The eruption started on March 27 but was considerably smaller than the scientists had thought it would be. Activity continued, and seismic activity was continuous, while chemical samples (taken from the sides of the mountain but also in the crater (!) by daredevil scientists) didn't show signs of imminent danger. In May, the sesimic activity in the mountain seemed to settle again and public pressure to lift warnings and security measures started to mount. So did, however, the magma pressure in the mountain. It was even visible for the naked eye, as a buldge had formed on the northern slope. Laser measurements assessed the growth to one meter and a half per day (!). In this situation, volcanologists could persuade the governor to keep access to the “red zone” closed, the area around the mountain thought to be directly hit if the mountain had a Plinian eruption, explosive like that described by Pliny the younger at Mt Vesuvius in 79, which destroyed Herculaneum and Pompeii. However, some residents and lumber companies refused to evacuate, tourists continued to flock around the mountain, trying to evade road blocks, and the scientists kept watch from several observation spots.
At St Helens, volcanologists worked with three types of predictors when assessing the risk for an eruption: seismology (earthquakes), chemistry (gas and water emissions) and geology (deformation, previous eruptive history). By combining these factors in "event tree" algorithms, the accuracy of newer predictions is estimated to have moderate to good predictive ability for short term predictions (Areas under Receiver Operating Characteristics Curves of 0.78-0.81, Junek and co-workers, 2012, Algorithms, which is an estimate of the validity of the method rather than the security of a single prediction). Just as in psychiatry, a “clinical override” is also allowed for, so a senior volcanologist can have an experience-based opinion that takes precedence before the results of the algorithm. And Mt St Helens proved to be a testing ground for many new tools used routinely today to predict volcanic activity.
On Saturday, May 17, the mountain was so calm that residents in the red zone were allowed a brief entry from 10 AM to 4 PM to collect property. A similar opening was planned for the following Sunday. The weather was clear and sunny, with a very good visibility. Fortunately, Sunday meant the loggers had the day off. At 8 32 AM Sunday morning, the whole northern flank of Mt St Helens suddenly collapsed and gave way to a Plinian eruption, which also contained what is referred to as a “lateral blast”, a sort of eruptive column going side-ways instead of upwards. The consequences were terrible. Fifty-seven people died, among them several scientists and journalists, the most well-known being David Johnston, who had predicted just a lateral blast based on the bulge but nevertheless remained at an observation post overlooking the north face of the mountain from about 10 km. No film of the eruption survived, but several series of still shots taken in rapid succession made a very detailed reconstruction possible (below). A detailed description is given in the Wikipedia entry on the eruption. Several other interesting films describing the sequence of eruptions and its aftermath are available on YouTube. The events were also used in several movies, among them St Helens and Dante's Peak.
Due to the lateral blast, the zone of destruction was much larger than the predicted red zone, and many victims died in areas considered medium or even low danger. Rapidly after the blast, mud flows created by rapidly melting snow on the mountain hit further victims down the river beds. After his visit, US president Jimmy Carter said that, in comparison to the devastation wrought on the countryside to the north of the volcano, the “moon looked like a golf course”. Fortunately, as the area around Mt St Helens was sparsely populated and, at least partially, evacuated, the death toll was two-figure and not much worse. When volcanoes like the neighboring Mt Rainier, which overlooks Seattle and many smaller cities in its river valleys, come to live, the responsibility put on the scientists will be unfathomable.
A last word of hope. At first, scientists thought that the blast zone would remain sterile and life-less for a very long time, up to a century. For the 30th anniversary of the eruption, a nature TV documentary was produced. It really shows how amazingly fast life is to reclaim lost areas and fill them with wonders anew.
On Friday, the next blog entry will deal with the science of predicting future dangers in forensic psychiatry as compared to volcanology. I also plan to read some more pappers on predictions in volcanology, as their use of event trees and short-span predictions are very interesting, and to present it in some blog post. And if you followed the path to the 2014 Bardarbunga eruption, which is currently ongoing, seismic recordings posted on the internet made it possible to see how the magma rose in the volcano day by day, and where it would eventually erupt. But also in Iceland, major risk assessments are currently performed, and can be followed on the website linked to bardarbunga (The Icelandic Met Office)