Mount St Helens – Before the eruption
Mount St Helens explosion stated with a landslide
A giant landslide at Mount St. Helens in 1980 had an effect like pulling a cork off a bottle of severely shaken soda.
Once the side of the mountain was removed, the volcanic gases exploded out of the side of the volcano producing a lateral blast. This blast, traveling at speeds of up to 1,072 km/hr (670 mi/hr), quickly overtook the landslide and extended to up to 30.4 km (19 mi) from the volcano.
The eruption involved a complex series of events that unfolded over the next 12 hours, with many events going on simultaneously. These volcanic events buried some areas in debris avalanches and mudflows, scoured other areas with hot gases, blew down or scorched forests on slopes several miles away, and dusted forests farther away with volcanic ash.
In the areas closest to the volcano and up to about 13 km (8 mi) away the blast destroyed everything — trees, houses, wildlife — and the area was left barren as the moon.
Beginning about noon and lasting for several hours, super-hot (at least 1,300 degrees F), fast-moving, pumice-rich pyroclastic flows poured from the crater and covered six square miles north of the volcano with pumice many feet deep.
This sterile, desolate terrain was later called the Pumice Plain. Heat from the eruption melted snow and glaciers on the volcano’s slopes. The meltwater picked up soil, rocks, and logs, forming mudflows that traveled for tens of miles down river channels.
In the area between approximately 13 and 21 km from the volcano, the blast toppled trees and left them lying in neat rows like toothpicks. Still further from the volcano, the trees remained standing, but were singed brown by the hot gases of the blast leaving forests of “skeleton” trees with needles stripped.
The towering column of ash rose for more than nine hours, and reached a height of about 80,000 feet. Wind carried ash mostly to the northeast, where it darkened skies and covered the ground with gray, volcanic ash. Some ash remained aloft, and this part of the plume circled the Earth in 15 days.
Most of the 57 people killed in the eruption, including volcanologist David Johnston, were killed by asphyxiation from the lateral blast as the hot gases scorched their lungs.
The blast and the landslide removed the upper 396 m (1,306.8 feet) of the volcano. The blast devastated 596 square kilometers (229 square miles) and destroyed timber valued at several millions of dollars — an estimated 4 billion board feet of timber (enough to build about 300,000 two-bedroom homes).
It was the deadliest and most economically destructive volcanic event in the history of the United States.
THE VOLCANO AWAKENS: March 16 – May 17, 1980
The first sign of activity at Mount St. Helens in the spring of 1980 was a series of small earthquakes that began on March 16.
After hundreds of additional earthquakes, steam explosions on March 27 blasted a crater through the volcano’s summit ice cap. Within a week the crater had grown to about 1,300 feet in diameter and two giant crack systems crossed the entire summit area.
By May 17, more than 10,000 earthquakes had shaken the volcano and the north flank had grown outward at least 450 feet to form a noticeable bulge. Such dramatic deformation of the volcano was strong evidence that molten rock (magma) had risen high into the volcano.
CATACLYSMIC ERUPTION: May 18, 1980
Within 15 to 20 seconds of a magnitude 5.1 earthquake at 8:32 a.m., the volcano’s bulge and summit slid away in a huge landslide — the largest on Earth in recorded history.
The landslide depressurized the volcano’s magma system, triggering powerful explosions that ripped through the sliding debris. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north.
This lateral blast of hot material accelerated to at least 300 miles per hour, then slowed as the rocks and ash fell to the ground and spread away from the volcano; several people escaping the blast on its western edge were able to keep ahead of the advancing cloud by driving 65 to 100 miles an hour.
The blast cloud traveled as far as 17 miles northward from the volcano and the landslide traveled about 14 miles west down the North Fork Toutle River.
The lateral blast produced a column of ash and gas (eruption column) that rose more than 15 miles into the atmosphere in only 15 minutes. Less than an hour later, a second eruption column formed as magma erupted explosively from the new crater.
Then, beginning just after noon, swift avalanches of hot ash, pumice, and gas (pyroclastic flows) poured out of the crater at 50 to 80 miles per hour and spread as far as 5 miles to the north. Based on the eruption rate of these pyroclastic flows, scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m.
Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano.
During the first few minutes of this eruption, parts of the blast cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano.
The hot rocks and gas quickly melted some of the snow and ice capping the volcano, creating surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges.
|Elevation of summit||9,677 feet before; 8,363 feet after; 1,314 feet removed|
|Volume removed*||0.67 cubic miles (3.7 billion cubic yards)|
|Crater dimensions||1.2 miles (east-west); 1.8 miles (north-south); 2,084 feet deep|
|Crater floor elevation||6,279 feet|
|Eruption Column and Cloud|
|Height||Reached about 80,000 feet in less than 15 minutes|
|Downwind extent||Spread across US in 3 days; circled Earth in 15 days|
|Volume of ash*||0.26 cubic miles (1.4 billion cubic yards)|
|Ash fall area||Detectable amounts of ash covered 22,000 square miles|
|Ash fall depth||10 inches at 10 miles downwind (ash and pumice); 1 inch at 60 miles downwind; 1/2 inch at 300 miles downwind|
News report: St Helens blows stack sky-high
Walla Walla Union-Bulletin (Walla Walla, Washington) May 19, 1980
Copter locates nine survivors – The plume from Mount St Helens reached more than nine miles into the air
Nine persons, apparent survivors of Mount St Helens’ thunderous eruption, were spotted by rescue helicopters today near the mountain.
Five of the persons were found about eight miles from the volcano, which continues to spew huge amounts of ash and steam.
Four others were found four to five miles from the mountain, near Fawn Lake. They were described as two adults, a child and an infant. Helicopters could not land to pick up the nine because of poor visibility caused by volcanic ash. A 0130 plane was circling the area.
Other survivors were seen 20 miles from the volcano, searchers reported, and helicopters were sent to them.
“It’s really hard to say what they’ve undergone,” said FAA spokesman Marv Norman in Olympia, Wash. “It’s hard to say what heat they endured.”
In the Toutle River 45 miles below the mountain, hot mud and rock and ash reportedly heated the waters to 100 degrees, killing all fish. Red-hot sulfur gas, rocks and mud raced down the mountainside in Sunday’s eruption, incinerating everything in the way.
At least five persons were killed, caught in the gases and mudslides or by flooding that followed. There was an unconfirmed report that two other persons were killed by trees blown down by the hot blast.
About 1,200 feet was ripped off the top of the mountain by the pulverizing eruption. A volcanic crater at the summit was a mile and a half wide. Twenty-one persons have been listed as missing. Thousands were forced to flee from a mile-wide wall of steaming mud.
Among those missing were Harry Truman, 84-year-old patriarch of the mountain, a U.S. Geological Survey geologist and Reid Blackburn, a newspaper photographer for The Columbian in Vancouver.
Truman’s home at Spirit Lake Lodge was obliterated. The lake, a popular recreation area at the base of the volcano’s north side, was turned into a bubbling mud cauldron.
Day was turned into night in many areas Sunday when sky-borne volcanic ash blotted out the sun and created an eerie “midnight at noon.”
Many communities across eastern Washington and western Montana were virtually at a standstill, buried in ash up to 7 inches deep following Sunday’s convulsion.
Many areas of Washington, Idaho, Montana, Wyoming, Colorado and North Dakota were sprinkled with ash, ranging from a slight dusting to grit ankle deep.
A half-inch of ash layered the ground like black snow at Spokane, 250 miles from the volcano, where police were urging residents to stay indoors.
A meteorologist for the National Weather Service in Boise, Idaho, said the fallout could reach as far as New England in two or three days. “Our forecast calls for the ash to track across the Dakotas, Nebraska, northern Colorado, Kansas and Missouri, then swing back through to the northeast through the Midwest and on to New England,” said meteorologist Carl Keith.
Hundreds of schools in Washington and Montana closed today because of danger in driving in the slippery ash, which many motorists found frustrated even tire chains. Washington State University at Pullman and the University of Idaho in Moscow also closed.
A plume of steam and ash was still billowing 14,000 feet high from a crater a half-mile wide today, but there were no sightings of the rivers of mud, rock and gas which roared down the flanks of the peak earlier. There were no sightings of lava flows during the eruption.
Both the Cowlitz and the Toutle rivers were dropping after being swollen Sunday by the mudflows.
“It’s still perking, but it is not as violent,” said Sam Frear, a spokesman for the Forest Service said. “We hope we’ve seen the worst.” The Red Cross estimates between 2,500 and 3,000 persons have been evacuated.
The explosion early Sunday knocked 1,300 feet off the top of the once pristine and snow-covered 9,377- foot peak, which until March had been quiet since 1857.
“It looks like the aftermath of an atomic explosion,” said Dwight E Reber, a pilot for Columbia Helicopters Inc. of Aurora, Ore. Ash and flows of gas and newly formed rock poured from the mountain throughout Sunday.
The mudflow — the consistency of wet cement, moving at 50 mph — pushed floodwaters before it, swept up cars and houses and snapped concrete-and-steel bridges like toothpicks.
Officials late Sunday reported eight killed, but said today that three people had been counted twice.