Learning Objectives
After reading and studying this chapter, students should:
• Know the major types of volcanoes, the rocks they produce, and their plate tectonic setting.
• Understand the main types and effects of volcanic activity.
• Understand the methods of studying volcanic activity, in order to better predict volcanic
eruptions and minimize the hazard.
Chapter Summary
This chapter is an extensive treatment of volcanoes, volcanic processes, and volcanic hazards. The
chapter begins with a discussion of volcano types, of the origins of volcanoes within the plate tectonic
Chapter Outline
I. Introduction to volcanic hazards
A. 50 to 60 volcanoes erupt each year
1. Several hundred million people live close to volcanoes
a. as human population grows, more people live closer to volcanoes
2. Almost 100,000 people were killed by volcanic eruptions in last 100 years
a. nearly 23,000 people were killed between 1980 and 2000
3. Densely populated countries with many active volcanoes are particularly vulnerable
a. Japan
b. Mexico
II. Volcanism and volcanoes
A. Volcanism is directly related to plate tectonics
2. Two thirds of active volcanoes are on the Pacific “ring of fire”
3. (see A closer Look: How magma forms)
III. Volcano types
A. Style of activity related to viscosity of magma
1. Viscosity: resistance to flow
B. Shield volcanoes
2. Common in Hawaii, Iceland, Indian Ocean islands
4. Lava tends to flow down sides of volcano rather than exploding violently, because of low
viscosity
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Volcanic Activity
6. Also produce tephra (pyroclastic debris)
8. May have summit caldera
C. Composite volcanoes
2. United States examples include Mt. St. Helens, Mt. Rainier
4. Erupt with mixture of pyroclastic (explosive) activity and lava flows, producing alternating
layers of pyroclastic deposits and lava flows
D. Volcanic domes
1. Viscous magma (rhyolite) with relatively high silica content
3. Mt. Lassen
E. Cinder cones
2. Paricutin, Central Mexico, 1943
IV. Volcano origins
A. Mid-ocean ridge volcanism produces basalt
1. Wells up directly from asthenosphere
B. Shield volcanoes form above hot spots
C. Composite volcanoes
1. Andesitic rocks
3. Most common volcanoes on Pacific Rim
D. Caldera-forming eruptions
1. Extremely violent and explosive
V. Volcanic features
A. Craters, calderas, and vents
2. Calderas: gigantic, circular depressions resulting from explosive ejection of magma followed by
collapse of the volcanic cone
3. Volcanic vents: openings through which lava and pyroclastic debris are erupted at the surface of
the Earth
B. Hot springs and geysers
2. Geyser: hot groundwater circulation produces steam and hot water
C. Caldera eruptions
1. Produce rare but extremely violent eruptions
3. Most recent North American examples
4. Main events in caldera formation can occur quickly (days to weeks), but lesser-magnitude
activity can linger for a million years
VI. Volcanic hazards
A. Primary vs. secondary effects
B. Lava flows
C. Methods to control lava flows
1. Mixed results
3. Wall construction
D. Pyroclastic activity
2. Ash fall
4. Ash flows
E. Poisonous gases
2. Toxic concentrations rarely reach populated areas
4. Sulfur dioxide can produce acid rain
5. Vog (volcanic smog)
F. Debris flows and mudflows
1. Lahar
3. Mudflows
VII. Two case histories
A. Mt. Pinatubo, Philippines, 1991
1. Second largest volcanic eruption of the 20th century
3. Thousands saved by prediction and good communication
4. Ash cloud cooled the atmosphere during the year following the eruption
5. Contrast with Nevado del Ruiz, 1985
B. Mt. St. Helens
1. May 18, 1980, eruption exemplifies many types of volcanic events expected from a Cascade
volcano
3. Bulge grew on north flank
5. Lateral blast
7. Several mudflows
9. Eruption prompted extensive program to monitor volcanic activity
VIII. Forecasting volcanic activity
Chapter 8 – Instructor’s Manual
A. Forecast
1. A probabilistic statement concerning time, place, and character of an eruption
3. Forecasting of majority of eruptions is unlikely soon, but progress is being made
B. Seismic activity
2. Caused by moving magma
C. Thermal, magnetic, and hydrologic monitoring
1. Hot magma in reservoir beneath volcano changes local magnetic, thermal, hydrologic, and
geochemical conditions
D. Topographic monitoring
2. Summit tilts and swells before eruption
E. Monitoring volcanic gas emissions
2. Relative amounts of steam, carbon dioxide, sulfur dioxide
3. Gas emission rates
F. Geologic history
G. Volcanic alert or warning
1. At what point should public be alerted or warned?
2. U.S. Geological Survey (USGS) warning system coded by color, denoting increasing concern
IX. Adjustment to and perception of the volcanic hazard
A. Primary human adjustment is evacuation
B. People live near volcanoes for a variety of reasons, and may understand volcanic hazards to
Answers to Review Questions and Critical Thinking Questions
Review Questions
1. The type of volcano determines the hazards that are likely to occur.
Volcanic Activity
10. Primary effects: lava flows, pyroclastic activity, and gas release; secondary effects: lahars, floods,
fires, global cooling.
11. Lava flow control methods have included bombing, hydraulic cooling, and wall construction.
Critical Thinking Questions
1. An answer to this question should focus on the origins, settings, and processes associated with
2. The public relations program should focus on the potential hazards associated with the volcano, the