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Instructor’s Manual
GEOL
Chapter 5
Volcanoes and Volcanism
Chapter 5
Volcanoes and Volcanism
Table of Contents
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Chapter Outline
Learning Outcomes
Chapter Summary
Lecture Suggestions
Enrichment Topics
Common Misconceptions
Consider This
Key Terms
Internet Sites, Videos, Software, and Demonstration Aids
Chapter Outline
Introduction
 Sidebar: Eruptions in the Continental U.S.
LO1 Volcanism and Volcanoes
LO2 What Are the Types of Volcanoes?
LO3 Other Volcanic Landforms
LO4 Distribution of Volcanoes
LO5 Plate Tectonics, Volcanoes, and Plutons
LO6 Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions
Learning Outcomes
After reading this unit, the students should be able to do the following:
LO1 Understand volcanism and volcanoes
LO2 Identify the types of volcanoes
LO3 Identify other volcanic landforms
LO4 Identify the distribution of volcanoes
LO5 Understand the relationship between plate tectonics, volcanoes, and plutons
LO6 Understand volcanic hazards, volcano monitoring, and forecasting eruptions
Chapter Summary
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Volcanism encompasses those processes by which magma rises to the surface as lava
flows and pyroclastic materials and gases are released into the atmosphere.
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Gases make up only a few percent by weight of magma. Most is water vapor, but sulfur
gases may have far-reaching climatic effects.
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
Aa lava flows have surfaces of jagged, angular blocks, whereas the surfaces of pahoehoe
flows are smoothly wrinkled.
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Several other features of lava flows are spatter cones, pressure ridges, lava tubes, and
columnar joints. Lava that erupts under water typically forms bulbous masses known as
pillow lava.
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Volcanoes are found in various shapes and sizes, but all form where lava and pyroclastic
materials are erupted from a vent.
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The summits of volcanoes have either a crater or a much larger caldera. Most calderas
form following voluminous eruptions, and the volcano peak collapses into a partially
drained magma chamber.
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Chapter 5
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
Shield volcanoes have low, rounded profiles and are composed mostly of mafic flows
that cool and form basalt. Small, steep-sided cinder cones form around a vent where
pyroclastic materials erupt and accumulate. Composite volcanoes are made of lava flows
and pyroclastic materials of intermediate composition and volcanic mudflows.
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Viscous, bulbous masses of lava, generally of felsic composition form lava domes, are
dangerous because they erupt explosively.
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Fluid mafic lava from fissure eruptions spreads over large areas to form a basalt plateau.
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Pyroclastic sheet deposits result from huge eruptions of ash and other pyroclastic
materials, particularly when calderas form.
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Geologists have devised a volcanic explosivity index (VEI) to give semiquantitative
measure of the size of an eruption. Volume of material erupted and the height of the
eruption plume are criteria used to determine the VEI. Fatalities and property damage are
not considered.
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Approximately 80% of all active volcanoes are found in the circum-Pacific belt and the
Mediterranean belt, mostly at convergent plate boundaries. Most of the rest of the
eruptions occur along mid-oceanic ridges or their extensions onto land.

The two active volcanoes on the island of Hawaii and the one just to the south lie above a
hot spot over which the Pacific plate moves.
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To effectively monitor volcanoes, geologists evaluate several physical and chemical
aspects of volcanic regions. Of particular importance in monitoring volcanoes and
forecasting eruptions is detecting volcanic tremor and determining the eruptive history of
a volcano.
Lecture Suggestions
1. To demonstrate the concept of viscosity, it is helpful to show students the rates at which
different fluids flow down an inclined surface. You can do this by pouring a small
amount of each of several types of fluid onto a piece of plywood or similar flat surface.
Incline the plywood toward the class, so the fluids flow downward toward them.
Vegetable oil, honey, molasses, and oatmeal provide a good range of flow rates.
2. Differential viscosity can also be illustrated in this manner. Place three small samples of
honey or molasses in separate beakers. Put one sample in a bath of ice water and another
in a bath of hot water before class, and let them sit until they warm or cool significantly.
By pouring the three samples simultaneously, you can show the students how
temperature has a significant effect on the viscosity of a given substance. They may
remember this better if you remind them of the old phrase, “as slow as molasses in
January.”
3. Is the inside of Earth entirely molten? Aside from an argument based on the distribution
of active volcanoes (see Common Misconceptions), try this simple demonstration. Bring
two identical looking eggs to class, one of them hard-boiled and the other uncooked.
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During the discussion of magma sources, spin the hard-boiled egg on its long axis. Then,
you may invite two students to come forward, give each of them one of the eggs, and ask
each to spin his/her egg (not too close to the table edge). One student (with the cooked
egg) should be able to do this, without much trouble. The other student will find it
impossible. Some of the students by now may have guessed at the difference between the
eggs, or you can elicit the answer. You can then point out the analogy with Earth, which
spins very well (with a little wobble) on its axis.
4. Don’t forget the comparison of dissolved gases in magma coming out of solution as the
magma rises to the surface (and pressure is released) to what happens when a bottle of
club soda is opened. You can even bring a bottle to class, shake it up, and while pointing
it at the class, ask what will happen if you open the top. Even when you don’t go through
with it, they will get the point. It’s more fun when you “erupt” the bottle, though.
Enrichment Topics
Topic 1. Volcanism and Pollutants. In the years it’s erupting, the biggest single source of
air pollution in Washington State is Mount St. Helens, putting out between 50 and 250 tons of
sulfur dioxide a day. SO2 is a component of smog and also a contributor to acid rain. With
their pollution control devices, all of the state’s industries combined produce about 120 tons a
day. Mount Etna in Italy can produce 100 times more SO2 than Mount St. Helens. In 1991,
Mount Pinatubo’s enormous eruption in the Philippines spread haze and lowered average
surface temperatures throughout the Northern Hemisphere. In all though, SO2 emissions from
volcanoes account for only about 15 million tons a year, while emissions from human
activities account for about 200 million tons. Volcanoes also produce the noxious gases
hydrogen sulfide and hydrogen chloride. Carbon dioxide, a normal component of the
atmosphere, is a greenhouse gas and contributes to rising global temperatures. Volcanoes also
emit CO2. Mount St. Helens emits between 500 and 1,000 tons a day, and Mount Etna emits
around 25 million tons. “Mount St. Helens Volcano is Washington State’s No. 1 Air
Polluter,” The Seattle Times, 2004.
Topic 2. Undersea Eruption. An undersea eruption was caught on camera, producing 22
million cubic meters of new lava along the East Pacific Rise, one of the mid-ocean ridges.
The lava buried seismometers that were placed along the ridge to monitor earthquake activity.
Images show that the eruption was right along the ridge crest, just as predictions would
suggest new oceanic crust forms. http://dsc.discovery.com/news/2007/12/04/underseavolcano.html
Topic 3. How Large Was That Eruption? The volcanic explosivity index (VEI) combines
volume of material erupted, eruption cloud height, and other qualitative information to arrive
at a measure of the explosive intensity of an eruption. This is an open-ended scale, which
ranges at present from 0 to 7 for the largest historical eruption. The estimate of tephra
(pyroclastic material) volume produced is the most important of the contributing determinants
of the VEI for an eruption. View Figure 5.18 in your text, or view this image:
http://volcanoes.usgs.gov/images/pglossary/vei.php. Note that the most powerful eruptions
ever were long before people were around to experience them. What do you think would be
the result of an eruption as large as Yellowstone if it happened today?
Topic 4. Plateau Basalts and Mass Extinctions. The Deccan Traps, a region of plateau
basalts that erupted about 65 million years ago, may be the cause of the extinction of the
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dinosaurs and other contemporaneous late Cretaceous fauna, rather than the favored asteroid
impact hypothesis. The argument is that the climatic effects of such large scale eruptions
would be sufficient to cause extinctions. http://www.geosociety.org/news/pr/07-59.htm;
http://geoweb.princeton.edu/people/keller/deccan/deccan.html
Topic 5. Volcanism and Antarctic Ice Loss. Climate change may cause the West Antarctic
ice sheet to melt, which will raise global sea level. The recent discovery of evidence of a
volcanic eruption taking place beneath the ice is a big concern, since a volcanic eruption
beneath the ice will cause melting from the bottom as well as melting from warmer
temperatures at the top. Add to that the possibility that melting beneath the ice sheet can
cause water to lubricate the location where the ice meets the underlying rock and cause the
ice sheet to slip into the sea faster.
http://sciencenow.sciencemag.org/cgi/content/full/2008/122/3
Common Misconceptions
Misconception: Below the surface, the inside of Earth is entirely molten. Volcanoes are
places where this material “squirts out”.
Fact: Below the surface, Earth is solid—although the asthenosphere does behave like a
plastic—to a depth of about 1,800 miles. For now, it may be enough to note the distribution
of active and recently active volcanoes. These are in belts, or occasional isolated spots, but
are by no means randomly distributed, as one might expect if there were molten material
everywhere beneath the surface. See also Lecture Suggestion 3.
Misconception: Volcanic eruptions are responsible for global warming.
Fact: Some people who do not accept that human activities are causing the rise in greenhouse
gases that is causing global warming suggest the alternative that volcanoes emit more
greenhouse gases during eruptions and so are responsible for the bulk of the warming. In
truth, the amount emitted by volcanoes is relatively tiny. The annual contribution of CO2 to
the atmosphere by volcanoes is 200 million tons, less than one percent of the nearly 30 billion
tons emitted by human activities. This human enhanced greenhouse effect may cause an
average annual global warming of 3 to 5oC over the next 50 years.
Consider This
1. Why are felsic magmas more viscous than mafic magmas?
2. Where are the most hot spots found? Are they found everywhere on oceanic plates? Are
they found on continental plates? How does a mantle plume penetrate the thickness of the
continental lithosphere to create an eruption?
3. Do volcanic eruptions vary in frequency on a human timescale? Are there certain years or
decades in which eruptions are more common?
4. What different kinds of hazards can people living near volcanoes expect? Why do people
live on and near known or potentially active volcanoes? What are some benefits that
volcanoes may provide?
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Key Terms
aa
basalt plateau
caldera
Cascade Range
cinder cone
circum-Pacific belt
columnar jointing
composite volcano (stratovolcano)
crater
fissure eruption
lahar
lava dome
lava tube
Mediterranean belt
nuée ardente
pahoehoe
pillow lava
pyroclastic sheet deposit
shield volcano
volcanic ash
volcanic explosivity index (VEI)
volcanic tremor
volcanism
volcano
Internet Sites, Videos, Software, and Demonstration Aids
Internet Sites
1. Global Volcanism Program. Smithsonian Institution: http://www.volcano.si.edu/
This website looks at Holocene volcanoes and eruptions. You can also use this as a
gateway to Google Earth’s volcano map.
2. Deadly Shadow of Vesuvius. NOVA Online, PBS:
http://www.pbs.org/wgbh/nova/vesuvius/
The story of the ancient Roman city of Pompeii, which was destroyed by the eruption of
Mount Vesuvius, and the risk the people of Naples face today.
3. Hawaii Born of Fire. NOVA Online, PBS: http://www.pbs.org/wgbh/nova/hawaii/
A look at the Hawaiian Islands, from molten rock to verdant landscape.
4. Violent Hawaii. Nature, PBS Video: http://www.pbs.org/wnet/nature/episodes/violenthawaii/video-full-episode/1422/
The Hawaiian Islands create a chain that stretches 1,500 miles across the Pacific Ocean,
where volcanoes and living creatures thrive.
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5. Volcanoes, United States Geological Survey: http://www.usgs.gov/themes/volcano.html
The USGS looks at volcanoes as natural hazards, which can bring risks to life, property,
and infrastructure.
6. Volcano World, Oregon State: http://volcano.oregonstate.edu/
Hot topics in volcanology with an archive that covers many topics.
7. Cascades Volcano Observatory: http://vulcan.wr.usgs.gov/home.html
Mount St. Helens and other volcanoes of the Pacific Northwest, including current
activity.
8. Volcano Video Productions: http://www.volcanovideo.com/
Video clips and DVDs for sale of volcanic features and eruptions, focusing on Kilauea in
Hawaii. Includes eruption updates; Kilauea: Close-up of an Active Volcano; Lava Flows
and Lava Tubes: What They Are and How They Form; Eruption at Sea and others.
9. NASA, Jet Propulsion Laboratory, California Institute of Technology:
http://www.jpl.nasa.gov/video/index.cfm?id=858
Space Age Volcano Monitoring Network: If Mount St. Helens erupts again, a team of
scientists and engineers will be monitoring with space age technology from NASA.
Videos
1. Volcanoes of the Deep Sea. IMAX (2003, 40 min.)
Volcanoes at spreading centers 12,000 feet below the ocean’s surface with dramatic
landscapes and strange creatures.
2. Volcano! Nature’s Inferno. National Geographic, DVD (1990)
Learn about volcanoes as you travel around the world and join volcanologists in their
efforts to forecast eruptions.
3. Volcano under the City. NOVA, PBS, DVD (2005, 60 min.)
An active volcano in central Africa is the study site for a team of scientists.
4. Kilauea: Mountain of Fire. Nature, PBS, DVD (can be viewed online)
The world’s most active volcano has had eruptions continuously since 1983: Kilauea, on
Hawaii’s big island.
5. In the Path of a Killer Volcano. NOVA, PBS, DVD (1993, 60 min.)
The massive evacuation that was prompted by the imminent eruption of Mount Pinatubo
in the Philippines.
6. Mystery of the Megavolcano. NOVA, PBS (2006, 56 min.)
Evidence of the greatest volcanic eruption of the last 100,000 years is found in a remote
lake in Southeast Asia.
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7. Supervolcano: It’s Under Yellowstone and It’s Overdue. Discovery Communications
(105 min.)
Yellowstone National Park, with its amazing geysers and wildlife, is above an enormous
mantle plume that could erupt at any time.
8. The Earth Revealed. Annenberg/CPB Collection.
 # 13: Volcanism. What volcanoes tell scientists about the Earth’s interior, the
processes that create them, and the various types of volcanic features and phenomena
are explored.
 # 25: Living with Earth, Part I, Destructive Natural Phenomena.
9. Everything You Need to Know: Volcanoes. Insight Media (2007, 56 min.).
An introduction to volcanoes and the parts of an eruption.
10. Forces That Shape the Earth. Insight Media (2000, 29 min.)
Forces that form and reform rocks and landforms on Earth.
11. Volcanism. Insight Media (1999, 15 min.)
The formation of volcanoes and the importance of volcanic activity to the Earth.
12. Volcanoes. Insight Media (2004, 15 min.)
Introduction to basaltic and andesitic volcanism.
13. Volcanoes of the United States. Insight Media (2004, 24 min.)
Ancient and current volcanoes of the U.S. explored.
Slides
1. Geologic Hazards Slide Sets: Volcanoes. National Geophysical Data Center, 8 sets
available.
2. Geothermal Activity and Energy. Educational Images, Ltd.
3. Volcanism. GeoPhoto Publishing, digital images.
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