The location of a volcano and the composition of magma that feeds it determines the frequency and types of eruptions. Oceanic volcanoes fed by "hot spots" in the crust commonly have frequent non-explosive eruptions of slow-moving basaltic lava. These eruptions often begin with spectacular lava fountains that construct spatter ramparts and cinder cones. Examples are the Island of Hawaii, located above a hot spot, and Iceland positioned astride the mid-Atlantic Ridge. The eruption at Laki, Iceland in 1783 poured out 12 cubic kilometers of lava from a 65-kilometer-long (40-mile) fissure, in the process creating a European environmental disaster.
Composite volcanoes in chains such as the Cascade Mountains, erupt less frequently but usually with more explosive violence. Eruptions may come along only decades apart, but not always from the same volcano. Individual active volcanoes can be dormant for a few hundred years at a time. Mount St. Helens has erupted approximately every 100 to 150 years or so since 1400 A.D. Mount Lassen erupted in 1915. Crater Lake formed in a gigantic cataclysmic eruption about 5000 B.C. Mount Baker showed increased activity in the 1970's. Warm spots and steam rise in the summit areas of Mount Rainier, Mount Shasta and Mount Hood. Any one of those Cascades volcanoes could turn deadly as did Mount St. Helens in 1980.
Even though individual volcanoes are different, they can be grouped into a few families according to shape, size, and types of volcanic material. Probing a bit deeper, volcanologists have learned that the composition of the original magma, or mixing with surface water, are the principal reasons for similarities and differences. Volcanoes within each family may resemble one another but they differ in physical details and behavior. It is not easy to predict eruptive behavior until the behavior has been witnessed or determined by examining the resulting deposits.
Mount St. Helens and Mount Rainier are typical of the graceful solitary cones known as composite cones (or stratovolcanoes) with slopes made of innumerable pyroclastic layers interspersed with lava flows. Some attain 2,000 to 3,000 meters (6,000 to 8,000 feet) above their bases. Depending upon latitude, seasonal winter snows whiten the slopes to the base, and the heat of summer melts all but the highest snows. On the upper slopes of some volcanoes, such as Mount Rainier, the perennial snows spawn glaciers that descend for many miles as rivers of ice.
Most composite cones occur alone or are separated by several tens of kilometers. Some of the most majestic in the world form a chain down the backbone of the Cascade Mountains of British Columbia, Washington, Oregon, and northern California; the chain continues around the Pacific in the "Ring of Fire." Composite volcanoes are constructed from multiple eruptions that occur over periods ranging from hundreds to hundreds of thousands of years. Andesite magma, the most common, but not the only magma type forming composite cones, is more explosive than basalt magma because it contains more silica, but also produces lava flows. The explosive eruptions provide abundant pyroclastic debris in the stratocones.
But, not all of the magma rises to the summit. Some of it penetrates pyroclastic layers of the volcano to form dikes (sheetlike bodies of intrusive rock) or sills (intrusive rock that parallels the boundary between two rock layers). In this way, multiple intrusive events build a structurally strong three-dimensional dike-and-sill network that knits together the voluminous accumulation of volcanic ash, sediment and lava flows. Composite cones can therefore grow to great heights before collapsing under the weight of the Earth's gravitational field, as happened at Mount St. Helens.
Lave domes exist on the outside slopes of composite cones or inside their craters. These protrusions result from the slow extrusion of highly viscous silica-rich magma. Most domes are rather small, but some have volumes exceeding 25 cubic kilometers. Although they end up as rather benign eruptions, dome-producing eruptions may start very explosively, forming reamed-out pits blanketed by pyroclastic debris. The explosive activity wanes as the gas content decreases; the magma is extruded slowly as rhyolite lava that is so viscous it only forms a bulbous dome. If extrusions continue, the dome enlarges and its margins slowly creep outward as a lava flow with steep cliff-like margins and a rubbly surface. The steep dome fronts can collapse in a dangerous mass of hot rubble that can form pyroclastic flows.
Domes can be solitary volcanoes, form in clusters, grow in craters or along the flanks of composite cones. A dome has been growing slowly within the crater of Mount St. Helens since the eruption of 1980. Domes also have filled the crater of Mt. Pelee, Martinique, and many other volcanoes.
In the jargon of some volcanologists a volcanic eruption is divided into
Individual volcanoes can erupt for a short time once every thousand years, but we are most concerned with eruptions on a human time scale of weeks, months or years. On this scale, some eruptions last a very long time. Stromboli, Italy, the "lighthouse of the Mediterranean," has been erupting for over 2,500 years. Some eruptions are exceedingly short, with 10 percent lasting no longer than a single day, most ending in less than 100 days, and a few lasting longer than 1,000 days. The average duration of recorded eruptions at Stromboli is 7 weeks.
History's most damaging eruption occurred in 1815. Tambora volcano, Indonesia, experienced 3 years of mild activity, lulling the people into complacency. But following the mild activity, Tambora violently exploded with an eruption cloud that reached 44 kilometers. Pumice and volcanic ash amounting to 100 to 300 cubic kilometers were ejected -- the largest known eruption in history and one that caused famine and serious economic repercussions in Europe.
This is an image showing part of Isla Isabella in the western Galapagos Islands. The western Galapagos Islands, which lie about 1,200 kilometers (750 miles) west of Ecuador in the eastern Pacific, have six active volcanoes similar to the volcanoes found in Hawaii. These volcanoes reflect the volcanic processes that occur where the ocean floor is created. Since the time of Charles Darwin's visit to the area in 1835, there have been over 60 recorded eruptions on these volcanoes. This SIR-C/X-SAR image of Alcedo and Sierra Negra volcanoes shows the rougher lava flows as bright features, while ash deposits and smooth pahoehoe lava flows appear dark. A small portion of Isla Fernandina is visible in the extreme upper left corner of the image. (Courtesy NASA/JPL)
Isla Isabella in 3D
This is a three-dimensional view of Isabela, one of the Galapagos Islands located off the western coast of Ecuador, South America. (Courtesy NASA/JPL)
This image of the Galapagos Islands was taken from the space shuttle using a hand held camera. There are seven shield volcanoes in this area (Fernandina, Ecuador, Wolf, Darwin, Alcedo, Sierra Negra, and Azul) which collectively have erupted more than sixty times this century. Unlike Hawaii, these volcanoes are infrequently studied due to their inaccessibility and delicate ecology. In addition, the rugged terrain, lack of water and field support make these volcanoes difficult to map and study in the field. (Courtesy NASA/JPL)
This is a false color image of the area around Mount Pinatubo in the Philippines. The area shown is approximately 45 by 68 kilometers (28 by 42 miles). The main volcanic crater on Mount Pinatubo produced by the June 1991 eruptions, and the steep slopes on the upper flanks of the volcano, are easily seen in this image. The red color on the high slopes show the rougher ash deposited during the 1991 eruption. The dark drainages are the smooth mudflows which continue to flood the river valleys after heavy rain.
The 1991 eruption of Mount Pinatubo in the Philippines is well known for its near-global effects on the atmosphere and climate due to the large amount of sulfur dioxide that it injected into the upper atmosphere. What is less widely known is that even today the volcano continues to be a major hazard to the people who have returned to the area around the volcano. Dangerous mudflows (called "lahars") are often generated by heavy rains, and these can still sweep down river valleys and wash out roads and villages, or bury low lying areas in several meters of mud and volcanic debris. These mudflows will continue to be a severe hazard around Pinatubo for the next 10 to 15 years. (Courtesy NASA/JPL)
Virunga Volcano Chain
This is a false-color radar image of Central Africa, showing the Virunga volcano chain along the borders of Rwanda, Zaire, and Uganda. This area is home to the endangered mountain gorillas. The image was acquired on October 3, 1994.
The dark area at the top of the image is Lake Kivu, which forms the border between Zaire (to the right) and Rwanda (to the left). In the center of the image is the steep cone of Nyiragongo volcano, rising 3,465 meters (11,369 feet) high, with its central crater now occupied by a lava lake. To the left are three volcanoes, Mount Karisimbi, rising 4,500 meters (14,800 feet) high; Mount Sabinyo, rising 3,600 meters (12,000 feet) high; and Mount Muhavura, rising 4,100 meters (13,500 feet) high. To their right is Nyamuragira volcano, which is 3,053 meters (10,017 feet) tall, with radiating lava flows dating from the 1950s to the late 1980s. These active volcanoes constitute a hazard to the town of Goma, Zaire, and the nearby Rwandan refugee camps, located on the shore of Lake Kivu at the top left. (Courtesy NASA/JPL)
This is a radar image of Mount Rainier in Washington state. The volcano last erupted about 150 years ago and numerous large floods and debris flows have originated on its slopes during the last century. Today the volcano is heavily mantled with glaciers and snowfields. More than 100,000 people live on young volcanic mudflows less than 10,000 years old and, consequently, are within the range of future, devastating mudslides. North is toward the top left of the image. Forested regions are pale green in color; clear cuts and bare ground are bluish or purple; ice is dark green and white. The round cone at the center of the image is the 14,435-foot (4,399- meter) active volcano, Mount Rainier. On the lower slopes is a zone of rock ridges and rubble (purple to reddish) above coniferous forests (in yellow/green). (Courtesy NASA/JPL)
This is an image of the area of Kliuchevskoi volcano, Kamchatka, Russia, which began to erupt on September 30, 1994. Kliuchevskoi is the blue triangular peak in the center of the image, toward the left edge of the bright red area that delineates bare snow cover. The image was acquired on October 5, 1994. It shows an area approximately 75 kilometers by 100 kilometers (46 miles by 62 miles) that is centered at 56.07 degrees north latitude and 160.84 degrees east longitude. North is toward the bottom of the image.
The Kamchatka volcanoes are among the most active volcanoes in the world. The volcanic zone sits above a tectonic plate boundary, where the Pacific plate is sinking beneath the northeast edge of the Eurasian plate. In addition to Kliuchevskoi, two other active volcanoes are visible in the image. Bezymianny, the circular crater above and to the right of Kliuchevskoi, contains a slowly growing lava dome. Tolbachik is the large volcano with a dark summit crater near the upper right edge of the red snow covered area. The Kamchatka River runs from right to left across the bottom of the image. The current eruption of Kliuchevskoi included massive ejections of gas, vapor and ash, which reached altitudes of 15,000 meters (50,000 feet). Melting snow mixed with volcanic ash triggered mudflows on the flanks of the volcano. Paths of these flows can be seen as thin lines in various shades of blue and green on the north flank in the center of the image. (Courtesy NASA/JPL)
Unzen Volcano, Japan
This is a space radar image of the area around the Unzen volcano, on the west coast of Kyushu Island in southwestern Japan. Unzen, which appears in this image as a large triangular peak with a white flank near the center of the peninsula, has been continuously active since a series of powerful eruptions began in 1991. The image was acquired on April 15, 1994. The image shows an area 41.5 kilometers by 32.8 kilometers (25.7 miles by 20.3 miles) that is centered at 32.75 degrees north latitude and 130.15 degrees east longitude. North is toward the upper left of the image. The city of Shimabara sits along the coast at the foot of Unzen on its east and northeast sides. At the summit of Unzen a dome of thick lava has been growing continuously since 1991. Collapses of the sides of this dome have generated deadly avalanches of hot gas and rock known as pyroclastic flows. (Courtesy NASA/JPL)
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