Description:

CHAPTER 1 Geology and the Earth Imagine walking on a rocky shore. You can see the pounding surf, hear stones clink together as waves recede, feel the wind blowing in your hair. But the cliffs don’t move and the ground doesn’t shake. Even though the Earth appears to be a firm foundation beneath your feet, it is a dynamic planet. Continents slowly shift position; mountains rise and then erode away. These motions escape casual observation because they are generally slow, although every year events such as volcanic eruptions and earthquakes remind us that geologic change can be rapid. A storm-driven wave crashes against the Oregon coast. (H. Richard Johnston Tony Stone Images) 1 2 CHAPTER 1 GEOLOGY AND THE EARTH 1.1 The Science of Geology Geology is the study of the Earth, including the materials it is made of, the physical and chemical changes that occur on its surface and in its interior, and the history of the planet and its life forms. The Earth and Its Materials The Earth’s radius is about 6370 kilometers, nearly one and a half times the distance from New York to Los Angeles (Fig. 1–1). If you could drive a magical vehicle from the center of the Earth to the surface at 100 kilometers per hour, the journey would take more than two and a half days. Most of the Earth is composed of rocks. Rock outcrops form some of our planet’s most spectacular scenery: white chalk cliffs, pink sandstone arches, and the grey granite of Yosemite Valley. Rocks, in turn, are composed of minerals (Fig. 1–2). Although more than 3500 different minerals exist, fewer than a dozen are common. Geologists study the origins, properties, and compositions of both rocks and minerals. Geologists also explore the Earth for resources needed in our technological world: fossil fuels such as coal, petroleum, and natural gas; mineral resources such as metals; sand and gravel; and fertilizers. Some search for water in reservoirs beneath Earth’s surface. Figure 1–2 This granite rock is composed of different minerals, primarily quartz, feldspar, and hornblende. The mineral grains are a few millimeters in diameter. Internal Processes Processes that originate deep in the Earth’s interior are called internal processes. These are the driving forces that raise mountains, cause earthquakes, and produce volcanic eruptions. Builders, engineers, and city planners might consult geologists, asking, “What is the probability that an earthquake or a volcanic eruption will damage our city? Is it safe to build skyscrapers, a dam, or a nuclear waste repository in the area?” Kilometers Biosphere All life is concentrated at or within a few 20 kilometers of the surface. Hydrosphere Deep ocean floor 5 km deep. Fresh water and ice exist on land. Atmosphere 99% of atmosphere lies within 30 km of the surface. Earth Radius 6370 km Figure 1–1 Most of the Earth is solid rock, surrounded by the hydrosphere, the biosphere, and the atmosphere. The Science of Geology Surface Processes Surface processes are all of those processes that sculpt the Earth’s surface. Most surface processes are driven by water, although wind, ice, and gravity are also significant. The hydrosphere includes water in streams, wetlands, lakes, and oceans; in the atmosphere; and frozen in glaciers. It also includes ground water present in soil and rock to a depth of at least 2 kilometers. Most of us have seen water running over the ground during a heavy rain. The flowing water dislodges tiny grains of soil and carries them downslope. If the rain continues, the water may erode tiny gullies into a hillside (Fig. 1–3). A gully may form in a single afternoon; over much longer times, the same process forms canyons and spacious river valleys. People build cities along rivers to take advantage of the flat land, fertile soil, and abundant water. But the erosion continues. Rivers wear away at their banks and bed and periodically flood adjacent land. Geologists seek to understand these processes and advise builders and planners to minimize loss of life and property. The oceans cover more than 70 percent of our planet. Although oceanography is a separate scientific discipline, it overlaps with geology. Geologic processes form the ocean basins and alter their size and shape. Weathering and erosion of continents carry mud, sand, and salts to the sea. Earth is the only planet in the Solar System that has oceans. It is also the only planet that supports life. Oceanographers examine the oceans’ influence on climate, the atmosphere, life, and the solid Earth. The Atmosphere The atmosphere is a mixture of gases, mostly nitrogen and oxygen (Fig. 1–4). It is held to the Earth by gravity and thins rapidly with altitude. Ninety-nine percent is concentrated within 30 kilometers of the Earth’s surface, but a few traces remain even 10,000 kilometers above the surface. A brief look at our neighbors in space reminds us that the interactions among air, rock, and life affect atmospheric composition, temperature, and movement. The solid Earth, Venus, and Mars are approximately identical in composition. Yet the three planets have radically different atmospheres and climates. Today, the Venusian atmosphere is hot, acidic, and rich in carbon dioxide. The surface temperature is 450°C, as hot as the interior of a self-cleaning oven, and the atmospheric pressure is 90 times greater than that of the Earth. In contrast, Mars is frigid, with an atmospheric pressure only 0.006 that at the surface of the Earth. Venusian water has boiled off into space; almost all Martian water lies frozen in vast underground reservoirs. Figure 1–3 Over long periods of time, running water can carve deep canyons, such as this tributary of Grand Canyon in the American southwest. Figure 1–4 This storm cloud over Mt. Robson, British Columbia, is a visible portion of the Earth’s atmosphere. 4 CHAPTER 1 GEOLOGY AND THE EARTH The Biosphere The biosphere is the thin zone near the Earth’s surface that is inhabited by life. It includes the uppermost solid Earth, the hydrosphere, and the lower parts of the atmosphere. Land plants grow on the Earth’s surface, with roots penetrating at most a few meters into soil. Animals live on the surface, fly a kilometer or two above it, or burrow a few meters underground. Sea life also concentrates near the ocean surface, where sunlight is available. Some aquatic communities live on the deep sea floor, bacteria live in rock to depths of a few kilometers, and a few windblown microorganisms are found at heights of 10 kilometers or more. But even at these extremes, the biosphere is a very thin layer at the Earth’s surface. Paleontologists are geologists who study the evolution and history of life by examining fossils and other evidence preserved in rock and sediment. The study of past life shows us that the solid Earth, the atmosphere, the hydrosphere, and the biosphere are all interconnected. Internal processes such as volcanic eruptions and migrating continents have altered the Earth’s climate and atmospheric composition. Life has altered the atmosphere. The atmosphere reacts with rocks. 1.2 Uniformitarianism and Catastrophism James Hutton was a gentleman farmer who lived in Scotland in the late 1700s. Although trained as a physician, he never practiced medicine and instead turned to geology. Hutton observed that a certain type of rock, called sandstone, is composed of sand grains cemented together (Fig. 1–5). He also noted that rocks slowly decompose into sand, and that streams carry sand into the lowlands. He inferred that sandstone is composed of sand grains that originated by the erosion of ancient cliffs and mountains. Hutton tried to deduce how much time was required to form a thick bed of sandstone. He studied sand grains slowly breaking away from rock outcrops. He watched sand bouncing down streambeds. Finally, he traveled to beaches and river deltas where sand was accumulating. Hutton concluded that the sequence of steps that he had observed must take a long time. He wrote that on us who saw these phenomena for the first time, the impression will not easily be forgotten. We felt ourselves necessarily carried back to the time when the sandstone before us was only beginning to be deposited, in the shape of sand and mud, from the waters of an ancient ocean. The mind seemed to grow giddy by looking so far into the abyss of time. Hutton’s conclusions led him to formulate a principle now known as uniformitarianism. The principle states that geologic change occurs over long periods of time, by a sequence of almost imperceptible events. Hutton surmised that geologic processes operating today also operated in the past. Thus, scientists can explain events that occurred in the past by observing changes occurring today. Sometimes this idea is summarized in the statement “The present is the key to the past.” For example, we can observe today each individual step that leads to the formation of sandstone. Even though it would take too long for us to watch a specific layer of sandstone form, we can infer that the processes occur slowly—step by step—over great periods of time. If we measure current rates of geologic change, we must accept the idea that most rocks are much older than human history. Taking his reasoning one step further, Hutton deduced that our planet is very old. He was so overwhelmed by the magnitude of geological time that he wrote, “We find no vestige of a beginning, no prospect of an end.” William Whewell, another early geologist, agreed that the Earth is very old but argued that geologic change was sometimes rapid. He wrote that the geologic past may have consisted of epochs of paroxysmal and catastrophic action, interposed between periods of comparative tranquility. Whewell was unable to give examples of such catastrophes. He argued that they happen so infrequently that none had occurred within human history. Today, geologists know that both Hutton’s uniformitarianism and Whewell’s cata' Ключевые слова: pressure, channel, potassiumargon method, individual particle, plate, chapter, magmatic arc, continental margin, columbia plateau, hot, magma, glacial episode, single mass, occur, roche moutonne, beneath, sedimentary rock, north america, alluvial fan, metamorphism, area, primary objective, wind, alternating normal, earthquake, meter, hydrologic cycle, fracture, desert, ridge, san francisco, geologic structures, monetary benefit, table, layer, tectonic activity, boundary, percent, clay, mount everest, atom, organic component, doe, sand, mineral, year, chemical, age, neutron-absorbing alloy, fine-grained matrix, fundamental particle, south dakota, oil, erosion, metamorphic, narrow neck, appalachian mountains, sea-floor spreading, sedimentary, andean-type margin, niagara falls, small, sea, crust, changing burden, southeastern canada, quartz, time-travel curve, fine grained, chemical bonds, continent, western canada, phanerozoic eon, dune, sea level, ocean, seismic station, greek god, change, form, igneous, mass wasting, sedimentary bedding, volcanic, land, tibetan plateau, western mountains, alfred wegener, commonly, mountain, marine invertebrates, geologic hazard, ha, mount kenya, pressure-release fracturing, lake, structure, war, oceanic island, rock, lithospheric plate, geologist, ground, slope, flood, result, global warming, formed, british columbia, kilometer, volcanic hazard, ice, tectonic, ancient civilization, lithosphere, billion, subduction, control rod, carbonate platform, volcanic activity, western margin, beach, zone, artificially augmenting, passive margin, type, south america, volcanic island, magmatic activity, limestone, wa, explain, marine evaporite, sediment, volcanic neck, mantle, ground water, lithospheric spreading, america, alternate compression, recording station, water, rise, original horizontality, material, fault, nonmetallic resource, soil, jigsaw puzzle, pressurerelease fracturing, latin word, plate tectonics, streams load, unit, north, crystal, common, continental, region, high, ash-flow tuff, magnetic stripe, seasonal snow, sierra nevada, deposit, glacier, particle, valley, oceanic crust, southern hemisphere, unit cell, outwash plain, point, echo sounder, oceanic, energy, coastal plain, plume tectonics, time, accreted terrain, frost wedging, bedrock, asthenosphere, abyssal fan, mount baker, wave, exponential expression, southwestern pacific, level, volcano, temperature, los angeles, floor, river, geologic, representative reactions, surface, stream, coherent mass, process, yellow-orange-red hue, island, capillary fringe, snow survives, granite, earth, flow, main component, eruption, large, range