The deadline for Accretionary Wedge #43 – “my favorite geological illustration” was extended, which finally kicked me into deciding what to post. There are a number of diagrams I love that people have already used (Erik’s bubble figure, the Wooster fun with chemographic diagrams, MK’s subduction zone — which I’ve drawn on tables at Italian restaurants & on onsies at baby showers). And then there were the ones that were finalists (Tharp ocean map is awesome, the USGS Volcanic Hazards poster is basically reproduced by students during intro classes, the different types of silicates), but in the end, I had to go for what I truly know — metamorphic thin sections. Though photomicrographs are gorgeous, to truly “see” what’s going on with textures, you need to draw them by hand. The old-fashioned pen & ink drawings draw your eyes to the key features — ah, for the days when every department had an in-department scientific illustrator.
The following illustrations are of the progressive syntectonic metamorphism of a volcanic graywacke from New Zealand. The original illustrations are from Best (1982): Igneous and Metamorphic Petrology (W. H. Freeman, San Francisco).
burial of our volcanic rocks, which turns up the heat & pressure a bit:
rock continues to be buried, which increases the amount of metamorphism:
as metamorphism continues, we finally get to the “good stuff” i.e. garnet 🙂
My students don’t really appreciate my insistence that they have to draw fields of view during mineralogy & petrology, but the process really helps them “see” what’s going on so much better. And though most of them are not in the running to become scientific illustrators in the long run, I do really enjoy grading those labs.