Serpentinite in Subduction Zones: How do we find it, and how common is it?

Jeffrey G. Ryan, University of South Florida
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One of the interesting discoveries arising out of the MARGINS Subduction Factory Initiative is the recognition that serpentinites - metamorphically hydrated products of ultramafic rocks that are rich in serpentine group minerals - are significant constituents of both the mantle wedge and downgoing plate. Serpentine minerals are interesting mineralogically because of their distinctive physical properties, habits and appearance; and for their commonly close affinities for olivine and Mg-rich pyroxenes. As well, serpentine group minerals are interesting rheologically because, as sheet silicates, they can behave in a plastic fashion in rocks that are undergoing deformation.

Photomicrograph of South Chamorro seamount serpentinite This activity leads students through a range of literature sources and activities, including GeoMapApp manipulations and some "back of the envelope" calculations about isostatic compensation and volumetric expansion during serpentinization to help them appreciate the different ways that the presence of serpentinite can be observed or inferred in subduction zone settings. A second but equally important goal is to familiarize students with some of the basic kinds of data and interpretive approaches used by geochemists and geophysicists in studying modern subduction zone environments.

This activity is targeted for use in Junior- or Senior-level mineralogy and/or petrology courses. It makes use of published data and information from both of the MARGINS Subduction Factory focus sites (Izu-Bonin-Mariana and Central America), as well as information from meeting presentations posted on the NSF-MARGINS homepage site. It presumes a basic familiarity with examining geospatial data (primarily comparative elevations) using GeoMapApp. It is suggested that students be provided with thin sections or hand samples of actual serpentinites to give them a better understanding of what these materials are, and how they compare to olivine-rich ultramafic rock samples more typical of 'fresh' mantle.

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Learning Goals

The biggest learning goal for this activity is to get students to integrate mineralogical, petrological and geophysical information to draw conclusions about the nature and occurrences of serpentinites in subduction zones. To be able to do this, they will need familiarity with mineralogy, with the reading of phase diagrams and related petrologic plots, and with examining and understanding various 2D interpretive images of the seismological characteristics of subduction zones.

Context for Use

I designed this activity with a Junior-level mineralogy/petrology course in mind. The time necessary to complete the activity varies a lot with the way one chooses to approach it (see Teaching Notes and Tips below) and with the overall background of the students in the course. The listed references and access to (and the ability to manipulate) GeoMapApp are all that is really necessary to do this activity, though having some handsamples and thin sections of actual serpentinites, for comparison to "fresh" ultramafic rocks, can be helpful.

Description and Teaching Materials

Teaching Notes and Tips

Given that this activity draws upon at least three different geoscience subdisciplines, it is a safe bet that many students will be absolutely lost trying to do this independently. My preferred approach with this activity is to make it a longer duration assignment (i.e., give it a due data a week to 10 days hence) and to cover the different "problem" points in the work along the way as students identify them.

What these problems are likely to be will vary with student group: my students, as an example, are comfortable with the mineralogical and petrologic aspects of the activity, but have difficulty conceptualizing the seismological aspects and with doing the suggested calculations. Also, in general, getting students to read technical papers for information (as opposed to the way one reads a novel) is challenging.


Again, this can vary a lot depending on how the activity is approached. Having students provide answers to all the questions listed is one option, though very time consuming. I will often work through a section or a couple questions with students in class, and then assign other sections for homework, which we discuss as a class and hand in for evaluation.

References and Resources

All relevant literature resources are listed in the activity worksheet.

Students will need access to GeoMapApp ( for one part of the activity.