Viscosity Experiments: Physical Controls and Implications for Volcanic Hazards

Ben Edwards
Department of Geology, Dickinson College
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This activity is a laboratory-style exercise that involves investigating the physical controls on viscosity by pouring different syrup mixtures down an inclined plane and using Jeffreys equation to calculate viscosity. Students time the syrup flows and then calculate viscosities. The four sets of experiments include examining the effects of temperature, dissolved water, solids and bubbles on syrup viscosity. Students then use Jeffreys equation to calculate the velocities of a variety of lava flow compositions. They also investigate the effects of composition, temperature, dissolved water and solids numerically using the freeware program MAGMA by K. Wohletz.

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This activity is designed for a sophomore or junior level required course in petrology.

Skills and concepts that students must have mastered

Students should have a basic understanding of viscosity and should be able to do basic algebra.

How the activity is situated in the course

This is a stand-alone exercise.


Content/concepts goals for this activity

This activity is designed to help students understand viscosity and how to calculate viscosities of different types of lava using the Jeffreys equation.

Higher order thinking skills goals for this activity

Other skills goals for this activity

Description of the activity/assignment

Teaching students about viscosity is easy, effective and fun. It is a topic that is conducive to a range of teaching and learning styles, and allows for the integration of theory, experiments, and calculations. During the course of this exercise, students are required to make predictions about the outcomes of experiments, quantitatively document the results of their experiments, calculate viscosities using the Jeffreys equation (Jeffreys 1925; Nichols 1939; Cas and Wright 1987), and extrapolate the concepts learned from their laboratory results to natural conditions appropriate for silicate magmas and lavas. Students are also introduced to Ken Wohletz's freeware program MAGMA (no longer available), which allows them to determine viscosities for magma and lava compositions, and are required to do some simple graphical analysis of the effects of composition, dissolved H2O, and % solids on magma and lava viscosity using the MAGMA calculations. Viscosity is important for students at all levels of earth science to understand because it is a critical control on morphologies of volcanoes, velocities of lava flows, eruptive styles (effusive versus explosive), and ascent velocities of magmas within the earth.

The objectives of the lab are for students to:
  • learn about the rheological property called viscosity and some of the factors that affect it;
  • think about and discuss ways in which viscosity controls styles of eruptions and relates to volcanic hazards; and
  • practice quantitative skills.
I have used the viscosity experiments as a classroom demonstration in introductory geology courses, as one part of a more extensive lab on volcanoes in introductory geology courses, and as a more intensive viscosity lab for introductory petrology courses. Generally the students do this exercise after they have had at least one introductory lecture on volcanoes, so that they are familiar with several basic terms, including viscosity, lava, magma, as well as some basic igneous rock terms (basalt, andesite, rhyolite). Over the fives years that I have been using the experiments, students at all levels have commented that the experiments are some of the most memorable, interesting and fun parts of my courses. I would welcome any direct student or instructor feedback for improvements or additions to the exercises (edwardsb AT

Determining whether students have met the goals

Students have met the goals for this activity if they complete the laboratory assignment (see lab handout download below) thoroughly and accurately.

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