Mount St. Helens

Mount St. Helens

On May 18, 1980, after lying dormant 123 years Mount St. Helens erupted powerfully and had a profound impact on the Pacific Northwest. On that summer day in 1980 Mount St. Helens produced a huge debris avalanche, an explosive lateral blast, lahars and an eruption column. In an instant the countryside and lakes surrounding a great distance around became victims of devastation.

Located in the state of Washington, St. Helens is considered to be the youngest and most active volcano in the Cascade Range. Although only 12th in height among the major Cascade volcanoes, scientists predict a significant increase in size in the future because the mountain is still in its cone building stage. Mount St. Helens stands atop an older volcano which historically, was probably one of the most explosive peaks in the Pacific Northwest. This assumption is evident through the amount of debris that is scattered for miles over the countryside. The oldest recognized products of the ancestral cone are a pumice layer which is dated at 37,600 years and a weathered mudflow deposit dated at approximately 36,000 years (Harris 1980). Evidence of glacial sediments containing fragments of the earlier mountain are dated at approximately 18,000 years and indicate that Mount St. Helens experienced at least one episode of glaciation. Present day lava content of St. Helens is composed of olivine basalt, dacite, and pyroxene andesite. The ancestral St. Helens, although almost buried beneath the recent core, has left behind much evidence of its previous eruptions. Pumiceous ash blankets the land for hundreds of miles and many valleys are filled with accumulations of explosive rubble. Modern day Mount St. Helens is a result of avalanche debris, explosion rubble and mudflow deposits erupted by its predecessor.

Modern day St. Helens also grew “through the extrusion of at least two large dacite domes and the production of glowing avalanches similar to those erupted by the ancestral volcano” (Harris 1980:171). The famous Spirit Lake surrounding Mount St. Helens is also a result of violent activity. The lake formed when mudflows form the volcano blocked the headwaters of a fork of the Toutle River and the lake was raised over 60feet by mudflows.

Prior to 1980, after over one hundred years of lying dormant, St. Helens had the opportunity to recuperate from its previous eruptions and develop its beautiful shape, rising approximately 3000 meters above sea level. (Sanders 2002) The landscape surrounding the mountain consisted of dense, temperate and coniferous rain forests.

Large areas had been partially modified by timber harvest activity and sparse alpine vegetation occurred at high elevations. Lakes and streams were also common to the area. However, by 1949 endless miles of roads covered the previously solitude mountain. By 1975 there were few roadless areas for hikers and wildlife and most of the trails had been converted to logging road (Williams 1988). According to the U.S Forest Service, timber harvesting had been “quite extensive, primarily on the south and east sides” (Williams 1988:32). The timber industry claimed that clearcutting was an economic necessity essential for providing timber and jobs. Clearcutting moved up Mount. St. Helen streams and efforts to replant the timberline clearcuts failed and once the vegetation was destroyed, the pumice became a desert, and few plants were able to grow. Only the north side of the mountain prior to the 1980 eruption remained in a semi-natural state, although fisheries were damaged by dams and erosion from logging (Williams 1988).

The Cascade Mountains, St. Helen’s being one of them, represent a volcanic arc that is created where the Juan the Fuca tectonic plate is moving eastward and subsiding beneath North America. The plate originated as magma, through rifts in the Juan de Fuca Ridge. The Juan de Fuca Ridge is the remaining northern segment of a larger oceanic feature called the Farallon Ridge. Most of this ridge has been over ridden by the westward drifting North American Plate. The rock that makes up the Juan de Fuca plate is heated to a degree that hot fluids are driven out. These hot fluids, which are mainly composed of water, then penetrate into the overlying wedge of mantle rock and cause chemical interactions that lower melting temperatures in the wedge. This causes blobs of magma to rise through the fractures in the crust and accumulate in chambers beneath the volcanic peaks of the Cascade Range and results in eruptions (Sanders et al. 2002).

Studies by members of the U.S Geological Survey were conducted and the results were published in 1978 by authors Crandell and Mullineaux. Based on the eruptive history and frequency of St. Helens eruptions the authors warned of the likelihood of future eruptions.

In the future, Mount St. Helens probably will erupt violently and intermittently just as it has in the recent geologic past, and these future eruptions will affect human life and healthy, property, agriculture and general economic welfare over a broad area…an eruption is… likely to occur within the next hundred years, and perhaps even before the end of the century” (Sanders et al. 2002:232)

These warnings however, did not reach a wide audience and were not taken very seriously by a majority of the public.

The issue was clouded by misunderstandings by the media, government officials, and even geologists. Two years after the publication by Crandell and Mullineaux Mount St. Helens erupted.

The first sign of activity at Mount St. Helens began On March 16, 1980. A series of small earthquakes were detected and on March 27, after hundreds of earthquakes had been detected, steam explosions blasted a crater through the volcano’s summit ice cap. The crater was estimated to have grown about 1,300 feet in diameter within a week (Brantley et al. nd). Two enormous crack systems had also crossed the entire summit area. 10,000 earthquakes had shaken the volcano by May 17, and the north flank had grown outward to form a noticeable bulge. This indicated to geologists that magma had risen high into the volcano. On the morning of May 18, 1980 and earthquake of magnitude 5.1, shook the volcano and within 20 seconds, the volcano’s bulge and summit slid away in the largest landslide in recorded history. The landslide was about 2.5km^3 (Pringle 1990). Powerful explosions were triggered because the landslide depressurized the volcano’s magma system. “Rocks, ash, volcanic ash and steam were blasted upward and outward to the north. This lateral blast of hot material accelerated to at least 300 miles per hour, the slowed as the rocks and ash fell to the ground and spread away from the volcano.” (Brantley et al. nd) The blast cloud is reported to have traveled as far as 17 miles northward from the volcano. An eruptive column was a result of the blast and rose more than 15 miles in about 15 minutes. The destruction continued as magma erupted from the new crater and a second eruption column was formed.

Just after 12:00pm pyroclastic flows consisting of ash, pumice and gas poured out of the crater, reaching speeds as fast as 80 miles per hour (Brantley et al. nd). The scorching ash quickly mixed with existing ice and snow and began to flow down all sides of the mountain. Debris was picked up on the way and included boulders reaching sizes of up to 20 feet in diameter (Volcano World nd). Strong winds then proceeded to blow the 520 million tons of ash eastward across the United States. 250 miles from the volcano, the city of Spokane, was enclosed in darkness (Brantley et al. nd).

Effects of the 1980 eruption were disastrous. There were 57 human fatalities, mostly due to suffocation from ash inhalation, and thousands of animals were killed including, 7000 big game animals and 12 million salmon. Hundreds of houses in the valley were destroyed, bridges were wrecked and vehicles were demolished. Deposits of mud and debris in the upper reaches of the North Fork Toutle Rive valley were so great that its floor was raised as much as 180 meters and debris dams formed lakes where none had existed before. A flood of water filled with mud, poured into the Cowlitz River and enormous amounts of sediment were dumped into the Columbia River and clogged the shipping channel. “Upstream in the Columbia, more than twenty oceangoing ships were trapped at Portland and Vancouver until the U.S. Army Corps of Engineers deepened the channel with dredges working around the clock” (Sanders et al. 2002:239) Ash clouds were seen drifting across three states, Washington, Idaho and Montana, and the volumes of rock, ice, mud, ash and debris were estimated in the billions of cubic yards.

Ash fall of up to 8cm in Washington and Idaho caused harmful problems clogged air filters that disabled vehicles and short-circuited electrical transformers, resulting in power outages. Breathing was so difficult in some areas that many had to wear masks. Needless to say much of the wildlife in the surrounding area was destroyed. Few trees and plants survived the eruption. “The volcano blew out a large portion of the mountainside and baked several hundred square kilometers of adjacent countryside to 400600 oC. The result was a lifeless moonscape punctuated by the ash-gray, leafless trunks of thousands of blown-down trees” (Life erupts at Mount St. Helens 1997:400)

Following the eruption, thousands of people were evacuated, some by helicopter. Emergency responses that included salvaging lives and property were the first priority. Work crews worked for weeks removing tons of ash from roofs, streets, sidewalks and parking lots. The U.S Army Crops of Engineers worked around the clock, deepening the channel of the Columbia River so that ships could be freed. People sought immediate need for information regarding volcanic events and hazards. As a result an Emergency Coordination Center (ECC) was established in Washington. The ECC consisted of experienced emergency response personal and was staffed 24 hours a day. Information was also provided through public meetings, press conferences, and briefings with government agencies and private businesses. Updates on the status of the volcano were also provided through “volcanic and seismic activity reports.”

Despite all the effort, the destruction and loss resulting from the volcano caused many to experience emotional problems and psychological problems.

“Some people made a scapegoat of Mount St. Helens, blaming the volcano for personal problems unrelated to the eruption” (Sanders et al. 2002:242-243)

Bibliography:

Zeilinga de Boer, Jelle and Donald Theodore Sanders. 2002. Volcanoes in Human History. New Jersey: Princeton University Press.

Harris, Stephen L. 1980. Fire & Ice The Cascade Volcanoes. Revised Edition. Seattle:

The Mountaineers.

Williams, Chuck. 1988. Mount St. Helens National Volcanic Monument. Seattle: The Mountaineers.

Pringle, Patrick. Mount St Helens: A Ten-Year Summary. Washington Geologic Newsletter. Vol. 18, No. 2, May 1990.

Brantley, Steve and Bobbie Myers. Mount St. Helens-From the 1980 Eruption to 2000.