|| Day Becomes Night || Volcanic Ash's Deadly Effects || Impact on Climate || How Much Ash Fell? || Activity 1 || Activity 2 ||

In less than 10 minutes after the onset of the cataclysmic eruption of Mount St. Helens, a column of tephra, steam, aerosols, and gases reached an altitude of 19 kilometers (12 miles). Although the largest fragments of tephra fell back to the ground close to the volcano, the smallest fragments, ash and dust, were carried eastward by the prevailing winds. Five days after the eruption, monitoring instruments in New England detected ash from Mount St. Helens. Some of the ash eventually circled the globe and the smallest fragments and aerosols remained suspended for years in the stratosphere.

Day Becomes Night

Moving at an average speed of 95 kilometers per hour (60 miles per hour), the ash cloud reached Yakima, Wash., by 9:45 a.m. Pacific Daylight Time and Spokane, Wash., about 2 hours later. In Yakima, a city of 51,000, day became night. Automobile and street lights remained on for the rest of the day as the eruption continued for more than 9 hours. Ash as fine as talcum powder clogged engine air filters and choked people - face masks or handkerchiefs were a necessity for those who ventured out of doors.

Ash blanketed the ground like snow, but snow that would not melt. Residents shoveled and bulldozed ash from streets, sidewalks, and roofs; an estimated 600,000 tons of ash were removed from the city. It took 10 weeks to haul it away!

Volcanic Ash's Deadly Effects

Ash fall, however, is more than an inconvenience. It can be lethal to plants, wildlife, and humans. Swirling particles of ash in the atmosphere generated lightning, which in turn ignited hundreds of forest fires near the volcano. Autopsies revealed that most of the human deaths in the blast area resulted from asphyxiation, from inhaling hot volcanic ash during the first few minutes of the eruption.

As the ash settled to the ground, it also took its toll. Eastern Washington became known as the "ash belt" where many farm crops were destroyed in areas of thick accumulation. Volcanic ash can also affect aircraft operations. Jet engines are susceptible to damage: the volcanic ash coats and melts turbine blades, often causing the engines to stall.

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Impact on Climate

Volcanic eruptions can also affect climate and weather patterns. Mount St. Helens' 1980 eruptions did not have a significant effect on global climate, but the 1982 eruption of El Chichòn in Mexico, for example, had measurable effects. El Chichòn's magma was much richer in sulfur than Mount St. Helens'. As a result, the Mexican volcano produced sulfuric acid aerosols (a fine mist of particles) that formed a layer of haze in the stratosphere. This haze, which can remain in the atmosphere for years, reflects the sun's radiation and reduces surface temperatures. For example, more than a year after the April 1815 eruption of Indonesia's Tambora volcano, its effects were felt. In the northeastern United States, 1816 was so cold that snow fell in some New England States in June and July. It was known in New England as the "year without a summer."

How Much Ash Fell?

In comparison to other historic eruptions, the volume of ash fall from the 1980 eruption of Mount St. Helens was relatively small (fig. 1). The eruption of Tambora ejected 150 times more ash than Mount St. Helens in 1980. And ash ejected by Mount Mazama (now Crater Lake), located about 125 kilometers (200 miles) south of Mount St. Helens, was even greater than Tambora. The 1980 eruption of Mount St. Helens was only an inkling of the destructive potential of a volcanic eruption.

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Activity 1—Tracking an Ash Cloud

15-minute demonstration
45-minute work session
Students observe a demonstration of a "dust box" to help them understand that some volcanic ash (tephra) may be difficult to see. In a work session, they will then use the equation, D = RxT (distance is equal to the rate of speed times the time of travel) to calculate the time it took volcanic ash erupted into the atmosphere to travel to different parts of the United States following the May 18, 1980, eruption of Mount St. Helens.

Key teaching points

1. Explosive volcanoes can erupt large quantities of tephra, gases, and aerosols into the atmosphere.

2. The smallest of these particles are suspended in the atmosphere and are sometimes carried by wind great distances from the immediate site of an eruption. Some of these particles are too small to see.

Materials

Demonstration

1. Large cardboard box such as a photocopy paper box
2. Black construction paper, dark cloth bed sheet, or black spray paint
3. Glue, tape, or staples
4. Scissors
5. Pencil or pen
6. Flashlight
7. Two erasers with chalk dust

Work Session

1. Activity Sheets 3.1 a— b
2. Atlas
3. Colored pens or pencils
4. Calculators (optional)

Procedures

Preparation:

1. Remove the flaps from one side of the box.
2. Line the interior of the box with black construction paper or a dark sheet, or spray paint it black.
3. On one side, cut a hole the same size as the lamp end of the flashlight.

Introduction

Use the photograph on the VOLCANOES! teacher packet table of contents (side 1 of the poster) and point out the column of ash rising above Mount St. Helens. Remind students that in Lesson 1 they learned that explosive volcanoes like Mount St. Helens erupt rock and lava fragments, gases, and aerosols into the atmosphere. The rock fragments, which are called tephra, can range in size from car-sized boulders to ash or dust that is so small it is invisible to the naked eye.

Demonstration

1. With the lights on, beat two erasers together inside of the "dust box."

2. Ask students to raise their hands when they can no longer see any "dust" in the air. When the last student has raised his or her hand, turn on the flashlight. Ask students to put their hands down if they now see dust.

Work Session

1. Discuss the photographs and captions in the atmosphere section of the poster (poster figs.12—14.). Remind students of the "dust box" demonstration. Like chalk dust, the smallest volcanic particles stay suspended in the atmosphere. Once in the atmosphere, they can be carried by wind great distances from the volcano. (fig. 2)

2. Some of these particles are so small that they form invisible clouds, which are particularly dangerous to airplanes that unknowingly encounter volcanic ash clouds. Volcanic ash sucked into jet engines can cause the engines to stall, as the following story illustrates:

   Following the 1989 eruption of Mount Redoubt in Alaska, a Boeing 747 aircraft lost power in all four of its engines after flying into a volcanic ash cloud. Finally after losing 4,267 meters (14,000 feet) in altitude the pilot was able to restart the engines. The airliner landed safely, but the damage to the aircraft has been estimated to exceed $80 million. Because of the potential hazard posed by ash clouds to aircraft, air traffic controllers need to issue warnings to aircraft flying in the air space they monitor.

3. Tell the students that they are air traffic controllers. They have just received word that Mount St. Helens has had a major eruption. They should stand by to receive critical data so that they can calculate when airborne ash will reach the air space they monitor. Discuss with the class the type of data they will need to gather, such as

   • the time of the eruption,
   • the wind direction,
   • the rate of speed the airborne tephra is traveling, and
   • how far their city is located from the eruption.

4. Divide the class into teams of air traffic controllers for each of the following locations:

   • Great Falls, Mont.
   • Rapid City, S. Dak.
   • Madison, Wis.
   • Minneapolis, Minn.
   • Chicago, Ill.
   • Detroit, Mich.
   • Pittsburgh, Pa.
   • Boston, Mass.

5. On their activity sheet, each team will use the data to calculate the time the ash is expected to arrive in the air space they monitor. (The ash cloud is moving at a rate of 96 kilometers per hour.)

6. Distribute the Activity Sheets.

7. Before students begin, review with the class the formula (D=RxT). Demonstrate how to use the formula to calculate both rate and time.

   

8. As a homework assignment, ask each student to calculate the time it took the ash cloud to circle the Earth. The Earth is 40,000 kilometers (20,500 miles) at the Equator.

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Activity 2—In the Rain Shadow

45 minutes
By recording the annual precipitation (rain and snow) for cities on the east and west sides of the Cascade Mountains, students will discover that volcanic mountains do not have to erupt to affect the atmosphere.

Key Teaching Points

1. The Cascade Range comprises a 1,130-kilometers (about 700 miles) long chain of volcanoes lying about 160 to 240 kilometers (100 to 150 miles) inland from the coast of the Pacific Ocean. Their location affects the climate of the Pacific Northwest region.

2. Because the Cascades act as a geographic barrier to moisture-laden masses of air arriving from the Pacific Ocean, cities on the west side of the mountain receive more precipitation annually than those on the east side. The cities on the east side are in the "rain shadow" created by the mountains.

Materials

1. Activity Sheets 3.2 a— b
2. Wall map of the United States
3. Glass of ice water

Procedures

1. Using a large wall map, locate the Cascade Range. Remind students that Mount St. Helens is one of the volcanic mountains that make up the Cascade Range.

2. Tell students that in the Pacific Northwest region, the prevailing winds blow from west to east. That means that most of the weather that affects the region forms over the Pacific Ocean where it picks up a great deal of moisture. Given this fact, ask students if they think annual precipitation is greater or lesser on the west or east side of the mountains.

3. Distribute Activity Sheets 3.2 a-b.

4. After the students have completed their activity sheets, discuss with them the reasons why cities on the west side of the Cascade Range have a greater precipitation than those on the east side of the mountains.

   • The mountains act as a barrier: Air must rise to get over the mountains. As the air rises, the temperature of the air falls and moisture in the air condenses. As the moisture condenses, it falls as rain or snow. By the time the air reaches the top of the mountain, most of the moisture has been lost as rain or snow.
   • As the air descends on the other side of the mountain, most of the moisture that remains is lost through evaporation instead of falling as precipitation.
   • That is why cities on the western, or windward, side of the Cascades receive a higher annual precipitation than those on the eastern, or leeward, side of the Cascades. The mountains have produced a "rain shadow" on the leeward side (fig. 4).

Extension

Have students prepare reports discussing how the differences in precipitation in the Pacific Northwest affect the natural resources and economy of the region.

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This teacher packet was originally published and printed in 1997. The online edition contains full text from the original publication. Some images have been modified or added to improve the scientific visualization of information. All poster text and images are available under the Poster section. A paper copy of the poster is for sale by calling 1-888-ASK-USGS. The Master Sheets and some Activity Sheets are in PDF format. The PDF format reproduces the printed publication and will be better for creating overlay and overhead transparencies. To view and print PDF's install the Acrobat® Reader, available at no charge from Adobe Systems. This document has undergone official review and approval for publications established by the National Mapping Division, U.S. Geological Survey.