On May18, 1980, at 8:32a.m., Mount St. Helens in Skamania County, Washington, United States, experienced a catastrophic explosive eruption which had a volcanic explosivity index of5. It was the first to occur in the contiguous United States since the much smaller 1915 eruption of Lassen Peak in California. The main eruption was preceded by a series of volcanic explosions, pyroclastic flows, and phreatic blasts beginning in March 1980. It has often been considered the most disastrous volcanic event in U.S. history.
The eruption was preceded by a series of earthquakes and steam-venting episodes caused by an injection of magma at shallow depth below the volcano that created a large bulge and a fracture system on the mountain's north slope. An earthquake at 8:32:11am PDT (UTC−7) on May 18, 1980, caused the entire weakened north face to slide away, a sector collapse which was the largest subaerial landslide in recorded history. This allowed the partly molten rock, rich in high-pressure gas and steam, to suddenly explode northward toward Spirit Lake in a hot mix of lava and pulverized rock, overtaking the landslide. An eruption column rose into the atmosphere and deposited ash in 11 U.S. states and various Canadian provinces. At the same time, snow, ice, and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly to the southwest. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. The thermal energy released during the eruption was equal to 26 megatons of TNT.
About 57 people were killed, including innkeeper and World War I veteran Harry R. Truman, photographers Reid Blackburn and Robert Landsburg, and volcanologist David A. Johnston. Hundreds of square miles were reduced to wasteland, causing over $1billion in damage (equivalent to $ billion in ). Thousands of animals were killed, and Mount St. Helens was left with a crater on its north side. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but the United States Forest Service acquired the land from the railroad after the eruption. (adapted public domain text). The area was later preserved in the Mount St. Helens National Volcanic Monument and the State of Washington recognized the month of May as "Volcano Awareness Month". Events are held at Mt. St. Helens and within the region to discuss the eruption and safety concerns, and to commemorate lives lost during the disaster.
Build-up to the eruption Mount St. Helens remained dormant from its last period of activity in the 1840s and 1850s until March 1980. Several small earthquakes, beginning on March 15, indicated that magma might have begun moving below the volcano. On March 20, at 3:45pm Pacific Standard Time, a shallow 4.2-magnitudeearthquake, centered below the volcano's north flank, signaled the volcano's return from 123years of hibernation. A gradually building earthquake swarm saturated area seismographs and started to climax at about noon on March 25, reaching peak levels in the next two days, including an earthquake registering 5.1 on the Richter scale among a total of 174 shocks of magnitude2.6 or greater.
Shocks of magnitude 3.2 or greater occurred at a slightly increasing rate during April and May, with five earthquakes of magnitude4 or above per day in early April, and eight per day the week before May 18. Initially, no direct sign of eruption was seen, but small earthquake-induced avalanches of snow and ice were reported from aerial observations.
At 12:36 pm on March 27, phreatic eruptions (explosions of steam caused by magma suddenly heating groundwater) ejected and smashed rock from within the old summit crater, excavating a new crater and sending an ash column about into the air. By this date, a eastward-trending fracture system had also developed across the summit area. This was followed by more earthquake swarms and a series of steam explosions that sent ash above their vent. Most of this ash fell between from its vent, but some was carried south to Bend, Oregon, or east to Spokane, Washington.
A second, new crater and a blue flame were observed on March 29. The flame was visibly emitted from both craters and was probably created by burning gases. Static electricity generated from ash clouds rolling down the volcano sent out lightning bolts that were up to long. Ninety-three separate outbursts were reported on March 30, and increasingly strong harmonic tremors were first detected on April 1, alarming geologists and prompting Governor Dixy Lee Ray to declare a state of emergency on April 3. Governor Ray issued an executive order on April 30 creating a "red zone" around the volcano; anyone caught in this zone without a pass faced a $500 fine () or six months in jail. This precluded many cabin owners from visiting their property.
By April 7 the combined crater was and deep. A USGS (United States Geological Survey) team determined in the last week of April that a section of St. Helens' north face was displaced outward by at least . For the rest of April and early May, this bulge grew by per day, and by mid-May, it extended more than north. As the bulge moved northward, the summit area behind it progressively sank, forming a complex, down-dropped block called a graben. Geologists announced on April 30 that sliding of the bulge area was the greatest immediate danger and that such a landslide might spark an eruption. These changes in the volcano's shape were related to the overall deformation that increased the volume of the volcano by by mid-May. This volume increase presumably corresponded to the volume of magma that pushed into the volcano and deformed its surface. Because the intruding magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true lava dome exposed at the surface.
Volcanologist Barry Voight was contacted by Rocky Crandell,Thompson, p. 76. a USGS employee working in the Vancouver office near the mountain.Thompson, p. 31. Crandell sought Voight's expertise in landslides, hoping Voight would opine on the growing bulge.Harris, p. 204.Olson, p. 71. In his reporting to Crandell and his associates, Voight, who observed the mountain April 11–19, said that the bulge could fail and collapse the volcano's entire north sector. He suggested they begin monitoring the rate of movement of the bulge, worried that the collapse could trigger an eruption. He also advised hiring a local surveyor to take measurements, offending several of the geologists.Thompson, p. 77. Shortly after, Voight left the mountain and returned to teaching classes at Pennsylvania State University. On May 1, he sent his full report to the USGS, summarizing his predictions, predicting the failure of the bulge and the collapse of the mountain's north side followed by a violent eruption that would require an extensive evacuation zone.Thompson, p. 97.Olson, p. 72.
On May 7, eruptions similar to those in March and April resumed, and over the following days, the bulge approached its maximum size. All activity had been confined to the 350-year-old summit dome and did not involve any new magma. About 10,000 earthquakes were recorded before the May 18 event, with most concentrated in a small zone less than directly below the bulge. Visible eruptions ceased on May 16, reducing public interest and consequently the number of spectators in the area. Mounting public pressure then forced officials to allow 50 carloads of property owners to enter the danger zone on Saturday, May 17, to gather whatever property they could carry. Another trip was scheduled for 10am the next day, a Sunday, when more than three hundred loggers who would normally be working in the area were not present.
By the time of the climactic eruption, dacite magma intruding into the volcano had forced the north flank outward nearly and heated the volcano's groundwater system, causing many steam-driven explosions (phreatic eruptions).
Landslide and climactic phase
At dawn May 18, Mount St. Helens' activity did not show any change from the pattern of the preceding month. The rates of bulge movement and sulfur dioxide emission, and ground temperature readings did not reveal any changes indicating a catastrophic eruption. USGS volcanologist David A. Johnston was on duty at an observation post around north of the volcano. At 6:00am Johnston's measurements did not indicate any unusual activity.
At 8:32 am, a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide, approximately 7–20seconds after the shock, followed a few seconds later by the main volcanic blast. The sector collapse, the largest subaerial landslide in recorded history, traveled at and moved across Spirit Lake's west arm. Part of it hit a ridge about north. Some of the slide spilled over the ridge, but most of it moved down the North Fork Toutle River, filling its valley up to deep with avalanche debris. An area of about was covered, and the total volume of the deposit was about .
Scientists were able to reconstruct the motion of the landslide from a series of rapid photographs by Gary Rosenquist, who was camping away from the blast . Rosenquist, his party, and his photographs survived because the blast was deflected by local topography short of his location.
Most of St. Helens' former north side became a rubble deposit long, averaging thick; the slide was thickest at below Spirit Lake and thinnest at its western margin. The landslide temporarily displaced the waters of Spirit Lake to the ridge north of the lake, in a giant wave about high. This, in turn, created a avalanche of debris consisting of the returning waters and thousands of uprooted trees and stumps. Some of these remained intact with roots, but most had been sheared off at the stump seconds earlier by the blast of superheated volcanic gas and ash that had immediately followed and overtaken the initial landslide. The debris was transported along with the water as it returned to its basin, raising the surface level of Spirit Lake by about .
Four decades after the eruption, floating log mats persist on Spirit Lake and nearby St. Helens Lake, changing position with the wind. The rest of the trees, especially those that were not completely detached from their roots, were turned upright by their own weight and became waterlogged, sinking into the muddy sediments at the bottom where they are in the process of becoming petrified in the anaerobic and mineral-rich waters. This provides insight into other sites with a similar fossil record.
Pyroclastic flows Initial lateral blast
The landslide exposed the dacite magma in Mount St. Helens' neck to much lower pressure, causing the gas-charged, partially molten rock and high-pressure steam above it to explode a few seconds after the landslide started. Explosions burst through the trailing part of the landslide, blasting rock debris northward. The resulting blast directed the pyroclastic flow laterally. It consisted of very hot volcanic gases, ash, and pumice formed from new lava, as well as pulverized old rock, which hugged the ground. Initially moving about , the blast quickly accelerated to around , and it may have briefly passed the speed of sound.
Pyroclastic flow material passed over the moving avalanche and spread outward, devastating a fan-shaped area 23 miles across by 19 miles long (37kmby 31km). In total, about of forest were knocked down, and extreme heat killed trees miles beyond the blow-down zone. At its vent, the lateral blast probably did not last longer than about thirty seconds, but the northward-radiating and expanding blast cloud continued for about another minute.