Further Reading

Alvarez, Walter. T Rex and the Crater of Doom. Princeton,

N.J.: Princeton university Press, Princeton. 1997. Eldredge, N. Fossils: The Evolution and Extinction of Species. Princeton, N.J.: Princeton university Press. 1997.

stanley, steven m. Earth and Life Through Time New York: W. H. Freeman 1986.

crust Thin, low-density rock material called crust makes up the outer layer of the solid Earth, ranging in thickness from about three miles (five km) and less near the midocean ridges, to more than 50 miles (70 km) beneath the tallest mountain ranges. This is followed inward by the mantle, a solid rocky layer extending to 1,802 miles (2,900 km). The outer core is a molten metallic layer extending to 3,169-mile (5,100-km) depth, and the inner core is a solid metallic layer extending to 3,958 miles (6,370 km).

The temperature increases with depth with a gradient of 30°C per kilometer (139°F per mile) in the crust and upper mantle, and with a much smaller gradient deeper within the Earth. The heat of the Earth comes from residual heat trapped from initial accretion, radioactive decay, latent heat of crystallization of outer core, and dissipation of tidal energy of the sun-Earth-Moon system. Heat flows from the interior of the Earth toward the surface through convection cells in the outer core and mantle. The top of the mantle and the crust compose a relatively cold and rigid boundary layer, or lithosphere, which is about 65 miles (100 km) thick. heat escapes through the lithosphere largely by conduction or transport in igneous melts, and in convection cells of water through midocean ridges.

The Earth's crust is divisible broadly into continental crust of granodioritic composition and oceanic crust of basaltic composition. Continents make up 29.22 percent of surface, whereas continental crust underlies 34.7 percent of the Earth's surface, with continental crust under continental shelves accounting for the difference. The continents are in turn divided into orogens, made of linear belts of concentrated deformation, and cratons, making the stable, typically older interiors of the continents.

The distribution of surface elevation is strongly bimodal, as reflected in the hypsometric diagrams. Continental freeboard is the difference in elevation between the continents and ocean floor and results from difference in thickness and density between continental and oceanic crust, tectonic activity, erosion, sea level, and strength of continental rocks.

See also continental crust; craton; lithosphere; ocean basin.

crystal, crystal dislocations Crystals are homogeneous solid structures composed of chemical elements or compounds having a regularly repeating arrangement of atoms, as well as a large number of defects in the crystal lattice, such as dislocations. All minerals are solids, and in all minerals the atoms are arranged in a very regular geometric form that is unique to that mineral. Every mineral of that species has an identical crystalline structure. This regular structure gives each mineral its characteristic color, chemistry, hardness, and crystal form.

Many minerals may not have a well-developed external crystal form, but still must have a regularly repeating crystal lattice composed of the constituent atoms. Crystal forms and the internal atomic lattice exhibit symmetry, which may be of several different varieties. Crystals exhibit four main types of symmetry. In mirror plane symmetry, the simplest, the crystal can be divided by an imaginary mirror plane that would result in two halves that are mirror images of one another. Crystals may have symmetry about

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The seven basic types of crystal forms an axis that runs through the center of the crystal, in which the crystal lattice would be rotated into an identical configuration two, three, four, or six times in a 360° circuit. These symmetry systems are known as diads, triads, tetrads, and hexads. A more complex form of symmetry is known as roto inversion, characterized by a rotational operation followed by inversion of the lattice across its center point. Finally, a simple inversion across the center of the crystal leads to a crystal face diametrically opposite to every other crystal face. In various combinations of these symmetry operations, all crystals belong to one of seven crystal systems. These include cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal.

Crystals may appear to be close to perfect, but they always have millions of atomic defects. These include vacancies in the crystal lattice, various types of defects in the arrangement of the atoms and lattice, and replacements of one type of atom or ion by another with a similar charge and size.

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