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More Plate Tectonics

Continental lithosphere is less dense and thicker than oceanic lithosphere. When a continental plate meets an oceanic plate, the oceanic plate bends and is forced under the continental plate. Subduction is the process of one plate being forced under another plate.

When heat along a subduction zone partially melts rock, magma forms and rises to the surface. It travels into a volcanic arc that runs along side the subduction zone. A deep sea trench also runs parallel to a subduction zone. The Andes mountain range in South America is a subduction zone.

Convergent plate boundaries also exist between two slabs of oceanic lithosphere. The colder, denser plate sub ducts. Magma that erupts there creates chains of volcanic islands called island arcs. Japan is an example of an ocean-ocean convergent boundary.

Along some convergent plate boundaries, two continental slabs of low density collide but do not subduct. Since both are low in density, they both buckle upward to form a high range of folded mountains. The Himalaya of Asia are an example.


Plate Movement

Research shows that plates are driven by a combination of forces. One force is ridge push at the mid-ocean ridge system. Because divergent boundaries are higher at the center of the ridge, gravity forces material down the slopes of the mid-ocean ridge system.

Slab pull is a process that occurs when a plate subducts back into Earth at some convergent boundaries. You probably have experienced an analogy to slab pull. When you wake up and your bed covers are on the floor, something like slab pull has occurred. Here’s what happens. While you tossed and turned at night, your covers began to move off the bed. Eventually, enough of the covers were over the side that gravity pulled the rest of them to the floor. Subducting plates act in much the same way. Portions of descending plates pull the rest of the plate down with them.

Internal convection of mantle material is the driving force for all plate motion. The main source for the heat in Earth’s mantle is the decay of radioactive elements.