In geology Geology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is a major academic discipline, and is also, rock is a naturally occurring solid aggregate of minerals A mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific chemical composition. Minerals range in composition and/or mineraloids A mineraloid is a mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amorphous glass and not a crystal. Jet is derived from decaying wood under extreme pressure. Opal is another mineraloid because.

The Earth's outer solid layer, the lithosphere The lithosphere is the rigid outermost shell of a rocky planet, is made of rock. In general rocks are of three types, namely, igneous Igneous rock is one of the three main rock types (the others being sedimentary and metamorphic rock). Igneous rock is formed by magma (molten rock) cooling and becoming solid. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can, sedimentary Sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by, and metamorphic Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic. The scientific study of rocks is called petrology Petrology is the branch of geology that studies rocks, and the conditions in which rocks form. Lithology once was approximately synonymous with petrography, but in current usage, lithology is a subdivision of petrology focusing on macroscopic hand-sample or outcrop-scale description of rocks, while petrography is the speciality that deals with, and petrology is an essential component of geology.

Rock classification

See also: Formation of rocks The synthetic investigation of rocks, which proceeds by experimental work to reproduce different rock types and in this way to elucidate their origin and explain their structures. In many cases no experiment is necessary. Every stage in the origin of clays, sands and gravels can be seen in process around us, but where these have been converted

Rocks are classified by mineral and chemical In chemistry, a chemical substance is a material with a specific chemical composition composition, by the texture Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level. This includes the geometric aspects and relations amongst the component particles or crystals which is called the crystallographic texture or preferred of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous Igneous rock is one of the three main rock types (the others being sedimentary and metamorphic rock). Igneous rock is formed by magma (molten rock) cooling and becoming solid. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can, sedimentary Sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by and metamorphic Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic. They are further classified according to particle size Particle size or grain size refers to the diameter of a grain of granular material, such as sediment or the lithified particles in clastic rock. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders. The transformation of one rock type to another is described by the geological model called the rock cycle The rock cycle is a fundamental concept in geology that describes the dynamic transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. As the diagram to the right illustrates, each type of rock is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as.

Igneous rocks Igneous rock is one of the three main rock types (the others being sedimentary and metamorphic rock). Igneous rock is formed by magma (molten rock) cooling and becoming solid. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can are formed when molten magma Magma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano. Magma is capable of intrusion into adjacent rocks, extrusion cools and are divided into two main categories: plutonic rock A pluton in geology is an intrusive igneous rock body that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically kilometers in dimension, without and volcanic Volcanic rocks are usually fine-grained or aphanitic to glassy in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth Earth is the third planet from the Sun, and the fifth-largest of the eight planets in the Solar System. It is also the largest, most massive, and densest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 3] or Terra.[note 4]'s crust (example granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry), while volcanic or extrusive rocks result from magma reaching the surface either as lava Lava is molten rock expelled by a volcano during eruption. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700 °C to 1,200 °C . Although lava is quite viscous, with about 100,000 times the viscosity of water, or fragmental ejecta (examples pumice Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid cooling and rapid depressurization. The depressurization and basalt Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey) .^[1]

Sedimentary rocks Sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by are formed by deposition of either clastic Clastic rocks are composed of fragments, or clasts, of pre-existing rock. The term is most commonly, but not uniquely, applied to sedimentary rocks sediments Sediment is naturally-ocurring material that is broken down by processes of weathering and erosion and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself, organic matter, or chemical precipitates (evaporites Evaporites are water-soluble mineral sediments that result from the evaporation of bodies of surficial water. Evaporites are considered sedimentary rocks), followed by compaction of the particulate matter and cementation during diagenesis In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% (mudstone Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the appearance of fissility or parallel layering. This, shale Shale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by thin laminae or parallel layering or bedding less than one centimeter in thickness and siltstone Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones); sandstones Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white. Since sandstone beds often form 20 to 25% and carbonate rocks Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone and dolomite, composed of calcite and the mineral dolomite (CaMg(CO3)2) respectively. Chalk and tufa are also minor sedimentary carbonates 10 to 15% (limestone Limestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized and dolostone Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is resistant to erosion and can either contain bedded layers).^[1]

Metamorphic rocks Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature There are basically two equivalent concepts of temperature, the thermodynamic concept and the statistical physics concept. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in macroscopically measurable variables. Statistical physics and pressure Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient 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).^[1]

The three classes of rocks — the igneous, the sedimentary and the metamorphic — 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 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 selected points (more or less arbitrary) in a continuously graduated series.^[2]

Impact on society

Rocks have had a huge impact on the cultural and technological advancement of the human race. Rocks have been used by Homo sapiens Humans are bipedal primates belonging to the species Homo sapiens in Hominidae, the great ape family. They are the only surviving members of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving. This mental capability, combined with an erect body carriage that frees the and other hominids The Hominidae form a taxonomic family, including four extant genera: chimpanzees, gorillas, humans and orangutans for more than 2 million years Lower Paleolithic (genus Homo). Lithic technology In archeology, lithic technology refers to a broad array of techniques and styles to produce usable tools from various types of stone. The earliest stone tools were recovered from modern Ethiopia and were dated to between two-million and three-million years old. The archaeological record of lithic technology is divided into three major time marks some of the oldest and continuously used technologies. The mining Mining is the extraction of valuable minerals or other geological materials from the earth, usually from an ore body, vein or seam. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock salt and potash. Any material that cannot be grown through agricultural processes, or of rocks for their metal A metal is a chemical element that is a good conductor of both electricity and heat and forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions (cations). Those ions are surrounded by ore An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element 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.

The prehistory and history of civilization is classified into the Stone Age, Bronze Age, and Iron Age. Although the stone age has ended virtually everywhere, rocks continue to be used to construct buildings and infrastructure. When so used, rocks are called dimension stone.

See also

• Dimension stone
• Geologic time scale
• Geomorphology
• Kekur
• List of rock types
• List of rocks in Western Australia
• Oldest rock
• Quarry
• Rauk
• Riprap

References

1. ^ ^{a b c} Blatt, Harvey and Robert J. Tracy, 1996, Petrology, W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3
2. ^ This article incorporates text from the article "Petrology" in the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.

Categories: Petrology | Rocks | Materials

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