How Volcanoes Work - controls on eruption style
The magma types vary from mafic magmas, which have relatively low silica and high Fe and Mg contents, to felsic A magma's viscosity is largely controlled by its temperature, composition, and gas content (see Vesicle-rich flow top. Silicate-rich magmas are typically formed at destructive plate boundaries, by partial melting and/or assimilation of crustal rocks (which are richer in silica than the. Discuss viscosity, silica content, volatiles, and temperature as each relates to magma composition. Explain the relationship between igneous activity and plate tectonics. . Most are associated with explosive eruptions of gas-rich magma.
Chemical Composition of Magmas The chemical composition of magma can vary depending on the rock that initially melts the source rockand 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. 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. 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. 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. 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.
Geological Society - Viscosity of Magmas
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 SiO2. 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 SiO2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature.
At high degrees of partial melting of the mantle, komatiite and picrite are produced. Migration and solidification of magmas[ edit ] Magma develops within the mantle or crust where the temperature and pressure conditions favor the molten state.
After its formation, magma buoyantly rises toward the Earth's surface. As it migrates through the crust, magma may collect and reside in magma chambers though recent work suggests that magma may be stored in trans-crustal crystal-rich mush zones rather than dominantly liquid magma chambers . Magma can remain in a chamber until it cools and crystallizes forming igneous rock, it erupts as a volcanoor moves into another magma chamber.
Viscosity of Magmas
There are two known processes by which magma changes: Plutonism[ edit ] When magma cools it begins to form solid mineral phases. Some of these settle at the bottom of the magma chamber forming cumulates that might form mafic layered intrusions. Magma that cools slowly within a magma chamber usually ends up forming bodies of plutonic rocks such as gabbrodiorite and granitedepending upon the composition of the magma. Alternatively, if the magma is erupted it forms volcanic rocks such as basaltandesite and rhyolite the extrusive equivalents of gabbro, diorite and granite, respectively.
Volcanism During a volcanic eruption the magma that leaves the underground is called lava. Lava cools and solidifies relatively quickly compared to underground bodies of magma. This fast cooling does not allow crystals to grow large, and a part of the melt does not crystallize at all, becoming glass. Rocks largely composed of volcanic glass include obsidianscoria and pumice. Before and during volcanic eruptions, volatiles such as CO2 and H2O partially leave the melt through a process known as exsolution.
Magma with low water content becomes increasingly viscous. If massive exsolution occurs when magma heads upwards during a volcanic eruption, the resulting eruption is usually explosive.Factors Affecting Volcanic Eruptions
Magma usage for energy production[ edit ] The Iceland Deep Drilling Projectwhile drilling several 5,m holes in an attempt to harness the heat in the volcanic bedrock below the surface of Iceland, struck a pocket of magma at 2,m in Keszthelyi observed the temperature at the base of pahoehoe lava flow lobes extruded on Kilauea Volcano, Hawaii.
The delay was attributed to crystallization kinetics and the heating to latent heat release. Because the initial cooling is so rapid, the magma is forced into a supercooled state with few crystals grown.
When nucleation does begin it is as a sudden pulse which then liberates a large amount of latent heat. For andesitic magma the latent heat of solidification is approximately 3. In this paper we consider flow in an axisymmetric cylindrical conduit of infinitesimal and finite length to simulate flow of a cooling magma in a conduit. The non-linear governing equations are solved using the finite element method. The lava rheology is described by an empirical expression for the viscosity considering crystal and water content, as well as temperature dependence appropriate for lava dome building eruptions.
Newtonian and non-Newtonian i.
Volcanoes, Magma, and Volcanic Eruptions
Either adiabatic or isothermal boundary conditions are used at the conduit wall to represent the two end-member states. An isothermal boundary condition most appropriately represents the conduit wall at the start of an eruption. As the eruption continues, the conduit walls will heat up and tend towards an adiabatic condition.
We consider first the effects of viscous heating alone, and then in combination with crystal-growth to account for the latent heat released. Crystal growth is considered using the theory of Hort as developed by Melnik and Sparkswhich account for changes in magma liquidus in the melt phase.
Our models maintain a constant mass flux because it would not be appropriate to use a constant pressure in a conduit segment since in later models we consider flow along a conduit length and here the pressure gradient is not expected to remain constant. However, lava dome-forming eruptions typically experience steady state extrusion rates over extended periods of time Harris