Lecture #8 notes; Geology 3950, Spring 2006; CR Stern May 1980 eruption of Mt St Helens volcano (text pages 183-192 in the 4th edition and 206-222 in the 5th edition) Mt St Helens in southwest Washington State is one of approximately 15 stratovolcanoes that form the Cascade volcanic arc, which extends from northern California into southernmost Canada (Figure 1 below). Other important volcanoes in this arc include Mt Shasta and Lassen in northern California, Crater Lake (formally Mt Mazama) and Hood in Oregon, Mt Rainer and Adams in Washington, and Mt Garibaldi in Canada.

The volcanoes in the Cascade volcanic arc result from the slow subduction of the small Juan de Fuca oceanic plate into the Cascadia subduction zone below the northwest US, the same tectonic activity that produces the risk of large subduction earthquakes in this part of the country (Fig 2 below).

Subduction produces volcanism because water-rich oceanic sediments and crust is tectonically carried down into the mantle. When this water is released from the subducting lithosphere plate, it rises into the overlying mantle and fluxes melting and magma formation, since water lowers the melting temperature of mantle rock (Figure 3)

Note that there is no volcanism associated with the San Andreas Strike-slip plate boundary between the Pacific and North American plates, and that there is no volcanism associated with the thrust faults below Los Angeles, as both these fault systems are only in the crust, and magmas are derived by tectonic disturbances in the mantle. Volcanoes do occur in association with the extension taking place in the Basin and Range and Rio Grande rift in the area between eastern California and western Colorado, and also in association with the Yellowstone hot spot in Wyoming, but the Cascade arc of stratovolcanoes and calderas is generally considered the area of highest volcanic risk in the US (figure 4)

Mt St Helens was know to have entered into active periods that lasted from 20-50 years once every 100-150 years over the last 500 years, (Figure 5).

Mt St Helens was known to be one of the most active of all the Cascade volcanoes (Figure 6). In the 1970’s it was predicted that it would erupt again before the year 2000.

Mt St Helens (figure 7) entered into a new phase of activity in early March 1980 with a swarm of small earthquakes below the volcano

These earthquakes resulted from magma rising from the underlying subduction zone into the base of the volcano, and the breaking of rocks to make room for this magma. The magma heated ground water in the volcano and by mid-March plumes of steam and fragments of rocks from the top of the volcano were being blasted into the air, creating craters in the snow and ice cover of the volcano. However, this activity is not considered an “eruption” since only steam is involved, not magma itself. This activity continued so that by late April the volcano was covered with the rock fragments blasted out of the top of the volcano by the release of the plumes of steam (figure 8) .

The earthquake swarms persisted from early March into May, with their greatest density at a depth of a 10,000 ft below the north flank of the volcano (figure 9), interpreted as the chamber into which the new magma coming from the mantle was gathering

The magma caused the north flank of the volcano to bulge upwards by more than 100 feet and over-steepened (figure 10)

Because of the many indication of an impending eruption, the US Geologic Survey and the Washington State authorities evacuated the resort areas around the volcano in early May. On May 18th at about 8:30 AM a larger than usual earthquake caused the unstable north flank of the volcano to landslide down into the valley below, a process called “sector collapse”. This released pressure off the top of the magma chamber below the volcano, allowing volatiles (water) dissolved in the andesite magma in this chamber to form bubbles. The expansion of water, by >30 times its volume, and the formation of these bubbles is what caused the ensuing volcanic explosion that generated first a lateral blast to the north (figures 11 and 12) followed by a vertical Plinean eruption column of volcanic ash that rose >60,000 feet into the stratosphere both because of the force of the eruption and convection of the surrounding air heated by the hot volcanic fragments generated by the explosion (figure 13)

The eruption continued for 9 hours, generating pyroclastic flows of hot volcanic ash on the flanks of the volcano (figure 14)

The next day the volcano was covered these deposits, and the lake at the base of the volcano was filled with debris from the landslide and the lateral blast (figure 15 below)

Damage resulted from:

1) the lateral blast the downed trees and killed >60 people, including the USGS geologist monitoring the eruption, in a wide arc extending more than 20 km to the north. People in the area of the blast died because of the heat of the volcanic ash they inhaled (figures 16 and 17) burnt their lungs.

2) a series of mudflows that carried the debris from the landslide down the Tuttle into the Cowlitz and ultimately the Columbia River (figure 16)

3) the fallout of volcanic ash from the plume of ash carried into the stratosphere and blown eastward over Washington into Idaho by the jet-stream (fig 18 and 19)

The fall-out from this ash cloud resulted in tephra (volcanic glass fragments) deposits all over the state of Washington (figure 19)

Magnitudes of volcanic eruptions are measured by the volume of magma erupted, since the heat content of this magma is the main component of energy that drives the eruption. The total volume of andesite magma erupted in 8 hours by Mt St Helens was approximately 1 km3, which is not all that much when compared with the eruption of Vesuvius in 79 AD that covered Pompei, the eruption of Krakatoa in 1883, the eruption of Mt Mazama in 4600 BC that created Crater Lake, the 1550 BC eruption of Santorini island in the Mediterranean sea from which derives the myth of Atlantis (about the twice the size of the Mt Mazama eruption), or the eruption in 1815 of Tambora volcano in Indonesia, the largest historic eruption observed in the last few hundred years (figure 20)

Activity at Mt St Helens continued with the eruption of volatile poor magma that formed a small dome inside the summit crater (figure 21)

This activity continued through the 1980’s (figure 22) into the 1990’s and Mt St Helens was again active in September and October of 2004 as the crater dome grew larger. This extended period of activity is similar to the long eruptive cycles in the past

It is unlikely Mt St Helens will have another explosive event until a new batch of volatile rich magma rises from the underlying subduction zone into the base of the volcano, but this will happen again in the future as the life-span of subduction zone plate boundary stratovolcanoes like Mt St Helens is 100’s of thousands of years.