Ch. 4.2 The Theory of Ch. 4.2 The Theory of Plate TectonicsPlate Tectonics

The theory of The theory of plate tectonicsplate tectonics explains the explains the what, howwhat, how and and whywhy of continental of continental movement.movement.

Two types of earth’s crust: Two types of earth’s crust: oceanic crust oceanic crust (makes up the ocean floor), and (makes up the ocean floor), and continental crustcontinental crust (makes up the (makes up the continental landmasses).continental landmasses).

LithosphereLithosphere—the oceanic and continental —the oceanic and continental crust and the rigid upper mantle.crust and the rigid upper mantle.

AsthenosphereAsthenosphere—a layer of slowly flowing, —a layer of slowly flowing, deformable rock under the lithosphere.deformable rock under the lithosphere.

Lithosphere is broken into separate plates Lithosphere is broken into separate plates that “float” on the denser asthenosphere.that “float” on the denser asthenosphere.

The continents and oceans are carried The continents and oceans are carried along on the moving lithospheric plates.along on the moving lithospheric plates.

There are about 30 identified There are about 30 identified lithospheric plates, most consisting lithospheric plates, most consisting

of both types of crust.of both types of crust.

Plates moving in relation to each other Plates moving in relation to each other have created earth’s major surface have created earth’s major surface features.features.

Plates can either move towards each Plates can either move towards each other, away from each other (mid ocean other, away from each other (mid ocean ridges), or they can slide past each other.ridges), or they can slide past each other.

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

Divergent BoundariesDivergent Boundaries

When two plates move away from each When two plates move away from each other.other.

Melted asthenosphere rises to fill the void.Melted asthenosphere rises to fill the void.

It forms mid-ocean ridges, with a narrow It forms mid-ocean ridges, with a narrow rift valleyrift valley in the center. in the center.

Rift valleys are wide where continents are Rift valleys are wide where continents are separating (Example: the Red Sea).separating (Example: the Red Sea).

Convergent BoundariesConvergent Boundaries

When one plate collides with another.When one plate collides with another.

Three possible types of collisions, Three possible types of collisions, depending on if the plate has oceanic depending on if the plate has oceanic crust or continental crust as its leading crust or continental crust as its leading edge.edge.

When oceanic crust collides with When oceanic crust collides with continental crust, the denser oceanic crust continental crust, the denser oceanic crust is is subducted subducted (forced under) the less dense (forced under) the less dense continental crust.continental crust.

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

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

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

Transform BoundariesTransform Boundaries

Where two plates are grinding past each Where two plates are grinding past each other.other.

Not smooth movement. Series of sudden Not smooth movement. Series of sudden movements follow by long periods of movements follow by long periods of inactivity (Ex. San Andreas Fault is on a inactivity (Ex. San Andreas Fault is on a major transform boundary)major transform boundary)

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Causes of Plate MotionCauses of Plate Motion

ConvectionConvection—the transfer of heat through —the transfer of heat through the movement of heated material.the movement of heated material.

Convection CurrentConvection Current—the cycle of warm —the cycle of warm material rising and cool material sinking.material rising and cool material sinking.

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