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46-452: The batholith is composed of many individual masses of rock called plutons , which formed deep underground during separate episodes of magma intrusion , millions of years before the Sierra itself first began to rise. The extremely hot, relatively buoyant plutons, also called plutonic diapirs , intruded through denser, native country rock and sediments, never reaching the surface. At

92-400: 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

138-402: 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

184-506: 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

230-518: 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

276-407: A period of crystallization that may be very distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Later crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals. The former case

322-450: 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

368-887: A wide variety of forms and compositions, illustrated by examples like 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

414-482: 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

460-501: 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

506-399: 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

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552-499: Is common in lavas but very rare in plutonic rocks. Muscovite is confined to intrusions. These differences show the influence of the physical conditions under which crystallization takes place. Hypabyssal rocks show structures intermediate between those of extrusive and plutonic rocks. They are very commonly porphyritic, vitreous , and sometimes even vesicular . In fact, many of them are petrologically indistinguishable from lavas of similar composition. Plutonic rocks form 7% of

598-408: Is described as miarolitic texture . Because their crystals are of roughly equal size, intrusive rocks are said to be equigranular . Plutonic rocks are less likely than volcanic rocks to show a pronounced porphyritic texture, in which a first generation of large well-shaped crystals are embedded in a fine-grained ground-mass. The minerals of each have formed in a definite order, and each has had

644-429: Is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions , such as batholiths , dikes , sills , laccoliths , and volcanic necks . Intrusion is one of the two ways igneous rock can form. The other is extrusion , such as a volcanic eruption or similar event. An intrusion is any body of intrusive igneous rock, formed from magma that cools and solidifies within

690-404: 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

736-401: Is said to be idiomorphic (or automorphic ); the latter is xenomorphic . There are also many other characteristics that serve to distinguish plutonic from volcanic rock. For example, the alkali feldspar in plutonic rocks is typically orthoclase , while the higher-temperature polymorph, sanidine , is more common in volcanic rock. The same distinction holds for nepheline varieties. Leucite

782-1110: The QAPF diagram . Dioritic and gabbroic rocks are further distinguished by whether the plagioclase they contain is sodium -rich, and sodium-poor gabbros are classified by their relative contents of various iron - or magnesium -rich minerals ( mafic minerals) such as olivine , hornblende , clinopyroxene , and orthopyroxene, which are the most common mafic minerals in intrusive rock. Rare ultramafic rocks , which contain more than 90% mafic minerals, and carbonatite rocks, containing over 50% carbonate minerals, have their own special classifications. Hypabyssal rocks resemble volcanic rocks more than they resemble plutonic rocks, being nearly as fine-grained, and are usually assigned volcanic rock names. However, dikes of basaltic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as diabases or dolerites. Rare ultramafic hypabyssal rocks called lamprophyres have their own classification scheme. Intrusive rocks are characterized by large crystal sizes, and as

828-731: The Cenozoic. Geologic evidence in the form of erosion surfaces, paleo-canyons, and related deposits suggests the majority of the uplift was achieved prior to 4-10Ma. The plutons associated with the Sierra Nevada Batholith intruded into pre-existing rocks on the North American Continent. As the plutons intruded into these rocks, many were altered or metamorphosed. The "host rocks" include passive margin sequence units, deep water sediments, and shallow-water passive margin units. There are several locations that

874-517: The Earth are called abyssal or plutonic while those that form near the surface are called subvolcanic or hypabyssal . Plutonic 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 are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in

920-464: The Earth's crust over the span of 100 Ma , forming several plutons , or a chain of volcanoes if the magma reached the surface. Most of the granitic rocks formed between 105 and 85 Ma, during the Cretaceous , with pluton formation ending around about 70 Ma. Erosion from 85 until 15 Ma removed the volcanic rocks and exposed the granitic core. Around 80-76 million years ago, subduction beneath

966-463: The Sierra Nevada batholith transitioned from steep-angle to shallow-angle . This shut down arc magmatism , moving the volcanic arc westward and leaving the Sierra Nevada block in a forearc setting. Apatite and Sphene fission track thermochronology done by Dumitru (1990) revealed a period of rapid decrease in geothermal gradient (>270 °C to <70 °C from 80Ma to 60-50Ma) as

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1012-461: The beginning of unroofing and uplift of the Sierra Nevada block to approximately 30-15Ma. Fission tracks – destructive remnants of radioactive decay in Uranium -bearing minerals – were shorter than expected in samples taken from several Sierra Nevada plutons . This implied a late-Cenozoic residency at depth, meaning the unroofing and uplift of the Sierra Nevada block happened rapidly near the end of

1058-569: The block cooled, followed by a relatively stable period of subnormal geothermal gradients (5-15 °C/km) throughout the Cenozoic . Modeling of the rapid decrease in geothermal gradient returned a crude estimate of depth to the subducting plate of about 35–50 km, with a hard upper limit of 60 km. This is much shallower than the more typical ~120 km depth to the subducting plate in volcanic arc regimes. Using data from his thermochronology analysis, Dumitru (1990) also constrained ages for

1104-485: 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

1150-423: 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

1196-461: 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

1242-451: The contact between the granitic intrusions and the now metamorphosed sedimentary units can be seen. These unique contacts are called roof pendants. 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

1288-416: 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

1334-468: 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

1380-451: 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

1426-510: 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

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1472-481: The crust in dikes and sills). Because the solid country rock into which magma intrudes is an excellent insulator, cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained ( phaneritic ). However, the rate of cooling is greatest for intrusions at relatively shallow depth, and the rock in such intrusions is often much less coarse-grained than intrusive rock formed at greater depth. Coarse-grained intrusive igneous rocks that form at depth within

1518-501: 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

1564-455: The crust of the planet . In contrast, an extrusion consists of extrusive rock, formed above the surface of the crust. Some geologists use the term plutonic rock synonymously with intrusive rock, but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock (typically formed deeper in the Earth's crust in batholiths or stocks ) and medium-grained subvolcanic or hypabyssal rock (typically formed higher in

1610-401: 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

1656-407: 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

1702-448: The individual crystals are visible, the rock is called phaneritic . There are few indications of flow in intrusive rocks, since their texture and structure mostly develops in the final stages of crystallization, when flow has ended. Contained gases cannot escape through the overlying strata, and these gases sometimes form cavities , often lined with large, well-shaped crystals. These are particularly common in granites and their presence

1748-541: 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

1794-494: The magma is extremely slow, and intrusive igneous rock 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

1840-517: 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

1886-608: The material which had covered the batholith for millions of years. The exposed portions of the batholith became the granite peaks of the High Sierra, including Mount Whitney , Half Dome and El Capitan . Most of the batholith, however, remains below the surface. The Sierra batholith was formed when the Farallon Plate subducted below the North American Plate . The resultant molten rock rose through

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1932-687: 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 Intrusive rock Intrusive rock

1978-487: The same time, some magma managed to reach the surface as volcanic lava flows, but most of it cooled and hardened below the surface and remained buried for millions of years. The batholith – the combined mass of subsurface plutons – became exposed as tectonic forces initiated the formation of the Basin and Range geologic province, including the Sierra Nevada. As the mountains rose, the forces of erosion eventually wore down

2024-405: 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

2070-401: 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,

2116-401: 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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