Magma Viscosity Demos
This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
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.
Context for Use
Description and Teaching Materials
- Powerpoint for Magma Viscosity Activity (PowerPoint 190kB Aug25 09)
- Water-Honey Viscosity Movie (Quicktime Video 3.7MB Aug25 09)
- Chain Silicate Crystal Structure Movie (Quicktime Video 7.3MB Aug25 09)
- Quartz Crystal Structure Movie (Quicktime Video 3.5MB Aug25 09)
- Water-Toothpaste Viscosity Movie (Quicktime Video 1.6MB Aug25 09)
Teaching Notes and Tips
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.