Volcanoes and Volcanic Hazards - Investigations in Environmental Geology - page 55

QUESTIONS 4, INTRODUCTION 5. Use the graph below, and build a histogram to illustrate the

data you calculated in answering question 4 above. Deter-
mine your own vertical scale.

1. Sketch representative topographic cross sections of volca-
noes with high-viscosity, intermediate-viscosity, and low-
viscosity lavas. Which type of magma is most explosive?
Least explosive? Which type of magma has the highest silica
content? The lowest?

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2. Refer to Figure 4.1 and Table 4.2, and describe briefly the
volcanic products and hazards for both people and property
that are likely to exist in the following locations.
a. If people or property are on a ridge top, close to a com-
posite volcano?

b. If people or property are on a valley bottom close to a
composite volcano?

c. If people or property are on a valley bottom 25 km from
a composite volcano?

6. From the data on your graph (question number 5), describe
the relationship between eruption size and eruption fre-
quency (the typical number of eruptions per 100 years).

d. If people or property are on a ridge top 10 km from a
rhyolitic volcano?

PART A. RHYOLITIC VOLCANOES
Rhyolitic volcanoes present many different kinds of
volcanic hazards (Table 4.1). Since no major eruptions
have occurred recently in the United States, the inter-
pretation of possible hazards from rhyolitic volcanoes
in the United States must be based on other data,
including eruptions from elsewhere around the world
and U.S. eruptions in the geologic past.
Two of the major rhyolitic volcanic centers in the
United States are the Long Valley Caldera in California
and the Yellowstone Caldera in Wyoming. These sites are
shown in Figure 4.3, which is a map that also shows the
distribution of volcanic ash from these volcanic centers.
Rhyolitic volcanoes may have different scales of
eruptions. They may erupt small plugs and domes of
rhyolite, or they may erupt as "supervolcanoes" that
can have global impacts. Both Long Valley and Yellow-
stone have had these different scales of eruptions.
Geophysical evidence suggests that there is active
magma in the subsurface at both volcanoes.

3. Do basaltic lava flows present a greater hazard to people
or to property? Why?

4. Refer to Figure 4.2. If you live to be 100 years old:
a. how many eruptions the size of Kilauea or Unzen are
likely in your lifetime?

b. how many eruptions the size of Etna are likely to occur
in your lifetime?

c. how many eruptions the size of Mount St. Helens are
likely to occur in your lifetime?

d. how many eruptions the size of Pinatubo or Katmai are
likely to occur in your lifetime?

A1. Supervolcano Eruptions

Table 4.3 presents data from Long Valley and Yellow-
stone eruptions. We focus on one aspect of these erup-
tions, their tephra distribution. It is important to note,

a A