How friction melts sliding rock

When two rock formations slide past one another during an earthquake, the friction generated melts the rock to create a liquid layer that eases further movement. At 1000–1550 °C, interface temperatures during sliding exceed the melting points of most minerals. Even so, examination of exhumed faults does not reveal evidence of the uniform, equilibrium melting that occurs at high temperature. In the case of quartzite rock (shown in the figure), the melting is especially puzzling. Minority components in the quartzite that have lower melting temperatures than their host will melt and resolidify. But so too does some of the quartzite, whose melting temperature is 1726 °C.

To identify the processes at play, Sung Keun Lee of Seoul National University and Raehee Han of Gyeongsang National University—both in South Korea—and their colleagues cleaved a cylindrical sample of Brazilian quartzite perpendicular to its axis and loaded the two disk-shaped pieces into a rotary shear apparatus. The bottom piece was held stationary while the top piece, in contact with the bottom, was spun at a slip rate at the circumference of 1.3 m/s. Friction between the two pieces created a molten interface at the rim that was 0.7 mm thick and attained a temperature of about 1400 °C. When the researchers studied the interface after it had cooled, they found evidence of the same lack of uniform melting around the rim as in samples from exhumed faults. Toward the center of the samples, where the slip rate was lower than at the rim, melting did not occur. But the friction did cause the samples to break up into powder.

Here was a key clue: As embodied by the Gibbs–Thomson equation, the smaller a piece of material is, the greater the energy of its curved surface with respect to the bulk. The upshot is to lower the powdered particles’ melting temperature. What’s more, the rapid temperature rise brought on by the sudden onset of friction had another effect: It triggered the metastable melting at 1400 °C of β-quartz, one of quartzite’s principal crystalline phases. So when quartzite formations slide past each other, the friction does not have to reach the mineral’s melting temperature for a lubricating layer to develop. (S. K. Lee et al., Nat. Geosci. 10, 436, 2017, doi:10.1038/ngeo2951 .)