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 humankind throughout history. The minerals and metals 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.
1.1 Igneous rock 1.2 Sedimentary rock 1.3 Metamorphic rock
2 Human use
3 See also 4 References 5 External links
Classification See also: Formation of rocks
Rock outcrop along a mountain creek near Orosí, Costa Rica.
Rocks are composed of grains of minerals, which are homogeneous solids formed from a chemical compound 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 it 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 names and properties. Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. These physical properties are the result of the processes that formed the rocks. Over the course of time, rocks can transform from one type into another, as described by a geological model called the rock cycle. This transformation produces three general classes of rock: igneous, sedimentary, and metamorphic. Those three classes are subdivided into many groups. There are, however, no hard-and-fast boundaries between allied rocks. By increase or decrease in the proportions of their minerals, they pass through gradations from one to the other; the distinctive structures of one kind of rock may thus be traced gradually merging into those of another. Hence the definitions adopted in rock names simply correspond to selected points in a continuously graduated series. Igneous rock Main article: Igneous rock
Sample of igneous gabbro
Volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta, forming minerals such as pumice or basalt.
The chemical abundance and the rate of cooling of magma typically
forms a sequence known as Bowen's reaction series. Most major igneous
rocks are found along this scale.
About 64.7% of the
Sedimentary sandstone with iron oxide bands
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. Metamorphic rock 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
Ceremonial cairn of rocks, an ovoo, from Mongolia
Mi Vida uranium mine near Moab, Utah
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. Mining Main article: Mining
Bay of Fires, Tasmania
History of Earth
Geologic time scale Geomorphology Human timeline Life timeline List of rock types Nature timeline Oldest rock Stone industry
^ Roberts, Dar. "Rocks and classifications". Department of Geography,
University of California, Santa Barbara. Archived from the original on
31 October 2012. Retrieved 11 November 2012.
^ a b c d Wilson, James Robert (1995), A collector's guide to rock,
mineral & fossil localities of Utah, Utah Geological Survey,
pp. 1–22, ISBN 1557913366, archived from the original on
19 November 2016.
^ a b c d Blatt, Harvey; Tracy, Robert J. (1996).
The Wikibook Historical
Media related to rocks at Wikimedia Commons The dictionary definition of rock at Wiktionary
GND: 4020734-1 NDL: 00562239