Starting Point-Teaching Entry Level Geoscience > Interactive Lectures > Longer Activity Examples > Magma Viscosity Demos
Explore Teaching Examples | Provide Feedback

Magma Viscosity Demos

This page authored by Barry Bickmore, Brigham Young University.
Author Profile

This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are

  • Scientific Accuracy
  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
  • Robustness (usability and dependability of all components)
  • Completeness of the ActivitySheet web page

For more information about the peer review process itself, please see http://serc.carleton.edu/NAGTWorkshops/review.html.

This page first made public: Nov 10, 2010

This material was originally developed within the Pedagogy in Action Portal


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.

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.

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

See related activities from the Cutting Edge petrology collection:
Viscosity Experiments: Physical Controls and Implications for Volcanic Hazard
The Nature of Volcanism as Controlled by Viscosity


Geoscience:Geology:Igneous and Metamorphic Petrology:Igneous Associations and Tectonic Settings, Igneous Processes, Igneous Rocks, Geoscience:Geology:Igneous and Metamorphic Petrology, Geoscience, Geology:Tectonics, Igneous and Metamorphic Petrology:Volcanology, Environmental Science:Natural Hazards:Volcanism

Resource Type

Activities, Classroom Activity:Short Activity:Demonstration, Audio/Visual:Animations/Video

Special Interest


Grade Level

College Lower (13-14), Introductory Level, High School (9-12), Middle (6-8)

Earth System Topics

Solid Earth:Volcanoes, Earth Materials:Rocks, Solid Earth:Igneous Processes, Plate Tectonics, Human Dimensions:Natural Hazards


Solid Earth:Petrology:Igneous Rocks , Igneous Associations and Tectonic Settings, Igneous Processes, Volcanology


Teach the Earth:Course Topics:Environmental Science, Teach the Earth:Teaching Topics:Volcanoes, Plate Tectonics, Teach the Earth:Incorporating Societal Issues:Hazards, Teach the Earth:Teaching Environments:K12, Intro Geoscience, Teach the Earth:Course Topics:Petrology

See more Longer Activity Examples »