Plate Tectonics

Continental drift, earthquakes, volcanoes, and mountain building

Earth's outer shell (lithosphere) is divided into about 15 major tectonic plates that float on the semi-molten asthenosphere. These plates move 2-10 cm per year driven by convection currents in the mantle. Convergent boundaries (plates collide): continental-continental creates mountain ranges (Himalayas); oceanic-continental creates subduction zones, ocean trenches, and volcanoes; oceanic-oceanic creates island arcs. Divergent boundaries (plates separate): creates rift valleys (East Africa) on land, mid-ocean ridges under the sea (seafloor spreading). Transform boundaries (plates slide past each other): creates strike-slip faults and earthquakes (San Andreas Fault). About 250 million years ago, all continents were joined in one supercontinent called Pangaea, which has since broken apart.