5 RISK ASSESSMENT: PREDICTING VOLCANIC ERUPTIONS

䊳 5.5 RISK ASSESSMENT: PREDICTING VOLCANIC ERUPTIONS

Approximately 1300 active volcanoes are recognized globally, and 5564 eruptions have occurred in the past 10,000 years. Many volcanoes have erupted recently, and we are certain that others will erupt soon. How can ge- ologists predict an eruption and reduce the risk of a vol- canic disaster?

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REGIONAL PREDICTION

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Volcanoes concentrate near subduction zones, spreading centers, and hot spots and are rare in other places. Thus,

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the first step in assessing the volcanic hazard of an area

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is to understand its tectonic environment. Western Washington and Oregon are near a subduction zone and in a region likely to experience future volcanic activity. Kansas and Nebraska are not.

Furthermore, the potential violence of a volcanic eruption is related to the environment of the volcano. If an active volcano lies on continental crust, the eruptions may be violent because granitic magma may form. In contrast, if the region lies on oceanic crust, the eruptions may be gentle because basaltic volcanism is more likely. Violent eruptions are likely in Western Washington and Oregon, but less so on Hawaii or Iceland.

Risk assessment is based both on frequency of past eruptions and on potential violence. However, regional

Figure 5–24 Calderas (red dots) and ash-flow tuffs (orange prediction just outlines probabilities and cannot be used areas) in western North America.

to determine when a particular volcano will erupt or the intensity of a particular eruption.

volcanic gas––have escaped from the vicinity of the caldera since 1994. As in the case of Yellowstone, an-

SHORT-TERM PREDICTION

other eruption would not surprise most geologists. In contrast to regional predictions, short-term predictions attempt to forecast the specific time and place of an im- pending eruption. They are based on instruments that monitor an active volcano to detect signals that the vol- cano is about to erupt. The signals include changes in the shape of the mountain and surrounding land, earthquake swarms indicating movement of magma beneath the mountain, increased emissions of ash or gas, increasing temperatures of nearby hot springs, and any other signs that magma is approaching the surface.

In 1978, two United States Geological Survey (USGS) geologists, Dwight Crandall and Don Mullineaux, noted that Mount St. Helens had erupted more frequently and violently during the past 4500 years than any other vol- cano in the contiguous 48 states. They predicted that the volcano would erupt again before the end of the century.

Figure 5–25 California’s popular Mammoth Mountain Ski In March 1980, about two months before the great Area lies on the edge of the Long Valley Caldera.

May eruption, puffs of steam and volcanic ash rose from

Summary 89

Figure 5–26 Eruption of Mount St. Helens, May 18, 1980. (USGS)

the crater of Mount St. Helens, and swarms of earth- showed signs of an impending eruption. The U.S. Forest quakes occurred beneath the mountain. This activity con-

Service and local law enforcement officers quickly evac- vinced USGS geologists that Crandall and Mullineaux’s

uated the area surrounding the mountain and averted a prediction was correct. In response, they installed net-

much larger tragedy that might have occurred when the works of seismographs, tiltmeters, and surveying instru-

mountain exploded (Fig. 5–26). Using similar kinds of ments on and around the mountain.

information, geologists predicted the 1991 Mount In the spring of 1980, the geologists warned gov-

Pinatubo eruption in the Philippines, saving many lives. ernment agencies and the public that Mount St. Helens

SUMMARY

Basaltic magma usually erupts in a relatively gentle man- tic eruption may form a cinder cone. Alternating erup- ner onto the Earth’s surface from a volcano. In contrast,

tions of fluid lava and pyroclastic material from the same granitic magma typically solidifies within the Earth’s

vent create a composite cone. When granitic magma crust. When granitic magma does erupt onto the surface,

rises to the Earth’s surface, it may erupt explosively, it often does so violently. These contrasts in behavior of

forming ash-flow tuffs and calderas. the two types of magma are caused by differences in sil-

Volcanic eruptions are common near a subduction ica and water content.

zone, near a spreading center, and at a hot spot over a Any intrusive mass of igneous rock is a pluton. A

mantle plume but are rare in other tectonic environments. batholith is a pluton with more than 100 square kilo-

Eruptions on a continent are often violent, whereas those meters of exposure at the Earth’s surface. A dike and a

in oceanic crust are gentle. Such observations form the sill are both sheetlike plutons. Dikes cut across layering

basis of regional predictions of volcanic hazards. in country rock, and sills run parallel to layering.

Short-term predictions are made on the basis of earth- Magma may flow onto the Earth’s surface as lava or

quakes caused by magma movements, swelling of a vol- may erupt explosively as pyroclastic material. Fluid lava

cano, increased emissions of gas and ash from a vent, forms lava plateaus and shield volcanoes. A pyroclas-

and other signs that magma is approaching the surface.

90 CHAPTER 5 P L U TO N S A N D VO L C A N O E S

KEY WORDS

viscosity 75 columnar joint 79 vent 81 volcanic neck 85 pluton 75 pillow lava 79 crater 81 pipe 85 batholith 75 pyroclastic rock 80 active volcano 81 kimberlite 85 stock 77 volcanic ash 80 dormant volcano 81 pumice 86 dike 77 cinder 80 extinct volcano 81 ash flow 86 sill 78 volcanic bomb 80 shield volcano 81 nuée ardente 86 lava 79 fissure 81 cinder cone 82 tuff 86 pahoehoe 79 flood basalt 81 composite cone 84 welded tuff 86 aa 79 lava plateau 81 stratovolcano 84 caldera 87

vesicle 79 volcano 81

REVIEW QUESTIONS

1. Describe several different ways in which volcanoes and 12. How do a shield volcano, a cinder cone, and a composite volcanic eruptions can threaten human life and destroy

cone differ from one another? How are they similar? property.

13. Which type of volcanic mountain has the shortest life 2. What has been the death toll from volcanic activity dur-

span? Why is this structure a transient feature of the ing the past 2000 years? During the past 100 years?

landscape?

3. How much silica does average granitic magma contain? 14. How does a composite cone form? How much does basaltic magma contain?

15. What is a volcanic neck? How is it formed? 4. Why does magma rise soon after it forms?

16. Explain why and how granitic magma forms ash-flow 5. What happens to most basaltic magma after it forms?

tuffs and calderas.

6. What happens to most granitic magma after it forms? 17. What is pumice, and how does it form? 7. Explain why basaltic magma and granitic magma behave

18. How does welded tuff form?

differently as they rise toward the Earth’s surface.

19. How does a caldera form?

8. Many rocks, and even entire mountain ranges, at the 20. How much pyroclastic material can erupt from a large Earth’s surface are composed of granite. Does this obser-

caldera?

vation imply that granite forms at the surface? 21. Explain why additional eruptions in Yellowstone Park

9. Do batholiths and stocks differ chemically or physically, seem likely. Describe what such an eruption might or both chemically and physically?

be like.

10. Explain the difference between a dike and a sill. 11. How do columnar joints form in a basalt flow?

Discussion Questions 91

DISCUSSION QUESTIONS

1. How and why does pressure affect the melting point of 6. Parts of the San Juan Mountains of Colorado are com- rock and, conversely, the solidification temperature of

posed of granite plutons, and other parts are volcanic magma? How does the explanation differ for basaltic and

rock. Explain why these two types of rock are likely to granitic magma?

occur in proximity.

2. Why does water play an important role in magma gener- 7. Compare and contrast the danger of living 5 kilometers ation in subduction zones, but not in the other two major

from Yellowstone National Park with the danger of living environments of magma generation?

an equal distance from Mount St. Helens. Would your 3. How could you distinguish between a sill exposed by

answer differ for people who live 50 kilometers or those erosion and a lava flow?

who live 500 kilometers from the two regions? 4. Imagine that you detect a volcanic eruption on a distant

8. Use long-term prediction methods to evaluate the vol- planet but have no other data. What conclusions could

canic hazards in the vicinity of your college or university. you draw from this single bit of information? What types

9. Discuss some possible consequences of a large caldera of information would you search for to expand your

eruption in modern times. What is the probability that knowledge of the geology of the planet?

such an event will occur?

5. Explain why some volcanoes have steep, precipitous faces, but many do not.

CHAPTER

Weathering and Soil

E arly in Earth history, between 4.5 and 3.5 billion years

ago, swarms of meteorites crashed into all of the plan- ets and their moons. Today, the craters created by these impacts are abundant on the Moon but are completely gone from the Earth’s surface. Why has the Moon retained its craters, and why have the craters vanished from the Earth?

Tectonic activity such as mountain building and volcanic eruptions has continually renewed the Earth’s surface over geologic time. In addition, Earth has an atmosphere and water, which decompose and erode bedrock. The combina- tion of tectonic activity, weathering, and erosion has elimi- nated all traces of early meteorite impacts from the Earth’s surface. In contrast, the smaller Moon has lost most of its heat, so tectonic activity is nonexistent. In addition, the Moon has no atmosphere or water to weather and erode its surface. As a result, the lunar surface is covered with meteorite craters, many of which are billions of years old.

Delicate Arch, in Utah, formed as sandstone weathered and eroded.

94 CHAPTER 6 W E AT H E R I N G A N D S O I L

wind or water slows down and loses energy or, in the case of glaciers, when the ice melts, transport stops and sediment is deposited. These four processes—weather- ing, erosion, transportation, and deposition—work to- gether to modify the Earth’s surface (Fig. 6–2).

MECHANICAL AND CHEMICAL WEATHERING The environment at the Earth’s surface is corrosive to

most materials. An iron tool left outside will rust. Even stone is vulnerable to corrosion. As a result, ancient stone cities have fallen to ruin. Over longer periods of time, rock outcrops and entire mountain ranges wear away. Weathering occurs by both mechanical and chemical processes. Mechanical weathering reduces solid rock to rubble but does not alter the chemical composition of rocks and minerals. In contrast, chemical weathering occurs when air and water chemically react with rock to alter its composition and mineral content. These chemi- cal changes are analogous to rusting in that the final

Figure 6–1 This boulder weathered in place. products differ both physically and chemically from the starting material.