The combined with the pyroclastic flow reduced

The eruption of Mount St Helens 1980On the 18th may 1980 Mount St Helens in Washington erupted, it was the most powerful eruption in the recorded history of the United States. The eruption killed 57 people. The initial blast was an estimated 24 megatons as powerful as 1,600 times the nuclear bomb dropped on Hiroshima.The Mount is situated in Washington, about 100 miles from Seattle. And 50 miles from Oregan. This makes the ash and other gases ejected from the crater of significant risk to two large cities in the event of an eruption.

It is most noted for the eruption in 1980.The first indication that something was awry at Mount St Helens was an earthquake of the magnitude 4.2 on the Richter scale. Then a week later steam venting began. 52 days later, on the 18th of May a 5.2 magnitude earthquake brought the entire northern side down, this was the largest avalanche ever recorded. Following this an eruption of magma and Sulphur dioxide spewed out into the atmosphere forming a super-massive pyroclastic flow.

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This flow spread out over an area of 230 square miles, flattening wildlife and burying homes. The eruption carried on for nine hours, ejecting a total of 3,900,000 cubic yards of ash, dust and sulphur dioxide. This debris all formed pyroclastic flows, which devastated the area, moving over the debris from the avalanche it picked up speed and flash heated river water, causing further explosions. The estimated temperature of the pyroclastic flow I estimated to be 1200oF.The cloud of ash that was ejected from the crater ascended to the height of 12 miles. The ash caused lightening, which in turn started forest fires, this in turn created smoke. This, combined with the pyroclastic flow reduced the visibility in nearby towns to 10 feet, causing gridlock and the automatic street lights to come on at noon.

The sudden increase in temperature of the volcano melted the glacial water on the summit, these mixed with the pyroclastic to form a Lahr, these are hot mudflows that can travel for hundreds of miles. The major risk from these flows is the damage to infrastructure they cause. They will damage electricity connections and phone lines, they will cover roads and surround houses. All this will lead to chaos. One of the other key volcanic hazards are the effect of ground stability.

The eruption at Mount st Helens is a key example of this, the eruption was caused by the instability of the ground.   One of the key ways to accurately predict and prevent volcanic disasters is the accurate and informed use of seismology. This study if the movement of rocks works effectively to predict the movements and potential for eruption. A key example of this is at MT st Helens where the beginning of the steam eruptions was preluded by the vigorous seismic activity. This may have been caused by the ascending larva being forced to intrude upon other rocks after being blocked by an area of congealed magma on its route to the surface. Within three minutes the blast was traveling at three hundred miles per hour the thick clouds of ash and debris ploughed down almost everything in its way, this cause massive damage to the tree population with 4,000,000,000 trees being blown down.

This intrusion caused a bulge in the north face, this bulge would eventual for the vast majority of the Lahr that came from the debris mixing with the melted glacial water.A Lahr is a flow of water and debris they cause chaos wherever they flow. This is due to the sheer quantity of mud that gets into the infrastructure. This particular Lahr was responsible for burying 123 homes under 200 feet of mud. Landslides were a particularly prominent feature of the eruption of Mount st Helens. The tremors caused a landslide which in turn released the magma.

The great cloud of debris that came from the landslide mixed with the pyroclastics and was responsible for the fierceness and strength of the flow.The change in global climate due to the eruption was negligible, however significant damage was done to the local area, damage such as the forests destroyed by the pyroclastic flow, and the ash that fell upon local cities and caused power cuts and caused millions of dollars of damage to the electricity and infrastructure.The US geological survey had been doing work to monitor the volcano after the re-awakening of the peak in March 1980. They had been measuring the temperatures and pressures at the summit of the volcano. Their advice had been listened to by the US state department. For this reason there were remarkably few deaths for an eruption of this size, the local residents had been evacuated.

And holidaymakers and hikers had not been allowed into the area. Luckily the eruption happened at the weekend, otherwise hundreds of loggers would have been working right under the north face. The local wildlife was not as lucky, with as many as 200 brown bear dying in the eruption.The most powerful eruption of the 20th century was at MT st Helens. Yet it had one of the lowest death tolls of any volcanic eruption in the 20th century.

The use of volcanic measuring techniques allowed them to avert disaster and prevent fatalities. However the cost of repairs was still huge. Infra-structure had to be rebuilt, bridges repaired and houses repaired and powerlines reconstructed. No attempt was made to divert the pyroclastic flow, this would have required civil engineering on a scale not seen since the construction of the Hoover dam, with the great forces of nature being tamed by an architect’s pencil, but such a construction would have reduced damage done by the eruption.


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