Magma Degassing and Eruption Style 2

Jessica F. Larsen, University of Alaska, Fairbanks (
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Initial Publication Date: December 16, 2020 | Reviewed: March 12, 2023


This activity provides an opportunity for students to work with real data collected from samples from the 2008 eruption of Okmok volcano, Alaska. The homework activity explores how magma gas content can be estimated from the porosities of volcanic rock samples from explosive eruptions. Students are given weight data collected using Archimedes method for numbered samples from the 2008 eruption, collected from prior class sessions held at UAF. From the dry and submerged weights of individual samples, students calculate bulk density and porosity (or % vesicularity) as a proxy for the gas content of the magma at the moment of fragmentation (with a few assumptions). Students then can estimate the permeability of the magma via percolation theory, applied to their porosity data. Finally, students use their data and models to answer questions about the mechanisms of eruption and eruption style. This activity was inspired by the Porosity and Permeability Module by Chuck Connor of USF as part of his Spreadsheets Across the Curriculum project.

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This activity is aimed at upper division (junior or senior level) undergraduate students majoring in geology or geophysics. The activity can be scaled up or down in level to apply to lower division geology or geophysics students, or for graduate students in volcanology. An entry level version of this module for college freshmen, high school students, or non-science major courses can be found in the Magma Degassing 1 module. Another version of this type of activity can be found in the Spreadsheets Across the Curriculum, Porosity and Permeability activity by Chuck Connor.

Skills and concepts that students must have mastered

  • Students need to have a background in geology or geophysics, including the basics of volcanic processes
  • Students should be able to use a spreadsheet program to make calculations using a series of data given to them.
  • The activity requires students to be able to make plots of their data, including x and y error bars.
  • A basic knowledge of statistics is required, involving calculating the mean and standard deviation from three sets of data.
  • Students should have basic physics knowledge, including forces, buoyancy, density, mass, and volume relationships, as applied to Archimedes principle.

How the activity is situated in the course

This activity is best suited for a focused course covering physical volcanology at the junior or senior undergraduate level. The activity should come after the course covers the basics of magmatic gas exsolution, gas bubble growth, and simple conduit processes including fragmentation and steady state Plinian style eruptions.


Content/concepts goals for this activity

  1. Recall (from lecture) the reasons why magma degassing helps control whether volcanoes erupt explosively or effusively.
  2. Review Archimedes Principle, and how this simple physics law is quite useful and accurate to apply to a modern physical volcanology problem.
  3. Apply your knowledge of Archimedes Principle by calculating the gas content (ie vesicularity) preserved in natural pyroclasts and lava samples, through bulk density and porosity estimates.
  4. Apply statistics to derive the best value of porosity and vesicularity from each clast and an error estimate, based on three replicate measurements provided to you.
  5. Use Percolation theory to estimate the gas permeability of each clast, based on the calculated porosity (after XX reference).
  6. Compare the Percolation theory model against permeability data collected from the same clasts in the UAF Petrology Lab.
  7. Demonstrate your knowledge by answering questions about the 2008 eruption based on your calculations and models.


Higher order thinking skills goals for this activity

  • This activity provides and opportunity for students to work with real data collected by the author, and apply to models of magma degassing
  • Using data collected by prior classes will provide students with practice applying statistics to their calculations to observe variations in data and different sources of error.
  • Students will gain practice applying models they calculate to an eruption case study (2008 Okmok)
  • Data computed and modeled by students will be used to apply to conduit processes from an actual eruption.

Other skills goals for this activity

  • Students will gain practice using Excel (or another spreadsheet program) to make calculations on arrays of data. This skill can be useful in future courses or even in graduate school.
  • The activity involves computing statistics on real data points, and students will gain experience plotting and examining error bars and sources of error on repeat measurements.
  • The application of percolation theory to estimate permeability from their porosity data provides experience working with theoretical models applied to real world data.

Description and Teaching Materials

The teaching materials include a PowerPoint slide show describing the activity and background about the eruption. The word document describes the activity and can be used as is, or can be modified in any way the instructor wishes. The excel file provided includes the dry and submerged (wet) weights of a series of numbered scoria samples from the Okmok 2008 eruption. There are three sets of measurements included on the same set of numbered samples, so that the mean and standard deviation of the different measurements can be computed. The references link includes more information about the 2008 Okmok eruption that can be used as background information.

Because the data are provided with these materials, this activity is well-suited for either in person or online courses. The activity can be modified easily to include measurements of bulk density from samples in an in person lab course, but the instructor would need to find their own samples for this purpose.

PowerPoint SlideShow Magma Degassing 2_Serc_Final.ppt (PowerPoint 12.8MB Nov24 20)

Word Document Outlining the Activity Magma Degassing #2 SERC.docx (Microsoft Word 2007 (.docx) 19kB Nov24 20)

Excel File with Sample Data Okmok_2008_Sample_Data.xlsx (Excel 2007 (.xlsx) 13kB Nov24 20)

Teaching Notes and Tips

This exercise will be used as a homework assignment (augmented with additional lecture material) in an online version of the UAF GEOS 406 course Spring 2021. It is written to be used primarily as homework, which should take students on the order of a few hours to complete. It can be modified to include hands on measurements of class densities using Archimedes method if an instructor would like to include that activity. The instructor would need to find their own samples to do this. Parafilm wax sheets are needed to seal the exteriors of the samples against water filling the pores. Dry sample weights are easy to obtain by weighing the pumice directly on a scale. A scale and a sample hangar (with ballast) is needed but can be made very economically by using a wire (or coat hangar), a plastic water bottle (cut in half), and ballast (rocks or even nuts/bolts) filling the bottle. For the submerged weight measurement, place the ballast-filled bottle on a scale and attach the wire hangar to it (with a sample holder fashioned on the end - coil up the end of the wire to make a little coiled cup). You can capture the parafilm coated sample in the sample holder by pinching it between two coils. Then, place the sample in the water attached to the holder on the scale (tared to account for the holder/ballast weight). Record the wet weight measurement.


The instructor should perform all of the activity calculations ahead of time to make up an answer key. Students are graded on correct calculations, correctly inputting the formulas into their spreadsheet, and obtaining the same answers to the bulk density, porosity, and permeability calculations. Text-based answers can be graded at the discretion of the instructor, but students should be able to discuss the application of their calculations to the 2008 eruption clearly, and accurately, based on prior background information given in the class.

References and Resources

Larsen, Jessica F., Maciej G. Śliwiński, Christopher Nye, Cheryl Cameron, and Janet R. Schaefer. "The 2008 eruption of Okmok Volcano, Alaska: Petrological and geochemical constraints on the subsurface magma plumbing system." Journal of Volcanology and Geothermal Research 264 (2013): 85-106.

Larsen, Jessica, Christina Neal, Peter Webley, Jeff Freymueller, Matthew Haney, Stephen McNutt, David Schneider, Stephanie Prejean, Janet Schaefer, and Rick Wessels. "Eruption of Alaska volcano breaks historic pattern." Eos, Transactions American Geophysical Union 90, no. 20 (2009): 173-174.

Okmok Volcano Information Page on the Alaska Volcano Observatory Website

Porosity and Permeability activity by Chuck Connor (USF).