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Ch. 4.2 The Theory of Plate Tectonics

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1 Ch. 4.2 The Theory of Plate Tectonics

2 The theory of plate tectonics explains the what, how and why of continental movement.
Two types of earth’s crust: oceanic crust (makes up the ocean floor), and continental crust (makes up the continental landmasses).


4 Lithosphere—the oceanic and continental crust and the rigid upper mantle.
Asthenosphere—a layer of slowly flowing, deformable rock under the lithosphere. Lithosphere is broken into separate plates that “float” on the denser asthenosphere. The continents and oceans are carried along on the moving lithospheric plates.


6 There are about 30 identified lithospheric plates, most consisting of both types of crust.

7 Plates moving in relation to each other have created earth’s major surface features.
Plates can either move towards each other, away from each other (mid ocean ridges), or they can slide past each other.


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10 Lithospheric plate boundaries don’t necessarily correspond to the outlines of continents. They may extend out into oceans, or lie within continents.

11 Divergent Boundaries When two plates move away from each other.
Melted asthenosphere rises to fill the void. It forms mid-ocean ridges, with a narrow rift valley in the center. Rift valleys are wide where continents are separating (Example: the Red Sea).

12 Convergent Boundaries
When one plate collides with another. Three possible types of collisions, depending on if the plate has oceanic crust or continental crust as its leading edge. When oceanic crust collides with continental crust, the denser oceanic crust is subducted (forced under) the less dense continental crust.

13 This area is called a subduction zone, and results in deep ocean trenches and volcanic mountains.

14 When two plates of continental crust come together, neither is subducted. Instead, the leading edges are crumpled and uplifted, forming high mountains.

15 When two plates of oceanic crust come together, the result is again a subduction zone, with deep ocean trenches and a chain of volcanic islands called an island arc.

16 Transform Boundaries Where two plates are grinding past each other.
Not smooth movement. Series of sudden movements follow by long periods of inactivity (Ex. San Andreas Fault is on a major transform boundary)


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19 Causes of Plate Motion Convection—the transfer of heat through the movement of heated material. Convection Current—the cycle of warm material rising and cool material sinking.


21 Rising asthenosphere material at divergent boundaries forms new lithosphere.
As it moves away from the divergent boundary, it eventually cools and sinks. The downward pull of the sinking plate is believed to be the main driving force for tectonic plate movement.


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