Volcanoes and Other Mountains
As a scientifically literate citizen, what 3 questions would you
ask about this volcano if you moved to the city in the
foreground (Tacoma, Washington)?
The Good Earth/Chapter 6: Volcanoes and Other Mountains
The Volcano Commandos
• 1,500 active
volcanoes
worldwide
− a third have
records of
previous
eruptions
− 500 million people live near active volcanoes
− Fewer than 200 volcanoes have instruments
to assess potential for eruption
− 2 or 3
eruptions per
decade are
major
disasters
The Good Earth/Chapter 6: Volcanoes and Other Mountains
What exactly is a Volcano?
Location where
molten rock and
other mantle
materials, are
released, to the
surface.
Parts of the Volcano
• Magma:
– Molten rock
– Less dense than crust
• Lava:
– Magma that has
reached the surface
and erupted out of the
volcano
Crater
Pipes
Lava
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Parts of the Volcano
• Magma Chamber
– Underground pools of magma
• Pipes
– Pathways for magma
• Vents
– Area where magma reaches the surface
• Crater
– Depression at the top of a volcanic form after an
eruption
2 Types of volcanoes
1. Shield
Made from fast flowing basaltic lava
(Basaltic: A hard, dense, dark volcanic
rock)
Large and flat due to movement of the lava
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Hawaiian Islands formed this way
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http://www.history.com/shows/how-the-earth-was-made/videos/how-washawaii-formed - how-was-hawaii-formed
Volcano types
2. Strato-volcanoes
majority of the
worlds volcanoes
are these
most explosive
and destructive
Associated with
subduction
zones
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Where do they occur?
• All boundaries except transform
Famous Volcanoes / areas of volcanoes
The Mount St. Helens Eruption
• Cascade Mountains – volcanic
arc in Pacific Northwest
− Major cities within 100 km of
active volcanoes
− Mount St. Helens eruption of
May 18, 1980
The Good Earth/Chapter 6: Volcanoes and Other Mountains
The Mount St. Helens Eruption
• Cascade Mountains
− Volcanoes formed above
subduction zone where Juan
de Fuca plate slides beneath
North America
− Mount St. Helens is most active
volcano in conterminous US
The Good Earth/Chapter 6: Volcanoes and Other Mountains
The Mount St. Helens Eruption
Prior Activity
• Early (March) unrest featured
− Minor eruptions
− Earthquakes
− Release of volcanic gases
• Followed by change in shape
of cone (bulge on North
flank)
• Increasing frequency of
earthquakes
The Good Earth/Chapter 6: Volcanoes and Other Mountains
The Mount St. Helens Eruption
May 18 Eruption
• A moderate earthquake
triggered a massive
landslide (debris avalanche)
on the North side of the
volcano
− Debris clogged streams
− Pressure released on
near-surface magma
− Lateral blast produces an
initial sideways eruption to
North
− Later vertical eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Eruption of Mount St. Helens reduced height
of volcano by 400 meters
• Features near volcano were blown over or
carried away by products of eruption
Geologist David Johnston (right) died at this site (Johnston’s
Ridge) located 10 km from the volcano. (this is 6.25 miles or from
here to the pro-football hall of fame.
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Major products of volcanic eruptions:
• Airborne – lateral blast, tephra, volcanic gases
• Flows on land – lava, pyroclastic flows, lahars
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Airborne Eruption Products
• Rare lateral blasts
can destroy objects
up to 12 km away
and knock down
trees more than 25
km distant
− Effect of lateral
blast only seen on
North flank of
Mount St. Helens
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Airborne Eruption Products
• Tephra represents particles blasted into air by eruption
− Volcanic bombs and ash are found near and far from eruption
source, respectively
Blobs of magma
solidify to form lava
bombs
Wind can transport fine
volcanic ash for hundreds
of kilometers downwind
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Airborne Eruption Products
• Volcanic gases (water
vapor, sulfur dioxide,
carbon dioxide) may
affect climate patterns
− Sulfur dioxide may
block insolation,
temporarily (up to 1
year) reducing global
temperatures
Trees killed by excessive carbon dioxide released by
magma under Mammoth Mountain, California.
− Widespread release of
carbon dioxide and
higher temperatures
due to faster rates of
volcanic activity
approximately 120-80
million years ago
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Eruption Products on Land
• Low viscosity lava can
flow up to 50 km from its
source
− Lava transported to
front of lava flows in
long lava tubes
− Lava flows build up in
a series of layers
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Eruption Products on Land
• Low viscosity lava can
flow up to 50 km from its
source
− Lava transported to
front of lava flows in
long lava tubes
− Lava flows build up in
a series of layers
Walter’s Kalapana store, Hawaii,
was buried in lava within a few
weeks in 1990
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Eruption Products on Land
• Higher viscosity lava remains
within volcano crater
− Lava dome formed in crater of
Mount St. Helens
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Eruption Products on Land
• Pyroclastic flow – dense
cloud formed from combination
of tephra and volcanic gases
− Fast moving, up to 700 C
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Products of Volcanic Eruptions
Eruption Products on Land
• Lahars – mudflows formed when
volcanic debris mixes with streams
or melting ice
− Often confined to stream channels
Lahar along Muddy
River reached
depths of 20 meters
following Mount St.
Helens eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Ring of fire
area surrounding the pacific plate
with VERY high activity of Volcanoes
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