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Transcript
34. The Changing Face of the Earth In Chapter 32 we considered interactions among tectonic plates that result in mountain ranges like the Appalachians on the North American continent and the Himalayas of Asia. Plate tectonics alone can provide us only with the generalities—the broad locations and orientations of some landforms. The specific details of mountains, valleys, plains, and plateaus arise from the interaction of the plate tectonic system with the hydrologic system. As a preface to an examination of that system, we introduce the hydrologic cycle. (the water that flows downhill to eventually join streams) from the discharge of the Seine. After allowing for the amount of water evaporated from the surface, he showed that there was easily enough precipitation to feed both the streams and springs in the basin. If this fact seems contradictory to your intuition, just realize that precipitation falls over a very large area, but the actual area occupied by streams is comparatively small. The “water-flow budget” of the earth is depicted in Figure 34.1. The hydrologic cycle, shown schematically in Figure 34.2, consists largely of those phenomena mentioned above and is powered by energy from the sun. Note, however, that the plate tectonic system, powered by energy from radioactive decay, plays a role in the hydrologic cycle because subducted seafloor rock and sediment carry H2O and CO2, which are chemically bound in minerals, deep into the earth where metamorphism and partial melting result in their return to the atmosphere. (Quantitatively, in cubic kilometers per year, the plate-tectonic contribution is relatively small, but over geologic time it has likely accounted for the presence of enough CO2 in our atmosphere to keep the planet warm—a very important benefit, indeed.) The Hydrologic Cycle If you were a traveler in space and had particularly acute eyesight, one of the most pervasive features you would observe as you approached the earth would be moving water. You would see vast oceans covering three-quarters of the planet, and throughout them the water would be moving. On the continents water would be flowing downhill in millions of streams, varying in size from the very largest rivers to the smallest brooks. Water would be slowly percolating through the pores of rocks underground. You would see roots of plants absorbing it from thin coatings around soil grains and transporting it into the leaves, where it would be transpired into the atmosphere by evaporation from the surfaces of the leaves. Evaporation would move great quantities of water into the atmosphere from the oceans, lakes, streams, and even falling raindrops, where it would again condense and eventually fall toward the surface of the planet as precipitation. In a general way, the water would circulate from the surface to the atmosphere and back again. You are aware of the existence of rivers like the Mississippi, the Amazon, and the Nile, and you may wonder where all of that water comes from. Is precipitation sufficient to provide the water for large rivers like these and for the countless smaller streams that drain the land, as well as for springs that bring water to the surface from some unseen source? That question was answered in the mid-17th century, before geology became a formal science, by the French naturalist Pierre Perrault (1611-1680). For a period of years he kept track of the amount of precipitation that fell on the Seine River basin and calculated the mean annual runoff The Hydrologic System In discussing the ways in which the hydrologic system modifies the surface of the earth, we shall consider surface drainage, glaciers, subsurface water, wind (not only because it carries water vapor but, more importantly, because it also has the power to erode and transport sediment), and oceans. Most of the earth’s water, over 97 percent, is in the oceans, with about 2 percent frozen as ice, 0.6 percent underground, and only 0.01 percent in surface water on land. Unlikely as it may sound, that 0.01 percent has had the most widespread effect of all on the shape of our land surfaces. Running Water Virtually every location on the surfaces of the earth’s landmasses have been shaped, to some extent, by running water. Color Plate 15 is a satellite photograph of the Grand Canyon of Arizona, one of the more arid regions 337 Precipitation Evaporation Precipitation 9.6 6.0 28.4 Evaporation 32.0 Runo ff 3.6 Groundw ater Figure 34.1. The water-flow budget of the earth. Figures are in units of 104 km3, or tens of thousands of cubic kilometers, per year. (Each cubic kilometer is 264 billion gallons!) Evaporation from lakes, streams, and soil Evaporation H2O and CO2 Precipitation Transpiration ff Runo Groundwater H2 O and CO 2 H O and CO* 2 2 * Mostly as CaCO3 used by marine organisms to build shells. Figure 34.2. The hydrologic cycle, consisting largely of water moving in the hydrologic system (essentially a surface or near-surface system), and also in limited parts of the plate tectonic system (essentially a subsurface system). of the planet. Yet, observe the intricate network of stream valleys that dissect the plateau around the canyon. Stream valleys, in fact, are the most abundant landform on the continents. As water flows in them, particles of sand and other materials carried in the current act as “sandpaper” on the stream bed, abrading it and deepen- ing the valley. At the same time, water moving down the valley sides in sheets and rivulets erodes the valley walls, causing them to recede. Valleys are lengthened by a process called headward erosion, in which erosion is focused at the head of a valley by convergence of runoff from several directions (see Figs. 34.3 and 34.4). 338 source of the sediment while finer particles are carried farther. When the sediment eventually becomes sedimentary rock, its grain size provides insight into the distance of transportation of the sediment. A little thought will convince you that there must be a limit to the depth of stream erosion. For streams that empty into the ocean, that limit is essentially sea level, because a stream would have to flow uphill if it cut more deeply than that. In order to erode at all, a stream must flow fast enough to carry abrasive sediment, so this maximum depth of erosion must decrease away from the sea in order that there be some slope to the stream channel. (That is, the erosional limit must be at higher and higher elevations further and further from the sea.) The maximum possible depth of downcutting is called base level, and it is the level toward which all streams strive. They rarely attain base level, however, because tectonic movement of the land intercedes and changes it. Thus there is a constant competition between the hydrologic system and the tectonic system, the former striving to lower the land and the latter generally attempting to raise it. Figure 34.3. Headward erosion occurs when runoff converges on the head of a valley from several directions. Glaciers Glaciers are masses of ice that form either at high elevations (in mountains) or at high latitudes (far north or far south) where temperatures are perennially low. The process of forming a glacier requires that snow accumulation in the winter exceed loss in the summer, so that the deposit becomes deeper with time. Eventually, the snow deep in the snow field is compacted into a tough, granular form, similar texturally to the old, grainy snow that accumulates at the side of the road during a long winter cold spell. The final stage is the recrystallization of the granular snow into glacial ice, a mass of intergrown ice crystals many meters below the surface. As more and more ice forms, the mass becomes heavy enough to move downhill as the ice at the bottom slowly deforms under pressure. Glaciers that form in valleys originally carved by streams are called valley glaciers. They move slowly downhill both by sliding (basal slip) and by deformation of the ice near the base (plastic flow). Continental glaciers cover very large areas and are not confined to valleys. Color Plate 16 shows the Vatnajökull continental glacier in Iceland with outlet glaciers that are similar to valley glaciers near the fringe. Such glaciers spread out from a central area in all directions, much like pancake batter from the center of a griddle. Figures 34.5 and 34.6 are photographs of a valley glacier and a continental glacier, respectively. Figure 34.4. A section of the Colorado Plateau, showing a through-flowing stream with several tributary canyons occupied by streams during periods of rain. Headward erosion slowly extends the canyons into the plateau. Many stream-generated landforms are depositional rather than erosional. As streams empty into the ocean or lakes, they deposit sediment in deltas, fan-shaped or fingerlike deposits that record, in part, a history of the stream. In some areas streams do not flow to the ocean or to lakes but end in dry basins that have no outlets. In such places, the streams that intermittently flow from canyons deposit their sediment on the valley floors in alluvial fans, the subaerial analogs of the subaqueous deltas. Streams tend to be efficient at sorting sediment by size, so that coarser particles are deposited near the 339 however, even the slowest glaciers can carry huge boulders as well as smaller material. Not only does the ice itself abrade the land but it also carries embedded rocks of all sizes, and thus acts very much like sandpaper. Valley glaciers gouge and scoop out the valleys they occupy, modifying them into U-shaped troughs. The topography between adjacent glaciated valleys is often angular and sharp. Continental glaciers scrape and scour the rocks over which they flow, largely obliterating previously developed stream drainage systems. Glaciers do not flow uphill, of course, but they may melt faster at their lower ends than they advance. When this occurs, we say that a glacier is receding. Because they can carry sediment of all sizes, but are incapable of sorting it by size as is running water, glaciers leave deposits of rough, unsorted debris called moraines when they melt. Moraines deposited by ancient glaciers are common landforms in some areas of the northeastern and midwestern parts of the United States. More glaciers seem to be receding than advancing today, but there have been times in the past when glaciation was a much more active process than it is now. There have been several “ice ages” during geologic time, as far back as the Precambrian and as recently as the Great Ice Age of the Pleistocene Epoch in the Cenozoic Era. The latter involved recurring glacial and interglacial stages and was so recent that we cannot be sure that we are not simply in another interglacial stage of the same ice age now. Moreover, there is no general agreement about the cause of ice ages, although it may have much to do with the positioning of continent-size landmasses at high north or south latitudes by plate motion, and their interference with ocean currents that facilitate the worldwide transfer of heat. Figure 34.5. A typical valley glacier. Groundwater Considerably less than a percent of the total water in the hydrologic system resides underground, but it is of critical importance—20 percent of the freshwater requirements of the United States is met by it. Nearly all groundwater comes from precipitation that has seeped into the ground. Some precipitation remains in the upper soil layer as a film around soil particles, but most descends to a depth at which all of the pore spaces in the rock are filled with water—the region of groundwater. The upper surface of this region is the water table, and its shape tends to mimic the topography above (see Fig. 34.7). The zone of pore saturation exists under both humid and arid areas of the earth, but it is deeper in arid areas. In humid areas, the water table is shallow and groundwater contributes to stream flow, so that even during dry spells there is water in the streams. In arid areas, the water table is deep and does not contribute to stream flow; rather, the streams “leak” and provide water to the subsurface, so that most streambeds in such areas contain no water during dry Figure 34.6. Part of the continental glacier that covers Greenland. Like running water, glaciers generate both erosional and depositional landforms. Unlike running water, 340 Zone of saturation Water table when the water table has been lowered by deepening of nearby stream channels, carbonate-rich waters percolating into these caverns will deposit calcium carbonate (CaCO3) to make beautiful stalactites, stalagmites, and other cavern formations. Often, where limestone beds with dissolved voids exist near the surface, the roofs of the voids are too thin to support themselves and they collapse, forming sinkholes (Fig. 34.8). In populated areas these cause great damage when they collapse beneath and engulf houses, cars, and so forth. Figure 34.7. The water table. Wind periods. Like surface water and glaciers, groundwater also flows in response to gravity. It moves generally from areas of high elevation of the water table to areas of lower elevation (streams, springs, or lakes), but may move locally upward in order to reach regions of lesser hydrostatic pressure caused by “topography” on the water table itself. The flow of groundwater is slower than water flow on the surface, averaging only centimeters per day—a fortunate circumstance, for were it not so, wells drilled into groundwater would rapidly go dry because supply by rainfall could not keep up with depletion from pumping. The erosional and depositional work of groundwater produces some spectacular results, as anyone who has visited an underground cavern can attest. Rainwater absorbs small amounts of carbon dioxide from the atmosphere as it falls, creating a weak carbonic acid (H2CO3) that is very effective in slowly dissolving limestone. Consequently, in areas where limestone constitutes a major part of the near-surface rock, large caverns may be dissolved near or below the water table. Later, Wind is incapable of carrying the heavy particles that denser agents like running water or ice can carry, but anyone who has been caught in a dust storm or sandstorm recognizes that wind can lift and transport very large amounts of smaller particles. Wind plays its most important role in shaping the face of the land in deserts and near-desert regions, and these areas constitute about one-fifth of the land surface of the earth. Even in most of these, water is a very important, though infrequent, agent of erosion. Sand dunes are probably the best known of windgenerated landforms. Color Plate 17 shows large dunes on the Arabian Peninsula; some of them are 100 meters high (higher than a football field on end) and up to 200 kilometers long. Depending upon the abundance of sand, the density of plant cover, and the constancy and strength of winds, other types of dunes with different shapes may develop in other arid areas. Figure 34.8. Sinkholes produced by the collapse of underground voids in limestone. The voids develop as weak carbonic acid, formed when rain combines with atmospheric carbon dioxide, dissolves the limestone. Figure 34.9. A typical sea cliff, a feature created by marine erosion. 341 Motion of sand grains W av ed ire cti Sand migration on Figure 34.10. When waves impinge obliquely on the beach, sediment is carried in from an angle, but recedes directly downslope, resulting in migration of the sediment down a coastline. Oceans The crashing breakers during a storm at the seashore demonstrate the power of the ocean to modify the land, but of course the effects are limited to that narrow band where sea and land meet—the shoreline. Wave action can be very effective in modifying the shoreline, as seen in Figure 34.9, which shows a sea cliff carved by marine erosion. Many beach cottages that have been built well back from sea cliffs have eventually become unsafe because of receding shorelines. The sea can create depositional landforms as well as erosional ones. As sediment is carried by waves onto a beach at an angle oblique to the shoreline, it washes back toward the sea directly downslope, perpendicular to the shoreline (Fig. 34.10). Thus sediment migrates slowly down a coast. Such sediments are often spread into elongate landforms, as shown in Figure 34.11. Some of these become large enough to support buildings or communities. The various facets of the hydrologic system do not always work independently. Where a stream empties into the sea, it builds a delta, but the waves and currents of the sea may modify the form of that deposit. Within many areas in the world, one may see the combined effects of waves, wind, and running water; glaciers and waves; or running water and groundwater. Figure 34.11. An elongate landform created by the spreading of sediment transported in the way shown in Figure 34.10. which it has passed since its formation. Many of the details of this long history have yet to be worked out by geologists, but the general outline is probably correct. Accretion Stage The Face of the Earth Through Time Recall from Chapter 28 that the early history of the solar system involved the gravitational collapse of the condensing solar nebula into countless small clumps and that larger clumps gradually swept up smaller ones and grew into planetesimals. The process continued until there emerged from the mass of gas, dust, and While the interaction of the tectonic system and the hydrologic system explains the appearance of the earth today, our planet has not always appeared as it does now—even vaguely. To conclude this brief study of the earth, we summarize the various major stages through 342 chunks a star, nine planets, their assorted moons, and various swarms of material like meteors. One of the planets was ours, of course, but it bore little resemblance to today’s earth. There were neither continents nor oceans, and there was no internal structure—that is, it was homogeneous, the same all the way from surface to center. The earth was essentially age zero, about 4.6 billion years ago, and its temperature was around 1000 °C in the interior. an oxide-and-silicate mantle of mostly iron-magnesium-oxygen compounds, which in turn was surrounded by a basaltlike crust. The process of forming layers based on density is called planetary differentiation. Because the radioactive elements were chiefly involved in compounds with oxygen, they tended to concentrate in the outer layers of the planet where radiogenic heat was dissipated rapidly, and the heating therefore slowed down. By this point 400 or 500 million years had passed since the formation of the solar system, and the time was about 4.2 billion years ago. The face of the earth was still not recognizable. Bombardment and Heating After the initial formation of the planets there were countless small bits of matter in the nebular disk yet to be gravitationally swept up. Bombardment of the earth by meteors was intense and, as each meteor hit, its kinetic energy was transformed into internal energy— heat. (The record of the bombardment stage is largely absent from the earth now, but it is present on other bodies such as the moon, Mercury, Mars, and some satellites of the outer planets, none of which have had vigorous tectonic or hydrologic systems capable of obliterating it.) As gravity continued to contract the new planet, gravitational energy was likewise transformed into heat faster than it could be dissipated. In addition, radioactive elements (mostly uranium, thorium, and potassium), all of which must have been more abundant in the early earth than they are now, decayed—a third source of heat. The result of all this heating was that, by around 4.2 to 4.5 billion years ago, the temperature rose to the melting point of iron in a shell beginning at 400 kilometers deep and extending to 800 kilometers. (The temperature must have been hotter deeper than this, but the melting temperature of iron also increases with depth. The situation is analogous to the one for the origin of the asthenosphere, and the general geometry shown in Figure 30.9 applies—with different actual temperatures and depths, of course.) Onset of the Tectonic System The heat generated by the iron catastrophe must have repeatedly melted silicate minerals surrounding the developing core, including those constituting the thin primitive crust that was forming. Eventually, as the “distillation” process of planetary differentiation proceeded, the least dense materials must have accumulated on top and formed the earliest continents, perhaps around 4.2 billion years ago. It is difficult to know just when the tectonic system, as we now define it, had its beginning, but by 3.9 billion years ago there were probably thin, rather fragile plates moving comparatively rapidly over an asthenosphere, being subducted and recycled. These would gradually thicken, support volcanic activity and igneous intrusion, weather and erode, be metamorphosed, and become the shields of today’s continents. Not yet wholly familiar, the face of the earth at least had the beginnings of familiar features. Origin of the Atmosphere and Oceans The original atmosphere of the earth was probably very unlike the one we know today and was swept away by the vigorous solar wind of the young sun. The atmosphere we breathe is probably mostly of volcanic origin and was produced by release of gases from the interior. If you have seen pictures of volcanic eruptions, you might have noticed that large amounts of gases are emitted along with lava, ash, or other volcanic emanations. Most of this gas is water vapor, with some carbon dioxide and nitrogen, and was no doubt produced by the early volcanism of the heated earth just as it is today. Even at the present rate of volcanism (and the rate in the hot, differentiating earth would have been significantly higher), enough water would have been produced in this way over the span of geologic time to fill the oceans, along with enough nitrogen, carbon dioxide, and other gases to create nearly all of the atmosphere. The part of the atmosphere not produced by this process—approximately 20 percent of it—is the oxygen on which life depends, but which is essentially absent in volcanic gases. The Iron Catastrophe and Differentiation As iron in the 400-800 kilometer depth range began to melt, it formed droplets that migrated gravitationally toward the center of the earth, displacing the less dense material. The process was self-accelerating: As iron sank, it raised the temperature through friction, and more iron melted and sank. As the temperature increased, minerals of all sorts melted as their melting points were reached. Most silicate minerals with low melting points also have relatively low densities, so they floated to the surface. The process finally elevated the earth’s temperature to around 2000 °C, and so a large fraction of the planet melted. This event, called the iron catastrophe, resulted in the complete reorganization of the interior. When it was over, the metallic iron was mostly in the center of the earth, surrounded by 343 Sunlight striking the upper layers of this oxygenpoor atmosphere dissociated some of the water molecules into hydrogen and oxygen. Hydrogen is so light that it escaped the earth’s gravity, but oxygen was carried by atmospheric turbulence downward toward the surface of the planet, to become part of the permanent gaseous envelope of the earth. Most of the oxygen must eventually have been produced by photosynthesis, however, and this required plant life. The first primitive cells could have developed in only a small fraction (perhaps as little as one five-thousandth) of the present oxygen level in the atmosphere and begun the slow, continual process of combining carbon dioxide and water to produce carbohydrates and oxygen. For a long time the atmosphere was too poor in oxygen to support complex forms of life; hence, the Precambrian is represented by few fossils, virtually all soft-bodied organisms. However, as simple plants (chiefly algae) built the oxygen supply, increasingly complex organisms developed, and the attainment of some critical level of oxygen concentration may have led to the almost explosive flowering of shelled forms (and hence fossils) at the beginning of the Paleozoic Era. That the chemistry of our atmosphere is highly dynamic and has evolved significantly over time is a fascinatingly troubling concept. To be sure, we are the beneficiaries of that long course of development, but we are also now its modifiers—and it is clear that it can be modified. By our industrial and cultural activities, we are adding to the levels of some noxious compounds in the air (such as carbon monoxide and sulfur dioxide), and we appear to be decreasing the abundance of at least one crucially necessary substance, ozone. Such tampering is not without consequences, and hence the current concern with control of atmospheric pollution. One may argue over the methods and priorities suggested by those on various sides of the issue, but its importance and urgency cannot be in doubt. reconstruct some of the plate interactions that occurred before Pangaea, although the details become less and less distinct as we look further back in time. For example, the Ural Mountains, which constitute the traditional geographic separation between Europe and Asia, testify of an ancient collision between those two continents that welded them into a single large landmass. It appears that there have been six periods of very intense and widespread mountain-building activity during the earth’s history. Some of these have been longer and more intense than others, but they may each represent a time when continents were converging to form a supercontinent. The times are about 2600, 2100, 1700, 1100, 650, and 250 million years ago. The 400- to 600million-year periodicity in these events has led some to suggest that there is a plate-tectonic cycle consisting of the repeated assembly and fragmentation of supercontinents, caused largely by the way in which heat builds up under large land masses. Regardless of whether this is the case, it is clear that mountain ranges have been built during several events—not only those mentioned above but also numerous more local events at other times— only to be leveled by the hydrologic system. Since Pangaea The division of the geologic account into periods before and after the break-up of Pangaea is somewhat artificial, and post-Pangaea history was included in the section on general continental evolution in Chapter 32. Here we mention only a major Cenozoic event that changed the face of much of the land—the Great Ice Age of the Pleistocene Epoch. For nearly two-million years, great sheets of ice up to two kilometers thick episodically advanced over northern North America and northern Europe, and then retreated during interglacial periods, exposing the scraped and scoured topography typical of continental glaciation. The ice obliterated stream drainage systems and left deposits of coarse, unsorted sediment. It created myriad lakes and clusters of low, streamlined hills (one of them Hill Cumorah near Palmyra, New York). It gouged out stream valleys to make the magnificent fjords of Scandinavia and lowered sea level to facilitate the carving of some immense canyons on the nowsubmerged continental shelves. While intense glaciation is by no means unique in the history of the earth, the Pleistocene Ice Age was recent enough to have clearly left its mark on the world we inhabit. In fact, it is quite possible that we are now living during an interglacial period, merely awaiting the return of the ice. Toward Pangaea In Chapter 31 we examined some of the evidence for the existence of the supercontinent Pangaea. We recognized that Pangaea was not the initial event in the history of plate motion, but simply a part of the continuum of plate tectonic history that spans nearly four billion years. The existence of two or more continental masses makes it virtually inevitable that there will be eventual continental collisions, and these produce large ranges of fold mountains. Even when such mountains have been completely eroded, their “roots” are still discernible through careful geologic mapping, and the large amounts of sediment shed from them show up as thick sequences of sedimentary rock layers that become thinner away from the sites of the former mountains. By understanding the significance of such geologic features, it is possible to Summary The hydrologic system consists of all fluids that move near the surface of the earth. Its various agents (running water, groundwater, wind, waves, and 344 ice) have eroded and deposited material to shape the face of the earth for many millions of years, but always in concert with the tectonic system. As plate motion has raised mountains, the hydrologic system has worked to wear them down. The conflict between the two systems, one driven by heat from radioactive decay within the earth and the other by the heat of solar radiation from outside the earth, has yielded the variety and beauty of our planet. The application of physical and chemical principles we have learned previously to the preferred model of the origin of the solar system leads us through a series of predictable stages for a developing earth, ending with a planet whose face is now familiar to us but which we realize is still undergoing slow and inexorable change. 8. 9. 10. 11. STUDY GUIDE Chapter 34: The Changing Face of the Earth 12. A. FUNDAMENTAL PRINCIPLES: No new fundamental principles. 13. B. MODELS, IDEAS, QUESTIONS, AND APPLICATIONS 1. What is the hydrologic system? What are the main elements of the system and what influence do they have on the features of the surface of the earth? 2. What has happened and what is now happening to determine the main features observed on the surface of the earth? 3. Sketch the geologic and biological history of the earth with approximate dates as we now understand it from interpretation of the physical data. 14. 15. 16. 17. C. GLOSSARY 1. Alluvial Fan: A fan-shaped deposit formed when a stream that flows intermittently from a canyon deposits its sediments on the valley floor. 2. Base Level: The maximum possible depth to which a stream can cut by erosion; the level to which each stream strives. 3. Continental Glacier: Large dollar shaped glacier covering an extended land mass (Greenland, Antarctica). Ice accumulates near the center and flows to the periphery. 4. Delta: A fan-shaped or fingerlike deposit formed when a stream or river empties into an ocean or lake. 5. Evaporation: The changing of water from its liquid state to its gaseous state. 6. Groundwater: Water which exists in the subsurface of the earth’s crust, usually in the pore spaces of the rock. 7. Headward Erosion: A process by which valleys are lengthened because erosion is focused at the head of a valley by convergence of runoff from sev- eral directions. Hydrological Cycle: The cyclic movement of the earth’s water supply as it moves among the oceans, the atmosphere, and the land. Hydrologic System: The methods by which the water moves through the hydrological cycle, i.e., running water, glaciers, subsurface water, wind, evaporation, etc. Iron Catastrophe: An early event in the earth’s history (about 4.2 billion years ago) during which the interior melted rather quickly and extensively and the denser elements (iron and nickel) moved to the center, displacing less dense oxides and silicates to the mantle and crust. Moraine: A deposit of rough, unsorted debris left by a glacier when it melts. Planetary Differentiation: The process by which the materials of a forming planet organize themselves into layers, with the more dense materials on the inside and the less dense materials on the outside. See Chapter 30. Precipitation: Water which condenses into liquid or solid form and falls toward the surface of the earth. Sand Dune: A wind-generated deposit of sand, commonly found in arid areas. Sinkhole: A hole or depression in the ground formed by the collapse of the roof above an underground void in limestone. Water Table: The top surface of the underground region where all of the pore spaces in the rock are filled with water. Valley Glacier: A glacier (mass of moving ice and snow) that forms in a valley originally carved by a stream. D. FOCUS QUESTIONS 1. Describe three main elements of the hydrologic system. Use examples to explain how individual elements of the cycle influence the appearance of the earth. 2. Describe the various stages in the history of the earth from its formation as a planet to its present stage according to current geologic thought. Include at least five important dates in your outline. E. EXERCISES 34.1. Discuss the general features of the hydrologic cycle. 34.2. The hydrologic system causes the most extensive changes in the surface of the earth through (a) running water. (b) glaciers. (c) shoreline processes. (d) wind. 345 34.3. Why is the central region of a large continental mass never eroded all the way to sea level? 34.4. Ice, running water, and wind are all able to transport sediment from a source area to an area of deposition. List these three in order of their ability to carry the largest particles. (a) Wind, ice, running water (b) Running water, wind, ice (c) Ice, wind, running water (d) Ice, running water, wind 34.5. It is believed that volcanic eruptions were the major source of most of the atmosphere of the earth. What major component would not have been contributed in this way, and where did it come from? 34.6. How do mountain ranges provide evidence that much plate tectonic activity occurred prior to the assembly of Pangaea? 346