Part 4

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 rapidly shrinks 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.

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 and People

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 in Suggested 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.

Suggested Reading

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.

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U.S. Geological Survey Information Services P.O. Box 25286 Denver, CO 80225

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Transcriber’s Notes

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