Geology Reference
In-Depth Information
FIGURE 5.4(A)
Vertical aerial stereographic photos of Mount
St. Helens prior to May 18, 1980 (USGS).
FIGURE 5.4(B)
Vertical aerial stereographic photos of Mount
St. Helens after the May 18, 1980, eruption (USGS).
Eruption-Related Problems Away From the Volcano
10.
Figure 5.6 shows the general area impacted by lahars
(mudflows). Since the major direction of the first volcanic
burst was to the north, what geologic processes caused
lahars to form on all sides of the volcano?
Note that this convention is opposite what you might expect,
in that a westerly wind (one coming out of the west) is plotted
toward the east, in the direction the wind is going.
13.
Based on the wind rose data, what percent of time would
winds blow toward the ENE and E? Are Olympia
(NNW) or Portland (SSW) at much risk of tephra from Mount
St. Helens, based on the data in the wind rose?
Explain.
11.
The lahars transported tremendous amounts of sediment
far from the volcano. Figure 5.7 shows the configuration of
the Columbia River bed before and after the eruption. What
differences in configuration are evident?
Figures 5.9 and 5.10 are maps of tephra deposits from
two Mount St. Helens eruptions. Note the little circles in the
corner of each figure. These are wind directions, toward which
winds are blowing at the times of eruption, given for different
elevations in the atmosphere. Remember that the posteruption
elevation of the top of Mount St. Helens is about 2,550 meters.
14. Compare the distributions of tephra, the wind directions
at the times of the eruptions, and the wind rose. Did the
eruptions occur at times of common or uncommon wind
directions? Explain.
12.
How thick did sediment deposited in 1980 in the Columbia
River get?
Tephra
Figure 5.8 is a wind rose, which shows the percent of time that
the wind blows in a particular direction. Imagine that you are
standing in the center of the circle. The letters on the outside of
the circle represent compass directions, and the numbers
inside the circle represent the percent of time that the wind is
blowing from you toward that particular direction. It can be
seen, for instance, that the wind blows 16% of the time toward
the east (E), and only 1% of the time toward the west (W).
Assessing the Likely Hazard
Figure 5.11 shows potential hazard areas of Mount St.
Helens; the map was published about 2 years before the
