Molecular dynamics simulations of quartz grains separated by a thin water film show that friction coefficient decreases monotonically as temperature rises from 300 to 500 Kelvin. The effect is not small --- the reduction is systematic and consistent across the full range. Heat weakens the grip that wet rock has on itself.
The mechanism operates through the water layer's hydrogen-bond network. At lower temperatures, water molecules near the quartz surface organize into ordered, tightly adsorbed layers that act as rigid bridges between the grain surfaces. These structured water films transmit shear stress efficiently, locking the grains in place. As temperature increases, the hydrogen bonds progressively break. The water transitions from an ordered, strongly bound state to a diffuse, weakly bound configuration. The rigid bridges become a lubricant. The same thin film of water that resisted motion at 300 K facilitates it at 500 K --- not because the water has changed chemically, but because its internal architecture has reorganized.
This has direct consequences for earthquake mechanics. Fault gouge --- the crushed rock filling fault zones --- is rarely dry. The water trapped between grains mediates the friction that determines whether a fault creeps or locks. The temperature dependence means that frictional heating during slip onset could trigger a feedback loop: initial slip generates heat, heat weakens the water-mediated friction, weakened friction permits faster slip, faster slip generates more heat. The fault does not simply overcome its friction --- it dissolves it.
The broader principle is that in any system where a thin intermediary layer controls the coupling between two surfaces, the layer's internal order --- not just its presence --- determines whether it locks or lubricates. The same substance can be either glue or grease, depending on how its own structure is organized.