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Volcanoes
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What is a volcano?
A volcano is an opening or vent in the earth’s surface
through which molten material erupts and solidifies as lava.
Volcanic vent
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Where are volcanoes found?
Around which plate do we find most volcanoes?
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Label this cross section of a volcano
•Volcanic bombs,
ash, lava, gases
•Magma chamber
•Parasitic cone
•Crater
•Main vent
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Cross section of a volcano
Crater
Volcanic
bombs, ash
and gases
Parasitic
cone
Main vent
Magma
chamber
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Why do they happen?
A destructive plate boundary is found
where a continental plate meets an
oceanic plate.
Destructive
Plate
Boundary
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The oceanic plate descends under the
continental plate because it is denser.
As the plate descends it starts to melt
due to the friction caused by the
movement between the plates. This
melted plate is now hot, liquid rock
(magma). The magma rises through
the gaps in the continental plate. If it
reaches the surface, the liquid rock
forms a volcano.
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Destructive plate boundary
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Why do they happen?
At a constructive plate
boundary, two plates move
apart.
Constructive
Plate Boundary
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As the two plates move apart,
magma rises up to fill the gap.
This causes volcanoes at this
type of boundary. However,
since the magma can escape
easily at the surface, the
volcano does not erupt with
much force.
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Constructive plate boundary
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At destructive plate boundaries the lava is viscous (thick
like treacle) and it cannot flow very far from the volcano’s
vent. This makes the volcano steep sided.
At constructive plate boundaries the lava is runny and it
can flow far away from the volcano’s vent. This makes the
volcano gentle sided.
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Volcano Shapes
Why are these volcanoes different shapes?
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Do all volcanoes erupt?
Active volcano – liable to erupt e.g. Mt Etna.
Dormant (sleeping) volcano – a volcano which
has not erupted for many years. For example, Mt
Pinatubo erupted in 1991 after 500 years of
dormancy.
Extinct volcano – a volcano which has not
erupted for many thousands or millions of years
e.g. Edinburgh.
However, it is often very difficult to tell whether a volcano
will erupt again…El Chichon, Mexico erupted in 1982
after being dormant for approximately 1200 years!
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The Hawaiian islands are a chain
of volcanoes in the Pacific Ocean.
Look at their location on the map
below. Why is this an unusual place for
them to be located?
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Hot spot volcanoes
In the animation above, why are the volcanoes to the left of
the ‘hot spot’ extinct?
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Mt St Helen’s eruption (May 1980)
Mt St Helens
located on the ‘Ring
of Fire’
Internet Links
http://volcano.und.nodak.edu/vwdocs/msh/msh.html
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Mt St Helens – causes of the eruption
Juan de Fuca plate
North American Plate
Which is the oceanic plate?
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Mt St Helens – the eruption
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Mt St Helens – consequences of the eruption
N
0
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10km
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Mt St Helens
AFTER
BEFORE
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What damage did the eruption cause?
Up to 70mm of ash falls across Washington and East Montana… 7000 school
students have an early summer vacation due to problems on the roads
An increase
Mudflows block and divert rivers. The US army takes
in tourism
18 months to dig out the debris from the River Toutle.
Two million birds, animals
and fish are killed.
A decrease in
tourism
Communications routes threatened – railway and
road bridges destroyed, Portland harbour blocked
Hot mudflows raise the temperature of
the rivers and lakes to 30ºC
Deer mice, chipmunk, vole,
gophers, salamander survive
the blast. Their population
steadily increases due to the
absence of predators.
60 people die from
the eruption.
Gophers, through their tunnelling, mix the soil
with the ash. This increases the fertility of the
soil.
Damage to crops=$175million
The post office and Lone Fir Motel shut
down in Cougar.
Separate the above into the short term and long term
impacts of the eruption.
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Deer mice, chipmunk, vole, gophers, salamander survive the blast.
Their population steadily increases due to the absence of
predators.
Why do you think animals such as the vole and gopher
survived the blast?
What benefits might the volcanic activity have brought to
Mt St Helens?
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Why do people live in volcanic areas?
This lava is
weathered (broken
down) to form a fertile
soil,
Can you think of any other reasons?
Tourists are attracted to
areas of volcanic activity.
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Geothermal energy
(heat from the earth)
can be produced in
many volcanic areas.
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Nevada del Ruiz, Colombia (November 1985)
Nevada del Ruiz is located in the Andes. The volcano
had been giving signs of increased activity for some
time and on the 13th November the volcano erupted,
producing a cloud of steam, ash and rocks. The heat
melted snow on the volcano and the water produced
combined with ash, creating a mudflow. This mudflow
rushed down the valley at 60km/hour towards the town
of Armero and engulfed the town with mud up to 5m
deep. It was just before midnight and most people
were in bed. By the time the rescue team reached
Armero, the next day, the mudflows had set hard.
23,000 dead
5,000 injured
5,000 homes
destroyed
The eruption started at 9.08pm. If the mudflows reached
Armeno at 11.30pm, how far is Armero from Nevada del Ruiz?
What could have been the volcano’s ‘signs of increased
activity’?
Why wasn’t Armero evacuated when the volcano had shown
these ‘signs of increased activity’?
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Internet links!
Volcano World - a fun and informative web site
http://volcano.und.nodak.edu/
Virtual field visits
http://educeth.ethz.ch/stromboli/
The Michigan Technological University Volcanoes Page
http://www.geo.mtu.edu/volcanoes/
Global Volcanism Program
http://www.volcano.si.edu/gvp/
Fallout: Eye on the Volcano
http://www.nationalgeographic.com/features/98/volcanoes/
Savage earth
http://www.pbs.org/wnet/savageearth/
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