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Biography
pp-semi|small=yespp-move-indefRedirect|Rocks||Rock (disambiguation)!RockRedirect|StoneRefimprove|date=November 2010ConstructionIn geology , rock or stone is a naturally occurring solid aggregate of mineral s and/or mineraloid s.
The Earth's outer solid layer, the lithosphere , is made of rock. In general, rocks are of three types, namely igneous , sedimentary , and Metamorphic rock|metamorphic . The scientific study of rocks is called petrology , and petrology is an essential component of geology.
Rock classification
See also|Formation of rocks Rocks are generally classified by mineral and chemical composition, by the Texture (geology)|texture of the constituent particles and by the processes that formed them. These indicators separate rocks into three types: igneous rock|igneous , sedimentary rock|sedimentary , and metamorphic rock|metamorphic . They are further classified according to Particle size (grain size)|particle size . The transformation of one rock type to another is described by the geological model called the rock cycle .
Igneous rock s are formed when molten magma cools and are divided into two main categories: Pluton|plutonic rock and Volcanic rock|volcanic . Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth 's crust (example granite ), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (examples pumice and basalt ).cite book |last=Blatt |first=Harvey |first2=Robert J. |last2=Tracy |year=1996 |title=Petrology |location= |publisher=W. H. Freeman |edition=2nd |isbn=0-7167-2438-3
Sedimentary rock s are formed by deposition of either clastic sediment s, organic matter , or chemical precipitates ( evaporite s), followed by compaction of the particulate matter and cementation during diagenesis . Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% ( mudstone , shale and siltstone ); sandstone s 20 to 25% and carbonate rock s 10 to 15% ( limestone and dolostone ).
Metamorphic rock s are formed by subjecting any rock type (including previously formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by Recrystallization (geology)|recrystallization ).
The three classes of rocks—the igneous, the sedimentary and the metamorphic—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 constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points (more or less arbitrary) in a continuously graduated series.1911|article=Petrology
Igneous rocks
Main|Igneous rock Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) is one of the three main rock types, the others being Sedimentary rock|sedimentary and metamorphic rock . Igneous rock is formed through the cooling and freezing|solidification of magma or lava . Igneous rock may form with or without crystallization , either below the surface as Intrusion|intrusive ( plutonic ) rocks or on the surface as extrusive (geology)|extrusive ( Volcanic rock|volcanic ) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planet 's mantle (geology)|mantle or crust (geology)|crust . Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth 's crust (geology)|crust . These have diverse properties, depending on their composition and how they were formed.
Sedimentary rocks
Main|Sedimentary rock Sedimentary rocks are types of rock that are formed by the Deposition_(sediment)|deposition of material at the Earth 's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic matter|organic particles ( detritus ) to settle and accumulate or for minerals to precipitation (chemistry)|precipitate from a solution . Particles that form a sedimentary rock by accumulating are called sediment . Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water , wind , ice , Mass wasting|mass movement or glacier s which are called agents of denudation .
Metamorphic rocks
Main|Metamorphic rock Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism , which means "change in form". The original rock ( protolith ) is subjected to heat and pressure, (temperatures greater than 150 to 200& nbsp;°C and pressures of 1500& nbsp;barsBlatt, Harvey and Robert J. Tracy, Petrology , W.H.Freeman, 2nd ed., 1996, p.355 ISBN 0-7167-2438-3) causing profound physical and/or chemical change. The protolith may be sedimentary rock , igneous rock or another older metamorphic rock.
Human use
Unreferenced section|date=October 2010Expand section|date=October 2010The use of rocks has had a huge impact on the cultural and technological development of the human race. Rocks have been used by humans and other hominids for more than paleolithic|2& nbsp;million years . Lithic technology marks some of the oldest and continuously used technologies. The mining of rocks for their metal ore content has been one of the most important factors of human advancement, which has progressed at different rates in different places in part because of the kind of metals available from the rocks of a region.
The prehistory and history of civilization is classified into the Stone Age , Bronze Age , and Iron Age . Although the Stone Age has ended virtually everywhere, rocks continue to be used to construct buildings and infrastructure . When so used, rocks are called dimension stone .
