Environmental Problems, Their Causes, and Sustainability 1

usually rides up over the denser oceanic plate andpushes it down into the mantle in a process calledsubduction. The area where this collision and subductiontakes place is called a subduction zone. Overtime the subducted plate melts and rises back to theearth’s surface as molten rock or magma. A trenchordinarily forms at the boundary between the twoconverging plates. Stresses in the plate undergoingsubduction cause earthquakes at convergent plateboundaries.The third type of boundary is a transform fault,which occurs where plates slide and grind past one anotheralong a fracture (fault) in the lithosphere (Figure16-5, bottom). Most transform faults are on theocean floor but a few are found on land. An exampleis the North American Plate and the Pacific Plate thatslide and rub past each other along California’s SanAndreas Fault. As a result, southern California isslowly moving along this transform fault towardnorthern California. Sometime perhaps 30 millionyears from now the geographical area we now call LosAngeles will probably slowly grind and slide by thegeographical area now known as San Francisco. But bythen neither of these areas will exist as we know themtoday. This is fascinating stuff from a geological perspectiveof the earth’s history but is not a problem thatwe need worry about.What Geologic Processes Occuron the Earth’s Surface? Erosion andWeatheringWater and wind move large amounts of soil andbroken-down pieces of rock from one place toanother.Geologic changes based directly or indirectly on energyfrom the sun and on gravity (rather than on heatin the earth’s interior) are called external processes. Internalprocesses generally build up the earth’s surface.In contrast, external processes tend to wear it downand produce a variety of landforms and environmentsformed by the buildup of eroded sediment.A major external process is erosion: the process bywhich material is dissolved, loosened, or worn awayfrom one part of the earth’s surface and deposited elsewhere.Flowing streams cause most erosion. WindEURASIAN PLATEMid-IndianOceanRidgeCHINASUBPLATEINDIAN-AUSTRALIAN PLATEPHILIPPINEPLATETransformfaultJUAN DEFUCA PLATEPACIFICPLATETransformfaultCOCOSPLATEEast PacificRiseNORTHAMERICANPLATENAZCAPLATECARIBBEANPLATEMid-AtlanticOceanRidgeSOUTHAMERICANPLATEReykjanesRidgeANATOLIAN PLATEARABIANPLATEAFRICANPLATESOMALIANSUBPLATEEURASIANPLATECarlsbergRidgeSoutheast IndianOcean RidgeTransformfaultSouthwest IndianOcean RidgeANTARCTIC PLATEConvergentplate boundariesPlate motionat convergentplate boundariesDivergent ( ) andtransform fault ( )boundariesPlate motionat divergentplate boundariesFigure 16-4 The earth’s major tectonic plates. These bands correspond to the patterns for the types of lithosphericplate boundaries shown in Figure 16-5. What plate do you live on?http://biology.brookscole.com/miller14335