so rapidly it partially empties the underlying magma chamber. When this happens the summit of the volcanic structure collapses into the emptied magma chamber.

Subduction Zones• Continental + Oceanic Crust: • The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which

forms the continents and the areas close to their shores, known as continental shelves. Continental crust makes up about 70% of the volume of Earth's crust.

• Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium.

• When two continental plates collide, they sometimes do not subduct beneath each other but instead collide "head on" into each other. Mountain ranges are formed when the continental crust "folds up", which are also called fold mountains. That is how landforms are produced.

This picture was taken at Crater Lake in 1941.

Subduction zones cont.• Oceanic + Oceanic crusts: • Oceanic Crusts -the relatively thin part of the earth’s crust which underlies the

ocean basins. It is geologically young compared with the continental crust and consists of basaltic rock overlain by sediments.

• Reyes Penof a bay mouth bar landform that acts as a barrier between the lagoon and the Pacific Ocean.

• Oceanic trench: Tonga Trench Pacific Ocean 10,882 m (35,702 ft)

Collision Zones• The continental crust is the layer of granitic, sedimentary and metamorphic rocks

which form the continents and the areas close to their shores, known as continental shelves.

• Flow of material under compression in the weak layers of continental crust can cause post-rift uplift of passive continental margins.

• Landforms Produced: Fold mountains are mountains formed mainly by the effects of folding on layers

within the upper part of the Earth's crust. Ex: The Jura mountains - A series of sub-parallel mountainous ridges formed by

folding over a Triassic period due to thrust movements in the foreland of the Alps• Plateaus in the canyon lands national park.

Transformed Boundary• The Strike – Slip fault surface is usually near vertical and the footwall moves either left or

right or laterally with very little vertical motion.• Reverse faults occur in areas undergoing compression.• Tension is a pulling force.• Shear stress is the stress component parallel to a given surface, such as a fault plane, that

results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock.

• Fault-block mountains are formed by the movement of large crustal blocks when forces in the Earth’s crust pull it apart. Some parts of the Earth are pushed upward and others collapse down.

• Sierra Nevada mountains Graben:

Part Two

• Chapter 12

Earthquakes• Elastic rebound: As rocks on opposite sides of a fault are subjected to force and

shift, they gain energy and slowly deform until their inner strength has exceeded. When is has exceeded a movement along the fault occurs, releasing the energy, and the rocks snap back to their original untouched shape.

• - - this shows the difference from the fence in the stages of an elastic rebound.

• • -this is a picture taken in Maric

Marin county which proves the earthqua the earthquake set the fence 8. 8.5 inches off.

Seismology• Seismology -is the scientific study of earthquakes.• Seismometer-Records seismic waves allow seismologists to map the interior of the

Earth, and locate and measure the size of these different sources.• Seismograph- records ground motion.

Earthquakes• Focus: epicenter of an earthquake

• Epicenter: point right above the focus, the point where an earthquake or underground explosion originates.

Seismic Waves• Body Waves: Body waves travel through the interior of the Earth. They create a

geometric path between the transmitting and receiving locations, refracted by the density and stiffness of the Earth's interior. The density and stiffness, in turn, vary according to temperature, composition, and phase. This effect is similar to the refraction of light waves.

S & P Waves :• S Wave: secondary wave, or shear wave .• It shears the rock sideways at right angles to the direction of travel. If a

liquid is sheered sideways or twisted, it will not spring back. S waves can not join in liquid parts of the earth, oceans and lakes.

• P Waves: motion is the same as a sound wave as it spreads out , it pushes and pulls the rock. The P waves can travel through both solid and liquid material.

Surface waves

• Or ground waves, of mechanical or electromagnetic nature.

• A seismic wave that is trapped near the surface of the earth.

How do seismic waves lead to determine earth’s interior?

• Three different types of waves travel away from the focus of a seismic event. These waves travel at different speeds. In addition to surface waves, the waves that are proving critical to providing evidence about the interior of the Earth are 'P' waves, and 'S' waves P-waves are faster than S-waves.

• P-waves can travel through solid rock and fluids, and their speed will vary depending on the density of the rock through which they travel; increased speed will indicate increased density, thus providing information about the density and possible mineralogy of Earth's various layers.

• S-waves cannot travel through fluids, thus when an S-wave encounters the outer core of the Earth (liquid iron and nickel), it is disrupted, indicating the boundary between the mantle and outer core and the boundary between the inner core and outer core. The effect observed from the motion of S-waves reveals a “shadow zone”, opposite to where they originate.

Earthquakes not resulting from movement along plate boundaries.

• Distribution plots reveal that many earthquakes are associated with volcanic action and ocean trenches, and occur over subduction zones and over pacific belt.

Magnitude

• A Richter magnitude scale is one of a number of ways that have been developed to assign a single number to quantify the energy contained in an earthquake.

Intensity

• Intensity describes the severity of an earthquake and affects of earth’s surface• The Mercalli intensity scale is a seismic scale used for measuring the intensity of an

earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude

Tsunami

• A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake.

• Tōhoku earth2011 quake and tsunami.

Related Landforms• Dome mountains- when magma builds up but doesn’t push through and forms a dome

mountain.

• South Dakota• Hot Spots- spots from volcanic activity