Molecular dynamics simulations of quartz-water interfaces reveal that heating from 300 to 500 Kelvin monotonically reduces the friction coefficient in fault gouge. The mechanism is not thermal expansion or pressure changes but a structural transition in the interfacial water itself: heat disrupts the hydrogen-bond network, transforming the water from an ordered, tightly bound lattice into a diffuse, weakly bound film. The water shifts from acting as mechanical glue -- locking particles together through structured hydrogen bonds -- to acting as lubricant, enabling particles to slide past each other with minimal resistance.
The insight is that the same thin film of water can either stabilize or destabilize a fault, and the switch is governed entirely by the water's internal structure, not its quantity. There is no more or less water; there is differently organized water. The hydrogen-bond network is load-bearing when intact, and the heat that disrupts it does not remove the water but rearranges it from a solid-like architecture into a liquid-like one. The fault does not become wet; it becomes differently wet.
This principle -- that a material's structural organization matters more than its composition -- operates wherever interfaces determine system behavior. The same substance in the same quantity at the same location can serve opposite mechanical functions depending on its internal ordering, and the transition between those functions can be triggered by a continuous variable crossing an organizational threshold.
(arXiv:2603.16921)