EENS 3050 | Natural Disasters |
Tulane University | Prof. Stephen A. Nelson |
Volcanoes, Magma, and Volcanic Eruptions |
Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth's surface, we must first discuss the characteristics of magma and how magmas form in the Earth. Characteristics of Magma Types of Magma
Gases in Magmas At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the
gas forms a separate vapor phase when pressure is decreased as magma rises toward the
surface of the Earth. This is similar to carbonated beverages which are bottled at
high pressure. The high pressure keeps the gas in solution in the liquid, but when
pressure is decreased, like when you open the can or bottle, the gas comes out of solution
and forms a separate gas phase that you see as bubbles. Gas gives magmas their
explosive character, because volume of gas expands as pressure is reduced. The
composition of the gases in magma are:
The amount of gas in a magma is also related to the chemical composition of the magma. Rhyolitic magmas usually have higher gas contents than basaltic magmas. |
Temperature of Magmas
Viscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity).
Viscosity depends on primarily on the composition of the magma, and
temperature.
Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their
viscosity is still 10,000 to 100,0000 times more viscous than water.
Rhyolitic magmas tend to have even higher viscosity, ranging between 1
million and 100 million times more viscous than water. (Note that
solids, even though they appear solid have a viscosity, but it very high,
measured as trillions times the viscosity of water). Viscosity is an
important property in determining the eruptive behavior of magmas. |
Summary Table |
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Magma Type | Solidified Rock | Chemical Composition | Temperature | Viscosity | Gas Content |
Basaltic | Basalt | 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na | 1000 - 1200 oC | Low | Low |
Andesitic | Andesite | 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K | 800 - 1000 oC | Intermediate | Intermediate |
Rhyolitic | Rhyolite | 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na. | 650 - 800 oC | High | High |
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, magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH. 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. |
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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: |
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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. 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.
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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. 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. In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas. 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.
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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 |
Volcanic Eruptions
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Effusive (Non-explosive) Eruptions
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. Lava Flows 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 6.17 in your text). Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent. Explosive Eruptions
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Tephra and Pyroclastic Rocks
Average Particle Size (mm) Unconsolidated Material (Tephra) Pyroclastic Rock >64 Bombs or Blocks Agglomerate 2 - 64 Lapilli Lapilli Tuff <2 Ash Ash Tuff
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Pyroclastic Deposits Pyroclastic material ejected explosively from volcanoes becomes deposited on the land surface. The process of deposition leaves clues that allow geologists to interpret the mode of ejection from the volcano. Fall Deposits
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Pyroclastic Flows Pyroclastic flows are also sometimes called pyroclastic density currents (PDCs). They can range from surges which can have a range of clast densities from low to high with generally low concentration of of solid clasts (high amonts of gases) to high clast concentration clouds of ash and gas (pyroclastic flows). |
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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. |
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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. |
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Surge Deposits
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Surges tend to hug the ground as they flow over the surface and thus tend to produce thicker deposits in valleys with thinner deposits over ridges. This helps to distinguish surge deposits from flow deposits and fall deposits.
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Because they move close to ground, friction with ground tends to produce cross stratification in the deposits. Individual layers can be well-sorted, but overall the deposits tend to be poorly sorted. |
Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the behavior of the eruption is continually changing throughout the course of the eruption. This makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified according to the principal types of behavior that they exhibit. An important point to remember, however, is that during a given eruption the type of eruption may change between several different types.
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Questions on this material that could be asked on an exam
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