My threshold concepts poll is closed now, and the winner is... going from 2D to 3D. (Geologic time was a close second, though.)
I know I had a hard time making sense of 2-dimensional diagrams. I may be a structural geologist now, but it still takes some effort to interpret a complicated geologic map. (And mineral structure models? I have to stop and think and draw my own pictures of those.)
I like using physical objects to help students make that leap. Silly putty, play dough, pencils, pieces of paper, students' hands... in my opinion, there's nothing like being able to pick up and move an object, and really see it from all sides. But it's easier to share 2D images, whether in a book or on a computer screen, so 2D images just aren't going to go away.
Computers can help make the bridge between paper and physical models, and they're getting better all the time. It's hard to sort through them all, though, so here are some sites that I like.
- Webmineral.com. I found this just last week, by googling a mineral name with a student. It's like having a combination of Deer Howie and Zussmann and a good optical mineralogy reference book on the web, for free. But that's not all - it also includes crystal structure models that you can rotate or zoom into. (Check out magnetite and labradorite and quartz. Extra credit for rotating the quartz structure model until you are looking down the 3-fold symmetry axis...) If I were still teaching mineralogy, I would try to come up with some way to adapt the old pearwood model lab to include this.
- Geoblocks 3D. Steve Reynolds and colleagues at Arizona State University have been working on ways to help students visualize maps and cross-sections. This page links to a series of exercises using Quicktime movies to help students imagine what's inside block diagrams of dipping layers, folds, faults, unconformities and intrusions. I use the full set of exercises as pre-lab assignments in structure; there is also a short version designed for intro classes.