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41-498: The batholith is composed of at least seven, and possibly as many as 14, discrete rock masses called plutons , which had formed beneath the Earth's surface during a period of magma intrusion about 73 to 78 million years ago ( Late Cretaceous time). The rising buoyant plutons resulted from subduction along what was then the west coast of North America, which was near today's border between Montana and Idaho . Regional uplift brought

82-400: A chilled margin is often found on the intrusion side of the contact, while a contact aureole is found on the country rock side. The chilled margin is much finer grained than most of the intrusion, and may be different in composition, reflecting the initial composition of the intrusion before fractional crystallization, assimilation of country rock, or further magmatic injections modified

123-402: A layered intrusion . The ultimate source of magma is partial melting of rock in the upper mantle and lower crust . This produces magma that is less dense than its source rock. For example, a granitic magma, which is high in silica, has a density of 2.4 Mg/m , much less than the 2.8 Mg/m of high-grade metamorphic rock. This gives the magma tremendous buoyancy, so that ascent of the magma

164-399: A chamber for a long period, then it can become stratified with lower density components rising to the top and denser materials sinking. Rocks accumulate in layers, forming a layered intrusion . Any subsequent eruption may produce distinctly layered deposits; for example, the deposits from the 79 AD eruption of Mount Vesuvius include a thick layer of white pumice from the upper portion of

205-506: A contact aureole, and often contain xenolithic fragments of country rock suggesting brittle fracturing. Such intrusions are interpreted as occurring at shallow depth, and are commonly associated with volcanic rocks and collapse structures. An intrusion does not crystallize all minerals at once; rather, there is a sequence of crystallization that is reflected in the Bowen reaction series . Crystals formed early in cooling are generally denser than

246-518: A crystallized magma chamber . A pluton that has intruded and obscured the contact between a terrane and adjacent rock is called a stitching pluton . Intrusions are broadly divided into discordant intrusions , which cut across the existing structure of the country rock, and concordant intrusions that intrude parallel to existing bedding or fabric . These are further classified according to such criteria as size, evident mode of origin, or whether they are tabular in shape. An intrusive suite

287-450: A relatively shallow level in the crust. However, the rate of magma production in tectonic settings that produce supervolcanoes is quite low, around 0.002 km year , so that accumulation of sufficient magma for a supereruption takes 10 to 10 years. This raises the question of why the buoyant silicic magma does not break through to the surface more frequently in relatively small eruptions. The combination of regional extension, which lowers

328-468: A specific United States geological feature is a stub . You can help Misplaced Pages by expanding it . Plutons In geology , an igneous intrusion (or intrusive body or simply intrusion ) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth . Intrusions have a wide variety of forms and compositions, illustrated by examples like

369-450: A volcanic neck, suggesting that necks tend to form at intersections of dikes where passage of magma is least obstructed. Diatremes and breccia pipes are pipe-like bodies of breccia that are formed by particular kinds of explosive eruptions . As they have reached the surface they are really extrusions, but the non erupted material is an intrusion and indeed due to erosion may be difficult to distinguish from an intrusion that never reached

410-415: A volcano may have a deep magma chamber many kilometers down, which supplies a shallower chamber near the summit. The location of magma chambers can be mapped using seismology : seismic waves from earthquakes move more slowly through liquid rock than solid, allowing measurements to pinpoint the regions of slow movement which identify magma chambers. As a volcano erupts, surrounding rock will collapse into

451-482: Is 1,100 kilometers (680 mi) long and 50 kilometers (31 mi) wide. They are usually formed from magma rich in silica , and never from gabbro or other rock rich in mafic minerals, but some batholiths are composed almost entirely of anorthosite . A sill is a tabular concordant intrusion, typically taking the form of a sheet parallel to sedimentary beds. They are otherwise similar to dikes. Most are of mafic composition, relatively low in silica, which gives them

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492-501: Is a group of intrusions related in time and space. Dikes are tabular discordant intrusions, taking the form of sheets that cut across existing rock beds. They tend to resist erosion, so that they stand out as natural walls on the landscape. They vary in thickness from millimeter-thick films to over 300 meters (980 ft) and an individual sheet can have an area of 12,000 square kilometers (4,600 sq mi). They also vary widely in composition. Dikes form by hydraulic fracturing of

533-443: Is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma , in such a chamber is less dense than the surrounding country rock , which produces buoyant forces on the magma that tend to drive it upwards. If the magma finds a path to the surface, then the result will be a volcanic eruption ; consequently, many volcanoes are situated over magma chambers. These chambers are hard to detect deep within

574-399: Is called the room problem , and it remains a subject of active investigation for many kinds of intrusions. The term pluton is poorly defined, but has been used to describe an intrusion emplaced at great depth; as a synonym for all igneous intrusions; as a dustbin category for intrusions whose size or character are not well determined; or as a name for a very large intrusion or for

615-438: Is coarse-grained ( phaneritic ). Intrusive igneous rocks are classified separately from extrusive igneous rocks, generally on the basis of their mineral content. The relative amounts of quartz , alkali feldspar , plagioclase , and feldspathoid is particularly important in classifying intrusive igneous rocks. Intrusions must displace existing country rock to make room for themselves. The question of how this takes place

656-404: Is inevitable once enough magma has accumulated. However, the question of precisely how large quantities of magma are able to shove aside country rock to make room for themselves (the room problem ) is still a matter of research. The composition of the magma and country rock and the stresses affecting the country rock strongly influence the kinds of intrusions that take place. For example, where

697-872: The Palisades Sill of New York and New Jersey ; the Henry Mountains of Utah ; the Bushveld Igneous Complex of South Africa ; Shiprock in New Mexico ; the Ardnamurchan intrusion in Scotland; and the Sierra Nevada Batholith of California . Because the solid country rock into which magma intrudes is an excellent insulator , cooling of the magma is extremely slow, and intrusive igneous rock

738-508: The Butte quartz monzonite and its contact with adjacent rocks, resulted in major ore deposits of lead, zinc, and silver. The Boulder Batholith was named for the prominent rounded boulders that typify its landscape, the result of spheroidal weathering of fractured granite. It measures approximately 75 miles (121 km) north-south by about 25 miles (40 km) east-west, rather small in comparison to most batholiths. This article about

779-485: The Earth, and therefore most of those known are close to the surface, commonly between 1 km and 10 km down. Magma rises through cracks from beneath and across the crust because it is less dense than the surrounding rock. When the magma cannot find a path upwards it pools into a magma chamber. These chambers are commonly built up over time, by successive horizontal or vertical magma injections. The influx of new magma causes reaction of pre-existing crystals and

820-485: The classification is meaningful for bodies which do not change much in area with depth and that have other features suggesting a distinctive origin and mode of emplacement. Batholiths are discordant intrusions with an exposed area greater than 100 square kilometers (39 sq mi). Some are of truly enormous size, and their lower contacts are very rarely exposed. For example, the Coastal Batholith of Peru

861-423: The composition of the rest of the intrusion. Isotherms (surfaces of constant temperature) propagate away from the margin according to a square root law, so that if the outermost meter of the magma takes ten years to cool to a given temperature, the next inward meter will take 40 years, the next will take 90 years, and so on. This is an idealization, and such processes as magma convection (where cooled magma next to

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902-461: The contact between intrusion and country rock give clues to the conditions under which the intrusion took place. Catazonal intrusions have a thick aureole that grades into the intrusive body with no sharp margin, indicating considerable chemical reaction between intrusion and country rock, and often have broad migmatite zones. Foliations in the intrusion and the surrounding country rock are roughly parallel, with indications of extreme deformation in

943-416: The contact is given by the relationship T / T 0 = 1 2 + 1 2 erf ⁡ ( x 2 k t ) {\displaystyle T/T_{0}={\frac {1}{2}}+{\frac {1}{2}}\operatorname {erf} ({\frac {x}{2{\sqrt {kt}}}})} where T 0 {\displaystyle T_{0}} is the initial temperature of

984-468: The contact sinks to the bottom of the magma chamber and hotter magma takes its place) can alter the cooling process, reducing the thickness of chilled margins while hastening cooling of the intrusion as a whole. However, it is clear that thin dikes will cool much faster than larger intrusions, which explains why small intrusions near the surface (where the country rock is initially cold) are often nearly as fine-grained as volcanic rock. Structural features of

1025-451: The country rock by magma under pressure, and are more common in regions of crustal tension. Ring dikes and cone sheets are dikes with particular forms that are associated with the formation of calderas . Volcanic necks are feeder pipes for volcanoes that have been exposed by erosion . Surface exposures are typically cylindrical, but the intrusion often becomes elliptical or even cloverleaf -shaped at depth. Dikes often radiate from

1066-510: The country rock. Such intrusions are interpreted as taking placed at great depth. Mesozonal intrusions have a much lower degree of metamorphism in their contact aureoles, and the contact between country rock and intrusion is clearly discernible. Migmatites are rare and deformation of country rock is moderate. Such intrusions are interpreted as occurring at medium depth. Epizonal intrusions are discordant with country rock and have sharp contacts with chilled margins, with only limited metamorphism in

1107-501: The crust is undergoing extension, magma can easily rise into tensional fractures in the upper crust to form dikes. Where the crust is under compression, magma at shallow depth will tend to form laccoliths instead, with the magma penetrating the least competent beds, such as shale beds. Ring dikes and cone sheets form only at shallow depth, where a plug of overlying country rock can be raised or lowered. The immense volumes of magma involved in batholiths can force their way upwards only when

1148-399: The deep-seated granite to the surface, where erosion exposed the rocks and the extremely rich mineral veins they contained. Hundreds of millions of dollars of copper, silver, gold, zinc, lead, and other metals have been mined from the batholith, both using underground mining and pit mining. The Butte Granite makes up the bulk of the batholith (about 70%), with additional named intrusions along

1189-401: The field, there is geochemical evidence. Zircon zoning provides important evidence for determining if a single magmatic event or a series of injections were the methods of emplacement. Large felsic intrusions likely form from melting of lower crust that has been heated by an intrusion of mafic magma from the upper mantle. The different densities of felsic and mafic magma limit mixing, so that

1230-407: The hot material, k is the thermal diffusivity (typically close to 10 m s for most geologic materials), x is the distance from the contact, and t is the time since intrusion. This formula suggests that the magma close to the contact will be rapidly chilled while the country rock close to the contact is rapidly heated, while material further from the contact will be much slower to cool or heat. Thus

1271-541: The low viscosity necessary to penetrate between sedimentary beds. A laccolith is a concordant intrusion with a flat base and domed roof. Laccoliths typically form at shallow depth, less than 3 kilometers (1.9 mi), and in regions of crustal compression. Lopoliths are concordant intrusions with a saucer shape, somewhat resembling an inverted laccolith, but they can be much larger and form by different processes. Their immense size promotes very slow cooling, and this produces an unusually complete mineral segregation called

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1312-510: The lower melting point components will tend to make the magma more viscous (by increasing the concentration of silicates ). Thus, stratification of a magma chamber may result in an increase in the amount of gas within the magma near the top of the chamber, and also make this magma more viscous, potentially leading to a more explosive eruption than would be the case had the chamber not become stratified. Supervolcano eruptions are possible only when an extraordinarily large magma chamber forms at

1353-413: The magma chamber overlaid with a similar layer of grey pumice produced from material erupted later from lower in the chamber. Another effect of the cooling of the chamber is that the solidifying crystals will release the gas (primarily steam ) previously dissolved when they were liquid, causing the pressure in the chamber to rise, possibly sufficiently to produce an eruption. Additionally, the removal of

1394-517: The magma is highly silicic and buoyant, and are likely do so as diapirs in the ductile deep crust and through a variety of other mechanisms in the brittle upper crust. Igneous intrusions may form from a single magmatic event or several incremental events. Recent evidence suggests that incremental formation is more common for large intrusions. For example, the Palisades Sill was never a single body of magma 300 meters (980 ft) thick, but

1435-467: The maximum attainable overpressure on the chamber roof, and a large magma chamber with warm walls, which has a high effective viscoelasticity , may suppress rhyolite dike formation and allow such large chambers to fill with magma. If the magma is not vented to the surface in a volcanic eruption, it will slowly cool and crystallize at depth to form an intrusive igneous body, one, for example, composed of granite or gabbro (see also pluton ). Often,

1476-583: The periphery consisting of granodiorite , monzogranite , and minor syenogranite . The batholith intruded into Mesoproterozoic and Mesozoic sedimentary rocks , including the Elkhorn Mountains volcanics . The origin of these igneous rocks is related to subduction and continental magmatic arc processes associated with the Late Cretaceous western margin of North America. Later hydrothermal vein activity along fault and shear zones within

1517-549: The pressure in the chamber to increase. The residing magma starts to cool, with the higher melting point components such as olivine crystallizing out of the solution, particularly near to the cooler walls of the chamber, and forming a denser conglomerate of minerals which sinks (cumulative rock). Upon cooling, new mineral phases saturate and the rock type changes (e.g. fractional crystallization ), typically forming (1) gabbro , diorite , tonalite and granite or (2) gabbro , diorite , syenite and granite . If magma resides in

1558-690: The remaining magma and can settle to the bottom of a large intrusive body. This forms a cumulate layer with distinctive texture and composition. Such cumulate layers may contain valuable ore deposits of chromite . The vast Bushveld Igneous Complex of South Africa includes cumulate layers of the rare rock type, chromitite, composed of 90% chromite, Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Magma chamber A magma chamber

1599-405: The silicic magma floats on the mafic magma. Such limited mixing as takes place results in the small inclusions of mafic rock commonly found in granites and granodiorites. An intrusion of magma loses heat to the surrounding country rock through heat conduction. Near the contact of hot material with cold material, if the hot material is initially uniform in temperature, the temperature profile across

1640-401: The surface when magma/lava. The root material of a diatreme is identical to intrusive material nearby, if it exists, that never reached the then surface when formed. A stock is a non-tabular discordant intrusion whose exposure covers less than 100 square kilometers (39 sq mi). Although this seems arbitrary, particularly since the exposure may be only the tip of a larger intrusive body,

1681-401: Was formed from multiple injections of magma. An intrusive body is described as multiple when it forms from repeated injections of magma of similar composition, and as composite when formed of repeated injections of magma of unlike composition. A composite dike can include rocks as different as granophyre and diabase . While there is often little visual evidence of multiple injections in

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