Background (Encyclopedia of Global Resources)
The earth consists of a nested set of spheres of different composition and of decreasing density with distance from the center of the Earth. The crust is the outermost and lowest-density hard shell, significantly less dense (2.7 to 3.0 grams per cubic centimeter) than the underlying mantle (3.3 grams per cubic centimeter). The earth’s two distinct types of crust—continental and oceanic—differ in five fundamental aspects: thickness, density, composition, age, and mode of formation.
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Continental and Oceanic Crust (Encyclopedia of Global Resources)
Continental crust is generally found beneath the exposed parts of the Earth’s surface known as continents. In addition, continental crust is submerged and makes up the continental shelves and submerged continental platforms. Correspondingly, a larger proportion of the Earth’s surface is composed of continental crust (40 percent) than is exposed above sea level as continents (25 percent). Oceanic crust makes up the floor of the oceans; in rare cases it rises above sea level, such as in Iceland and Ethiopia. Our store of nonrenewable natural resources is produced and kept in the crust. hydrothermal systems associated with oceanic crust formation at mid-ocean ridges produce metal deposits. Nearly all economic ore deposits are extracted from the continental crust. Basins in the continental crust and along the continental margins are the principal sites for the formation and storage of oil and gas deposits.
Typical continental crust is about 40 kilometers thick, has a density of about 2.7 grams per cubic centimeter, and has a bulk composition similar to the volcanic rock andesite; it is about 60 percent silicon dioxide (SiO2). Continental crust as old as 4 billion years has been found, and 2.5 billion-year-old continental crust is common. The earth is about 4.5 billion years old, and continental crust from the Earth’s first 500 million years has not been preserved. This contrasts with the situation for...
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Metal and Hydrocarbon Deposits (Encyclopedia of Global Resources)
The two types of crust play different roles in the formation of nonrenewable natural resources such as metallic ores and hydrocarbons. Metallic ores are predominantly produced at divergent or convergent plate boundaries—that is, where oceanic crust is either produced or destroyed. Vast deposits of manganese and cobalt exist on the deep-sea floor in the form of manganese nodules. Hydrocarbon deposits form principally in basins on continental crust or beneath continental margins, at the boundary between oceanic and continental crust. The configuration of continents may also be important for controlling oil and gas deposits, because it can cause the formation of restricted basins where oxygen-poor waters allow organic matter to be preserved and buried. The relatively thin sedimentary sequences typically deposited on oceanic crust are not conducive to formation and preservation of hydrocarbon deposits.
The distribution of mineral and hydrocarbon resources is strongly controlled by the age of the crust and the sedimentary basins that these harbor. In spite of the fact that the oceanic crust is the principal factory for generating ore deposits, a minuscule proportion of these are presently exploited, largely for economic reasons. Because of its age and mode of formation, the continental crust acts as a warehouse for ore deposits produced over Earth’s history, especially those deposits produced at convergent plate...
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Resource Frontiers (Encyclopedia of Global Resources)
A wide range of mineral and hydrocarbon resources are sought on all continents except Antarctica. This search benefits increasingly from abundant technological resources, including satellite remote sensing, geophysical surveys, geochemical studies, and traditional field mapping, and from the tremendous increase in computing power available to process large and complex data sets. These nonrenewable resources are likely to be depleted in the future, leading to a rise in prices that will reward exploitation of “frontier” deposits. Resource frontiers pertaining to the Earth’s crust include mining and drilling for oil deeper below the continental surface, drilling for oil in deeper water offshore, the mining of deep-sea resources, and exploiting geothermal and hydrothermal resources for energy, including the tremendous heat energy stored in the deep continental crust and vented from hydrothermal sites along the midocean ridges.
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Further Reading (Encyclopedia of Global Resources)
Brown, Michael, and Tracy Rushmer, eds. Evolution and Differentiation of the Continental Crust. New York: Cambridge University Press, 2006.
Condie, Kent C. Earth as an Evolving Planetary System. Boston: Elsevier Academic Press, 2005.
Davis, Earl E., and Harry Elderfield, eds. Hydrogeology of the Ocean Lithosphere. New York: Cambridge University Press, 2004.
Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed. New York: Cambridge University Press, 2005.
Grotzinger, John P., et al. Understanding Earth. 5th ed. New York: W. H. Freeman, 2007.
Mathez, Edmond A., and James D. Webster. The Earth Machine: The Science of a Dynamic Planet. New York: Columbia University Press, 2004.
Rogers, John J. W., and M. Santosh. Continents and Supercontinents. New York: Oxford University Press, 2004.
Taylor, Stuart Ross, and Scott M. McLennan. The Continental Crust: Its Composition and Evolution, an Examination of the Geochemical Record Preserved in Sedimentary Rocks. Boston: Blackwell Scientific, 1985.
U.S. Geological Survey. The Earth’s Crust. http://earthquake.usgs.gov/research/structure/crust/index.php
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Overview (The Solar System)
The crust of the Earth is the outermost layer of the Earth. It is distinct from the region of rock lying beneath it, called the mantle, in that the rocks that comprise the crust have different compositions and a lower density. Continental crust is composed mostly of granitic rocks with densities around 2.7 grams per cubic centimeter, and oceanic crust is composed mostly of basaltic rocks with densities around 3.0 grams per cubic centimeter. In contrast, rocks from the upper mantle have densities around 3.3 grams per cubic centimeter, and probably are composed mostly of peridotite.
The rocks of the Earth’s crust are quite varied. They can be classified as belonging to one of three broad groups, depending on how they formed: igneous, sedimentary, and metamorphic. These three groups are parts of what is referred to as the rock cycle, which depicts the way in which rocks from each of these groups can provide the raw material to form rocks in any other group.
Igneous rocks are formed by cooling and Crystallization from molten material called lava (if on the surface) or Magma (if below the surface). Igneous rocks that cool and harden on the surface are said to be extrusive, and those that cool and harden below the surface are said to be intrusive. Intrusive igneous rocks cool more slowly and thus usually contain larger mineral crystals (large enough to be seen with the unaided eye). Extrusive igneous rocks cool more rapidly and...
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Methods of Study (The Solar System)
Seismic waves created by both earthquakes and artificial explosions are used to probe the interior of the Earth, including its top layer, the crust. One type of seismic wave that travels through the Earth is a longitudinal compressional wave called a P wave (for primary wave); this type of wave is analogous to an acoustic or sound wave, alternately compressing and stretching the material it travels through. The density, rigidity, and compressibility of the material determine the wave speed in it. P waves travel at speeds between about 2 and 6 kilometers per second near the surface, because of the wide range of compositions of surface rocks as well as the presence of open space and fluids within them. (For comparison, sound waves travel at about 0.3 kilometers per second through the lower atmosphere.) P-wave speed reaches about 6 to 7 kilometers per second in the lower crust just above the Moho. It has been found in the laboratory that metamorphic rocks known as granulites, which can form when basalts and gabbros are subjected to the pressures and temperatures of the lower crust, have P-wave speeds in the proper range. Furthermore, granulites are similar to some rock samples brought to the surface in volcanic pipes that are thought to have originated in the lower crust.
The thickness of continental crust has been determined by the study of seismic waves that are reflected off the Moho, as well as those that refracted by it....
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Context (The Solar System)
The Earth’s crust is in a state of dynamic evolution, with rock materials being created, deformed, and destroyed. This dynamic evolution of the crust is brought about by the movement of large lithospheric plates composed of the crust and upper mantle, up to 100 kilometers thick. Processes at plate boundaries can produce volcanism and earthquakes.
Volcanic activity over the billions of years of the Earth’s existence has provided the water vapor and other gases necessary to form the oceans and atmosphere by the release of gases trapped in lavas that reach the surface. An understanding of volcanoes, including how, why, and where they occur, requires an understanding of the Earth’s crust and crustal dynamics. It is especially important in the areas with active volcanoes to be able to assess the hazards they pose.
Plate motion also produces earthquakes where two plates rub against each other. Usually the strongest occur at transform plate boundaries and at subduction zones along convergent boundaries. Eventually, knowledge of how crustal rocks change and respond before impending earthquakes may allow their prediction.
Exploration for important economic minerals is guided by knowledge about the evolution and composition of the crust. The concentration of valuable metal deposits, such as gold and copper, occurs during volcanic activity at ocean ridge sites where new oceanic crustal rocks are being created. Consequently,...
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Further Reading (The Solar System)
Bally, A. W. Seismic Expression of Structural Styles. Tulsa, Okla.: American Association of Petroleum Geologists, 1983. An excellent visual treatment of the structure and layering primarily of the upper crust throughout the world. Sections into the crust of offshore Scotland and northwest Germany show the Moho. Suitable for a broad audience from general readers to scientific specialists.
Bott, M. H. P. The Interior of the Earth. New York: Elsevier, 1982. This book was intended for undergraduate and graduate students of geology and geophysics as well as for other scientists interested in the topic. The plate tectonic framework of the outer part of the Earth is strongly emphasized.
Brown, G. C. The Inaccessible Earth: An Integrated View to Its Structure and Composition. 2d ed. New York: Chapman and Hall, 1993. A good general introduction geared toward the undergraduate college student. The primary topics are the internal state and composition of the Earth. Included is background material on seismology and three chapters discussing the Earth’s crust.
Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed. New York: Cambridge University Press, 2004. An updated version of a widely used textbook for introductory geophysics courses. Designed for students with some knowledge of physics and calculus.
Knapp, Ralph E. Geophysics. Exeter, England:...
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