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. When gases begin to escape exsolve from the magma, the effect of gas bubbles on the bulk viscosity is variable. Although the growing gas bubbles will exhibit low viscosity, the viscosity of the residual liquid will increase as gas escapes. The overall bulk viscosity of the bubble-liquid mixture depends on both the size and distribution of the bubbles.
Although gas bubbles do have an effect on the viscosity, the more important role of these exsolving volatiles is that they provide the driving force for the eruption. This is discussed in more detail below. As dissolved gases are released from the magma, bubbles will begin to form. Bubbles frozen in a porous or frothy volcanic rock are called vesicles , and the process of bubble formation is called vesiculation or gas exsolution.
The dissolved gases can escape only when the vapor pressure of the magma is greater than the confining pressure of the surrounding rocks. The vapor pressure is largely dependent on the amount of dissolved gases and the temperature of the magma. Gas escape through vertical vesicle cylinders Vesicle-rich flow top. Explosive eruptions are initiated by vesiculation, which in turn, can be promoted in two ways: 1 by decompression , which lowers the confining pressure, and 2 by crystallization, which increases the vapor pressure.
In the first case, magma rise can lead to decompression and the formation of bubbles, much like the decompression of soda and the formation of CO 2 bubbles when the cap is removed.
This is sometimes referred to as the first boiling. Alternatively, as magma cools and anhydrous minerals begin to crystallize out of the magma, the residual liquid will become increasingly enriched in gas. In this case, the increased vapor pressure in the residual liquid can also lead to gas exsolution. This is sometimes referred to as second or retrograde boiling.
Both mechanisms can trigger an explosive volcanic eruption. The amount of dissolved gas in the magma provides the driving force for explosive eruptions. No, quiet eruptions. That would normally be magma with a low silica content. Shield volcanoes usually produce "quiet" eruptions, but explosive eruptions can occur if water gets into the magma chamber.
It produces a quiet eruption. The viscosity of magma or lava will determine whether or not the eruption is explosive or quiet. Higher viscosity magma can result in explosive eruptions. Lower viscosity magmas tend to flow more freely. Quiet eruptions. It depends on how much silica is in the magma. If there is low-silica in the magma then the volcano will erupt quietly. If there is high-silica in the magma then the volcano will erupt explosively.
If the composition of the magma is high in silica, the eruption will be explosive. The Eruption of Mt. Helens was an explosive eruption. If the composition of the magma is low in silica, it will produce a quiet eruption. The eruption s of Mt. Kilauea are quiet eruptions. Shielf volcanoes generally produce "quiet" eruptions. Explosive eruptions are rare. The volcano usually erupt quietly if the magma is low viscosity. That kind of lava is thin and flow easily and gases in the magma bubble out gently.
The mountain varies between the years depending on the current pressure inside and the positioning of the magma beneath. It changes in a fairly unpredictable way. No, they produce very explosive eruptions. Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in three types of lava flow coming from effusive eruptions Figure below. The lava cools very quickly to roughly spherical rocks.
Pillow lava is common at mid-ocean ridges. Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building or road Figure below.
Volcanologists attempt to forecast volcanic eruptions, but this has proven to be nearly as difficult as predicting an earthquake. Many pieces of evidence can mean that a volcano is about to erupt, but the time and magnitude of the eruption are difficult to pin down. This evidence includes the history of previous volcanic activity, earthquakes, slope deformation, and gas emissions.
Which of these categories does the volcano fit into? Mount Vesuvius destroyed Pompeii in 79 AD. Fortunately this volcano is dormant because the region is now much more heavily populated. Moving magma shakes the ground, so the number and size of earthquakes increases before an eruption. A volcano that is about to erupt may produce a sequence of earthquakes.
Scientists use seismographs that record the length and strength of each earthquake to try to determine if an eruption is imminent. Most ground deformation is subtle and can only be detected by tiltmeters, which are instruments that measure the angle of the slope of a volcano.
But ground swelling may sometimes create huge changes in the shape of a volcano. Mount St. Helens grew a bulge on its north side before its eruption. Ground swelling may also increase rock falls and landslides. Gases may be able to escape a volcano before magma reaches the surface. Scientists measure gas emissions in vents on or around the volcano.
Gases, such as sulfur dioxide SO 2 , carbon dioxide CO 2 , hydrochloric acid HCl , and even water vapor can be measured at the site Figure below or, in some cases, from a distance using satellites. The amounts of gases and their ratios are calculated to help predict eruptions. Some gases can be monitored using satellite technology Figure below. 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.
Types of Magma Types of magma are determined by chemical composition of the magma. Three general types are recognized:.
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. Rhyolitic magmas usually have higher gas contents than basaltic magmas. Temperature of Magmas Temperature of magmas is difficult to measure due to the danger involved , but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:.
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, to , times more viscous than water.
Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 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.
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. 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.
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