Chapter 3 Plate Tectonics
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Transcript of Chapter 3 Plate Tectonics
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Plate Tectonics
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Earth’s Drifting Continents
• Theory of Continental drift- the theory, proposed by Alfred Wegener, that the continents were once joined together and have since drifted apart.
– Alfred Wegener concept in 1912.
– He thought some of the continents coasts look like they could have fit together at one time.
– Discredited by many scientists until the 1950’s.
– Went against many geological basic principles.
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• Pangaea- means all Earth; is the name of the super continent from millions of years ago.
– Fossil records showed that the same kind of animals and plant remains were separated by the Atlantic Ocean.
– Evidence from Rocks- when you line South America and Africa up together there is rock formations that line up and are the same age as each other.
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• Rock deposits left from glaciers.
– Many of the same rock deposits have been found in South America, Africa, India, Australia, and Antarctica.
– Ancient glacial deposits have been found in areas with very warm climates.
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Earth’s Spreading Floor
• During the 1950’s, new mapping techniques discovered midocean ridges.
– Midocean Ridges- an undersea mountain chain where new ocean floor is produced.
• This gave the final piece of evidence for the Theory of Continental Drift to be expected.
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• Midocean Ridges form the worlds longest mountain chain (80,000 km).
– Great deal of volcanic activity occurs in this area.
– When the ocean floor moves apart, lava wells up and hardens causing new oceanic crust.
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• Ocean-floor spreading- the process in which old ocean floor is pushed away from a midocean ridge by the formation of new ocean floor.– Helped explain Continental Drift.
– When the ocean floor moves, it takes the continent with it.
• Transform fault- a fault that runs across a midocean ridge.
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• New deep-sea drilling also provides evidence to support the idea of ocean-floor spreading.
• Rock samples from the ocean floor indicate that rocks next to a midocean ridge are younger than rocks farther away.
• Magnetic stripes in ocean-floor rocks further convinced scientists of ocean-floor spreading.
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• Trenches- deep V-shaped valley that lie along the bottom of the ocean.
– They are found near some continents or near strings of islands.
• Subduction- process in which crust plunges back into the Earth.
– The older oceanic crust is pushed under the continental crust.
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Homework
• Page 59 1-3
• Page 64 1-3
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Earth’s Moving Plates
• Theory of Plate Tectonics- links together the ideas of continental drift and ocean-floor spreading, explains how the Earth has evolved over time. It helps to explain the formation, movements, collisions, and destruction of the Earth’s crust.
• Provides a framework for understanding mountains, volcanoes, earthquakes, and other landforms and processes of the physical Earth.
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Plate Tectonics
• Gives scientists insight into how and why life on earth has evolved.
• Helps people to understand the past and to predict the future.
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Lithospheric Plates
• Lithosphere- the topmost solid part of the Earth.
– Made up of a number of plates.
– Plates contain a thin layer of crust above a thick layer of relatively cool, rigid mantle rock.
– Plates usually contain both oceanic and continental crust.
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7 Major Lithospheric Plates
• Pacific Plate- covers 1/5 of the Earth’s surface.
• North American
• South American
• Eurasian
• African
• Indo-Australian
• Antarctic
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Plate Boundaries
Three Main Types of Boundaries
• Divergent Boundaries- a plate boundary at which plates move apart.
– This type occurs at the midocean ridges.
– Also called constructive boundaries.
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• Convergent Boundaries- a plate boundary at which plates come together.
– This plate boundary has trenches.
– They are also called destructive boundaries.
– Cause tremendous pressure and friction.
– Causes severe earthquakes.
– When the plate material melts in the Earth’s mantle, some of it surges upward to produce volcanoes.
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• Strike-Slip Boundaries- two plates grind together and slip past each other horizontally.
– No new plate material is made, and no plate material is destroyed.
– It is also called the conservative boundary.
– Earthquakes often occur along these boundaries.
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Plate Motion
• Convection Currents- a movement of material caused by differences in temperatures.
– convection currents move air in the atmosphere and water in the ocean. And they may move the plates of the lithosphere as well.
– A rising and sinking cycle repeats over and over.
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• Subduction- the process in which crust plunges back into the interior of the Earth.– The denser plate is subducted, and the other plate
edge floats over it.
– When two oceanic plates collide, the older oceanic plate is subducted under the younger plate.
– When two continental plates collide that have the same density, neither plate is subducted, and they push upward to form a mountain.
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Homework
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