Creepy Landscapes and Critical Points: How Rivers and Hillslopes Behave like Glass

Soil on hillslopes slowly and imperceptibly creeps downhill, but suddenly liquefies to produce landslides. The transition between creeping and flowing is a critical point, often defined in terms of the shear stress, that depends on the characteristics of the soil and the geologic environment. We show that the nature of this transition, however, is general. Creep is the localized and erratic motion of soil grains below the critical point; because this kind of fragility is a generic consequence of disorder (no minimum energy state can be achieved because there is no crystal), soil creep should be similar to amorphous glass. Indeed, we find that the transition from creeping to landsliding is a continuous phase transition that follows predictions from glass transition models. The generality of this transition suggests that the onset of sediment transport in rivers should behave in a similar manner, and we demonstrate that this is the case using laboratory experiments and simulations. Because the sediment transport rate rapidly increases for stresses above critical, many landscapes such as rivers organize to be close to the critical point. In essence, landscapes flicker back and forth across the glass transition. We show that this critical behavior has consequences for how landscapes respond to external forcings such as climate. In particular, self-organization of near-critical river channels filters the climate signal evident in discharge, blunting the impact of extreme rainfall events on landscape evolution.