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Plate Tectonics
Tectonic plates move atop the molten magma of the Earth's mantle, interacting with each other in three primary ways. When plates move apart, they create a divergent boundary. Divergent boundaries are responsible for creating new crust. When plates move laterally across one another, they create transform boundaries. These boundaries neither create nor destroy crust. Finally, when plates collide together, they form a convergent boundary. A special type of convergent boundary, called a subduction zone, is responsible for destroying crust.
Crustal Destruction
When plates collide along a convergent zone, the denser plate is forced under the more buoyant plate. This process is known as subduction. This occurs when either oceanic plates collide or an oceanic plate collides with a continental plate. It should be noted that this does not occur along convergent zones involving two continental plates, as both are equally buoyant. As the subducting plate plunges beneath the other plate, it encounters the Earth's asthenosphere. The heat and pressure of this layer causes the plate to slowly melt, destroying the crust.
Crustal Creation
When plates move apart along divergent zones, underlying magma is able to flow into the cracks created by the growing rift. The magma quickly cools as it is exposed to the cold oceanic waters, forming new crust. This process typically occurs between oceanic plates, forming massive mid-ocean ridges. However, the process can also involve continental plates, forming rift valleys. While volcanic activity along subduction zones does contribute to the creation of new crust, the vast majority of new crust is created along the planet's network of mid-ocean ridges.
Conservation of Crust
If volcanic activity around the planet creates new crust and the Earth consists of a finite amount of space, then something must give. Conservation of crust must be maintained, and this means an equal amount of crust must be created and destroyed. Divergent boundaries and subduction zones are the mechanisms by which Earth achieves this balance. New crust is created along mid-ocean ridges, slowly travels along the ocean floor as if on a conveyor belt and is then destroyed in subduction zones. The building materials are recycled, ultimately to become crust again in the future.
Examples of Creation and Destruction
The East Pacific Rise is the fastest spreading mid-ocean ridge on the planet. Off the coast of South America, the Pacific and Nazca plates are moving apart at a rate as high as 6 inches per year, creating new crust. This crust is being recycled along a number of subduction zones. At the Mariana Trench, the Pacific plate is subducting under the Philippine plate at a rate of around 1.5 inches per year. At the Peru-Chili Trench, the Nazca plate is subducting on the South American plate at a rate of around 3 inches per year.