Igneous Rock Compositions and Plate Tectonics

Allen Glazner
University of North Carolina, Chapel Hill
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In this exercise, students use whole-rock major- and trace-element compositions of igneous rocks from a variety of tectonic settings and locations to explore the importance of plate setting in determining magma compositions. Students are split into groups and assigned different tectonic settings to examine and compare with other groups. Datasets are obtained from the GEOROC database, imported into Excel spreadsheets, and graphed to learn how igneous rock compositions are a function of plate tectonic setting.

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undergraduate- or graduate-level petrology course

Skills and concepts that students must have mastered

Basic knowledge of plate tectonic settings and processes of igneous differentiation is assumed for this exercise. The students should be familiar with the theory and practice of variation diagrams and normalized REE plots, and should know how to manipulate and plot data in Excel spreadsheets.

How the activity is situated in the course

This activity could be used fairly early in an igneous petrology course to introduce the importance of plate tectonics in generating igneous rock diversity. It is recommended that this activity follow material on igneous processes, such as fractional crystallization and magma mixing.


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, make useful geochemical plots (variation and normalized REE diagrams), and interpret geochemical plots to suggest or rule out possible petrogenetic processes

Higher order thinking skills goals for this activity

This exercise requires students to formulate hypothesis based on geochemical data and to compare/contrast datasets.

Other skills goals for this activity

Students gain practice in using an Excel program to plot geochemical data, interpreting geochemical plots, working in groups, and making presentations in class. In addition, the act of obtaining and using data from online databases like GEOROC informs the students about the powerful resources that have recently become available to the scientific community via the creation of digital cyberinformatics and cyberinfrastructure. A carefully guided approach of guiding students into these databases, through the various steps required to screen, download, import, and use their data, empowers the students in a tangible, practical way to think and act like scientists.

Description of the activity/assignment

In this exercise, students are split into groups to gather whole-rock geochemical data (major-, trace-, and rare-earth elements) from the GEOROC database for igneous rocks sampled from four different plate tectonic settings: mid-ocean ridges, subduction zones, oceanic islands, and oceanic plateaus. Each group is assigned a different plate tectonic setting and collects three datasets from different locations for their tectonic setting. Geochemical data is graphed as major-element variation and REE diagrams to quantify igneous diversity both within the same tectonic setting and between different tectonic settings. The main goal of this exercise is to demonstrate that igneous rock compositions are a strong function of plate tectonic setting.

Determining whether students have met the goals

This is up to the instructor.

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