Mount St. Helens erupted 40 years ago.

The 1980 eruption of Mount St. Helens was the culmination of a prolonged period of volcanic unrest. We will go through the events that lead, exactly 40 years ago, to one of the largest eruptions in the history of the United States. 

Text: Kathelijne Bonne

The early morning of Sunday 18 May 1980 was one like any other on the U.S. west coast. But rest is a relative concept in areas where two tectonic plates meet. The ocean floor of the Pacific Ocean slowly plunges below the North American continent, in a process known as subduction. Earthquakes and volcanic eruptions are common in such areas. Mount St-Helens, together with Mount Baker and Mount Rainier, lies in a string of volcanoes that runs along the entire west coast of the American continent. 

Increased unrest

Months before the fatal Sunday morning, Mt St. Helens started showing signs of unrest. From the 15th until the 21rst of March that year, seismographs registered more than 100 earthquakes. Ash and vapor rose from the summit. The volcano continued rumbling, and snow avalanches slid down its slopes. At some places large fissures started to open up. 

After a few stronger quakes, the US Geological Survey urged people to stay away from the volcano and its immediate environment, including the area of Spirit Lake. A bulge started to form on the north slope of the volcano. That was a sign that it was being injected with magma from beneath. Ash clouds rose, up to three kilometers high. Ash that had rained down was studied in the lab, and was apparently not formed from 'fresh' magma, but from older volcanic material. Maybe that was an indication that the newly injected fresh magma, was still residing at a considerable depth. Nevertheless, volcanologists were not at ease. 

State of emergency

Mount St. Helens was increasingly restless. Quakes became more intense and the intervals between the quakes became shorter. Gas explosions occurred as the volcano became hotter on the inside, and the hot material caused the groundwater to evaporate. 

On the 3rd of April, the state of emergency was declared. The area was marked as a red zone and whoever hung around without permission was given a 500-dollar fine or six months of jail. Meanwhile, the bulge on the north flank had reached a thickness of 90 meter, only to further increase in size. The area behind it was sinking. 

Geologists realized that this protruding, convex slope, was at risk of failing, and capable of producing a landslide. If that would happen, the magma underneath would be released. But nobody could predict if, and when that would happen. They could only wait. And by the start of May, the rumbling increased again. 

Silence before the storm

Then, on the 15th of May, Mount St. Helens fell silent. Not much more happened and the attention the volcano had received in the last few weeks, weakened. Homeowners became nervous, desiring to visit their properties. On May 17th, they were allowed to go to their homes to collect belongings. They on the following day, they were given permission to enter the area. 

In the early morning of May 18th, the volcano looked the same as the day before. A geologist who made observations from 10 kilometers away saw nothing unusual compared to the previous days and weeks. 

North slope collapses

At 8.32 am that day, an earthquake of magnitude 5.1 hit just below the protruding bulge. It just sat there for a few seconds, but then the entire north flank of the volcano began to move. 

The largest landslide ever observed by humans, moved in the direction of Spirit Lake. The sliding mass followed a river valley all the way down, at speeds of 175 to 250 kilometers per hour, over a distance of 20 kilometers.

Lateral blast

The magma underneath was released in an explosive way, producing a gigantic lateral eruption, now forever known as the infamous lateral blast. The explosivity is due to the gasses that were locked under high pressure in the magma. You can compare it to a bottle of Champaign you first shake and then open. 

Gasses and shattered volcanic material formed a huge could, a pyroclastic flow that cascaded down the slopes, reaching an incredible speed and quickly overtaking the huge landslide. Entire forests were destroyed over a large area. For decades to come, tree trunks were seen floating on Spirit Lake. The lake itself obviously looked completely different after the eruption. 

Direct blast zone

The destroyed area was later categorized into three concentric areas with different levels of destruction. Near the volcano was the direct blast zone. Everything was destroyed and gone, even soil had been blown off. A bit further was the channelized blast zone. In some areas, especially behind topographic obstacles, soil with seeds was protected. Fallen and broken trees lay all in the same direct away from the blast. In the searing zone or standing dead zone, trees still stood upright, but were charred. 

57 people died. Amongst them were a few geologists, photographers and homeowners. Tragically, Robert Landsburg lay curled up around his camera; he had died, but the film roll was undamaged. 

Molten glacier water

Ten minutes after the lateral blast, an ash column rose from the summit to a height of more than 20 kilometers. For ten hours, Mt St-Helens spewed volcanic material, in the form of ash and debris. Lightning bolts in the ash cloud added to the spectacle. And more landslides, of a mixture of ash, lava and mud shoved off the slopes. Melted glacier water created deadly lahars, a kind of mudflow, that raged through the river valleys. 

The finest ash stayed in the atmosphere and drifted to the east, carried by the winds. In the hours following the eruption, the ashes were reported in increasingly distant areas. In Yellowstone, then in Colorado and Minnesota, and finally the ashes circled the earth. 

The water of the river and part of the lake evaporated in a loud bang and hissing sound, which could be heard as far away as Canada.

Aftermath

The eruption of Mount St-Helens was the most devastating in the recent history of the United States. In addition to the people who died, huge expanses of forest and timber were destroyed, elk and deer were killed, and more than ten million salmon in the rivers surrounding the mountain also died. Roads, bridges and crop land were destroyed, and air traffic was disrupted. Cleaning up the ashes was a huge job that lasted for weeks. 

And Mount St-Helens itself? It looked completely different, lower, and with a large horseshoe-shaped crater. The top had literally been blown away. The volcano remained active and several smaller eruptions have occurred since then. In the meantime, there's a new, small mountain rising from the big crater. 

This story reminds us once more that nature always has the last word.

Source: 

Francis, P., and Oppenheimer, C. (1993) Volcanoes. Oxford University Press. 521 p.