VOLCANOES AND PLATE TECTONICS
Lava Video
VOLCANOES, MAGMA, AND LAVA
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Volcanoes are weak spots
in the crusts where
molten material, or
magma, comes to the
surface.
When magma reaches the
surface it is called lava.
The lava released during
volcanic activity builds up
Earth’s surface, it is a
constructive force.
LOCATION OF VOLCANOES
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There are about 600 active
volcanoes on land, many more
lie beneath the sea.
Most volcanoes occur along
volcanic belts which lie along
plate boundaries.
Most volcanoes occur along
diverging plate boundaries, such
as the mid-oceanic ridge.
The biggest volcanic belt is the
Ring of Fire that lies along the
edges of the Pacific Ocean.
ISLAND ARC’S
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Many volcanoes occur on
islands, near plate boundaries
where two oceanic plates
collide.
When the magma breaks
through the ocean floor it
creates volcanoes.
The resulting volcanoes create
a string of islands called island
arcs.
Examples of island arcs are
Japan, New Zealand,
Indonesia, and the Caribbean
Islands.
Birth of a Volcano
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HOT SPOT VOLCANOES
Some volcanoes result from hot
spots in Earth’s mantle.
A hot spot is an area where
magma from deep within the
mantle melts through the crust
like a blow torch.
Hot spots often lie in the middle
of plates far from any plate
boundaries.
A hot spot volcano in the ocean
floor can gradually form a series
of volcanic mountains. (HAWAII)
Hot spots can also form under the
continents. (YELLOW STONE
NATIONAL PARK) Last volcanic
eruption there occurred 75,000
year ago.
Under Water Hot Spots
WHERE DOES LAVA COME FROM
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Lava begins as magma in
the earth’s mantle.
Because magma is less
dense than the surrounding
solid material, magma flows
upward into any cracks in
the rock above.
Magma rises until it reaches
the surface, or until it
becomes trapped beneath
layers of rock.
All materials go from areas
of high pressure to areas of
low pressure. Magma
reacts in the same way.
VOLCANIC ERUPTION
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During a volcanic
eruption, the gases
dissolved in magma
rush out, carrying the
magma with them.
Picture to the right is
Mount St. Helens.
INSIDE A VOLCANO
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Beneath a volcano magma
collects in a pocket called a
Magma Chamber.
The magma moves through a
Pipe, a long tube in the ground
that connects the magma
chamber to the Earth’s surface.
The volcanoes Vent is the point
on the surface where magma
leaves the volcanoes pipe.
Lava can collect in the Crater
the bowl shaped area that forms
around the volcanoes vent.
LAVA CONSISTENCY
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Some types of magma are thick
and flow very slowly. Other
types of magma are fluid and
flow almost as easily as water.
The hotter the magma, the more
fluid it is.
The amount of silica in magma
also helps determine how easily
magma flows. Silica is a
material formed the from the
elements of oxygen and silicon.
The more silica magma contains
the thicker it is.
Obsidian is high in silica
QUIET ERUPTIONS
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A volcano erupts quietly if
its magma flows easily.
Thin, runny lava oozes
quietly from the vent.
Quiet eruptions produce two
different types of lava
known as “pahoehoe” which
is lava that flows like a solid
mass of wrinkles, billows,
and chunks.
The second type of lava is
called “aa.” Which
translated means “painful
to walk on.”
EXPLOSIVE ERUPTIONS
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If a volcanoes magma is
thick and sticky, a volcano
erupts explosively.
The thick magma does not
flow out of the crater and
down the mountain.
Instead it slowly builds up in
the volcano’s pipe, like
putting your finger over a
garden hose.
Dissolved gases cannot
escape from the thick
magma. The trapped gases
build up pressure until they
explode.
PYROCLASTIC FLOW
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Explosive volcanoes create
volcanic ash, cinders, and
large pieces of lava called
bombs.
These bombs can be as big as
cars.
Pyroclastic flows occur when
an explosive eruption hurls
out ash, cinders, and bombs.
Pyroclastic flows are quick
they can travel between 100
and 200 mph. You will be
cooked if you are around one
of these.
Pyroclastic Flow Video
VOLCANIC DAMAGES
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During a quiet eruption, lava
flows pour from vents,
setting fire to and then
burying everything in their
path.
During an explosive
eruption, a volcano can
throw out hot, burning
clouds of volcanic gases as
well as cinders and bombs.
Volcanic ash can bury entire
towns, damage crops, and
collapse the roofs of homes.
STAGES OF A VOLCANOES LIFE
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The activity of a volcano may
last from less than a decade to
more than 10 million years.
An active, or live, volcano is one
that is erupting or has shown
signs that it may erupt in the
near future.
A dormant, or sleeping, volcano
may be awaken in the future
and become active. (Mt.
Taranaki-New Zealand Pictured
to the right)
An extinct, or dead, volcano is
unlikely to erupt again.
Dating Lava Flows
HOT SPRINGS
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Hot springs and geysers are
two examples of volcanic
activity that do not involve
the eruption of lava.
These features may occur in
any volcanic area-even
around an extinct volcano.
A hot spring forms when
groundwater heated by a
nearby body of magma rises
to the surface and collects
into a natural pool.
Japanese Macaque is
relaxing in a hot spring to
the right in a cold winter day
GEYSERS
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Sometimes, rising hot
water and steam
become trapped
underground in a
narrow crack. Pressure
builds until the mixture
suddenly sprays above
the surface as a geyser.
A geyser is a fountain of
water and steam that
erupts from the ground.
LANDFORMS FROM LAVA AND ASH
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Rock and other
materials formed from
lava create a variety of
landforms including
shield volcanoes,
composite volcanoes,
cinder cone volcanoes,
calderas and lava
plateaus.
SHIELD VOLCANOES
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At some places on
Earth’s surface, thick
layers of lava pour out
of a vent and harden on
top of previous layers.
Such lava flows
gradually build a wide,
gently sloping mountain
called a shield volcano.
CINDER CONE VOLCANOES
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Cinder Cone Volcanoes
are steep, cone shaped
hills or mountains.
If a volcanoes lava is
thick and stiff, it may
produce ash, cinders,
and bombs.
These materials pile up
around the vent in a
steep, cone shaped pile.
COMPOSITE VOLCANOES
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Sometimes, lava flows
alternate with explosive
eruptions of ash, cinder,
and bombs.
This creates a Composite
Volcano which are
generally tall, cone
shaped mountains in
which layers of lava
alternate with layers of
ash.
Example is Mount Fuji or
Mount St. Helens.
CALDERAS
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Enormous eruptions may
empty the main vent and
the magma chamber
beneath the volcano.
The mountain becomes a
hollow shell with nothing
to support it. This hollow
shell is known as a
Caldera.
Crater Lake is pictured to
the right
LAVA PLATEAUS
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Instead of forming
mountains, some
eruptions of lava form
high, level areas called
Lava Plateaus.
This is usually caused
by thin, runny lava
traveling far before
cooling and solidifying.
PREDICTING VOLCANIC ERUPTIONS
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Before an eruption, magma
moves into the area
beneath the volcano and
collects in a magma
chamber, or reservoir. As it
comes closer to the surface,
the magma releases gases.
These events can offer
valuable clues about the
likelihood of an eruption.
For example, the movement
of magma produces small
earthquakes and vibrations
(seismicity).
PREDICTING VOLCANIC ERUPTIONS
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Magma gathering in a chamber
causes slight swelling of the
volcano's slopes. Gases released
near the volcano can be
measured for changes in quantity
and makeup.
A number of tools can be used to
record these warning signs.
Seismographs can detect small
earthquakes, while tiltmeters and
geodimeters can measure the
subtle swelling of a volcano.
Correlation spectrometers
(COSPECS) can measure amounts
of sulfur dioxide--a telltale gas
that is released in increasing
quantities before an eruption.
VOLCANO VIDEOS
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Lava Sampling Video in
Hawaii
Dating Lava Flows in
Hawaii
Mount Pinatubo
Prediction
Mount Pinatubo
Aftermath