Mount Sinabung, Sumatra (By J. Baylor Roberts (c) National Geographic Society).Mount Sinabung, Sumatra (By J. Baylor Roberts (c) National Geographic Society).
Mount Sinabung, Sumatra (By J. Baylor Roberts (c) National Geographic Society).
Island-arc environmentIsland-arc environment
Island-arc environment
In a typical “oceanic” environment, volcanoes are aligned along the crest of a broad ridge that marks an active fracture system in the oceanic crust. Basaltic magmas, generated in the upper mantle beneath the ridge, rise along fractures through the basaltic layer. Because the granitic crustal layer is absent, the magmas are not appreciably modified or changed in composition and they erupt on the surface to form basaltic volcanoes.
Mauna Kea Volcano, Hawaii.Mauna Kea Volcano, Hawaii.
Mauna Kea Volcano, Hawaii.
Oceanic environmentOceanic environment
Oceanic environment
In the typical “continental” environment, volcanoes are located in unstable, mountainous belts that have thick roots of granite or granitelike rock. Magmas, generated near the base of the mountain root, rise slowly or intermittently along fractures in the crust. During passage through the granitic layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of nonbasaltic rocks.
Mount Adams, Washington.Mount Adams, Washington.
Mount Adams, Washington.
Continental environmentContinental environment
Continental environment
In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate, locally melting at depth; the magma rises to feed and form the Cascade volcanoes.In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate, locally melting at depth; the magma rises to feed and form the Cascade volcanoes.
In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate, locally melting at depth; the magma rises to feed and form the Cascade volcanoes.
According to the now generally accepted “plate-tectonics” theory, scientists believe that the Earth’s surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world’s active volcanoes are located along or near the boundaries between shifting plates and are called “plate-boundary” volcanoes. However, some active volcanoes are not associated with plate boundaries, and many of these so-called “intra-plate” volcanoes form roughly linear chains in the interior of some oceanic plates. The Hawaiian Islands provide perhaps the best example of an “intra-plate” volcanic chain, developed by the northwest-moving Pacific plate passing over an inferred “hot spot” that initiatesthe magma-generation and volcano-formation process. The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates, are dotted by many active volcanoes that form the so-called “Ring of Fire.” The “Ring” provides excellent examples of “plate-boundary” volcanoes, including Mount St. Helens.
The accompanying figure shows the boundaries of lithosphere plates that are presently active. The double lines indicate zones of spreading from which plates are moving apart. The lines with barbs show zones of underthrusting (subduction), where one plate is sliding beneath another. The barbs on the lines indicate the overriding plate. The single line defines a strike-slip fault along which plates are sliding horizontally past one another. The stippled areas indicate a part of a continent, exclusive of that along a plate boundary, which is undergoing active extensional, compressional, or strike-slip faulting.
Major tectonic plates of the Earth.Major tectonic plates of the Earth.
Major tectonic plates of the Earth.
Mariner 9 imagery of Olympus Mons Volcano on Mars compared to the eight principal Hawaiian Islands at the same scale (Mariner 9 Image Mosaic, NASA/JPL).Mariner 9 imagery of Olympus Mons Volcano on Mars compared to the eight principal Hawaiian Islands at the same scale (Mariner 9 Image Mosaic, NASA/JPL).
Mariner 9 imagery of Olympus Mons Volcano on Mars compared to the eight principal Hawaiian Islands at the same scale (Mariner 9 Image Mosaic, NASA/JPL).
Volcanoes and volcanism are not restricted to the planet Earth. Manned and unmanned planetary explorations, beginning in the late 1960’s, have furnished graphic evidence of past volcanism and its products on the Moon, Mars, Venus and other planetary bodies. Many pounds of volcanic rocks were collected by astronauts during the various Apollo lunar landing missions. Only a small fraction of thesesamples have been subjected to exhaustive study by scientists. The bulk of the material is stored under controlled-environment conditions at NASA’s Lunar Receiving Laboratory in Houston, Tex., for future study by scientists.
From the 1976-1979 Viking mission, scientists have been able to study the volcanoes on Mars, and their studies are very revealing when compared with those of volcanoes on Earth. For example, Martian and Hawaiian volcanoes closely resemble each other in form. Both are shield volcanoes, have gently sloping flanks, large multiple collapse pits at their centers, and appear to be built of fluid lavas that have left numerous flow features on their flanks. The most obvious difference between the two is size. The Martian shields are enormous. They can grow to over 17 miles in height and more than 350 miles across, in contrast to a maximum height of about 6 miles and width of 74 miles for the Hawaiian shields.
Voyager-2 spacecraft images taken of Io, a moon of Jupiter, captured volcanoes in the actual process of eruption. The volcanic plumes shown on the image rise some 60 to 100 miles above the surface of the moon. Thus, active volcanism is taking place, at present, on at least one planetary body in addition to our Earth.
Spacecraft image, made in July 1979, shows volcanic plume rising some 60 to 100 miles above the surface of Io, a moon of Jupiter (Voyager 2 photo, NASA).Spacecraft image, made in July 1979, shows volcanic plume rising some 60 to 100 miles above the surface of Io, a moon of Jupiter (Voyager 2 photo, NASA).
Spacecraft image, made in July 1979, shows volcanic plume rising some 60 to 100 miles above the surface of Io, a moon of Jupiter (Voyager 2 photo, NASA).
It has been said that the science of “volcanology” originated with the accurate descriptions of the eruption of Vesuvius in A.D. 79 contained in two letters from Pliny the Younger to the Roman historian Tacitus. Pliny’s letters also described the death of his uncle, Pliny the Elder, who was killed in the eruption. Actually, however, it was not until the 19th century that serious scientific inquiry into volcanic phenomena flourished as part of the general revolution in the physical and life sciences, including the new science of “geology.” In 1847, an observatory was established on the flanks of Vesuvius, upslope from the site of Herculaneum, for the more or less continuous recording of the activity of the volcano that destroyed the city in A.D. 79. Still, through the first decade of the 20th century, the study of volcanoes by and large continued to be of an expeditionary nature, generally undertaken after the eruption had begun or the activity had ceased.
The U.S. Geological Survey’s Hawaiian Volcano Observatory, on the crater rim of Kilauea Volcano.The U.S. Geological Survey’s Hawaiian Volcano Observatory, on the crater rim of Kilauea Volcano.
The U.S. Geological Survey’s Hawaiian Volcano Observatory, on the crater rim of Kilauea Volcano.
Perhaps “modern” volcanology began in 1912, when Thomas A. Jaggar, Head of the Geology Department of the Massachusetts Institute of Technology, founded the Hawaiian Volcano Observatory (HVO), located on the rim of Kilauea’s caldera. Initially supported by an association of Honolulu businessmen, HVO began to conduct systematic and continuous monitoring of seismic activity preceding, accompanying, and following eruptions, as well as a wide variety of other geological, geophysical, and geochemical observations and investigations. Between 1919 and 1948, HVO wasadministered by various Federal agencies (National Weather Service, U.S. Geological Survey, and National Park Service), and since 1948 it has been operated continuously by the Geological Survey as part of its Volcano Hazards Program. The more than 75 years of comprehensive investigations by HVO and other scientists in Hawaii have added substantially to our understanding of the eruptive mechanisms of Kilauea and Mauna Loa, two of the world’s most active volcanoes. Moreover, the Hawaiian Volcano Observatory pioneered and refined most of the commonly used volcano-monitoring techniques presently employed by other observatories monitoring active volcanoes elsewhere, principally in Indonesia, Italy, Japan, Latin America, New Zealand, Lesser Antilles (Caribbean), Philippines, and Kamchatka (U.S.S.R.).
What does “volcano monitoring” actually involve? Basically, it is the keeping of a detailed “diary” of the changes—visible and invisible—in a volcano and its surroundings. Between eruptions, visible changes of importance to the scientists would include marked increase or decrease of steaming from known vents; emergence of new steaming areas; development of new ground cracks or widening of old ones; unusual or inexplicable withering of plant life; changes in the color of mineral deposits encrusting fumaroles; and any other directly observable, and often measurable, feature that might reflect a change in the state of the volcano. Of course, the “diary” keeping during eruptive activity presents additional tasks. Wherever and whenever they can do so safely, scientists document, in words and on film, the course of the eruption in detail; make temperature measurements of lava and gas; collect the eruptive products and gases for subsequent laboratory analysis; measure the heights of lava fountains or ash plumes; gage the flow rate of ash ejection or lava flows; and carry out other necessary observations and measurements to fully document and characterize the eruption. For each eruption, such documentation and data collection and analysis provide another building block in constructing a model of the characteristic behavior of a given volcano or type of eruption.
Volcano monitoring also involves the recording and analysis of volcanic phenomena not visible to the human eye, but measurable by precise and sophisticated instruments. These phenomena include ground movements, earthquakes (particularly those too small to be felt by people), variations in gas compositions, and deviations in local electrical and magnetic fields that respond to pressure and stresses caused by the subterranean magma movements.
Some common methods used to study invisible, volcano-related phenomena are based on:
1. Measurement of changes in the shape of the volcano—volcanoes gradually swell or “inflate” in building up to an eruption because of the influx of magma into the volcano’s reservoir or “plumbing system”; with the onset of eruption, pressure is immediately relieved and the volcano rapidlyshrinks or “deflates.” A wide variety of instruments, including precise spirit-levels, electronic “tiltmeters,” and electronic-laser beam instruments, can measure changes in the slope or “tilt” of the volcano or in vertical and horizontal distances with a precision of only a few parts in a million.
2. Precise determination of the location and magnitude of earthquakes by a well-designed seismic network—as the volcano inflates by the rise of magma, the enclosing rocks are deformed to the breaking point to accommodate magma movement. When the rock ultimately fails to permit continued magma ascent, earthquakes result. By carefully mapping out the variations with time in the locations and depths of earthquake foci, scientists in effect can track the subsurface movement of magma, horizontally and vertically.
Scientist, wearing asbestos gloves and gas mask, samples volcanic gases from active vent.Scientist, wearing asbestos gloves and gas mask, samples volcanic gases from active vent.
Scientist, wearing asbestos gloves and gas mask, samples volcanic gases from active vent.
3. Measurement of changes in volcanic-gas composition and in magnetic field—the rise of magma high into the volcanic edifice may allow some of the associated gases to escape along fractures, thereby causing the composition of the gases (measured at the surface) to differ from that usually measured when the volcano is quiescent and the magma is too deep to allow gas to escape. Changes in the Earth’s magnetic field have been noted preceding and accompanying some eruptions, and such changes are believed to reflect temperature effects and/or the content of magnetic minerals in the magma.
Recording historic eruptions and modern volcano-monitoring in themselves are insufficient to fully determine the characteristic behavior of a volcano, because a time record of such information, though perhaps long in human terms, is much too short in geologic terms to permit reliable predictions of possible future behavior. A comprehensive investigation of any volcano must also include the careful, systematic mapping of the nature, volume, and distribution of the products of prehistoric eruptions, as well as the determination of their ages by modern isotopic and other dating methods. Research on the volcano’s geologic past extends the data base for refined estimates of the recurrence intervals of active versus dormant periods in the history of the volcano. With such information in hand, scientists can construct so-called “volcanic hazards” maps that delineate the zones of greatest risk around the volcano and that designate which zones are particularly susceptible to certain types of volcanic hazards (lava flows, ash fall, toxic gases, mudflows and associated flooding, etc.).
A strikingly successful example of volcano research and volcanic-hazard assessment was the 1978 publication (Bulletin 1383-C) by two Geological Survey scientists, Dwight Crandell and Donal Mullineaux, who concluded that Mount St. Helens was the Cascade volcano most frequently active in the past 4,500 years and the one most likely to reawaken to erupt, “... perhaps before the end of this century.” Their prediction came true when Mount St. Helens rumbled back to life in March of 1980. Intermittent explosions of ash and steam and periodic formation of short-lived lava domes continued throughout the decade. Analysis of the volcano’s past behavior indicates that this kind of eruptive activity may continue for years or decades, but another catastrophic eruption like that of May 18, 1980, is unlikely to occur soon.
On 18 May 1982, the U.S. Geological Survey (USGS) formally dedicated the David A. Johnston Cascades Volcano Observatory (CVO) in Vancouver, Washington, in memory of the Survey volcanologist killed two years earlier. This facility—a sister observatory to the Hawaiian Volcano Observatory—facilitates the increased monitoring and research on not only Mount St. Helens but also the other volcanoes of the Cascade Range of the Pacific Northwest. More recently, in cooperation with the State of Alaska, the USGS established the Alaska Volcano Observatory in March 1988. The work being done at these volcano observatories provides important comparisons and contrasts between the behavior of the generally non-explosive Hawaiian shield volcanoes and that of the generally explosive composite volcanoes of the Cascade and Alaskan Peninsula-Aleutian chains.
Volcanoes both harass and help mankind. As dramatically demonstrated by the catastrophic eruption of Mount St. Helens on May 1980 and of Pinatubo in June 1991, volcanoes can wreak havoc and devastation in the short term. The types of volcanic and associated hazards are not described in this booklet but treated in several of the publications listed inSuggested Reading. However, it should be emphasized that the short-term hazards posed by volcanoes are balanced by benefits of volcanism and related processes over geologic time. Volcanic materials ultimately break down to form some of the most fertile soils on Earth, cultivation of which fostered and sustained civilizations. People use volcanic products as construction materials, as abrasive and cleaning agents, and as raw materials for many chemical and industrial uses. The internal heat associated with some young volcanic systems has been harnessed to produce geothermal energy. For example, the electrical energy generated from The Geysers geothermal field in northern California can meet the present power consumption of the city of San Francisco.
The challenge to scientists involved with volcano research is to mitigate the short-term adverse impacts of eruptions, so that society may continue to enjoy the long-term benefits of volcanism. They must continue to improve the capability for predicting eruptions and to provide decision makers and the general public with the best possible information on high-risk volcanoes for sound decisions on land-use planning and public safety. Geo-scientists still do not fully understand how volcanoes really work, but considerable advances have been made in recent decades. An improved understanding of volcanic phenomena provides important clues to the Earth’s past, present, and possibly its future.
Decker, Robert, and Decker, Barbara, 1989, Volcanoes: W.H. Freeman and Company, New York, 285 p. (Revised edition).Editors, 1982, Volcano: in the series Planet Earth, Alexandria, Virginia, Time-Life Books, 176 p.Harris, S.L., 1988, Fire mountains of the West: The Cascade and Mono Lake Volcanoes: Missoula, Montana, Mountain Press Publishing Company, 379 p.Heliker, Christina, 1990, Volcanic and seismic hazards on the Island of Hawaii: Reston, Virginia, U.S. Geological Survey, 48 p.Macdonald, G.A., 1972, Volcanoes: Englewood Cliffs, New Jersey, Prentice-Hall, Inc., 510 p.Simkin, Tom, Tilling, R.I., Taggart, J.N., Jones, W.J., and Spall, Henry, compilers, 1989, This dynamic planet: World Map of volcanoes, earthquakes, and plate tectonics: U.S. Geological Survey, Reston, Virginia, prepared in cooperation with the Smithsonian Institution, Washington, D.C. (scale 1:30,000,000 at equator).Tilling, R.I., Heliker, Christina, and Wright, T.L., 1989, Eruptions of Hawaiian volcanoes: Past, present, and future: Reston, Virginia, U.S. Geological Survey, 54 p.Tilling, R.I., Topinka, Lyn, and Swanson, D.A., 1990, Eruptions of Mount St. Helens: Past, present, and future: Reston, Virginia, U.S. Geological Survey, 56 p. (Revised edition).Tilling, R.I., 1991, Monitoring active volcanoes: Reston, Virginia, U.S. Geological Survey, 13 p. (Revised edition).Wood, C.A., and Kienle, Jurgen, 1990, Volcanoes of North America: United States and Canada: New York, Cambridge University Press, 354 p.
Decker, Robert, and Decker, Barbara, 1989, Volcanoes: W.H. Freeman and Company, New York, 285 p. (Revised edition).
Editors, 1982, Volcano: in the series Planet Earth, Alexandria, Virginia, Time-Life Books, 176 p.
Harris, S.L., 1988, Fire mountains of the West: The Cascade and Mono Lake Volcanoes: Missoula, Montana, Mountain Press Publishing Company, 379 p.
Heliker, Christina, 1990, Volcanic and seismic hazards on the Island of Hawaii: Reston, Virginia, U.S. Geological Survey, 48 p.
Macdonald, G.A., 1972, Volcanoes: Englewood Cliffs, New Jersey, Prentice-Hall, Inc., 510 p.
Simkin, Tom, Tilling, R.I., Taggart, J.N., Jones, W.J., and Spall, Henry, compilers, 1989, This dynamic planet: World Map of volcanoes, earthquakes, and plate tectonics: U.S. Geological Survey, Reston, Virginia, prepared in cooperation with the Smithsonian Institution, Washington, D.C. (scale 1:30,000,000 at equator).
Tilling, R.I., Heliker, Christina, and Wright, T.L., 1989, Eruptions of Hawaiian volcanoes: Past, present, and future: Reston, Virginia, U.S. Geological Survey, 54 p.
Tilling, R.I., Topinka, Lyn, and Swanson, D.A., 1990, Eruptions of Mount St. Helens: Past, present, and future: Reston, Virginia, U.S. Geological Survey, 56 p. (Revised edition).
Tilling, R.I., 1991, Monitoring active volcanoes: Reston, Virginia, U.S. Geological Survey, 13 p. (Revised edition).
Wood, C.A., and Kienle, Jurgen, 1990, Volcanoes of North America: United States and Canada: New York, Cambridge University Press, 354 p.
The port city of St. Pierre on the island of Martinique; Mont Pelée is in the background. In 1902, this city was entirely destroyed by pyroclastic flows; about 30,000 people died.The port city of St. Pierre on the island of Martinique; Mont Pelée is in the background. In 1902, this city was entirely destroyed by pyroclastic flows; about 30,000 people died.
The port city of St. Pierre on the island of Martinique; Mont Pelée is in the background. In 1902, this city was entirely destroyed by pyroclastic flows; about 30,000 people died.
As the Nation’s principal conservation agency, the Department of the Interior has responsibility for most of our nationally owned public lands and natural and cultural resources. This includes fostering sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the environmental and cultural values of our national parks and historical places; and providing for the enjoyment of life through outdoor recreation The Department assesses our energy and mineral resources and works to ensure that their development is in the best interests of all our people by encouraging stewardship and citizen participation in their care. The Department also has a major responsibility for American Indian reservation communities and for people who live in island territories under U S administration
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