Along the boundary of the Earth's tectonic plates a fiery drama of catastrophic proportions is played out. Movement of the Earth's crust creates earthquake activity and in some places great outpourings of lava extrude onto the surface.
Four types of plate boundaries are recognized and are depicted in Figure 15.17.A spreading or divergent boundary occurs where plates are moving away (diverging) from one another, like that occuring along the midocean ridge where new crust is formed . Convergent boundaries are located where subduction is active and the lithosphere is being "consumed". This occurs where two plates collide with one another. Transform boundaries occur where plates are grinding past one another, like the San Andreas fault. Plate boundary zones are broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. Understanding plate motions (USGS) is important in deciphering land forming processes that occur along plate boundaries.
Figure 15.17 Tectonic Settings