Like the pieces of a giant jigsaw puzzle, slabs of rocky crust known as tectonic plates fit together to form the Earth's outer shell. The puzzle changes as the plates slide over the hotter, softer rocks beneath them. Moving by mere inches annually, they reshape continents and ocean basins over millions of years by colliding, separating, and scraping past one another with relentless force. These interactions set off earthquakes, fire up volcanoes, wrinkle the earth's crust into mountains, valleys, and deep-sea trenches. ...
SUBDUCTION ZONES : When two plates collide, one often dives beneath the other – a process called subduction. Oceanic plates subduct under other oceanic plates as well, forming trenches that mark their descent. Where continental plates run into each other, they buckle, creating ranges such as the Himalaya.
TRANSFORM FAULTS : Sometimes tectonic plates shift past each other horizontally at a boundary known as a transform fault – a distinct type of strike-slip fault. The San Andreas, one example, accommodates movement fitfully along its several hundred miles – slow creep in some places, frequent small jumps or rare big ones in others.
SPREADING CENTERS : As tectonic plates move apart, magma rises in the resulting rift, hardens, and is pulled aside, making way for more magma. Under the oceans this process has created the 46,600-mile-long Mid-Ocean Ridge system, segmented by faults, that snakes around the globe. On land the process has created – and contines to widen – the East African Rift System.
VOLCANOES : Magma rises to the surface from inside the Earth mainly at subduction ones and spreading centers. The edge of the Pacific Basin – the 30,000-mile-long Ring of Fire – is especially volatile, with about 70 percent of the world's more than 500 historically active volcanoes.
HOT SPOTS : In a scattering of places, magma burns through a tectonic plate. Each hot spot likely marks the top of a plume of semimolten rock that rises many hundreds of miles inside the Earth. As a plate slowly passes above, the plume melts into it, creating a chain of volcanoes such as those in the Hawaiian Islands.
EARTHQUAKES : Most quakes occur near plate boundaries, where rock grinds past rock. Friction keeps the plate edges from sliding smoothly. the longer they remain stuck, the more strain builds and the more violent that snap and resulting vibrations – the earthquake.
