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rock,aggregation of solid matter composed of one or more of the mineralsmineral,
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 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 batholithsbatholith,
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 floors, and
..... 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., granitegranite,
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., basaltbasalt
, fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state.
..... 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 porphyryporphyry
, igneous rock composed of large, conspicuous crystals (phenocrysts) and a groundmass in which the phenocrysts are embedded. Some authorities consider the expression "porphyritic rock" better usage than porphyry, since the term refers only to the texture of the
..... 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 pumicepumice
, volcanic glass formed by the solidification of lava that is permeated with gas bubbles. Usually found at the surface of a lava flow, it is colorless or light gray and has the general appearance of a rock froth.
..... 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 originate from the consolidation of sedimentssediment,
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 stratificationstratification
(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, sandstonesandstone,
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 sandstones
..... Click the link for more information. , tillite, sedimentary breccia, shaleshale,
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. , marlmarl
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. , chalkchalk,
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 foraminiferans.
..... Click the link for more information. , limestonelimestone,
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. , coalcoal,
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, volatile matter,
..... Click the link for more information. , lignitelignite
or brown coal,
carbonaceous fuel intermediate between coal and peat, brown or yellowish in color and woody in texture. It contains more moisture than coal and tends to dry and crumble when exposed to the air; the flame is long and smoky and the heating power
..... Click the link for more information. , gypsumgypsum
, mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO4·2H2O. It is the most common sulfate mineral, occurring in many places in a variety of forms. A transparent crystalline variety is selenite.
..... 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 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 metamorphismmetamorphism,
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 essentially in the
..... Click the link for more information. ). Some (e.g., marblemarble,
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 quartzitequartzite,
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. It results from the metamorphism of pure quartz sandstone.
..... 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 cleavagecleavage,
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 face.
..... Click the link for more information. . Among the common metamorphic rocks are schistschist
, metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are visible to the naked eye. Schists are distinguished from the other foliated rocks, slates and gneisses, by the size of their mineral crystals; these are
..... Click the link for more information. (e.g., mica schist and hornblende schist), gneissgneiss
, coarse-grained, imperfectly foliated, or layered, metamorphic rock. Gneiss is characterized by alternating light and dark bands differing in mineral composition and having coarser grains than those of schist.
..... Click the link for more information. , quartzite, slateslate,
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.
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).
What does it mean when you dream about a rock?
Rocks usually embody stability and permanence, as signified by the expression “solid as a rock.” A large rock or boulder in a dream may indicate the dreamer is making a commitment to a relationship or contemplating some change that will provide a more solid foundation.