ROCKS AND THEIR CLASSIFICATION 453 Table The three main classes of rock, their origins, and examples Class of rock Origin Examples Sedimentary Deposition of particles of pre existing rocks Chemical replacement and precipitation Heated by igneous intrusions Heated and subjected to high pressure by deep burial Volcanic eruptions Deep cooling of magma Gravel, sand, and mudstone Limestone dolomite and salt Hornfels Slate, schist, and gneiss Basalt, andesite, and rhyolite Granite, diorite, and gabbro Metamorphic Igneous weathered, eroded, transported, and deposited is discussed in detail elsewhere (see Weathering, Sedimentary Processes: Erosional Sedimentary Structures, Sedimentary Environments: Depositional Systems and Facies) Sediments possess a wide range of particle sizes, ranging from boulders to clay, and of chemical compositions, including silica, lime, and ferromagnesian volcanic detritus These parameters of particle size and composition are used to classify sedimentary rocks (see Sedimentary Rocks: Mineralogy and Classification) Sedimentary rocks commonly exhibit two properties that may be used to differentiate them from igneous and metamorphic rocks Where they crop out at the surface of the Earth, sedimentary rocks generally show stratification The strata indicate successive episodes of deposition Layering is usually absent from igneous rocks, as discussed above, but is found in some metamorphic rocks, as discussed below When examined under the microscope, sedimentary rocks are generally seen to consist of particles Void space (porosity) is commonly present between the constituent grains If there are interconnected pores, these give the rock permeability Permeability allows fluids to migrate through the rock and enables rock and soil to drain Fossils are found only in sedimentary rocks, some of which are, indeed, made up of nothing else Sedimentary rocks include sands, including terrigenous silica-rich sandstone, and carbonates, termed limestone Sediments composed of rounded pebbles are termed conglomerates, while those composed of angular clasts (fragments) are termed breccias Metamorphic Rocks Metamorphic rocks are so termed from the Greek meta-, meaning altered, and morphos-, meaning shape The word is applied to rocks, of whatever previous origin, that have been changed as a result of being exposed to high temperature, high pressure, or both Two types of metamorphism are recognized, regional metamorphism, and thermal or contact metamorphism When magma moves up through the crust it creates a metamorphic aureole in the adjacent cover rocks This is termed contact or thermal metamorphism By contrast, rocks that are subjected to high temperatures and pressures during deep burial are said to have undergone regional metamorphism Common thermally metamorphosed rocks include hornfels and quartzite Regionally metamorphosed rocks include schist and gneiss Thermally metamorphosed rocks are often crystalline and devoid of porosity They may be massive or layered, according to the character of the original rock Regionally metamorphosed rocks are also commonly crystalline and devoid of porosity They are, however, often banded The banding seldom resembles the regular stratification of sediments, but often takes the form of curvaceous foliation and cleavage (see Regional Metamorphism) Rock Classification: A Simple Summary Igneous and metamorphic rocks are both normally composed of a mosaic of interlocking crystals that formed at high temperatures and pressures; they thus usually lack porosity and permeability In the petroleum industry igneous and metamorphic rocks are generally grouped together under the term ‘basement’, indicating that it is pointless to continue drilling deeper into such tight rocks Igneous rocks are generally massive and structureless, while metamorphic rocks are often banded Sediments, on the other hand, are deposited with space between the constituent particles Thus, most sedimentary rocks, by their very nature, tend to be porous and permeable Sediments are deposited episodically and thus characteristically exhibit stratification They are also the only one of the three main classes of rock to contain fossils Igneous, metamorphic, and sedimentary rocks may therefore be differentiated from one another using macroscopic structure and microscopic texture as shown in Figure Complications and Anomalies When the tripartite classification of rocks was introduced at the beginning of this article it was pointed out that, like all classifications, it has complications