Friction between large moving tectonic plates is thought to be high but the lack of corresponding frictional heat suggests otherwise. The lack of friction could be explained by smaller rock fragments being rounded and acting as ball-bearings between the plates. To study rock fragmentation and friction a special apparatus called Stonegrinder was built. It grinds two drill core rock samples against each other and measures the changes in friction.

Rough surfaced gneiss samples were ground under two conditions: constant normal force, and constant normal pressure. Smooth surfaced gneiss samples were ground under constant normal force and pressure. Torque and the magnitude of temperature change were found to be inversely dependent on the compressive pressure. The coefficient of friction was independent of pressure and temperature. Wear particles imaged with X-ray tomography were found to be angular instead of spherical. Smooth surfaced samples had high mean torques due to the lack of wear particles.