QUESTIONS 5, PART A
Mount St. Helens: Topography Before and After
1. Figure 5.3a is a topographic map of Mount St. Helens prior to the eruption, and Figure 5.3b is a
map of the mountain after the eruption. Compare these maps with both the oblique photographs
(Figure 5.2) and with the stereo photographs (Figure 5.4). Use a stereoscope, if available, to study
Figure 5.4.
Draw topographic profiles of the mountain from point A to point B on Figure 5.3a, and from
2. Describe the main differences in these profiles. How are these differences reflected in the
stereo photographs? Why does it help (or not) to have the oblique photos for reference in
drawing these profiles?
The profile from 5.3a shows a conical mountain, and the profile from 5.3b shows a crater where
3. What were the peak elevations of Mt. St. Helens before and after the eruption? How much
elevation was lost from the old peak to the new crater floor? (Hint: Use the average elevation
between the two contours that define the 1980 crater floor.)
Students may give answers that are from former peak to current ridgecrest or to the floor of the
4. From the shape of the mountain prior to eruption, is Mount St. Helens closer in profile to
Hawaiian vokanoes or to composite vokanoes? (See Introduction, Exercise 4.)
5. What does the shape of Mt. St. Helens suggest about the composition of rock that you
would expect to find here?
It is not likely to be either basaltic, as the mountain is not a shield, or rhyolitic, as the overall
6. After the eruption, much of the volcano was missing. Where did the material go?
a. Compare the elevation of Spirit Lake on Figures 5.3a and 5.3b.
What is the elevation before the eruption? 3198 feet
What is the elevation after the eruption? 3408 feet (Note that the elevation of Spirit Lake on
b. Follow the channel of the North Fork of the Toutle River west from Spirit Lake. On Figure
5.3a, in section 18, just east of where Studebaker Creek enters the Toutle River, what is the
c. After the eruption (Figure 5.3b), how much elevation change occurred in the elevation of the
northern third of section 18?
d. What geologic deposit occurs in this area southwest of Johnston Ridge (refer to Figure 5.5 in
the colored maps section)? What is the origin of this deposit? What other eruption impacts
occurred in this area?
7. Examine Figures 5.3 and 5.5 (and photos 5.2 and 5.4) to determine the distribution of glaciers
near the top of Mt. St. Helens both before and after the eruption.
a. What changes took place? All the ice on the north and northwest sides is gone, and the ice on
the south side may be smaller.
b. Where did the ice go? It made part of the landslide and blast deposits.
c. What river channel(s) was (were) heavily impacted by waters from the melting ice? Especially
Volcanic Deposits Near the Mountain
8. Figure 5.5 (in maps section at back of book) is a map of proximal deposits and features of
1980 eruptions of Mt. St. Helens published by the U.S. Forest Service. What are the major types
of volcanic deposits and eruption-related features depicted on this map?
Lava dome, crater (collapse caldera), pyroclastic flow areas, debris avalanche, mudflows and
9. What are the relative geographic and topographic positions of these deposits and features in
relation to the main cone of the mountain (i.e., which are nearby and which are farther away)?
lava dome – at the site of the former cone
Which are confined to valleys and which are found on hills?
The debris avalanche actually climbed over Johnston Ridge, as shown on Figure 5.5. This was
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?
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?
12. How thick did sediment deposited in 1980 in the Columbia River get? The
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
13. Based on the wind rose data, what percent of time would winds blow toward the:
ENE – 16%
E – 16%
Are Olympia (NNW) or Portland (SSW) at much risk of tephra from Mt. St. Helens, based on the
data in the wind rose? No
Explain. Olympia is to the north-northwest where the wind blows from Mt. St. Helens about 2%
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.
The May 18 eruption was at a time of typical wind directions, while the May 25 eruption was at a
15. List the predicted events for each flowage–hazard zone shown in Figure 5.11.
Zone 1 – lava flows, pyroclastic flows, mudflows, floods, and ash
16. a. Was a preferred direction predicted for events in flowage zone 1? No
b. What eruption processes and products that occurred, if any, were not included in the
predictions?
17. a. How closely do the predictions in Figure 5.11 match the locations of actual mudflows or
lahars in the southern half of zone 1 at end of the following valleys (see Figure 5.5 and 5.6).
Muddy River – Lahars followed the predicted pattern, although the prediction suggested that the
lahars would travel further downstream; Pine Creek – Lahars followed the predicted pattern,
although the prediction suggested that the lahars would travel further downstream; Swift
18. a. How closely do the predictions for lahars in Figure 5.11 match the actual distribution of
lahars as shown in Figure 5.6 for each of the following valleys? Toutle River (North) –
b. Do you think that future lahars will follow the patterns of May 18, 1980? Why or why not?
QUESTIONS 5, PART B
a. How many eruptions are shown for Mt. St. Helens? 21
c. What is the average interval between eruptions of Mt. St. Helens for the past 4,000 years?
d. Prior to the 1980 eruption (shown at the present on Figure 5. 1), approximately how many
years before was the previous eruption?
This cannot really be distinguished on the figure. Oops.
e. Does the interval between the previous eruption and 1980 fit the pattern for average intervals
during the past 4,000 years?
Again, this cannot really be determined by looking at the graphic.
If the question was changed to “How does the interval for the past 1000 years compare with the
interval between 1000 and 4000 years ago?” the answer is that the current interval of 111 years
2. The dome-building eruption that began in 2004 is continuing in 2008. The eruption is
producing about 0.6 cubic yards of new magma every second. Scientists with the U.S. Geological
Survey estimate that about 3.5 billion cubic yards of the mountain was lost in the original eruption
in 1980. Since 1980, dome-building eruptions have produced almost 200 million cubic yards of
new rock. At the rate of 0.6 cubic yards per second, how long will it take the mountain to rebuild
itself to its pre-1980 eruption shape?
First, note that the eruption continued until about the middle of 2008. Check the Cascade
3. A record of the growth of the 2004 dome is seen in a series of images from a LIDAR-sensing
mission flown by NASA in conjunction with the U.S. Geological Survey (Figure 5.12a–f). The
5. For each of Figures 5.12c, d, e, and f, describe the change from the previous image. Consider
the area or size of the dome, the appearance of the surface of the dome, and the position of the
dome in the crater. The circles in Figures 5.12a, b, c, and d provide a reference for the change.
6. As a journalist or a scientist, you have been asked to describe, on camera near the site, the
changes that have taken place between September 24 and November 20. You think that
percentage increase in the area of the new dome would be one interesting and important fact to
report for viewers. You set out to determine the change in area. You recall that areas can be
The increase in dome size between 5.12c (very approximately 17,500 square meters) and 5.12f
(very approximately 91,500 square meters) is more than 400% during the time frame. Of
Optional online access required for the next question.
7. Compare your answer with new images that may have been posted on Mt. St. Helens’ web sites