OverviewStudents view distribution patterns of tephra layers found around Mount Rainier and discover their source. This lesson plan is part of the "Living with a Volcano in Your Backyard" curriculum, created through a partnership between Mount Rainier National Park and the US Geological Survey Cascades Volcano Observatory.
- Understand general distribution patterns of tephra dispersal by wind at Mount Rainier.
- Understand how to interpret an isopach map.
- Recognize that some tephra at Mount Rainier originated at other Cascade volcanoes.
BackgroundTephra is a general term for fragments of rock and lava regardless of size that are blasted into the air by explosive eruptions. Tephra includes large rocks and small fragments, such as scoria, pumice, and volcanic ash. At Mount Rainier, tephra is conspicuous as a sandy material in colorful shades of orange, tan, yellow, gray, brown, and white. Each tephra layer represents an eruptive event.
During an eruption, large pieces of dense rock, pumice and scoria drop on the slopes of the volcano while volcanic ash often remains aloft and is transported laterally. When volcanic ash and surrounding air cools, and air speed is insufficient to support it, the ash drops to the ground and forms a layer often in the shape of an elongated oval.
While Mount Rainier is the source for at least 40 recognizable tephra layers, not all of the tephra found within Mount Rainier National Park originated there. Wind carried volcanic ash from eruptions at Mount St. Helens to the slopes of Mount Rainier on at least two occasions, once between 3,700 and 3,800 years ago and in A.D. 1480 (layers Yn and W respectively), and from Mount Mazama (Crater Lake) in Oregon to broad regions of the Pacific Northwest around 7,700 years ago (layer O).
These rogue layers puzzled early researchers who found the ash in unexpected locations and who recognized that their chemical composition differed from rocks at Mount Rainier. By carefully mapping the thickness of each ash layer, geologists were able to see that the ash layer thickened away from Mount Rainier and thereby were able to trace the ash and pumice to its source. They confirmed their hypothesis, thereby matching the chemical composition of the tephra layers with their volcanic origin.
Far-traveled tephra from Mount St. Helens and Mazama have a silica content that typically ranges between 62 and 67 percent. Tephra from these two volcanoes appears white, gray, tan, or orange in color. Mount Rainier rocks and tephra contain 55 to 64 percent silica. Its tephra typically darker in color than tephra from Mount St. Helens and Mount Mazama.
Teacher and Student pages for the Tephra Explorer lesson plan; and supplemental graphics.
Teacher information for the "Exploring Tephra Layers at Mount Rainier" portion of the lesson plan. Download
Teacher answers for the "Make Your Own Tephra Isopach Map" student handout. Download
Student handout to learn more about tephra layers at Mount Rainier. Download
Student instructions on making a tephra isopach map. Download
A map illustrating the locations of Cascade volcanoes. Download
Exploring Tephra Layers at Mount Rainier
Determine the origin of tephra layers on the flanks of Mount Rainier by studying maps and developing hypotheses based on tephra thickness and distribution. These tephra layers are some of the most prominent layers observable by visitors on the south side of Mount Rainier National Park.
You will need:
- Copies of Student Page: "Make Your Own Tephra Isopach Map"
- Copies of Student Page: "Exploring Tephra Layers at Mount Rainier (Tephra Maps 1-6)"
- Graphic: "Map of Cascade Volcanoes"
- Before class begins, determine whether you wish to project maps for all-class viewing or distribute a set of maps to each student or student group, and then make preparations. All students should receive a copy of the student page "Make Your Own Tephra Isopach Map".
- Tell students that some tephra layers on the flanks of Mount Rainier originated at other volcanoes and explain why this at first perplexed geologists who assumed that the tephra had erupted from Mount Rainier. Explain to students that they will observe maps of tephra layers at Mount Rainier. Encourage them to develop hypotheses about how wind direction influences the pattern of tephra fallout.
- Begin (and end) the activity with the "Tephra Isopach Map" student page, where students read that at Cascade volcanoes, the prevailing direction of air flow is towards the east, northeast, or southeast approximately 85 percent of the time. They are instructed to draw a hypothetical tephra layer–a partly teardrop shape that delineates the outermost limit of a recognized tephra layer. This tephra layer originates at the summit crater and extends away from the volcano, as blown by prevailing winds. Be sure that students recognize this as a vertical view, with the observer looking down upon a tephra layer on the ground. This step should advance students' understanding of the general pattern of tephra fallout at volcanoes, and help them to interpret tephra maps 1 and 2.
- Instruct the students to draw circles (contours) to separate the zeros from the ones, the ones from the twos, the twos from the threes, and the threes from the fours (see example on teacher page, 10). These contours indicate areas of equal tephra thickness, known as isopachs. Explain that a tephra isopach map is a standard tool used by geologists who study volcanic eruptions.
- Use overhead projection to display tephra maps 1 through 6 or provide paper copies to each student or student group. For each map, students should identify the wind direction at the time of the tephra eruption and a probable source volcano. Students should make note of the location of their community relative to the probable volcanic source and assess whether tephra from each eruption has fallen there. Use the "Map of Cascade Volcanoes" as a guide to this investigation.
- Promote further discussion about what happens when the wind direction changes during the course of an eruption or when rising tephra reaches atmospheric levels where winds blow in opposing directions. Tephra fallout patterns can become complex, such as during the eruptions of Mount St. Helens in the 1980s, when complex atmospheric conditions resulted in complicated small-scale patterns of tephra fall across Washington State.
- Return to the student page "Make Your Own Tephra Isopach Map". Instruct students to use a different color marker or pen to add to their student page maps, by drawing 3 to 5 hypothetical isopachs for a potential future tephra fall on Mount Rainier from a large eruption at Crater Lake. (Isopachs should appear as curved lines from the south, with decreasing thickness towards the north.) Provide additional scenarios for students' drawing of isopachs, such as the pattern of a tephra layer that results from easterly winds (which would result with a teardrop shape facing west).
- Invite discussion about how this information can assist geologists who investigate a volcano's history and hazards. Explore the types of information that geologists can obtain from these maps, such as the volcanic source, area and volume of tephra erupted, and wind direction at the time of eruption.
- Lead a discussion of energy transformation, where rising tephra gains potential energy as it rises and looses energy as it falls.
Optional: Provide students with information about some Pacific Northwest tephra layers by distributing the information on the teacher pages.
Adaptions: Use protractors with your study of tephra layers. Provide students with protractors and instruct them to determine precise compass directions of tephra fall for the "Tephra Maps," 1 through 6. For the student page "Make Your Own Tephra Isopach Map," choose some compass directions in degrees and instruct students to produce a tephra layer on the axis of the compass directions provided. Explore together what regions or communities would be affected by these hypothetical tephra eruptions
Look for students' understanding of the following concepts:
- That wind disperses tephra.
- Tephra particle sizes are greatest near the volcano.
- Some tephra on the slopes of one volcano might have erupted from a different volcano.
Students should understand that tephra layers provide a valuable record of previous eruptions. Students might indicate that this knowledge helps scientists determine the kinds of eruptions that happen at a given volcano, and ultimately the most likely eruption types for the future. Understanding what types of eruptions that can happen in the future ultimately saves lives and property and improves the well being of nearby communities.
While Mount Rainier is the source for at least 40 recognizable tephra layers, not all of the tephra found within Mount Rainier National Park originated there. This activity demonstrates how wind has the ability to carry volcanic ash from surrounding eruptions.
- Students research the tephra layers discussed in this activity. Students conduct a library or Internet search for information about the tephra layers discussed in this activity.
- Use kitchen ingredients to make a cross-sectional representation of tephra layers stacked one upon the other. Assemble cornmeal, cinnamon, oatmeal, flour, decorative sprinkles, and different colors of sugar, paper and glue sticks. Students draw a rectangle across one-half of a piece of construction paper. They take a pinch of each ingredient, and glue it on the paper from the bottom of the rectangle to the top. Each ingredient represents a layer of tephra. Students should label each layer, and then while observing the width of each layer on the paper and its relative particle size, add a few words about the eruptive event, such as "large eruption," "short eruption," "tephra from nearby eruption" and "tephra from far away."
Additional ResourcesMullineaux, D.R., 1974, Pumice and other pyroclastics in deposits at Mount Rainier National Park: U.S. Geological Survey Bulletin 1326, 83 p.
Crandell, D.R., 1969, Surficial geology of Mount Rainier National Park Washington: U.S. Geological Survey Bulletin 1288, 41 p.