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Mystery of the Megavolcano

Classroom Activity

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Activity Summary
Students use volcanic ash data to determine the source of a possible supervolcanic eruption that occurred in the western United States.

Learning Objectives
Students will be able to:

• describe some of the characteristics of a supervolcanic eruption.

• better understand how additional data can help support or refute a hypothesis.

• copy of the "CSI: Ashfall Fossil Beds" student handout (PDF or HTML)
• pencil or pen

• copy of the "Volcanic Identification" student handout (PDF or HTML)
• copy of the "Volcano Suspects: Location" student handout (PDF or HTML)
• copy of the "Volcano Suspects: Description" student handout (PDF or HTML)
• copy of the "Volcano Suspects: Ash Composition" student handout (PDF or HTML)
• access to print and Internet U.S. map resources

Background
Supervolcanic eruptions are extremely large eruptions that produce at least 1,000 cubic kilometers of magma and pyroclastic material (a hot, dry, fast-moving mixture of ash, pumice, rock fragments, and gas). These eruptions could destroy virtually all life within a radius of hundreds of kilometers from the site and could bury areas as far away as 1,500 kilometers in meters of ash. Very large-scale explosive eruptions of this type produce calderas, large depressions formed by the collapse of the summit or flanks of a volcano.

Volcanic ash consists of rock, mineral, and glass fragments smaller than two millimeters in diameter. Ash is formed by the catastrophic drop in pressure on magma brought about by the volcanic eruption (breaking up of volcanic edifice results in atmospheric pressure inside volcano). This causes gases in the magma to expand violently, fragmenting the magma into tiny pieces, which instantly solidify on ejection into the atmosphere (lower temperature compared to magmatic temperatures).

Ash from a particular volcano has its own unique characteristics, much like a person's fingerprints. These characteristics include chemical composition, and the size and shape of crystals and glass shards. They can be used to determine not only the particular volcano that produced the ash, but the particular eruption from that volcano as well.

The characteristics, along with the age of the ash, help scientists identify the source of material. Volcanic rocks are typically divided into four basic types—basalt, andesite, dacite, and rhyolite—according to the average concentration of major compounds in the rock. These compounds include silicon dioxide (SiO₂), titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), iron oxide (FeO or Fe₂O₃), manganese oxide (MnO), magnesium oxide (MgO), calcium oxide (CaO), sodium oxide (Na₂O), potassium oxide (K₂O), and phosphorous pentaoxide (P₂O₅).

In 1971, Michael Voorheis, a paleontologist at the University of Nebraska State Museum, made a startling discovery at a farm in northeastern Nebraska. He uncovered the bones of 200 fossilized rhinos, together with the prehistoric skeletons of camels, lizards, horses, and turtles. They had been killed millions of years ago by suffocating amounts of volcanic ash (the site later became known as Ashfall). But there are no volcanoes in Nebraska, nor had there ever been. In fact, there are no volcanoes in the continental United States east of Colorado. So where did the ash come from?

Scientists discovered that the ash originated in an extremely explosive eruption approximately 10 million years ago. By analyzing and comparing ash samples from the Ashfall site to those produced by volcano eruptions of the same age, the scientists were able to identify the source of the eruption as the Bruneau-Jarbridge volcano in southwestern Idaho, some 1,500 kilometers away.

In this activity, students will use real volcanic data to identify the likely source of volcanic ash found in an area of Nebraska.

Key Terms

caldera: Large depression formed by collapse of the ground following an explosive volcanic eruption of a large body of stored magma.

pyroclastic flow: Hot, dry, fast-moving mixture of ash, pumice, rock fragments, and gas from a volcano.

supervolcano: A volcano that has produced an exceedingly large explosive eruption involving the ejection of huge amounts of ash into the atmosphere, causing formation of a giant caldera.

tephra: The general term now used by volcanologists for airborne volcanic ejecta of any size.

1. Brainstorm with the class about the possible types of information they might need to identify a source of volcanic ash. (The age of a sample is normally a key identifying characteristic; however, because supervolcanic explosions are so rare, knowing the age of the ash sample would allow students to immediately identify the source. Therefore it is not included as a characteristic in this activity.)

2. Organize students into teams and distribute copies of the "CSI: Ashfall Fossil Beds" and "Volcanic Identification" student handouts.

3. Ask students to use the information on the "Volcanic Identification" handout to identify the type of ash at the Nebraska site and the type of eruption and volcano that might have produced it.

4. Explain to students that they will be provided with three sets of data, one at a time. (You may want to point out to students that this is actual real volcanic data.) For each data set, they will be asked to analyze the information and— based on that data—eliminate "suspect" volcanoes they think least fit the profile for the Ashfall event. They also will be asked to rate how certain they are of their conclusions. Explain that they will review their conclusions again when all the data are in.

5. Distribute the three data handouts in the following order, having students complete each one before starting the next:

   Volcano Suspects: Location
   Volcano Suspects: Description
   Volcano Suspects: Ash Composition

6. After students complete each handout in the order given above, have them mark their "Volcano Suspects Table" on their main "CSI: Ashfall Fossil Beds" handout with the volcano or volcanoes they feel is or are the least likely suspect(s) based on the data, and rate their confidence level in their answer.

7. After all students have completed all three handouts and filled out their "Volcano Suspects Table," ask them to identify their main suspect and answer the questions listed on their "CSI: Ashfall Fossil Beds" handout. Lead a class discussion about which volcano was the main suspect and why. Did students' opinions change as they got more data? Why or why not? Which data were most relevant? Which were least relevant?

8. As an extension, ask students to research and report on the eruptions of the different "suspect" volcanoes. When did the major eruptions occur at each site? What was the result of those eruptions? Should students be concerned about living in an area where supervolcanic eruptions might have occurred? Why or why not?

Volcanic Identification

Type of ash at Ashfall: felsic, because the ash contains a silica content of more than 65 percent

Type of eruption most likely to have created Ashfall: highly explosive

Type of volcano form most likely to have created Ashfall: caldera or dome volcano

Volcano Suspects: Location

1. Which volcano is located closest to the Ashfall site? Yellowstone

2. Which volcanoes are farthest away? Mount St. Helens, Lassen, Crater Lake, and Long Valley

Volcano Suspects: Description

1. The eruptive volume corresponds to the explosiveness of a volcano. Which volcano had the most explosive eruption? Which volcano had the least explosive eruption? Yellowstone had the most explosive eruption; Mount St. Helens had the least.

2. Does there seem to be a relationship between the size of a volcano's crater/caldera and an eruption's explosiveness? Why or why not? Yes, more explosive volcanoes seem to have larger calderas because of the more powerful eruptions they create.

Volcano Suspects: Ash Composition

1. For each suspect volcano, would you characterize the magma that produced the ash sample as mafic, intermediate, or felsic? Why? All the given samples correspond to felsic eruptions based on their silica content.

2. Which ash seems to be most similar in composition to the Ashfall sample? The Bruneau-Jarbridge ash is most similar in composition.

CSI: Ashfall Fossil Beds

Student answers will vary but should indicate the following:

Location: most-distant volcanoes (Mount St. Helens, Lassen Peak) least likely

Description: stratovolcanoes (Mount St. Helens, Lassen) and less explosive volcanoes (Crater Lake, Mount St. Helens, Lassen) least likely

Ash Composition: those with greatly differing amounts of silica, aluminum, sodium, and potassium from the Ashfall site (Mount St. Helens, Crater Lake, Lassen Peak, Long Valley—and to a lesser extent, Valles Caldera, and La Garita) least likely

Sample Volcano Suspects Table

CSI: Ashfall Fossil Beds Student Handout Questions

1. Most of the listed "suspect" volcanoes have calderas, which are large depressions formed by the collapse of the summit or flanks of a volcano during a large-scale, highly explosive eruption. Why would a caldera-forming eruption be the most likely source of the ash found in Nebraska? Caldera-forming eruptions are felsic eruptions and would produce massive amounts of ash that could reach a great distance from the source of the eruption.

2. The most explosive volcanoes have magma with a very high silica (SiO₂) content. Based on this information, which of the suspect volcanoes is most likely to have had the most explosive eruption? Yellowstone and La Garita

3. Which volcano do you think was the most likely source of the eruption that killed the animals in Nebraska? Why? Bruneau-Jarbridge, because the ash is very similar in composition. Even though the caldera is not the closest, it had a very explosive eruption that might have produced enough ash to reach Nebraska.

Web Sites

NOVA—Mystery of the Megavolcano
www.pbs.org/nova/megavolcano
Discover what a supervolcano eruption might mean today, find out what lessons can be learned from the Toba eruption, see the impact Toba had 75,000 years ago, and explore a map of supereruptions around the world.

Ashfall Fossil Beds State Historical Park
ashfall.unl.edu
Features information about the Ashfall Fossil Beds as well as the history and geology of the area.

Smithsonian Institution: Global Volcanism Program
www.volcano.si.edu
Describes volcanoes around the world and eruptions that have occurred during the past 10,000 years.

Books

Volcanoes and Earthquakes
by Susanna Van Rose. Dorling Kindersley, 2004.
Explains how volcanoes and earthquakes occur.

Volcanoes
by Robert and Barbara Decker. W.H. Freeman and Company, 1997.
Provides detailed information about the geology of volcanoes.

The "Investigating Evaporation" activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).

Grades 5-8
Science Standard D
Earth and Space Science
Structure of the Earth system

Science Standard F
Science in Personal and Social Perspectives
Natural hazards

Grades 9-12
Science Standard D
Earth and Space Science
The origin and evolution of the Earth system

Science Standard F
Science in Personal and Social Perspectives
Natural and human-induced hazards

Classroom Activity Author

Margy Kuntz has written and edited educational materials for more than 20 years. She has authored numerous educational supplements, basal text materials, and trade books on science, math, and computers.