In 1980, a major volcanic eruption occurred at Mount St. Helens, a volcano located in state of Washington, in the United States. The eruption (which was a VEI 5 event) was the only significant one to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California.[1] The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a huge bulge and a fracture system on the mountain's north slope.
Prior to the eruption, USGS
scientists convinced local authorities to close Mount St. Helens to the
general public and to maintain the closure in spite of local pressure
to re-open it; their work saved thousands of lives. An earthquake at
8:32:17 a.m. PDT (UTC−7) on Sunday, May 18, 1980, caused the entire weakened north face to slide away, suddenly exposing the partly molten, gas- and steam-rich rock in the volcano to lower pressure. The rock responded by exploding a hot mix of lava and pulverized older rock toward Spirit Lake so fast that it overtook the avalanching north face.
An eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U.S. states.[2] 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 50 miles (80 km) to the southwest. Less-severe outbursts
continued into the next day, only to be followed by other large, but not
as destructive, eruptions later in 1980.
Fifty-seven people were killed, including innkeeper Harry R. Truman, photographer Reid Blackburn and geologist David A. Johnston.[3] Hundreds of square miles were reduced to wasteland causing over a billion U.S. dollars in damage ($2.88 billion in 2014 dollars[4]),
thousands of game 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 afterward the land passed to the United States Forest Service.[5] The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument.
Everyone in the quiet zone could see the huge ash cloud that was sent
skyward from St. Helens' northern foot. The near-supersonic lateral
blast, loaded with volcanic debris, caused devastation as far as 19
miles (31 km) from the volcano. The area affected by the blast can be
subdivided into three roughly concentric zones:[5]
- Direct blast zone, the innermost zone, averaged about 8 miles (13 km) in radius, an area in which virtually everything, natural or artificial, was obliterated or carried away.[5] For this reason, this zone also has been called the "tree-removal zone." The flow of the material carried by the blast was not deflected by topographic features in this zone. The blast released energy equal to 24 Megatons of TNT.
- Channelized blast zone, an intermediate zone, extended out to distances as far as 19 miles (31 km) from the volcano, an area in which the flow flattened everything in its path and was channeled to some extent by topography.[5] In this zone, the force and direction of the blast are strikingly demonstrated by the parallel alignment of toppled large trees, broken off at the base of the trunk as if they were blades of grass mown by a scythe. This zone was also known as the "tree-down zone."
- Seared zone, also called the "standing dead" zone, the outermost fringe of the impacted area, a zone in which trees remained standing but were singed brown by the hot gases of the blast.[5]
By the time this pyroclastic flow hit its first human victims, it was
still as hot as 360 °C (680 °F) and filled with suffocating gas and
flying angular material.[23] Most of the 57 people known to have died in that day's eruption succumbed to asphyxiation while several died from burns.[5] Lodge owner Harry R. Truman was buried under hundreds of feet of avalanche material. Volcanologist David A. Johnston was one of those killed, as was Reid Blackburn, a National Geographic photographer.
Direct results
The May 18, 1980, event was the most deadly and economically destructive volcanic eruption in the history of the United States.[5]
Fifty-seven people were killed and 200 houses, 27 bridges, 15 miles
(24 km) of railways and 185 miles (298 km) of highway were destroyed.
U.S. President Jimmy Carter surveyed the damage and said it looked more desolate than a moonscape.[26][27] A film crew was dropped by helicopter on St. Helens on May 23 to document the destruction. Their compasses, however, spun in circles and they quickly became lost.[28]
A second eruption occurred the next day (see below), but the crew
survived and were rescued two days after that. The eruption ejected more
than 1 cubic mile (4.2 km3) of material.[29] A quarter of that volume was fresh lava in the form of ash, pumice and volcanic bombs while the rest was fragmented, older rock.[29]
The removal of the north side of the mountain (13% of the cone's
volume) reduced St. Helens' height by about 1,280 feet (390 m) and left a
crater 1 to 2 miles (2 to 3 km) wide and 2,100 feet (640 m) deep with
its north end open in a huge breach.[29]
More than 4,000,000,000 board feet (9,400,000 m3) of timber was damaged or destroyed, mainly by the lateral blast.[5]
At least 25% of the destroyed timber was salvaged after September 1980.
Downwind of the volcano, in areas of thick ash accumulation, many
agricultural crops, such as wheat, apples, potatoes and alfalfa, were destroyed. As many as 1,500 elk and 5,000 deer were killed, and an estimated 12 million[5] Chinook and Coho salmon fingerlings died when their hatcheries were destroyed. Another estimated 40,000 young salmon were lost when they swam through turbine blades of hydroelectric generators when reservoir levels were lowered along the Lewis River to accommodate possible mudflows and flood waters.[5]
In all, Mount St. Helens released 24 megatons of thermal energy, 7 of
which was a direct result of the blast. This is equivalent to 1,600
times the size of the atomic bomb dropped on Hiroshima.[30]
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