Earthquake instability has long been attributed to fault weakening during accelerated slip, and a central question of earthquake physics is identifying the mechanisms that control this weakening. Even with much experimental effort, the weakening mechanisms have remained enigmatic. Here we present evidence for dynamic weakening of experimental faults that are sheared at velocities approaching earthquake slip rates. The experimental faults, which were made of room-dry, solid granite blocks, quickly wore to form a fine-grain rock powder known as gouge. At modest slip velocities of 10-60 mm s(-1), this newly formed gouge organized itself into a thin deforming layer that reduced the fault's strength by a factor of 2-3. After slip, the gouge rapidly 'aged' and the fault regained its strength in a matter of hours to days. Therefore, only newly formed gouge can weaken the experimental faults. Dynamic gouge formation is expected to be a common and effective mechanism of earthquake instability in the brittle crust as (1) gouge always forms during fault slip; (2) fault-gouge behaves similarly to industrial powder lubricants; (3) dynamic gouge formation explains various significant earthquake properties; and (4) gouge lubricant can form for a wide range of fault configurations, compositions and temperatures.