Paper No. 1
Presentation Time: 9:00 AM-6:30 PM


HIRSCH, David M., Geology, Western Washington University, 516 High St, Bellingham, WA 98225,

Phase diagrams are tools we use to make sense of petrologic observations. Ever since Eskola published the first diagram of this type for metamorphic rocks, we have been using them to organize our thinking.

Qualitative differences exist between metamorphic and igneous phase diagrams. Because most metamorphic suites include a larger range of bulk composition, compositional variation is more important in metamorphic phase diagrams and petrologic thinking.

Students who have learned phase diagrams think differently about rocks than those who haven’t. Phase-diagram-educated students show a deeper understanding of metamorphic processes and textures, are better able to organize rocks into progressions along a gradient, and are less distracted by variations in bulk composition. A complicating factor in these results is that other learning happens in the same courses in which phase diagrams are taught, so it is challenging to identify the effects of phase diagram education alone.

Each of the different types of metamorphic phase diagrams is valuable for learning different aspects of metamorphic petrology. Compositional diagrams (e.g., AFM, ACF) are excellent for teaching students the effect that bulk composition has on reaction progress. Petrogenetic grids are the easiest to learn, and are useful for putting bounds on conditions of metamorphism, but can be challenging to the student attempting to interpret rocks in a field area. Pseudosections are perhaps the most useful in guiding students through rock interpretations and for instilling the effects of bulk compositional variation, and modern computational tools allow their production quickly and easily. In order for the most learning to be accomplished, these three types (and others, in some cases) must be combined. Insufficient attention is typically given to pseudosections in most undergraduate courses.