Geology Reference
In-Depth Information
Exercise 5 • Hazards of Mount St. Helens
79
b.
Which is the second most active volcano?
How many eruptions does it have?
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.
c.
What is the average interval between eruptions of
Mount St. Helens for the past 4,000 years?
Image c (changes from b)
d.
Prior to the 1980 eruption (shown at the present on
Figure 5.1), approximately how many years before was
the previous eruption?
Image d (changes from c)
Image e (changes from d)
e.
Does the interval between the previous eruption and
1980 fit the pattern for average intervals during the past
4,000 years?
Image f (changes from e)
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 estimated from a formula, either for a
rectangle [A = L X W] or for a circle [A =
irr
2
],
where
77
= 3.14 and r is the radius. Begin by calculating the areas of
the dome in Figures 5.12c and 5.12f. Then calculate the per-
centage increase between the area of the dome in c and in f.
Show your work.
(Note: The diameter of the circle is 300 m. You can use
this as a scale to determine the sizes of the dome in Figures
5.12c and f.)
2.
The dome-building eruption that began in 2004 is continu-
ing in 2008. The eruption is producing about 0.6 cubic yards
of new magma every second. Scientists with the U.S. Geolog-
ical 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?
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 dates of each image are indicated in the upper left-hand
corner of each image. The LIDAR image has a horizontal
resolution of 2 m and a vertical resolution of 20-30 cm.
a.
Examine Figures 5.12a (September 3) and 5.12b (Sep-
tember 24). Mark on Figure 5.12b any changes in topogra-
phy that you see.
Optional online access required for the next question.
7. Compare your answer with new images that may have
been posted on Mount St. Helens web sites http:/ /vulcan.wr
mshnvm/. Describe below the current status of the volcano.
What possible events could occur today? What do the alert
level and aviation color code mean?
b.
Describe what seems to have happened in the time
between these two images.
4. On each of Figures 5.12c, d, e, and f, trace the outline of the
dome that is building.
