One of the most surprising aspects of the May 18th eruption of Mount St. Helens was the devastating lateral blast that ravaged such a large area. We’ll be spending the next few posts on that subject. It’s a complicated aspect of a very complex eruption, so before we dive in, let’s have a look at historic lateral blasts, what we knew before the whole side of Mount St. Helens blew out, and some of what we learned from her.
Lateral blasts weren’t completely unknown before 1980. In 1888, Bandai-san in Japan experienced a catastrophic eruption that removed 1.5 cubic kilometers (.36 cubic miles) of its summit. Its former Fuji-like summit was reduced to a shattered remnant – much like another volcano we’ve become intimate with. Imperial University of Tokyo geologists Seikei Sekiya and Y. Kikuchi thought the deposits left at base of Bandai-san’s north slope were the result of a landslide; Soviet volcanologist G. S. Gorshkov put them down to a directed blast. Could it have been both? Mount St. Helens tipped us off to the possibility that such blasts were very much related to landsliding: an earthquake knocks an unstable slope loose, the resulting landslide depressurizes a magma chamber (and/or hydrothermal system?) beneath, and boom.