Everything about 1980 Eruption Of Mount St Helens totally explained
The
1980 eruption of Mount St. Helens, a volcano located in
Washington state, in the United States, was a major
volcanic eruption. The eruption was the most significant to occur in the contiguous 48
U.S. states (
VEI = 5, 0.3 cu mi, 1.2 km
3 of material erupted), in terms of power and volume of material released, since the 1915 eruption of
California's
Lassen Peak. 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 mountain that created a huge bulge and a fracture system on
Mount St. Helens' north slope. An earthquake at 8:32 a.m. on
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 into a very hot mix of pulverized
lava and older rock that sped toward
Spirit Lake so fast that it quickly passed the avalanching north face.
A
volcanic ash column rose high into the
atmosphere and deposited ash in 11 U.S. states. At the same time, snow, ice, and several entire
glaciers on the mountain melted, forming a series of large
lahars (volcanic
mudslides) that reached as far as the
Columbia River, nearly fifty miles (eighty kilometers) to the south. Less severe outbursts continued into the next day only to be followed by other large but not as destructive eruptions later in 1980. By the time the ash settled, 57 people (including innkeeper
Harry Truman and geologist
David A. Johnston; a full list is available here:
(External Link
)) and thousands of animals were dead. Hundreds of square miles were reduced to wasteland, over a billion
U.S. dollars in damage had occurred ($2.74 billion in 2007 dollars), and the face of Mount St. Helens was scarred with a huge 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. Several small earthquakes beginning as early as
March 15,
1980, indicated that magma may have been moving below the volcano. Then on
March 18 at 3:45 p.m.
Pacific Standard Time (all times will be in PST), a shallow
Richter magnitude 4.2
earthquake (the initial reading was 4.1), centered below the mountain's north flank, 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 4.5 on the Richter scale. A total of 174 shocks of magnitude 2.6 or greater were recorded during those two days. Shocks of magnitude 3.2 or greater occurred at a slightly increasing rate during April and May with five earthquakes of magnitude 4 or above per day in early April, and 8 per day the week before
May 18. This was followed by more earthquake storms and a series of steam explosions that sent ash 10,000 to 11,000 feet (3,000 to 3,400 m) above their vent.
A second, new crater and a blue flame was observed on
March 29.
By
April 7 the combined crater was 1,700 feet (520 m) long, 1,200 feet (365 m) wide and 500 feet (150 m) deep. A
USGS team determined in the last week of April that a diameter section of St. Helens' north face was displaced out at least 270 feet (82 m). This volume increase presumably corresponded to the volume of magma that pushed into the volcano and deformed its surface. Because the intruded magma remained below ground and wasn't directly visible, it was called a
cryptodome, in contrast to a true
lava dome exposed at the surface.
On
May 7, eruptions similar to those in March and April resumed, and over the next days the bulge approached its maximum size. All activity had been confined to the 350-year-old summit dome and didn't involve any new
magma. A total of about 10,000 earthquakes were recorded prior to the
May 18 event, with most concentrated in a small zone less than 1.6 miles (2.6 km) directly below the bulge. Mounting public pressure then forced officials to allow 50 carloads of property owners to enter the danger zone on
May 17 to gather whatever property they could carry. approximately 7–20 seconds (about 10 seconds seems most reasonable) after the shock.
Montana,
Idaho, and
Northern California. Yet many areas closer to the eruption (
Portland, Oregon, for example) didn't hear the blast. This so-called "quiet zone" extended radially a few tens of miles from the volcano and was created by the complex response of the eruption's
sound waves to differences in temperature and air motion of the atmospheric layers and, to a lesser extent, local
topography. Most of the 57 people known to have died in that day's eruption succumbed to
asphyxiation while several died from burns.
Strong high-
altitude wind carried much of this material east-northeasterly from the volcano at an average speed of about 60 mph (100 km/h). By 9:45 a.m. it had reached
Yakima, Washington, 90 miles (145 km) away, and by 11:45 a.m. it was over
Spokane, Washington. Lahars traveled as fast as 90 mph (145 km/h) while still high on the volcano but progressively slowed to about 3 mph (5 km/h) on the flatter and wider parts of rivers. Ultimately more than 65 million cubic yards (50 million m³) of sediment were dumped along the lower Cowlitz and Columbia Rivers. Fifty-seven people were killed and 200 homes, 47 bridges, 15 miles (24 km) of railways and 185 miles (300 km) of highway were destroyed. U.S. President
Jimmy Carter surveyed the damage and stated it looked more desolate than a
moonscape. A film crew was dropped by helicopter on St. Helens on
May 23 to document the destruction. Their
compasses, however, span in circles and they quickly became lost. A second eruption occurred the next day (see below), but the crew survived and were rescued two days after that.
In all, St. Helens released an amount of
energy equivalent to 27,000
Hiroshima-sized
nuclear weapons and ejected more than 1
cubic mile (4 km³) of material. A quarter of that volume was fresh lava in the form of ash, pumice, and volcanic bombs while the rest was fragmented, older
rock.
Cost
Early estimates of the cost of the eruption ranged from
US$2–3
billion. This event caused the Portland area, previously spared by wind direction, to be thinly coated with ash in the middle of the annual Rose Festival. A
dacite dome then oozed into existence on the crater floor, growing to a height of 200 feet (60 m) and a width of 1,200 feet (370 m) within a week.
Seismic activity and gas emission steadily increased in early August, and on
August 7 at 4:26 p.m., an ash cloud slowly expanded 8 miles (13 km) into the sky.
Small pyroclastic flows were sent through the northern breach and weaker outpouring of ash rose from the crater. This continued until 10:32 p.m. when a second large blast sent ash high into the air.
A second dacite dome filled this vent a few days later.
Two months of repose were ended by an eruption lasting from
October 16 to
October 18. This event obliterated the second dome, sent ash 10 miles (16 km) in the air and created small, red-hot
pyroclastic flows.
A third dome began to form within 30 minutes after the final explosion on
October 18, and within a few days, it was about 900 feet (270 m) wide and 130 feet (40 m) high. In spite of the dome growth next to it,
a new glacier formed rapidly inside the crater.
All of the post-1980 eruptions were quiet dome-building events, beginning with the
December 27,
1980, to
January 3,
1981, episode. By 1987 the third dome had grown to be more than 3,000 feet (900 m) wide and 800 feet (240 m) high.
At that rate, assuming additional destructive eruptions don't occur, St. Helens' summit would be restored to its previous height sometime in the mid to late 22nd century.
Further eruptions occurred over a few months during 1989–1991, and the mountain became active again in late 2004.
Summary table
Eruption Summary
May 18, 1980 Eruption of Mount St. Helens
|
| Volcano |
Elevation of Summit:
| Before eruption:
After eruption:
Total removed:
|
Crater dimensions:
| East-West:
North-South:
Depth:
|
Crater floor elevation:
|
|
|
|
| Eruption |
Date:
| May 18, 1980
|
Time of initial blast:
| 8:32 a.m. Pacific Daylight Time (UTC-7)
|
Eruption trigger:
| A magnitude 5.1 earthquake about beneath the volcano
|
|
|
Landslide /
Debris Avalanche |
Area covered:
|
|
Volume:
(uncompacted deposits) | 0.67 mi³ (2.8 km³)
|
Depth of deposit:
| Buried North Fork Toutle River to average depth of with a maximum depth of
|
Velocity:
| to
|
|
|
| Lateral Blast |
Area covered:
| ; reached northwest of the crater
|
Volume of deposit:
(uncompacted deposits) | 0.046 mi³ (0.19 km³)
|
Depth of deposit:
| From about at volcano to less than at blast edge
|
Velocity:
| At least
|
Temperature:
| As high as
|
Energy release:
| 24 megatons thermal energy (7 by blast, rest through release of heat)
|
Trees blown down:
| 4 billion board feet (9.4 million m³) of timber (enough to build about 300,000 two-bedroom homes)
|
Human fatalities:
| 57
|
|
|
| Lahars |
Velocity:
| About to and over on steep flanks of volcano
|
Damaged:
| 27 bridges, nearly 200 homes. Blast and lahars destroyed more than of highways and roads and of railways.
|
Effects on Cowlitz River:
| Reduced carrying capacity at flood stage at Castle Rock from 76,000 ft³/s (2,150 m³/s) to less than 15,000 ft³/s (225 m³/s)
|
Effects on Columbia River:
| Reduced channel depth from to ; stranded 31 ships in upstream ports
|
|
|
Eruption Column
And Cloud |
Height:
| Reached about in less than 15 minutes
|
Downwind extent:
| Spread across U.S. in 3 days; circled Earth in 15 days
|
Volume of ash:
(based on uncompacted deposits | 0.26 mi³ (1 km³)
|
Ash fall area:
| Detectable amounts of ash covered
|
Ash fall depth:
| at downwind (ash and pumice)
at downwind
at downwind
|
|
|
| Pyroclastic Flows |
Area covered:
| ; reached as far as north of crater
|
Volume and depth:
(volume based on uncompacted deposits) | 0.029 mi³ (0.12 km³); multiple flows to thick; cumulative depth of deposits reached in places
|
Velocity:
| Estimated at to
|
Temperature:
| At least
|
|
|
| Other |
Wildlife:
| The Washington State Department of Game estimated nearly 7,000 big game animals (deer, elk, and bear) perished as well as all birds and most small mammals. Many burrowing rodents, frogs, salamanders, and crawfish, managed to survive because they were below ground level or water surface when the disaster struck.
|
Fisheries:
| The Washington Department of Fisheries estimated that 12 million Chinook and Coho salmon fingerlings were killed when hatcheries were destroyed. Another estimated 40,000 young salmon were lost when forced to swim through turbine blades of hydroelectric generators as reservoir levels along the Lewis River were kept low to accommodate possible mudflows and flooding.
|
| Brantley and Myers, 1997, Mount St. Helens -- From the 1980 Eruption to 1996: USGS Fact Sheet 070–97, accessed 2007-06-05; and Tilling, Topinka, and Swanson, 1990, Eruption of Mount St. Helens - Past, Present, and Future: USGS General Interest Publication, accessed 2007-06-05.
|
| Table compiled by Lyn Topinka, USGS/CVO, 1997
|
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