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).
original volcanic rock
burial of our volcanic rocks, which turns up the heat & pressure a bit:
breakdown of some of the hydrous material, recrystallization of material to start to form definite foliation
rock continues to be buried, which increases the amount of metamorphism:
continued breakdown of hydrous phases; beginning of segregation into "felsic" vs. "mafic" lithologies
as metamorphism continues, we finally get to the “good stuff” i.e. garnet 
down to just muscovite & biotite as hydrous phases, with a higher mode of anhydrous garnet + garnet + oligoclase dominating; segregation is more pronounced
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.
Elli Goeke
I, too, love those sorts of drawings. I did my PhD studying the Cambrian Metamorphic rocks in Tasmania. The last person to have worked on those was Alan Spry, who did his PhD in the 1960′s. The line drawings in his thesis often showed me more details than looking at the same thin sections using the student microscope I had on my desk. If anyone wants to see his drawings, he re-used many of his thesis drawings for his 1969 book _Metamorphic Textures_, published by Pergamon, New York.
Glad you got something in for AW-43. I was studying geology in the late seventies and just about all of my textbooks had illustrations instead of photos. These prints bring back the memories along with the drawing we had to do. It’s nice to see you are keeping the tradition going of drawing what you see, because I do think you learn it better by using as many different learning styles as possible.