Mount St. Helens - Natural disasters.

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MOUNT ST HELENS

Where is Mount St. Helens?

Mount St Helens was 9,667 feet high volcano located at 46.20 N by 122.18 W           in southwest Washington State, approximately a 3-hour drive from Seattle, 90 miles away and a 2.5 hour drive from Portland, Oregon 65 miles away. The volcano is in The Mount St. Helens National Volcanic Monument. On the North American plate, located near a convergent plate boundary. In an area called the cascades. I saw the volcano in the year 2000 when I went on a holiday around America.

Formed from an earlier volcano that existed 25,000 years ago, but St. Helens is relatively new. In fact, younger than the pyramids of Egypt that are 4,000 years old

Mount St Helens was a stratovolcano, made of layers of lava, pyroclastic deposits also known as tephras, and mudflow deposits.                

To the natives- The Sanpoil Indians the volcano was sacred; they had seen its previous eruptions.

They had different names for the volcano, Some of the names given to the mountain were Lawelatla ("One From Whom Smoke Comes"), Louwala-Clough ("Smoking Mountain"), Tah-one-lat-clah ("Fire Mountain") and the most commonly used name today Loo-wit ("Keeper of the Fire").  The local tribes would not fish in Spirit Lake, believing the fish, with heads like bears, held the souls of the evilest people who had ever lived.  They also believed the lake shores were populated by a band of rogue demons.  Only young warriors out to prove their bravery dared climb to the timberline and spend the night.  Later, legends claimed the evil spirits of the mountain were punishing the local tribes for allowing the white men to settle near them.

The modern name comes from Captain George Vancouver of the British Royal Navy 1972, he spotted the peak from his ship discovery and named it in October 20, 1792 after his friend Alleyne Fitzherbert, who held the title Baron St. Helens and was at the time the British Ambassador to Spain. He had also named three other volcanoes in the cascades; Mounts Baker, Hood, and Rainier.

What is the history of eruptions at Mount St.Helens?

St. Helens has erupted 23 times before 1831, after that it has erupted in 1835, 1842, 1847, 1848, 1849, 1853, 1854, and 1857.

In the last 515 years, it is known to have produced 4 major explosive eruptions

Two of the major eruptions were separated by only 2 years. One of those, in 1480 A.D., was about 5 times larger than the May 18, 1980 eruption, and even larger eruptions are known to have occurred during Mount St. Helens' brief but very active 50,000-year lifetime.

What was the most recent eruption of Mount St.Helens and what caused it?

In spring 1980 the where the first signs of activity; There was a series of small earthquakes that began on March the 16th.

On the 20th of March at 3:47 pm there was a magnitude 4.2 earthquake, the following week there was more activity. On the 25th and 26th there was 174 shocks with magnitudes greater than 2.6. During the week aerial views showed there was small avalanches of snow, but no sign of eruption. Two eruptions widely heard on march the 27th at 12:36, Mount St. Helens began to spew ash and steam, with a huge column about 6,000 foot high, this small eruption blew a crater 250ft wide in the ice, in a week the crater grew to be 1,300 ft in diameter. As well as to huge cracks across the summit. From the 31st of march seismographs where recording occasional spasms of volcanic tremor, a type of continuous, rhythmic ground shaking different from the discrete sharp jolts characteristic of earthquakes. Such continuous ground vibrations, commonly associated with eruptions at volcanoes in Hawaii, Iceland, Japan, and elsewhere, are interpreted to reflect subsurface movement of fluids, either gas or magma. The combination of sustained strong earthquake activity and volcanic tremor at Mount St. Helens suggested to scientists that magma and associated gases were on the move within the volcano, thereby increasing the probability of magma eruption.

From April the 21st there was regularly ash and steam explosions lasting from a few seconds to several minutes. All of the ash that the volcano had spewed out was From the 350 year old summit dome. It was broken by phreatic (steam-blast) processes driven by the expanding, high temperature gases. No magma was released during the eruptions.

Visible activity stopped in late April and early may, small steam explosions restarted on may the 7th, and continued until may the 16th, during the gap in between activity magma had risen high in the volcano and was continuing to rise, seismographs showed the activity, but this activity was much more noticeable by the eye, a huge bulge had grown out of the north face. By accurate measurement taken in late April found that a bulge had been growing, about 5 feet per day this huge bulge had grown out to 450 feet, this bulge is called a cryptodome. Through mid may there had been 10,000 eruptions, the activity was concentrated in a small 1.6 mile area about half a mile below the volcano. Continuing this explosion there was more eruptions of steam and debris from the summit dome, while these eruptions the cryptodome was increasing in size dramatically and was splitting the volcano in two, this activity was extremely dangerous, because of the hot magma in the volcano this melted the snow which resulted in small streams and steam coming from the north face of the volcano.      The bulge had become increasingly unstable.

Local governments had made areas that where restricted access. The USGS wanted the restricted areas to be larger. Loggers had persuaded the local government that millions of dollars in revenue were going to be lost if loggers could not get in to retrieve lumber. The local people also where annoyed by the lack of access to their property. On May the 19th the volcano had become very quiet, and most people thought that the danger had passed. They wanted the blockades removed.

A few hours before the eruption there was 330 lumberjacks in the area trying to retrieve the lumber.

For a very detailed lead up to the May 1980 eruption see

Log of Mount St. Helens Precursory Activity from the USGS on page 15

What damage was caused by eruption?

On may the 18th at 8:32am there was a Within 15 to 20 seconds of a magnitude 5.1 earthquakes about a mile below the volcano. 10 seconds after the earthquake the

Volcano’s bulge and summit slid away in a huge landslide. During the next 15 seconds another huge block slid off, then another and even another.

All these blocks slid down to from the largest landslide on Earth in recorded history, about 23 square miles, it turned into a huge avalanche travelling at 155-180 mph, part of the avalanche went down and across spirit lake, but most of it flowed west to the north fork of the Toutle river. The avalanche had enough power to flow over a ridge 1,150 feet high 4 miles away. And carried on until 13 miles down the river. It filled the valley 150ft deep with debris and ice. The total volume was 0.7 cubic miles. The avalanche was mainly debris, glacial ice and possibly water from Spirit Lake. Covering an area of 24 square miles, Spirit Lake’s bottom was raised by 295ft and the water by 200ft.

It collapsed at 110-240 Kph for 24 kilometres down the North Fork Toutle River; arms of the avalanche entered spirit Lake, 8 kilometres from the summit, and overtopped 300-380 meter high Johnston Ridge north of the Toutle.

The avalanche buried the Toutle valley to a depth of nearly 50 meters.

Its maximum velocity may have been supersonic.

This avalanche removed the top 1,312 feet (400 meters) of the cone and left a crater 2,050 feet (625 meters) deep, 1.7 miles (2.7 kilometres) long, and 1.3 miles (2.0 kilometres) wide. 

What was once the 9th highest peak in Washington State was suddenly reduced to the 30th highest peak. 

When the summit had slid off the magma trapped beneath was released, the explosion pushed the summit debris, magma, ash and steam into the air.

Its maximum velocity may have been supersonic and a temperature exceeding 350ºC.  

It instantly turned more than 70% of the snow and glacial ice on the mountain to water. 

The blast covered 230 square miles and 17 miles to the north of the crater. The blast had extreme energy; 24 megatons thermal energy (7 by blast, rest through release of heat).

The 0.046 square miles of ash and debris fell about 3 feet thick around the volcano.

The pyroclastic flows covered 6 square miles, and 5 miles north of the crater.

These pyroclastic flows where 1,300ºC and travelled at 50 to 80 Mph.

Lahars damaged 27 bridges, nearly 200 homes. It also destroyed more than 185 miles of highways and roads and 15 miles of railways. It also destroyed 31 ships down river of it. The areas where there had been mudflows needed expensive dredging to remove the ash and debris. They travelled at 10-50 Mph.

Losses to property and crops were set at more than $1.8 billion. There was a 600-meter-deep crater 2 kilometres wide rim-to-rim.

Ash damage

In 10 minutes the ash column reached 12 miles in altitude, the moving ash particles caused lightening and forest fires. The ash was been blown at 60 mph, and reached Yakima at 9:45am and Spokane at 11:45am. The ash cloud was dense enough to block out all light and turning on the darkness sensitive switches on streetlights.

During the 9 hours of vigorous eruptive activity, about 540 million tons of ash fell over an area of more than 22,000 square miles

Interstate 90 was closed because of poor visibility, many other roads where closed some for hours, others for weeks. The whole road system in Vancouver was closed down for a week. Airports in eastern Washington were closed down for 2 weeks and nearly 1000 commercial flights were cancelled.

The ash was a fine, gritty type that caused problems for car engines and air conditioning, and any kind of air filters got blocked up. It scratched moving surfaces

And the fine ash short-circuited many electrical transformers, which caused regular power cuts. The ash blocked sewage systems.

Damage to the environment:

The eruption had made an 80,000 feet eruption column or cloud in less than 15 minutes. Spread across U.S. in 3 days and circled Earth in 15. This dark, grey ash, made up mostly of lithic debris from this column, fell more than 930 miles (1,500 kilometres) away. Then a lighter ash fell, made mainly of pumice.

Enough trees to build 300,000 houses where knocked down by the blast.

Huge amounts of wildlife where all killed as well, The Washington State Department of Game estimated that 5,000 black-tailed deer, 1,500 Roosevelt elk, 200 black bears, and 15 mountain goats fell victim that’s 7,000 big game animals; deer, elk, and bear were killed like small birds and most mammals, killing most wildlife in a 212-square-mile.

The Washington Department of Fisheries estimated that 12 million Chinook and Coho salmon fingerlings were killed when hatcheries were destroyed. Another estimated 40,000 salmon were lost when forced to swim through turbine blades of hydroelectric generators as reservoir levels along the Lewis River were kept low to allow possible mudflows and flooding. Fifty-seven people were killed in the eruption, of those 21 b bodies were never recovered.

Victims Recovered:

Victims Never Recovered:

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First Hand Accounts:

Keith stofell:

"Noticed land sliding of rock and ice debris in-ward into the crater... the south-facing wall of the north side of the main crater was especially active. Within a matter of seconds, perhaps 15 seconds, the whole north side of the summit crater began to

move instantaneously. ... The nature of movement was eerie.... The entire mass began to ripple and churn up, without moving laterally. Then the entire north side of the summit began sliding to the north along a deep-seated slide plane. I (Keith Stoffel)

was amazed and excited with the ...

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