Why Are Igneous Rocks The Best Type Of Rock To Use For Radiometric Dating?
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Igneous rock derived from the Latin word ignis meaning fireor magmatic rockis one of the three main rock typesthe others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet 's mantle or crust. Typically, the melting is caused by one or more of three processes: 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. In terms of modes of occurrence, igneous rocks can be either intrusive plutonic and hypabyssal or extrusive volcanic. Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, 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 Radiometric Hookup Of An Igneous Rock Provides the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholithsstocks, laccolithssills and dikes. When the magma solidifies within the earth's crust, it cools slowly forming coarse textured rocks, such as granite, gabbro, or diorite.
The central cores of Radiometric Hookup Of An Igneous Rock Provides 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.
Intrusive igneous rocks that form at depth within the crust are termed plutonic or abyssal rocks and are usually coarse-grained.
Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are usually medium-grained. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopolithsor phacoliths.
From Wikipedia, the free encyclopedia. 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. 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.
Extrusive igneous rocks, also 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 igneous rocks cool and solidify quicker than intrusive igneous rocks.
They are formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and fine-grained.
Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. The molten rock, with or without suspended crystals and gas bubbles, is called magma.
Radiometric or Absolute Rock Dating
It rises because it is less dense than the rock from which it was created. When magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerialwhereas 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 read article proportions: Magma that erupts from a volcano behaves according to its viscositydetermined by temperature, composition, crystal content and the amount of silica. 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.
Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows are typically of limited extent and have steep margins, because the magma is so viscous. 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 from airfall tuffs, which may be formed from volcanic ash. The continental crust is composed primarily of sedimentary rocks resting on a crystalline basement formed of a great variety of metamorphic and igneous rocks, including granulite and granite.
Intermediate composition magma, such as andesitetends to form cinder cones of intermingled ashtuff and lava, and may have a viscosity similar to thick, cold molasses or even rubber when erupted.
Felsic magma, such as rhyoliteis usually erupted at low temperature and is up to 10, times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows are typically 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 the release of dissolved gases—typically water vapour, but also carbon dioxide.
Explosively erupted pyroclastic material is called tephra and includes tuffagglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas. Because lava usually cools and crystallizes rapidly, it is usually 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 more slowly, the please click for source would be coarse-grained. Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous Radiometric Hookup Of An Igneous Rock Provides.
Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscopelink only an approximate classification can usually be made in the field.
Igneous rocks 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 Radiometric Hookup Of An Igneous Rock Provides the classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock.
Feldsparsquartz or feldspathoidsolivinespyroxenesamphibolesand micas are all important minerals in the formation of almost all igneous rocks, and they are basic Radiometric Hookup Of An Igneous Rock Provides 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 read article are separated on the basis of the type of feldspar present, the presence or absence of quartzand 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 in a stable association with quartz. Igneous rocks that 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 source 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.
Http://1dating.info/z/im-white-and-hookup-a-haitian-men-bathing-myvidster.php rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of the mineral grains or crystals of which the rock is composed. Texture is an important criterion for the naming of volcanic rocks.
The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuffa pyroclastic lava or a simple lava. However, Radiometric Hookup Of An Igneous Rock Provides 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 from 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 also tend to be please click for source 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 dikes apophyses.
Mineralogical classification is most often used to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e. Chemical classification also extends to differentiating rocks that are chemically similar according to the TAS diagram, for instance:.
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. Ina group of American petrographers proposed that all existing classifications of igneous rocks should be discarded and replaced by a "quantitative" classification based on chemical analysis. They showed how vague, and often unscientific, much of the existing terminology was 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 Radiometric Hookup Of An Igneous Rock Provides 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.
For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criterion is the phenocryst species, followed by the groundmass mineralogy. Often, where the groundmass is aphaniticchemical classification must be used to properly identify a volcanic rock.
For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least via microscopethe mineralogy is used to classify the rock. This usually occurs on ternary diagramswhere the relative proportions of three minerals are used to classify the rock. The following table is a simple subdivision of igneous rocks according to both their composition and mode of occurrence. For a more detailed classification see QAPF diagram.
Granite is an igneous intrusive rock crystallized at depthwith felsic composition rich in silica and predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase and phaneritic, subeuhedral texture minerals are visible to the unaided eye and commonly some of them retain original crystallographic shapes. The Earth's crust averages about 35 kilometers thick under the continentsbut averages only some 7—10 kilometers beneath the oceans.
The continental crust is composed primarily of sedimentary rocks resting on a 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 waterto an increase in temperature, or to a combination of these processes.
Other mechanisms, such as melting from a meteorite impactare less important today, but impacts during the accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth was probably an ocean of magma.
Impacts of large meteorites in the last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces. Decompression melting occurs because of a decrease in pressure. The solidus temperatures of most rocks the temperatures below which they are completely solid increase with increasing pressure in the absence of water.
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 processbut the click the following article is only about 0. If the rock rises far enough, it will begin to melt.
Melt droplets can coalesce into larger volumes and be intruded upwards.
This process of melting from the upward movement of solid mantle is critical in the evolution of the Earth. Decompression melting creates the ocean crust at mid-ocean ridges.