igneous rock


Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
Related to igneous rock: Intrusive igneous rock

rock

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.

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).

The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. Licensed from Columbia University Press. All rights reserved.

igneous rock

A class of rock formed by change of the molten material to the solid state; generally termed granite if coarse-grained.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
References in periodicals archive ?
B is mainly igneous rock formed through consolidated molten magma from volcanic activities before the late Triassic or in the late to early Permian [22], which contains coarse-grained granite, textured with tough and rough surface.
365-355 Ma bimodal intrusive and extrusive alkalic igneous rocks in the Cobequid Highlands of central Nova Scotia (Dunning et al.
This age also suggests that some of the granitic clasts incorporated into the Del Raton Formation conglomerates come from the erosion of early Carboniferous igneous rocks related to Chanic magmatism.
(1) The Permian-age coal from the Haizi Mine of the Huaibei Coalfield is intruded by Yanshan Period igneous rock. Thermal alteration occurs down to 160 m (~1.33 x sill thickness).
Use the drawing tool to write Yosemite under Igneous Rock on the diagram.
Granite is an igneous rock, but its youthful enthusiasms and passions have long since cooled.
Diabase is a fine- to medium-grained, dark gray to black intrusive igneous rock. It is extremely hard and is commonly quarried for crushed stone, under the name of trap.
For example, his claim that granite was an igneous rock, that is, of volcanic origin, was eventually accepted, but his belief that phlogiston, a hypothetical substance, accounted for the heat of subterranean earth was rejected.
The K-Ar method is also useful for dating biotite, muscovite, hornblende, and feldspar in coarse-grained igneous rock bodies such as granite or gabbro that solidify far beneath Earth's surface.
Which is the only igneous rock that can float on water?
"That evokes the gravitational pull of planets." In another section, fiery, high-caliber tricks illustrate the volcanic explosion of igneous rock into lava.