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Volcanoes
What is a volcano?
• A volcano is a
mountain that
forms when molten
rock erupts or
flows as lava from
an opening in
Earth’s surface.
How do volcanoes form?
• A volcano forms when molten rock erupts or
flows as lava from an opening in earth’s surface
and builds up a volcanic cone.
• These openings are called vents.
• Volcanoes release molten rock, ash, and
poisonous gases. All these products result from
melting in the mantle or in the crust.
Where do they occur?
• Like earthquakes, most volcanoes occur at
plate boundaries.
• Along a mid-ocean rift, where two plates
move apart, divergent boundary, a ridge of
underwater volcanoes is formed—
Iceland’s volcanoes are part of this kind of
ridge.
• At convergent boundaries, where an
oceanic plate collides with a continental
plate—the Andes Mountains in South
America.
Ring
of Fire
• Not all of Earth’s volcanoes form at the edges
of the plates.
• Sometimes a volcano forms when a narrow
column of hot molten rock breaks through the
lithosphere in the middle of a plate. This
narrow column of magma is called a hot spot.
• A hot spot does not move. However, the plate
above it moves forming a chain of volcanoes.
• The island farthest from the hot spot is the
oldest.
• The Pacific Plate is moving over the hot spot,
which remains stationary.
• As the plate moves, the first volcano formed
dies out and a second volcano forms over the
hot spot, at the new location on the moving
plate.
• This process continues as the plate continues
to move over the hot spot.
The Hawaiian Islands were
formed by a hot spot under the
Pacific Plate.
Today, only the island of Hawaii,
located directly over the hot spot,
has active volcanoes.
Types of Volcanoes
• The type of eruption
determines the type of
volcano.
• Runny lava spreads far
from the vent, building up
a gentle slope.
• Thicker lava erupts more
violently and produces
more ashes and cinders
that pile up to form a
steep cone-shape.
Types of lava:
1. Pahoehoe: hot, thin, flowing,
7512play
and forms ropes
2. Aa: cooler, much thicker,
slow, forms jagged, sharp,
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blocks.
How magma affects a
volcanic eruption.
• There are three main factors that
determine whether a volcano extrudes
magma violently or quietly.
• The factors are:
– magma’s composition—silica content
– temperature
– amount of dissolved gases it contains.
Quiet Eruption
In general, the hot , basaltic magmas flow
easily and gently. It is dark-colored and
contains a lot of water. This is rich in iron
and magnesium and cools to form igneous
rocks such as basalt. This lava is thin and
runny and most tends to flow. The islands
of Hawaii and Iceland were formed by
many lava flows.
Explosive Eruption
• Thicker, cooler magmas that contain more
silica are more difficult to force through the
vent. They may even plug up the vent,
causing gases to collect in bubbles and
pockets that increase in size and pressure.
Eventually that increasing pressure ejects
the molten rock from the volcano in a
violent explosion. Ex. Mt. St. Helens 1980
Composite Volcanoes
Composite volcanoes are
made up of alternating layers
of ash, cinders, and lava.
Composite volcanoes switch
between quiet eruptions of flowing
lava and violent eruptions of thick
gas-rich lava.
This type of volcano has the most
powerful eruptions of all.
Their magma is rich in silica
and therefore thick. Gases
are trapped in the magma,
causing eruptions that
alternate between flows and
explosive activity that
produces cinders and ash.
Composite volcanoes are
typically thousands of meters
high, with steep slopes.
When Mount St. Helens
erupted on 18 May
1980,the top
1,300 ft. disappeared
within minutes.
Mount St. Helens is a stratovolcano.
. Image taken on 10/16/94 from the
Space Shuttle.
The blast area covered an
area of more than 150 sq.
miles and sent thousands
of tons of ash into the
upper atmosphere
Mt. St. Helens
• Mt. St. Helens before 5/18/80
• Mt. St. Helens - after
5/18/80
Shield Volcanoes
• Shield Volcanoes look like a warrior’s shield. These broad, slightly
dome-shaped volcanoes are the world’s largest.
• Volcanoes with broad, gentle slopes and built by eruption of fluid
basalt lava are called shield volcanoes.
• Basalt lava tends to build enormous, low angle cones because it
flows across the ground easily.
• The largest volcanoes on Earth are Shield Volcanoes.
Cinder Cones
• A cinder cone volcano is
built up from ashes,
cinders, and rocks that
burst from Earth during
violent eruptions.
• The rocks fall back to
Earth near the opening.
• At the same time, hot
ash covers a larger
surrounding area.
• Cinder cones erupt for a
very short amount of
time.
Most cinder cones have a bowlshaped crater at the summit and
rarely rise more than a thousand
feet or so above their surroundings.
Volcanic Activity
• Volcanoes are rather
unpredictable . Some
erupt regularly, others
have not erupted in
modern history.
Scientists classify
them as active,
dormant or extinct.
Active
Volcanoes
• An active volcano is one that erupts wither
continually or periodically such as Mount
Katmai in Alaska and Mount St. Helens in
the Cascade Range.
Dormant Volcano
A volcano that has been known to erupt
within modern times but is now inactive is
classified as a dormant volcano. Mount
Rainier in Washington state are example
of dormant volcanoes in the United States.
Extinct Volcano
A volcano not known to have erupted
within modern history is classified as an
extinct volcano. They have been worn
away almost to the level of their magma
chamber. Scientists can be wrong. Mount
St. Helens was considered to be dormant
but erupted after long periods of inactivity.
Predicting Eruptions
different from predicting a flood or
an earthquake in that volcanoes
usually shows signs of
"awakening" before they erupt,
giving geologists time to monitor
the precursory events.
Predicting Eruptions
• Geologists use sensitive instruments to monitor
an active volcano to determine if magma is
approaching the surface.
Signs that a volcano is about to erupt include:
• increasing temperature of hot springs near
the volcano,
• changes in the shape of the volcano or
surrounding land,
• an increase in hot gas and ash from the vent,
• and small earthquakes around the volcano.