Published by the American Geological Institute
and Trends in the Geosciences
During the 1980s, three calderas (Long Valley,
California; Campi Flegrei, Italy; and Rabaul, Papua New Guinea) were restless,
experiencing increased seismicity, higher gas emissions, and ground uplift
as great as two meters. The Rabaul caldera crisis from 1983 to 1985 led
scientists and officials to anticipate an eruption within a few months
after declaring an alert in October 1983. However, the anticipated eruption
did not materialize until September 1994, more than a decade later. While
anomalous activity at Campi Flegrei largely ceased by 1985, the unrest
at Long Valley caldera continues.
The clear lesson from such caldera unrest is that state-of-the-art volcanology, even with abundant monitoring data, still does not identify diagnostic, unambiguous precursors to an eruption from a restless large caldera system. Long Valley is being intensively monitored by the USGS Long Valley Observatory and other research groups.
Volcanic ash and airplanes
Explosive eruptions of volcanoes, even in remote
and sparsely populated regions, can pose a serious, but heretofore little-known,
indirect volcano hazard: In-flight encounters between jet aircraft and
drifting volcanic ash clouds. Ingestion of gritty volcanic ash into jet
engines produces severe damage, degrading engine performance and, in some
cases, causing engine flame-out and power loss.
The primary mission of the Alaska Volcano Observatory — established in 1984 under a cooperative program between the USGS, University of Alaska-Fairbanks, and the state of Alaska — is to provide timely warnings of explosive eruptions of volcanoes in the northern Pacific region and, in cooperation with the National Weather Service, to provide information on the movement of resulting ash clouds to air-traffic controllers and the airline industry.
‘Vog’ from Kilauea
The eruption on the east-rift zone of Kilauea Volcano in Hawaii, which began in January 1983, is continuing unabated into the 21st century. This eruption is the longest rift eruption of Kilauea in recorded history and is affording the staff of the USGS Hawaiian Volcano Observatory, and other investigators, unprecedented opportunities to study basaltic lava-field development, formation and evolution of lava-tube systems, lava entry into the ocean, and the origin and adverse health effects of “vog” (volcanic smog). Since Kilauea’s eruption pattern shifted from an episodic to a continous flow in 1986, it has released a steady stream of sulfur dioxide and other volcanic gases that travel downwind and interact chemically in the atmosphere to produce a hazy atmosphere called “vog. ”
Gases as clues to eruptions
The 1990s witnessed increasing recognition that
many magmatic systems are gas-saturated and contain a free gas phase within
the magma reservoir prior to an eruption. Despite earlier controversy over
the role of gas in initiating an eruption, most investigators now believe
that such gas is commonly present in the magma.
The recognition of free gas as an important agent in driving eruptions was made possible by a combination of refined analytical procedures for study of melt inclusions and phenocrysts and modern remote-sensing techniques to measure emission of volcanic gases during eruptions (e.g., the satellite-based Total Ozone Mapping Spectrometer, or TOMS, and airborne or ground-based Correlation Spectrometer, or COSPEC, for sulfur dioxide.) Since 1980, scientists also have increasingly recognized the importance of subsurface magma degassing in modulating eruptive behavior.
Low emission rates of sulfur dioxide must be interpreted with caution. Because sulfur dioxide is highly soluble in water, a low-emission rate may simply reflect that, as the gas escapes to the surface from the degassing magma, it is taken up or “scrubbed” by the surrounding hydrothermal envelope or groundwater. Such scrubbing can mask a high rate of sulfur dioxide emission from subsurface magma. Thus, monitoring the emission rates of carbon dioxide and other gases will yield a better estimate of their magmatic component.
Soufrière Hills, Montserrat
The 1995-present eruptive activity of Soufrière Hills Volcano on the Island of Montserrat (British West Indies) has caused severe and sustained socio-economic hardships for the approximately 12,000 inhabitants of this tiny island (less than 160 square kilometers in area). Pyroclastic flows and surges generated by a series of collapses of actively growing, gravitationally unstable lava domes have devastated all flanks of the volcano, which make up the entire southern half of the island. Because of intensive volcano monitoring by the Montserrat Volcano Obervatory, effective communications of hazard warnings to government officials, and timely evacuations of high-risk areas, relatively few people (19) have been killed. However, hundreds of homes and nearly all of the island’s means of livelihood have been destroyed. Plans for restoring the island’s infrastructure and allowing the 8,000 people evacuated to return remain undecided.
Real-time and near-real-time monitoring capabilities
at numerous volcanoes around the world are a major advance in volcano-hazards
studies. For example, systematic volcano monitoring enabled the accurate
prediction, from hours to even a few weeks in advance, of nearly all the
post-May 18, 1980, dome-building eruptions of Mount St. Helens before magmatic
activity stopped in 1986.
Real-time tracking of seismicity has been, and doubtless will remain, the most widely used diagnostic technique for monitoring restless volcanoes. Improved locating of earthquakes recorded by permanent seismic networks have made possible informative 3-dimensional imaging of the magmatic plumbing systems beneath some volcanoes. Also, the increasing use of broadband seismometers has facilitated the complete recording and comprehensive analysis of long-period seismic signals, which nearly always precede and accompany eruptions. A more quantitative understanding of long-period seismicity not only refines short-term forecasts of volcano hazards, but also improves our knowledge of magma transport and eruption dynamics.
Summaries and collected works, not individual topical papers
Mount St. Helens
Crandell, D.R., and Mullineaux, D.R., 1978, Potential hazards from future eruptions of Mount St. Helens Volcano, Washington: U.S. Geological Survey Bulletin 1383-C, 26 p. [Little noticed when first published, this became the report most widely read by scientists and emergency-management officials between the onset of activity (27 March) and the climactic eruption on 18 May 1980.]
Foxworthy, B.L., and Hill, Mary, 1982, Volcanic eruptions of 1980 at Mount St. Helens: The first 100 days: U.S. Geological Survey Professional Paper 1249, 125 p. [A detailed but non-technical summary with many color photographs and illustrations.]
Hickson, C.J., and Peterson, D.W., editors, 1990, Proceedings of a symposium commemorating the 10th anniversary of the Eruption of Mount St. Helens, May 18, 1980: Geoscience Canada, v. 17, no. 3, p. 125-187. [A collection of 14 papers focused on studies of Mount St. Helens as well as of some other volcanic systems.]
Keller, S.A.C., editor, 1982, Mount St. Helens: One Year Later: Eastern Washington University Press, 243 p. [Proceedings of a symposium held 17-18 May 1981 and contains 34 papers covering geologic, biologic, and socio-economic topics.]
Keller, S.A.C., editor, 1986, Mount St. Helens: Five Years Later: Eastern Washington University Press, 441 p. [Proceedings of a symposium held 16-18 May 1985 and contains 47 papers covering geologic, biologic, environmental, and socio-economic topics.]
Lipman, P.W., and Mullineaux, D.R., editors, 1981, The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, 844 p. [A collection of 62 papers on diverse aspects of the 1980 eruptions; includes a 1:50,000 scale map of the proximal deposits and features.]
Science, 1983, Reports: Science, v. 221, no. 4618, p. 1369-97. [A collection of 9 papers dealing with volcano-monitoring and other topical studies on Mount St. Helens activity through 1982.]
JGR, 1987, Mount St. Helens Special Section: Journal of Geophysical Research, v. 92, no. B10, p. 10,149-10,334. [A collection of 12 papers encompassing geology, geophysics, tectonic setting, petrology, and numerical modeling studies of Mount St. Helens Volcano.]
Tilling, R.I., Topinka, Lyn, and Swanson, D.A., 1990, Eruptions of
Mount St. Helens: Past, Present, and Future: U.S. Geological
Survey series of general-interest publications, 56 p. [A jargon-free,
non-technical summary, illustrated by many color photographs and diagrams,
of the abundant scientific data for the volcano, with emphasis on the climactic
eruption of 18 May 1980; updated from version published originally in 1984.]