Compositional Diversity in Volcanic Suites

Kent Ratajeski
Montana State University
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In this exercise, students use geochemical data downloaded from the GEOROC database to explore the origins of compositional variation among volcanic rocks associated with the Yellowstone and Crater Lake calderas.

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undergraduate- or graduate-level petrology course

Skills and concepts that students must have mastered

Some basic knowledge of plate tectonic settings (e.g., arcs and continental hotspots) is assumed for this exercise, but no advanced knowledge of the mineralogy and petrology of volcanic rocks beyond what is normally covered in introductory courses is required. No advanced knowledge of chemical variation diagrams is necessary, but it would probably be best if the instructor had covered the basics of whole-rock data and variation diagrams previously in lecture. This exercise does not assume that every student knows how to plot data in Excel spreadsheets, so I have included several pull-down text boxes (marked by 'Show me' tabs), as well as a separate page on plotting chemical variation diagrams in Excel, in order to help these students.

How the activity is situated in the course

This activity could supplement class lectures on compositional diversity or volcanic caldera complexes.


Content/concepts goals for this activity

Students who complete this exercise should be able to use the GEOROC online geochemical database to extract useful whole-rock major- and trace-element data from precompiled datasets, make useful geochemical plots (Harker diagrams), and interpret geochemical plots to suggest or rule out possible petrogenetic models.

Higher order thinking skills goals for this activity

This exercise requires students to formulate hypothesis based on geochemical data, and to compare/contrast two different volcanic suites.

Other skills goals for this activity

Students gain practice in using an Excel program to plot geochemical data, interpreting geochemical plots, and writing answers to open-ended questions.

Description of the activity/assignment

In this exercise, students use whole-rock major- and trace-element compositions of volcanic rocks to explore the origins of compositional variation in igneous suites. With the help of detailed step-by-step instructions, datasets from the Yellowstone and Crater Lake calderas are downloaded from the GEOROC database, imported into Excel spreadsheets, and graphed in the form of "Harker" diagrams to learn about the different petrogeneses of these two volcanic suites.

Determining whether students have met the goals

This activity is formatted as a self-paced exercise where students can check their own answers by clicking on "Show answer" tabs. The exercise could be reformatted as a normal homework assignment without the answers given and graded by the professor, using his/her own evaluation scheme.

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