HR: 0830h
AN: ED41C-1179    [PDF]
TI: Improving Student Understanding of Magmatic Differentiation Using an M\&M Magma Chamber
AU: * Wirth, K R
EM: wirth@macalester.edu
AF: Geology Department, Macalester College, 1600 Grand Ave., St. Paul, MN 55105 United States
AB: Many students, especially those in introductory geology courses, have difficulty developing a deep understanding of the processes of magmatic differentiation. 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. Students then fractionate the magma by moving M\&M's to the bottom of the magma chamber forming a series of cumulus layers; the M\&M's are removed in 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. Other models (e.g., paths of equilibrium and fractional crystallization on phase diagrams) can also be used to illustrate differentiation processes in upper level courses (e.g., mineralogy and petrology).
DE: 3640 Igneous petrology
DE: 6605 Education
SC: Education and Human Resources [ED]
MN: 2003 Fall Meeting