Rock (geology)
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A rock is a naturally occurring mixture of minerals and/or mineraloids. The Earth's lithosphere is made of rock. Petrology is the scientific study of rocks.
[change] Rock classification
Rocks are classified by mineral and chemical composition, by the texture of their particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic.[1] The transformation of one rock type to another is described by the geological model called the rock cycle.
Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic rock. 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) .[2]
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. Mud rocks comprise 65% (mudstone, shale and siltstone); sandstones 20 to 25% and carbonate rocks 10 to 15% (limestone and dolostone).[2]
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).[2]
[change] See also
[change] References
- ↑ They may also be classified according to particle size, in the case of conglomerates and breccias or in the case of individual stones.
- ↑ 2.0 2.1 2.2 Blatt, Harvey and Robert J. Tracy, 1996, Petrology, W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3
