At a depth of 100 kilometers or more the subductedoceanic sediments, together with part of the upper basaltic crust,melt to an andesitic magma, which rises to the surface and gives birthto a line of volcanoes a few hundred kilometers behind the subductingboundary. This boundary is usually marked by anoceanic deep, or trench, where the overriding plate scrapes off theupper crust of the lower plate to create a zone of highly deformed,largely sedimentary rock. If both plates are oceanic, the deformedsediments and volcanoes form two island arcs parallel to the trench. If one plate is continental, the sediments are usually accretedagainst the continental margin and the volcanoes form inland, as theydo in Mexico or western South America. Along the third type of plate boundary, two plates movelaterally and pass each other without creating or destroying crust.
Large earthquakes are common along such strike-slip, or transform,boundaries.
Also known as fracture zones, these plate boundaries areperhaps best exemplified by the San Andreas fault in California andthe North Anatolian fault system in Turkey. Most of the seismic and volcanic activity on Earth is thereforeconcentrated along plate boundaries where mid-ocean ridges, trencheswith island arcs, and mountain ranges are generated. Some seismic andvolcanic activity also occurs within plates. Interesting examples ofthis interplate activity are linear volcanic chains in ocean basins,such as the Hawaiian Islands and their westward continuation as astring of reefs and submerged seamounts. An active volcano usuallyexists at one end of an island chain of this type, with progressivelyolder extinct volcanoes occurring along the rest of the chain.