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Volcanoes Case Studies
Mount St Helens – May 18 1980
What
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Where
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Mount St Helens is on the plate boundary between the Juan de
Fuca plate and North American plate. When it erupted it
permanently changed the surrounding landscape
It is one of five volcanoes in the cascade range in Washington
An Earthquake of 5.0 on the Richter scale triggered the
eruption
Huge lateral (sideways) blast of hot volcanic gases team dust
and rock fragments released at over 100mph
Some lava erupted but mainly as pyroclastic (lava bombs and
pumice stones)
Glaciers melted
Huge quantities of ash, steam and gas released
Biggest ever landside of soil, snow and rock ran down the
mountainside and filled Spirit Lake - led to a huge mudslide
Located in Washington state, NW USA, the Rocky mountains
Located on a destructive plate boundary
N. American Plate (continental crust) and Juan de Fuca plate
(Pacific plate(oceanic crust) collided – a subduction zone
Pacific plate is sub ducted and pressure/heat causes rock to
"melt" into magma
increased pressure caused Mt St Helens to erupt above the
magma hot spot
The area around the mountain is a National Park which is
mainly used for tourism, forestry, salmon farming and some
agriculture – therefore the population density of the area is
relatively low
The USA is an MEDC (and was in 1980) – with a GDP per capita
of over $25,000 (in 1980)
Mt. St. Helens is a typical composite cone volcano - formed on a
destructive plate margin where the Juan de Fuca plate is being
Effects
Short
term
Social
 Ash and mud blocked
roads, chocked rivers
 Many injuries and 61
people died mainly after
choking on hot volcanic
ash and burns and
poisonous gas
 Many people left
homeless after 200 log
cabins destroyed
 Flooding destroyed
communications such as
road and railway bridges.
 Sediment carried
downstream ruined
barge transport on the
Columbia River
 Lahars (mudflows of ash
and water) covered an
extensive area
surrounding the volcano.
 Helicopters used to
search for survivors
 Air conditioning systems
cleaned as clogged by
Ash
 Masks ordered (2
million) to protect lungs
 2000 people evacuated.
 The National Guard
issued clean water for
Economic
Environmental
 Livestock,
 520km of pine
wildlife and
forests flattened
crops destroyed
up to 20km away
many killed e.g.
from the
deer, bear and
eruption –
birds
landscape totally
changed
 Ash blocked
rivers destroying
 Ash deposited
popular fishing
over a huge area
sites and causing
(over 1cm deep
flooding. This in
120km away at
turn destroyed
Yakima)
crops and
 Ash cloud took
livestock.
17 days to travel
round the world
 The national park
was closed as
 Swirling ash
partly destroyed
triggered
so loss of tourism
lightening which
 Prime farm land
started many
covered in ash so
forest fires
no production of
 When it erupted
crops (Nuée
it permanently
ardente (hot ash
changed the
and gas
surrounding
 Massive loss of
landscape.
commercial
 Caused biggest
logging land and
landslide ever
trees destroyed
recorded.
 Local tourism
 The sideways
disrupted i.e.
blast of
camp sites,
pulverised rock,
subducted under the North American plate
people to drink.
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Cause
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The eruption on Mount St Helens was caused by an
earthquake caused by the feeble north face, which made the
volcano to release its underground pressure.
Series of earthquakes that initially took place at a rate of 15
per hour. Each of these earthquakes was of a different
magnitude and they took place beneath the volcano. The
northern flank of the mountain then began to slide downward
and a series of steam eruptions followed.
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When
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May 18 1980 at 8.30am (had not erupted for 100 years before)
From the spring of 1980, scientist started to record 100s of
small earthquake tremors coming from the mountain.
In March there were small eruptions of stream and ash from
the volcano. These events attracted a large number of news
reporters and tourist to the area.
By early May there was a visible bulge on the northern flank of
the volcano where magma was being pushed to as a result of a
volcanic plug that had formed in the main vent.
At 8:32 am on Sunday 18th May 1980 the eruption began .As
well as the landslide, lateral blast, pyroclastic flows and ash
cloud, the heat from the eruption melted snow which covered
the mountain creating lahars (as secondary hazard)
Long
Term
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The Volcano is now more
closely monitored
Drains cleared to prevent
flooding.
More people employed
in tourism.
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hotels etc.
Huge
compensation
issue as flights
were cancelled
etc.
Some positives souvenir ash
trays/T-shirts
made a lot of
income and the
site became an
even bigger
tourist attraction
some financial
compensation
Buildings and
bridges and roads
rebuilt – allow
tourism
Farming
community and
locals suffered
huge losses-land,
property
Overall cost of
destruction and
damage estimated
at $1.1 billion
Tourism increased
– local economy
helped. Many
tourist centres.
glacier ice and
ash wiped out all
living things up
to 27km north of
the volcano
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Trees replanted
(over 10 million)
Fertile soil after a
time due to ash
adding nutrients
Rapid natural
recovery
Mount Pinatubo – 3 June 1991
What
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Where
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Pinatubo lies near a subduction zone and has been
inactive for 600 years
The first formal evacuations were ordered from the
10 km (6.2 mi) zone on April 7. Followed by ones on
June 7, 13 and 15.
Ten times larger than the eruption of Mount St.
Helens in 1980, Pinatubo’s eruption threatened the
lives of millions of people.
A giant ash cloud rose 35 kilometres into the sky
Typhoon Yunya, coincidentally, occurred the same
time as the eruption and also saved many lives. The
typhoon brought heavy rain, which saturated
accumulating tephra and stopped its spread.
Second largest eruption this century – Not erupted
for 4 centuries.
Mount Pinatubo is a stratovolcano found in the
north east Philippines. It formed where the Eurasian
plate is being subducted beneath the Philippine
plate.
One of the 22 active volcanoes dotting the
Philippines
Mount Pinatubo is one of a chain of composite
volcanoes that constitute the Luzon volcanic arc.
It is among the highest peaks in west-central Luzon,
in the Philippines.
Effects
Social
Short
term
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847 people killed 300 by
collapsing roofs, 100 from
mud flows (lahars) and rest
from disease including
measles.
Ash and pumice destroyed
42,000 homes
58,000 people had to be
evacuated from a 30km
radius of the volcano
100 people killed by lahars
Manila airport closed
(houses and bridges
destroyed).
Electricity went off
200000 people relocated
Red Cross, Action Aid and
Oxfam provided food and
blankets in evacuation camps
23 USGS personnel helped
advise the community and
government officials for 8
weeks
Lack of clean water (as
contaminated)
Aviation problems
Economic
Environmental
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Ash cloud covered 125000km2
bringing darkness to central
Luzon.
Volcanic ash smothered
80000 hectares of land.
150km2 of reforestation
projects destroyed.
Water was contaminated
Fast flowing volcanic
mudflows (lahars) because
severe river bank erosion,
undercut bridges etc.
Until June 1991, Mount
Pinatubo rose 1,745 metres
above sea level. The new
summit elevation of Pinatubo
is approximately 1,485 metres
above sea level.
Huge avalanches of hot ash,
gas, with fresh volcanic
deposits as much as 200
metres thick.
The eruption removed so
much magma and rock from
below the volcano that the
summit collapsed to form a
large volcanic depression or
caldera
Weight of the rain caused
roofs to collapse.
5 billion cubic meters of ash
and pyroclastic debris were
ejected from its fiery bowels
producing eruption columns
18 kilometres wide at the
base and heights reaching up
to 30 kilometres above the
volcano’s vent.
When
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Cause
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15th March 1991 earthquakes felt
Earthquakes of increasing intensity for the next 2
weeks.
2nd April 1991 phreatic eruptions along a 1.5km
fissure on the summit.
Small eruptions of volcanic ash over the next few
weeks.
3rd May first eruption of magma.
7th June Large explosion generated an ash column
7km high.
12th June more violent eruptions with small
earthquakes producing ash columns 24km high.
15th June ash was ejected 34km into the
atmosphere. The eruption lasted for 3 hours.
A series of earthquakes that lasted over two weeks
caused Mount Pinatubo to erupt.
Mount Pinatubo erupted because the Philippine
plate started to slide underneath the Eurasian
plate. This caused the lave to be pushed to the
surface and causing a big explosion
A build-up of pressure due to tectonic movement
over time.
Long
Term
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500 people died from
diseases such as measles in
refugee camps
Many indigenous Aeta
people had to move into
government organised
resettlement areas because
their homes were destroyed.
This caused the Aeta society
to become fragmented.
President Ramos created the
Mount Pinatubo Commission
to raise 10 billion Filipino
pesos in aid.
Overseas Filipino Workers
provided clothing for victims
The volcano continues to be
a major hazard to the people
who have returned to the
area around the volcano.
1.2 million people lost their
homes and had to migrate to
shanty towns in Manila
Telephone links cut and
communications were
damaged or destroyed by
pyroclastic flows
Heavy rainfall from Typhoon
Yunga causes buildings to
collapse
New houses have been built
on stilts so future lahars will
not bury them.
Road links destroyed
More than 8,000 houses
were completely destroyed,
and a further 73,000 were
damaged.
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Full economic recovery
cost £10 billion.
Norway and UK
government sent
millions in aid
5 billion pesos spent on
used to build dykes for
lahars (later washed
away by the lahars)
Farmland destroyed by
falling ash and pumice,
unusable for years, the
1991 harvest was
destroyed
800km2 agricultural
land destroyed and
800000 livestock and
poultry killed costing
1.5 billion pesos.
650,000 workers lost
jobs
Total losses in 1991 and
1992 alone were
estimated at 10.6 and
1.2 billion pesos
respectively, including
damage to public
infrastructure
estimated at 3.8 billion
pesos.
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Lahars caused severe erosion
to rivers.
Global cooling caused by the
ash. Temperatures dropped
by 0.5°C.
Acid rain due to the 22million
tonnes of SO2 erupted.
Since the climactic 1991
eruption, ash deposits have
also been remobilised by
monsoon and typhoon rains
to form giant “lahars