Ingeology,rock (orstone) is any naturally occurring solid mass or aggregate ofminerals ormineraloid matter. It is categorized by the minerals included, itschemical composition, and the way in which it is formed. Rocks form the Earth's outer solid layer, thecrust, and most of its interior, except for the liquidouter core and pockets ofmagma in theasthenosphere. The study of rocks involves multiple subdisciplines of geology, includingpetrology andmineralogy. It may be limited to rocks found on Earth, or it may includeplanetary geology that studies the rocks of other celestial objects.
Rocks are usually grouped into three main groups:igneous rocks,sedimentary rocks andmetamorphic rocks. Igneous rocks are formed whenmagma cools in the Earth's crust, orlava cools on the ground surface or the seabed. Sedimentary rocks are formed bydiagenesis andlithification ofsediments, which in turn are formed by theweathering, transport, anddeposition of existing rocks. Metamorphic rocks are formed when existing rocks are subjected to such high pressures and temperatures that they are transformed without significant melting.
Humanity has made use of rocks since the time the earliest humans lived. This early period, called theStone Age, saw the development of many stone tools. Stone was then used as a major component in the construction of buildings and earlyinfrastructure.Mining developed to extract rocks from the Earth and obtain the minerals within them, includingmetals. Modern technology has allowed the development of new human-made rocks and rock-like substances, such asconcrete.
Geology is the study of Earth and its components, including the study of rock formations. Petrology is the study of the character and origin of rocks. Mineralogy is the study of the mineral components that create rocks. The study of rocks and their components has contributed to the geological understanding of Earth's history, thearchaeological understanding of human history, and the development ofengineering andtechnology in human society.[1]
While thehistory of geology includes many theories of rocks and their origins that have persisted throughout human history, the study of rocks was developed as a formal science during the 19th century.Plutonism was developed as a theory during this time, and the discovery ofradioactive decay in 1896 allowed for theradiometric dating of rocks. Understanding ofplate tectonics developed in the second half of the 20th century.[2]
Rocks are composed primarily of grains of minerals, which arecrystalline solids formed from atomschemically bonded into an orderly structure.[4]: 3 Some rocks also containmineraloids, which are rigid, mineral-like substances, such asvolcanic glass,[5]: 55, 79 that lack crystalline structure. The types and abundance of minerals in a rock are determined by the manner in which it was formed.
Most rocks containsilicate minerals, compounds that include silica tetrahedra in theircrystal lattice, and account for about one-third of all known mineral species and about 95% of theearth's crust.[6] The proportion ofsilica in rocks and minerals is a major factor in determining their names and properties.[7]
Rocks are classified according to characteristics such as mineral and chemical composition,permeability,texture of the constituent particles, andparticle size. These physical properties are the result of the processes that formed the rocks.[5] Over the course of time, rocks can be transformed from one type into another, as described by a geological model called therock cycle. This transformation produces three general classes of rock:igneous,sedimentary andmetamorphic.
These three classes are subdivided into many groups. There are, however, no hard-and-fast boundaries between allied rocks. By increase or decrease in the proportions of their minerals, they pass through gradations from one to the other; the distinctive structures of one kind of rock may thus be traced, gradually merging into those of another. Hence the definitions adopted in rock names simply correspond to selected points in a continuously graduated series.[8]
Igneous rock (derived from theLatin wordigneus, meaningof fire, fromignis meaningfire)[9] is formed through the cooling andsolidification ofmagma orlava. This magma may be derived from partial melts of pre-existing rocks in either aplanet'smantle orcrust. Typically, the melting of rocks is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.[10]: 591–599
Igneous rocks are divided into two main categories:
Volcanic orextrusive rocks result from magma reaching the surface either as lava orfragmental ejecta, forming minerals such aspumice orbasalt.[5]
Magmas tend to become richer in silica as they rise towards the Earth's surface, a process calledmagma differentiation. This occurs both because minerals low in silica crystallize out of the magma as it begins to cool (Bowen's reaction series) and because the magma assimilates some of the crustal rock through which it ascends (country rock), and crustal rock tends to be high in silica. Silica content is thus the most important chemical criterion for classifying igneous rock.[7] The content ofalkali metal oxides is next in importance.[11]
Sedimentary rocks are formed at the earth's surface by the accumulation and cementation of fragments of earlier rocks, minerals, and organisms[14] or as chemical precipitates and organic growths in water (sedimentation). This process causesclasticsediments (pieces of rock) ororganic particles (detritus) to settle and accumulate or for minerals to chemicallyprecipitate (evaporite) from asolution. The particulate matter then undergoes compaction andcementation at moderate temperatures and pressures (diagenesis).[5]: 265–280 [15]: 147–154
Before being deposited, sediments are formed byweathering of earlier rocks byerosion in a source area and then transported to the place of deposition bywater,wind,ice,mass movement orglaciers (agents ofdenudation).[5] About 7.9% of the crust by volume is composed of sedimentary rocks, with 82% of those being shales, while the remainder consists of 6% limestone and 12% sandstone andarkoses.[13] Sedimentary rocks often containfossils. Sedimentary rocks form under the influence of gravity and typically are deposited in horizontal or near horizontal layers orstrata, and may be referred to as stratified rocks.[16]
Sediment and the particles ofclastic sedimentary rocks can be further classified bygrain size. The smallest sediments areclay, followed bysilt,sand, andgravel. Some systems includecobbles andboulders as measurements.[17]
Metamorphic rocks are formed by subjecting any rock type—sedimentary rock, igneous rock or another older metamorphic rock—to differenttemperature andpressure conditions than those in which the original rock was formed. This process is calledmetamorphism, meaning to "change in form". The result is a profound change in physical properties and chemistry of the stone. The original rock, known as theprotolith, transforms into other mineral types or other forms of the same minerals, byrecrystallization.[5] The temperatures and pressures required for this process are always higher than those found at the Earth's surface: temperatures greater than 150 to 200 °C and pressures greater than 1500 bars.[18] This occurs, for example, whencontinental plates collide.[19]: 31–33, 134–139 Metamorphic rocks compose 27.4% of the crust by volume.[13]
The three major classes of metamorphic rock are based upon the formation mechanism. An intrusion of magma that heats the surrounding rock causes contact metamorphism—a temperature-dominated transformation. Pressure metamorphism occurs when sediments are buried deep under the ground; pressure is dominant, and temperature plays a smaller role. This is termed burial metamorphism, and it can result in rocks such asjade. Where both heat and pressure play a role, the mechanism is termed regional metamorphism. This is typically found in mountain-building regions.[7]
Depending on the structure, metamorphic rocks are divided into two general categories. Those that possess a texture are referred to asfoliated; the remainders are termed non-foliated. The name of the rock is then determined based on the types of minerals present.Schists are foliated rocks that are primarily composed oflamellar minerals such asmicas. Agneiss has visible bands of differinglightness, with a common example being the granite gneiss. Other varieties of foliated rock includeslates,phyllites, andmylonite. Familiar examples of non-foliated metamorphic rocks includemarble,soapstone, andserpentine. This branch containsquartzite—a metamorphosed form ofsandstone—andhornfels.[7]
Though most understanding of rocks comes from those of Earth, rocks make up many of the universe's celestial bodies. In theSolar System,Mars,Venus, andMercury are composed of rock, as are manynatural satellites,asteroids, andmeteoroids.Meteorites that fall to Earth provide evidence of extraterrestrial rocks and their composition. They are typically heavier than rocks on Earth. Asteroid rocks can also be brought to Earth through space missions, such as theHayabusa mission.[20]Lunar rocks andMartian rocks have also been studied.[21]
The use of rock has had a huge impact on the cultural and technological development of the human race. Rock has been used by humans and otherhominids for at least2.5 million years.[22]Lithic technology marks some of the oldest and continuously used technologies. The mining of rock for its metal content has been one of the most important factors of human advancement, and has progressed at different rates in different places, in part because of the kind of metals available from the rock of a region.
Anthropic rock is synthetic or restructured rock formed by human activity. Concrete is recognized as a human-made rock constituted of natural and processed rock and having been developed sinceAncient Rome.[23] Rock can also be modified with other substances to develop new forms, such asepoxy granite.[24]Artificial stone has also been developed, such asCoade stone.[25] Geologist James R. Underwood has proposed anthropic rock as a fourth class of rocks alongside igneous, sedimentary, and metamorphic.[26]
Rock varies greatly in strength, fromquartzites having atensile strength in excess of 300MPa[27] to sedimentary rock so soft it can be crumbled with bare fingers (that is, it isfriable).[28] (For comparison,structural steel has a tensile strength of around 350 MPa.[29]) Relatively soft, easily worked sedimentary rock was quarried for construction as early as 4000 BCE in Egypt,[30] and stone was used to build fortifications inInner Mongolia as early as 2800 BCE.[31] The soft rock,tuff, is common in Italy, and theRomans used it for many buildings and bridges.[32]Limestone was widely used in construction in the Middle Ages in Europe[33] and remained popular into the 20th century.[34]
Mining of rock and metals has been done sinceprehistoric times. Modern mining processes involveprospecting for mineral deposits, analysis of the profit potential of a proposed mine, extraction of the desired materials, and finally reclamation of the land to prepare it for other uses once mining ceases.[37]
Mining processes may create negative impacts on the environment both during the mining operations and for years after mining has ceased. These potential impacts have led to most of the world's nations adopting regulations to manage negative effects of mining operations.[38]
Stone tools have been used for millions of years by humans and earlierhominids. The Stone Age was a period of widespread stone tool usage.[39]Early Stone Age tools were simple implements, such ashammerstones and sharp flakes.Middle Stone Age tools featured sharpened points to be used asprojectile points, awls, orscrapers.Late Stone Age tools were developed withcraftsmanship and distinct cultural identities.[40] Stone tools were largely superseded by copper and bronze tools following the development ofmetallurgy.
^""igneous, adj."".OED Online. Oxford University Press. March 2021. Retrieved17 April 2021.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press.ISBN9780521880060.
^Le Maitre, R. W.; Streckeisen, A.; Zanettin, B.; Le Bas, M. J.; Bonin, B.; Bateman, P.; Bellieni, G.; Dudek, A.; Efremova, S.; Keller, J.; Lamere, J.; Sabine, P. A.; Schmid, R.; Sorensen, H.; Woolley, A. R., eds. (2002).Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks (2nd ed.). Cambridge University Press.ISBN0-521-66215-X.
^Blatt, Harvey and Robert J. Tracy,Petrology, W.H.Freeman, 2nd ed., 1996, p. 355ISBN0-7167-2438-3
^Lillie, Robert J. (2005).Parks and plates : the geology of our national parks, monuments, and seashores (1st ed.). New York: W.W. Norton.ISBN0393924076.
^Jackson, Julia A., ed. (1997). "Friable".Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute.ISBN0922152349.
^Bjorhovde, Reidar (2004). "Development and use of high performance steel".Journal of Constructional Steel Research.60 (3–5):393–400.doi:10.1016/S0143-974X(03)00118-4.
^Shelach, Gideon; Raphael, Kate; Jaffe, Yitzhak (2011). "Sanzuodian: the structure, function and social significance of the earliest stone fortified sites in China".Antiquity.85 (327):11–26.doi:10.1017/S0003598X00067405.S2CID163488276.
^Botin, J.A., ed. (2009).Sustainable Management of Mining Operations. Denver, CO: Society for Mining, Metallurgy, and Exploration.ISBN978-0-87335-267-3.
^Wilson, Arthur (1996).The Living Rock: The Story of Metals Since Earliest Times and Their Impact on Developing Civilization. Cambridge, England:Woodhead Publishing.ISBN978-1-85573-301-5.
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