Where is andesitic magma found

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, and thus, volcanoes are only found on the Earth's surface in areas above where these special circumstances occur.

Volcanoes don't just occur anywhere, as we shall soon see. To understand this we must first look at how rocks and mineral melt. To understand this we must first look at how minerals and rocks melt. As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.

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. The Mantle is made of garnet peridotite a rock made up of olivine, pyroxene, and garnet -- evidence comes from pieces brought up by erupting volcanoes. In the laboratory we can determine the melting behavior of garnet peridotite.

Decompression Melting - Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it.

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.

Rhyolitic magma can also be produced by changing the chemical composition of basaltic magma as discussed later. 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. Chemical Composition of Magmas. 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. Now let's imagine I remove 1 MgO molecule by putting it into a crystal and removing the crystal from the magma.

Now what are the percentages of each molecule in the liquid? If we continue the process one more time by removing one more MgO molecule. Thus, composition of liquid can be changed. This process is called crystal fractionation. A mechanism by which a basaltic magma beneath a volcano could change to andesitic magma and eventually to rhyolitic magma through crystal fractionation, is provided by Bowen's reaction series, discussed next.

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.

In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks. As they rise they may encounter a depth or pressure where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase i. When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution.

In other words, the gas bubbles begin to expand. If the liquid part of the magma has a low viscosity, then the gas can expand relatively easily. When the magma reaches the Earth's surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a non-explosive eruption will occur, usually as a lava flow Lava is the name we give to a magma when it on the surface of the Earth. If the liquid part of the magma has a high viscosity, then the gas will not be able to expand very easily, and thus, pressure will build up inside of the gas bubble s.

When this magma reaches the surface, the gas bubbles will have a high pressure inside, which will cause them to burst explosively on reaching atmospheric pressure. This will cause an explosive volcanic eruption. Effusive Non-explosive Eruptions. Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas.

If the viscosity is low, non-explosive eruptions usually begin with fire fountains due to release of dissolved gases. When magma reaches the surface of the earth, it is called lava. Since it its a liquid, it flows downhill in response to gravity as a lava flows. Different magma types behave differently as lava flows, depending on their temperature, viscosity, and gas content. Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere.

This causes a surface skin to form, although it is still very hot and behaves in a plastic fashion, capable of deformation. Such lava flows that initially have a smooth surface are called pahoehoe flows. Initially the surface skin is smooth, but often inflates with molten lava and expands to form pahoehoe toes or rolls to form ropey pahoehoe.

See figure 6. Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent. A'A' Flows - Higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, but this is quickly broken up by flow of the molten lava within and by gases that continue to escape from the lava.

This creates a rough, clinkery surface that is characteristic of an A'A' flow see figure 6. Rhyolitic magma forms as a result of wet melting of continental crust. Rhyolites are rocks that contain water and minerals that contain water, such as biotite. The continental crust must be heated above the normal geothermal gradient in order to melt.

The most common cause of a rise in temperature of continental crust is basaltic magma rising from the mantle. Basaltic magma is usually very dense and gets stopped in the continental crust rather than reaching the surface, causing it to crystallize. Andesitic magma is formed through wet partial melting of the mantle.

The mantle under the ocean has contact with water. When subduction, or continental plates pulling away from one another, occurs, the mantle will heat up and water is pushed into it. This causes the melting temperature of the mantle to decrease, causing the mantle to begin partially melting due to the heat.

Basaltic magma with a high water content is the result. If this type of basaltic magma melts with continental crust that has a high density of dioxide silicon, andesitic magma will form. Lindsey Fisher began writing professionally in Facts on Convergent Boundaries.

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Most are located around the Pacific Ocean in what is commonly called the Ring of Fire. A volcano is defined as an opening in the Earth's crust through which lava, ash, and gases erupt. The term also includes the cone-shaped landform built by repeated eruptions over time. Teach your students about volcanoes with this collection of engaging material. Magma is a mixture of molten and semi-molten rock found beneath the surface of the Earth. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.

Skip to content. Image lava flow Lava magma that has erupted onto the Earth's surface is visually mesmerizing — as the molten rock flows downhill, lava exposed to the air cools to a deep black color, while the molten rock beneath glows bright orange.

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