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Volcanoes
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Learning objectives
What is a volcano?
Why do volcanoes occur?
Where are volcanoes found?
What happened in the 1980 Mount St
Helens eruption?
Can volcanic eruptions be predicted?
Why do people live in volcanic areas?
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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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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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Cross section of a volcano
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Volcanic emissions
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Volcanic emissions
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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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Learning objectives
What is a volcano?
Why do volcanoes occur?
Where are volcanoes found?
What happened in the 1980 Mount St
Helens eruption?
Can volcanic eruptions be predicted?
Why do people live in volcanic areas?
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Why do they happen?
A destructive plate boundary is found where a
continental plate meets an oceanic plate.
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.
As the two plates move apart, magma rises up to fill the
gap. This causes volcanoes. 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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Volcano shapes
Why do volcanoes have different shapes?
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Volcano shapes
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Volcanic activity at plate margins
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© Boardworks Ltd 2005
Learning objectives
What is a volcano?
Why do volcanoes occur?
Where are volcanoes found?
What happened in the 1980 Mount St
Helens eruption?
Can volcanic eruptions be predicted?
Why do people live in volcanic areas?
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Where are volcanoes found?
Around which plate do we find most volcanoes?
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Hawaii
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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Learning objectives
What is a volcano?
Why do volcanoes occur?
Where are volcanoes found?
What happened in the 1980 Mount St
Helens eruption?
Can volcanic eruptions be predicted?
Why do people live in volcanic areas?
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Mt St Helens eruption (May 1980)
Mt St Helens is 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
Mt St Helens is located on a destructive plate boundary
where a continental plate (North American) meets an
oceanic plate (Juan de Fuca).
Juan de Fuca plate
North American plate
Which plate is denser?
Describe what happens when the oceanic plate
descends under the continental plate.
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Mt St Helens – the eruption
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Mount St. Helens woke up on March 20, 1980, with a Richter magnitude 5.1 earthquake. Steam
venting started on March 27. By the end of April, the north side of the mountain started to bulge.
With little warning, a Richter magnitude 5.1 earthquake triggered a massive collapse of the north face
of the mountain on May 18. This was the largest known debris avalanche in recorded history. The
magma inside of St. Helens burst forth into a large-scale pyroclastic flow which flattened vegetation
and buildings in an area of over 230 square miles (600 km²). This eruption was a 5 on the Volcanic
Explosivity Index scale.
For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20 to
25 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (95
km/h), with ash reaching Idaho by noon.
The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars
(volcanic mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying
bridges and lumber camps. A total of 3.9 million cubic yards (3.0 million cubic meters) of material
was transported by the lahars.
By around 5:30 PM on May 18 the vertical ash column declined in stature but less severe outbursts
continued through the night and the following several days. In all, St. Helens released an amount of
energy equivalent to 27,000 Hiroshima-sized atomic bombs and ejected more than a cubic kilometre
of material. The removal of the north side of the mountain reduced St. Helens' height by about 1300
feet (400 m) and left a 1 to 2 mile (1.6 to 3.2 km) wide and 0.5 mile (800 m) deep crater with its north
end open in a huge breach. Fifty-seven people were killed along with 1500 elk, 5000 deer, and an
estimated 11 million fish. In addition, 200 homes, 47 bridges, and 185 miles (300 km) of highway
were destroyed.
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VEI
Description
Plume
Height
Volume
Classification
How often
Example
0
non-explosive
< 100 m
1000s m3
Hawaiian
daily
Kilauea
1
gentle
100-1000 m
10,000s m3
Haw/Strombolian
daily
Stromboli
2
explosive
1-5 km
1,000,000s m3
Strom/Vulcanian
weekly
Galeras, 1992
3
severe
3-15 km
10,000,000s m3
Vulcanian
yearly
Ruiz, 1985
4
cataclysmic
10-25 km
100,000,000s m3
Vulc/Plinian
10's of years
Galunggung, 1982
5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens, 1981
6
colossal
>25 km
10s km3
Plin/Ultra-Plinian
100's of years
Krakatau, 1883
7
super-colossal
>25 km
100s km3
Ultra-Plinian
1000's of years
Tambora, 1815
8
mega-colossal
>25 km
1,000s km3
Ultra-Plinian
10,000's of years
Yellowstone, 2 Ma
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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 – consequences of the eruption
AFTER
BEFORE
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What damage did the eruption cause?
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Mt St Helens – consequences of the eruption
Why do you think animals such as
the vole and gopher survived the
blast?
How did their survival benefit the
area?
What benefits might the volcanic activity have brought to
Mt St Helens?
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Learning objectives
What is a volcano?
Why do volcanoes occur?
Where are volcanoes found?
What happened in the 1980 Mount St
Helens eruption?
Can volcanic eruptions be predicted?
Why do people live in volcanic areas?
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Predicting eruptions
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The problem of prediction
Volcanologists (people who study volcanoes)
are skilled at predicting the likelihood of an
eruption.
However, it's very difficult to pinpoint
exactly when an eruption will happen.
Often, moving magma doesn't result in an
eruption, but instead cools below the
surface.
Monitoring potential eruptions is expensive.
With many volcanoes erupting only every
few hundred years, it's not possible to
monitor every site.
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Precautions during an eruption
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Learning objectives
What is a volcano?
Why do volcanoes occur?
Where are volcanoes found?
What happened in the 1980 Mount St
Helens eruption?
Can volcanic eruptions be predicted?
Why do people live in volcanic areas?
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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 can be
produced in many volcanic areas.
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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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