Mt St Helens Volcanic Eruption Case Study - Mount St Helens, Rocky Mountain Fold, North America.

On the 18th May, 0830 hours, Ash and steam erupted. Two minutes later, an earthquake shook the mountain. This earthquake measured 5 on the Richter scale and also gave the bulge a move forwards and downwards. 1000's of tonnes of rocks, glaciers, ice, soil and mud raced downhill within a minute. It filled Spirit Lake and all the water in there was displaced and filled up the Toutle Valley. The water went further and reached Baker Camp and Portland Port on the Columbia River - All in a minute.

At 0833 hours, the exposed magma exploded sideways sending out blast waves of Volcanic gas, steam and dust (called a nuée ardente). This all moved Northwards for 25KM and everything within this range: Animals and Vegetation, Buildings and houses were destroyed.

Mt St Helens

Spirit Lake after the eruption

Why: A minor plate called the Juan de Fuca Plate (oceanic crust) started moving eastwards, colliding with the North American Plate which was also Oceanic crust. The Oceanic crust from the Juan de Fuca plate went below the North American Plate into the mantle. The extreme pressure and friction melted the crust and turned it to magma. As more magma is built up into the mantle, pressure increases and the magma starts to rise to the surface. Eventually the magma explodes out the top. All the earthquakes come from the excessive rubbing of the plates when movement occurs. Although this is what happens in every volcano, this is a very good example.

Who were affected: Many People were effected during and after the eruption and the earthquakes. A little damage was done when all the earthquakes occurred but big damage was done when the eruption occurred. Several Days after the main earthquake, the ash that exploded out of the bulge had encircled the world. The volcanic plume of fine ash reached the East Coast of the USA. Sunlight was blocked from the ash and for the rest of the morning, a series of eruptions ejected gas, ash and volcanic bombs (rocks) from the earthquake. The thicker ash rose 20km and drifted eastwards before settling.

1000's of people's homes or properties were damaged in some way or another. Some were incinerated, others had windows and cars damaged from ash all over USA. Spirit Lake was a lake that was seriously damaged from volcanic waste.

Facts about Mt St Helens:

• 1314 feet was lost of the top of the Mountain

• The Landslide Debris Avalanche travelled at 150 miles per hour

• The lateral blast had 250 million cubic yards of waste and deposit

• The temperature of the blast was 350 degrees Celsius

• 24 megatons of thermal energy was released (7 megatons by blast the rest through release of heat)

• If all the lost timber was saved from the avalanche, it could build 150 000 houses, 4 bedroom houses.

• 185 miles of highways & roads and 15 miles of railways were lost in the blast.