COMPUTER MODELING IN THE STRUCTURAL GEOLOGY CLASSROOM: UNDERSTANDING THE IMPORTANCE OF STRUCTURE TO OTHER DISCIPLINES

At a recent NSF-sponsored workshop on Teaching Structural Geology in the 21st Century, working groups formed to explore the creation of course materials that would illuminate the importance of structural geology to other disciplines within the Earth Sciences and to consider the role of numerical modeling in instruction. Two STELLA models, one of the flow of ice within glaciers and the other of isostasy and crustal rebound, have been created at Vassar College and meet both of these goals. STELLA is an iconographical box-modeling software package that represents reservoirs as boxes and flows between them as arrows. A click of the mouse allows initial conditions, boundary conditions, and mathematical relationships between variables to be specified. A built-in graph pad allows the modeler to watch the progress of variables throughout the run.

The glacier model consists of 10 reservoir boxes, each of which represents the volume of ice within a 1-km-long down-valley section. Accumulation and ablation add and remove ice from the surface of each section, and the Glen flow law, relating shear stress within the ice to strain rate, governs the flow of ice from each section to the next. To simplify the model, shear stress is calculated based on the assumption that the glacier has a uniform cross-valley width in order to determine ice thickness. Variations in bed topography lead to thickening and thinning of the ice over lower and higher slope sections respectively. Students can explore the roles of viscosity and the power law relationship between stress and strain rate on the resulting temporal and spatial behavior of the modeled ice.

In the isostasy and crustal rebound model, the maximum deflection expected for a given load is first determined from the relative densities of the load, crust, and mantle and the thickness of the load. Thereafter, a crustal relaxation routine involving mantle viscosity is invoked to show the gradual response of the crust to the imposition or removal of the load. In the model developed at Vassar, a large lake, such as Lake Bonneville in Utah, is used as the load, and students experiment with the impact of changing lake levels and mantle viscosities on isostatic compensation.

Both modeling exercises lend themselves to use in structural geology, geomorphology, or geophysics courses and show the importance of rheological behavior to geodynamics.