Steam used at The Geysers is produced from a greywacke sandstone reservoir, capped by a heterogeneous mix of low permeability rocks and underlaid by a silicic intrusion.[2][7] Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 miles (6.4 km) beneath the surface, and greater than 8 miles (13 km) in diameter