Ch 06 - Volcanic Activity

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 06 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 framework, and of volcanic features. Subsequent sections focus on primary and secondary volcanic hazards and on two case studies, Mt. Pinatubo and Mt. St. Helens. The chapter closes with a discussion of volcanic forecasting and of human adjustments to volcanic hazards.

Chapter 06 Outline
I. Introduction to volcanic hazards
A. Fifty to sixty volcanoes erupt each year
1. Several hundred million people live close to volcanoes
2. Almost 100,000 people killed by volcanic eruptions in last 125 years
3. Densely populated countries with many active volcanoes are particularly vulnerable
a. Japan
b. Mexico
c. Philippines
d. Indonesia
II. Volcanism and volcanoes
A. Volcanism is directly related to plate tectonics
1. most active volcanoes located near plate boundaries
2. two thirds of active volcanoes are on the Pacific “ring of fire”
III. Volcano types
A. Style of activity related to viscosity of magma
1. Viscosity: resistance to flow
a. determined by silicon content and temperature
B. Shield volcanoes
1. the largest volcanoes
2. common in Hawaii, Iceland, Indian Ocean islands
3. shaped like a shield
4. lava tends to flow down sides of volcano rather than exploding violently, because of low viscosity
5. common rock type is basalt
6. also produce tephra (pyroclastic debris)
7. lava tubes often move magma underground for many kilometers
8. typically have summit caldera

C. Composite volcanoes
1. known for beautiful cone shape
2. U.S. examples include Mt. St. Helens, Mt. Rainier
3. characterized by magma with intermediate silicon content (andesite)
4. erupt with mixture of pyroclastic activity and lava flows, producing alternating layers of pyroclastic deposits and lava flows
5. composite volcanoes responsible for most of deadly and destructive volcanic hazards
D. Volcanic domes
1. viscous magma (rhyolite) with relatively high silica content
2. activity is mostly explosive
3. Mt. Lassen
E. Cinder cones
1. formed from tephra accumulated near volcanic vent
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
1. example: Hawaiian Islands
C. Composite volcanoes
1. andesitic rocks
2. subduction zones: rising magma mixes with oceanic and continental crust
3. most common volcanoes on Pacific Rim
D. Caldera-forming eruptions
1. extremely violent and explosive
a. typically rhyolitic magma produced when magma moves upward and mixes with continental crust
V. Volcanic features
A. Craters, calderas and vents
1. craters: depressions at top of volcanoes, formed by explosion or collapse of upper portion of volcanic cone
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
a. circular conduits
b. elongate fissures
B. Hot springs and geysers
1. hot spring: groundwater contacts hot water, becomes heated, and discharges at surface
2. geyser: hot groundwater circulation produces steam and hot water
C. Caldera eruptions
1. produce rare but extremely violent eruptions
a. at least 10 caldera eruptions in last 1 million years
b. three in North America
2. large eruption may extrude up to 1000 km3 of pyroclastic debris, mostly ash
3. most recent North American examples
a. Yellowstone, approximately 600 ka ago
b. Long Valley, California, approximately 700 ka ago
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
1. flow speed and characteristics vary
C. Methods to control lava flows
1. bombing
2. hydraulic chilling
3. wall construction
D. Pyroclastic activity
1. ash fall
2. lateral blasts
3. ash flows
E. Poisonous gases
1. several types of gases emitted during volcanic activity
2. toxic concentrations rarely reach populated areas
3. Lake Nyos
4. sulfur dioxide can produce acid rain
5. vog (volcanic smog)
F. Debris flows and mudflows
1. lahar
2. debris flows
3. mudflows
VII. Two case histories
A. Mt. Pinatubo, Philippines, 1991
1. second largest volcanic eruption of the twentieth century
2. 300 people died from ash fall, debris flows, mudflows, and typhoon
3. thousands saved by prediction and good communication
4. ash cloud cooled the atmosphere during the year following the eruption
B. Mt. St. Helens
1. May 18, 1980, eruption exemplifies many types of volcanic events expected from a Cascade volcano
2. awoke in March 1980 after 120 years dormancy
3. bulge grew on north flank
4. M5.1 earthquake triggered large avalanche
5. lateral blast
6. one hour after blast, a large, vertical ash cloud rose to ca. 19 km
7. several mudflows
8. altitude of volcano reduced by 450 m
9. eruption prompted extensive program to monitor volcanic activity
VIII. Forecasting volcanic activity
A. Forecast
1. a probabilistic statement concerning time, place, and character of an eruption
2. forecasting of majority of eruptions is unlikely soon, but progress is being made
B. Seismic activity
1. often provides earliest warning
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
1. used successfully at Kilauea
2. summit tilts and swells before eruption
E. Monitoring volcanic gas emissions
1. primary objective is to recognize changes in geochemical composition
2. relative amounts of steam, carbon dioxide, sulfur dioxide
3. gas emission rates
F. Geologic history
1. useful in predicting types of future eruptions likely to occur
2. includes geologic mapping, dating of lava flows and pyroclastic deposits
G. Volcanic alert or warning
1. at what point should public be alerted or warned?
2. 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 varying degrees
C. Science of volcanoes is becoming well known
1. biggest probable progress in hazard reduction will be in understanding human and societal issues that arise during a volcanic crisis