Igneous rock originates from the cooling and solidification of molten matter from the earth's interior. If the rock is formed on the earth's surface (i.e., from the solidification of lava), it is called extrusive rock; igneous rock that has cooled and solidified slowly beneath the earth's surface is intrusive rock. Among the forms commonly taken by intrusive rocks are batholiths, which are enormous, irregular masses cutting or displacing older rocks; stocks, irregular and smaller than batholiths; necks, or plugs, columnar in form and probably the result of the hardening of magma in the necks of extinct volcanoes; dikes, more or less vertical, filling fissures in previously existing rock; sills, more or less horizontal, forced between layers of previously existing rock; and laccoliths, modified domelike sills that arch under the overlying rock.
Igneous rocks are commonly divided into classes by texture. Some rocks are markedly granular (e.g., granite, syenite, diorite, gabbro, peridotite, and pyroxenite), while others (e.g., basalt, trachite, dacite, and andesite) are composed of grains visible only under a microscope. Both fine-grained and coarse-grained igneous rocks frequently contain grains called phenocrysts that are larger than the surrounding grains; such rocks are said to be porphyritic in texture (see porphyry). Rocks with grains of uniform size are called equigranular.
Igneous rocks are commonly light in color if their constituent minerals are predominantly alkali feldspars and dark in color if the feldspars are calcic or if magnesia and iron minerals are abundant. The glassy igneous rocks include obsidian, pitchstone, and pumice, which contain few or no phenocrysts, and vitrophyre, or glass porphyry, which does contain phenocrysts. Rocks such as tuff and volcanic breccia, which are formed from fragmental volcanic material, are sometimes grouped as pyroclastic rocks.
Sedimentary rocks originate from the consolidation of sediments derived in part from living organisms but chiefly from older rocks of all classes (ultimately the mineral elements are derived from igneous rocks alone). The sediments of inorganic origin are chiefly removed from older rocks by erosion and transported to the place of deposition; chemical precipitation from solution is a secondary cause of deposition of inorganic matter. Sedimentary rocks are commonly distinguished, according to their place of deposition, by a great variety of terms, such as continental, marine (i.e., oceanic), littoral (i.e., coastal), estuarine (i.e., in an estuary), lacustrine (i.e., lakes), and fluviatile, or fluvial (i.e., in a stream).
The characteristic feature of sedimentary rocks is their stratification; they are frequently called stratified rocks. Sedimentary rocks made up of angular particles derived from other rocks are said to have a clastic texture, in contrast to pyroclastic sediments, which are particles of volcanic origin. Among the important varieties of sedimentary rock, distinguished both by texture and by chemical composition, are conglomerate, sandstone, tillite, sedimentary breccia, shale, marl, chalk, limestone, coal, lignite, gypsum, and rock salt. Characteristic occurrences in sedimentary rocks are fossils, footprints, raindrop impressions, concretions, oolites, ripple marks, rill marks, and crossbedding. Some of these features are useful in determining the antiquity of sedimentary formations and in interpreting geologic history.
Metamorphic rocks originate from the alteration of the texture and mineral constituents of igneous, sedimentary, and older metamorphic rocks under extreme heat and pressure deep within the earth (see metamorphism). Some (e.g., marble and quartzite) are massive in structure; others, and particularly those which have been subject to the more extreme forms of metamorphism, are characterized by foliation (i.e., the arrangement of their minerals in roughly parallel planes, giving them a banded appearance). A distinguishing characteristic of many metamorphic rocks is their slaty cleavage. Among the common metamorphic rocks are schist (e.g., mica schist and hornblende schist), gneiss, quartzite, slate, and marble.
See H. Blatt et al., Origin of Sedimentary Rocks (1972); A. F. Deeson, ed., The Collector's Encyclopedia of Rocks and Minerals (1973); N. Cristescu, Rock Rheology (1988).
Any igneous rock derived from magma that is poured out or ejected at the Earth's surface. Extrusive rocks are usually distinguished from intrusive rocks on the basis of their texture and mineral composition. Lava flows and pyroclastic debris (fragmented volcanic material) are extrusive; they are commonly glassy (e.g., obsidian) or finely crystalline (e.g., basalts and felsites).
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Any of a group of sedimentary rocks that consist largely or almost entirely of silicon dioxide (SiO2), either as quartz or as chert, the most common siliceous rock. It occurs in beds and in nodules. Bedded cherts may be an original organic or inorganic precipitate. Nodular cherts appear to be produced by the alteration of preexisting sedimentary rock. In this process silica distributed throughout the rock dissolves and reprecipitates to form nodules.
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Rock formed at or near the Earth's surface by the accumulation and lithification of fragments of preexisting rocks or by precipitation from solution at normal surface temperatures. Sedimentary rocks can be formed only where sediments are deposited long enough to become compacted and cemented into hard beds or strata. They are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust. Their defining characteristic is that they are formed in layers. Each layer has features that reflect the conditions during deposition, the nature of the source material (and, often, the organisms present), and the means of transport. Seealso sedimentary facies.
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Naturally occurring sodium chloride (NaCl), common or rock salt. Halite occurs on all continents, in beds that range from a few feet to more than 1,000 ft (300 m) in thickness. Termed evaporite deposits because they formed by the evaporation of saline water in partially enclosed basins, they characteristically are associated with beds of limestone, dolomite, and shale. Halite is found in large deposits in New York and in Russia, France, India, and Canada.
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American pika (Ochotona princeps).
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Musical style that arose in the U.S. in the mid-1950s and became the dominant form of popular music in the world. Though rock has used a wide variety of instruments, its basic elements are one or several vocalists, heavily amplified electric guitars (including bass, rhythm, and lead), and drums. It began as a simple style, relying on heavy, dance-oriented rhythms, uncomplicated melodies and harmonies, and lyrics sympathetic to its teenage audience's concerns—young love, the stresses of adolescence, and automobiles. Its roots lay principally in rhythm and blues (R&B) and country music. Both R&B and country existed outside the mainstream of popular music in the early 1950s, when the Cleveland disc jockey Alan Freed (1921–65) and others began programming R&B, which until then had been played only to black audiences. Freed's success gave currency to the term rock and roll. The highly rhythmic, sensual music of Chuck Berry, Bill Haley and the Comets, and particularly Elvis Presley in 1955–56 struck a responsive chord in the newly affluent postwar teenagers. In the 1960s several influences combined to lift rock out of what had already declined into a bland and mechanical format. In England, where rock's development had been slow, the Beatles and the Rolling Stones were found to have retained the freshness of its very early years and achieved enormous success in the U.S., where a new generation had grown up unaware of the musical influences of the new stars. At the same time, Bob Dylan, Joni Mitchell, the Byrds, and others were blending the traditional ballads and verse forms of folk music with rock, and musicians began to explore social and political themes. Performers such as the Grateful Dead, Jim Morrison of the Doors, and Frank Zappa of the Mothers of Invention combined imaginative lyrics with instrumental virtuosity, typically featuring lengthy solo improvisation. Janis Joplin and Jimi Hendrix won large followings with their exotic elaborations on R&B. The 1970s saw the rise of singer-songwriters such as Paul Simon, Neil Young, Elton John, David Bowie, and Bruce Springsteen, and rock assimilated other forms to produce jazz-rock, heavy metal, and punk rock. In the 1980s the disco-influenced rock of Madonna, Michael Jackson, and Prince was balanced by the post-punk “new wave” music of performers such as Laurie Anderson, Talking Heads (led by David Byrne), and the Eurythmics—all of whom illustrated their songs with music videos. By the 1990s rock music had incorporated grunge, rap, techno, and other forms.
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Tonguelike body of coarse rock fragments, found above the timberline on mountains, that moves slowly down a valley. The rock material usually has fallen from the valley walls and may contain large boulders; it resembles the material left at the end (terminus) of a true glacier. A rock glacier may be 100 ft (30 m) deep and nearly a mile (1.5 km) long.
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Rock crystal from the Dauphiné region of France.
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Aggressive form of rock music that coalesced into an international (though predominantly Anglo-American) movement in 1975–80. Originating in the countercultural rock of artists such as the Velvet Underground and Iggy (Pop) and the Stooges, punk rock evolved in New York City in the mid-1970s with artists such as Patti Smith and the Ramones. It soon took root in London—where distinctly “punk” fashions, including spiked hair and ripped clothing, were popularized—with bands such as the Sex Pistols and the Clash, and later in California, with X, Black Flag, and the Dead Kennedys. It is often marked by a fast, aggressive beat, loud guitar with abrupt chord changes, and nihilistic lyrics. Variants include new wave (more pop-oriented and accessible) and hardcore (characterized by brief, harsh songs played at breakneck speed); the latter continued to thrive through the 1990s.
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Rock with a high concentration of phosphates in nodular or compact masses. The phosphates may be derived from a variety of sources, including marine invertebrates that secrete shells of calcium phosphate and the bones and excrement of vertebrates. Typical phosphorite beds contain about 30percnt phosphorus pentoxide (P2O5) and constitute the primary source of raw materials for phosphate fertilizers. Significant deposits in the U.S. include the Phosphoria Formation in Idaho and the Monterey Formation in California. Major deposits also occur in the Sechura Desert in Peru.
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Any of a class of rocks that result from the alteration of preexisting rocks in response to changing geological conditions, including variations in temperature, pressure, and mechanical stress. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. The structure and mineralogy reflect the particular type of metamorphism that produced the rock and the composition of the parent rock. Metamorphic rocks are commonly classified by type of facies, predictable mineral assemblages associated with certain temperature and pressure conditions (see, e.g., granulite facies).
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In geology, any igneous rock dominated by the silicates pyroxene, amphibole, olivine, and mica. These minerals are high in magnesium and ferrous iron, and their presence gives mafic rock its characteristic dark colour. It is usually contrasted with felsic rock. Common mafic rocks include basalt and gabbro.
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Any of various crystalline or glassy, noncrystalline rocks formed by the cooling and solidification of molten earth material (magma). Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary rocks. Though they vary widely in composition, most igneous rocks consist of quartz, feldspars, pyroxenes, amphiboles, micas, olivines, nepheline, leucite, and apatite. They may be classified as intrusive or extrusive rocks.
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Any rock composed entirely of crystallized minerals without glassy matter (matter without visible crystals). Intrusive igneous rocks (see intrusive rock) are nearly always crystalline; extrusive igneous rocks (see extrusive rock) may be partly to entirely glassy. Metamorphic rocks are also always completely crystalline and are termed crystalline schists or gneisses. Sedimentary rocks can also be crystalline, such as crystalline limestones that precipitate directly from solution; the term is not generally applied to clastic sediments (made of fragments of preexisting rock), even though they are formed largely from the accumulation of crystalline materials.
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River, north-central U.S. It rises in southeastern Wisconsin and flows across the northwestern corner of Illinois, emptying into the Mississippi River at Rock Island, Ill.; it is 300 mi (480 km) long. The bottomlands along the lower course are subject to spring floods and require levee protection.
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City (pop., 2000: 183,133), capital of Arkansas, U.S., located on the Arkansas River. In 1722 Bernard de la Harpe, a French explorer, named the site La Petite Roche for a rock formation on the riverbank. It became the capital of Arkansas in 1821. It was strongly anti-Union at the outbreak of the American Civil War; Federal troops occupied the city in 1863. It grew as the commercial centre of a farming region and as a hub of railway and river transportation. In 1957 federal troops were sent there to prevent state authorities from interfering with desegregation at Central High School. The state's largest city, it has many institutions of higher learning, including the University of Arkansas at Little Rock (1927).
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Oldest existing Islamic monument. It is located on Temple Mount, previously the site of the Temple of Jerusalem. The rock over which it is built is sacred to both Muslims and Jews. In Islam, Muhammad is believed to have ascended into heaven from the site. In Judaism it is the site where Abraham prepared to sacrifice his son Isaac. Built in 685–91 as a place of pilgrimage, the octagonal building has richly decorated walls and a gold-overlaid dome mounted above a circle of piers and columns.
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