You can also get bubbles of lava filled with volcanic gasses that burble and pop on the surface of the lava. And over time, volcanoes made from low lava viscosity are wide and have a shallow slope; these are known as shield volcanoes. Instead of rivers of lava, you can get crumbling piles of rock flowing down hill. It can also clog up the volcanic vent and form blocks that resist the flow of lava. Viscous lava will trap pockets of gas within the rock, and not let them pop as bubbles on the surface.
The extensional deformation occurs because the underlying mantle is rising from below and stretching the overlying continental crust. Upwelling mantle may melt to produce magmas, which then rise to the surface, often along normal faults produced by the extensional deformation. Basaltic and rhyolitic volcanism is common in these areas. In the same area, the crust has rifted apart along the Red Sea, and the Gulf of Aden to form new oceanic ridges.
This may also be the fate of the East African Rift Valley at some time in the future. Other areas where extensional deformation is occurring within the crust is Basin and Range Province of the western U. These are also areas of recent basaltic and rhyolitic volcanism. All around the Pacific Ocean, is a zone often referred to as the Pacific Ring of Fire, where most of the world's most active and most dangerous volcanoes occur. The Ring of Fire occurs because most of the margins of the Pacific ocean coincide with converging margins along which subduction is occurring.
These are all island arcs. The Hawaiian Ridge is one such hot spot trace. Here the Big Island of Hawaii is currently over the hot spot, the other Hawaiian islands still stand above sea level, but volcanism has ceased.
Northwest of the Hawaiian Islands, the volcanoes have eroded and are now seamounts. Plateau or Flood basalts are extremely large volume outpourings of low viscosity basaltic magma from fissure vents. The basalts spread huge areas of relatively low slope and build up plateaus. Many of these outpourings appear to have occurred along a zone that eventually developed into a rift valley and later into a diverging plate boundary.
Examples of questions on this material that could be asked on an exam. Physical Geology. Volcanoes and Volcanic Eruptions. Magmas and Lava Since volcanic eruptions are caused by magma a mixture of liquid rock, crystals, and dissolved gas expelled onto the Earth's surface, we'll first review the characteristics of magma that we covered previously. Viscosity of Magmas Viscosity is the resistance to flow opposite of fluidity. Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas Lower Temperature magmas have higher viscosity than higher temperature magmas.
Solidified Volcanic Rock. Solidified Plutonic Rock. Intermediate or Andesitic. 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 9. 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 9.
Lava Tubes - Once the surface skin becomes solid, the lava can continue to flow beneath the surface in lava tubes. The surface skin insulates the hot liquid lava form further cooling. When the eruption ends, liquid lava often drains leaving an open cave see figure 9. Pillow Lavas - When lava erupts on the sea floor or other body of water, the surface skin forms rapidly, and, like with pahoehoe toes inflates with molten lava.
Eventually these inflated balloons of magma drop off and stack up like a pile of pillows and are called pillow lavas. Ancient pillow lavas are readily recognizable because of their shape, their glassy margins and radial fractures that formed during cooling see figure 9.
Columnar Jointing - When thick basaltic or andesitic lavas cool, they contract. The contraction results in fractures and often times results in a type of jointing called columnar jointing. The columns are usually hexagonal in shape. This often happens when lavas pool in depressions or deep canyons see figure 9.
Lava Domes or Volcanic Domes - result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent.
Blocks of nearly solid lava break off the outer surface of the dome and roll down its flanks to form a breccia around the margins of domes. Pyroclastic Material If the magma has high gas content and high viscosity, the gas will expand in an explosive fashion and break the liquid into clots that fly through the air and cool along their path through the atmosphere. Blocks are angular fragments that were solid when ejected. Volcanic Landforms Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed.
Shield Volcanoes A shield volcano is characterized by gentle upper slopes about 5 o and somewhat steeper lower slopes about 10 o. Most shield volcanoes have a roughly circular or oval shape in map view. Long periods of repose times of inactivity lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.
Cinder Cones Cinder cones are small volume cones consisting predominantly of ash and scoria that result from mildly explosive eruptions. They usually consist of basaltic to andesitic material. They are actually fall deposits that are built surrounding the eruptive vent. Slopes of the cones are controlled by the angle of repose angle of stable slope for loose unconsolidated material and are usually between about 25 and 35 o.
On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters are usually eroded away on older cones. Craters and Calderas Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and ash.
Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure. The collapse results from evacuation of the underlying magma chamber.
Crater Lake Caldera in southern Oregon is an 8 km diameter caldera containing a lake The caldera formed about years ago as a result of the eruption of about 75 km 3 of rhyolite magma in the form of tephra, found as far away as Canada, accompanied by pyroclastic flows that left thick deposits of tuff on the flanks of the volcano. Subsequent eruptions have built a cinder cone on the floor of the caldera, which now forms an island called Wizard Island.
Larger calderas have formed within the past million years in the western United States. The Yellowstone caldera is an important example, as it illustrates the amount of repose time that might be expected from large rhyolitic systems, and the devastating effect caldera forming eruptions can have on widespread areas. Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the third caldera to form in the area within the past 2 million years.
The three calderas formed at 2. Thus the repose time is on the average about , years. Tephra fall deposits from the latest eruption are found in Louisiana and into the Gulf of Mexico, and covered much of the Western part of the United States. The eruption , years ago produced about km 3 of rhyolite in comparison, the eruption of Mt. Helens in May of produced only 0. Magma still underlies Yellowstone caldera, as evidenced by the large number of hot springs and geysers in the area. 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.
When the magma reaches the Earth's surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a effusive or non-explosive eruption will occur, usually as a lava flow 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.
Effusive Eruptions Effusive or 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. Lava flows are produced on the surface, and these run like liquids down slope, along the lowest areas they can find.
If the magma emerges along a fracture, it results in a fissure eruption, often called a "curtain of fire" Lava flows produced by eruptions under water are called pillow lavas. 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. Pillow Lavas - When lava erupts on the sea floor or other body of water, the surface skin forms rapidly, and, like with pahoehoe toes inflates with molten lava. However, if the magma is viscous, like rhyolite, its high polymerization will impede the upward mobility of the gas bubbles.
As gas continues to exsolve from the viscous melt, the bubbles will be prevented from rapid escape, thus increasing the overall pressure on the magma column until the gas ejects explosively from the volcano. As a general rule, therefore, nonexplosive eruptions are typical of basaltic-to-andesitic magmas which have low viscosities and low gas contents, whereas explosive eruptions are typical of andesitic-to-rhyolitic magmas which have high viscosities and high gas contents.
There are, however, two exceptions to this general rule. Andesitic-to-rhyolitic lavas that have been degassed often erupt at the surface nonexplosively as viscous lava domes or obsidian flows. Similarly, many of the so-called hydrovolcanic eruptions involve basaltic-to-andesitic magmas that erupt explosively in the presence of groundwater or surface water. For more information on the variability of explosivity, see the Volcano Explosivity Index.
Intermediate to advanced users may be interested in the following programs for calculating viscosity. Click image to download. Viscosity for Windows. Developed by Dr. Jon Dehn, this program calculates the viscosity of silicate magmas from the magma composition and temperature.
Nonexplosive eruption with effusive lava flows. Explosive eruption with voluminous plume of tephra. Felsic low Si. Felsic high Si. Gas escape through vertical vesicle cylinders. Vesicle-rich flow top. SiO 2.
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