Eruption of Mt. Pinatubo, Philippines, June 1991.

Still, under ideal conditions, volcanologists have recently met with a great deal of success in foretelling eruptions. While they were caught off guard by the exact timing and magnitude of the 1980 Mt. St. Helens eruption, for example, their timely warnings of an impending blow prompted the U.S. Forest Service to evacuate people from dangerous areas near the volcano. Though 57 people died in the eruption, perhaps 20,000 lives were saved, says Dr. William Rose, a volcanologist at Michigan Technological University. Similarly, a USGS SWAT team that rushed to the Philippines' Mt. Pinatubo in the spring of 1991 successfully augured the June eruption, leading to evacuations that saved thousands if not tens of thousands of lives and millions of dollars worth of military equipment at the nearby Clark Air Force Base.

A Kilauea lava fountain spews from Puu Oo vent on March 13, 1985.

Not surprisingly, volcanologists have had the most success at volcanoes that host their own observatories. In 1912, Thomas A. Jaggar, head of the Geology Department at the Massachusetts Institute of Technology, founded the first volcano observatory in the United States on Kilauea. (There are now three others—in Menlo Park, California; Anchorage, Alaska; and Vancouver, Washington, near Mt. St. Helens.) Over the succeeding decades, researchers at the Hawaiian Volcano Observatory developed many of the techniques used today and can now predict Kilauea's eruptions to a tee.

They know when and how Kilauea will erupt because it does so frequently and predictably, and because after decades of intensive study they know the volcano inside and out. Learning as much as possible about a volcano's previous behavior is the essential first step in anticipating future blows, just as knowing a career criminal's record can help indicate what he might do next. "There is no doubt that the eruptive history of a volcano is the main key for long-term prediction," says Dr. Yuri Doubik, a Russian volcanologist who has studied past eruptions on the Kamchatka Peninsula for 35 years. Such work entails laboriously picking through the physical remains of previous eruptions. And mapping such old lava flows, pyroclastic deposits, and other volcanic debris distributed around a crater can reveal much about the timing, type, direction, and magnitude of previous blows.

Close-up of a seismograph drum.

Satellite data can greatly aid such mapping, and volcanologists are looking forward to using images generated by the Earth Observing System after it is launched in 1999. The satellite's purpose is to study environmental ills such as global warming and depletion of the ozone layer, but it will also gather information of use to volcanologists, including gas concentrations in the atmosphere over volcanoes and images clear enough to reveal the fallout from former eruptions.

The Volcanologist's Toolkit

Tavurvur volcano erupts in the distance as workers install a tiltmeter at Rabaul, Papua New Guinea, September 1994.

While seismicity is the workhorse, monitoring ground deformation is another up-and-coming technique that allows three-dimensional mapping of what's occurring underground. Magma rising from the depths often pushes the skin of a volcano up and out, like a balloon filling with air. Sensitive tiltmeters and surveying instruments can measure and record the slightest changes, which help volcanologists determine, for example, roughly how deep a magma source is, how fast it is moving, and where on a volcano it might erupt. Such monitoring has helped scientists anticipate eruptions at Hawaii's Kilauea and Mauna Loa volcanoes, which deform in predictable ways and at predictable rates.

A USGS team prepares to fly a gas-measuring flight at Montserrat, August 1995.

One drawback is that studying ground deformation has required scientists to climb volcanoes to take measurements—a perilous undertaking. But USGS volcanologists are now testing a prototype of a fully automated ground-deformation system. Flown aboard satellites or aircraft, the so-called "synthetic aperture radar" can automatically and continuously transmit information on a volcano's ground movements to remote observatories. Though it will not penetrate dense vegetation and is sensitive to moisture, the radar provides a resolution of less than an inch under ideal conditions. "It's a tremendous tool because it gives a complete map of ground movements, and we don't have to go into the field to get it," says Dr. Dan Dzurisin, a geologist with the USGS Volcano Hazards Program (VHP) who is helping to perfect the new device. His colleague at the VHP, the volcanologist Dr. Robert Tilling, is equally optimistic: "We're confident that by the turn of the century, we'll have such a system and at low enough cost that it can be applied easily everywhere in the world."

Measuring Vapors

Alaska's Spurr volcano blows its top on August 19, 1992.

Concurrently, Williams and others are working on infrared telescopes to monitor concentrations of gases escaping from volcanic vents. Williams's version is modeled after the correlation spectrometer, a device originally developed in the 1970s to monitor SO₂ and other toxic gases from factory smokestacks. His prototype unit measures the amount of infrared light absorbed by CO₂ molecules, from which an estimate of CO₂ concentrations in the air can be made. Dr. Kenneth McGee, a volcanologist at the Cascades Volcano Observatory in Vancouver, Washington, is perfecting an infrared spectrometer that he says will detect still other volcanic gases that absorb infrared light, including hydrochloric acid gas, carbon monoxide, methane, and water vapor.

Mt. St. Helens erupting, May 18, 1980.

Calling the Next Big One

Tilling says volcanologists also need to get a better handle on the basic mechanisms behind precursory signals, such as the long-period earthquakes that often precede eruptions. Dr. Bernard Chouet, a VHP volcano seismologist, says these quakes provide a "direct window" into the magmatic fluid moving about beneath a restless volcano. "These earthquakes are like stress gauges that light up and reflect the pressurization going on below," he says. Careful monitoring of such natural gauges can help forecast eruptive activity. The USGS team that successfully predicted Pinatubo's burst did so in part by watching the build-up of long-period quakes.

USGS Volcanologist Ken Yamashita surveys on the dome, Mt. St. Helens, March 1986.

The urgent need to improve methods to call the next Big One holds especially true for large caldera-forming eruptions. These true earth-shakers explode with such Herculean force that they leave behind vast, basin-like depressions—calderas—that can stretch many miles across. The largest caldera-forming eruptions, which fortunately have not occurred in human history, make the explosive eruption of Mt. St. Helens in 1980 seem like a firecracker. In the mid-1980s, three volcanic fields believed to hold the potential for one of these monumental cataclysm—California's Long Valley, Papua New Guinea's Rabaul, and Italy's Campi Flegrei—turned on almost simultaneously, throwing the volcanological community into a bit of a frenzy. All three centers calmed down without further ado, though Rabaul erupted a decade later (see Planning for Disaster).