Water and friction play central roles in controlling the mechanics of earthquakes, slow-slip behavior, glacier dynamics, and the style of some types of volcanic eruptions. In each case, the underlying micro-scale physical interactions can be homogenized to pro- duce tractable models for the macroscopic behavior that is observed at geological scales and inferred from geophysical data. Simple continuum treatments that provide useful insights into some aspects of the physical system are prone to failure in describing other key observations, emphasizing the need for re ned constitutive laws that incorporate additional experimental, numerical (e.g. particle dynamics), and theoretical constraints.

I discuss how my collaborators and I are attempting to identify and address such is- sues for a variety of geophysical systems, with a particular focus on understanding how water transport and frictional rheology in uence shear-zone widths during earthquakes, slow-slip transients in a subduction melange, temporal and spatial transitions in sliding beneath glaciers, and the quasi-steady-state extrusion of volcanic spines.