.PLATE TECTONICS

Plate Tectonics- Scientists think the earth formed about 4.6 billion

years ago. It was originally a large ball of molten rock. As the earth cooled the heavier, denser materials such as iron, sank towards the center of the earth. Other layers formed around that heavy, dense core.

- Earth has 4 main layersCrust- thin, coolMantle- thick, liquidOuter core- liquid, iron and nickelInner core- solid,iron

.- The crust and the top of the mantle together form the

lithosphere. (litho=rock). - The lithosphere sits on top of the asthenosphere

(astheno= weak) a layer of softer rock in the upper mantle. Imagine the lithosphere as a solid layer of pavement sitting on a hot gooey tar layer.

.- The lithosphere is made up of many plates. These are called tectonic plates. These plates fit together like a jigsaw puzzle. (like the cracked shell of a hard boiled egg).

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Continental Drift- How do we know the continents move?-In 1912, a scientist named Alfred Wegener, proposed the hypothesis of continental drift. He said that all the continents were once joined as one big continent he called pangaea. He said that they have moved over millions of years to their present locations.- Wegener had evidence for his theory in 4 ways:1. Fossils- fossils of an ancient reptile called Mesosaurus had been found in Africa and South America.

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Mesosaurus

.2. Climate- Places like Greenland and Antarctica, which are cold today, have fossils and evidence of a warm climate in the past. So….they must once have been in a warmer place.3. The “fit”- some continents seem to fit together perfectly. South America and Africa

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4. Geology- The kinds of rock in widely seperated continents are identical. Rocks in Maine match rocks in Europeand Africa

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So how do we think the continents moved?The original Supercontinent- Pangaea (means one-earth)Seperated about 200 million years ago into 2 continents…

Laurasia (north)Gondwanaland (south)

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laurasia- North America, Europe, AsiaGondwanaland- Africa, Australia, Antarctica, India

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PLATE TECTONICS 2

Plate Tectonics Part II

How do Earth’s Plates Move?Plate Boundaries- a plate boundary is where the edges of 2 plates meet.

a. Divergent boundary: where plates move apart

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a. Divergent boundary: where plates move apartexample Mid-Ocean Ridge

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Divergent Boundary

At the Mid Ocean Ridge, the plates are spreading apart and new molten rock is coming up from in between the plates (sea-floor spreading)

.b. Convergent boundary- where plates push together

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.b. Convergent boundary- where plates push together

Subduction Zone- one plate moves under the other

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.b. Convergent boundary- where plates push together

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c. Transform boundary- where plates scrape against each other. Move sideways to each other

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c. Transform boundary- where plates scrape against each other. Move sideways to each other

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c. Transform boundary- where plates scrape against each other. Move sideways to each other

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c. Transform boundary- where plates scrape against each other. Move sideways to each other

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Divergent BoundariesIn the ocean, divergent boundaries are where the ocean bottom is spreading apart. A mid-ocean ridge forms along the rift valley. Here, molten rock rises to build new crust. Video: sea floor spreading

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Mid-ocean ridges are the longest chains of mountains in the world.

The Mid-Atlantic ridge is 6000 miles long and it’s rift valley is 6 miles deep. That’s 4 miles deeper than the Grand Canyon!!

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-video: underwater lava

Magnetic Reversal- the sea floor is younger near a mid-ocean ridge because that’s where new lava is coming out. - Scientists have discovered that they can look at these

new rocks to see what the magnetic field of the earth looked like in the past. They have discovered that the magnetic field has reversed many times in the earth’s history!!

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Continents also have divergent boundaries where they spread apart. This happens as magma rises from deep in the mantle and bulges up the crust. As the crust stretches it cracks and forms a rift valley. Many earthquakes and volcanoes occur in these areas.

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Hot Spots- in some places, called hot spots, heated magma rises from the mantle. Volcanoes often form in these spots. - Hot spots stay in the same place, but the plates are

moving. The most famous examples of hot spots are the islands of Hawaii and Yellowstone National Park.

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I I

J

H

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• E- Convection Currents in the mantle cause the lithosphere plates to move. Hot rock rises and cools and then sinks

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H- Hot rock rising in mantle (less dense)

J- Cool rock sinking back into mantle (more dense)

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F- New molten rock coming to surface making new crust. Forms : G- Mid Ocean Ridge

C- Rift Valley

rift valley

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D- Subduction Zone: Ocean plate goes under the continent plate. The rock melts as it sinks into the mantle The ocean plate PUSHES UP the continent to make mountains (India/Himalayas)

I- Melting rock rises to surface to make volcanoes

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B Trench- a deep “canyon” where ocean crust goes under the continent

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A Volcanoes form from melted rock when it reaches the surface

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The “Whole Thing” of Plate Tectonics: Driven by convection currents in the mantle