Rock (geology)
A
rock is a naturally occurring aggregate of
minerals and/or
mineraloids. Rocks are classified by mineral and
chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into
igneous,
sedimentary and
metamorphic. They may also be classified according to
grain size, in the case of
conglomerates and
breccias or in the case of individual
stones.
Igneous rocks are formed from molten
magma and are divided into two main categories:
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 (when the molten rock is in the mantle, it is called magma, when it is above the crust, it is called lava) or fragmental ejecta. Intrusive igneous rocks usually take a few thousand years to cool whereas extrusive igneous rocks take only a few days or weeks to cool and solidify.
Sedimentary rocks are formed by deposition of either
clastic sediments, organic matter, or chemical precipitates (
evaporites), followed by compaction of the particulate matter and cementation during
diagenesis. Sedimentary rocks form at or near the Earth's surface.
Metamorphic rocks 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).
The transformation of one rock type to another is described by the geological model called the
rock cycle.
The
Earth's crust (including the
lithosphere) and
mantle are formed of rock.
Petrology is the study of rocks.
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List of minerals*
List of rocks (geological)
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List of stone (building stone)
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Quarrying
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Rock formations (a list of scenic features)
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Megalith (Archaeology)
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Riprap (civil engineering use)
