Lesson 4 Activity 2: The Dangers of Snow and Ice

• 45-minute demonstration
• 45-minute work session
• Students observe a demonstration of how melting snow and ice can contribute to mudflows and then learn in a work session why some volcanic mountains have permanent snow and ice.

Key teaching points

• Some volcanic mountains have permanent snowpacks - snow that does not melt during the summer months. The lowest elevation at which snow remains on a mountain during the summer is called the snowline. (A mountain that has no snow in the summer has no snowline.) The snowline moves up and down a mountain seasonally - lowest in late winter and highest in late summer.

• The snowline is related to air temperature, which in turn is influenced by elevation and distance from the Equator: air temperature drops as elevation increases and distance from the Equator increases. Even in areas near the Equator, there are some mountains that have snow year-round at their highest elevations, whereas in polar regions permanent snow can be found close to sea level during the summer months.

This diagram shows how the elevation of the snowline changes with latitude. The approximate elevation of the snowline is indicated on this diagram where the white and black areas meet.

• The snowline is highest - that is, less of the mountain is covered with snow in the summer - on mountains closest to the Equator. The snowline is lowest on land closest to the poles. The greater the distance from the Equator, the less elevation is necessary to establish a snowline. (fig. 2)

• The snowline also is influenced by the amount of yearly snow fall. Thus, the snowline may not be the same for all mountains at the same latitude. (Generally, mountains closest to an ocean receive the greatest amounts of precipitation.)

• Melted snow and glacial ice significantly contributed to creating the mudflows that followed the May 18, 1980, eruption of Mount St. Helens.

Materials

• Potting soil, gravel, and water
• Baking pan
• Freezer
• Bunsen burner

• Magazines
• Large world map and push pins
• Activity Sheet 4.2 a - b
• Transparency of "Snowline Diagram" (fig. 2)

Demonstration

1. The day before the demonstration: In a baking pan, mix potting soil, gravel, and water to make a slurry. Place the baking pan in a freezer for at least 8 hours.
   

2. The next day: Bring the frozen slurry into class. Set it up at a steep angle. Hold a Bunsen burner under the high end of the baking pan. (fig. 3). Observe the pan at 3-minute intervals. Wait for the slurry to melt.

3. Point out to the students that this demonstration is similar to what happens when the heat of an eruption melts snow and ice on a volcanic mountain: water mixes with volcanic debris and creates mudflows.

4. Look at poster figures 10 and 11 that show Mount St. Helens before and after the eruption.

Work Session

1. Make a transparency of the “Snow Line Diagram” (fig. 2).

2. As a homework or library assignment, have students collect pictures of snow- and ice-covered mountains. Ask them to make a list of the names of the mountains and the countries or continents where they are located. (Collect your own group of photographs to make sure that all of the continents are represented.)

3. In class, make a list of continents on the chalkboard and compile the number of snow- and ice-covered mountains that the class found on each continent. Ask students if they expected to find snow-covered mountains in every continent.

4. Remind students that when Mount St. Helens erupted on May 18, 1980, it was capped by snow and numerous glaciers. Ask students if they would expect to see snow in May.

5. Distribute Activity Sheets 4.2a–b.

6. Students first label a group of volcanoes on a blank map. After they complete this part of the activity, ask them to stop.

7. Using a transparency, explain the "Snowline Diagram." This chart shows how the snowline varies with elevation and latitude. To plot a volcano and find its snowline:

   1. Find the approximate latitude of the volcano along the bottom of the chart and put a mark.

   2. Keep one finger on that spot and then find the approximate elevation along the right hand side of the chart.

   3. Put a mark where the two points come together. (For younger students, you may need to do this exercise as a class.)

8. Explain the concept of a snowline to the class.