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

Must know how to read topographic maps and draw topographic profiles and have a general knowledge of volcanic eruptions

This is a laboratory activity that follows a lecture on the 1980 eruption. It falls in about the middle of the course.

Describe the shape of Mt. St. Helens before and after the eruption by studying topographic maps, drawing two topographic profiles across the volcano, and looking at the mountain on Google Earth (optional).

Compare the two topographic profiles of the mountain, calculate their vertical exaggeration, and estimate the volume of material removed by the eruption.

Visualize the material removed by the eruption as a cone, apply the formula for the volume of a cone, compare result with published values, analyze sources of error in the computation.

This activity is designed as a laboratory exercise and to take ~1-1.5 hours 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. The power of the initial blast was directed upward and laterally, snapping off trees for miles in the blast zone. In the years since 1980, many people – geologists, biologists, environmentalists – have been observing and studying how the landscape recovers after a major volcanic eruption.

In this exercise, students study simplified topographic maps of Mt. St. Helens to interpret the shape of the mountain before and after the 1980 eruption. An option is to have them look at the volcano on Google Earth at this point. Student materials include a graph on which to plot two topographic profiles across Mt. St. Helens to illustrate the change in its shape. The accompanying Instruction file includes calculation of the vertical exaggeration of the profiles, but this section of the exercise may be omitted. Assuming that the material removed by the eruption was in the form of a perfect cone, students use their profiles to measure the height and diameter of the cone to calculate the volume of material removed. Students then compare the result of their calculation with published values for the eruptive material removed from the mountain and identify possible sources of error in their work.

As originally designed for a traditional face-to-face course, this activity is assessed by the quality of the topographic profiles and the answers to the questions. In the online version, only the answers to questions are used to assess student understanding of the exercise, but these questions include choosing the correct shapes for the profiles.

Brugman, M.M., and A. Post, 1981, Effects of volcanism on the glaciers of Mt. St. Helens: U.S. Geological Survey Circular 850D, 11 pp.

Mt. St. Helens Volcano Camera

Mt. St. Helens Fact Sheet

U.S. Geological Survey website about Mt. St. Helens