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.
In 1956, Bezymianny volcano on the Kamchatka Peninsula exploded in 1956 - an eruption Gorshkov classified as "really gigantic." And it was - the eruption destroyed a large part of the volcano. Gorshkov's Figure 16 shows the pre-eruption outline superposed over the post-eruption edifice. Fans of the Mount St. Helens series should find this eerily familiar.
After May 18th, 1980, volcanologists looked back on Bezymianny and found the destruction there making a certain amount of sense.
Shiveluch, also in Kamchatka, erupted violently in 1964. Gorshkov wrote a seminal paper on the blast entitled "Gigantic Directed Blast at Shiveluch Volcano (Kamchatka)," referenced often by USGS geologists as they worked to make sense of the events at Mount St. Helens. The lateral (directed) blast here removed several domes painstakingly built by the mountain in previous eruptive periods and left a shell.
"It is necessary to add," Gorshkov wrote, "that the formerly picturesque volcano foot having been rich in animals and vegetation, turned into a barren stony desert, covered with thousands of large and small hills." He could have been describing the devastated area at Mount St. Helens.
The two lateral blasts, happening within less than a decade of each other on the same peninsula, led Gorshkov and his fellow geologists to define directed blasts as a new kind of eruption, and caused volcanologists to take a closer look at previous eruptions that hadn't been formerly understood as lateral eruptions.
Geologists even found evidence of previous lateral eruptions at Mount St. Helens. Crandell and Mullineaux noted in their hazard assessment that "such blasts often are associated with the formation of a volcanic dome, and they generally only affect the side of the volcano on which the dome is being erupted." They advised that lateral blasts could affect areas out to at least 10 kilometers (6.2 miles). They were very aware that Mount St. Helens was capable of producing a directed blast, perhaps even a sizable one - but no one predicted the scale of what she ultimately unleashed. The only confirmed lateral blasts she'd been responsible for previously had happened just over 1,000 years previously, and those were just two piddly things not even a tenth of the size of her May 18th extravaganza.
There were other directed blasts at other volcanoes - Lassen Peak in California in 1915, for instance, where a lateral blast destroyed a large swath of the northeast slope, and Mount Lamington in Papua New Guinea, where an eruption similar in type but smaller in size to Mount St. Helens occurred in 1951 - but they all had something else in common: their directed blasts, for various reasons, weren't nearly as well observed. Mount St. Helens, however, was crawling with geologists, bristling with sophisticated equipment, and festooned with photographers. Her eruption happened on a sunny morning on a clear day near a populated area. And that lateral blast became perhaps the best-documented, most closely-studied directed blast in history.
Geologists were overflying the mountain the instant it erupted, were photographing and flying around it throughout the eruption, and were on the ground before the ash had a chance to settle. They dug into the fresh deposits, documented and sampled and measured, took eyewitness statements, studied their photographs, and collected readings from a variety of instruments that had recorded the whole event. They could follow the whole sequence in series of photographs, correlate those photos to observations on the ground, and figure out just what causes volcanoes to sometimes blow out laterally.
Mount St. Helens also provided a laboratory where we learned how to recognize the eruptive products of directed blasts. One of the major axioms of geology is "the present is the key to the past." When it comes to volcanoes, the past is also the key to the future. But you have to know what you're looking at in order to decipher what those deposits are trying to tell you. The May 18th directed blast left plenty of deposits for geologists to study. They provided a nice compare-and-contrast that helped geologists understand the difference between classic pyroclastic surges and this bizarre horizontal eruptive style. Knowing what to look for would help geologists recognize similar landslides, now called sector collapses, at hundreds of other volcanoes.
We're now far more aware that those big, beautiful stratovolcanoes have a distressing tendency to fall down in major ways, and don't always blow up so much as blow out. This helps us understand the hazards we face as more people live and play near and on these dangerous but captivating mountains. The lessons we learned from Mount St. Helens help us save lives.
Crandell, D. R., and Mullineaux, D. R., 1978: Potential hazards from future eruptions of Mount St. Helens, Washington. U.S. Geological Survey Bulletin 1383-C.
Gorshkov, G.S. (1959): Gigantic eruption of the volcano Bezymianny. Bulletin Volcanologique, ser. 2, v. 20, p. 77-109.
Gorshkov, G.S. and Dubik, Y.M. (1970): Gigantic Directed Blast at Shiveluch Volcano (Kamchatka). Bulletin Volcanologique, v. 34, p. 261-268.
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.