Mt. St. Helens Volcanic Ashfall Eruption

Eileen Herrstrom, University of Illinois at Urbana-Champaign
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This page first made public: Jun 7, 2017

Summary

This activity takes place outside of the classroom and requires ~1 hour to complete. Students recreate the map of the Mt. St. Helens ash plume of 1980 and use their maps to answer a series of questions about this ashfall.

Context

Audience

Undergraduate class on introductory physical geology, volcanoes, natural disasters, or geology and society.

Skills and concepts that students must have mastered

Must know how to read maps, have a general knowledge of volcanic eruptions, and understand how to use the formula "distance = rate x time".

How the activity is situated in the course

This is an extra credit homework activity that comes after a lecture and a laboratory session on the 1980 eruption. It falls in about the middle of the course.

Goals

Content/concepts goals for this activity

Describe the ashfall aspect of the 1980 eruption, draw the outlines of the ash cloud, and duplicate the U.S. Geological Survey ash plume map.

Higher order thinking skills goals for this activity

Deduce the arrival of the ashfall in different areas on the map, calculate the velocity of ash movement, distinguish the affected and unaffected cities.

Other skills goals for this activity

Predict the thickness of the ash deposits in various places, explore online the effect of volcanic ashfalls on people and their activities, and compare the Mt. St. Helens ashfall with others around the world and through time.

Description and Teaching Materials

This activity is designed to be completed outside of class and to take ~1 hour to finish.
On May 18, 1980, Mt. St. Helens in the state of Washington exploded in a cloud of ash, plus lava and mud flows. What had been a beautiful symmetrical snow-covered mountain with heavily forested slopes became a startling landscape of ash, mud, and downed trees surrounding a broken, irregular peak. Geologists had been monitoring the mountain, and access to the area was restricted; still, 63 people were killed by the eruption, including a geologist who had been observing from a nearby ridge. The ash cloud reached 19 km (12 miles) into the atmosphere, where strong east winds scattered it across numerous states and cities.
In this exercise, students receive a map of the northwestern United States and illustrations of the ash cloud in map view at hourly intervals. These drawings show the changing size and shape of the ash cloud after the eruption. Students trace the outlines of the ash cloud on a separate sheet of paper, one superimposed upon another, making a composite image. Then they lay the tracing over the map to see where the ash was carried.
The exercise is completed by answering a series of questions, which can include cities affected, velocity of ash movements, possible effects of ash on people and their activities, and comparisons with other ashfall eruptions.
Mt. St. Helens Volcanic Ashfall Instructions (Acrobat (PDF) 434kB May30 17)
Mt. St. Helens Map (Acrobat (PDF) 578kB May30 17)
Mt. St. Helens Ash (Acrobat (PDF) 186kB May30 17)
Mt. St. Helens Volcanic Ashfall Key (Acrobat (PDF) 620kB May30 17)

Teaching Notes and Tips

This is a flexible assignment. It is described here as an optional extra credit exercise, but it can also be incorporated into a laboratory assignment on volcanoes, the 1980 eruption, or volcanic hazards. When used in a traditional face-to-face format course, students may be asked to submit their tracings as well as answer the questions. When used in an online format course, it is easier not to require submission of the tracings.
One of the difficulties students sometimes experience is comprehending the concept of superimposing the various shapes of the ash cloud to produce a composite drawing. A more thorough explanation of the process was added to the general description of the activity to address this problem.
Possible questions for the exercise in addition to or in place of the ones in the accompanying teaching material include the following: Which city affected by the ashfall would likely have the thickest (thinnest) accumulation of ash? Describe three ways that ashfalls affect people's health. Describe three ways ashfalls affect society in general. How does the Mt. St. Helens ashfall of 1980 compare with ashfalls in other times and places (may specify ashfalls to check: Long Valley Caldera 760,000 years ago; Yellowstone Caldera 665,000 years ago; Glacier Peak 14,500 years ago; Mt. Mazama 7600 years ago; Vesuvius in 79 CE; Novarupta in 1912; Eyjafjallajökull in 2010)?
Note that students need access to a printer to complete this exercise, as they must print some of the accompanying teaching materials.

Assessment

As originally designed, this activity is assessed by the quality of the superimposed tracings and the answers to the questions. In the online version, only the answers to questions are used to assess student understanding of the exercise. If the web component is added to the assignment, then a brief written summary of the hazards of ashfalls or comparisons of different ashfalls is appropriate. This option is more suitable for a small class than a large one.

References and Resources