In geology, a pluton is a body of intrusive igneous rock, crystallized from magma cooling below the surface of the Earth. While pluton is a general term to describe an intrusive igneous body, there has been some confusion around the world as to what is the definition of a pluton. Pluton has been used to describe any non-tabular intrusive body, batholith has been used to describe systems of plutons. In other literature and pluton have been used interchangeably. In central Europe, smaller bodies are described as larger bodies as plutons. In practice the term pluton most means a non-tabular igneous intrusive body; the most common rock types in plutons are granite, tonalite and quartz diorite. Light colored, coarse-grained plutons of these compositions are referred to as granitoids. Examples of plutons include Denali in Alaska. Intrusive bodies of igneous rock can be classified from one distinctions. If the body is tabular or not; the bodies can be further classified based on their shape and their concordancy with the surrounding country rocks.
A tabular body is magma that has filled in another plane of weakness. A non-tabular body however, can vary in shape much more than tabular bodies and tend to be much larger. A concordant body is one that does not cross a pre-existing fabric in the country rock, a sill is an example of a concordant tabular intrusive body. A discordant body is one that does cross pre-existing fabrics in the country rock, a dike is an example of a discordant tabular body. A non-tabular intrusive body is further classified by size. Stock is a term, used for a non-tabular body, exposed for less than 100 Km2, batholith is used to describe anything exposed for larger than 100 Km2; this size classification does not take into account the true size of the body, why some ambiguity in the use of pluton came about. A non-tabular body can be classified based on shape, if the bottom of the body is parallel with the underlying country rock it is termed a laccolith. If the bottom of the body is a basin and the top of the body is flat it is a lopolith.
A laccolith is thought to be formed. The horizontal movement of the magma is limited by the viscosity, which leads to the magma pushing the rock above it up creating a dome shape. Lopoliths are believed to have a more mafic, therefore less viscous, source. Lopoliths tend to be larger than laccoliths, are believed to get their lenticular shape from the weight of the intruding magma compressing the underlying country rock, or the shape comes from the evacuation of a magma chamber below the intruding magma, causing the country rock to collapse and creating a basin; some of these terms might be outdated, not describe the shape of a pluton but they are still used. Plutons are believed to be formed from either one single magmatic event, or several incremental events. Recent evidence suggests. While there is little visual evidence of multiple injections in the field, there is geochemical evidence. Zircon zoning is a key part to determining if a single magmatic event or a series of injections were the methods of emplacement.
Another side of the incremental theory is that plutons formed from the amalgamation of small intrusions. The incremental model suggests that there is more time in-between injections to account for the fractional crystallization that allows the newest injection to go in to the least crystallized part of the body. Methods of pluton emplacement Subvolcanic rock Volcanic rock Glazner, A. F. Bartley, J. M. Coleman, D. S. Gray, W. and Taylor, R. Z.. "Are plutons assembled over millions of years by amalgamation from small magma chambers?" GSA Today, 14, pp. 4–11. Young, Davis A.. Mind Over Magma: the Story of Igneous Petrology. Princeton University Press. ISBN 0-691-10279-1. Best, Myron G.. Igneous and Metamorphic Petrology. San Francisco: W. H. Freeman & Company. Pp. 119 ff. ISBN 0-7167-1335-7
A diapir is a type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–Taylor-instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dikes of material that move along tectonically induced fractures in surrounding rock; the term was introduced by the Romanian geologist Ludovic Mrazek, the first to understand the principle of salt tectonics and plasticity. The term "diapir" may be applied to igneous structures, but it is more applied to non-igneous cold materials, such as salt domes and mud diapirs. In addition to Earth-based observations, diapirism is thought to occur on Neptune's moon Triton, Jupiter's moon Europa, Saturn's moon Enceladus, Uranus's moon Miranda. Diapirs intrude vertically upward along fractures or zones of structural weakness through denser overlying rocks because of density contrast between a less dense, lower rock mass and overlying denser rocks.
The density contrast manifests as a force of buoyancy. The process is known as diapirism; the resulting structures are referred to as piercement structures. In the process, segments of the existing strata can be pushed upwards. While moving higher, they retain much of their original properties such as pressure, which can be different from that of the shallower strata they get pushed into; such overpressured Floaters pose a significant risk. There is an analogy to a Galilean thermometer. Rock types such as evaporitic salt deposits, gas charged muds are potential sources of diapirs. Diapirs form in the earth's mantle when a sufficient mass of hot, less dense magma assembles. Diapirism in the mantle is thought to be associated with the development of large igneous provinces and some mantle plumes. Explosive, hot volatile rich magma or volcanic eruptions are referred to as diatremes. Diatremes are not associated with diapirs, as they are small-volume magmas which ascend by volatile plumes, not by density contrast with the surrounding mantle.
Diapirs or piercement structures are structures resulting from the penetration of overlaying material. By pushing upward and piercing overlying rock layers, diapirs can form anticlines, salt domes and other structures capable of trapping petroleum and natural gas. Igneous intrusions themselves are too hot to allow the preservation of preexisting hydrocarbons. Geyser – Hot spring characterized by intermittent discharge of water ejected turbulently and accompanied by steam Granite – A common type of intrusive, igneous rock with granular structure Granite dome – Rounded hills of bare granite formed by exfoliation Hydrothermal vent – A fissure in a planet's surface from which geothermally heated water issues Methods of pluton emplacement – The ways magma is accommodated in a host rock where the final result is a pluton Mud volcano – Landform created by the eruption of mud or slurries and gases Salt tectonics
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that a sill does not cut across preexisting rock beds. Stacking of sills builds a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source; the rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations. Sills parallel beds and foliations in the surrounding country rock.
They can be emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills into near vertical orientations. Sills can be confused with solidified lava flows. Intruded sills will show partial incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed. Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will show evidence of vesicles where gases escaped into the atmosphere; because sills form at shallow depths below the surface, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock do not. Certain layered intrusions are a variety of sill that contain important ore deposits. Precambrian examples include the Bushveld and the Great Dyke complexes of southern Africa, the Duluth intrusive complex of the Superior District, the Stillwater igneous complex of the United States.
Phanerozoic examples are smaller and include the Rùm peridotite complex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusions contain concentrations of gold, platinum and other rare elements. Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by short dike-like segments; such sills are known as transgressive, examples include the Whin Sill and sills within the Karoo basin. The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic reflection data; such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive. Aquatic sill Batholith Dike Laccolith Sheet intrusion Sill swarm Stock
Intrusive rock is formed when magma crystallizes and solidifies underground to form intrusions, for example plutons, dikes, sills and volcanic necks. Intrusive rock forms within Earth's crust from the crystallization of magma. Many mountain ranges, such as the Sierra Nevada in California, are formed from large granite intrusions. Intrusions are one of the two ways igneous rock. Technically an intrusion is any formation of intrusive igneous rock. In contrast, an extrusion consists of extrusive rock. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Plutonic rocks form 7% of the Earth's current land surface. Coarse-grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called subvolcanic or hypabyssal. Intrusive structures are classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel the body is concordant, otherwise it is discordant.
An intrusive suite is a group of plutons related in time and space.. Intrusions vary from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes: Batholith: a large irregular discordant intrusion Chonolith: an irregularly-shaped intrusion with a demonstrable base Cupola: a dome-shaped projection from the top of a large subterranean intrusion Dike: a narrow tabular discordant body nearly vertical Laccolith: concordant body with flat base and convex top with a feeder pipe below Lopolith: concordant body with flat top and a shallow convex base, may have a feeder dike or pipe below Phacolith: a concordant lens-shaped pluton that occupies the crest of an anticline or trough of a syncline Volcanic pipe or volcanic neck: tubular vertical body that may have been a feeder vent for a volcano Sill: a thin tabular concordant body intruded along bedding planes Stock: a smaller irregular discordant intrusive Boss: a small stock A body of intrusive igneous rock which crystallizes from magma cooling underneath the surface of the Earth is called a pluton.
If the pluton is large, it may be called a stock. Intrusive rocks are characterized by large crystal sizes, as the individual crystals are visible, the rock is called phaneritic; this is as the magma cools underground, while cooling may be fast or slow, cooling is slower than on the surface, so larger crystals grow. If it runs parallel to rock layers, it is called a sill. If an intrusion makes rocks above rise to form a dome, it is called a laccolith. How deep-seated intrusions burst through the overlying strata causes intrusive rock to be recognized: Veins spread out into branches, or branchlike parts result from filled cracks, the high temperature is evident in how they alter country rock; as heat dissipation is slow, as the rock is under pressure, crystals form, no vitreous chilled matter is present. The intrusions did not flow. Contained gases could not escape through the thick strata, thus form cavities, which can be observed; because their crystals are of the rough equal size, these rocks are said to be equigranular.
There is no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. The minerals of each have formed in a definite order, each has had a period of crystallization that may be 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. Crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals; the former case is said to be idiomorphic. There are many other characteristics that serve to distinguish the members of these two groups. For example, orthoclase is feldspar from granite, while its modifications occur in lavas of similar composition; the same distinction holds for nepheline varieties. Leucite is common in lavas but rare in plutonic rocks. Muscovite is confined to intrusions; these differences show the influence of the physical conditions under which consolidation takes place.
Intrusive rocks formed at greater depths are called abyssal. Some intrusive rocks solidified in fissures as dikes and intrusive sills at shallow depth and are called subvolcanic or hypabyssal, they show structures intermediate between those of plutonic rocks. They are commonly porphyritic and sometimes vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition. Ellicott City Granodiorite Guilford Quartz Monzonite Methods of pluton emplacement Norbeck Intrusive Suite Volcanic rock Woodstock Quartz Monzonite
Stoping is a process accommodating the ascent of magmatic bodies from their sources in the mantle or lower crust to the surface. The theory was independently developed by Canadian geologist Reginald Aldworth Daly and American geologist Joseph Barrell; the process involves the mechanical disintegration of the surrounding country/host rock through fracturing due to pressure increases associated with thermal expansion of the host rock in proximity of the interface with the melt. After fractures are formed, melt and/or volatiles will invade, widening the fracture and promoting the foundering of host rock blocks. Once suspended in the melt, stoped blocks may either sink or float depending upon the density of the block relative to that of the melt. Additionally, blocks submerged within melt are subject to further thermally-induced fracturing which may account for the observed "lack of evidence" for the process of stoping. Methods of pluton emplacement
Magmatic foliation is a term in geology, for foliation in granitoids that form by magmatic flow, "submagmatic flow," by high-temperature solid-state deformation and moderate- to low-temperature solid-state deformation. Remember, granitoids are igneous rocks. Foliation
Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, evidence of magmatism has been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may contain suspended crystals and gas bubbles. Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers or trans-crustal crystal-rich mush zones. During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion. While the study of magma has relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects—twice in Iceland, once in Hawaii.
Most magmatic liquids are rich in silica. Silicate melts are composed of silicon, aluminium, magnesium, calcium and potassium; the physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition. Viscosity is a key melt property in understanding the behaviour of magmas. More silica-rich melts are more polymerized, with more linkage of silica tetrahedra, so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous. Speaking, more mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to less explosive eruptions. Characteristics of several different magma types are as follows: Ultramafic SiO2 < 45% Fe–Mg > 8% up to 32%MgO Temperature: up to 1500°C Viscosity: Very Low Eruptive behavior: gentle or explosive Distribution: divergent plate boundaries, hot spots, convergent plate boundaries.
At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay within the rock; the geothermal gradient averages about 25 °C/km with a wide range from a low of 5–10 °C/km within oceanic trenches and subduction zones to 30–80 °C/km under mid-ocean ridges and volcanic arc environments. It is very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure; the composition of a rock may be considered to include volatile phases such as water and carbon dioxide. The presence of volatile phases in a rock under pressure can stabilize a melt fraction; the presence of 0.8% water may reduce the temperature of melting by as much as 100 °C. Conversely, the loss of water and volatiles from a magma may cause it to freeze or solidify.
A major portion of all magma is silica, a compound of silicon and oxygen. Magma contains gases, which expand as the magma rises. Magma, high in silica resists flowing, so expanding gases are trapped in it. Pressure builds up until the gases blast out in a dangerous explosion. Magma, poor in silica flows so gas bubbles move up through it and escape gently. Melting of solid rocks to form magma is controlled by three physical parameters: temperature and composition; the most common mechanisms of magma generation in the mantle are decompression melting and lowering of the solidus. Mechanisms are discussed further in the entry for igneous rock; when rocks melt, they do so and because most rocks are made of several minerals, which all have different melting points. As a rock melts, for example, its volume changes; when enough rock is melted, the small globules of melt soften the rock. Under pressure within the earth, as little as a fraction of a percent of partial melting may be sufficient to cause melt to be squeezed from its source.
Melts can stay in place long enough to melt to 20% or 35%, but rocks are melted in excess of 50%, because the melted rock mass becomes a crystal-and-melt mush tha