Serpentinite is a soft rock made of serpentine minerals. They are in the process of being stretched and absorbed and broken down in the fault zone. Not only is serpentine a soft, slippery ingredient of the material in this setting, but it is also prone to being altered by deep fluids rich in silica, yielding the exceptionally soft mineral talc. Minerals like these serve to lubricate the fault, enabling it to move in an earthquake even when the stresses across the fault are relatively small. This may explain a longstanding puzzle of the San Andreas faultwhy it has such energetic earthquakes even though the ambient stresses on it are low. The presence of fresh serpentinite shows that the fault zone is constantly renewing its lubricating properties rather than growing hard and freezing up, or relying on high fluid pressures or other postulated mechanisms to trigger quakes.