St. Helens: Restless ...'Unpredictable'

Published: 05/17/1995
Byline: By LORETTA CALLAHAN, Columbian staff writer

Fifteen years after its devastating explosion, Mount St. Helens still ranks as the Cascade Range mountain most likely to blow.

But no need to start packing. The eruption ratio is just part of what geologist Rick Hoblitt calls "playing the probability game."

"That doesn't mean Mount St. Helens WILL be the next to erupt," Hoblitt said.

It means there is a statistical likelihood.

Volcanologists' odds, a kind of Jimmy the Geek gamble.

According to the U.S. Geological Survey, there is a 1 percent probability of 10 centimeters or more of tephra ashes from Mount St. Helen's falling on your head sometime during a year. The chance of that happening at Mount Rainier is 0.2 percent.

Beyond that, Hoblitt said, not enough is known to predict when Mount St. Helens will reawaken.

But a new study, which Hoblitt is about to publish in the Geological Society of America Bulletin, shows the mountain has spent a restless past 500 years.

Mount St. Helens has barely napped.

Its May 18, 1980, blast provided Hoblitt and other scientists at the USGS's David A. Johnston Cascades Volcano Observatory in Vancouver with a well-equipped, 150-square-mile research laboratory.

"This," Hoblitt said, "is where you can make quantum leaps in science. In an eruption you can learn in minutes what would otherwise take years."

Hoblitt and David Yamaguchi, a University of Washington doctoral graduate, seized the opportunity by examining cores taken from subfossil trees buried for hundreds of years until the May 18, 1980 eruption.

"Tree rings are sort of like fingerprints," Hoblitt said. "Weather patterns affect the ring width, and they do it over a broad area."

Hoblitt and Yamaguchi, now employed by a forest research institute in Japan, began their study by collecting cores of trees exhumed from at least 500-year-old pyroclastic flows and lahar deposits, mostly on the south flank of Mount St. Helens.

Back in the laboratory, the wood samples were dried and sanded. Fragile pieces were impregnated with warm paraffin. Then the wood was sliced into cross sections and matched against a master tree ring, supplied by one huge, old Douglas fir.

The results shed new light on Mount St. Helens' recent past, from 1479 to the present.

"Past behavior is sort of the best predictor for what will happen in the future," Hoblitt said. "By studying, we learn frequency, types of eruptions and extent of the area affected."

Prior to the tree ring study, scientists believed Mount St. Helens was most active from the 1480s to the mid-1500s known as the Kalama period followed by long years of quiet.

But the study shows little time, maybe 50 years, elapsed between the end of one eruptive period and the beginning of the next. And the eruptive periods lasted a longer time, as much as 300 years long, with short breaks in between sporadic explosions and lava flows.

Since 1479, the mountain's longest sleep appears to have lasted 123 years, not long at all for a volcano. "If you look over the long haul, St. Helens has periods in the order of thousands of years when it's active, other periods of the same duration when it's inactive," Hoblitt said. "We're currently in one of those active periods."

Hoblitt dates the Kalama cycle, the first in the mountain's most recent active period, from 1479 to 1750. He places the second, the Goat Rocks eruptive cycle, at 1800 to 1857.

The most recent cycle, Hoblitt said, began in 1980 and ended about 1986.

"One problem is the further back you get, the more clouded the record becomes," Hoblitt said.

Hoblitt has been fascinated with Mount St. Helens since he was a graduate student on a part-time survey crew in 1975.

By 1980, Hoblitt was on the USGS volcano monitoring team as a full-time scientists, studying the mountain's shifting and groaning.

Everyone wanted news of what was happening. Everyone including Hoblitt's supervisors, who told him to return to Denver to write the paper on the recent eruptive history for which the science world and public were eager.

"I didn't want to go," Hoblitt said.

Hoblitt was in Denver on May 18, 1980. He rushed back. Within hours of the eruption, he was flying above the crater in a small aircraft.

Hoblitt stayed at St. Helen's side for seven days a week until October.

Asked to describe the volcano, Hoblitt smiled and said, "Unpredictable."