VOLCANOES, MANTLE PLUMES, and HOT SPOTS

The Electronic Volcano (http://www.dartmouth.edu/~volcano/) is a good site for additional information on volcanoes, even though the site has numerous broken links. It is rich in pictures and written information, and is a good place to start exploring volcanoes. The United States Geological Survey (http://vulcan.wr.usgs.gov/Volcanoes/) also has an extensive site dedicated to volcanoes, and its links are more likely to be active. Another excellent source for volcano information is Volcano World (http://volcano.und.nodak.edu/), which is directed towards educational use of volcano information.

Composite Volcanoes

The type of molten lava controls the type of volcano created, and the type of lava produced is related to plate boundaries. In subduction zones, ocean lithosphere is subducted beneath the earth's surface and down into the upper mantle, where the heat and pressure cause the oceanic lithosphere, mainly basalt, to begin partially melt. The lava thus produced is relatively high in silica (SiO2), which makes it viscous. The lava is also relatively high in gas, mainly H2O but also CO2 and others. The combination of high viscosity and high gas content make this type of lava thick and sticky, and on occasion, explosive. Volcanoes produced by this type of magma are called Composite, because they are build up from alternating layers of lava flows and pyroclastics, "fire fragmented" fine pieces called ash, as well as larger fragments.

Many of the world's well known volcanoes are of the composite type: Mt. St. Helens, Mt. Vesuvius, and Mt Fuji, for examples. When people bring up a mental image of a volcano, it is usually the composite type.

Figure 1. Popocatepetl Volcano, Mexico, an example of a currently active composite volcano.

Composite volcanoes can be very explosive, destructive, and deadly. Mt. St. Helens is a good, recent example. Take some time to go through the web site information at http://vulcan.wr.usgs.gov/Volcanoes/MSH/framework.html to get a good overview of this well-studied volcano.

Figure 2. "Mount St. Helens on May 17, 1980, one day before the devastating eruption. The view is from Johnston's Ridge, six miles (10 kilometers) northwest of the volcano. Photo by Harry Glicken, May 17, 1980." Courtesy of USGS/Cascades Volcano Observatory.

Figure 3. "Mount St. Helens four months after the May 18, 1980 eruption, as viewed from Johnston's Ridge. Photo by Harry Glicken, September 10, 1980." Courtesy of USGS/Cascades Volcano Observatory.

In extreme cases, nearly the entire volcano can blow away, collapsing back into the emptied magma chamber, leaving only the base. A clear example of this, called a Collapsed Caldera, is Crater Lake in Oregon. Yellowstone Park is a series of collapsed calderas, although not so readily apparent as Crater Lake.

Figure 4. "Aerial view of Crater Lake caldera. Photo by W.E.Scott." Courtesy of USGS/Cascades Volcano Observatory.

Shield Volcanoes

Shield volcanoes form from lava that is relatively low in SiO2 and not very viscous. The origin of this lava, which produces basalt volcanic rock, is from partial melting of the upper mantle due to Hot Spots. Shield volcanoes are very broad compared with their height, because the lava is very fluid, flows relatively rapidly, and spreads out. The fluid lava may pour out through a fissure or crack in the earth's lithosphere and spread out so broad and flat that you were standing on it, you would have a difficult time visualizing that you were standing on a volcano.

Hot Spots are seemingly fixed areas in the upper mantle that are hotter than average. They produce a plume of hot mantle that convects towards the surface. Hot spots are assumed to be more or less fixed in the mantle, although they may be slowly moving. Hot mantle from hot spots is less dense than the surrounding mantle, causing it to rise towards the surface. Some geologists think that the hot material rises as a relatively narrow plume, while others believe that nearly the entire mantle is involved in general mantle convection. In both models, hot mantle spreads out beneath the overlying lithosphere plate. The image below depicts plumes and hot spots, based on data gathered from the earth's interior.

Figure 5. Depiction of mantle convection and plumes. Image produced by Jamie Painter, Visualization Scientist.

Copyright (c) 1997, 1998, 1999 The Regents of the University of California. Unless otherwise indicated, this information has been authored by an employee or employees of the University of California, operator of the Los Alamos National Laboratory under Contract No. W-7405-ENG-36 with the U.S. Department of Energy.

If you have a player that can view animations, you can view a 3-D depiction of mantle convection and plumes at http://www.acl.lanl.gov/~jamie/terra.html, from which the above image was taken.

Several hot spots are located along divergent boundaries, and they may have provided the convection necessary to rift the lithosphere. Other hot spots are beneath plate interiors, such as the ones thought to lie beneath Yellowstone Park (http://vulcan.wr.usgs.gov/Volcanoes/Yellowstone/framework.html) and the Hawaiian Islands (http://vulcan.wr.usgs.gov/Volcanoes/Hawaii/framework.html).

These intraplate hot spots produce a chain of volcanoes on the overlying plate. There is a line of volcanoes stretching from the west coast of North America almost due eastward, ending at Yellowstone Park. The volcanic activity is oldest in the west, and youngest at Yellowstone, and is thought to represent movement of the North American plate westward over the more or less fixed location of the Yellowstone hot spot. Spend some time visiting the Snake River Plain web site at http://vulcan.wr.usgs.gov/Volcanoes/Idaho/SnakeRiverPlain/description_snake_river_plain.html because it has a good discussion of the connection between the volcanic activity in the Snake River Plain, Yellowstone Park volcanism, and hot spots.

The Hawaiian Island chain of volcanoes stretches in a straight line northwestward, and continues as a chain of submerged and eroded volcanoes (seamounts) in the same direction. This chain makes an abrupt change in direction to nearly due north, and continues towards the Aleutian trench as the Emperor Seamount chain. The southeastern-most big island of Hawaii is the youngest and most volcanically active of all the Hawaiian Islands. They get progressively older and smaller towards the northwest until finally, past Kauai, the volcanic mountains are eroded off to below sea level.

Figure 6. "Mauna Loa shield volcano as seen from Mauna Kea. Photo by Tom Casadevall." Courtesy of USGS/Cascades Volcano Observatory. Note the circular cinder cone covered with snow (yes, snow in Hawaii, at this elevation!) on the lower right. The entire Hawaiian Island chain is made up of volcanoes like this; their bases rest on the floor of the ocean.

Figure 7. "Aerial view, Pu'u O'o eruption and lava flows. Photo by Lyn Topinka, USGS/CVO, April 14, 1986." Courtesy of USGS/Cascades Volcano Observatory. Note the fountain and glowing ribbons of lava flowing off to the right. This fluid lava pours and fountains freely and abundantly from vents, coming to rest and cooling at the low angles typical of shield volcanoes.