sedimentary rock: see rock; sediment.

rock temple: see temple.

rock salt: see sodium chloride.

rock rabbit: see pika.

rock music, type of music originating in the United States in the mid-1950s and increasingly popular throughout much of the world.

Origins of Rock

Essentially hybrid in origin, rock music includes elements of several black and white American music styles: black guitar-accompanied blues; black rhythm and blues, noted for saxophone solos; black and white gospel music; white country and western music; and the songs of white popular crooners and harmony groups. Emerging in 1954-55, rock music was initially referred to as "rock 'n' roll." After 1964 it was simply called "rock music." The change in terminology indicates both a continuity with and a break from the earlier period; rock music was no longer just for dancing. After 1964 the music was influenced by British groups such as the Beatles.

The 1950s—Bill Haley and Rock 'n' Roll

The first rock 'n' roll record to achieve national popularity was "Rock Around the Clock" made by Bill Haley and the Comets in 1955. Haley succeeded in creating a music that appealed to youth because of its exciting back beat, its urgent call to dance, and the action of its lyrics. The melody was clearly laid down by electric guitar; the lyrics were earthy and simple. Haley abruptly ended the ascendancy of the bland and sentimental ballads popular in the 1940s and early 50s. He also succeeded in translating black rhythm and blues into a form that adolescent white audiences could understand.

Blues, and rhythm and blues, were too adult, sexual, angry, and solely identified with black culture to be acceptable either emotionally or commercially without adaptation. Major record companies had for years been producing records for black audiences called "race records." The emergence of rock 'n' roll signified a slight weakening in resistance to black culture. The unadulterated black rock 'n' roll that Haley transformed can be heard in the sexually adult work of such artists as Hank Ballard and the Midnighters ("Work with Me, Annie") or "Big" Joe Turner ("Shake, Rattle, and Roll"), the latter song adapted by Haley for white audiences and the former transformed into "Dance with Me, Henry."

Rock 'n' roll was for and about adolescents. Its lyrics articulated teenage problems: school, cars, summer vacation, parents, and, most important, young love. The primary instruments of early rock 'n' roll were guitar, bass, piano, drums, and saxophone. All aspects of the music—its heavy beat, loudness, self-absorbed lyrics, and raving delivery—indicated a teenage defiance of adult values and authority. Influential performers of the 1950s include Chuck Berry ("Johnny B. Goode"), Little Richard ("Good Golly Miss Molly"), Sam Cooke ("You Send Me"), Buddy Holly ("Peggy Sue"), Jerry Lee Lewis ("Great Balls of Fire"), and Carl Perkins ("Blue Suede Shoes").

The Late 1950s and Early 60s—Elvis, Motown, and the British Invasion

The greatest exponent of rock 'n' roll from 1956 to 1963 was Elvis Presley, a truck driver and aspiring singer from Tupelo, Miss., whose plaintive, wailing, dynamic delivery and uninhibited sexuality appealed directly to young audiences while horrifying older people. As rock 'n' roll became a financial success, record companies that had considered it a fad began to search for new singers; they generally succeeded in commercializing the music, robbing it of much of its gutsy, rebellious quality. In the late 1950s, for example, there was a fad for sentimentally morbid songs such as "Laura" and "Teen Angel."

At the turn of the decade Detroit became an important center for black singers, and a certain type of sound known as "Motown" [motor town], named for Motown Records, developed. The style is characterized by a lead singer singing an almost impressionistic melody story line to the accompaniment of elegant, tight, articulate harmonies of a backup group. Popular exponents of this style are the Temptations, Smokey Robinson and the Miracles, Diana Ross and the Supremes, and Gladys Knight and the Pips.

Rock music again surged to popularity in 1962 with the emergence of the Beatles, a group of four long-haired lads from Liverpool, England. They were initially acclaimed for their energy and appealing individual personalities rather than for any innovations in their music, which was derived from Berry and Presley. Their popularity inevitably produced other groups with unusual names. One of the most important of these was the Rolling Stones, whose music derived from the black blues tradition. These British bands instigated a return to the blues orientation of rock 'n' roll, albeit in ever louder and more electric reincarnations.

The Late 1960s and Early 70s—Rock's Golden Age

Folk Rock

An important transformation of rock occurred in 1965 at the Newport Folk Festival when Bob Dylan, noted as a composer and writer of poetic folk songs and songs of social protest like "Blowin' in the Wind," appeared, playing electric guitar and backed by an electrified rock band. A synthesis of the folk revival and rock subsequently took place, with folk groups using rock arrangements and rock singers composing poetic lyrics for their songs (e.g., the Beatles' "Norwegian Wood," "Eleanor Rigby"). The Byrds' arrangement of Dylan's "Mr. Tambourine Man" is a folk-rock classic. Performers like the Mamas and the Papas; Peter, Paul, and Mary; Donovan; and the Lovin Spoonful sang a kind of music designated "folk rock."

Protest Songs and the Drug Culture

In the 1960s music mirrored the tensions of the Vietnam War era and played an important role in American culture. The verbal content of rock songs turned toward rebellion, social protest, sex, and, increasingly, drugs. Many groups, among them Jefferson Airplane and the Grateful Dead, tried to approximate in music the aural experience of psychedelic drugs, producing long, repetitive, occasionally exquisite songs with surreal lyrics (known as "acid rock" or "hard rock").

In 1967 the Beatles again made history with their album Sgt. Pepper's Lonely Hearts Club Band, which, in addition to including drug-oriented songs, presented a body of interrelated pieces that constituted an organic whole. This is considered the first "concept album." Subsequent products of this trend were rock musicals such as Hair (1968) and rock operas like Tommy, composed and sung by the Who.

Rock Comes of Age

By the late 1960s rock was widely regarded as an important musical form. Musicians such as Miles Davis and John McLaughlin and groups like Traffic or Blood, Sweat, and Tears tried to fuse rock and jazz, while such disparate artists as Leonard Bernstein and Frank Zappa attempted to connect rock and classical music. Groups featuring virtuoso guitarists such as Jimi Hendrix, Eric Clapton, Duane Allman, and Jimmy Page continued to perform variations on classic blues themes using the traditional instruments of rock 'n' roll.

From 1967 onward, the rock festival was regarded as the ideal context in which to hear rock music, and thousands of fans attended. The most successful and peaceful rock festival, Woodstock, was held near Bethel, N.Y., in Aug., 1969. Later, however, a similar event, featuring the Rolling Stones, was held at Altamont, Calif., and was marked by several violent incidents caught on film, including a murder. By 1970 several of rock's top performers—Janis Joplin, Jim Morrison, and Jimi Hendrix—were dead from substance abuse. The dangerous, androgynous quality projected by the Rolling Stones was taken to extremes by performers such as Alice Cooper and David Bowie, who were perhaps as famous for their sexual ambiguity and outrageous behavior as for their music.

The Late 1970s to the Present—Punk Rock, the Music Video, and Middle-aged Rockers

A turning point in rock music occurred in the mid-1970s in the form of punk rock, which was a response to the stagnation of the genre and a nihilistic political statement. The music was filled with contempt for previous styles; its fast-tempoed songs, usually propelled by electric guitar, featured irreverant lyrics often obscured by the clangerous music. Evident in Great Britain, performed by such bands as the Sex Pistols and the Clash, punk also quickly became popular in the United States, played by the Ramones and other American groups. By the early 1980s, it had changed rock music considerably as Black Flag, the Dead Kennedys, and other groups adopted political protest themes as the core of their music.

During the 1980s music videos became a popular form of promotion and entertainment. In the late 1980s, however, several bands, including Nirvana and Pearl Jam, continued to follow the path of early punk rock by focusing on political themes and celebrating their own lack of technical virtuosity. Punk persisted into the 1990s with such bands as Green Day and the Offspring. Also in the 90s the continuing popularity of older bands, such as the Grateful Dead and the Rolling Stones, bore witness to the enduring appeal of this form among both the young and the increasingly middle-aged. The appeal of older and past rock bands was also evident in the fanfare surrounding the opening (1995) of the Rock and Roll Hall of Fame in Cleveland, Ohio. Meanwhile, a young audience made rap music, with its pounding rhythms and strong, sometimes shocking spoken lyrics, a popular phenomenon, and other young rockers, largely club-goers, made the dance-based, electronically sophisticated techno another, though less pervasive, popular form.

rock garden, garden planned around natural rock formations or rocks artificially arranged to simulate natural (often mountainous) conditions. The concept of rock gardens is believed to have been introduced from China and Japan into the Western world in the 17th cent.; they have since gained wide popularity as an ideal method for the cultivation of mountain flora and for beautifying hilly, stony, or other awkward terrain. Rock plants usually have long roots that enable them to obtain moisture even when the surface is hot and dry. Low plants requiring well-drained conditions are suited to rock gardens: besides alpine plants, these include stonecrops and species of columbine, phlox, bluebell, and rockrose.

rock crystal: see quartz.

rock crawler, name applied to the slender, wingless insects of the family Grylloblattidae in the order Orthoptera. They have long antennae and range in length from 1/2 to 1 in. (15-30 mm). Rock crawlers occur at altitudes of 1,500 to 6,500 ft (450-2,000 m), where they live in caves, under rocks, or on snow and ice; they thrive at temperatures just above freezing. They appear to be nocturnal and omnivorous. Rock crawlers were not discovered until 1911, and only five species are now known in the United States. Other species occur in Canada, Japan, and Siberia. They are very primitive insects, considered closely related to the basic stock from which the Orthoptera (including the cockroaches, mantids, and locusts) arose. Rock crawlers are classified in the phylum Arthropoda, class Insecta, order Orthoptera, suborder Grylloblattodea, family Grylloblattidae.

rock carvings and paintings, designs inscribed on rock surfaces and huge stone monuments in many parts of the world by primitive peoples. They have been found on every continent and are usually from prehistoric times. Petroglyphs (rock carvings) are more widespread than pictographs (rock paintings), which are preserved chiefly in dry regions, inside caves, and under overhanging cliffs. It is thought that these designs were created for purposes of religious propitiation and sympathetic magic. Whatever the motive, the prehistoric artist often reached great aesthetic heights, as in the Paleolithic art of Western Europe, the rock figures attributed to the San of S Africa, and the Tassili cliff paintings discovered in the central Sahara that suggest that this was once a fertile area. Similar evidence was found in the Alps of N Italy. Successive styles and phases were found, and several layers of designs were often superimposed. Wild animals and hunting scenes abound, while the scenes of daily life were depicted alongside representations of ceremonies and deities. In Neolithic times herders and cows appeared, but rock art seems to have declined and disappeared with the advent of agriculture. In Europe and Africa the style was largely naturalistic, while in Australia and the Americas designs were more often symbolic and geometric, and sometimes approached a primitive form of writing. Carvings were usually incised or chipped out with a stone. Sometimes they were deeply gouged out in intaglio technique. The paintings, made with charcoal and earth pigments mixed with grease, gum, or water, vary from crude outlines to fully developed polychrome compositions. Engraving and painting techniques were sometimes combined. Stenciled human hands were found in numerous places.

rock and roll: see rock music.

rock 'n' roll: see rock music.

rock, aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.

Igneous Rocks

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

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

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.

igneous rock: see rock.

cock-of-the-rock: see cotinga.

calcareous rock: see limestone; marble.

White Rock, city (1991 pop. 16,314), SE British Columbia, Canada, on Georgia Strait and on the U.S. border. The city is a customs port and resort center with a residential area.

Steep Rock Lake, 4 mi (6.4 km) long and 3 mi (4.8 km) wide, SW Ont., Canada, W of Lac des Mille Lacs. It is the site of important iron ore mining. Part of the lake was drained to facilitate the mining, and the flow of the Seine River was diverted.

Starved Rock, cliff, 140 ft (43 m) high, overlooking the Illinois River between La Salle and Ottawa, N Ill. Legend says that in the 18th cent. the Ottawas drove a band of Illinois onto the cliff, where they died of thirst and starvation. Later, brigands and outlaws found refuge nearby in canyons and caves that were included in Starved Rock State Park (est. 1912). Starved Rock has been designated a national historic landmark.

Rock Springs, city (1990 pop. 19,050), alt. c.6,270 ft (1,910 m), Sweetwater co., SW Wyo., on Bitter Creek; inc. 1888. It is a cattle and sheep shipping point and the center of large natural trona mines that produce soda ash. Oil and gas production, electric-power distribution, a revived coal industry, food processing, a chemical industry, and tourism are also important. Rock Springs is the gateway to many recreational areas and natural wonders, and an annual rodeo is held there. It was settled around a trading post and stage station established on the Overland Trail in the 1860s.

Rock River, c.285 mi (460 km) long, rising in SE Wisc. and flowing SW through NW Ill. to the Mississippi River near Rock Island. It flows through a fertile farm area. Rockford, Ill., is the chief city on the river. Lake Koshkonong (c.20 sq mi/50 sq km) is a natural widening of the river.

Rock Island, town (1991 pop. 1,067), S Que., Canada, at the Vermont border. It is the seat of Stanstead College. Machine tools, dies, and textiles are made.

Rock Island, city (1990 pop. 40,552), seat of Rock Island co., NW Ill., on the Mississippi and Rock rivers, adjacent to Moline and opposite Davenport, Iowa; inc. 1841. These three cities, with Bettendorf, Iowa, are called the Quad Cities. Rock Island is a trade, transportation, and industrial center. Machinery, metal and wood products, and clothing are among its many manufactures. Railroad maintenance is also an important industry. Rock Island is the seat of Augustana College, and Black Hawk State Park adjoins the city. On a 1,000-acre (405-hectare) island in the Mississippi River is a huge U.S. arsenal (est. 1862), one of the largest manufacturing arsenals in the world and a major source of local employment. Arsenal Island was fortified by the British in the War of 1812 and by the Americans in 1816. In 1833, George Davenport built a palatial house there that still stands. During the Civil War, Arsenal Island was the site of a large northern military prison. A Confederate cemetery and a national cemetery are there. Bridges connect Arsenal Island, Rock Island, Davenport, and Moline.

Rock Hill, city (1990 pop. 41,643), York co., N S.C.; inc. 1870. An important textile center, it also has industries producing paper, glass, metal products, pharmaceuticals, chemicals, textiles, and food. A beautifully landscaped flower garden is in the center of the city. Rock Hill is the seat of Winthrop Univ. The county fair and a county horse show are held there annually. A 25-mi (40-km) long impoundment of the nearby Catawba River provides water recreation.

Rock Falls, city (1990 pop. 9,654), Whiteside co., NW Ill., on the Rock River opposite Sterling; inc. 1867. It is an industrial center in a farm region with corn, soybeans, cattle, and hogs. Electronic equipment, feeds, and plastics are manufactured.

Rock Creek Park: see National Parks and Monuments (table).

Red Rock chicken, the only chicken still popular to any large extent in the United States today for both meat and eggs. It resulted from a cross between a Rhode Island Red male and a Plymouth Rock female. The breed recently acquired new value when it was discovered that the sex of the day-old chicks could be determined by the differences in the color of their down. The variety is raised primarily in New England.

Plymouth Rock chicken, one of the most popular early breeds of poultry. The Barred Plymouth Rock was a favorite farm chicken since it was both a good egg producer and also developed a large quantity of meat. As the demand for white eggs increased, the breed lost popularity. It is still retained for breeding purposes, a popular cross being that of a Barred Plymouth Rock female and a Rhode Island Red male. See Red Rock chicken.

Percé Rock, E Que., Canada, just off the tip of the Gaspé Peninsula. It is a massive rock (1,420 ft/433 m long; c.300 ft/90 m wide; and 290 ft/88 m at the highest point), rising sheer from the Atlantic. It takes its name from an arch 50 ft (15 m) high near its seaward end. With nearby Bonaventure Island, it is a well-known tourist attraction.

North Little Rock, city (1990 pop. 61,741), Pulaski co., central Ark., on the Arkansas River opposite Little Rock; settled c.1856, inc. as a city 1903. North Little Rock lies in a cotton, rice, soybean, dairy-cattle, and truck-farm area. There is food processing and printing and the manufacture of consumer goods, fiberglass and electronic products, building materials, hospital garments, bakery products, feed, furniture, fertilizers, and chemicals. In the early 19th cent. the discovery of a small silver vein drew settlers to the area, which was then called Silver City. Most of the area later became part of Little Rock, but in 1903 local citizens pushed a bill through the Arkansas legislature permitting a part of Little Rock to secede and join the small village of North Little Rock. Nearby is Camp Joseph T. Robinson, the headquarters of the Arkansas National Guard.

Little Rock Central High School National Historical Site: see National Parks and Monuments (table).

Little Rock, city (1990 pop. 175,795), state capital and seat of Pulaski co., central Ark., on the Arkansas River; inc. 1831. It is a river port and the administrative, commercial, transportation, and cultural center of the state. The city's industries process agricultural products, fish, beef, poultry, and bauxite and timber. Its manufacturing industries are closely related with those of North Little Rock across the river.

The settlement was a well-known river crossing when Arkansas Territory was established in 1819. It became territorial capital in 1821 and state capital when Arkansas entered the Union in 1836. In the Civil War the battle of Little Rock (1863) was fought there. The city became a center of world attention in 1957, when federal troops were sent there to enforce a 1954 U.S. Supreme Court ruling against segregation in the public schools.

Little Rock is the seat of Philander Smith College, Arkansas Baptist College, the Univ. of Arkansas at Little Rock, and several other branches of the university, including the law and medical schools. Of interest are the beautiful Old State House, which served as capitol from 1836 to 1910 and is now a museum; several other museums, including the Arkansas Arts Center; and the Clinton presidential library. The present capitol building was built in 1911. The city also contains several state institutions and has a noteworthy symphony orchestra. Little Rock Air Force Base is in nearby Jacksonville.

Glen Rock, borough (1990 pop. 10,883), Bergen co., NE N.J., a residential suburb of New York City; settled c.1710, inc. 1896. George Washington's army used the area for camping grounds during the Revolutionary War.

Fastnet Rock, islet, Co. Cork, S Republic of Ireland. Its 160-ft (49-m) lighthouse is near the most southerly point of Ireland. It is the marker for the furthest leg of the Fastnet Cup, a yacht race in the Admirals Cup.

Dome of the Rock: see Islamic art and architecture.

Black Rock Desert, arid region of lava beds and alkali flats, NW Nev., in Toiyabe National Forest, stretching c.70 mi (110 km) NE from Gerlach. The Jackson Mts. rise to the east; the Black Rock Range to the west. A land speed record was set there in 1983. The desert has been the site of the Burning Man festival, a self-styled temporary community and weeklong arts festival that attracts thousands, since 1991. Smoke Creek Desert, to the SW of Gerlach, is an extension of Black Rock.

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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Any of numerous round-eared, tailless members (genus Ochotona, family Ochotonidae) of the rabbit order (Lagomorpha), found in Asia, eastern Europe, and parts of western North America. Though not hares, they are sometimes called mouse hares. The hind legs are less developed than a rabbit's; pikas scamper rather than bound. Their brownish or reddish fur is soft, long, and thick. Most pikas weigh between 4.5 and 7.1 oz. (125 and 200 g) and are about 6 in. (15 cm) long. Many species live in rocky, mountainous areas, but some Asian species inhabit burrows. Pikas do not hibernate, but in summer and autumn they “harvest” vegetation and store it in protected places (e.g., under rocks) to be eaten in winter.

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or rock and roll

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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Transparent variety of the silica mineral quartz that is valued for its clarity and total lack of colour or flaws. Rock crystal formerly was used extensively as a gemstone, but it has been replaced by glass and plastic; rhinestones originally were quartz pebbles found in the Rhine River. The optical properties of rock crystal led to its use in lenses and prisms; its piezoelectric properties (see piezoelectricity) are used to control the oscillation of electrical circuits.

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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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or phosphate rock

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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Division of igneous rocks on the basis of their silicate mineral content, these minerals usually being the most abundant in such rocks. Rocks are described as felsic, intermediate, mafic, and ultramafic, in order of decreasing silica content, and, in general, the gradation from felsic to mafic corresponds to an increase in colour (i.e., light to dark). Due to the erroneous belief that silica present in rock magmas occurred in the form of silicic acid, high- and low-silica rocks were once known as acid and basic rocks, respectively.

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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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or Mosque of Omar

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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