Geologist Recounts Mount St. Helens' Eruptions

Katharine Cashman
Katharine Cashman
Katharine Cashman became an expert on Mount St. Helens largely by accident. She isn’t a great geologist, she said during her presentation on April 13 in the Science Center, because she likes studying geologic processes that unfold on a time scale that she can watch. Volcanic activity is one such process, and Cashman, the head of the geosciences department at the University of Oregon, has devoted much of her life to studying the science behind the eruptions of Mount St. Helens.

Mount St. Helens is an active volcano located in southern Washington and is part of the Cascade Range. Cashman’s involvement with Mount St. Helens began shortly after its catastrophic eruption in early 1980, when she was hired by the U.S. Geological Survey as a public information scientist. Although her area of expertise was petrology, the study of rocks and how they form, Cashman enjoyed working in public information because of the amount of contact she had with all the different groups monitoring seismic activity around Mount St. Helens after the eruption.

The first indication of a stir beneath Mount St. Helens in 1980 was a 4.2 magnitude quake on March 20. Earthquakes concentrated near the surface became more frequent throughout March, and by April it became clear that a bulge was forming on the north side of the mountain. Finally, on the morning of May 18, the bulge on the north flank gave way to a landslide, coinciding with a 5.1 magnitude earthquake.

Low-level activity seismic activity continued for several hours following the landslide, and around noon Mount St. Helens began spewing gas and hot ash, creating an eruption column miles high that traveled east all the way into Idaho.

Though there was never another eruption as catastrophic as that of 1980, effusive eruptions (characterized by the outpouring of magma on the ground) continued into 1986. Scientists continued to closely monitor the activity of Mount St. Helens by noting seismic activity, observing the physical deformation of volcano and measuring gas emissions near the surface.

Between 1986 and 2004 there was little activity around Mount St. Helens; Cashman referred to it as a “period of recharge.” There was another eruption in 2004, for which Cashman flew in from Italy, where she was on her first sabbatical in 18 years. The 2004 eruption was gentle compared to the 1980 eruption. This was due to the crystallization of the magma within the earth so that it reached the surface as a solid. As the magma neared the surface, the pressure on it was lessened which decreased its ability to hold water. With less water, the magma solidified and pushed through the surface slowly as a solid.

Cashman showed a timelapse sequence of the growing bulge on Mount St. Helens in 2004, which showed the magma protruding over a stretch of several weeks as solid crystal. There is not a lot of certainty as to why the magma crystallized in 2004 when it did not in 1980—one hypothesis is that the cooler temperature of the magma facilitated the crystallization, though that does not explain why the magma was released slowly and in pockets rather than all at once in a powerful eruption.

Cashman said that nobody is entirely sure what will happen in the future with Mount St. Helens simply because there is still too much about volcanic eruptions that we do not know, but science is improving and increasing the chances that any major eruption in the future will not be unexpected and catastrophic.

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