Geology 101 – Lab #4 – Volcanic Hazards
Spring, 2006
Introduction: This week’s lab will involve an examination of volcanic hazards in the
Cascadia volcanic system; specifically we will compare and contrast the effects of the
1980 eruption of Mt. St. Helens with present-day hazards in the Mt. St. Helens and Mt.
Rainier areas. We will also examine hazards associated with Hawaiian volcanoes, and use
data that help us understand the motion of the Pacific plate relative to the Hawaiian hot
spot.
You will want to visit the USGS volcanoes website and read relevant materials for the
Cascades and Hawaii volcanic systems
Part A
1. Cascadia Volcanic System:
(useful website: http://vulcan.wr.usgs.gov/)
a. Briefly describe the plate tectonic origin of the Cascade Volcanic Arc.
b. What is the general composition (mineral and chemical) of the Mt. Rainier and Mt. St.
Helens magmas and lavas?
c. How does the composition of these magmas and lavas influence the eruptive style of
the volcanoes?
2. Comparison of Mt. St. Helens 1980 eruption with hazards predicted for future
eruptions of Mt. St. Helens and Mt. Rainier: Please refer to the hazards maps available
in the lab room, and refer to the website listed above. Also examine the stereo pairs of
Mt. St. Helens (taken shortly after the 1980 eruption) and the aerial photo of Mt. Rainier.
a. What are the main types of hazards that are predicted for an eruption of Mt. Rainier?
Where, generally, are each of these types of hazards predicted to occur?
b. Note that the current hazards map for Mt. St. Helens shows less extensive hazard zones
than the actual areas that were affected by the 1980 eruption. Why is this so?
b. Compare the extent of predicted hazards for Mt. Rainier vs. Mt. St. Helens.
What factors account for this difference?
Part B: Hawaii: Volcanic Risks
(useful website: http://hvo.wr.usgs.gov/)
a. What is the general composition of the Hawaiian volcanic system magmas and lavas?
b. How does this composition control the eruptive style of Hawaii volcanoes?
c. What are the major types of hazards associated with Hawaiian volcanoes?
d. Where, generally, are each of these types of hazards expected to occur?
Part C: Hawaiian Hot Spot Track and Pacific Plate motion
The linear chain of islands that extends northwest from the ‘Big Island’ through Midway
Island has been interpreted as a “hot spot track” that defines the motion of the Pacific
Plate over a hot spot. The hot spot is assumed to be fixed relative to the moving plate.
The Hawaiian chain continues as a submerged seamount chain that changes orientation
abruptly to form the Emperor Seamounts (see the map on the next page). This series of
volcanic features give us an unusual opportunity to calculate rates of plate motion, since
the ages of the extrusive basalt which forms each island or seamount can be determined
using radiometric dating techniques.
Use the maps that follow and the related information to determine the rate of motion of
the Pacific Plate over the Hawaiian hot spot. Note that the volcano that formed the Island
of Niihau is 4.89 million years old.
1. What is the distance from the southeastern end of Hawaii to Niihau?
_______________ kilometers
2. What is the rate of plate motion?
_________________ km/million years
_________________ cm /year
3. How far does the Pacific Plate move in 50 years?
____________________ meters
4. Repeat the exercise using the island of Molokai. What is the rate in
__________________ cm/year
What might explain the difference in the calculated rates?
5. How do these calculated rates compare with typical plate motion rates given in your
textbook?
6. What direction is the Pacific Plate traveling relative to the hot spot.?
7. The distances, measured along the hot spot track from Hawaii, and ages of other
islands and seamounts is given in the table below.
Seamount or Island
Suiko
Koko
Midway
Necker
Kauai
Distance (km)
4860
3758
2432
1058
519
Age in My
65
48
28
10
5
Use graph paper to plot age versus distance. Plot distance on the y axis and age on the xaxis. Use a scale of ~ 1inch = 1000 km; ~ 1 inch = 10 million years. (note: you may use
Excel to graph the data and determine the best-fit line. I will demonstrate this in class).
Draw a line through the points which is the approximate best fit to the points. Determine
the slope of the line (remember, slope is dy/dx); this slope is equal to the average rate of
plate motion. The rate is
_____________ kilometers/million years
_____________ cm/year
8. Has the rate of plate motion been faster or slower in the last 5 million years? Explain.
9. How long will it take for the big island of Hawaii to be consumed beneath the Japan
arc subduction zone? (Hawaii is about 6300 km from Hokkaido).
Show your work.