Geology Reference
In-Depth Information
FIGURE 4.7
Porkchop Geyser erupted in a small hydrothermal explosion in 1989. The blocks that surround the pool were thrown out
of the ground during the explosion. This is one of the smaller such explosions that have occurred. Other hydrothermal explosions have
been documented that are up to 2 miles in diameter. (Photograph © 2008 Duncan Foley, reprinted with permission)
PART B. COMPOSITE VOLCANOES
volcano and, ideally, when the events will occur. If it
cannot be determined when an event is going to occur
(i.e., a forecast or prediction cannot be made), it is
helpful to know when the last episode of a particular
event occurred and how often similar events have
occurred in the geologic past. In this section of this
exercise we look at past geologic events at Mt. Rainier
that have created hazards, and evaluate the risks that
exist for people living in nearby towns.
Mount Rainier, Washington
Mount Rainier (Figure 4.8) is located in western Wash-
ington, near the Puget Sound lowlands that include
the cities of Tacoma and Seattle (Figure 4.9, in the color
map section at the back of the topic, is a shaded topo-
graphic map of Mount Rainier). The proximity of this
active volcano to major population areas has led the
U.S. Geological Survey to designate Mount Rainier as
potentially one of the most hazardous volcanoes in the
world (U.S. Geodynamics Committee, 1994).
Potential geologic hazards from Mount Rainier
are particularly great, due to the combination of the
height of the volcano (4393 m; 14,410 feet), its active
nature, steep upper slopes, and heavy cover of the
mountain by glaciers. Hazards at Mount Rainier exist
both during and between magma eruptions (Hoblitt
and others, 1995; U.S. Geological Survey, 1996).
People who live near a volcano need to know the
types of volcanic events that create risks to people
and/or property, the probable size of those events,
how far the events are likely to extend beyond the
QUESTIONS 4, PART B
1.
Table 4.4 lists some hazards (adapted from U. S. Geodynam-
ics Committee, 1994) that have been identified as existing at
Mount Rainier. Fill in the blanks in the table, using "H" for
high risk, "M" for moderate risk, and "L" for low risk. Refer to
Tables 4.1 and 4.2 and Figure 4.1 to help you fill in the blanks
in the table. You may also wish to refer to your textbook.
2.
A major concern for people living below the slopes of Mount
Rainier is the possibility of large landslides and lahars. These
could occur without warning during dormant periods of the
volcano, if the rocks become too weakened by heat and fluids
