The surface of our planet is covered by seven large and numerous smaller tectonic plates. Up to 50 miles thick, these plates are in constant motion, governed by the opening and closing of oceans. Their movement, too slow to observe during our brief life spans, is manifest by earthquakes and volcanic eruptions.
All of the larger plates are composed of both continental and oceanic crust; the latter is more dense and, in areas where continental and oceanic crust collide, the oceanic crust subducts (dips) beneath the continental crust. The line of subduction is called a trench and, as the leading edge of the oceanic crust approaches the Earth's mantle, it melts to form magma, which, in turn, rises to form volcanoes on the overlying continental plate; the Aleutian Chain, the Andes, the Cascades, Japan, the Philippines and many other island chains are examples of this process. The edge of the continental plate is pulled down by friction with the subducting, oceanic plate but rebounds intermittently, producing earthquakes in these subduction zones; tidal waves (as occured in Indonesia) may be generated if ocean water overlies the rebounding edge.
Where two areas of continental crust collide, mountain ranges form; the Himalayas, still rising as the India Plate smashes into the Eurasian Plate, is our best current example; earthquakes are common in these collision zones. In other areas, tectonic plates are scraping along one another; earthquakes, like the one in Haiti this week and the regular quakes along the San Andreas Fault of Southern California, occur when pressure is suddenly released along these transform faults. While most earthquakes occur near the margins of tectonic plates, some occur in the middle of plates, the result of rifting (e.g. the East African Rift), settling along old suture lines, volcanism above mantle plumes (e.g. Yellowstone) or stretching of crust between areas of uplift (e.g. the Great Basin of the U.S.). In reality, few, if any, areas of our planet are immune to earthquakes.
