The earth consists of air, water, and land. We recognize these more technically as the atmosphere, a gaseous envelope surrounding the earth; the hydrosphere, the waters filling the depressions and covering almost 71 percent of the earth; and the lithosphere, the solid part of the earth which underlies the atmosphere and hydrosphere.
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The Atmosphere. The atmosphere, or gaseous portion of the earth, extends upward for hundreds of miles above sea level. It is a mixture of nitrogen, oxygen, carbon dioxide, water vapor, and other gases (see Table 1).
|GAS||PERCENT BY VOLUME|
TABLE 1. Analysis of gases present in pure dry air. Notice that nitrogen and oxygen comprise 99 percent of the total volume of atmospheric gases.
Of great importance to humans, the elements of the atmosphere make life possible on our planet. Moreover, the atmosphere acts as an insulating agent to protect us from the heat of the sun and to shield us from the bombardment of meteorites, and it makes possible the evaporation and precipitation of moisture. The atmosphere is an important geologic agent and is responsible for the processes of weathering which are continually at work on the earth’s surface.
The Hydrosphere. The hydrosphere includes all the waters of the oceans, lakes, and rivers, as well as ground water, glaciers, and ice – which exist within the lithosphere. Most of this water is contained in the oceans, which cover 70.8 percent of the earth’s surface to an average depth of about two and a half miles. The waters of the earth are essential to the existence of life and they are also of considerable geologic importance. Running streams and oceans are actively engaged in eroding, transporting, and depositing sediment. Indeed, running water, working in conjunction with atmospheric agents, has been the major force in forming the earth’s surface features throughout geologic time. The geologic work of the hydrosphere is discussed in some detail in later chapters of this book.
The Lithosphere. Of prime importance to the geologist is the lithosphere. This, the solid portion of the earth, is composed of rocks and minerals which, in turn, comprise the continental masses and ocean basins. The rocks of the lithosphere are of three basic types, igneous, sedimentary, and metamorphic. Igneous rocks were originally in a molten state but have since cooled and solidified to form rocks such as granite and basalt. Sedimentary rocks are formed from sediments (fragments of pre-existing rocks) deposited by wind, water, or ice. Limestone. sandstone, and clay are typical of this group. The metamorphic rocks have been formed from rocks that were originally sedimentary or igneous in origin. This transformation takes place as the rock is subjected to great physical and chemical change. Marble, which in its original form was limestone, is an example of a metamorphic rock. Most of what we know about the lithosphere has been learned through the study of the surface materials of the earth. However, by means of deep bore holes and seismological studies, geologists have gathered much valuable information about the interior of the earth. Additional geologic data are derived from rocks which were originally buried many miles beneath the ground but have been brought to or near the surface by violent earth movements and later exposed by erosion.
The Biosphere. Thanks to the life-giving qualities of air and water, Earth is populated by countless species of plants and animals. This horde of organisms comprises the biosphere-an organic realm of the earth that is intricately interrelated with the atmosphere, hydrosphere, and lithosphere. Plants and animals are involved in a number of earth processes. Moreover, such valuable economic products as coal and petroleum have been formed from the remains of prehistoric organisms. Many rocks, especially certain limestones, are also of organic origin. In addition, there are bacteria that play a key role in the development of certain types of iron ore, and perhaps in the formation of petroleum. Our interest in the biosphere reaches far back into the past. Geologists study the record of prehistoric life as revealed by fossils in an attempt to learn more about the history of the earth. They have also used these millions-of-years-old plant and animal remains to trace the development of life on earth. The spheres of the earth are rather clearly defined and each has its own special role in the overall composition of the planet. However, the boundaries between them are not always as distinct. Instead, they are continually intermingling as air touches rock, rock comes in contact with water, and water mixes with air. There is constant interaction between Earth’s spheres of matter, and important changes occur at the interface, or boundary, between them. These changes have been taking place for billions of years and will probably continue for billions more.