Rotary friction experiments have been performed on Westerly granite at start velocities of 2.0--4.0 m s-1, under loads of 250--500 N until the system stalls (up to ~4 s). Interface temperature, velocity and force were measured at intervals of 0.2 milliseconds. The results show that kinetic friction increases in direct proportion to temperature up to a critical value Tcr, whereupon the system changes from boundary lubrication (slip strengthening) to hydrodynamic lubrication (slip weakening) with an associated decrease in friction and temperature. For Westerly granite Tcr is ~1150¿C. This corresponds to the melting point of feldspar, which constitutes ~50% of the rock. If Tcr is exceeded in natural slip systems, which will depend on rock type, this can result in the generation of friction melt. Melt lubrication can explain the scaled energy of large earthquakes being 10 to 100 times more than that of small earthquakes.