Plate Tectonics

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.

• 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.

• 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.

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.

• 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.

• 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.

• 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.

• 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.

Homework

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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.

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.

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.

7 Major Lithospheric Plates

• Pacific Plate- covers 1/5 of the Earth’s surface.

• North American

• South American

• Eurasian

• African

• Indo-Australian

• Antarctic

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.

• 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.

• 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.

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.

• 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.

Homework

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