Volcanic Eruptions
MEDC Case Study - Mount St.Helens, USA
Mount St. Helens, USA – MEDC Case Study
• Is part of the ring of fire.
• Well known for ash explosions and pyroclastic flows.
• Most famous eruption – 18th May 1980.
• 57 people were killed, 250 homes destroyed.
• The 1980 eruption was the first time that it was possible to use
modern technology to carefully monitor a volcano as it built up
towards a violent eruption.
• Trees were pushed flat pointing away from the source of the
blast. Such was the speed of the moving air and debris that it
overtook cars as people tried to escape.
• Its last eruption was 10th July 2008!
The little Juan de Fuca ocean plate slides under the
continental plate.
As the ocean plate descends the rock melts and magma
rises.
This has created the Cascade Range of volcanoes.
Mt St Helens – causes
of the eruption
Juan de Fuca plate
North American Plate
The Volcano at Mt St Helens was formed because the _________ plate is
________ under the _________ plate. When this happens _________ occurs
and _______is formed under ____________ . The magma rises and when
the pressure is _________ a volcanic eruption occurs.
Juan de Fuca
pressure
subducted
North American
Melting
magma
released
http://www.youtub
e.com/watch?v=xP2
dreOI8gI&feature=r
elated
April May 1980
A bulge was growing on the
north flank.
May 18th at 8.32 a.m.
An earthquake occurs under the
volcano and triggers a huge
landslide.
May 18th at 8.33 a.m.
With the pressure released a huge
lateral blast heads northwards
May 18th rest of the day
More and more ash shoots up high
into the atmosphere. It gradually
blows around the world.
PRIMARY EFFECTS
Material from a massive landslide sped down the mountain, filling Spirit Lake and,
mixed with lake water, racing down the Toutle and Kalama rivers as a mudflow (or
lahar).
Explosions of gas and steam flatened everything in its path
Snow melt mixed with ash and mud carried huge amounts of debris, trees, cars etc
Ash clouds circulated the earth for 7 days.
The volcano was reduced in height by 400 meters.
57 DEAD.
Pyroclastic
Clouds
The pyroclastic
cloud caused
hundreds of trees to
be completely
destroyed beneath
the volcano.
Pyroclastic Flows – Mt. St.Helens May 1980
The pyroclastic flow
contained large boulders
up to 1 metre in diameter
Large bulldozer wrecked by the
pyroclastic flow of May 18th 1980
Pyroclastic Flows – Mt. St.Helens May 1980
Before
After
The same marker pole
Ash Clouds
SECONDARY EFFECTS
Road accidents
Plane crash
Plant and animal life wiped out
Rivers polluted
$860 million costs to the economy
$450 in the timber trade
$150 Tourism
Rivers polluted, bridges wiped out.
Today’s Tasks
(1) On your diagram from earlier, add some of the
labels/numbers from diagram B.
(2) Copy and complete the table below using
diagram C
(3) Complete the Mount St. Helens worksheet
Social Impacts
Economic Impacts
Environmental Impacts