Rock

Rock

To the geologist, the term rock means a naturally occurring aggregate of minerals that may include some organic solids (e.g., fossils ) and/or glass . Rocks are generally subdivided into three large classes: igneous, sedimentary, and metamorphic. These classes relate to common origin, or genesis. Igneous rocks form from cooling liquid rock or related volcanic eruptive processes. Sedimentary rocks form from compaction and cementation of sediments. Metamorphic rocks develop due to solid-state, chemical and physical changes in pre-existing rock because of elevated temperature , pressure, or chemically active fluids.

With igneous rocks, the aggregate of minerals comprising these rocks forms upon cooling and crystallization of liquid rock. As crystals form in the liquid rock, they become interconnected to one another like jigsaw puzzle pieces. After total crystallization of the liquid, a hard, dense igneous rock is the result. Also, some volcanic lavas, when extruded on the surface and cooled instantaneously, will form a natural glass. Glass is a mass of disordered atoms, which are frozen in place due to sudden cooling, and is not a crystalline material like a mineral. Glass composes part of many extrusive igneous rocks (e.g., lava flows) and pyroclastic igneous rocks. Alternatively, some igneous rocks are formed from volcanic processes, such as violent volcanic eruption. Violent eruptions eject molten, partially molten, and non-molten igneous rock, which then falls in the vicinity of the eruption. The fallen material may solidify into a hard mass, called pyroclastic igneous rock. The texture of igneous rocks (defined as the size of crystals in the rock) is strongly related to cooling rate of the original liquid. Rapid cooling of liquid rock promotes formation of small crystals, usually too small to see with the unaided eye. Rocks with this cooling history are called fine-textured igneous rocks. Slow cooling (which usually occurs deep underground) promotes formation of large crystals. Rocks with this cooling history are referred to as coarse-textured igneous rocks.

The mineral composition of igneous rocks falls roughly into four groups: silicic , intermediate, mafic , and ultramafic. These groups are distinguished by the amount of silica (SiO₄), iron (Fe), and magnesium (Mg) in the constituent minerals. Mineral composition of liquid rock is related to place of origin within the body of the earth. Generally speaking, liquids from greater depths within the earth contain more Fe and Mg and less SiO₄ than those from shallow depths.

In sedimentary rocks, the type of sediment that is compacted and cemented together determines the rock's main characteristics. Sedimentary rocks composed of sediment that has been broken into pieces (i.e., clastic sediment), such as gravel, sand , silt, and clay , are clastic sedimentary rocks (e.g., conglomerate, sandstone , siltstone, and shale, respectively). Sedimentary rocks composed of sediment that is chemically derived (i.e., chemical sediment), such as dissolved elements like calcium (Ca), sodium (Na), iron (Fe), and silicon (Si), are chemical sedimentary rocks. Examples of chemical sedimentary rocks are limestone (composed of calcium carbonate), rock salt (composed of sodium chloride), rock gypsum (composed of calcium sulfate), ironstones (composed of iron oxides), and chert (composed of hydrated silica). Biochemical sedimentary rocks are a special kind of chemical sedimentary rock wherein the constituent particles were formed by organisms (typically as organic hard parts, such as shells), which then became sedimentary particles. Examples of this special kind of sedimentary rock include chalk, fossiliferous limestone, and coquina. Sedimentary rocks are formed from sediment in two stages: compaction and cementation. Compaction occurs when sediments pile up to sufficient thickness that overlying mass squeezes out water and closes much open space . Cementation occurs when water flowing through the

compacted sediment deposits mineral crystals upon particles thus binding them together. The main cement minerals are calcite (CaCO₃), hematite (Fe₂O₃), and quartz (SiO₂).

With metamorphic rocks, the nature of the pre-existing rock (protolith) determines in large part the characteristics of the ultimate metamorphic rock . Regardless of protolith, however, almost all metamorphic rocks are harder and more dense than their protoliths. A protolith with flat or elongate mineral crystals (e.g., micas or amphiboles) will yield a metamorphic rock with preferentially aligned minerals (due to directed pressure). Such metamorphic rocks are called foliated metamorphic rocks (e.g., slate and schist ). Non-foliated metamorphic rocks (e.g., marble and quartzite) come from protoliths that have mainly equidimensional mineral crystals (e.g., calcite and quartz, respectively). For example, a protolith shale will yield a foliated metamorphic rock, and a protolith limestone will yield marble, a non-foliated metamorphic rock. Metamorphic rocks possess distinctive grades or levels of metamorphic change from minimal to a maximum near total melting . Low-grade metamorphic rocks generally have fine-textured crystals and low-temperature indicator minerals like the mica chlorite. High-grade metamorphic rocks generally have coarse-textured crystals and very distinctive foliation, plus high-temperature indicator minerals like the silicate mineral staurolite.

Rock is a brittle natural solid found mainly in the outer reaches of Earth's crust and upper mantle. Material that would be brittle rock at such shallow depths becomes to one degree or another rather plastic within the body of the earth. The term "rock" is not generally applied to such non-brittle internal Earth materials. Therefore, rock is a concept related to the outer shell of the earth. The term rock may also be properly applied to brittle natural solids found on the surfaces of other planets and satellites in our solar system . Meteorites are rock. Naturally occurring ice (e.g., brittle water ice in a glacier, H₂O) is also a rock, although we do not normally think of ice this way.

Rock has been an important natural resource for people from early in human evolution . Rocks' properties are the key to their specific usefulness, now as in the past. Hard, dense rocks that could be chipped into implements and weapons were among the first useful possessions of people. Fine-textured and glassy rocks were particularly handy for these applications. Later on, rock as building stone and pavement material became very important, and this continues today in our modern world. All of Earth's natural mineral wealth, fossil energy resources, and most groundwater are contained within rocks of the earth's crust.

See also Lithification; Metamorphism