ERUPTION OF Mt. St. HELENS

Map of Mt. St. Helens

Overview of Mt. St. Helens Mt. St. Helens is a composite volcano within the Cascade Range in what is now southern Washington, approximately 50 miles northwest of Portland, Oregon. Mt. St. Helens is approximately 40,000-years old, it is considered a relatively young, active volcano. Mt. St. Helens historically has had four extended periods of volcanic activity (each lasting hundreds of years), interspersed with dormant periods (often lasting thousands of years). The volcano is currently in one of its active periods. It was named "Louwala-Clough,” meaning “smoking mountain" in Native American. [1]

Mt. St. Helens erupted somewhere between 1600 and 1700, again in 1800, and then quite frequently during the 26-year span of 1831 to After 1857, the volcano grew quiet. People who viewed the 9,677-foot tall mountain saw a picturesque backdrop rather than a potentially deadly volcano. Thus, not fearing an eruption, many people built houses around the base of the volcano. On March 20, 1980, a 4.1 magnitude earthquake struck underneath Mt. St. Helens. This was the first warning sign that the volcano had reawakened. On March 27, a small explosion blew a 250-foot hole in the mountain and released a plume of ash. This caused fears of injuries from rockslides so the entire area was evacuated. On May 18, 1980, a 5.1 magnitude earthquake struck under Mt. St. Helens. Within seconds, the bulge and surrounding area fell away in a gigantic, rock avalanche. The avalanche created a gap in the mountain, allowing the release of pent-up pressure that erupted laterally in a huge blast of pumice and ash. [1]

The avalanche, huge to begin with, quickly grew in size as it crashed down the mountain, travelling around 70 to 150 miles per hour and destroying everything in its path. The blast of pumice and ash travelled northward at 300 miles per hour and was a raging hot 660° F (350° C). The blast killed everything in a 200-square mile area. The eruption lasted nine hours. Fifty-seven people were killed. It is estimated that about 7,000 large animals such as deer, elk, and bears were killed and thousands of small animals died from the volcanic eruption. Mt. St. Helens had been surrounded by a lush forest of coniferous trees and numerous clear lakes before the blast. The eruption felled entire forests, leaving only burned tree trunks.

Eruption of Mt. St. Helen

View looking north of Mount St. Helens on the afternoon of 18 May U.S. Geological Survey

View into the crater of Mount St. Helens on 12 September 2006 showing the 1980– 1986 lava dome, the complex dome that began growing in mid- October 2004, and the greatly deformed arms of Crater Glacier

Economic Impact of the May 18, 1980 Eruption Mount St. Helens' eruption in a matter of hours caused loss of lives and widespread destruction of valuable property, primarily by the debris avalanche, the lateral blast, and the mudflows. Early estimates were too high and ranged from $2 to $3 billion, primarily reflecting the timber, civil works, and agricultural losses. Unemployment in the immediate region of Mount St. Helens rose tenfold in the weeks immediately following the eruption, then nearly returned to normal once timber salvaging and ash-cleanup operations were underway. Only a small percentage of residents left the region because of lost jobs owing to the eruption.

The May 18 eruption nearly dealt a crippling blow to tourism, an important industry in Washington. Not only was tourism down in the Mount St. Helens-Gifford Pinchot National Forest area, but conventions, meetings, and social gatherings also were cancelled or postponed to areas not affected by the eruption. T The largest loss, reported to be $450 million, was standing timber in areas affected by the lateral blast and mudflows. Losses in agricultural output, which apparently are less than officials had expected, may range from $40 to $100 million. [2]

Eruption's impact on wildlife Killed in the blast were 7,000 deer, elk and bear; all birds and most small mammals; 12 million chinook and coho salmon fingerlings in hatcheries and an estimated 40,000 young salmon in the wild. The Toutle River got so hot that witnesses reported seeing fish jump out of the water to escape the heat. More than five tons of insects are blown onto the mountain every summer to help build up the nutrient-poor "tephra," volcanic ash and dust. [3][4]

Impact of the Mount St. Helens Eruptions on Agriculture Ash from Mount St. Helens has fallen over a diverse agricultural area, with deposits of up to 30 kilograms per square meter. Crop losses in eastern Washington are estimated at about $100 million in 1980— about 7 percent of the normal crop value in the affected area and less than was expected initially. The ash as an abrasive is lethal to certain insects, such as bees and grasshoppers, but populations are recovering. The ash has increased crop production costs by necessitating machinery repairs and increased tillage. On soil, the ash reduces water infiltration, increases surface albedo, and may continue to affect water runoff, erosion, evaporation, and soil temperature even when tilled into the soil. [5]

Ash on plant leaves reduced photosynthesis by up to 90 percent.

References [1] [1 Foxworthy and Hill, 1982, Volcanic Eruptions of 1980 at Mount St. Helens: The First 100 Days: U.S.Geological Survey Professional Paper 1249 [2] Blong, R.J., 1984, Volcanic hazards: A source book on the effects of eruptions: Academic Press, Orlando, Florida, 424 p.[3] Del Moral, R., 1981, Life returns to Mount St. Helens, Natural History, v. 90, no. 5, p [4] Geological Society of America Bulletin 1 July 2006: [5]