Explosive Or Effusive?

Home , Cinder cone, Explosive eruption, Lava dome

1 Explosive or Effusive?

Why do some volcanoes erupt violently and others do not? In this activity, students will learn how lava chemistry affects the type of eruption.

Adapted from the lessons “Are We Falling Apart? Exploding Volcanoes” (http://www.thirteen.org/edonline/lessons/volcanoes/b.html) and “Exploding Volcano” (http://eosweb.larc.nasa.gov/EDDOCS/Aerosols/exploding_volcano.html).

Grades: 3-5

Time frame: Two 30-45 minute lessons, plus additional time for lesson extension activities

Student Grouping: Pairs or small groups (3-4 students per group)

National Science Education Standards: Science as Inquiry, Physical Science, Earth and Space Science content standards

Teacher Background Knowledge

Volcanic eruptions may be explosive (violent) or effusive (passive), depending on the lava chemistry (amounts of silica and dissolved gases.) Silica is a mineral found in nature as sand or quartz. High levels of silica mean very viscous (thick) lava, and low levels mean more fluid lava. Dissolved gases (sulfur dioxide, water vapor, and carbon dioxide) build up inside the volcano, similar to how carbon dioxide builds up inside a can of soda that has been shaken. The higher the level of gas, the more pressure that builds – and the more violent an explosion. The combination of silica and dissolved gas levels determine the type of eruption and shape of the volcano.

Volcanoes are classified into four types, based on their lava chemistry and shape.

Shield Volcano: A shield volcano has low levels of dissolved gas and silica in its magma. Its eruptions are effusive, and the fluid lava moves quickly away from the vent, forming a gently sloping volcano. Mauna Loa in Hawaii is an example.

Cinder Cone Volcano: A cinder cone volcano has low silica levels and high levels of dissolved gas, resulting in fluid lava that erupts explosively as a result of the immense pressure built in the magma chamber. A cinder cone volcano erupts by shooting fountains of fiery lava high in the air. The lava cools and forms a steep-sided conical structure. Lava Butte in Oregon is an example.

Lava Dome Volcano: A lava dome volcano has high silica levels and low levels of dissolved gases in its magma. This results in effusive, viscous lava that forms a rounded, steep-sided

http://beyondpenguins.nsdl.org/ Issue 9: Earth’s Changing Surface (December 2008) Copyright 2008: Ohio State University. Funded by the National Science Foundation. Licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License.

2 Explosive or Effusive? mound. Lava domes are often created after an explosive eruption, which released much of the dissolved gas in the magma. The lava slowly continues to flow out of the volcano, forming a rounded, steep-sided mound. Since the 1981 eruption of Mt. St. Helens, a lava dome has been forming inside the crater of the volcano.

Composite Volcano: A composite volcano has high levels of dissolved gas and silica and erupts explosively. Before erupting, composite volcanoes often resemble steep-sided mountains before erupting. During violent eruptions, it can seem as if the whole top of the mountain has been blown off. Eruptions often include pyroclastic material (ash and lava fragments), leaving the volcano to collapse inward and form a crater. Mt. St. Helens and Mt. Rainier in Washington are examples.

Materials

2 graduated cylinders or cups per pair/small group

Water

Corn syrup

Straws (1 per student)

Bottle of clear soda or carbonated water

Plastic film canisters with lids (Black cylinders with gray lids are best. These are often available for free from photo-processing centers.)

Water

Small graduated cylinders

Alka Seltzer (regular strength) tablets, cut into halves and quarters

Lots of paper towels or rags for clean up

Powder (baby powder, talc) – optional

Fine sand - optional

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Procedure

Part One: Lava Viscosity

1. Explain that not all volcanoes have the same type of lava. Some volcanoes have very thick, sticky lava, while others have thin, runny lava.

2. Ask students to predict what type of explosion would happen with thick lava; thin lava. Have students record predictions.

3. Have students pour water into one graduated cylinder or cup. Pour corn syrup into the other. Ask them which represents thick, sticky lava (the corn syrup) and which represents thin, runny lava (the water).

4. Have students take turns placing straws into the cylinders and blowing to create bubbles. In which liquid was it more difficult to blow bubbles? Have students record their observations. For younger students, record observations as a class on the board or a chart.

5. Ask students to use their observations to explain which type of lava would produce a violent explosion. Have students record their explanations. For younger students, record conclusions as a class on the board or a chart.

Part Two: Dissolved Gases

1. Using a cross section of a volcano, explain that gases trapped inside the magma build up pressure inside the magma chamber. Demonstrate by shaking a bottle of clear soda so students can see the formation of bubbles. Discuss what would happen if you were to open the top of the soda bottle.

2. Explain that some volcanoes have more gas in the magma than others. Ask students to predict the types of eruptions in a volcano with high or low levels of dissolved gases.

3. Have students put about 20 ml (1 tablespoon) of water into a film canister.

4. Add a quarter tablet of an Alka Seltzer and quickly put on cap.

5. Observe what happens for about two minutes. The film cap should swell as the Alka Seltzer bubbles inside.

4 Explosive or Effusive?

6. Repeat using half of an Alka Seltzer tablet. Watch out – half a tablet will create greater pressure inside the canister and will cause the film canister’s cap to fly off!

Variations Repeat using half of a tablet and a lid with perforations that allows gas to escape. Repeat with sand or powder on top of the canister top.

7. Have students record their observations. For younger students, record conclusions as a class on the board or chart. 8. Ask students to use their observations to explain which type of lava would produce a violent explosion. Have students record their explanations. For younger students, record conclusions as a class on the board or a chart.

Lesson Extensions

1. National Geographic’s Forces of Nature web page (click on the Volcanoes icon) includes an interactive activity in which you change the dissolved gas and silica content of the magma and observe the simulated eruption. Use this on an interactive white board as a whole class activity to review the differences in eruptions based on lava chemistry.

Forces of Nature: http://environment.nationalgeographic.com/environment/natural- disasters/forces-of-nature.html

2. Watch video footage of volcanic eruptions and ask students to predict the viscosity and amount of dissolved gas in each type of lava. Videos are easily discovered through a web search. A few are listed below.

Hawaiian Volcano Observatory: Kilauea Volcano Time Lapse Movies http://hvo.wr.usgs.gov/gallery/kilauea/volcanomovies/#crater

Mt Erebus Volcano Observatory, Antarctica http://erebus.nmt.edu/

Mt. St. Helens Eruption Events Movies http://www.fs.fed.us/gpnf/volcanocams/msh/library/movies-home.php

3. Discuss how lava chemistry and the various types of eruptions lead to the formation of different types of volcanoes. Students could draw pictures or use clay to create models of cinder cone, shield, lava dome, and composite volcanoes. http://beyondpenguins.nsdl.org/ Issue 9: Earth’s Changing Surface (December 2008) Copyright 2008: Ohio State University. Funded by the National Science Foundation. Licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License.

5 Explosive or Effusive?

Volcano Types: http://eosweb.larc.nasa.gov/EDDOCS/Aerosols/Volcano_Types_Lesson.html

Assessment

This lesson provides many options for assessment, including: Student engagement and participation in the lesson (observations and anecdotal notes) Understanding of scientific concepts (student-recorded predictions, observations, and conclusions) Application of knowledge (student responses from lesson extension activities)