Speaking Of: : Volcanoes : When Will a Volcano Blow? Quakes, Ground Tilt Are Clues

While Californians are aware of the frustrating inability of seismologists to predict earthquakes, fewer know of the progress of scientists in predicting volcanic eruptions, another of nature’s threats to the Pacific Coast.

The crucial precursory volcanic events have become known, volcanologists say.

Randy White of the U.S. Geological Survey explained: “The scenario for really big explosive eruptions is, first, basalt (magma) coming into the bottom of the magma chamber, and that gives itself away as deep, long-period earthquakes. Then that stirs the pot and starts producing shallow, long-period earthquakes and probably sulfur-dioxide emissions. Then, as the pressure builds, you would start seeing the ground deformation, the tilt.”

In 1991, by conservative estimates, at least 10,000 lives were saved in the Philippines because a two-day warning that Mt. Pinatubo would undergo a major eruption allowed timely evacuations of American-operated Clark Air Base and nearby Philippine cities.

And last September, 30,000 people living in and around the port of Rabaul in the South Pacific nation of Papua New Guinea were evacuated the night before two vents on opposite sides of the caldera, the broad crater-like basin of the Rabaul volcano, erupted with great fury. Two people were killed, compared to about 500 fatalities in the last major eruptions at Rabaul in 1937.

These successes, however, do not mean that volcanic predictions are always precise or reliable.

If prediction, as defined by White, includes time, place and size, then scientists have been much more successful with time and place than size. The Geological Survey scientist said this amounts to “a forecast, not a prediction.”

At Mt. St. Helens in Washington state in 1980, scientists, noting earthquake and, particularly, harmonic tremors--the long-period quakes indicating the upward movement of molten material--warned of a serious eruption, and the authorities established an evacuation zone.

But the huge eventual eruption of May 18, 1980, exceeded their expectations.

A Geological Survey scientist, David Johnston, was killed at his observation station, six miles from the eruption site. His supervisors believed that he would be safe, but the blast extended more than three times farther than they expected.

More recently, it now appears that some scientists may have initially overestimated the dangers of a huge eruption at Popocatepetl, the 17,887-foot volcano 39 miles southeast of Mexico City, when it spewed out ash last Dec. 21.

Michael Sheridan, chairman of the Geology Department at the State University of New York’s Buffalo campus, declared at the time that “the chamber is loaded,” and, at another point, said: “That volcano is getting ready to do something pretty big.”

Dan Miller, a research geologist at the Cascade Volcanic Observatory in Vancouver, Wash., said, “It’s a volcano that has frequently erupted violently in the past, and it’s to be considered very dangerous now.”

Sheridan, Miller and others cautioned, however, that careful observations had to be made before the situation was clear, and the record since Dec. 21 indicates that, for now, no big eruption is in the offing.

On the other hand, Miller said, there are continuing high emissions of sulfur dioxide gas from the volcano, ranging from 1,000 to 12,000 metric tons per day. Only Sicily’s Mt. Etna “has those kinds of numbers during non-eruptive periods,” Miller noted.

“I think it’s a system that bears watching,” he concluded.

Bill Rose, professor of geology at Michigan Technological University in Houghton, observed that “people are very conscious of false alarms, so there is a sensitivity (among volcanologists) about the way predictions are made.”

In the longer term, cautioned Stanley Williams, a volcanologist at Arizona State University, different mountains seem to have different patterns, and for a general evaluation of the potential for an eruption, scientists need to understand a volcano’s history, if it is available.

“St. Helens is an example of the best we could hope for,” Williams said. “Don Mullineaux and Dwight Crandall (two Geological Survey scientists) had selected it (in 1978) as the most likely next eruption of the 20th Century, and they were right.

“With another 40 volcanoes, it is very, very uncertain. All kinds of attention is being paid to Rainier (in Washington state), because of the hazard to metropolitan centers, yet we have a very poor sense of the likelihood of an eruption there within the next 10 or 100 years.”

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Speaking Of: Vocanoes

* WHERE THEY ARE: The world has about 1,500 active or potentially active volcanoes. There are thousands more cinder cones in volcanic vicinities. About 75% of the world’s volcanoes are in the so-called Ring of Fire around the Pacific ocean.

* ANATOMY OF AN ERUPTION

Sulfur dioxide’s effects on the ozone layer: Some volcanoes emit high quantities of sulfur, forming natural aerosols that, added to man-made aerosols, contribute to depletion of the ozone layer protecting the earth from ultraviolet radiation.

Global cooling caused by ash clouds and gas emissions: The most violent eruptions send materials high into the stratosphere where they circle the globe. Several times this century, eruptions have been responsible for temporarily, over a period as long as several years, lowering the world’s average temperature by as much as one degree centigrade.

Ash fall: Even falls of several inches can cause great disruption, and occasionally deaths when roofs collapse. Aircraft must avoid such clouds to prevent engine failure aloft.

Nuee ardente: These “glowing clouds” of often superheated ash, gas and solid debris ranging from dust to boulders first became widely known to scientists in the Pelee eruption of 1902. The most immediately lethal volcanic effect, it kills through both blast and heat.

Tsunami: Great surges of water displaced by either undersea eruptions or sudden volcanic landslides have infrequently killed thousands of people, some of them many miles from the point of origin. Most tsunamis, however, are caused by ordinary earthquakes.

Blast zone: There are various kinds of blasts from volcanoes, some cold, some hot, some of light pumice, others of heavier solids as well as gas. A deadly hot blast was responsible for virtually all the 60 deaths at Mt. St. Helens in 1980.

Lava flow: These are among the most famous of volcanic effects, but they are generally not as lethal as nuee ardentes, other blasts, mud flows or tsunamis. Usually, there is time for people to get out of the way.

Mud flow: Known to volcanologists as lahars , these can move fast over long distances and can clog rivers and bays with debris more than 100 miles from the point of origin. Populated areas between Seattle and Tacoma overlie an ancient lahar from Mt. Rainier.

Some of the Most Celebrated Volcanic Eruptions in History

1. About 1500 B.C.--Thera is believed to have destroyed the Minoan civilization around the Aegean sea in what may have given rise to the legend of the lost continent of Atlantis. The site is now Santorini island in Greece.

2. A.D. 79--Vesuvius destroyed the city of Pompei with ash falls and Herculaneum with a mudflow.

3. 1669--In one of its 200 recorded eruptions, Mt. Etna in Sicily caused 20,000 deaths.

4. 1815--Tambora in Indonesia caused a tsunami that killed 56,000 people. Its worldwide ash cloud was blamed for a cold summer in Europe and North America the following year.

5. 1883--Krakatau, often called Krakatoa, in the Sunda Strait between Java and Sumatra, erupted so loudly the explosion was heard 2,900 miles away. The tsunami killed 36,000 people, and a wave was noticed half a world away in the Thames river in England.

6. 1902 -- A nuee ardente, a cloud of hot gases, swept downhill from Mt. Pelee on the island of Martinique, killing 29,933 people in the city of St. Pierre. There was one survivor, a condemned man in a dungeon. Authorities later decided to spare his life.

7. 1912--Katmai in Alaska deposited ash as deep as 50 feet in what became known as the Valley of Ten Thousand Smokes.

8. 1943--Paricutin appeared in a Mexican corn field, eventually building a cone 1,400 feet high.

9. 1980--Mt. St. Helens in Washington state blew its top off, sending a life-destroying blast outward 19 miles.

10. 1985--Nevado del Ruiz in Colombia generated a mudflow, known to volcanologists as a lahar, down its slopes, sweeping into the city of Armero and killing 22,000 people.

11. 1991--Pinatubo in the Philippines, in its first major eruption in 600 years, temporarily cooled the world’s climate by an average half degree and contributed to ozone depletion.

* ON THE NORTH AMERICAN MAP

Mt. Katmai: 6,715 ft.

Mt. Baker: 10,778 ft.

Glacier Peak: 10,541 ft.

Mt. Adams: 12,307 ft.

Lassen Peak: 10,457 ft.

Mammoth Mt.: 11,053 ft.

Mt. Rainier: 14,410 ft.

Mt. St. Helens: 8,364 ft.

Mt. Shasta: 14,162 ft.

Popocatepeti: 17, 887 ft.

Sources: Volcanoes, a Planetary Perspective; Encyclopaedia Britannica; World Book Encyclopedia; Encyclopedia Americana; World Almanac and Book of Facts; Information Please Almanac