EPT - Plate Tectonic Theory Lesson

 

Plate Tectonic Theory

Using the information provided by Wegener, Hess, and other geologists, scientists have a strong theory to explain the motion of the crust.

The surface of the Earth is broken into pieces, similar to an orange that has been peeled and then had the pieces of peel reattached. These pieces are known as tectonic plates. The motion of the sections of crust across the Earth's surface is affected by several different forces. The first is the pressure caused by new sea floor that forms at the mid ocean ridges; as this magma cools and hardens, it expands, pushing the old crust outward from the rift. The second force that causes the plates to move are the convection currents that are occurring in the upper mantle (the asthenosphere). The lower asthenosphere is heated by the core, expands and becomes less dense, and then rises to the upper asthenosphere. At the same time, material in the upper asthenosphere is cooling off and becoming denser, causing it to sink. These circulating convection currents act like conveyor belts below the tectonic plates, pushing the plates in various directions.

Without destruction of crust, the Earth would continue to grow as new crust is formed at the mid-ocean ridges. This crustal destruction occurs at subduction zones, usually where oceanic crust meets continental crust. The denser oceanic crust sinks below the less dense continental crust, plunges to the mantle and is recycled. This keeps the amount of crust on the surface constant - as new crust is being formed, old crust is being destroyed.

Since the continental crust is made of the least dense minerals, the continents never subduct. As a result, the rocks found on continents have proven to be some of the oldest discovered on Earth; there are rocks in Jack's Hill, Australia that contain minerals dating back 4.4 billion years. Their low density, however, doesn't protect them from collisions with other continents, and many of the mountain ranges that exist today are remnants of previous or ongoing collisions between continents.

 

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