At 7:30 am of May 18th, 1980, Don Swanson of USGS claimed that “the bulge still bulges”. The weird bulge grown on the northern flank of Mount St. Helens in the past weeks was still there; since days there were warning signs of a possible eruption. It would have begun exactly one hour later.
To remember the events that brought to the eruptive event, the USGS (U.S. Geological Survey) reported an accurate day-by-day chronicle of what happened fourty years ago on its Facebook page. Pictures and fascinating memories, exclusive video recordings, curiosities and details that had gone lost through time. A brief geological history of the volcano is presented here, from its origins until the day of the eruption. For more insights, we also suggest the reading of the chronicle on Facebook and the USGS scientist Richard Waitt’s book In the Path of Destruction: Eyewitness Chronicles of Mount St. Helens.
Mount St. Helens is an active stratovolcano, part of the Cascade Range volcanic arch (Cascades, State of Washington, USA), North-American section of the Pacific Ring of Fire. A volcanic arch forms by magma rising in proximity of subduction zones, i.e. when a tectonic plate (oceanic in this case) moves under a continental one. The descend of a fluid-rich crust into the mantle reaching increasing pressure and temperature creates instability, resulting in the rise of melted material towards the surface. The tectonic movements of the Pacific and surrounding plates generated several subduction zones and volcanic arches along the so-called Ring of Fire.
The magmatism by subduction usually originates at 100 km depth, since certain pressure and temperature conditions are necessary for the material to start melting and rising. However, Mount St. Helens is oddly located at only 67 km above the subduction zone. The results of the iMUSH (Imaging Magma Under St. Helens) research suggest that the volcano might seat above a cold magmatic rocks wedge (serpentinite) originating when the mantle reacts with oceanic water. The source of magmatims would be instead located eastward, under other volcanoes of the same chain. According to the research, the magma would migrate laterally feeding the Mount St. Helens eruptions.
The Geological history
The structure of a stratovolcano is originated by overlapping products emitted during its eruptive history. The shape of Mount St. Helens before the 1980 eruption was finalized during the Spirit Lake stage (Holocene, 3900 years – present day). The stage is divided in six eruptive periods: Smith Creek, Pine Creek, Castle Creek, Sugar Bowl, Kalama, Goat Rocks and the Modern one, begun with the 1980 activity. These periods were characterized mainly by explosive eruptions, proved by oldest products. The most significant event occurred during the Kalama period with the growth of the dome at the summit. This shaped the final volcano appearance. It took 100 years for the dome to grow and after the 1720 eruption it reached 1800 m above the sea level. At this altitude, an ice cap started covering the summit. The volcano remained quiet until mid-March 1980.
On March 20th 1980, after sleeping 123 years, a 4.2 degree Richter earthquake was registered in the volcanic area. It happened after days of weaker tremors, its hypocenter (the origin point inside Earth) was shallow. During the following days, the instruments recorded low magnitude seismic swarms, not perceived by humans. The USGS experts struggled to define the origin of the earthquakes (tectonic or volcanic), nor they could say if or when an eruption would occur.
The volcanic activity started on March 27th: thanks to an areal view the experts identified a 70 m wide newborn crater in the ice and a plume rising up to 2 km. Moreover, they mapped two East-West fracture systems and recorded more than 50 tremors with magnitude higher than 3.5 in one day. During the following days both the eruptive and seismic activities increased, while the plume rose up to 3 km above the mountain top.
In the first days of April the instruments recorded some low-frequency harmonic tremors. Compared to usual earthquakes caused by sudden energy release from the rocks, the harmonic tremors were associated to volcanic activity. They are usually caused by magma movements under the surface or by gas release from magma. Therefore, they can indicate an imminent eruption. Nevertheless, the wait continued.
On April 3rd the experts noticed a 450 m wide and 90 m deep crater on the northern flank, some fractures visible in the snow and an odd area of exposed terrain, probably the first signs of the bulge. The scientists collected eruptive samples and measured low concentration of sulfide dioxide (SO2), which can indicate sudden magma rise, if found in high concentrations. Water and ice surrounding the crater had formed muddy lakes on the crater floor. When new fluids entered the crater from outside, they warmed up and were ejected as steam and ashes in the atmosphere.
The activity seemed to settle in the first half of April: moderate explosions generated small plumes, some harmonic tremors were recorded, fractures on the northern flank extend and the bulge swells. The latter attracted the attention of the experts: at the end of the month it was 1.5 kilometer long, 2.5 kilometers wide and 100 m high. Its origin was a magmatic intrusion into the volcano (cryptodome), that occurred at the end of March. At the end of April its growth rate was 1.5 meter a day. The danger its existence and its fast growth posed was a material (snow, ice and rock) detachment from the flank and a rapid descent along the slope.
Despite the fact the scientists agreed about the danger on the northern flank, nobody knew exactly when it would have happened: the yield of the underlying rocks was unknown as well as the deformation depth. On May 12th a 5.0 magnitude earthquake caused an ice and rock avalanche, validating experts’ concerns.
May 18th, 1980
Few minutes after 8:30 am, a 1.5 East-West fracture opened north of the main crater. The resulting seism caused a detachment of 2.5 cubic kilometers of material from the northern flank and its descent for 14 kilometers. The volume was equal to the sum of a million of olympic swimming pools.
The volume detached from the flank was maintaining pressure on the magmatic system. After the cryptodome removal, the boiling water in the system was transformed into steam and triggered lateral hydrothermal explosions from the fracture exposed by the bulge. The explosion generated an eruptive plume that reached the height of 24 kilometers in less than 15 minutes and a pyroclastic flow that travelled for 8 kilometers at 130 km/h. The flow teared down trees in a 10 km radius.
Moreover, the pyroclastic flow melted part of the ice cap generating several lahars, mud rivers fast flowing towards the valley. The most destructive one travelled up to 80 km from the volcano. The lahars destroyed 27 bridges and almost 200 households.
The total amount of victims killed by May 18th 1980 explosions were 57. Most of the population and tourists had been previously evacuated. Because of its magnitude, the eruption has been classified as plinian on the scale of eruption types.
The eruptive event continued until October 1981, characterized by small explosive events with plumes up to 15 km high and pyroclasitc flows from the northern flank. Their products reached metropolitan areas in the states of Washington and Oregon that had not been affected by May 18th eruption.