452 ROCKS AND THEIR CLASSIFICATION ROCKS AND THEIR CLASSIFICATION R C Selley, Imperial College London, London, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction: What on Earth is a Rock? To a geologist the term ‘rock’ is applied to all the solid materials of the Earth, whether they are hard enough to hit with a hammer or soft unconsolidated gravel, sand, or mud The nomenclature and classification of rocks is a prerequisite to understanding them One of several definitions of science is that it is organized knowledge It is the organization and classification of knowledge that provides insight into processes and adds predictive value The Periodic Table of the Elements illustrates these points admirably Classification is an integral part of geology in all its forms It extends from palaeontology, where the classification of fossils according to the binomial nomenclature of Linnaeus is a prerequisite to demonstrating evolution and understanding palaeoecology, to mineralogy, where minerals are grouped into like families according to their chemistry and crystallography The object of this chapter is to review the fundamental classification of rocks as a foundation for discussing igneous, sedimentary, and metamorphic rock nomenclature and classification (see Sedimentary Rocks: Mineralogy and Classification, Metamorphic Rocks: Classification, Nomenclature and Formation) Rocks are traditionally grouped into three main classes, according to their genesis These will now be defined and described All generalizations are dangerous, however, including this one Thus, a detailed examination of the classification (see Igneous Processes) of rocks soon reveals discrepancies and inconsistencies (see Metamorphic Rocks: Classification, Nomenclature and Formation) It may, therefore, be helpful to examine these subsequently discussed elsewhere in this encyclopaedia, magma may move up through the crust towards the surface It may cool very slowly beneath the surface to form coarsely crystalline rocks These include silica-rich rocks, such as granite (see Igneous Rocks: Granite), and ferromagnesian-rich rocks, such as gabbro If the magma reaches the surface, it erupts in volcanoes (see Volcanoes), and the melt cools quickly to form microcrystalline or even glassy rocks Depending on their chemistry and texture, volcanic rocks may be classified as silica-rich rhyolite or ferromagnesian-rich basalt Thus, owing to their mode of formation, igneous rocks tend to be structureless when viewed at outcrop or in a hand specimen Under the microscope they are seen to consist of a random arrangement of interlocking crystals of a range of different minerals, which formed when the melt crystallized Porosity is generally absent Sedimentary Rocks Sedimentary rocks are formed from the detritus of pre-existing rocks, which may be igneous, metamorphic, or sedimentary The way in which rock is A Classification of Rocks Rocks are of three genetic types: igneous (formed from cooling magma), sedimentary (formed by the breakdown of pre-existing rock), and metamorphic (formed by the action of heat and pressure on pre-existing rock) (Figure and Table 1) Igneous Rocks Magma is molten material from deep beneath the surface of the Earth Through a variety of processes Figure The Rocks Display’d An illustration of the modes of formation of sedimentary, igneous, and metamorphic rocks, ac companied by their classical deities Neptune, Vulcan, and Pluto (together with his synonymous dog) Reproduced by permission of The Geologists’ Association from Proceedings of The Geologists’ Association, Wilson G, in Read HH, Meditations on Granite 2, volume 55, pp 93 Fig ß 1944 The Geologists’ Association