Viscosity of Magmas. Viscosity is the resistance to flow opposite of fluidity. As we have seen the only part of the earth that is liquid is the outer core. But the core is not likely to be the source of magmas because it does not have the right chemical composition. The outer core is mostly Iron, but magmas are silicate liquids. Thus, since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present.
We know that temperature increases with depth in the earth along the geothermal gradient. The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth.
Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid. Thus, magmas form only under special circumstances. To understand this we must first look at how rocks and mineral melt. As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.
For a pure dry no H 2 O or CO 2 present mineral, the melting temperate increases with increasing pressure. For a mineral with H 2 O or CO 2 present, the melting temperature first decreases with increasing pressure. Since rocks mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures.
Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma. The two general cases are:. From the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way. The geothermal gradient can be raised by upwelling of hot material from below either by uprise solid material decompression melting or by intrusion of magma heat transfer.
Lowering the melting temperature can be achieved by adding water or Carbon Dioxide flux melting. If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. Basaltic magmas appear to originate in this way.
Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys. Thus, generation of magma in these three environments is likely caused by decompression melting. Transfer of Heat - When magmas that were generated by some other mechanism intrude into cold crust, they bring with them heat. Upon solidification they lose this heat and transfer it to the surrounding crust. Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas.
Transfer of heat by this mechanism may be responsible for generating some magmas in continental rift valleys, hot spots, and subduction related environments. Flux Melting - As we saw above, if water or carbon dioxide are added to rock, the melting temperature is lowered. If the addition of water or carbon dioxide takes place deep in the earth where the temperature is already high, the lowering of melting temperature could cause the rock to partially melt to generate magma.
One place where water could be introduced is at subduction zones. Here, water present in the pore spaces of the subducting sea floor or water present in minerals like hornblende, biotite, or clay minerals would be released by the rising temperature and then move in to the overlying mantle. Introduction of this water in the mantle would then lower the melting temperature of the mantle to generate partial melts, which could then separate from the solid mantle and rise toward the surface.
The chemical composition of magma can vary depending on the rock that initially melts the source rock , and process that occur during partial melting and transport. The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting.
Melting of crustal sources yields more siliceous magmas. In general more siliceous magmas form by low degrees of partial melting.
As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic peridotite source yields a basaltic magma. But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.
These processes are referred to as magmatic differentiation and include assimilation, mixing, and fractional crystallization. Assimilation - As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous.
Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks. Fractional Crystallization - When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change.
The processes is called magmatic differentiation by Fractional Crystallization. Because mafic minerals like olivine and pyroxene crystallize first, the process results in removing Mg, Fe, and Ca, and enriching the liquid in silica.
Thus crystal fractionation can change a mafic magma into a felsic magma. Crystals can be removed by a variety of processes. If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind.
Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case. Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO 2. If we continue the process one more time by removing one more MgO molecule. Bowen's Reaction Series Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature. As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first.
Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO 2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature. The intrusive or plutonic environment is below the surface of the earth.
This environment is characterized by higher temperatures which result in slow cooling of the magma. Intrusive or plutonic igneous rocks form here. Where magma erupts on the surface of the earth, temperatures are lower and cooling of the magma takes place much more rapidly. This is the extrusive or volcanic environment and results in extrusive or volcanic igneous rocks. When magmas reach the surface of the Earth they erupt from a vent called a volcano. They may erupt explosively or non-explosively.
They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture.
Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite , found often in the U. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand. Rock texture with crystals that are invisible without magnification. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.
Sills parallel beds layers and foliations in the surrounding country rock. They can be originally emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills into near vertical orientations.
Sills can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock.
On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed contact metamorphism. Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles bubbles where gases escaped into the atmosphere.
Because sills generally form at shallow depths up to many kilometers below the surface, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not. Figure 7.
Certain layered intrusions are a variety of sill that often contain important ore deposits. Precambrian examples include the Bushveld, Insizwa and the Great Dyke complexes of southern Africa, the Duluth intrusive complex of the Superior District, and the Stillwater igneous complex of the United States.
These intrusions often contain concentrations of gold, platinum, chromium and other rare elements. Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by relatively short dike-like segments. Such sills are known as transgressive, examples include the Whin Sill and sills within the Karoo basin. Around the Pacific Rim is Indonesia, a nation built from the dotted volcanoes of an island arc.
Indonesia is distinctive for its rich volcanic soil, tropical climate, tremendous biodiversity, and volcanoes. These volcanoes are in Java, Indonesia. The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes. Eruptions also take place through other types of vents, commonly from fissures Figure 8. The eruptions that created the entire ocean floor are essentially fissure eruptions.
Figure 8. Viscous lava flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome. Because it is so thick, the lava does not flow far from the vent. Figure 9. Lava domes are large, round landforms created by thick lava that does not travel far from the vent. Lava flows often make mounds right in the middle of craters at the top of volcanoes, as seen in the Figure Figure Lava domes may form in the crater of composite volcanoes as at Mount St.
A lava plateau forms when large amounts of fluid lava flow over an extensive area Figure When the lava solidifies, it creates a large, flat surface of igneous rock. Layer upon layer of basalt have created the Columbia Plateau, which covers more than , square kilometers 63, square miles in Washington, Oregon, and Idaho.
Lava creates new land as it solidifies on the coast or emerges from beneath the water Figure Over time the eruptions can create whole islands.
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