Continental Crust Mass Balance Calculation

Jennifer Wenner, University of Wisconsin - Oshkosh
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Summary

Quartz Monzonite outcrop - Black Canyon of the Gunnison National Park This problem set is designed for a junior/senior level course and addresses several quantitative skills that are important in geochemistry and petrology. It involves conversions (i.e., cm to m), calculation and application of data to "real-life" problems in geology. It also requires that students use some higher order skills -- including making connections to other courses they have taken in geology. I use it in my upper level geochemistry course as a homework assignment but it could also be used in petrology or as an in-class activity. The exercise requires some understanding of rock type, geochemistry and geophysics.

The exercise uses geochemical data for average quartz monzonite and diorite from the Mineral Mountains in Utah. The students do mass balance calculations and are asked to relate their calculations to continental crust formation.

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Learning Goals

  • to connect arithmetic calculations to physical reality and real questions in geology
  • to apply quantitative geochemical techniques to other sub-disciplines of geology
  • to increase student familiarity with spreadsheet programs

Context for Use

I use this exercise as a homework assignment in my geochemistry course (giving them 4 days to a week to complete it). It also addresses issues common in petrology and geophysics and might easily be adapted to either of those courses.

The exercise requires some familiarity with the concepts of density and buoyancy. It also requires some understanding of incompatibility of elements (distribution coefficients). By the time they take my course, students have taken Lithology (the introductory "rock" course at Oshkosh) -- a key point since this exercise requires some familiarity of the IUGS classification scheme.

This exercise helps to lead the class into the second half of the course -- isotopes and their uses in studying the crust, etc.

This exercise can be used in class as a group exercise. Working through the entire problem can take up to an hour, if you include a discussion of the implications of the calculations (see question #4 in Student handout).

Description and Teaching Materials

Student Handout (with questions, data and hints) (Microsoft Word PRIVATE FILE 28kB Jul17 04)
  • This file is the handout I give to my students. It includes the exercise, data and some hints for working through the problems
Continental Crust Mass Balance Work-through (Microsoft Word PRIVATE FILE 28kB Jul15 04)
  • This file walks one through the steps of calculation involved in the exercise
Instructor Notes (Microsoft Word PRIVATE FILE 25kB Jul17 04)
  • This file contains more comprehensive notes on teaching this exercise.
Answers (Acrobat (PDF) PRIVATE FILE 85kB Jul17 04)
  • This PDF file has answers in Excel format.

Teaching Notes and Tips

  • Students struggle with this exercise because often they do not know how to apply the math they already know to a new problem. Help them to recognize and enjoy the challenge of applying geochemical and quantitative skills to real problems.
  • A workthrough of each step is provided in the Teaching materials section below. This document does not provide answers; thus, it may be provided to the students.
  • Because this exercise uses data from a real place, the mathematics can be made even more accessible with maps and field shots.
  • This problem set can be adapted to use data for diorites and intermediate rocks from other places (for example, I have used the Sierra Nevada in CA).
  • Data and/or figures from papers about continental crust such as Rudnick and Fountain, 1995, or Taylor and McClennan, 1985, may help students to make connections between the calculations they have made and what the crust looks like.
  • This is an excellent exercise to reinforce the idea that Excel (and other spreadsheet programs) can be a powerful tool in the geosciences. Bob McKay, Clark College, has an excellent website that gives a number of tutorials on the use of Excel.

References and Resources

Material for the classification of the residual "rock" was taken from Igneous Petrology , 2001, by Myron G. Best and Eric H. Christiansen. The classification scheme is from IUGS.

Several excellent tutorials on Excel can be found on pages designed by Robert McKay.