The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
Name: ___________________________________________________ Date: ____________________
This contains material adapted from Richard Abbot (Appalachian State University, Department of
Geology) and from the USGS Volcanoes! 1997 Teacher packet.
Part I. Mt. St. Helens
In the illustration at the
right is a vertical crosssection (from A to A’) of
Earth’s crust and mantle
from west of the Cascadia
subduction zone to east of
Mt. St. Helens and a map
showing this region and the
location of the vertical
cross-section.
1. What is the distance
between the Volcano and
the Cascadia subduction
zone fault? 360 km ______
2. What is the depth of the
Juan de Fuca plate beneath
Mt. St. Helens? 85 km_____
The cross-section (above the map) show the relations between the convergent plate boundary and
volcanism. In the case of Mount St. Helens, like the other stratovolcanoes of the Cascades, the
production of the andesitic/rhyolitic magma is ultimately related to the subduction of the Juan de Fuca
plate beneath the western edge of the North America plate. Oceanic crust of the Juan de Fuca plate and
sedimentary material are being shoved downward, toward the east, underneath North America. This
down-going material undergoes metamorphism as it becomes exposed to progressively higher pressures
and temperatures in the interior of the earth beneath North America. Temperatures at depths of the
top of the Juan de Fuca plate directly below Mt. St. Helens can be as high as 1,200° C, hot enough to
cause some partial melting of the metamorphosed basalt of the subducting oceanic crust. The
metamorphism also releases volatile (gas) components such as H2O, CO2, and SO2. The relatively small
amounts of basaltic magma and rather large amounts of volatile gasses migrate upward, ultimately
encountering the base of the overlying continental crust. Here, the basaltic magma and volatile gasses
contribute to partial melting of the base of the continental crust. The magma thus formed is andesitic
and rhyolitic, reflecting the composition of the lower part of the continental crust. Generally, only very
small amounts of the basaltic magma ever reach the surface. The greater part of the basaltic magma
becomes mixed, sort of homogenized, with the andesitic/rhyolitic magma. This andesitic/rhyolitic
magma readily dissolves, quite literally soaks up, the volatile components. Under the very high pressures
at such depths near the base of the continental crust these dissolved gasses are effectively trapped in
the magma. However, the buoyant, gas-charged magma tends to work its way upward toward the
surface (intrusion). Most of the andesitic/rhyolitic magma cools and crystallizes before it ever reaches
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The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
the surface, forming sometimes vast plutonic (intrusive) bodies of granodiorite/granite. Only
comparatively small amounts of the gas-charged andesitic/rhyolitic magma ever reach the surface, but
when this happens - LOOK OUT! The rapid formation of gas bubbles in the magma and their explosive
expansion can have catastrophic effects.
3. Stratovolcanoes are typically circular in map view. The radius of Mount St. Helens is about 6 km at
the base, and the elevation of the base is about 1 km above sea level. Before the eruption of May, 1980,
the elevation at the top of Mount St. Helens was about 3 km.
Using this information, and modeling the stratovolcano as a simple cone-shape, estimate the volume of
volcanic material in Mount St. Helens, in cubic kilometers (km3). To remind you, the volume of a cone is
given by the following formula, Volume = (1/3)*pi*r2*h, where pi = ~ 3.14, r = radius of the cone, and h =
height of the cone. Volume = _________________
(1/3) X (3.14) X (62) X 2 = ~75 km3
The next questions refer to these maps of Mt. St. Helens from before and from after the eruption. This
map is adapted by the USGS from Brugman and Post, 1981 http://pubs.er.usgs.gov/publication/cir850D
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The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
4. What is the slope and angle of the slope of the surface of Mount St. Helens along the thick black line
in the "before" map above? The length of the thick black line is 2,000 m. Recall that slope = rise/run.
Convert this from a fraction to a percent:
1,100/2,000 = 0.55 = 55%
5. Shield Volcanoes have slopes that range from 9 % along their lower slopes to 18 % along their higher
slopes. How does the slope you calculated in step 3 compare with shield volcanoes?
Mt. St. Helens is steeper than the average shield volcano.
6. Why are these slopes different?
Shield volcanoes are built from lava that has a lower Silica content, so the viscosity is lower than that
of stratovolcanoes.
7. One way to examine the effects of the eruption is to construct "before" and "after" topographic
profiles of the volcano.
a. Construct “before” and “after” topographic profiles on the grids that are placed along the A-A’ profile
lines. The elevation contours are in meters.
b. On your copy of the post-eruption profile use colored pencils to denote the following regions, (1)
where there has been no change, (2) where material has been removed, and (3) where material has
been added (deposited). Create a legend that lists what color you used for each of the three regions.
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The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
8. The amount of material (6.5 km3) that was removed from Mount St. Helens represents what
percentage of the original volcano?
6.5 km3 / 75 km3 = 0.0867 = 8.67%
9. What is the vertical exaggeration of your profiles?
1,000 m / 1,000 m = 1, therefore 1 X (no vertical exaggeration)
Part II. The Age of a volcanic deposit
Have you ever looked at a tree stump and noticed its rings? Count the rings and you will know how old
the tree is. Each ring represents 1 year in the life of the tree. If you look closely at tree rings, however,
you will see that the spaces between rings vary in width. Trees do not grow the same amount each year.
You can “read” these tree rings and find out what year there was an eruption of Mount Katmai in Alaska.
What you know:
1.
2.
3.
4.
This tree was growing 48 kilometers (29 miles) northwest of Katmai Volcano.
After the eruption, the forests were blanketed in ash.
This tree’s growth decreased for some years after the eruption, but then it increased.
This tree was cut down in 1962.
What you want to find out:
1. The tree’s age: (Count the number of rings from the center of the tree to the bark. Each dark
band represents 10 years, but not all decades get a dark ring.) 120 yrs______________
2. The year the tree started to grow:
a. (the year the tree was cut) - (the age of the tree) = (the year the tree started to grow)
b. 1962______ - 120______ = 1842________
3. The year of the eruption: (Count the number of rings from the center to the first thin ring.)
1842 + 71 = 1913_________
4. The number of years the tree’s growth decreased: (Count the number of thin tree rings)
3_____________
5. The number of years the tree’s growth increased: (Count the number of wide rings.)
12____________
6. Why do you think the tree’s growth increased?
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The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
Part III. Recurrence of Cascades Volcanic Eruptions
Volcanic eruptions pose a number of hazards that put people and their belongings at risk. Part of
evaluating these hazards includes making estimates of the likelihood of an eruption. Many are familiar
with the concept of whether a volcano is active or if it is dormant. Many volcanologists in the USA and
internationally consider a volcano to be active if it has erupted in the last 10,000 years. Likewise, if a
volcano has not erupted in the last 10 ka, we consider it dormant.
Below is a plot showing the eruptions of the major Cascade volcanoes over the past 4 ka. In order to
evaluate the relative hazards that each of these volcanoes may pose to people, we would like to make
an estimate of the recurrence of their eruptions. We will do this by finding the average time between
eruptions.
1. Calculate the mean RI for these Cascade volcanoes.
Step 1. Determine the number of eruptions for each volcano. Also, estimate the years ago of the last
eruption
Step 2. Calculate the Recurrence Interval (RI) for each Cascades volcano by dividing the number of intereruption times by the time span (4 ka). List these values below:
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The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
Volcano
Years Ago of Number of
Last Eruption Eruptions
Mount Baker:
150 years
1
Glacier Peak:
250 years
Mount Rainer:
Math
Recurrence
Interval (RI)
Ratio
4000 / 1 = 4,000 years
4,000 years
0.0375
6
4,000 / 6 = 667 years
667 years
0.375
150 years
11
4,000 / 11 = 364 years
364 years
0.41
Mount St. Helens:
35 years
21
4,000 / 21 = 190 years
190 years
0.18
Mount Adams:
1,100 years
2
2,000 years
0.55
Mount Hood:
150 years
3
100 years
1.5
Mount Jefferson:
>4,000 years 0
4,000 / 0 = >4,000 years
>4,000 years
>1
Three Sisters:
1,500 years
5
4,000 / 5 = 800 years
800 years
1.875
Newberry Volcano:
1,300 years
2
4,000 / 2 = 2,000 years
2,000 years
0.65
Crater Lake:
>4,000 years 0
4,000 / 0 = >4,000 years
>4,000 years
>1
4,000 / 2 = 2,000 years
300 / 3 = 100 years
Medicine Lake Volcano: 900 years
7
4,000 / 7 = 570 years
570 years
1.58
Mount Shasta:
200 years
7
4,000 / 7 = 570 years
570 years
0.35
Lassen Peak:
100 years
3
4,000 / 3 = 1,330 years
1,330 years
0.075
2. One general way to evaluate how likely a volcano might erupt is to compare the RI with the timing of
the last eruption. For example, let’s consider two volcanoes. Volcano A: the RI = 500 years and it has
been 1,000 years since the last eruption. Volcano B: the RI = 500 years and it has been 100 years since
the last eruption. Volcano A is probably closer to having an eruption sooner than volcano B.
Given your knowledge of the eruptive history, your calculations of RI, and your estimate of the age of
the last eruption,
a. What Cascade volcano is the most likely to erupt next (A or B)? Volcano A
b. What Cascade volcano is the least likely to erupt next (A or B)? Volcano B
3. Rank the three Cascade volcanoes that are closer to eruption, based upon your statistical analysis
above. One way to estimate which is the volcano that is most likely to erupt is to divide the “Years Ago
of Last Eruption” by the Recurrence Interval. The larger the value for this ratio, the more likely the
volcano might erupt.
1. Three Sisters
2. Medicine Lake Volcano
3. Mount Hood
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The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 3: Cascade Volcanoes
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