Mount Saint Helens

Introduction

The volcano, prior to 1980, was conical in shape. However, since the 1980 eruption, the summit was removed by an avalanche leaving a 2 by 3.5 Km horseshoe-shaped crater, which is filled by lava domes and a glacier.

Mount Saint Helens is a rhyodacite to basaltic stratovolcano on a continental-oceanic collision plate boundary (North American plate to Pacific plate). Mount Saint Helens began formation a long 275,000 years ago and has been the most active volcano in the cascade range during the Holocene.

The Volcano is now a popular tourist destination and a popular place for climbers.

Location

Mount Saint Helens is located in the northwest of the USA. It is located in the southwest part of the state of Washington. Mount Saint Helens is a part of the Cascade mountain range.

Formation

Until 12800 years ago, tephra pyroclastic flows and lava domes erupted to form the older edifice. This was mainly comprised of dacite lava domes surrounded by tephra.

Most of the more recent edifice was created in the last 3000 years. This occurred when volcanic products consisting of andesite and basalt erupted from the main and parasitic vents between periods of dacitic activity. These lava flows covered previous dacite lava cones and crystallised to form the conical structure.

1980 Eruption

Introduction

This eruption event occurred from March 1980 to May 18th 1980. the may eruption is said to be the most disastrous eruption in the USA's history. This was due to its volcanic explosivity index score of 5.

Lead up to the major eruption

Mount Saint Helens had been dormant since the 1840s and 1850s. However, on March 15th 1980, its long sleep was cut short, as small tremors/earthquakes had begun to occur around the volcano this was caused by and indicated the movement of magma in the magma chamber. on March 20th a 4.2 magnitude earthquake occurred signalling the end of the volcano's dormancy. This was followed by an earthquake swarm peaking on March 25th at 5.1 on the Richter scale.

At 12:36 PM on the 27th of march phreatic eruptions (explosions of steam caused by the sudden heating of groundwater by magma), due to an increase in pressure, fractured and ejected rock from within the volcano which created a rather large 75m wide crater and formed a giant 2.1km ash column. After merely 1 week the crater had become a massive 400m wide in diameter and 2 giant crack systems crossed the summit.

Eruptions in March occurred at a large frequency of once an hour and in April they slowed to occurring once per day. these ended on April 22nd and restarted on May 7th to May 17th. By this time there had been 10,000 small earthquakes and the north flank had grown a large 140m to create a sizeable bulge which had grown at a rapid rate of 2m a day. this bulge suggested magma had risen high in the volcano. Furthermore, beneath the bulge, a crypto dome (body of magma that rises from the depth and intrudes into the edifice of the volcano, but does not erupt on the surface) had intruded into the edifice but had not yet erupted to the surface.

May 18th events

At 8:32 Pm a large 5.1 magnitude earthquake occurred. During this time, the northern bulge and summit violently split from the volcano in a huge landslide. The debris avalanche travelled quickly around ridges to the north and mostly turned west and ran a massive 23km down the valley of the North Fork Toutle River and formed a hummocky deposit. the total avalanche volume came to a huge 2.5 cubic km

An ash-rich dark plume rose from the avalanche scarp and a 200m high plume erupted from the crater.

When the bulge separated it contained part of the crypto dome, which is a very hot and highly pressurised magma body, the removal caused a vast and fast decrease in pressure in the magmatic system causing violent lateral eruptions or blasts through the sliding debris. This removed a vast 300m of the volcano's summit. This lateral blast overtook the debris avalanche and travelled at an extremely fast 480km per hour.

The lateral blast destroyed an area of 30km from east to west and 20km north from the former summit. the inner zone was extended 10km from the summit. in these zones, the surrounding forest was annihilated with all trees toppled. 600km2 was covered by hot debris which was deposited by the blast.

A Plinian eruption (extremely explosive eruption that launches ash into the stratosphere and can cause thick pyroclastic flows) occurred in the volcano. This was due to the exposure of the conduit of the volcano ( as a result of the removal of the crypto dome and flank). This caused a depressurisation wave to cascade down the pipeline to the magma storage area, which caused the enclosed mama to expand upward (due to reduced pressure) towards an open parasitic vent, after just under 1 hour the massive loss of pressure resulted in an explosive and violent Plinian eruption sending a huge amount of tephra into the stratosphere. this phase continued for 9 hours and sent several pyroclastic flows travelling at 80 to 130 km per hour and covering 8km north this caused the deposit of pyroclastic material which consolidated to form the pumice plain. Prevailing winds carried 540 million tons of ash across the USA, it caused complete darkness in Spokane which was 400km from the volcano. the ash spread across the USA in merely 3 days and it encircled the earth in only 15 days.

During the eruption, hot debris and gas melted the snow and ice in the volcano, which created vast surges of water and loose debris called lahars, these destroyed parts of the forest and laid waste to local infrastructure.

The aftermath of May 18th

After the eruption, a once ecologically rich forest landscape had been transformed into a barren annihilated landscape, and the forest was levelled completely. However, a rather large amount of species managed to survive for example fish and amphibious life. As a result of the eruption 150 new lakes and bonds were formed and new river courses were created showing how much the eruption completely changed the natural landscape

Mount Saint Helens destroyed many roads bridges and hundreds of homes. The ash fall created major sewage and transport problems. due to ash and poor visibility, many highways were shut. there was also major air travel disruption as several easter washing airports closed due to poor visibility. This would have caused major economic issues as people cannot get to 3work so companies cannot operate and make money, also due to airport closures many people would have been out of work for a while and not being paid.

It was a huge effort to clear the ash, 1800000m cubed of ash was cleared from roads and airports in Washington. this would have been a long and costly endeavour.

The economic cost was an estimated $860 million. The eruption also caused major damage to tourism fisheries timber and farming industries. the timber industry took a massive hit with 4.7 board feet of timber destroyed which would be very costly to recover from.

This shows the true destructive power of Mount Saint Helens as it can completely change the surrounding landscape in a single day leaving very little.

Bibliography

https://www.usgs.gov/volcanoes/mount-st.-helens

https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens

https://www.nps.gov/articles/000/plinian-eruptions.htm

https://www.usda.gov/media/blog/2020/05/18/past-present-and-future-research-mount-st-helens#:~:text=57%20people%20lost%20their%20lives,with%20sediment%2C%20flooding%20their%20banks.

https://www.ncsl.org/bookstore/state-legislatures-magazine/my-district-is-in-the-shadow-of-mount-st-helens-magazine2021.aspx