Plate Tectonics: The Restless Earth
The Earth's surface is divided into several major crustal plates, layers of light rock 30 to 100 miles ( 48– 160 km) thick floating on top of the Earth's heavy, iron-rich mantle. Currents that originate in the molten, super-dense core trigger inexorable currents in the mantle above. The currents in the mantle, in turn, push up against the light, brittle rock of the crust. Those forces create cracks in the crust that can run for thousands of miles. Where the mantle current wells up against the underside of the crust it creates a crack through which molten rock oozes from below. When the magma reaches the surface, it builds great chains of volcanoes and mountain ranges like the Mid-Atlantic Ridge, which runs down the middle of the Atlantic Ocean. Here, the extrusion of new molten oceanic crust from below forces two crustal plates apart. At the opposite side of such a crustal plate, those same mantle currents can shove light crustal rock back down into the Earth. Here, a five- to seven-mile-deep undersea trench forms. So the rock of the Earth's oceanic crust is continually created at one edge of a crustal plate and destroyed at the other edge. This system of plate tectonics dominates the surface of the Earth and controls the destiny of all living things.
The even lighter rocks of the Earth's continents “float” on top of the oceanic crust. That means the continents are like rafts, carried along on the conveyor belt of the moving crustal plates in which they are embedded. The collision of continents and crustal plates largely determine the distribution of the world's deserts.