Magma in a Beaker: Analog Experiments with Water and Various Salts or Sugar for Teaching Igneous Petrology

Solutions of water and a salt or sugar make excellent experimental analog magmas for teaching concepts of igneous petrology because of the comparatively low temperatures involved, the simplicity of the apparatus needed, and the responsiveness of familiar chemical systems. Boiling of these aqueous solutions on a hot plate can be used to increase the concentration of a dissolved salt or sugar to levels that may be predicted by steam-saturation curves. Sufficiently concentrated solutions will crystallize, partially or completely, upon cooling to room temperature. Binary temperature–composition phase diagrams for H2O and KCl, NaCl, MgCl2, CaCl2, or C12H22O11 have been drawn to provide guidance for experiments, and equations are given for the saturation curves. Possible instructional activities with these simple systems include: (1) determination of saturation (liquidus) curves on binary phase diagrams, (2) measurement of the relative proportions of liquid and solid in a system that has partially crystallized, and comparison with predictions of the lever rule, (3) observation of some consequences of peritectic reactions on crystallization, (4) observation of the kinetic effects of temperature and concentration on crystallization, (5) simulation of a magma chamber with crystals settling because of their density and rising owing to convection, and (6) observation of simultaneous boiling and crystallization that buffer temperature, which can lead to a solid with vapor cavities. Movies of interesting aspects of these experiments are available online as supplementary documents.