A magma's resistance to flow is a function of its "internal friction" derived from the generation of chemical bonds within the liquid. Chemical bonds are created between negatively charged and positively charged ions anions and cations , respectively. Of the ten most abundant elements found in magmas see above , oxygen is the only anion. Silicon, on the other hand, is the most abundant cation. Thus, the Si-O bond is the single most important factor in determining the degree of a magma's viscosity.
These two elements bond together to form "floating radicals" in the magma, while it is still in its liquid state i. These floating radicals contain a small silicon atom surrounded by four larger oxygen atoms SiO 4. This atomic configuration is in the shape of a tetrahedron. The radicals are therefore called silicon-oxygen tetrahedra , as shown here.
These floating tetrahedra are electrically charged compounds. As such, they they are electrically attracted to other Si-O tetrahedra. The outer oxygen atoms in each tetrahedron can share electrons with the outer oxygen atoms of other tetrahedra.
The sharing of electrons in this manner results in the development of covalent bonds between tetrahedra. In this way Si-O tetrahedra can link together to form a variety shapes: double tetrahedra shown here, C , chains of tetrahedra, double chains of tetrahedra, and complicated networks of tetrahedra. As the magma cools, more and more bonds are created, which eventually leads to the development of crystals within the liquid medium.
Thus, the Si-O tetrahedra form the building blocks to the common silicate minerals found in all igneous rocks. However, while still in the liquid state, the bonding of tetrahedra results in the polymerization of the liquid, which increases the "internal friction" of the magma, so that it more readily resists flow. Magmas that have a high silica content will therefore exhibit greater degrees of polymerization, and have higher viscosities, than those with low-silica contents.
The amount of dissolved gases in the magma can also affect it's viscosity, but in a more ambiguous way than temperature and silica content. Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in size, style, and composition of erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines 1 the eruption style, 2 the type of volcanic cone that forms, and 3 the composition of rocks that are found at the volcano.
Different minerals within a rocks melt at different temperatures and the amount of partial melting and the composition of the original rock determine the composition of the magma. Magma collects in magma chambers in the crust at kilometers miles beneath the surface of a volcano.
The words that describe composition of igneous rocks also describe magma composition. Mafic magmas are low in silica and contain more dark, magnesium and iron rich mafic minerals, such as olivine and pyroxene. Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar. The higher the amount of silica in the magma, the higher is its viscosity. Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface.
Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move and so it gets stuck.
Dissolved gases become trapped by thick magma and the magma chamber begins to build pressure. Since rocks are 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:. Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.
Melting of wet rocks is similar to melting of wet minerals, except there is range of temperature range over which partial melting occurs. Again, the temperature of beginning of melting first decreases with increasing pressure or depth, then at high pressure or depth the melting temperatures again begin to rise. Three ways to Generate Magmas 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.
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.
Initial Composition of Magma The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting.
Magmatic Differentiation But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.
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. Crystal Fractionation - When magma solidifies to form a rock 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.
Depending on how many minerals are lost in this fashion, a wide range of compositions can be made. The processes is called magmatic differentiation by crystal fractionation.
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.
Volcanic Eruptions In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks. Effusive Non-explosive Eruptions Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas. Lava Flows Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere. Explosive Eruptions Explosive eruptions are favored by high gas content and high viscosity andesitic to rhyolitic magmas.
Blocks are angular fragments that were solid when ejected. Bombs have an aerodynamic shape indicating they were liquid when ejected. Bombs and lapilli that consist mostly of gas bubbles vesicles result in a low density highly vesicular rock fragment called pumice. Clouds of gas and tephra that rise above a volcano produce an eruption column that can rise up to 45 km into the atmosphere.
Eventually the tephra in the eruption column will be picked up by the wind, carried for some distance, and then fall back to the surface as a tephra fall or ash fall. If the eruption column collapses a pyroclastic flow will occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites if they contain pumice or pyroclastic flow deposits if they contain non-vesicular blocks.
Pyroclastic Deposits Pyroclastic material ejected explosively from volcanoes becomes deposited on the land surface. Fall Deposits. Material ejected into an eruption column eventually falls back to the earth's surface and blankets the surface similar to the way snow blankets the earth. The thickest deposits occur close to vent and get thinner with distance from the vent.
By measuring the thickness at numerous locations one can construct an isopach map. Such isopach maps help to locate the source volcanic vent if it is not otherwise known and provides information about wind direction in the upper levels of the atmosphere during the eruption. Fall deposits are usually fairly well-sorted, meaning that the clast size does not vary too much within the individual deposit.
The clast size can be ash as in a cinder cone. They may also contain clasts of rock fragments called lithic fragments that are pieces of the volcanic structure ripped from the sides of the conduit during the explosive eruption. If the pyroclastic flows consist of solid clasts with high density along with ash fragments, they are called block and ash flows.
If the pyroclastic flows have low density clasts pumice along with ash, they are called ignimbrites. There are no definitive boundary between pyroclastic flows and surges as they grade into one another continuously. Similarly, ignimbrites grade into block and ash flows as the clast density increases. Pyroclastic Flow Deposits Pyroclastic flows tend to follow valleys or low lying areas of topography. The material deposited, thus tends to fill valleys, rather than uniformly blanket the topography like fall deposits.
Block and Ash Flow Deposits. Ignimbrites Ignimbrites contain blocks of pumice in an unsorted mixture of ash, lapilli, pumice blocks, and lithic fragments. Sometimes one finds concentrated zones of pumice or lithic fragments in the deposits. Surge Deposits. What type of magma has the greatest silica content? Which type of rock has the greatest silica content? Which type of igneous rocks has the greatest silica content?
What does it mean by higher silica content magma in terms of viscosity? How does magma rise? What causes magma to rise at subduction zone? What causes magma to rise in a subduction zone?
What occurs at a subduction zone? Which side of Japan has deeper earthquakes? Why Is Japan a subduction zone? Does Japan sit on a tectonic plate? Is Japan located near a subduction zone? Why is Japan so tectonically active?
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