While geothermal activity has apparently persisted at shallow (3-4 km) levels for more than a million years (e.g., Moore and Gunderson, 1995), the nature of its heat source still remains poorly understood. At the surface, the Geysers reservoir is associated with small-volume volcanic centers (1 to 10 km 3) that range in age from ~2.7 to ~0.7 Ma.