rock, aggregation of solid matter composed of one or more of the minerals mineral, inorganic substance occurring in nature, having a characteristic and homogeneous chemical composition, definite physical properties, and, usually, a definite crystalline form. A few of the minerals (e.g.
..... Click the link for more information. 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 batholith, enormous mass of intrusive igneous rock, that is, rock made of once-molten material that has solidified below the earth's surface (see rock ). Batholiths usually are granitic (see granite ) in composition, have steeply inclined walls, have no visible
..... Click the link for more information. , 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 granite, coarse-grained igneous rock of even texture and light color, composed chiefly of quartz and feldspars. It usually contains small quantities of mica or hornblende, and minor accessory minerals may be present.
..... Click the link for more information. , syenite, diorite, gabbro, peridotite, and pyroxenite), while others (e.g., basalt basalt (bəsôlt`, băs`ôlt)
..... Click the link for more information. , 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 porphyry (pôr`fərē)
..... Click the link for more information. ). 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 pumice (pŭm`ĭs), volcanic glass formed by the solidification of lava that is permeated with gas bubbles.
..... Click the link for more information. , 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 sediment, mineral or organic particles that are deposited by the action of wind, water, or glacial ice. These sediments can eventually form sedimentary rocks (see rock ).
..... Click the link for more information. 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 stratification (Lat.,=made in layers), layered structure formed by the deposition of sedimentary rocks . Changes between strata are interpreted as the result of fluctuations in the intensity and persistence of the depositional agent, e.g.
..... Click the link for more information. ; 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 sandstone, sedimentary rock formed by the cementing together of grains of sand. The usual cementing material in sandstone is calcium carbonate, iron oxides, or silica, and the hardness of sandstone varies according to the character of the cementing material; quartz
..... Click the link for more information. , tillite, sedimentary breccia, shale shale, sedimentary rock formed by the consolidation of mud or clay, having the property of splitting into thin layers parallel to its bedding planes. Shale tends to be fissile, i.e., it tends to split along planar surfaces between the layers of stratified rock.
..... Click the link for more information. , marl marl or bog lime, soil, essentially clay mixed with carbonate of lime, highly valued as a dressing or fertilizer. It crumbles rapidly and easily. Marl in which the lime is in the form of invertebrate shells is called shell marl.
..... Click the link for more information. , chalk chalk, mineral of calcium carbonate , similar in composition to limestone, but softer. It is characteristically a marine formation and sometimes occurs in great thickness; the chief constituents of these chalk deposits are the shells of minute animals called
..... Click the link for more information. , limestone limestone, sedimentary rock wholly or in large part composed of calcium carbonate. It is ordinarily white but may be colored by impurities, iron oxide making it brown, yellow, or red and carbon making it blue, black, or gray. The texture varies from coarse to fine.
..... Click the link for more information. , coal coal, fuel substance of plant origin, largely or almost entirely composed of carbon with varying amounts of mineral matter.

Types

There is a complete series of carbonaceous fuels, which differ from each other in the relative amounts of moisture,
..... Click the link for more information. , lignite lignite (lĭg`nīt) or brown coal,
..... Click the link for more information. , gypsum gypsum (jĭp`səm), mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO₄
..... Click the link for more information. , 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 metamorphism, in geology, process of change in the structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock being changed remains
..... Click the link for more information. ). Some (e.g., marble marble, metamorphic rock composed wholly or in large part of calcite or dolomite crystals, the crystalline texture being the result of metamorphism of limestone by heat and pressure.
..... Click the link for more information. and quartzite quartzite, usually metamorphic rock composed of firmly cemented quartz grains. Most often it is white, light gray, yellowish, or light brown, but is sometimes colored blue, green, purple, or black by included minerals.
..... Click the link for more information. ) 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 cleavage, tendency of many minerals to split along definite smooth planar surfaces determined by their crystal structure. The directions of these surfaces are related to weaknesses in the atomic structure of the mineral and are always parallel to a possible crystal
..... Click the link for more information. . Among the common metamorphic rocks are schist schist (shĭst), metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are
..... Click the link for more information. (e.g., mica schist and hornblende schist), gneiss gneiss (nīs), coarse-grained, imperfectly foliated, or layered, metamorphic rock .
..... Click the link for more information. , quartzite, slate slate, fine-grained rock formed when sedimentary rocks such as shale are metamorphosed by great pressure. Slate splits into perfectly cleaved, broad thin layers; this characteristically regular and planar cleavage is called slaty cleavage.
..... Click the link for more information. , and marble.

Bibliography

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).

rock

In geology, a naturally occurring and coherent aggregate of minerals. The three major classes of rock—igneous, sedimentary, and metamorphic—are based on the processes that formed them. These three classes are further subdivided on the basis of various factors, especially chemical, mineralogical, and textural attributes (see e.g., acid and basic rocks; crystalline rock; extrusive rock). See also felsic rock; intrusive rock; mafic rock.

rock

1. Geology any aggregate of minerals that makes up part of the earth's crust. It may be unconsolidated, such as a sand, clay, or mud, or consolidated, such as granite, limestone, or coal

2. short for rock salmon

3. Slang another name for crack

4. on the rocks

a. in a state of ruin or destitution

b. (of drinks, esp whisky) served with ice

Rock

1. the. an informal name for Gibraltar

2. the. a Canadian informal name for Newfoundland

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Igneous Rocks

Sedimentary Rocks

Types

Metamorphic Rocks

Bibliography

rock

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