The Deep Convective Geothermal Systems of the Taupo Volcanic Zone, New Zealand

Abstract

Electrical resistivity surveys carried out over the last 30 years provide almost complete coverage of the Taupo Volcanic Zone (TVZ) an area of about 5000 km2. Low resistivity anomalies in these data show the locations and distribution of some 23 separate geothermal fields through which the entire heat output (about 4200 MW(1)) of the TVZ is passed. Of the 23 fields, 17 have high natural heat outputs (>20 MW(1)); the remaining 6 may represent formerly active fields now in decline. In addition there are at least two extinct fields. In the region between the geothermal fields, covering 94% of the total surface area, the heat flow is extremely small or zero.

A large scale system of convecting fluids underlies the entire TVZ. Cold waters of meteoric origin are collected over the whole region and permeate downwards to be heated by the deep hot volcanic rocks. These hot waters rise under buoyancy and become concentrated in narrow plumes with cross-sectional areas of, on average about 15 km2.

The geothermal fields are the near surface expressions of these up flowing plumes. Deep resistivity surveys suggest that the cross sectional areas of the plumes may increase the depth, consistent with the observed decrease in permeability. Chemical and detailed heat flow studies indicate that once the hot geothermal fluids reach the surface they join the local drainage and only a very small portion rejoins the deep circulating system.