Good Afternoon!
The 40th anniversary of the catastrophic eruption of Mount St Helens was on 18 May 2020. On 18 May 1980, 57 people are known to have been killed, most probably by the initial lateral blast, not taking into account anyone unwise enough to have ignored the exclusion zone. In addition, significant damage was done to the surrounding area.
Geologists Keith and Dorothy Stoffel, on a photography expedition, witnessed the catastrophic failure of the northern flank of the volcano: the north side of the volcano slid down then disintegrated in a massive debris avalanche. This was followed by the horizontal blast and the vertical ash column which reached a height of 20 km within 10 minutes. The threat of being engulfed in the ash cloud forced them to return to base.
Geological Setting
Mount St Helens lies at the front of the Cascades volcanic arc in Washington. She is the most active volcano in the arc. Her lavas are dacite, andesite / basaltic andesite, basalt / picro basalt, trachybasalt / tephrite basanite and trachyandesite / basaltic trachyandesite(1). The lavas are sourced from depths of 7 km – 14 km beneath the base of the edifice. Eruptions occurring after 1980 were fed from magma at a depth of less than 4 km.
Volcanism here is driven by the subduction of the Juan de Fuca plate under the North American plate. The arc starts in northern California and reaches up to British Columbia, including composite volcanoes and volcanic centres, calderas and back arc basalt shield volcanoes. Lava compositions range from intermediate to high silica lava domes to low silica lave flows in the southern end of the arc. The presence of low silica lavas at the southern end of the arc is attributed to extensional tectonics permitting the rise of more fluid magmas.
Seismicity in the Cascades
We downloaded the earthquakes between 39.470°N, 130.808°W and 50.870°N, 118.696°W from 1975 to 31 May 2020 to see what the subduction zone below the Cascades looks like. There were 18,173 quakes over 2.5, of which 48 were between 6.0 and 7.0 and 4 had magnitudes in excess of 7.0. We found two areas where there was some evidence of subduction, one at the northern end of the region (north of latitude 45°N) and the other in the south (south of latitude 42°N), otherwise the area was relatively quiet in this period.
In order to see the subduction zone clearly we have had to removed the “noise” from the Blanco Fracture Zone.
Eruptive History
Mount St Helens is a relatively young volcano, formed around 40,000 to 50,000 years ago. The current edifice was built in the last 2,200 years. Before the eruption Mount St Helens was conical stratovolcano made up of lava, ash, pumice and other deposits with layers of basalt and andesite. Dacite lava domes extruded from the summit and on the northern flank at Goat Rocks.
Mount St Helens has had 44 Holocene eruptions according to GVP(1), of which one was VEI 6, and five were VEI 5, including the 1980 eruption. Two years prior to the1980 eruption USGS volcanologists produced a hazard assessment, predicting an eruption with the next couple of decades.
The 18 May 1980 Eruption
A 4.2 earthquake on 20 March 1980 and increased seismic activity showed that the volcano was reawakening. Ash eruptions followed from 27 March 1980. A harmonic tremor, indicating magma ascent, set in. Between the initial earthquake and to the earthquake preceding the eruption on 18 May 1980 there were 228 earthquakes over 2.5 and thousands more smaller quakes.
By the end of April, a 2 km wide cryptodome (bulge) had deformed the northern flank, swelling upwards at a rate of 1 metre per day to 150 metres above the existing topography by 12 May 1980. Ground deformation was now presenting a serious hazard.
On the morning of 18 May 1980 magma had reached a level of 2 km below the edifice. The slope failure was triggered by a 5.1 earthquake on the morning of 18 May 1980 (or vice versa). A debris avalanche of rocks, glacier ice, soil and other debris ensued, reaching Spirit Lake and the Toutle River valley and ending up east of the Camp Baker logging base, 20 km further down the valley.
Depressurisation of the system, resulted in the violent explosive lateral blast. The blast outran the initial avalanche, felling trees in an area of 600km2 and causing the most loss of life. The blast included heated old lava from the volcano and left a relatively thin ash layer.
After the blast, pumice was erupted in a vertical eruption column and pyroclastic currents covered the northern side of the volcano. Pyroclastic deposits reached a depth of 40 metres in the Upper Toutle Valley. It is estimated that at the time of the eruption the pyroclastic flows had temperatures in the region of 700°C. Hot ash reaching Spirit Lake caused secondary eruptions as the water flashed to stream.
During the eruption, Mount St Helens lost its pristine cone and around 400 metres height; it was left with a 1.6 km wide crater, open on the northern side.
Post Eruption
From mid-June 1980, lava emerged into the crater forming a lava dome; subsequent eruptions have been crater-based lava dome.
Could Mount St Helens produce another catastrophic eruption?
As she has a history of VEI 5s and a VEI 6, the answer is probably, yes. Her lava types do produce explosive eruptions, including the dacite lava domes. If any of the remaining slopes are unstable, there could be future slope failure should ascending magma cause enough deformation but it’s possible that she may have to rebuild the cone first.
In the meantime, a risk would be to the unwary straying too near to the crater should she have a smaller e.g. phreatic eruption, which are unpredictable. Phreatic eruptions occur when ground water in the edifice heated by hot rocks or magma flashes to steam.
GVP(1) have added a commemorative feature to its website giving a lot more information, including footage of the eruption. We recommend that you take a look; it is well worth the visit(2).
The Armchair Volcanologist
2 June 2020
References & Further Reading
- Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu/
- Mount St Helens 40th Anniversary, GVP: https://volcano.si.edu/projects/sthelens40/
- Mount St Helens, Wikipedia: https://en.wikipedia.org/wiki/Mount_St._Helens
- 1980 Eruption of Mount St Helens, Wikipedia, https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens
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An Armchair Volcanologist 
