1. Unit 8 C: Plate Tectonics
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2. LAYERS OF THE EARTH Crust (brown/tan) Mantle (light & dark orange)
Core (yellow & white)

3. Beneath the lithosphere is the ASTHENOSPHERE
LAYERS OF THE EARTH
Lithosphere {
Lithosphere: Crust and upper mantle make up the LITHOSPHERE
rigid plates (OCEANIC & CONTINENTAL plates)
low temperature & pressure
Beneath the lithosphere is the ASTHENOSPHERE
plastic” mantle (flows, moves the plates)
higher temperature & pressure

4. Add Plate Tectonics to your index
The theory of plate tectonics states that Earth’s crust is broken into rigid plates that move slowly over Earth’s surface.
The rigid plates are called tectonic plates.
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Add Plate Tectonics to your index

5. The movement of one plate is described as either moving away from or toward another plate, or sliding past another plate.
Plates move at speeds of only a few centimeters per year.
At this rate, it takes moving plates millions of years to make new continents, new mountain ranges, or other landforms.

7. Long before geologists proposed the theory of plate tectonics, they discovered evidence of continental movement.
Photo by G.K. Gilbert, U.S. Geological Survey

8. In 1912, Alfred Wegener developed the hypothesis that all the continents were
once joined together and
have since drifted apart, called continental drift.

9. Types of Evidence
Wegener used different types evidence to back up his hypothesis:
Geological evidence
Fossil evidence 7

10. Geological Evidence
One piece of evidence is the shape of Earth’s continents.
The outlines of South America and Africa match up like pieces of a puzzle.
Deep scratches in rock showed that continental glaciers once covered South America.
Rocks that are made of similar substances and mountains that formed at similar times are present on continents that are now far apart.

11. Fossil Evidence
Geologists have discovered the same types of fossils on continents that are now separated by vast oceans.
Glossopteris, a fernlike plant was found in Africa, South America, Australia, India and Antarctica
Mesosaurus, a fresh water reptile, have been found in places separated by oceans

13. Wegener’s hypothesis of continental drift was not accepted for more than 50 years after it was proposed.
The main reason for this was because he could not explain how the continents could move.

15. Convection in the Mantle
Convection is the circulation of particles within a material caused by differences in thermal energy and density
Convection affects the mantle underneath tectonic plates.
Hotter mantle rises toward Earth’s surface and cooler mantle sinks deeper into the mantle.
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16. As the Earth’s mantle moves, it pushes and pulls tectonic plates over Earth’s surface.

17. PLATE BOUNDARIES Divergent boundaries  plates move apart
Convergent boundaries  plates move together
Transform boundaries plates slide past  

18. Divergent Boundaries
When plates move apart at divergent boundaries, a rift forms between the two plates.
A rift can form within continents when continental crust moves in opposite directions.

19. Divergent Boundaries
A rift also can form at divergent boundaries on the ocean floor.
As plates separate, molten rock can erupt from the rift.
As the molten rock cools, it forms new crust.

20. Convergent Boundaries
When two plates come together, the denser oceanic plate usually is forced down into the mantle and the less dense continental plate remains on Earth’s surface.
The area where one plate slides under another is called a subduction zone.

21. Convergent Boundaries
When two continents collide at a plate boundary, both continents remain on the surface.
As two continents push together, the crust rises up and large mountains form.

22. Transform Boundaries
Tectonic plates slide past each other at transform boundaries.
The two sides of the boundary move in opposite directions.
This can deform or break features such as fences, railways, or roads that cross the boundary.

23. Forces Changing Earth’s Surface
Forces within Earth cause plates to move.
The three types of plate boundaries experience different types of forces.

24. The squeezing force at a convergent boundary is called compression.
The pulling force at a divergent boundary is called tension.
The side-by-side dragging force at transform boundaries is called shear.

25. Even though plates move slowly, the forces at plate boundaries are strong enough to form huge mountains and powerful earthquakes.
Tensional forces pull the land apart and form rift valleys and mid-ocean ridges.
Compressional forces form mountains.

26. Add Sea-floor Spreading to your index
Harry Hess, an American geologist, was one of the scientists who studied mid-ocean ridges.
In 1960, he suggested the idea he called sea-floor spreading.
Sea-floor spreading: the sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added.
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27. Ocean floors move like conveyor belts.
Sea-floor spreading begins at the mid-ocean ridge, which forms along a crack in the oceanic crust.
The mountains that form as lava from a volcano at a divergent boundary builds up and cools are called mid-ocean ridges.

28. Molten material that forms beneath the surface rises and erupts.
At the same time, older rock moves outward on both sides of the ridge.
As the new molten rock cools, it forms a strip of solid rock in the center of the ridge.
When more molten material flows into the crack, it forms a new strip of rock.

29. At some convergent boundaries, one tectonic plate sinks, melts the mantle, and forms magma that erupts onto the plate that does not sink.