Using An M&M® Magma Chamber To Illustrate Magmatic Differentiation
Summary
Fractional crystallization by gravity settling can be illustrated using a model magma chamber consisting of M&M's®. In this model, each major cation (e.g., Si, Ti, Al, Fe, Mg, Ca, Na, K) is represented by a different color M&M®; other kinds of differently colored or shaped pieces could also be used.
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Context
Audience
This exercise is designed for a sophomore or junior level petrology course that is required for majors.
Skills and concepts that students must have mastered
Students should have a basic understanding of magma differentiation and fractional crystallization.
How the activity is situated in the course
This is a stand-alone exercise.
Goals
Content/concepts goals for this activity
This activity is designed to improve students' understanding of fractional crystallization.
Higher order thinking skills goals for this activity
Other skills goals for this activity
This activity involves group work.
Description of the activity/assignment
Magmatic differentiation is an important concept in the geology curriculum. Students are generally introduced to magmatic differentiation in the introductory course, whereas the details are typically developed in mineralogy and petrology courses. In particular, students often struggle to understand the processes of fractional crystallization.
Fractional crystallization by gravity settling can be illustrated using a model magma chamber consisting of M&M's®. In this model, each major cation (e.g., Si, Ti, Al, Fe, Mg, Ca, Na, K) is represented by a different color M&M®; other kinds of differently colored or shaped pieces could also be used. Appropriate numbers of each color M&M® are combined to approximate the cation proportions of a basaltic magma; this is the "parental magma". The M&M's® are then placed in a group on a tabletop to form a magma chamber. Students then fractionate the magma in ten crystallization steps. In each step the M&M's® are moved to the bottom of the magma chamber forming a series of cumulus layers; the M&M's® are removed in proportions that are identical to those of the stoichiometric proportions of cations in the crystallizing minerals (e.g., olivine, pyroxene, feldspars, quartz, magnetite, ilmenite). Students observe the changing cation composition (proportions of colors of M&M's®) in the cumulus layers and in the magma chamber and graph the results using spreadsheet software. Students classify the cumulates and resulting liquid after each crystallization step, and they compare the model system with natural magmatic systems (e.g., absence of important fractionating phases, volatiles). Students who have completed this exercise demonstrate increased understanding of fractionation processes exhibit greater familiarity with mineral stoichiometry, classification, solid-solution in minerals, element behavior (e.g., incompatibility), and chemical variation diagrams.
Fractional crystallization by gravity settling can be illustrated using a model magma chamber consisting of M&M's®. In this model, each major cation (e.g., Si, Ti, Al, Fe, Mg, Ca, Na, K) is represented by a different color M&M®; other kinds of differently colored or shaped pieces could also be used. Appropriate numbers of each color M&M® are combined to approximate the cation proportions of a basaltic magma; this is the "parental magma". The M&M's® are then placed in a group on a tabletop to form a magma chamber. Students then fractionate the magma in ten crystallization steps. In each step the M&M's® are moved to the bottom of the magma chamber forming a series of cumulus layers; the M&M's® are removed in proportions that are identical to those of the stoichiometric proportions of cations in the crystallizing minerals (e.g., olivine, pyroxene, feldspars, quartz, magnetite, ilmenite). Students observe the changing cation composition (proportions of colors of M&M's®) in the cumulus layers and in the magma chamber and graph the results using spreadsheet software. Students classify the cumulates and resulting liquid after each crystallization step, and they compare the model system with natural magmatic systems (e.g., absence of important fractionating phases, volatiles). Students who have completed this exercise demonstrate increased understanding of fractionation processes exhibit greater familiarity with mineral stoichiometry, classification, solid-solution in minerals, element behavior (e.g., incompatibility), and chemical variation diagrams.
Determining whether students have met the goals
More information about assessment tools and techniques.Teaching materials and tips
- Karl Wirth's notes to the instructor, activity handouts, spreadsheet template, and solution files for the activity can be downloaded from: https://sites.google.com/view/krwirth/about
Adaptations of this activity:
- Cynthia Fadem, at Earlham College, uses this activity as a lab in an undergraduate geochemistry course. She has modified the tables and questions to be accessible to undergraduates who have only had introductory geology and a brief introduction to thermodynamics. Here are her versions of the activity:
- Fadem Fractional Crystallization Lab Handout (Microsoft Word 2007 (.docx) 124kB Mar31 14)
- Fadem Fractional Crystallization Tables (Microsoft Word 194kB Mar31 14)
- Euan Mitchell, at Miami University, uses a modified version in large enrollment introductory courses. In this version the magma chamber is constructed in PowerPoint, eliminating the need for any physical supplies and making the activity accessible for classes of any size. He describes his adaptation further on this teaching demonstration page. Here are files for his versions of the activity:
- Student Handout (Microsoft Word 2007 (.docx) 22kB Jun22 17)
- Data Sheet (Excel 2007 (.xlsx) 31kB Jun22 17)
- Starting Magma Chamber (PowerPoint 2007 (.pptx) 44kB Jun22 17)
- Cynthia Fadem, at Earlham College, uses this activity as a lab in an undergraduate geochemistry course. She has modified the tables and questions to be accessible to undergraduates who have only had introductory geology and a brief introduction to thermodynamics. Here are her versions of the activity:
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Fractional Crystalization of an M&M Magma Chamber - Teaching Notes and Tips -- private instructor-only file
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