ROCK or STONE is a natural substance, a solid aggregate of one or more minerals or mineraloids . For example, granite , a common rock, is a combination of the minerals quartz , feldspar and biotite . The Earth's outer solid layer, the lithosphere , is made of rock.
Rock has been used by mankind throughout history. The minerals and metals found in rocks have been essential to human civilization.
Three major groups of rocks are defined: igneous , sedimentary , and metamorphic . The scientific study of rocks is called petrology , which is an essential component of geology.
* 1 Classification
* 2 Human use
* 3 See also * 4 References * 5 External links
At a granular level, rocks are composed of grains of minerals, which, in turn, are homogeneous solids formed from a chemical compound that is arranged in an orderly manner. The aggregate minerals forming the rock are held together by chemical bonds . The types and abundance of minerals in a rock are determined by the manner in which the rock was formed. Many rocks contain silica (SiO2); a compound of silicon and oxygen that forms 74.3% of the Earth\'s crust . This material forms crystals with other compounds in the rock. The proportion of silica in rocks and minerals is a major factor in determining their name and properties.
Rocks are geologically classified according to characteristics such as mineral and chemical composition, permeability , the texture of the constituent particles, and particle size . These physical properties are the end result of the processes that formed the rocks. Over the course of time, rocks can transform from one type into another, as described by the geological model called the rock cycle . These events produce three general classes of rock: igneous , sedimentary , and metamorphic .
The three classes of rocks are subdivided into many groups. However, there are no hard and fast boundaries between allied rocks. By increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to more or less arbitrary selected points in a continuously graduated series.
About 64.7% of the
Sedimentary rocks are formed at the earth's surface by the accumulation and cementation of fragments of earlier rocks, minerals, and organisms or as chemical precipitates and organic growths in water (sedimentation ). This process causes clastic sediments (pieces of rock) or organic particles (detritus ) to settle and accumulate, or for minerals to chemically precipitate (evaporite ) from a solution . The particulate matter then undergoes compaction and cementation at moderate temperatures and pressures (diagenesis ).
Before being deposited, sediments are formed by weathering of earlier rocks by erosion in a source area and then transported to the place of deposition by water , wind , ice , mass movement or glaciers (agents of denudation ). Mud rocks comprise 65% (mudstone , shale and siltstone ); sandstones 20 to 25% and carbonate rocks 10 to 15% (limestone and dolostone ). About 7.9% of the crust by volume is composed of sedimentary rocks, with 82% of those being shales, while the remainder consists of limestone (6%), sandstone and arkoses (12%). Sedimentary rocks often contain fossils . Sedimentary rocks form under the influence of gravity and typically are deposited in horizontal or near horizontal layers or strata and may be referred to as stratified rocks. A small fraction of sedimentary rocks deposited on steep slopes will show cross bedding where one layer stops abruptly along an interface where another layer eroded the first as it was laid atop the first.
Main article: Metamorphic rock Metamorphic banded gneiss
Metamorphic rocks are formed by subjecting any rock type—sedimentary rock , igneous rock or another older metamorphic rock—to different temperature and pressure conditions than those in which the original rock was formed. This process is called metamorphism ; meaning to "change in form". The result is a profound change in physical properties and chemistry of the stone. The original rock, known as the protolith , transforms into other mineral types or other forms of the same minerals, by recrystallization . The temperatures and pressures required for this process are always higher than those found at the Earth's surface: temperatures greater than 150 to 200 °C and pressures of 1500 bars. Metamorphic rocks compose 27.4% of the crust by volume.
The three major classes of metamorphic rock are based upon the
formation mechanism. An intrusion of magma that heats the surrounding
rock causes contact metamorphism—a temperature-dominated
Depending on the structure, metamorphic rocks are divided into two general categories. Those that possess a texture are referred to as foliated ; the remainders are termed non-foliated. The name of the rock is then determined based on the types of minerals present. Schists are foliated rocks that are primarily composed of lamellar minerals such as micas . A gneiss has visible bands of differing lightness , with a common example being the granite gneiss. Other varieties of foliated rock include slates , phyllites , and mylonite . Familiar examples of non-foliated metamorphic rocks include marble , soapstone , and serpentine . This branch contains quartzite —a metamorphosed form of sandstone —and hornfels .
The use of rocks has had a huge impact on the cultural and technological development of the human race. Rocks have been used by humans and other hominids for at least 2.5 million years . Lithic technology marks some of the oldest and continuously used technologies. The mining of rocks for their metal ore content has been one of the most important factors of human advancement, which has progressed at different rates in different places in part because of the kind of metals available from the rocks of a region.