Magmas, Igneous Rocks, Volcanoes and also Plutons

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Magmas, Igneous Rocks, Volcanoes, and Plutons ago to components of entire Course... type of Igneous absent varieties of Magma Eruption that Magma origin of Magma origin of Basaltic Magma origin of Granitic Magma beginning of Andesitic Magma Magmatic Differentiation Bowen"s Reaction series Volcanoes and Volcanic Rocks Plutons adapted to HTML from lecture note of Prof. Stephen A. Nelson Tulane university kinds of Igneous rock Igneous Rocks are formed by crystallization indigenous a liquid, or magma. They incorporate two types: volcano or extrusive igneous rocks within the magma cools and also crystallizes top top the surface ar of the earth Intrusive or plutonic igneous rocks wherein the magma crystallizes in ~ depth in the Earth.


You are watching: Granitic magma is very silica-rich.

Magma is a mixture of liquid rock, crystals, and also gas. Defined by a wide selection of chemical compositions, through high temperature, and properties that a liquid.
Magmas are less thick than bordering rocks, and also will thus move upward. If magma renders it come the surface ar it will certainly erupt and also later crystallize to type an extrusive or volcanic rock. If that crystallizes before it reaches the surface ar it will type an igneous rock at depth referred to as a plutonic or intrusive igneous rock. Because cooling of the magma takes place at a various rate, the crystals that form and your interrelationship (texture) exhibit different properties.
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rapid cooling ~ above the surface outcomes in many tiny crystals or quenching come a glass. Provides rise to aphanitic texture (crystals cannot be distinguished with the nude eye), or obsidian (volcanic glass).
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slow-moving cooling in ~ depth in the planet results in fewer much bigger crystals , offers rise to phaneritic texture.
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Porhpyritic texture develops when slow-moving cooling is followed by fast cooling. Phenocrysts = bigger crystals, procession or groundmass = smaller sized crystals. other rock textural state species of Magma chemical composition that magma is controlled by the diversity of aspects in the earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O comprise 99.9%. Because oxygen is therefore abundant, chemistry analyses space usually provided in regards to oxides. SiO2 is the many abundant oxide. Basaltic or gabbroic -- SiO2 45-55 wt%, high in Fe, Mg, Ca, short in K, Na Andesitic or Dioritic -- SiO2 55-65 wt%, intermediate. In Fe, Mg, Ca, Na, K Rhyolitic or Granitic -- SiO2 65-75%, short in Fe, Mg, Ca, high in K, Na. Gases - in ~ depth in the Earth almost all magmas save gas. Gas gives magmas your explosive character, due to the fact that the gas expands as press is reduced. higher SiO2 contents magmas have higher viscosity than reduced SiO2 content magmas lower Temperature magmas have greater viscosity than greater temperature magmas. Largely H2O through some CO2 Minor quantities of Sulfur, Cl , and also F Rhyolitic or granitic magmas generally have greater gas components than basaltic or gabbroic magmas. Temperature the Magmas Basaltic or gabbroic - 1000-1200oC Andesitic or dioritic - 800-1000oC Rhyolitic or granitic - 650-800oC. Viscosity of Magmas - Viscosity is the resistance to circulation (opposite of fluidity) relies on composition, temperature, gas content. higher SiO2 content magmas have higher viscosity than reduced SiO2 contents magmas reduced Temperature magmas have higher viscosity than greater temperature magmas. an introduction Table Magma form coagulation Volcanic absent coagulation Plutonic absent chemical Composition Temperature Viscosity Gas content
Basaltic Basalt Gabbro 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC low low
Andesitic Andesite Diorite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC intermediate intermediary
Rhyolitic Rhyolite Granite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na 650 - 800 oC High High
Eruption that Magma once magmas reach the surface of the earth they erupt from a vent. They might erupt explosively or non explosively. No explosive eruptions room favored by short gas content and low viscosity magmas (basaltic come andesitic magmas). Usually begin with fire fountains because of release of dissolved gases produce lava operation on surface produce Pillow lavas if erupted beneath water Explosive eruptions space favored through high gas content and also high viscosity (andesitic come rhyolitic magmas). Expansion of gas bubbles is resisted by high viscosity that magma - results in building of push High push in gas bubbles causes the balloon to burst as soon as reaching the low pressure at the Earth"s surface. Bursting that bubbles fragments the magma into pyroclasts and tephra (ash). Cloud of gas and tephra rises over volcano to develop an eruption pillar that deserve to rise as much as 45 km right into the atmosphere.
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If eruption obelisk collapses a pyroclastic flow may occur, within gas and also tephra rush under the flanks the the volcano in ~ high speed. This is the most dangerous type of volcanic eruption. The deposits the are created arecalled ignimbrites.
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Lateral blasts and debris avalanches take place when gas is released all of sudden by a big landslide or debris avalanche taking out component of the volcano beginning of Magma In order for magmas come form, some part of the earth must gain hot enough to melt the rocks present. Under common conditions, the geothermal gradient is not high enough to melt rocks, and thus through the exception of the outer core, most of the planet is solid. Thus, magmas form only under special circumstances. To understand this we must first look at just how rocks and also mineral melt. as pressure boosts in the Earth, the melting temperature alters as well. Because that pure minerals, there space two general cases.
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because that a pure dried (no H2O or CO2 present) mineral, the melting temperate boosts with increasing pressure.
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for a mineral v H2O or CO2 present, the melt temperature first decreases with boosting pressure due to the fact that rocks mixture of minerals, they behave rather differently. Unlike minerals, rocks execute not melt in ~ a solitary temperature, yet instead melt end a selection of temperatures. For this reason it is feasible to have partial melts, from which the liquid section might be extracted to kind magma. The two general cases are:
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melt of dry rocks is similar to melt of dry minerals, melt temperatures boost with enhancing pressure, other than there is a variety of temperature end which there exists a partial melt. The degree of partial melt can range from 0 come 100%
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melt of rocks comprise water or carbon dioxide is comparable to melting of wet minerals, melt temperatures initially decrease with boosting pressure, other than there is a variety of temperature over which over there exists a partial melt. origin of Basaltic Magma much evidence says that Basalticmagmas result from dried partial melt of mantle. Basalts make up most that oceanic crust and also only mantle underlies crust. Basalts contain minerals prefer olivine, pyroxene and plagioclase, nobody of i m sorry contain water. Basalts erupt non-explosively, indicating a short gas content and also therefore short water content. The Mantle is make of garnet peridotite (a rock made up of olivine, pyroxene, and garnet) -- evidence originates from pieces brought up by erupting volcanoes. In the activities we deserve to determine the melting habits of garnet peridotite. Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is reduced than the start of melting of the mantle. For this reason in order because that the mantle to melt there needs to be a device to advanced the geothermal gradient. When such device is convection, wherein hot mantle material rises to reduced pressure or depth, carrying its warm with it. If the increased geothermal gradient becomes greater than the initial melt temperature at any type of pressure, then a partial melt will form. Liquid from this partial melt can be separated indigenous the staying crystals because, in general, liquids have actually a lower density than solids. Basaltic or gabbroic magmas show up to originate in this way.
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origin of Granitic Magma many Granitic or Rhyolitic magma appears to an outcome from wet melt of continent crust. The evidence for this is: most granites and rhyolites are found in locations of continental crust. Once granitic magma erupts native volcanoes the does so really explosively, describe high gas content. solidified granite or rhyolite consists of quartz, feldspar, hornblende, biotite, and also muscovite. The latter minerals save water, denote high water content
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Still, the temperature in continent crust is typically not high sufficient to cause melting, and thus another heat source is necessary. In most instances it appears that this heat resource is basaltic magma. The basaltic magma is generated in the mantle, climate rises right into the continental crust. But, because basaltic magma has a high density it might stop in the crust and also crystallize, releasing warm into the neighboring crust. This raises the geothermal gradient and also may cause wet partial melting of the crust to create rhyolitic magmas. origin of Andesitic Magma average composition of continent crust is andesitic, however if andesite magma is created by melt of continent crust climate it requires finish melting that crust. Temperature in crust i can not qualify to acquire high enough. Andesitic magmas erupt in areas over subduction zones - says relation in between production the andesite and subduction. One theory involves wet partial melting of subducted oceanic crust. But, more recent theories suggest wet partial melting of mantle.
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Magmatic Differentiation when magma solidifies to type a absent it go so over a range of temperature. Each mineral begins to crystallize in ~ a different temperature, and if these minerals are somehow gotten rid of from the liquid, the liquid composition will change. Depending on how plenty of minerals are lost in this fashion, a wide variety of compositions can be made. The procedures is referred to as magmatic differentiation by decision fractionation. Crystals can be gotten rid of by a range of processes. If the crystals are more dense than the liquid, they might sink. If they space less thick than the liquid they will certainly float. If liquid is squeezed the end by pressure, climate crystals will be left behind. Remove of crystals have the right to thus adjust the ingredient of the liquid section of the magma. Let me illustrate this utilizing a very straightforward case. Imagine a liquid containing 5 molecule of MgO and also 5 molecules of SiO2. Initially the composition of this magma is expressed together 50% SiO2 and 50% MgO. I.e
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currently let"s imagine I eliminate 1 MgO molecule by putting it right into a crystal and removing the decision from the magma. Now what are the percentages of every molecule in the liquid?
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If we continue the procedure one much more time by removed one more MgO molecule
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Thus, ingredient of liquid deserve to be changed. Bowen"s Reaction series Bowen uncovered by experiment the the bespeak in which minerals crystallize indigenous a basaltic magma counts on temperature. As a basaltic magma is cooled Olivine and also Ca-rich plagioclase crystallize first. Upon more cooling, Olivine reacts with the liquid to develop pyroxene and Ca-rich plagioclase react through the liquid to develop less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are eliminated from the liquid by crystal fractionation, climate the staying liquid will be an ext SiO2 rich. If the procedure continues, an initial basaltic magma can adjust to very first and andesite magma then a rhyolite magma with falling temperature
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Volcanoes and Volcanic Rocks Basalts, Andesites, Dacites and Rhyolites space all varieties of volcanic rock identified on the basis of their mineral assemblage. Escape on conditions present during eruption and also cooling, any type of of these rock species may kind one of the following varieties of volcanic rocks. Obsidian - dark fancy volcanic glass mirroring concoidal fracture. Typically rhyolitic or dacitic. Pumice - light colored and also light weight rock consists of largely holes (vesicles) that were once occupied by gas, typically rhyolitic, dacitic or andesitic. Vesicular absent - rock filled v holes (Swiss cheese) or vesicles that were once lived in by gas. Commonly basaltic and also andesitic. Amygdaloidal basalt. If motor in a vesicular basalt are later on filled by precipitation that calcite or quartz, the fillings are termed amygdules and the basalt is termed an amygdaloidal basalt. Pyroclasts and also Tephra Pyroclasts = hot, damaged fragments. Result from explosively take it apart of magma. Loosened assemblages the pyroclasts called tephra. Relying on size, tephra deserve to be classified together bombs. Lapilli, or ash. Rock developed by build-up and cementation the tephra called a pyroclastic absent or tuff. Welding, compaction and deposition of various other grains reason tephra (loose material) to be converted in pyroclastic rock. Volcanoes
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Shield volcano - volcanoes that erupt low viscosity magma (usually basaltic) the flows long distances from the vent.
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Pyroclastic cone or cinder cone - a volcano developed mainly the tephra autumn deposits situated immediately about the vent.
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Stratovolcano (composite volcano) - a volcano developed of interbedded lava flows and also pyroclastic material. Crater - a depression led to by explosive ejection of magma or gas.
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Caldera - a depression led to by collapse of a volcano right into the cavity once lived in by magma
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Lava Dome - a steep face volcanic structure resulting indigenous the eruption the high viscosity, short gas contents magma thermal Springs and Geysers - warm water resulting from heating by magma in ~ depth in the Earth. Springs flow, geysers erupt. Fissure Eruptions - one eruption the occurs follow me a narrow crack or fissure in the Earth"s surface. Pillow Lava - Lavas developed by eruption in ~ the surface ar of the s or a lake. Plutons Igneous rocks cooled at depth. Name comes from Greek god the the underworld - Pluto. Dikes - a little tabular intrusion the cuts across layers of pre-existing rock. Sills - a tabular intrusion the intruded parallel to layers that pre-existing rock. Lacoliths - comparable to a sill, other than the layers above are bent increase by the intrusion. Volcano Necks - a previous volcanic conduit that has been exposed by erosion that the volcano. Batholiths - a big intrusion commonly of granitic or dioritic composition.

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Stocks - a somewhat smaller sized intrusion normally intruded off of a bigger batholith. methods of intrusion melting - crystallization Stoping - xenoliths Injection concern - Why execute we see intrusive igneous rocks in ~ the surface ar of the Earth? prize - They room exposed through erosion which has actually removed all of the material over the intrusion height