igneous rocks yahoo answers

igneous rocks? | Yahoo Answers

Mar 12, 2007· Best Answer: Contrast magma and lava. Describe how the rate of cooling influences the size of crystals in igneous rocks. List the different igneous rock textures (aphanitic, phaneritic, porphyritic, pegmatitic, vesicular, glass, and pyroclastic) and explain their origins.

Contrast magma and lava. Describe how the rate of cooling influences the size of crystals in igneous rocks. List the different igneous rock textures (aphanitic, phaneritic, porphyritic, pegmatitic, vesicular, glass, and pyroclastic) and explain their origins. Discuss the contributions of N. L. Bowen to the understanding of igneous rocks. Be able to list the minerals of Bowen's Reaction series in order. Explain the significance of Bowen's Reaction Series to melting and crystallization. Discuss the various types of magma and how they relate to igneous rocks. List the various kinds of intrusive igneous bodies (dike, sill, laccolith, stock, batholith) and describe each in terms of the criteria used to classify plutons. Origin "fire-formed rocks" Crystallize from molten material: Magma - below the Earth's surface Lava - erupts onto the Earth's surface through a volcano or crack (fissure) Lava cools more quickly because it is on the surface. Cooling Rates Cooling rates influence the texture if the igneous rock: Quick cooling = fine grains Slow cooling = coarse grains Igneous rocks are classified on their texture and their composition. Igneous textures: Glassy - instantaneous cooling Obsidian = volcanic glass Obsidian Aphanitic - fine grain size (< 1 mm); result of quick cooling Rhyolite Basalt Rhyolite Andesite Phaneritic - coarse grain size; visible grains (1-10 mm); result of slow cooling Granite - polished Granite Diorite Gabbro Pegmatitic - very large crystals (many over 2 cm) Granite pegmatite or pegmatitic granite Porphyritic- Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling. Terms for the textural components: Phenocrysts - the large crystals Groundmass or matrix - the finer crystals surrounding the large crystals. The groundmass may be either aphanitic or phaneritic. Types of porphyritic textures: Porphyritic-aphanitic Porphyritic-phaneritic Origin: mixed grain sizes and hence cooling rates, imply upward movement of magma from a deeper (hotter) location of extremely slow cooling, to either: a much shallower (cooler) location with fast cooling (porphyritic- aphanitic), or a somewhat shallower (slightly cooler) location with continued fairly slow cooling (porphyritic-phaneritic). Rock name = porphyry Granite porphyry or porphyritic granite (porphyritic-phaneritic) - phenocrysts usually potassium feldspar Granite porphyry Andesite porphyry or porphyritic andesite (porphyritic-aphanitic) - phenocrysts usually hornblende (amphibole) Rhyolite porphyry or porphyritic rhyolite (porphyritic-aphanitic) Rhyolite Porphyry Vesicular - contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma. Very porous. May resemble a sponge. Commonly low density; may float on water. Vesicular basalt - basalt with a vesicles, which may be quite large. Sometimes lined with crystals to form geodes. Vesicular basalt Vesicular basalt with olivine phenocrysts, building stone at Hawaiian Volcano Observatory, Big Island of Hawaii Pumice - light in color; white to gray; may be glassy or dull. Fully riddled with holes. Very sponge-like. Floats. Used as an abrasive. (Pumice stone, Lava Soap). Pumice Pumice Scoria - dark in color; brown, black, or dark red; similar to vesicular basalt but is fully riddled with holes to form a spongy mass. (May find in barbecue grills as lava rock). Scoria Pyroclastic or Fragmental - pieces of rock and ash come out of a volcano and get welded together by heat. May resemble rhyolite or andesite, but close examination shows pieces of fine-grained rock fragments in it. May also resemble a sedimentary conglomerate or breccia, except that rock fragments are all fine-grained igneous or vesicular. Tuff - made of volcanic ash Volcanic breccia - contains fragments of fine-grained igneous rocks that are larger than ash. Pyroclastic rock Composition of Igneous Rocks Igneous rocks can be placed into four groups based on their chemical compositions: Sialic (or granitic or felsic) Dominated by silicon and aluminum (SiAl) Usually light in color Characteristic of continental crust Forms a stiff (viscous) lava or magma Rock types include: Granite Granite Rhyolite Minerals commonly present include: potassium feldspar (generally pink or white) Na-plagioclase feldspar (generally white) quartz (generally gray or colorless) biotite amphibole? muscovite? Intermediate (or andesitic) Intermediate in composition between sialic and mafic Rock types include: Andesite (aphanitic) Diorite (phaneritic) Diorite Minerals commonly present include: plagioclase feldspar amphibole pyroxene biotite quartz Mafic (or basaltic) Contains abundant ferromagnesian minerals (magnesium and iron silicates) Usually dark in color (dark gray to black) Characteristic of Earth's oceanic crust, Hawaiian volcanoes Forms a runny (low viscosity) lava Also found on the Moon, Mars, and Venus Rock types include: Basalt (aphanitic) Basalt Gabbro (phaneritic) Gabbro Diabase - texture intermediate between basalt and gabbro; characteristic of Early Mesozoic dikes in eastern North America. Minerals commonly present include: Ca-plagioclase feldspar pyroxene olivine amphibole Ultramafic Almost entirely magnesium and iron silicates (ferromagnesian minerals) Rarely observed on the Earth's surface Believed to be major constituent of Earth's mantle Commonly found as xenoliths in basaltic lavas Rock types include: Peridotite (phaneritic) dominated by olivine - the birthstone is Peridot, which gives its name to Peridotite Peridotite Minerals commonly present include: Olivine is dominant. (Olivine is olive green). may have minor amounts of pyroxene and Ca-plagioclase Other types of igneous rock: Syenite A polished syenite called larvikite with centimeter- to inch-scale gray to blue plagioclase crystals. The industrial name for the rocks is "blue pearl". Photographed in an above-ground cemetery in New Orleans, LA Bowen's Reaction Series You have to know the series AND understand the concepts of how Bowen's Reaction Series relates to melting and to crystallization, and to the origin of igneous rocks of various composition. Plutons Subsurface igneous bodies Concordant Plutons sill laccolith Discordant Plutons dike stock batholithBest answer · 3igneous rocks0Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior. Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine. Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende. Pumice is a very light colored, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. Pumice is so light that it actually floats on water. Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals. Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes. Diorite Colour: Speckled black and white in hand specimen; occasionally shades of dark green or pink. The dark minerals are more noticeable than in gabbro. Colour index: 40 to 90, but very variable, often over short distances. Grain size: Coarse; may be pegmatitic. Texture: Equigranular or porphyrithic. In porphyritic varieties the feldspar or hornblende may form phenocrysts. Diorites often vary rapidly in texture; an equigranular variety may grade into a porphyritic one within a few centimeters. They are sometimes foliated due to the roughly parallel arrangement of the minerals. Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Notice in the photo to the left how it fractures. Obsidian is produced when lava cools very quickly. The lava cools so quickly that no crystals can form. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way. Obsidian is usually black or a very dark green, but it can also be found in an almost clear form. Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Look at the photo of granite above, notice the different crystals in the rock. Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. It is also used for monuments because it weathers slowly. Engravings in the granite can be read for hundreds of years, making the rock more valuable. Granite is quarried in many places in the world including the United States. The State of New Hampshire has the nickname "Granite State" because of the amount of granite in the mountains of that beautiful state. The Canadian Shield of North America contains huge outcroppings (surface rocks) of granite. Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Andecite Andesite is an extrusive igneous rock, light to dark gray, sometimes with reddish to greenish hues; fine-grained, sometimes displays bubbles (vesicular texture), frequently has a brecciated texture (tuff) and sometimes contains phenocrysts of feldspar (Ca and Na rich varieties), quartz, hornblende, pyroxene, and biotite mica. Scoria Scoria is the basltic equivalent of pumice. Certain scoriae, sometimes called cinders or volcanic cinder actually resemble cinders from a coal furnace as the photo shows above.0I second what Marine 52 said...I'll have to use some of the information for my Rocks and Minerals unit...thanks!2There are mainly two catigories of igneous rocks intrusive, and extrusive. Igneous rock From Wikipedia, the free encyclopedia (Redirected from Igneous rocks) Jump to: navigation, search Volcanic rock on North America Plutonic rock on North AmericaIgneous rocks are formed when rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes -- an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. Contents [hide] 1 Geologic significance 2 Morphology and setting 2.1 Intrusive igneous rocks 2.2 Extrusive igneous rocks 3 Classification 3.1 Texture 3.2 Chemical classification 3.3 History of classification 4 Mineralogical classification 4.1 Example of classification 5 Magma origination 5.1 Decompression 5.2 Effects of water and carbon dioxide 5.3 Temperature increase 5.4 Magma evolution 6 Etymology 7 References 8 See also 9 External links [edit] Geologic significance Igneous rocks make up approximately ninety five percent of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks. 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, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted; their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events; their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics); in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites, whereas ores of chromium and platinum are commonly associated with gabbros. [edit] Morphology and setting In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic) or extrusive (volcanic). [edit] Intrusive igneous rocks Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusive rocks can also 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, stocks, laccoliths, sills and dikes. The extrusive types usually are called lavas. The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface. Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal. Igneous rock - light colored tracks show the direction of lava flow [edit] Extrusive igneous rocks Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust. The melt, with or without suspended crystals and gas bubbles, is called magma. Magma rises because it is less dense than the rock from which it was created. When it reaches the surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of volcanoes under the 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. Magma which erupts from a volcano behaves according to its viscosity, determined by temperature, composition, and crystal content. High-temperature magma, most of which is 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, tuff and lava, and may have viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magama commonly erupt explosively, and rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt often do so violently, with explosions driven by release of dissolved gases -- typically water but also carbon dioxide. Explosively erupted material is called tephra, and volcanic deposits are called pyroclastic, and they include tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits which can often cover vast areas. Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would be coarse-grained. Because the minerals are fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope, so only an approximate classification can usually be made in the field. [edit] Classification Igneous rock are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends upon the cooling history, and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks include those with essential carbonates. In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist with in a stable association with quartz. Igneous rocks which have crystals large enough to be seen by the naked eye are called phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material. [edit] Texture main article Rock microstructure Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks, including the size, shape, orientation, and distribution of grains and the intergrain relationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simple lava. However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or which are airfall tuffs which may be formed from volcanic ash. Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic apophyses. Mineralogical classification is used most often to classify plutonic rocks and chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, eg; "olivine-bearing picrite" or "orthoclase-phyric rhyolite". see also List of rock textures and Igneous textures [edit] Chemical classification Igneous rocks can be classified according to chemical or mineralogical parameters: Chemical - Total alkali - silica content (TAS diagram) for volcanic rock classification used when modal or mineralogic data is unavailable: acid igneous rocks containing a high silica content, greater than 63% SiO2 (examples rhyolite and dacite) intermediate igneous rocks containing between 52 - 63% SiO2 (example andesite) * basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example basalt) ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite) alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with a molar ratio of alkali to silica greater than 1:6. (examples phonolite and trachyte) Note: the acid-basic terminology is used more broadly in older (generally British) geological literature. In current literature felsic-mafic roughly substitutes for acid-basic. Chemical classification also extends to differentiating rocks which are chemically similar according to the TAS diagram, for instance; Ultrapotassic; rocks containing molar K2O/Na2O >3 Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 >1 Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1 An idealized mineralogy (the normative mineralogy) can be calculated from the chemical composition, and the calculation is useful for rocks too fine-grained or too altered for identification of minerals that crystallized from the melt. For instance, normative quartz classifies a rock as silica-oversaturated; an example is rhyolite. A normative feldspathoid classifies a rock as silica-undersaturated; an example is nephelinite. texture-depends on the size, shape, and arrangement of its mineral crystals. [edit] History of classification In 1902 a group of American petrographers brought forward a proposal to discard all existing classifications of igneous rocks and to substitute for them a "quantitative" classification based on chemical analysis. They showed how vague and often unscientific was much of the existing terminology and argued that as the chemical composition of an igneous rock was its most fundamental characteristic it should be elevated to prime position. Geological occurrence, structure, mineralogical constitution, the hitherto accepted criteria for the discrimination of rock species were relegated to the background. The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when the magma crystallizes, e.g. quartz feldspars, olivine, akermannite, feldspathoids, magnetite, corundum and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another.[1][2] [edit] Mineralogical classification For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criteria is the phenocryst species, followed by the groundmass mineralogy. Often, where the groundmass is aphanitic, chemical classification must be used to properly identify a volcanic rock. Mineralogic contents - felsic versus mafic felsic rock, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite) are usually light coloured, and have low density. mafic rock, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt) are usually dark coloured, and have higher density than felsic rocks. ultramafic rock, with more than 90% of mafic minerals (e.g., dunite) For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least via microscope, the mineralogy is used to classify the rock. This usually occurs on ternary diagrams, where the relative proportions of three minerals are used to classify the rock. The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence. Composition Mode of occurrence Acid Intermediate Basic Ultrabasic Intrusive Granite Diorite Gabbro Peridotite Extrusive Rhyolite Andesite Basalt Komatiite For a more detailed classification see QAPF diagram. [edit] Example of classification Granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica and with more than 10% of felsic minerals) and phaneritic, subeuhedral texture (minerals are visible for the unaided eye and some of them retain original crystallographic shapes). Granite is the most abundant intrusive rock that can be found in the continents. [edit] Magma origination The Earth's crust averages about 35 kilometers thick under the continents, but averages only some 7-10 kilometers beneath the oceans. The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust rest on peridotite of the mantle. Rocks may melt in response to a decrease in pressure, to a change in composition such as an addition of water, to an increase in temperature, or to a combination of these processes. Other mechanisms, such as melting from impact of a meteorite, are less important today, but impacts during accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites in last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces. [edit] Decompression Decompression melting occurs because of a decrease in pressure. The solidus temperatures of most rocks increase with increasing pressure in the absence of water (The solidus temperature of a rock at a given pressure is the maximum temperature below which that rock is completely crystalline.). Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3°C to 4°C per kilometer. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3°C per kilometer. The rock may rise far enough so that its temperature is at the solidus at that shallower depth. If the rock rises higher, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from upward movement of solid mantle is critical in the evolution of the earth. Decompression melting creates the ocean crust at mid-ocean ridges. Decompression melting caused by the rise of mantle plumes is responsible for creating ocean islands like those of the Hawaiian islands. Plume-related decompression melting also is the most common explanation for flood basalts and oceanic plateaus (two types of large igneous provinces), although other causes such as melting related to meteorite impact have been proposed for some of these huge volumes of igneous rock. [edit] Effects of water and carbon dioxide The change of rock composition most responsible for creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, but near or above about 1500°C in the absence of water (Grove and others, 2006). Water is driven out of the ocean lithosphere in subduction zones, and it causes melting in the overlying mantle. Hydrous magmas of basalt and andesite composition are produced directly and indirectly as results of dehydration during the subduction process. Such magmas and those derived from them build up island arcs such as those in the Pacific ring of fire. These magmas form rocks of the calc-alkaline series, an important part of continental crust. The addition of carbon dioxide is relatively a much less important cause of magma formation than addition of water, but genesis of some silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200°C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. Magmas of rock types such as nephelinite, carbonatite, and kimberlite are among those that may be generated following an influx of carbon dioxide into a mantle volume at depths greater than about 70 km. [edit] Temperature increase Increase of temperature is the most typical mechanism for formation of magma within continental crust. Such temperature increases can occur because of the upward intrusion of magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary. The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust. Studies of electrical resistivity deduced from magnetotelluric data have detected a layer that appears to contain silicate melt and that stretches for at least 1000 kilometers within the middle crust along the southern margin of the Tibetan Plateau (Unsworth and others, 2005). Granite and rhyolite are types of igneous rock commonly interpreted as products of melting of continental crust because of increases of temperature. Temperature increases also may contribute to the melting of lithosphere dragged down in a subduction zone. [edit] Magma evolution Main article: Igneous differentiation Most magmas are only entirely melt for small parts of their histories. More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve. As magma cools, minerals typically crystallize from the melt at different temperatures (fractional crystallization). As minerals crystallize, the composition of the residual melt typically changes. If crystals separate from melt, then the residual melt will differ in composition from the parent magma. For instance, a magma of gabbro composition can produce a residual melt of granite composition if early formed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1200°C, and derivative granite-composition melt may have a liquidus temperature as low as about 700°C. Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma. Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions. There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements: silicon, oxygen, aluminium, sodium, potassium, calcium, iron, and magnesium. These are the elements which combine to form the silicate minerals, which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks is expressed differently for major and minor elements and for trace elements. Contents of major and minor elements are conventionally expressed as weight percent oxides (e.g., 51% SiO2, and 1.50% TiO2). Abundances of trace elements are conventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm). The term "trace element" typically is used for elements present in most rocks at abundances less than 100 ppm or so, but some trace elements may be present in some rocks at abundances exceeding 1000 ppm. The diversity of rock compositions has been defined by a huge mass of analytical data -- over 230,000 rock analyses can be accessed on the web through a site sponsored by the U. S. National Science Foundation (see the External Link to EarthChem). [edit] Etymology The word "igneous" is derived from the Latin igneus, meaning "of fire". Volcanic rocks are named after Vulcan, the Roman name for the god of fire. Intrusive rocks are also called plutonic rocks, named after Pluto, the Roman god of the underworld. [edit] References ^ Quantitative Classification of Igneous Rocks, Chicago, 1902 ^ This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Petrology", a publication now in the public domain. R. W. Le Maitre (editor), A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efremova, J. Keller, J. Lamere, P. A. Sabine, R. Schmid, H. Sorensen, and A. R. Woolley, Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press, 2002. ISBN 0-521-66215-X T. L. Grove, N. Chatterjee, S. W. Parman, and E. Medard, The influence of H2O on mantle wedge melting. Earth and Planetary Science Letters, v. 249, p. 74-89 (2006). M. J. Unsworth, A. G. Jones, W. Wei, G. Marquis, S. G. Gokarn, J. E. Spratt, and the INDEPTH-MT team, Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data. Nature, v. 438, p. 78-81 (2005). ( It has loads more inside info too!!!)3

Igneous rocks? | Yahoo Answers

Nov 03, 2007· Best Answer: When most people think about igneous rocks they envision a volcano erupting pumice and lava. The term igneous comes to us from the Latin word "Ignis" which means fire. Igneous rocks are produced this way but most igneous rocks are produced deep underground by the cooling and hardening of magma.

Status: ResolvedAnswers: 3Light-colored igneous rocks are called? | Yahoo AnswersNov 03, 2013Status: ResolvedExplain why dark coloured igneous rocks tend to be heavier ...May 11, 2013Status: ResolvedIn general, how do light colored and dark colored igneous ...Jan 26, 2009Status: ResolvedHow does the density of a light colored igneous rock ...Nov 01, 2008Status: ResolvedSee more results

Igneous rocks? | Yahoo Answers

Nov 04, 2007· Some of the more common types of extrusive igneous rocks are lava rocks, cinders, pumice, obsidian, and volcanic ash and dust. Millions and even billions of years ago molten rock was cooling and thus hardening into igneous rocks deep under the surface of the Earth.

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How are igneous rocks formed? | Yahoo Answers

Feb 27, 2009· Best Answer: Igneous rocks form from a melt of molten rock (magma, or lava) and can be Plutonic (intrusive) or Volcanic (extrusive). Plutonic rocks: * Generally form/solidify underground. Most commonly within magma tubes. * Have relatively large crystals. …

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What is an igneous rock? | Yahoo Answers

Jun 04, 2009· Igneous rock is formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

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Igneous rocks are formed from:? | Yahoo Answers

Jun 19, 2013· The formation of igneous rocks is more like freezing than anything else. As the magma cools, the ions in the magma get together, first forming molecules of minerals with a high melting point (those freeze first), then a lower melting point, and so on.

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what are uses of igneous rocks? | Yahoo Answers

Jun 03, 2011· For the best answers, search on this site https://shorturl.im/axYEg Many types of igneous rocks are used as building stone, facing stone, and decorative material, such as that used for tabletops, cutting boards, and carved figures.

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What are examples of intrusive and extrusive igneous rocks ...

Dec 05, 2006· 95% of the Earth's crust is made of igneous rock. An example of extrusive igneous rock is when magma errupts from a volcano and cools. They have small crystals and cool quickly.

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