USING AN M&M® MAGMA CHAMBER TO ILLUSTRATE MAGMATIC DIFFERENTIATION

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. However many students, especially those at the introductory level, typically do not develop a thorough understanding of differentiation processes. In particular, students often struggle to understand Bowen’s reaction series and fractional crystallization.

The process of 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. 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). Students who have completed this exercise generally indicate a positive experience and demonstrate increased understanding of Bowen’s Reaction Series and fractionation processes. They also exhibit greater familiarity with mineral stoichiometry, classification, solid-solution in minerals, element behavior (e.g., incompatibility), and chemical variation diagrams.