Environmental Problems, Their Causes, and Sustainability 1

Figure 16-8 A volcano erupts when molten magma in thepartially molten asthenosphere rises in a plume through thelithosphere to erupt on the surface as lava that can spill over orbe ejected into the atmosphere. Chains of islands can be createdby eruptions of volcanoes that then become inactive.extinctvolcanoesOther volcanoes erupt morequietly. They involve primarilylava flows, which can cover roadsand villages and ignite brush,trees, and homes.We tend to think negatively ofvolcanic activity, but it also providessome benefits. One is outstandingscenery in the form ofmajestic mountains, some lakes(such as Crater Lake in Oregon;see title page photo), and otherlandforms. Perhaps the most importantbenefit of volcanism is thehighly fertile soils produced bythe weathering of lava.SolidlithosphereWe can reduce the loss of human life and sometimesproperty from volcanic eruptions. One way is touse historical records and geologic measurements toidentify high-risk areas so that people can try to avoidliving in them. Other methods involve developing effectiveevacuation plans and trying to develop measurementsthat warn us when volcanoes are likely toerupt.Scientists are studying phenomena that precedean eruption. Examples include tilting or swelling ofthe cone, changes in magnetic and thermal propertiesof the volcano, changes in gas composition, and increasedseismic activity.centralventmagmaconduitUpwellingmagmamagmareservoirPartially moltenasthenosphereA mineral is an element or inorganic compoundthat occurs naturally and is solid with a regular internalcrystalline structure. Some minerals consist of asingle element, such as gold, silver, diamond (carbon),and sulfur. But most of the more than 2,000 identifiedminerals occur as inorganic compounds formed byvarious combinations of elements. Examples are salt,mica, and quartz.Rock is a solid combination of one or more mineralsthat is part of the earth’s crust. Some kinds of rock,such as limestone (calcium carbonate, or CaCO 3 ) andquartzite (silicon dioxide, or SiO 2 ), contain only onemineral, but most rocks consist of two or more minerals.16-4 MINERALS, ROCKS,AND THE ROCK CYCLEWhat Are Minerals and Rocks?Hard StuffThe earth’s crust consists of solid inorganicelements and compounds called minerals andmasses of one or more minerals called rock.The earth’s crust, still forming in various places, iscomposed of minerals and rocks. It is the source ofalmost all the nonrenewable resources we use: fossilfuels, metallic minerals, and nonmetallic minerals (Figure1-6, p. 9). It is also the source of soil (Figure 4-25,p. 73) and of the elements that make up your body andthe bodies of other living organisms. You are mostlywater mixed with chemically transformed particles ofminerals and rock and a small amount of air.What Are the Major Rock Types and How AreThey Recycled? Really Slow RecyclingThe earth’s crust contains igneous, sedimentary, andmetamorphic rocks that are recycled by the rock cycle.Based on the way it forms, rock is placed in threebroad classes. One is igneous rock, formed below oron the earth’s surface when molten rock (magma)wells up from the earth’s upper mantle or deep crust,cools, and hardens. Examples are granite (formed underground)and lava rock (formed aboveground whenmolten lava cools and hardens). Although often coveredby sedimentary rocks or soil, igneous rocks formthe bulk of the earth’s crust. They also are the mainsource of many nonfuel mineral resources.A second type is sedimentary rock. It is formedfrom sediment produced when existing rocks areweathered and eroded into small pieces, and trans-338 CHAPTER 16 Geology and Nonrenewable Mineral Resources