The first earthquakes struck on March 20, 1980. Seismologists quickly
determined that the quakes were centered beneath a snowy mountain, known to
them, but not to the general public, as a potentially dangerous volcano, which
had been dormant for more than a century. During the next week, the number of
earthquakes increased, and these quakes triggered snow avalanches, which in turn
forced closure of winter recreation areas around the mountain. Geologists and
geophysicists converged on the scene to monitor the activity and met with local
authorities to alert them to the possibility of an eruption. On March 27, steam
and ash exploded from the summit of the volcano and marked the beginning of
several small eruptions during the next two months.
Public authorities prudently closed the area surrounding the mountain after being informed of the volcano's past violent behavior by those who had conducted careful geological studies during the preceding 20 years. Although closure was a necessary precautionary measure, it created discontent and even anger on the part of some citizens who wanted access to their property and recreation sites. Continued monitoring of the volcano indicated that its north flank, which towered above the most popular recreation area, was becoming increasingly unstable. Warnings were issued for landslides and large-scale snow avalanches. These warnings supported the need for continued closure of the area, although public pressure eventually led to brief, authorized forays into the area by cabin owners to retrieve belongings. One such trip was scheduled for the morning of May 18, but it never took place.
At 8:32 a.m. on May 18, an earthquake triggered a gigantic landslide on the unstable north flank, which in turn unleashed a scorching, explosive blast of hot gas laden with rock fragments; massive floods of mud and rock down most river valleys; flows of hot, gas-rich volcanic rock; and an enormous plume of ash. The water-soaked landslide debris produced a series of dense slurries that raced downstream and nearly severed Interstate Highway 5 and the AMTRAK rail line connecting Portland, Oregon, and Seattle, Washington. These debris flows brought shipping on the Columbia River to a halt and came close to blocking cooling-water intakes at an operating nuclear powerplant. These events transformed a lush landscape of dense, green forest into a dusty volcanic wasteland and killed 57 people who were too close to the mountain. The eastern half of the State, where people were virtually unaware of any volcanic hazard, was blanketed with ash. The death toll, though large, could have been much, much higher without the previous warnings and resultant land closure. Luck also played a role in keeping the number of fatalities down. Had the eruption occurred on Monday rather than Sunday, several hundred loggers, working in an area near the volcano but outside the closed area, would have died. During the next decade, continued enforcement of restricted zones and careful observation and prediction of activity warned the public of impending eruptions, and no additional lives were lost. Research into what had caused the catastrophic eruption led to increased appreciation of the inherent instability of high, snow-covered volcanoes and the hazards they pose.
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