Granite is a common type of felsic intrusive igneous rock, granular and phaneritic in texture. Granites can be predominantly white, pink, or gray depending on their mineralogy; the word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Speaking, granite is an igneous rock with between 20% and 60% quartz by volume, at least 35% of the total feldspar consisting of alkali feldspar, although the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar; the term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks consist of feldspar, quartz and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole peppering the lighter color minerals; some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic.
A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids; the extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive and tough; these properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3, its compressive strength lies above 200 MPa, its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar on the A-Q-P half of the diagram.
True granite contains both alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite; when a granitoid contains less than 10% orthoclase, it is called tonalite. A granite containing both muscovite and biotite micas is called two-mica granite. Two-mica granites are high in potassium and low in plagioclase, are S-type granites or A-type granites. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: Granite containing rock is distributed throughout the continental crust. Much of it was intruded during the Precambrian age. Outcrops of granite tend to form rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite occurs as small, less than 100 km2 stock masses and in batholiths that are associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are associated with the margins of granitic intrusions.
In some locations coarse-grained pegmatite masses occur with granite. Granite is more common in continental crust than in oceanic crust, they are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granitoids have crystallized from felsic magmas that have compositions near a eutectic point. Magmas are composed of minerals in variable abundances. Traditionally, magmatic minerals are crystallized from the melts that have separated from their parental rocks and thus are evolved because of igneous differentiation. If a granite has a cooling process, it has the potential to form larger crystals. There are peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling.
Anatectic melts are produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. The composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, titanium and sodium, enrich the melt in potassium and silicon – alkali feldspar and quartz, are two of the defining constituents of granite; this process operates regardless of the origin of parental magmas to granites, regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was; the final texture and composition of a granite are distinctive as to its parental rock. For instance, a granite, derived from partial melting of meta
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
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava; the magma can be crust. The melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, orogens, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are geologically important because: their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted.
In terms of modes of occurrence, igneous rocks can be either extrusive. Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock; the mineral grains in such rocks can be identified with the naked eye. Intrusive rocks can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths and dikes; when the magma solidifies within the earth's crust, it cools forming coarse textured rocks, such as granite, gabbro, or diorite. The central cores of major mountain ranges consist of intrusive igneous rocks granite; when exposed by erosion, these cores may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic rocks and are coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are medium-grained.
Hypabyssal rocks are less common than plutonic or volcanic rocks and form dikes, laccoliths, lopoliths, or phacoliths. Extrusive igneous rocks known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive solidify quicker than intrusive igneous rocks, they are formed by the cooling of molten magma on the earth's surface. The magma, brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth and fine-grained. Basalt is lava plateaus; some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example; the molten rock, with or without suspended crystals and gas bubbles, is called magma. It rises; when magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary: 15% hotspot: 12%. Magma that erupts from a volcano behaves according to its viscosity, determined by temperature, crystal content and the amount of silica. High-temperature magma, most of, basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash and lava, may have a viscosity similar to thick, cold molasses or rubber when erupted. Felsic magma, such as rhyolite, is erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma erupt explosively, rhyolitic lava flows are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuff and ignimbrite. Fine volcanic ash is erupted and forms ash tuff deposits, which ca
The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth; the human eye can differentiate between about 10 million colors and is capable of detecting a single photon. Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil and suppression of the hormone melatonin and entrainment of the body clock; the eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea and lens; the cornea is transparent and more curved, is linked to the larger posterior segment, composed of the vitreous, retina and the outer white shell called the sclera.
The cornea is about 11.5 mm in diameter, 1/2 mm in thickness near its center. The posterior chamber constitutes the remaining five-sixths; the cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black; the size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles. Light energy enters the eye through the cornea, through the pupil and through the lens; the lens shape is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision. Dimensions differ among adults by only one or two millimetres, remarkably consistent across different ethnicities; the vertical measure less than the horizontal, is about 24 mm. The transverse size of a human adult eye is 24.2 mm and the sagittal size is 23.7 mm with no significant difference between sexes and age groups.
Strong correlation has been found between the width of the orbit. The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres, a mass of 7.5 grams.. The eyeball grows increasing from about 16–17 millimetres at birth to 22.5–23 mm by three years of age. By age 12, the eye attains its full size; the eye is made up of layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the sclera; the middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid as well as the retinal vessels; the spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid, contained in two areas: the anterior chamber between the cornea and the iris, the posterior chamber between the iris and the lens.
The lens is suspended to the ciliary body by the suspensory ligament, made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation. The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition; the approximate field of view of an individual human eye varies by facial anatomy, but is 30° superior, 45° nasal, 70° inferior, 100° temporal. For both eyes combined visual field is 200 ° horizontal, it is 13700 square degrees for binocular vision. When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle. About 15° temporal and 1.5° below the horizontal is the blind spot created by the optic nerve nasally, 7.5° high and 5.5° wide. The retina has a static contrast ratio of around 100:1; as soon as the eye moves to acquire a target, it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil.
Initial dark adaptation takes place in four seconds of profound, uninterrupted darkness. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; the human eye can detect a luminance range of 1014, or one hundred trillion, from 10−6 cd/m2, or one millionth of a candela per square meter to 108 cd/m2 or one hundred million candelas per square meter. This range does not include looking at the midday lightning discharge. At the low end o
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
Petrology is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous and sedimentary petrology. Igneous and metamorphic petrology are taught together because they both contain heavy use of chemistry, chemical methods, phase diagrams. Sedimentary petrology is, on the other hand taught together with stratigraphy because it deals with the processes that form sedimentary rock. Lithology was once synonymous with petrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the speciality that deals with microscopic details. In the petroleum industry, lithology, or more mud logging, is the graphic representation of geological formations being drilled through, drawn on a log called a mud log; as the cuttings are circulated out of the borehole they are sampled and tested chemically when needed. Petrology utilizes the fields of mineralogy, optical mineralogy, chemical analysis to describe the composition and texture of rocks.
Petrologists include the principles of geochemistry and geophysics through the study of geochemical trends and cycles and the use of thermodynamic data and experiments in order to better understand the origins of rocks. There are three branches of petrology, corresponding to the three types of rocks: igneous and sedimentary, another dealing with experimental techniques: Igneous petrology focuses on the composition and texture of igneous rocks. Igneous rocks include plutonic rocks. Sedimentary petrology focuses on the texture of sedimentary rocks. Metamorphic petrology focuses on the composition and texture of metamorphic rocks Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are useful for investigating rocks of the lower crust and upper mantle that survive the journey to the surface in pristine condition, they are one of the prime sources of information about inaccessible rocks such as those in the Earth's lower mantle and in the mantles of the other terrestrial planets and the Moon.
The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built. Important publications in petrology Ore Pedology Atlas of Igneous and metamorphic rocks and textures – Geology Department, University of North Carolina Metamorphic Petrology Database – Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute Petrological Database of the Ocean Floor - Center for International Earth Science Information Network, Columbia University