Laboratory Manual for Introductory Geology 4e

FIGURE 5.14 Comparison of areas in North America covered by ash from
volcanic eruptions.
Mt. St. Helens
Mt. Mazama ash
(Crater Lake)
Crater
Lake
Mt. St.
Helens ash
Yellowstone
Caldera
Lava Creek ash
(Yellowstone)
Long Valley
Caldera
Bishop ash
(Long Valley)
0 500 1000 miles
0 500 1000 km
roll downslope and speed across the surrounding countryside at velocities measured
between 400 and 500 miles per hour (Fig. 5.13c). A nuée ardente from the
1902 eruption of Mt. Pelée, on the Caribbean island of Martinique, flattened the
capital city of St. Pierre in minutes (FIG. 5.15), bent 20-foot-high iron gates like
pretzels, and killed nearly all of the 50,000 people who had fled to the city for safety.
■ Lahars (volcanic mudslides): Tiny ash particles shot into the atmosphere act as
seeds for rain, so heavy downpours commonly accompany stratovolcano eruptions.
The rain mixes with loose ash, forming a slurry that flows downhill with enormous
power, dams rivers, and can bury local communities (Fig. 5.13d).
Exercise 5.15 reviews selected natural disasters caused by stratovolcanoes and
explores a potential disaster in the making in Yellowstone National Park. The case
histories examine volcanoes rated at different levels of the USGS Volcanic Explosivity
Index (VEI) shown in TABLE 5.4.
FIGURE 5.15 St. Pierre, Martinique, before (left) and after (right) the 1902 nuée ardente.
136 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY