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Earth is a dynamic planet that has continuously changed during its 4.6-billion-year existence. The size, shape, and geographic distribution of continents and ocean basins have changed through time; the composition of the atmosphere has evolved; and life-forms existing today differ from those that lived during the past. We can easily visualize how landscapes are changed by the forces of wind, water, and ice. Volcanic eruptions and earthquakes reveal an active interior, and folded and fractured rocks indicate the tremendous power of Earth's internal forces.
Earth consists of three concentric layers: the core, the mantle, and the crust. This orderly division results from density differences between the layers as a function of variations in composition, temperature, and pressure.
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The core has a calculated density of 10 to 13 grams per cubic centimeter (g/cm³) and occupies about 16%of Earth's total volume. Seismic (earthquake) data indicate that the core consists of a small, solid inner region and a larger, apparently liquid, outer portion. Both are thought to consist largely of iron and a small amount of nickel.
The mantle surrounds the core and comprises about 83%of Earth's volume. It is less dense than the core (3.3-5.7 g/cm³) and is thought to be composed largely of peridotite, a dark, dense igneous rock containing abundant iron and magnesium. The mantle can be divided into three distinct zones based on physical characteristics. The lower mantle is solid and forms most of the volume of Earth's interior. The asthenosphere surrounds the lower mantle. It has the same composition as the lower mantle but behave plastically and slowly flows. Partial melting within the asthensphere generates magma (molten material), some of which rises to the surface because it less dense than the rock from which it was derived. The upper mantle consists of the asthenphere and the overlying solid mantle rocks up to the base of the crust. The solid portion of the upper mantle and the overlying crust constitute the lithosphere, which is broken into numerous individual pieces called plates that move over the asthenphere as a result of underlying convection cells. Interactions of these plates are responsible for such phenomena as earthquakes, volcanic eruptions, adn the formation of mountain ranges and ocean basins.
Earth's plates are thought to move as a result of underlying mantle convection cells in which warm material from deep within Earth rises toward the surface, cools, and then, upon losing heat, descends back into the interior. The movement of these convection cells is though to be the mechanism responsible for the movement of Earth's plates, as shown in this diagrammatic cross section.
The crust , the outer most layer of Earth, consists of two types. Continental crust is thick (20-90 km), has an average density of 2.7 g/cm³, and contains considerable silicon and aluminum. Oceanic crust is thin (5-10 km), denser than continental crust (3.0g/cm³), and is composed of the igneous rock basalt.
Since the widespread acceptance of plate tectonic theory more than 25 years ago, geologists have viewed Earth from a global perspective in which all its subsystems and cycles are interconnected. Thus, the distribution of mountain chains, major fault systems, volcanoes, and earthquakes, the origin of new ocean basins, the movement of continents, and several other geologic processes and features are all interrelated.
From Physical Geology Exploring the Earth 4th Edition, James S. Monroe, Reed Wicander, Center Michigan University, brooks/cole
Department of Earth and Atmospheric Science, York University