Magma Degassing 1

Jessica F Larsen, University of Alaska, Fairbanks ( Nathan A. Graham, University of Alaska, Fairbanks (


This exercise introduces students to concepts of how magma gas content is important for eruption style. Students use the vesicles preserved in different volcanic rocks to estimate gas content by calculating vesicularity. The results are applied to simple determinations of eruption style.

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This exercise is appropriate for high school students or for college freshman level, non-science majors in general education courses.

Skills and concepts that students must have mastered

Students should be able to perform simple math calculations, using the ideal gas law and simple statistics to compute average and standard deviation for a series of measurements. Students should be able to perform simple exponential and square root calculations. A basic level of chemistry and/or physics (e.g., from high school courses) is also required for students to feel comfortable with Archimedes method and the ideal gas law.

How the activity is situated in the course

This activity would go into a lab or course module or section focused on volcanoes and volcanic processes. This activity would fit well in the context of an intro level, freshman course on geology either for majors or non-majors (ie Geology 101, or similar).


Content/concepts goals for this activity

This activity covers concepts of how magma gas content, combined with permeable or impermeable conduit walls, contributes to explosive or effusive eruptions. The activity introduces students to the concepts that volcanic rocks could preserve evidence of how much gas a magma had before and/or during an eruption. Students gain experience working with density data from volcanic rocks collected using Archimedes method. Students use that data to estimate percent gas content in different magmas, through volcanic rocks as proxies. This activity also provides practice for students to compute simple statistics on a set of repeat measurements.

Higher order thinking skills goals for this activity

This activity provides practice for simple statistics evaluation of data through average and standard deviation calculations. The activity requires students to make an assessment of the goodness of data within each sample set, based on how close each value is to the average. Students also need to assemble several concepts into models for how explosive versus effusive eruptions happen. They need to put together information about magma gas content with the physical state of the conduit from the different exercises in the activity, to conclude whether each sample represented an explosive or effusive eruption.

Other skills goals for this activity

If available to the class, students will also be expected to examine examples of each type of rock to make an assessment of what the relative magma gas contents were. Students will be expected to synthesize their results and write briefly about their ideas, calculation and model results, and conclusions.

Description and Teaching Materials

Students will examine photos or physical samples to make an initial estimate of gas content in the magmas that created those rocks. Students will then be given data sets of bulk densities for three different types of volcanic rocks and asked to calculate vesicularities and simple statistics from the given data. The data were collected using Archimedes method and an option for this activity is to allow students to perform Archimedes measurements themselves using rock samples provided by the instructor. This activity is, however, written to be completed without physical samples, although most geology departments have a basic sect of volcanic rocks students can at least examine for this activity. Students will get the most out of it if they can examine hands on examples of each type of rock discussed. For physical examples, the activity would be enhanced with a few samples each of pumice, scoria (from cinder cone for example), and lava. If students use physical samples for their first initial estimates of vesicularity, and standard grain percentage chart would be useful to help them estimate percentages of vesicles.

The second part of the activity relies on a simple experiment using film canisters and Alka Seltzer tablets to create small models of "sealed" and "leaky" volcanic conduits. For this activity, the instructor will need a few plastic film canisters with lids and some Alka Seltzer (or equivalent) tablets and a little bit of water. Using a pin, poke holes through the walls of at least one of the canisters. Fill up the cans maybe 1/3 to 1/2 full, drop the tablet in, and seal up the lid. The sealed canister will pop and shoot up in the air. SAFETY GLASSES REQUIRED! It is helpful to have a couple towels and clean water to mop up the mess.

Teaching Notes and Tips

This activity should be easily completed within a 3 hour lab course time period. For an advanced class or with modifications, this activity could also probably be completed within an hour and a half long course lecture time period.


Evaluation of student learning should include checking the calculations of vesicularity for accuracy and by assessing the qualitative answers written by students. The last question of the activity asks students to put everything together to synthesize their learning, and apply to different volcanic eruption styles.

References and Resources

Archimedes' Principle visual, from Weber State University

Find-A-Feature: Vesicles, from USGS

Krakatau information, from the Smithsonian Institution National Museum of Natural History Global Volcanism Program

Popocatépetl information, from the Smithsonian Institution National Museum of Natural History Global Volcanism Program

Eruption Styles, from Oregon State University

USGS Volcano Hazards Program Glossary