Name ________________________________
The Eruption of Mount St. Helens Date ________ Period_______ Group______
Background:
1. Label the 4 plate boundaries shown on
the diagram to the right.
2. What type of plate boundary is causing
the formation of the Cascade Range (see
diagram)?
3. Name a volcano in the Cascade Range
that is also in California.
Mt. Baker
Mt. St. Helens
4. Name a Cascade volcano that is in
Washington State.
Introduction
Mt Shasta
Mt Lassen
On May 18, 1980 a scenic mountain in the
southwestern part of the state of Washington
erupted violently. Scientists can use data from
this eruption and others to help make models to
predict future eruptions.
Part A: Before and After
In this section of the lab, you and a partner will be creating a model of Mt. St. Helens before and
after the eruption on May 18, 1980.
Procedure:
 Locate your two small topographic maps of Mt. St. Helens.
 Place each map in a separate clear tray, facing up.
 Place another clear tray on top of the “Before the 1980 Eruption” map. Let the top tray
rest on the bottom one without compressing it.
 Using a transparency marker, trace the outermost contour line from the map onto the top
clear tray.
 Place another clear tray on top of the first two. Using the transparency pen, trace the
second outermost contour line onto this clear tray.
 Continue stacking clear trays and tracing contour lines until you have reached the center
of the mountain. You should have only one contour line on each clear tray.
 Repeat this procedure with the “After the 1980 eruption” map.
 After you answer the questions, check with your teacher to see the photos of the
mountain before and after the eruption.
Questions:
5. What is the contour interval on these maps?
6. What was the elevation of the highest contour line before the eruption?
7. What was the elevation of the highest contour line after the eruption?
8. Describe the shape of the mountain before the eruption.
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9. Describe the shape of the mountain after the eruption.
10. Why do you think there aren’t any hachure marks on the topo map showing the
mountain after the eruption?
Part B: How can we predict volcanic eruptions?
On March 20, 1980 a pattern of earthquakes different from any other ever recorded in the
Pacific Northwest began with a single moderate-sized earthquake beneath Mount St. Helens.
During the weeks that followed, hundreds more occurred in the same place.
Graph 1 below shows the number of earthquakes of greater than the indicated magnitude that
occurred on selected dates from March 22 to May 30, 1980. Magnitude refers to the amount of
energy released by an earthquake; an earthquake with a magnitude of 4.0 represents a larger
release of energy than an earthquake with a magnitude of 3.2.
11. What does earthquake
magnitude tell you
about the earthquake?
12. What can you say
about the change in the
number of earthquakes
per day greater than
3.2 between March 22
and May 18?
13. What can you say
about the change in the
number of earthquakes
per day greater than
4.0 between March 22
and May 18?
14. Do you think that the
above trends by
themselves could have
been used to predict
the May 18 eruption?
Explain your answer.
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Most of our present earthquake information about volcanoes is based on methods developed
since the early 1900’s at the Hawaiian Volcano Observatory. Many years of observations of
Kilauea and Mauna Loa, two of the world’s most active volcanoes, have given some of the best
understanding available about earthquakes associated with volcanoes.
The graph below (Figure 4) is a typical graph of the daily number of earthquakes that occur on
Hawaiian volcanoes. This graph has been developed using data from thousands of eruptions
occurring over several decades.
The volcano’s magma chamber begins to fill
during Stage 1. By stage 2 the magma chamber
is at its fullest which causes the surrounding rocks
to fracture, creating earthquakes. The eruption of
the volcano occurs as the magma chamber
empties in stage 3.
Figure 4
15. What two volcanoes have been used the
most to help scientists learn about how
earthquakes are related to volcanoes?
16. During which stage in Figure 4 would the eruption of a Hawaiian volcano occur?
17. Why do more earthquakes occur in stage 2?
18. How would geologists be able to predict an eruption from the information available in
Figure 4?
19. Look at your graph of the Mount St. Helens earthquakes leading up to the May 18, 1980
eruption. How was the pattern of earthquakes at Mount St. Helens different than at the
Hawaiian volcanoes?
20. What reason(s) can you suggest for the difference between the Mount St. Helens and
Hawaiian earthquake patterns? (Hint: think location!)
Part C: Will it Happen Again?
In 1978, an article titled. "Potential Hazards from Future Eruptions of Mount St. Helens Volcano,
Washington," was published by the United States Geological Survey. The following paragraph is
the introduction to that article.
“Mount St. Helens is a symmetrical volcanic cone in southwestern Washington about 75 km
northeast of Portland, Oregon. Most of the visible part of the cone has been formed within the
last thousand years but it overlies an older volcanic center that evidently came into existence
before 36,000 years ago. Mount St. Helens has had a long history of spasmodic explosive
activity, and we believe it to be an especially dangerous volcano because of its past behavior
and the relatively high frequency of its eruptions during the past 4,500 years (Table 2).
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“In the future, Mount St.
Helens probably will erupt
violently and intermittently
just as it has in the recent
geologic past, and these
future eruptions will affect
human life and health,
property, agriculture, and
general economic welfare
over a broad area.”
In discussing a forecast of
the next eruption, the
authors went on to state
that, "The volcano's
behavior pattern suggests
that the (present) quiet
interval will not last as long
as a thousand years;
instead, an eruption is
more likely to occur within
the next hundred years.
and perhaps even before
the end of this century,"
Crandell, Dwight R., and
Mullincaux. Donal R., U.S.
Geological Survey Bulletin,
No. 138.3 C, 1978, p.
C25).
Using the information
above and the diagram at
right, answer the following
questions.
Since about 1285 A.D., what has been the average length of time for an eruptive interval?
21. Since about 1285 A.D., what has been the average length of time for a dormant interval?
22. For what reason did the authors of the article identify Mount St. Helens as an "especially
dangerous volcano"?
23. Since May 18, 1980, Mt. St. Helens has been in an eruptive interval. Using your
answers to questions 20 and 21, make a prediction as to how long Mount St. Helens
may remain active.
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