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The Earth

The separation of the earth sciences into independent branches—such as geology, oceanography, and meteorology—was traditionally done for convenience. This made it easier to study a large and complex Earth. In the past, scientists in each branch studied their field without considering the earth as an interacting whole. Today, most earth scientists consider changes in the earth as taking place in an overall dynamic system. The parts of the earth's interior, the rocks on the surface, the oceans, the atmosphere, and the environmental conditions are today understood to be parts of a complex, interacting system with a cyclic movement of materials from one part to another.

How can materials cycle through changes from the interior to the surface, and to the atmosphere and back? The answer to this question is found in the unique combination of fluids of the earth. No other known planet has Earth's combination of (1) an atmosphere consisting mostly of nitrogen and oxygen, (2) a surface that is mostly covered with liquid water, and (3) an interior that is partly fluid, partly semifluid, and partly solid. Earth's atmosphere is unique both in terms of its composition and in terms of interactions with the liquid water surface. These interactions have cycled materials, such as carbon dioxide, from the atmosphere to the land and oceans of the earth. The internal flow of rock materials, on the other hand, produces the large-scale motion of the earth's continents and the associated phenomena of earthquakes and volcanoes. Volcanoes cycle carbon dioxide back into the atmosphere and the movement of land cycles rocks from the earth's interior to the surface and back to the interior again. Altogether, the earth's atmosphere, liquid water, and motion of its landmasses make up a dynamic cycling system that is found only on the planet Earth.

Earth also seems to be unique because there is life on Earth, but apparently not on the other planets. The cycling of atmospheric gases and vapors,waters of the surface, and flowing interior rock materials sustain a wide diversity of life on Earth. There are millions of different species of plants, animals, and other kinds of organisms. Yet, there is no evidence of even one species of life existing outside the earth. The existence of life on Earth must be related to Earth's unique, dynamic system of interacting fluids. This chapter is concerned with earth materials, the internal structure of the earth, the movement of landmasses across the surface of the earth, and how earth materials are recycled










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