Geobrowsers such as Google Earth and NASA World Wind are excellent tools for analyzing geological surface processes and constructing virtual field trips, but representation of the subsurface (the area of greatest interest to most geologists) is hampered by the non-transparent rendering of the terrain model's surface. Whereas maps draped over the surface may be rendered semi-transparent, the surface itself is always opaque. We have devised a solution using 3-D Collada solid models controlled by code written in Keyhole Markup Language (KML). Our models are first created in the negative z-space of Google SketchUp (a free 3-D modeling application distributed by Google Inc) and are placed at specified locations under the Google Earth terrain. Using the KML timespan function, these models may be made to "emerge" by continuously varying their altitude attribute with a slider user interface feature. Initial applications include geoseismic sections (southern Rocky Mountains region, Scotland), tectonic cross sections (West Virginia-Virginia Appalachians, Connemara region of western Ireland), and block diagrams representing current plate tectonic processes (Hawaiian hot spot, Andean subduction zone). Paleo-tectonic models are also possible with the aid of paleogeographic maps draped over the virtual globe so as to conceal the current distribution of land and sea. Unlike many idealized textbook representations of plate tectonic scenarios, our models directly incorporate modern research data and correspond to real present-day geography and/or spatially well-constrained paleogeography.

Many students are already comfortable with the intuitive Google Earth interface. Preliminary feedback suggests that the addition of interactive, emergent models of the subsurface will help students grasp the scale and three-dimensional geometry of plate tectonic processes and will reinforce fundamental concepts in global tectonics and geodynamics.