Using an M&M® Magma Chamber to Illustrate Magmatic Differentiation

submitted by Karl Wirth, Macalester College

The M&M® magma chamber exercise was developed to provide students first-hand experience with the process of magmatic differentiation by fractional crystallization using a simple experiment. The activity also reinforces concepts of stoichiometry, chemical variation diagrams, and use of spreadsheets.
GSA Poster (Acrobat (PDF) 41.2MB Nov11 03)

A 1:00 preview video of the activity.

Learning Goals


Higher Order Thinking Skills: working with data, visualizing processes, drawing conclusions from data

Other Skills: stoichiometry, spreadsheets, chemical variation diagrams, phase diagrams


Instructional Level: 9-12 Earth Science courses, undergraduate entry level, and undergraduate majors

Skills Needed: introductory earth science level minerals and rocks

Role of Activity in a Course: The activity is currently used in introductory geology courses during the regular lecture period instead of lecturing on Bowen's reaction series. A more sophisiticated version of the activity is used in the Petrology course as a two-hour lecture-laboratory activity.

Dr. Wirth conducting the M&M Magma Chamber activity with his class. The video is 9:42 minutes long.You may also download a Quicktime version (Quicktime Video 54.7MB Oct15 08) of the video.

Data, Tools and Logistics

Required Tools: Items needed include large sheets of paper (approx. 36" x 48"); M&M's®; instructions and spreadsheet template handouts.

All of the above, as well as notes for the instructor can be downloaded from

Logistical Challenges: M&M's® need to be sorted by color and counted before the exercise. This step can be facilitated by purchasing the M&M's® as individual colors and weighing them.


Evaluation Goals: Most students indicate that the exercise is both enjoyable and informative. Student knowledge of fractional crystallization and magmatic differentiation, as indicated by exam results and course projects, seems to have increased significantly since incorporating this exercise into the curriculum. However, this has not been rigorously tested.

Evaluation Techniques:


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. More advanced students (e.g., petrology course) can classify the cumulates and resulting liquid after each crystallization step, and they can compare the model system with natural magmatic systems (e.g., absence of important fractionating phases, volatiles).