Moving Mountains

Among the rocky planets in our solar system, only Earth shows plate tectonics. This essay explores a small but significant episode in the broader scientific quest to understand tectonics, that is, the dynamic behavior of Earth’s crust: how the study of glacier flow in the Swiss Alps came to serve as an analog for rock deformation. In the second half of the nineteenth century, geologists in Alpine Europe began to shift away from stratigraphy-oriented interpretations of geohistory. Instead, they embraced the study of the architecture of Earth’s crust and its structural changes over deep time. Albert Heim (1849–1937) reconceptualized geology as a materials science. Amid the smoothly bent rock folds—visible in the Alps in the hundreds, from the microscopically small to entire mountains—nothing intrigued him more than the idea of rigid masses in motion. Developed alongside the burgeoning research on glaciers and the study of ice ages, tapping into visceral encounters with glaciers, mountains, and tunnel construction, and underpinned by experimental workbench studies, Heim’s proposal was that glaciers offer an open-air illustration of the kind of rock flow that once occurred within Earth’s crust. On geological timescales, rocks must be conceived as soft; such was Heim’s idea—a physical condition he defined as the “latent plasticity” of rock masses.