The Earth’s surface is composed of a staggering diversity of particulate-fluid mixtures: dry to wet, dilute to dense, colloidal to granular, attractive to repulsive particles, laminar to turbulent flows, and steady to highly-unsteady forcing. This material variety is matched by the range of relevant stresses and strain rates, from rapid and catastrophic landslides to the slow relaxation of soil over geologic timescales. In this talk I illustrate the commonalities and challenges in understanding geophysical flows by highlighting two problems: gravity-driven downslope soil movement, and fluid-driven particle transport in rivers.

Soil on hillslopes slowly and imperceptibly creeps downhill, but suddenly liquefies to produce landslides. The transition between creeping and flowing is a yield condition, 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 yield; because this kind of fragility is a generic consequence of disorder, 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 yield, many landscapes such as rivers organize to be close to the yield point. In essence, landscapes flicker back and forth across the glass transition. We explore several consequences of these dynamics for the sculpting of landscapes.