Environmental Problems, Their Causes, and Sustainability 1

Civilization exists by geological consent, subject to changewithout notice.WILL DURANTThis chapter discusses the earth’s basic geologicalprocesses and the nonrenewable mineral resources weuse. It addresses the following questions:■■■■■What major geologic processes occur within theearth and on its surface?What are the hazards from earthquakes and volcaniceruptions?What are rocks, and how are they recycled by therock cycle?How do we find and extract mineral resourcesfrom the earth’s crust?Will there be enough nonrenewable mineralresources for future generations?16-1 GEOLOGIC PROCESSESIs the Earth a Stable or a Dynamic Planet?Change Is the NormThe planet we live on is constantly changingas a result of processes taking place on and belowits surface.Geology, the subject of this chapter, is the sciencedevoted to the study of dynamic processes occurringon the earth’s surface and in its interior. Geologistsstudy and analyze rocks and the features and processesof the earth’s interior and surface. Some of theseprocesses lead to geologic hazards such as earthquakesand volcanic eruptions, and others produce therenewable soil and nonrenewable mineral and energyresources that support life and economies.You probably think of the ground you stand on asstable and unmoving. Wrong. It is part of a dynamicplanet whose surface and interior are constantlychanging. Fortunately, most of these geologic changestake place very slowly on our short human time scaleand most occur out of sight within the earth’s interior.Over eons continents have moved to new positions,breaking apart and crunching into one another(Figure 5-8, p. 95). Inside the earth, huge cyclical flowsof molten rock break the earth’s surface into a series ofgigantic plates that move very slowly across theplanet’s surface. Go outside and you are standing onone of these moving plates. Hard to believe, isn’t it?Energy from the sun and from the earth’s interior,coupled with the erosive power of flowing water, havecreated continents, mountains, valleys, plains, andocean basins in an ongoing process that continues tochange the landscape. And every now and then thesolid earth under our feet shakes, rattles, and rollsduring an earthquake, or erupts like a punctured boilwhen a volcano forms or awakens after a long geologicalsleep.What Is the Earth’s Structure? Livingon a Layered SphereThe earth’s three major zones are its core, mantle,and crust.As the primitive earth cooled over eons, its interiorseparated into three major concentric zones, layerswhich geologists identify as the core, the mantle, andthe crust (Figure 4-7, p. 60). In other words, beneathyour feet is a crust of soil and rock floating on a mantleof partly melted and solid rock, which surrounds anintensely hot core. What we know about the earth’s interiorcomes mostly from indirect evidence such asdensity measurements, seismic (earthquake) wavestudies, measurements of interior heat flow, lavaanalyses, and research on meteorite composition.The core is the earth’s innermost zone. It is intenselyhot and has a solid inner part, surrounded by aliquid core of molten or semisolid material.A thick solid zone called the mantle surrounds thecore. Most of the mantle is solid rock, but under itsrigid outermost part is a zone—the asthenosphere—ofvery hot, partly melted rock that flows slowly and canbe deformed like soft plastic.The outermost and thinnest zone of the earth iscalled the crust. It consists of the continental crust,which underlies the continents (including the continentalshelves extending into the oceans), and theoceanic crust, which underlies the ocean basins andcovers 71% of the earth’s surface (Figure 16-2).16-2 INTERNAL AND EXTERNALGEOLOGIC PROCESSESWhat Geologic Processes Occur withinthe Earth’s Interior? Welcome to theHot ZoneHuge volumes of heated and molten rock movearound within the earth’s interior.We tend to think of the earth’s crust, mantle, and coreas fairly static. But geologic processes taking placewithin the earth and on its surface, mostly over thousandsto millions of years, bring about changes inthese components (Figure 16-3, p. 334).Geologic changes originating from the earth’s interior,called internal processes, generally build up theplanet’s surface. Heat from the earth’s interior providesthe energy for these processes, but gravity alsoplays a role.Residual heat from the earth’s formation is still beinggiven off as the inner core cools and the outer core332 CHAPTER 16 Geology and Nonrenewable Mineral Resources