THE SCIENCE OF GEOLOGY
Geology is the study of the Earth, including the materials that 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. 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 gray granite of Yosemite Valley. Rocks, in turn, are composed of minerals. 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 the 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.
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?”
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. 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 is a mixture of gases, mostly nitrogen and oxygen. 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.