The plate tectonics theory is a modern theory of continental drift that has revolutionized geologists’ understanding of earth history. It holds that the earth’s crust is divided into contiguous, moving plates that carry the embedded continents. Lines of earthquake and volcanic activity mark plate boundaries. One kind of boundary is at the mid-ocean ridges, where tensional forces open rifts, allowing new crustal material to well up from the earth’s mantle and become welded to the trailing edges of the plates. When a continent straddles such a rift it is split apart, forming a new ocean area (e.g., The Red Sea and the Gulf of California). The ocean trenches mark subduction zones, where plate edges dive steeply into the mantle and are reabsorbed. A third boundary type occurs where two plates slide past each other in a shearing manner along great transform faults (e.g., The San Andreas Fault in California). Mountain ranges form where two plates carrying continents collide (e.g., The Himalayas), or where ocean crust is subducted along a continental margin (e.g., The Andes). Geologists believe that bc.200 million years ago there was a supercontinent, Pangaea, which subsequent plate movements have split and resplit into the continents and islands we recognize today. Radar mapping of Venus has revealed some apparent effects of subduction on that planet’s surface.