Environmental Problems, Their Causes, and Sustainability 1

AbyssalhillsAbyssalfloorOceanicridgeAbyssalfloorTrench Folded mountain beltVolcanoesCratonOceanic crust(lithosphere)Abyssal plainContinentalriseContinentalslopeContinentalshelfAbyssal plainMantle (lithosphere)Continental crust(lithosphere)Mantle(lithosphere)Mantle (asthenosphere)Figure 16-2 Natural capital: major features of the earth’s crust and upper mantle. The lithosphere, composedof the crust and outermost mantle, is rigid and brittle. The asthenosphere, a zone in the mantle, can be deformedby heat and pressure.cools and solidifies. Continued decay of radioactiveelements in the crust, especially the continental crust,provides much of this heat flow from within. Heatfrom the core causes much of the mantle to deformand flow slowly like heated plastic.Measurements of heat flow within the earth suggestthat two kinds of movement occur in the mantle’sasthenosphere. One is convection cells or currents thatmove large volumes of rock and heat in loops withinthe mantle like a giant conveyer belt (Figure 16-3). Internalheat pushes soft rock in the mantle’s asthenosphereupward, then downward as the rock cools. Thispattern resembles convection in the atmosphere (Figure6-9, p. 107) or in a pot of boiling water (Figure 3-11,left, p. 45).The second type of movement involves mantleplumes, in which mantle rock flows slowly upward in acolumn (Figure 16-3), like smoke from a chimney on acold, calm morning. When the moving rock reachesthe top of the plume, it moves out in a radial pattern,as if it were flowing up an umbrella through the handleand then spreading out in all directions from thetip of the umbrella to the rim. There is a lot going onbeneath your feet.What Is Plate Tectonics? The EarthIs MovingHuge solid plates—called tectonic plates—moveextremely slowly across the earth’s surface.The flows of energy and heated material in the mantleconvection cells cause about 15 huge rigid plates,called tectonic plates, to move extremely slowly acrossthe earth’s surface (Figure 16-3 and Figure 16-4,p. 335). These plates are about 100 kilometers (60 miles)thick. They are composed of the continental andoceanic crust and the rigid, outermost part of the mantle(above the asthenosphere), a combination called thelithosphere.These plates move constantly, supported by theslowly flowing asthenosphere. They are somewhatlike large icebergs floating extremely slowly on thesurface of an ocean or like the world’s largest andslowest-moving surfboards. Some plates move about1 centimeter (slightly more than a third of an inch) ayear. Others at a seafloor spreading zone can move asmuch as 18 centimeters (7 inches) a year. You are ridingor surfing on one of these plates throughout yourentire life, but the motion is too slow for you to notice.The theory explaining the movements of theplates and the processes that occur at their boundariesis called plate tectonics. The concept, which becamewidely accepted by geologists in the 1960s, was developedfrom an earlier idea called continental drift.Throughout the earth’s history, continents have splitand joined as plates have very slowly drifted thousandsof kilometers back and forth across the planet’ssurface (Figure 5-8, p. 95).As these plates collide, break apart, and slide byone another over millions of years, they producemountains on land (such as the Himalayas and theAppalachian Mountains of the eastern United States),huge ridges and trenches on the ocean floor, and otherfeatures of the earth’s surface (Figures 16-2 and 16-3).These movements and geological processes continuetoday. Natural hazards such as volcanoes and earthquakesare likely to be found at plate boundaries. Inaddition, plate movements and interactions affect thehttp://biology.brookscole.com/miller14333