Rock (geology)

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.

The Earth's crust (including the lithosphere) and mantle are formed of rock.

The study of rocks is called petrology.

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 here.

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.

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.

Natural functions and human uses

The shape of the particles in sedimentary rocks has an important effect on the ability of microorganisms to colonize them. This interaction is studied in the science of geomicrobiology.

It is thought that the relatively low levels of carbon dioxide in the Earth's atmosphere (compared to that on Venus) is due to large amounts of carbon being trapped in limestone and dolomite sedimentary layers. The transfer of carbon from eroded sediments to marine deposits is part of the carbon cycle.

Sedimentary rocks are economically important in that they can be used as construction material. In addition, they often form porous and permeable reservoirs in sedimentary basins, in which petroleum and other hydrocarbons can be found.

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.

7777777777 Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form" (from the Greek prefix meta, "after", and the noun morphe, "form"). The protolith is subjected to heat (greater than 150 degrees Celsius) and extreme pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.

Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assembly (metamorphic facies). They are formed deep beneath the Earth's surface by great stresses from rocks above and high pressures and temperatures. They are also formed by the intrusion of molten rock, called magma, into solid rock and form particularly at the place of contact between the magma and solid rock where the temperatures are high.

The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides us with valuable information about the temperatures and pressures that occur at great depths within the Earth's crust.

Some examples of metamorphic rocks are slate, gneiss and schist.

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