Difference between revisions of "Rock (geology)" - New World Encyclopedia

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Sedimentary, volcanic, plutonic, metamorphic rock types of North America.

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

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

Sedimentary rocks cover 75% of the Earth's surface and include common types such as chalk, limestone, sandstone, clay, and shale. They are formed at or near the Earth's surface in three main ways: (a) deposition of the weathered remains of other rocks (known as "clastic" sedimentary rocks); (b) deposition of the results of biogenic activity; and (c) precipitation from solution. These processes are followed by compaction of the particulate matter and cementation.

The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only five percent of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.

Formation

The formation of sedimentary rock begins with the deposition of particles carried by water, wind, and glaciers to form a sediment. As the sediment builds up, pressure from the overburden ("lithostatic" pressure) squeezes the sediment into layered solids, and the liquids in the pores are expelled. This process is called "lithification" (rock formation). The term "diagenesis" is used to describe all the chemical, physical, and biological changes (including cementation) that a sediment undergoes after initial deposition and during and after lithification, excluding surface weathering.

Given their manner of formation, sedimentary rocks contain important information about the Earth's history. In particular, they contain fossils, the preserved remains of ancient plants and animals. Unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants. The composition of sediments provides clues about the original rock. Differences between successive layers indicate changes to the environment that occurred over time.

Three types of sedimentary rocks

Clastic sedimentary rocks

Lower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weathering and chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents.

Clastic sedimentary rocks are composed of discrete fragments or "clasts" of materials derived from other rocks. They are composed largely of quartz, with other common minerals including feldspars, amphiboles, and clay minerals. Sometimes there are more exotic igneous and metamorphic minerals.

Clastic sedimentary rocks may be considered according to their grain size. Shale consists of the finest particles, smaller than 0.004 mm; siltstone has slightly bigger particles, between 0.004 to 0.06 mm; sandstone is coarser still, with grain sizes of 0.06 to 2 mm; and conglomerates and breccias are the coarsest, with grains between 2 and 256 mm. Arenite is a general term for sedimentary rock with sand-sized particles.

All rocks disintegrate slowly as a result of mechanical and chemical weathering. Mechanical weathering is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. When water percolates into cracks in the rock and freezes, it expands. The force of expansion widens the cracks and breaks off pieces of rock. Heating and cooling of the rock, and its resulting expansion and contraction, also aids the process.

Chemical weathering is the breakdown of rock by chemical reaction. In this process, the minerals in the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions.

Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (water, ice, and wind) to new locations and redeposited in layers, generally at a lower elevation. These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown. Examples are quartz, zircon, rutile, and magnetite.

Biogenic sedimentary rocks

Biogenic sedimentary rocks contain materials generated by living organisms. They include carbonate minerals such as corals, molluscs, and foraminifera, which cover the ocean floor with layers of calcite that later forms limestone. Other examples include stromatolites, the flint nodules found in chalk (which is itself a biogenic sedimentary rock, a form of limestone), and coal (derived from the remains of tropical plants subjected to pressure).

Precipitate sedimentary rocks

Precipitate sedimentary rocks form when mineral solutions, such as seawater, evaporate. Examples include the minerals halite and gypsum.

Metamorphic rocks

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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@@ Line 16: / Line 16: @@
 The formation of sedimentary rock begins with the deposition of particles carried by water, wind, and glaciers to form a sediment. As the sediment builds up, pressure from the overburden ("lithostatic" pressure) squeezes the sediment into layered solids, and the liquids in the pores are expelled. This process is called "lithification" (rock formation). The term "diagenesis" is used to describe all the chemical, physical, and biological changes (including cementation) that a sediment undergoes after initial deposition and during and after lithification, excluding surface weathering.
-====Historical information====
 Given their manner of formation, sedimentary rocks contain important information about the Earth's history. In particular, they contain [[fossil]]s, the preserved remains of ancient [[plant]]s and [[animal]]s. Unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants. The composition of sediments provides clues about the original rock. Differences between successive layers indicate changes to the environment that occurred over time.
-===Classification===
+=== Three types of sedimentary rocks ===
 ====Clastic sedimentary rocks====
+[[Image:Lower_antelope_3_md.jpg|thumb|350px|right|[[Antelope Canyon|Lower Antelope Canyon]] was carved out of the surrounding [[Sandstone|sandstone]] by both mechanical weathering and chemical weathering.  Wind, sand, and water from [[Flash flood|flash flooding]] are the primary weathering agents.]]
 Clastic sedimentary rocks are composed of discrete fragments or "clasts" of materials derived from other rocks.  They are composed largely of [[quartz]], with other common minerals including [[feldspar]]*s, [[amphibole]]*s, and [[clay minerals]]*. Sometimes there are more exotic [[igneous]]* and [[metamorphic]]* minerals.
-Clastic sedimentary rocks may be considered according to their [[grain size]]*. [[Shale]]* consists of the finest with particles, less than 0.004 mm; [[siltstone]]* being a little bigger with particles between 0.004 to 0.06 mm, and [[sandstone]] being coarser still with grains 0.06 to 0.2 mm, and [[conglomerate (geology)|conglomerate]]s and [[breccia]]s being the coarsest with grains 2 to 256 mm. ''Arenite'' is a general term for sedimentary rock with sand-sized particles.
+Clastic sedimentary rocks may be considered according to their [[grain size]]*. [[Shale]]* consists of the finest particles, smaller than 0.004 mm; [[siltstone]]* has slightly bigger particles, between 0.004 to 0.06 mm; [[sandstone]] is coarser still, with grain sizes of 0.06 to 2 mm; and [[conglomerate (geology)|conglomerate]]*s and [[breccia]]*s are the coarsest, with grains between 2 and 256 mm. ''Arenite'' is a general term for sedimentary rock with sand-sized particles.
-The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration.
+All rocks disintegrate slowly as a result of mechanical and chemical weathering. Mechanical weathering is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. When water percolates into cracks in the rock and freezes, it expands. The force of expansion widens the cracks and breaks off pieces of rock. Heating and cooling of the rock, and its resulting expansion and contraction, also aids the process.
-Shales, which consist mostly of clay minerals, are generally further classified on the basis of composition and bedding.
+Chemical weathering is the breakdown of rock by chemical reaction. In this process, the minerals in the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions.
-Coarser clastic sedimentary rocks are classified according to their particle size and composition. Orthoquartzite is a very pure quartz sandstone; [[arkose]] is a sandstone with quartz and abundant feldspar; [[greywacke]] is a sandstone with quartz, clay, feldspar, and metamorphic rock fragments present, which was formed from the sediments carried by turbidity currents.
+Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (water, ice, and wind) to new locations and redeposited in layers, generally at a lower elevation. These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown. Examples are [[quartz]], [[zircon]]*, [[rutile]]*, and [[magnetite]]*.
-All rocks disintegrate slowly as a result of mechanical [[weathering]] and chemical weathering.
+====Biogenic sedimentary rocks====
-[[Image:Lower_antelope_3_md.jpg|thumb|350px|right|[[Antelope Canyon|Lower Antelope Canyon]] was carved out of the surrounding [[Sandstone|sandstone]] by both mechanical weathering and chemical weathering.  Wind, sand, and water from [[Flash flood|flash flooding]] are the primary weathering agents.]]
-'''Mechanical weathering''' is the breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock. Heating and cooling of the rock, and the resulting expansion and contraction, also aids the process. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents.
-'''Chemical weathering''' is the breakdown of rock by chemical reaction. In this process the minerals within the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions. The minerals in igneous rocks may be unstable under normal atmospheric conditions, those formed at higher temperatures being more readily attacked than those which formed at lower temperatures. Igneous rocks are commonly attacked by water, particularly acid or alkaline solutions, and all of the common igneous rock forming minerals (with the exception of quartz which is very resistant) are changed in this way into clay minerals and chemicals in solution.
+Biogenic sedimentary rocks contain materials generated by living organisms. They include carbonate minerals such as [[coral]]s, [[mollusc]]s, and [[foraminifera]], which cover the [[ocean floor]]* with layers of [[calcite]] that later forms [[limestone]]. Other examples include [[stromatolite]]*s, the [[flint]]* nodules found in [[chalk]] (which is itself a biogenic sedimentary rock, a form of limestone), and [[coal]] (derived from the remains of tropical plants subjected to pressure).
-Rock particles in the form of clay, silt, sand, and gravel, are transported by the agents of erosion (usually water, and less frequently by ice and wind) to new locations and redeposited in layers, generally at a lower elevation.
+====Precipitate sedimentary rocks====
-These agents reduce the size of the particles, sort them by size, and then deposit them in new locations. The sediments dropped by streams and rivers form alluvial fans, flood plains, deltas, and on the bottom of lakes and the sea floor. The wind may move large amounts of sand and other smaller particles. Glaciers transport and deposit great quantities of usually unsorted rock material as [[till]].
-These deposited particles eventually become compacted and cemented together, forming clastic sedimentary rocks. Such rocks contain inert minerals which are resistant to mechanical and chemical breakdown such as quartz, [[zircon]], [[rutile]], and [[magnetite]]. Quartz is one of the most mechanically and chemically resistant minerals.
+Precipitate sedimentary rocks form when mineral solutions, such as [[seawater]]*, evaporate. Examples include the minerals [[halite]]* and [[gypsum]]*.
-====Biogenic sedimentary rocks====
-Biogenic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as [[coral]]s, [[mollusc]]s, and [[foraminifera]], which cover the [[ocean floor]] with layers of [[calcite]] which can later form [[limestone]].  Other examples include [[stromatolite]]s, the [[flint]] nodules found in [[chalk]] (which is itself a biogenic sedimentary rock, a form of limestone), and coal (derived from the remains of tropical plants and subjected to pressure).
-====Precipitate sedimentary rocks====
-Precipitate sedimentary rocks form when mineral solutions, such as [[sea water]], evaporate.  Examples include the [[evaporite]] minerals [[halite]] and [[gypsum]].
 ==Metamorphic rocks==