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The Movement of Tectonic Plates
So far we have described the different types of plate material and plate boundaries and the processes that occur at the edge of the tectonic plates. This descriptive summary of plate tectonics is useful, however until we understand the mechanisms that drive the process, we cannot truly understand the entire concept of plate tectonics. The most important mechanisms are those that make the plates move around the surface of the earth. Ironically, in the early days of plate tectonics scientists believed that the continents were mobile and plowed across the oceanic crust. Later it became clear that the continental crust is relatively old and immobile, and it rides passively along with the younger and mobile oceanic crust.
The mantle is a key element in the movement mechanisms of plates. The mantle is a solid that behaves much like a liquid over the long term. The plates appear to move in response to movement of the upper mantle. There are thermal gradients and hotspots throughout the mantle, which result in thermal convection and the upward movement of basaltic magma. The magma rises to the surface at fissures that run along the ocean floors, called midocean ridges, and is extruded onto the surface to form oceanic crust. Some scientists believe that the emplacement of magma at the midocean ridges creates a "pushing" effect that moves the oceanic plates away from the midocean ridges. Other scientists believe that another process occurring at the other end of the oceanic plate, in the subduction zones, is also responsible for plate movement. The temperature and pressure increases with depth in the mantle. Therefore, when an oceanic plate is subducted into the mantle the increased pressure with depth causes the basaltic oceanic crust to undergo a phase transformation, resulting in a denser rock. This creates a "pulling" effect on the oceanic crust.
Seafloor spreading at the midocean ridges is the result of rising plumes of hot basaltic magma. The magma is extruded at the midocean ridge to create new oceanic crust, and additionally the heated magma spreads laterally away from the ridge and drags the plate along with it. Thus, seafloor spreading involves both the pushing effect of the magma being extruded at the ridges and the dragging effect of the magma moving away from the ridge. This process involves thermal convection, similar to what happens in a pot of boiling water, in which heat rises at the midocean ridges and moves downward at subduction zones. It is apparent that the convection does not involve large scale material convection because the mantle is not homogeneous vertically or horizontally.
At subduction zones realtively cold and dense oceanic crust is subducted under the lower density continental crust. As the oceanic crust is pushed down into the mantle it is subjected to increased temperature and pressure and undergoes a phase transformation, making it denser than normal oceanic crust. This density difference creates a "pulling" effect on the oceanic crust.
Plate movement mechanisms are a key to understanding plate tectonics. Click the link below to test your knowledge of what you have learned about plate movment mechanisms.
