Magma Viscosity Demos

This page authored by Barry Bickmore, Brigham Young University.
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
Initial Publication Date: November 10, 2010 | Reviewed: October 21, 2012


In this interactive lecture, geology or earth science students answer iClicker questions and have group discussions about two videos that demonstrate the links between magma composition, temperature, and viscosity, as well as how viscosity controls the explosiveness and morphology of volcanoes. The movies and a Powerpoint file with frames set aside for inserting the movies are provided.

Used this activity? Share your experiences and modifications

Learning Goals

Students should learn to recognize how silica content and temperature control magma viscosity, and magma viscosity controls the explosiveness and morphology of volcanoes. They should also link their prior knowledge about molecules, phases of matter, silicate crystal structures, and igneous rock classification with magma viscosity.

Context for Use

This is a good in-class activity for any introductory Earth Science or Geology course. It takes about 20-30 minutes of class time. I use it for college classes, but it might also be appropriate for high school or middle school. It is also possible to run the water-honey demo and the toothpaste-water demo as live demonstrations instead of videos. However, the movies always work, require no setup time, and the whole class can see them on a giant screen.

Description and Teaching Materials

I have included four movie files in Quicktime format, along with a Powerpoint in which to insert the movies.

Teaching Notes and Tips

One of the main problems I face is that many of my students treat material from different chapters in the textbook as completely separate information. It's just a mass of disconnected facts that they have to memorize. With this activity, I try to help them see that all the stuff they learned earlier about molecules, phases of matter, silicate crystal structures, and igneous rock classification can help them understand the different types of volcanoes. (Why are some volcanoes more explosive than others? Why are particular surface features associated with particular kinds of volcanoes? Why does basaltic magma have an easier time making it to the surface?) It's this kind of connected knowledge that students will have a better chance of retaining and being able to use after the class is over.

The first demo is a movie of me blowing air into a beaker of water, then a beaker of honey, and then a beaker of heated honey. The point is that sugar molecules make the liquid more viscous, because they like to stick together. They stick together less readily when heated. Likewise, silica molecules in magmas and minerals like to polymerize, so the more silica there is, the more viscous the magma, and when it's hotter, it becomes less viscous. More viscous magmas build up gas pressure more readily, and so are more explosive. They also have a harder time making it to the surface and don't spread out as much once they are erupted. The crystal structure demos illustrate the idea that more silica = more polymerization.

The second demo is a movie of me shooting water up out of an inverted funnel, and then shooting out toothpaste. The water runs out and spreads out because it isn't very viscous. The toothpaste comes out and piles up on itself, rather than running down the sides very far. This illustrates why runny lavas make shield volcanoes and flood basalts, but more viscous magmas make tall, pointy cones.

My ultimate goal is to get them to the point that they can pick up a piece of basalt, identify it, and make up a fairly complete story about how it originated, referring back to basic physical and chemical principles, when appropriate.

The Powerpoint should be pretty self explanatory in terms of the progression of ideas I take the students through. However, if you don't feel like you have the time to spend with all the "interactive" elements, like small-group discussions and iClicker questions, you can just import the movies into your own Powerpoint and use them however you like.

Having these demonstrations in a movie format is especially good for large classes, where it might be difficult for students in the back to see what the instructor is doing on the bench top.


I use the iClicker questions for informal assessment in class (so the students can take a stab and see if they were thinking right). Then I ask multiple choice questions on tests and quizzes that specifically target connections between concepts. For example, I might ask which type of magma (Felsic, Intermediate, Mafic) would be likely to be the most viscous, or associated with the most explosive volcanoes, or whatever.

References and Resources