Many fundamental geologic concepts are rooted in 3D spatial relationships. Well-crafted 2D perspective diagrams illuminate such relationships for some students, but interactive 3D models help a much wider range of students visualize complex geometries. In particular, students commonly learn how to solve specific problems using 2D projections but many fail to link the solution to the underlying 3D geometry. Trimble SketchUp is a particularly useful program for creating 3D models because it is relatively easy to learn and a free version exists. I have created interactive 3D models with SketchUp that illustrate how the stereographic projection works and how geologic maps and cross-sections are used to summarize the geology of an area. I also created a sequence of models showing how structure contours can help test if a contact is planar, determine the strike and dip of planar beds, find the true thickness of stratigraphic units, solve the 3-point problem, determine the depth of a target unit, estimate displacement across faults, and create cross-sections. SketchUp Pro offers the ability to create dynamic components, which can be used in the free version, and add another level of interactivity to SketchUp models. For example, models illustrating how the stereographic projection works can incorporate dynamic components so that the user can specify any values of strike and dip or trend and plunge to explore how planes and lines will plot on the stereonet.

Creating effective 3D models is, however, only the first step. Only motivated users will download a new program or plugin and take the time to learn how to use it. The models must, therefore, be easily accessible to instructors and students and be intuitive to use. The second challenge, then, is to provide interactive 3D models in a familiar format. SketchUp models can be exported as COLLADA digital asset exchange (.dae) files, and incorporated into an iBook as interactive 3D models using iBooks Author for IOS devices. The .dae files can also be uploaded to Sketchfab, a web service designed to publish and display 3D models. Once uploaded to Sketchfab, the models may be embedded in a webpage where anyone with a Web Graphics Library enabled browser can view them. A third promising approach is to export SketchUp models as 3D PDFs and incorporate them in digital instructional materials.