View source for Rock (geology)

{{Redirect|Rocks||Rock}}
[[Image:Balanced Rock.jpg|right|thumb|''Balanced Rock'' stands in [[Garden of the Gods]] park in [[Colorado Springs, CO]].]]

In [[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==
{{seealso|Formation of rocks}}

[[Image:DirkvdM rocks.jpg|left|thumb|The rocky side of a mountain creek near [[Orosí]], [[Costa Rica]].]]

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 [[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]].

[[File:GabbroRockCreek1.jpg|thumb|Sample of igneous [[gabbro]]]]
[[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]]) .<ref name=Blatt>Blatt, Harvey and Robert J. Tracy, 1996, ''Petrology'', W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3</ref>

[[File:SandstoneUSGOV.jpg|thumb|Sedimentary sandstone with iron oxide bands]]
[[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]]).<ref name=Blatt />

[[File:Skagit-gneiss-Cascades.jpg|thumb|Metamorphic banded gneiss]]
[[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)|recrystallisation]]).<ref name=Blatt />

The three classes of rocks &mdash; the igneous, the sedimentary and the metamorphic &mdash; 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.<ref name=EB1911>{{1911|article=Petrology}}</ref>

== Human use==
[[File:TallOvoo.JPG|thumb|left||Ceremonial [[cairn]] of rocks, an [[ovoo]], from [[Mongolia]]]]
[[File:UMine.JPG|thumb|[[Uranium]] mine near [[Moab, Utah]]]]
The 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]].

== See also ==
{{commonscat|rocks}}
{{wiktionary|rock}}
* [[Dimension stone]]
* [[Geologic time scale]]
* [[Geomorphology]]
* [[Kekur]]
* [[List of rock types]]
* [[Oldest rock]]
* [[Quarry]]
* [[Rauk]]
* [[Riprap]]

== References ==

{{refs}}

[[Category:Petrology]]
[[Category:Rocks| ]]
[[Category:Stone| ]]
[[Category:Materials]]

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