[[Image:DirkvdM rocks.jpg|thumb|300px|The rocky side of a mountain creek near [[Orosí]], [[Costa Rica]].]]

[[Image:North america rock types.jpg|thumb|Sedimentary, volcanic, plutonic, and metamorphic rock types of [[North America]].]]

{{otherusesof|rock}}

A '''rock''' is a naturally occurring [[aggregate]] of [[mineral]]s and/or [[mineraloid]]s. The [[Earth's crust]] (including the [[lithosphere]]) and [[Earth's mantle|mantle]] are formed of rock.

[[Petrology]] is the study of rocks.

==Rock classification==

Rocks are 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 may also be classified according to [[grain size]], in the case of [[conglomerate]]s and [[breccia]]s or in the case of individual '''stones'''. The transformation of one rock type to another is described by the geological model called the [[rock cycle]].

===Igneous===

{{details|Igneous rock}}

[[Igneous rock]]s are formed from molten [[magma]] and are divided into two main categories: [[Intrusion|plutonic rock]] and [[volcanic]]. Plutonic or intrusive rocks result when magma cools and crystallises slowly within the [[Earth]]'s crust, while volcanic or extrusive rocks result from magma reaching the surface either as [[lava]] or fragmental ejecta.<ref name=Blatt>Blatt, Harvey and Robert J. Tracy, 1996, ''Petrology'', W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3</ref>

===Sedimentary===

{{details|Sedimentary rock}}

[[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. Mudrocks comprise 65% ([[mudstone]], [[shale]] and [[siltstone]]); [[sandstone]]s 20 to 25% and [[carbonate rock]]s 10 to 15% ([[limestone]] and [[dolostone]]).<ref name=Blatt />

===Metamorphic===

{{details|Metamorphic rock}}

[[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 [[recrystallisation]]).<ref name=Blatt />

===Classification caveat===

The three classes of rocks: 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 diminution 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. This is frequently urged as a reason for reducing rock classification to its simplest possible terms, and using only a few generalized rock designations. But it is clear that many apparently trivial differences tend regularly to recur, and have a real significance, and so long as any variation can be shown to be of this nature it deserves recognition.<ref name=EB1911>{{1911|article=Petrology}}</ref>

==Coloration==

[[Iron oxide]]s and [[carbonate]]s play a large part in many sedimentary rocks and are especially important as coloring agents. The red sands and limestones, for example, which are so abundant, contain small amounts of [[iron(III) oxide]] ([[hematite]]), which in a

finely divided state gives a red hue to all rocks in which it is

present. [[Limonite]] and [[goethite]], on the other hand, makes rocks yellow or brown; [[manganese]] oxides, [[asphalt]] and other [[carbonaceous]] substances are the cause of the black color of many sediments. Bluish tints result sometimes from the presence of [[phosphate]]s or of [[fluorite]]; while green is most frequently seen in rocks which contain [[glauconite]] or [[Chlorite group|chlorite]].<ref name=EB1911 />

== Impact on Society ==

Rocks have had a huge impact on the cultural and technological advancement of the human race. The [[mining]] of rocks for their [[metal]] [[ore]] content has been one of the most important factors of human advancement. Humanity's advancement has been decided by the kind of metals available from the rocks of a region. The prehistory of civilization is classified into the [[stone age]], [[Iron Age]], and [[Bronze Age]]. Rocks have been and continue to be used to construct buildings and [[infrastructure]].

== See also ==

* [[Formation of rocks]]

* [[Geology]]

* [[Geologic time scale]]

* [[Geomorphology]]

* [[List of minerals]]

* [[List of rocks]] (geological)

* [[List of stone]] (building stone)

* [[Megalith]] (Archaeology)

* [[Quarry]]ing

* [[Riprap]] (civil engineering use)

* [[Rock balancing]]

* [[Rock formations]] (a list of scenic features)

==References==

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[[Category:Petrology]]

[[Category:Materials]]

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