The nature of volcanism as controlled by viscosity

Liz King
Illinois State University
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  • This activity is fun to include in a classroom.
  • This activity has the students design demonstrations using ketchup and peanut butter to document how viscosity differences between rhyolite and basalt control various behaviors.

Used this activity? Share your experiences and modifications



I use this activity for a portion of a lecture in petrology. This could very easily be used in an intro course and, much to the chagrin of the majors, I do a similar, but highly simplified, version for kindergarten classes.

Skills and concepts that students must have mastered

The full activity posted doesn't just have the students perform an activity, but rather they have to design the demonstration. I will give clues to the less creative groups but it does take a bit for them to step up and figure out how they are going to demonstrate the various volcanic properties and characteristics. Perhaps if used for an intro course the questions could be written in a little less open ended manner.

How the activity is situated in the course

This activity is done for a fun interlude in lecture.


Content/concepts goals for this activity

The goal of this activity is for students to develop a full understanding of exactly how different the magma viscosities are and how land forms, eruption style and crystal fractionation is controlled by this one property.

Higher order thinking skills goals for this activity

The development of an experiment that isn't laid out as cookbook steps for them encourages creative thinking. The students are not often faced with the creative side of designing experiments and lab exercises so this is a new experience for most of them.

Other skills goals for this activity

Other skill goals that are not unique to this exercise, but that are reinforced by it include group work, presentations to the rest of class, and critical thinking skills that are integrated throughout the course.

Description of the activity/assignment

This activity gives the students various properties or scenarios in magma chambers and volcanic eruptions that they need to demonstrate for the rest of the class. The students are told that basalt magmas have a similar viscosity to ketchup and rhyolite is similar to peanut butter. All they are given are the materials for the demonstration and a situation. The students then need to figure out how they are going to design a demonstration for the various situations as well as address the questions. They are not told that basalts make shield volcanoes and more felsic magma make the stratovolcanoes, they are asked to design a demonstration to document which lava will form which type of volcano.

Determining whether students have met the goals

If a year later the students can remember that basalt is the ketchup and rhyolite is the peanut butter.

More information about assessment tools and techniques.

Teaching materials and tips

Other Materials

Supporting references/URLs

Baker et al., 2004, The Viscosities of Foods as Analogs for Silicate Melts, Journal of Geoscience Education, v. 52, no. 4