Lesson 4 Activity 1: Forcasting the Path of Mudflows

• 30-minute demonstration
• 45-minute work session
• Students participate in a demonstration that will help them visualize the consistency of mudflows and how they move down stream valleys away from a volcano's summit or flanks. In a work session, students use topographic maps of Mount St. Helens before the 1980 eruptions to forecast the path mudflows might take during an eruption.

Key teaching points

• Volcanic avalanches and volcanic mudflows are somewhat similar, but different in one important respect. Both can contain
  1. volcanic debris, such as tephra of varying sizes ejected during an ongoing eruption, and
  2. lava and rocks from previous eruptions that were deposited on the volcano's slopes.

  Mudflows, however, are mixtures of volcanic debris and water. Volcanic avalanches lack the water of mudflows.

• Sources of water can include the
  1. breakout of a volcanic or glacial lake,
  2. melting snow and glacier ice,
  3. streams and rivers that flow down the flanks of a volcano, and
  4. intense rainfall.

• Mudflows behave differently than avalanches. Because water acts as a lubricant, mudflows travel farther than avalanches. Both avalanches and mudflows can move very fast.

• Mudflows move downslope and into stream valleys.

• Being able to forecast the path of mudflows is important to scientists who assess the potential hazards of volcanic eruptions. The chief threat to people is burial. Structures are at risk of being buried, carried away, or collapsing.

Materials

• Newspapers
• Large plastic tarp, 9' x 12'
• Rocks, bricks, or tent stakes to hold the tarp in place
• Buckets
• Large spoons or other sturdy stirring instruments
• One paper cup for each student
• Sand and gravel
• Water

• Activity Sheet 4.1 a - b
• Transparencies made from Master Sheet 4.1 and Activity Sheet 4.1b.
• Overhead projector

Procedures

1. Outdoors, construct a mockup of a volcano by crumbling up newspapers and piling them into the shape of a volcano.

2. Place a tarp over the newspapers. Make sure the tarp is large enough to simulate a flat area at the volcano's base. Also, create plenty of "hills" and "valleys." For ideas, refer to the photographs on the poster and to the topographic map on Activity Sheet 4.1b.

3. Place bricks, rocks, or tent stakes around the base of the tarp to keep it from moving.

4. Tell students that they will be creating an avalanche. Ask them to forecast the path the avalanche will take.

5. Create an avalanche:
   • Distribute paper cups and fill them with sand.
   • Pour the contents onto the top of the volcano to simulate an avalanche.
   • Repeat with gravel and then again with a mixture of sand and gravel.
   • Have students observe where the materials slide and which particles move fastest.
   • Discuss why. (Gravity pulls the materials downslope. The heaviest particles move fastest; the smallest particles move the farthest because they require less energy to move them.)

6. Create a mudflow:
   • In a bucket, mix water with sand and gravel to make a slurry.
   • Distribute paper cups and fill them with the slurry.
   • Pour the slurry onto the top of the volcano.
   • Discuss: Where did the mudflow flow? (It should go into the valleys.) Did it behave differently than the avalanche? What happens when the avalanche hits the flat area at the base of the volcano? (Slows down)

Work Session

Based on what they saw in the demonstration, students use a topographic map of Mount St. Helens before the 1980 eruption to forecast the paths mudflows might take as a result of an eruption. The students then compare their maps with the map that shows the path the flows actually took following the May 18, 1980, eruption. (Master Sheet 4.1)

Distribute Activity Sheets 4.1a-b. Note that the topographic map on Activity Sheet 4.1b is the same as Map A in Lesson 2 except that it (a) covers a more extensive area, (b) the contour interval is 150 meters instead of 100 meters, and (c) north is oriented toward the top of the map.

Discussion

1. Discuss the students' forecasts. Will mudflows follow stream valleys? Will mudflows occur on all sides of the volcano?

2. Show students a transparency of Master Sheet 4.1. Compare this map with the students' forecast maps.

3. Discuss why the south flank and the area to the south were relatively untouched by mudflows. (The lateral blast blew hot volcanic debris to the north.)

4. Using your transparencies, compare the extent of the glaciers on Mount St. Helens before and after the eruption. What happened to the glaciers? What happened to the ice in the glaciers? (It was melted by the heat of the eruption or was "ground up" by the avalanche)

5. Bring up the point that Mount St. Helens behaved in an unexpected way that even the scientists did not anticipate. (For example, David A. Johnston, a USGS volcanologist, was monitoring Mount St. Helens on a ridge north of the volcano, well outside of the anticipated danger zone, or so he thought. At 8:30 a.m. on May 18, 1980, Dr. Johnston made his last radio transmission: "Vancouver, Vancouver, this is it!" No trace of him or his equipment has ever been found.)