Environmental Problems, Their Causes, and Sustainability 1

AsthenosphereOceanic ridge at a divergent plate boundarySubductionzoneTrenchVolcanic island arcRisingmagmaTrench and volcanic island arc at a convergentplate boundaryFracture zoneTransformfaultLithosphereLithosphereAsthenosphereLithosphereAsthenosphereTransform fault connecting two divergent plate boundariesFigure 16-5 Types of boundaries between the earth’slithospheric plates. All three types occur both in oceans and oncontinents.blowing particles of soil from one area to another is anothercause. Human activities, particularly those thatdestroy vegetation that holds soil in place, accelerateerosion, as discussed in Section 14-3 (p. 279).Weathering consists of the physical, chemical, andbiological processes that break down rocks and mineralsinto smaller particles that can be eroded. There arethree types of weathering processes. One is physical ormechanical weathering, in which a large rock mass isbroken into smaller fragments. This is similar to whathappens when you hammer a rock into pieces. Themost important agent of mechanical weathering is frostwedging, in which water collects in pores and cracks ofrock, expands upon freezing, and splits off pieces ofthe rock. This is also what causes most of the potholeswe complain about in our roads and streets.A second process is chemical weathering, in whichone or more chemical reactions decompose a mass ofrock. Most chemical weathering involves a reaction ofrock material with oxygen, carbon dioxide, and moisturein the atmosphere and on the ground.A third process is biological weathering, the conversionof rock or minerals into smaller particles throughthe action of living things. For example, lichens produceacids that can chemically weather rocks. Androots growing into and rubbing against rock can physicallybreak it into small pieces.16-3 NATURAL GEOLOGICHAZARDS: EARTHQUAKESAND VOLCANIC ERUPTIONSWhat Are Earthquakes? Shake, Rattle,and RollEarthquakes occur when a part of the earth’scrust suddenly fractures, shifts to relieve stress,and releases energy as shock waves.Stress in the earth’s crust can cause solid rock to deformuntil it suddenly fractures and shifts along thefracture, producing a fault (Figure 16-5, bottom). Thefaulting or a later abrupt movement on an existingfault causes an earthquake that has certain featuresand effects (Figure 16-6).Relief of the earth’s internal stress releases energyas shock waves, which move outward from the earthquake’sfocus like ripples in a pool of water. Scientistsmeasure the severity of an earthquake by the magnitudeof its shock waves. The magnitude is a measure ofthe amount of energy released in the earthquake, as indicatedby the amplitude (size) of the vibrations whenthey reach a recording instrument (seismograph).Scientists use the Richter scale, on which eachunit represents an amplitude 10 times greater than thenext smaller unit. Thus a magnitude 5.0 earthquake is10 times greater than a magnitude 4.0, and a magnitude6.0 quake is 100 times greater than a magnitude4.0 quake. Seismologists rate earthquakes as insignifi-336 CHAPTER 16 Geology and Nonrenewable Mineral Resources