Assignment: Exploring Volcanic Features of the United States

Module 6 Assignment

Exploring Volcanic features of the U.S.

A volcanic cloud plume over an erupting volcano.

Figure 1. For more than nine hours a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20-25 kilometers) above sea level. The plume moved eastward at an average speed of 60 miles per hour (95 kilometers/hour), with ash reaching Idaho by noon. By early May 19, the devastating eruption was over. Shown here is a close-up view of the May 18 ash plume.



You can start the activity after you have completed the reading assignment.

Mount St. Helens Reawakens

The first earthquakes struck on March 20, 1980. Seismologists quickly determined that the quakes were centered beneath a snowy mountain, known to them, but not to the general public, as a potentially dangerous volcano, which had been dormant for more than a century. During the next week, the number of earthquakes increased, and these quakes triggered snow avalanches, which in turn forced closure of winter recreation areas around the mountain. Geologists and geophysicists converged on the scene to monitor the activity and met with local authorities to alert them to the possibility of an eruption. On March 27, steam and ash exploded from the summit of the volcano and marked the beginning of several small eruptions during the next two months.

A large mountain with a lake in the foreground

Figure 2. Mount St. Helens before the May 18, 1980 eruption. View from the northeast of Spirit Lake.


Public authorities prudently closed the area surrounding the mountain after being informed of the volcano’s past violent behavior by those who had conducted careful geological studies during the preceding 20 years. Although closure was a necessary precautionary measure, it created discontent and even anger on the part of some citizens who wanted access to their property and recreation sites. Continued monitoring of the volcano indicated that its north flank, which towered above the most popular recreation area, was becoming increasingly unstable. Warnings were issued for landslides and large-scale snow avalanches. These warnings supported the need for continued closure of the area, although public pressure eventually led to brief, authorized forays into the area by cabin owners to retrieve belongings. One such trip was scheduled for the morning of May 18, but it never took place.

A car, mangled, and buried in volcanic ash

Figure 3. Car after Mount St. Helens 1980 Eruption (May 31). Reid Blackburn’s (photographer, National Geographic, Vancouver Columbian) car, about 10 miles from Mount St. Helens.

For more than nine hours a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20-25 kilometers) above sea level. The plume moved eastward at an average spe ed of 60 miles per hour (95 kilometers/hour), with ash reaching Idaho by noon. By early May 19, the devastating eruption was over.

At 8:32 a.m. on May 18, an earthquake triggered a gigantic landslide on the unstable north flank, which in turn unleashed a scorching, explosive blast of hot gas laden with rock fragments; massive floods of mud and rock down most river valleys; flows of hot, gas-rich volcanic rock; and an enormous plume of ash. The water-soaked landslide debris produced a series of dense slurries that raced downstream and nearly severed Interstate Highway 5 and the AMTRAK rail line connecting Portland, Oregon, and Seattle, Washington. These debris flows brought shipping on the Columbia River to a halt and came close to blocking cooling-water intakes at an operating nuclear power plant. These events transformed a lush landscape of dense, green forest into a dusty volcanic wasteland and killed 57 people who were too close to the mountain. The eastern half of the State, where people were virtually unaware of any volcanic hazard, was blanketed with ash. The death toll, though large, could have been much, much higher without the previous warnings and resultant land closure. Luck also played a role in keeping the number of fatalities down.

The crater of Mt. St. Helens with a plume of smoke escaping.

Figure 4. Mount St. Helens in 1982> After the 1980 eruption, there was a massive debris avalanche that reduced the elevation of the mountain’s summit from 9,677 ft (2,950 m) to 8,363 ft (2,549 m), leaving a 1 mile (1.6 km) wide horseshoe-shaped crater.

Had the eruption occurred on Monday rather than Sunday, several hundred loggers, working in an area near the volcano but outside the closed area, would have died. During the next decade, continued enforcement of restricted zones and careful observation and prediction of activity warned the public of impending eruptions, and no additional lives were lost. Research into what had caused the catastrophic eruption led to increased appreciation of the inherent instability of high, snow-covered volcanoes and the hazards they pose. (Excerpts from Living With Volcanoes, by T.L. Wright and T.C. Pierson, The U.S. Geological Survey’s Volcano Hazards Program: U.S. Geological Survey Circular 1073, 57p. 1992.)

3 images of Mt. St. Helens showing before and after with lines to indicate where the top of the cone used to be.

Figure 5. Mount St. Helens before and after the 1980 eruption showing the reduction in height and the resulting crater.

How Rocks Melt

Most magma is generated at the base of the earth’s crust; the figure below is a pressure-temperature diagram. On the left side of the solid black line (called the solidus) is a region where the temperature is too low for a rock to melt. On the right side of the solidus line is the region where rock will melt. Notice that the solidus line is not a vertical line going straight down, but is sloped at an angle less than vertical, demonstrating that with increasing pressure the temperature must also increase in order for a rock to melt.

Figure 6. Pressure and Temperature graph of crustal rocks. The line that separates the region of solid versus liquid rock is called the solidus. Note that a rock at point X cannot melt unless one of the following conditions occur: either a temperature increase (arrow “a”), or a pressure decrease (arrow “b”), or any combination of these two changes (arrow “c”), or by adding water to change the melting conditions which shifts the solidus line to the left (arrow “d”).

Now take a look at the conditions at the base of the crust, at point “X”. This rock at “X” is not hot enough to melt; or it can be said that the rock at point X is under too much pressure to melt. To make this rock melt, either the temperature must increase (arrow “a”), or the pressure must decrease (arrow “b”), or we can have both hotter temperature and lower pressure occur simultaneously (arrow “c”). Regardless of the path taken, we can make this rock X cross the solid line and become magma. The only other way we can make rock X cross the solid line and become magma is to move this line (arrow “d” in the figure); in other words, change the melting temperature of the rock. This can be done by adding water, which lowers the melting temperature of rock, and now we can make rock X melt without actually having to change the temperature and pressure conditions.

Now let us think of plate tectonics and the types of boundaries that have magma associated with them (see figure below). Tectonic plates that are diverging (or pulling apart), causes the underlying region of the mantle to experience reduced pressure conditions. If the mantle is already fairly warm, the decreased pressure may just be enough for magma to be produced (arrow “b” in the figure below). Where tectonic plates are converging (coming together), one of the plates may subduct below the other plate; recall that subduction will only occur if the tectonic plate has an oceanic crust type. This subducting oceanic crust-topped plate will contain minerals that are hydrated (water in their crystal structure), and as the plate subducts, the hydrated minerals will become unstable and water will be released.

Figure 7. Areas of magma generation at certain plate boundaries (b and d), or within a plate due to a hotspot (a). Arrows a, b, and d correspond to the same arrows as shown in Figure 6.

This water will lower the melting temperature of the mantle region directly above the subducting plate, and as a result magma is produced (arrow “d” in the figure). The last way to melt rock is to just increase the temperature of the rock; this particular melting mechanism does not have to be associated with any particular plate boundary. Instead there must be a region known as a hot spot, caused by mantle plumes (arrow “a” in the figure above). Mantle plumes are thought to be generated at the core-mantle boundary, and are regions of increased temperature that can cause melting of the lithospheric region. With the lithosphere broken into several tectonic plates that have been migrating over these plumes throughout geologic time, the resulting hotspot-generated volcanoes can be found anywhere in the world.


Before your begin this activity, please revisit section 3.2 of Chapter 3 in Physical Geology on how magma is formed. You can also review the following excerpt on how rocks melt. This website briefly reviews the 3 main ways that rocks melt (decompression melting, flux melting [volatiles], and addition of heat [conduction]).

  1. Visit the USGS Geology of the National Parks. Select the link for each of the following parks and take the “standard” photo tour to explore the region:
  • Hawaii Volcanoes National Park
  • Devils Tower National Monument
  • Yellowstone National Park
  • Crater Lake National Park
  • Pinnacles National Park
  • Mount St. Helens National Volcanic Monument
  • Lava Beds National Monument
  1. Pick four sites from the list above that intrigue you the most. For each of the four sites, address the following questions. Feel free to use other credible websites (such as to help you explore the answers (be sure to cite your other sources):
  • Where is the park located (e.g. near which city, state, or specific region of the country?).
  • In 2 – 3 sentences, describe the central volcanic feature in the park. Is it an active, dormant, or extinct volcanic site, or is it another type of feature? If it is a volcano, what type of volcano is it (shield, stratovolcano, cinder cone, caldera, etc.)? If it is an active volcanic site, what specific type of volcanic activity does the USGS photo tour depict to help you determine this? (Include a screenshot of this activity from the photo tour where applicable.)
  • In 2 – 3 sentences, please describe the tectonic processes that resulted in the volcanic feature(s) in the park. For example, was the volcanic feature a result of activity at a plate boundary, a hot spot, or some other activity?
  • In 2 – 3 sentences, please give your own assessment of what type of melt process is occurring at each location. Is there more than one type of melt? Use the information you have learned in the text and from the excerpt above on How Rocks Melt.
  • What specific questions arose while you researched each feature, and were you able to find satisfactory answers to address your curiosity? Submit a short report (maximum 2 pages, 1.5 spacing) discussing your answers to these questions. You will be graded on this activity as described in the following rubric.

Note that points may be deducted for spelling or grammar errors, or for not following the line spacing and length rules.

Grading Rubric

15 points: Accurately responded to the questions for each of the four volcanic sites. Any additional references used were cited.

12 points: Accurately responded to the questions for each of the sites, missing or answering incorrectly only one or two questions. Any additional references used were cited.

8 points: More than two questions missed or answered incorrectly. References (if applicable) were not included.

5 points: Only very partial information provided.

0 points: Did not complete the assignment.