How Thick Is It: Viscosity and Volcanoes

Materials / Preparation

• For the teacher demonstration:
  • Liquids of different viscosity
    • Low viscosity: water, alcohol
    • Medium viscosity: mineral or vegetable oil
    • Higher viscosity: dish soap, shampoo
    • High viscosity: molasses
  • Bottles for each liquid - fill each bottle half way
  • Food dye color
• 5 lb. sack of flour
• Each group of students will need:
  • 3 cups (4-oz. each)
  • 1 plastic spoon
  • 3 paper plates
  • 1 ruler
  • Mountain Photo Archive (Volcanic mountains from this collection are: Avacha, Iliamna, Mauna Loa, and Fernandina.) These can be accessed online on the Mountain Building Student Web Page, projected in the classroom, or you can make copies of the pictures for each group.
• Students will use page 13 of their Mountain Building Journals.

Grouping

Groups of two or four

Teacher tips

If the liquid is hard to see in the bottles, add a little colored food dye to more easily the liquid's movement.

If time is an issue prepare cups of flour ahead of time (60 ml [1/4 cup] of flour in each cup) and just have the students add the water.

The viscosity of lava determines the shape a volcanic mountain will take. Viscosity is influenced by factors such as the mineral, water and gas content, and the temperature of the lava. The end result is that some lavas are thin and runny, others are thick and viscous, and many are in between. Thin, low viscosity lavas produce volcanoes that are low, flat, and shield-shaped. High viscosity lavas produce steep-sided lava domes (narrow bases but not very tall, usually just a few hundreds of meters high) and the mid-viscosity lavas form large, tall, stratovolcanoes (composite) with slope angles in between those of shields and domes.

Potential student misconception: In this student activity, adding more water to flour thins out the "lava" and makes it more runny. Interestingly enough, the opposite is true in rocks in nature. Magmas in nature with high water contents are higher in viscosity (thicker). This has to do with some complex chemistry. Water molecules are "polar" (have a slight electrical charge), and therefore, they attract one another and the silica molecules that make up the rocks. These attractions make the lava stick together more, which makes it thicker. Water also lowers the melting temperature of rock, so magma with high water content is usually cooler. In general, liquids are thicker when cooler and runnier when hotter. If you are teaching about different volcano types, just be aware that adding water has the opposite effect in nature than in this activity.

Potential student misconception: The high viscosity flour mixture will produce the tallest "volcano" in this experiment. Some of them may get so tall they collapse which actually happens to real volcanoes as well. In nature, viscosity is much less important for volcano height than the total amount of magma that has flowed out of the volcano over time. Remember that the volcanoes of Hawaii's Big Island, when measured from the sea floor, are greater than 30,000 feet tall and are made of the thinnest lavas of all (basalt). That's because they've had a whole lot of magma coming out for the last half million years! For this experiment, emphasize the STEEPNESS of the sides and not just the height. That is the same in both nature and the experiment.

Procedures

1. Begin by showing students a picture of a volcano and ask them what it is.
• Ask students to describe how volcanic mountains are formed. After hearing several student narratives, make sure to summarize with a story like the following: "Lava erupts from the ground, flows away from the place it erupted, and eventually solidifies. This happens over and over again, slowly building the volcano up."
• Ask students, "what exactly is lava?" ( melted rock – that means it is a LIQUID)
• Ask students to name other liquids that they know. Ask them what makes these liquids different? Lead them to the concept of viscosity (some liquids are thick and gooey, while others are thin and run like water)
2. Visually demonstrate the concept of viscosity by showing your students bottles filled with liquids of different viscosities.
• Use alcohol or water as your low-viscosity example.
• Other liquids to use, ranked by increasing viscosity, may include
  • Medium viscosity: mineral or vegetable oil
  • Higher viscosity: dish soap, shampoo
  • High viscosity: molasses
• Turn the bottles over and observe how quickly the air bubbles trapped in it move upwards.
3. Return to the concept of volcanoes. Ask students if they can think of a way that viscosity could affect volcanoes. Remind them that volcanoes grow when liquid lava flows away from the volcano's center during an eruption.
   • Tell them that they are going to be simulating volcano growth. Even though all lava is made out of rock, some lavas have a different viscosity than others, depending on what type of rock they are made of, how hot they are, and how much water they have mixed in.
   • Tell students that they are going to investigate how different viscosity magma affects volcano (and therefore mountain) shape.
4. Have students prepare the materials as follows:
   • Label one cup High Viscosity, one cup Low Viscosity, and one cup Medium Viscosity.
   • Label one plate High Viscosity; one plate Low Viscosity and one plate Medium Viscosity.
   • Measure about 60 ml. (1/4 cup) flour into each cup.
   • Add 30 ml. (2 tablespoons) water to cup of flour labeled High Viscosity and stir until mixed.
   • Add 52 ml (3 1/2 tablespoons) water to cup of flour labeled Low Viscosity and stir until mixed.
   • Add 37 ml (2 1/2 tablespoons) water to cup of flour labeled Medium Viscosity and stir until mixed.
5. Remind students that they should record their observations (speed of flow, area of base, and height) in the data table given on page 6 of their Mountain Building Journal as they complete their investigation.
6. Have students slowly pour the contents of the High Viscosity cup of flour onto the plate labeled High Viscosity.
   • Have students observe how fast the “lava” flowed from the cup using relative terms such as fast, medium or slow.
   • Students should record their observations in their journals in the 'speed of flow' column.
7. Repeat step #6 with the Medium and Low Viscosity lavas and their respective plates.
8. Tell the students to allow all three of their lava mountains to “rest” for 2 minutes (before measuring the base diameter in two directions).
• Have students determine the base diameter of the volcano in two directions: ("north-south" and "east-west. Students should multiply these two numbers together to determine how large of an area each mountain base covers. Record.
• Have the students measure and record the height of each mountain.
9. Review and Reflection: Have student answer the questions on page 13 of their journals.
   • Make sure that students leave knowing the concept: a volcano’s shape is determined by the viscosity of its lava.

Resources used

Mountain Photo Archive
http://www.teachingboxes.org/mountainBuilding/lessons/mountainSlides/MountainPhotoArchive.pdf

Mountain Building Journal
http://www.teachingboxes.org/mountainBuilding/journal/MBJournal.pdf

Mountain Building Journal: Teacher's Guide
http://www.teachingboxes.org/mountainBuilding/lessons/journal/MBJournal_teacherGuide.pdf