Geothermal Systems in Calderas

Large, young calderas and associated volcanic rocks are indicators of potentially immense geothermal resources. Smith and Shaw (1975) estimated that for every cubic kilometer of material erupted, between 3 and 9 km³ of partly molten rock resides below the volcanic field but within the upper 10 km of crust. The geothermal resource beneath a caldera exists as long as eruptive activity continues; in addition, if large silicic crustal magma bodies are

Fig. 4.15
Schematic cross section of the Valles/Toledo calderas of New Mexico. The caldera complex, which
appears to have a trapdoor shape, has a "hinge" on the northwest side where it overlies Paleozoic
sedimentary rocks and Precambrian metamorphic and igneous basement. Most collapse has
occurred along the eastern caldera margin where the caldera overlies the Rio Grande Rift
and a thick sequence of poorly consolidated rift sedimentary rocks.

formed, the resource has a lifetime of several million years after the final volcanic activity (Kolstad and McGetchin, 1978). As will be seen in the case histories discussed here, the magnitude of the accessible hydrothermal system depends entirely on the extent of permeable zones below the caldera or within the caldera deposits. In most calderas containing thick, densely welded tuff deposits, the hydrothermal systems are limited to caldera ring fractures and caldera-crossing faults, which form zones of fracture permeability. Evidence of hydrothermal activity is often seen in the acid-sulfate alteration that occurs at the surface. Acid fluids are formed when H₂ S and CO₂ escape from the underlying hot water reservoir and are oxidized at shallow depths. Water within these reservoirs is usually heated groundwater from recharge areas within the caldera.

Thermal models of calderas indicate that much of the elevated heat flow is conductive and that convective heat transfer is mostly limited to fault zones. Magma bodies below the larger calderas (>10-km diameter) cool slowly and may be heat sources for up to 2 million years. An example of such a system is the 15-km-diameter Valles caldera of New Mexico, where the most recent major eruption took place 1 million years ago and the most recent intracaldera eruption was 150,000 years ago. Along the keystone graben of the resurgent dome, the temperature is still 341°C at a depth of 3 km (ambient temperatures at a depth of 3 km in this region are ~110°C). Smith and Shaw (1975) estimated that within the uppermost 10 km below the Valles caldera the thermal energy is equivalent to 8425 × 10¹⁸ J. For the same caldera, Brook et al . (1975) estimated that the thermal energy of the hydrothermal systems is 81 × 10¹⁸ Joule, which is ~1% of the total heat in the system. The remaining heat is present in rock of low permeability (termed hot dry rock ) and in residual magma. For caldera systems, estimates of the amount of heat present as hydrothermal fluid range from 1 to 10%.

Nearly all hot-water circulation within known hydrothermal systems is located along active faults where there is fracture permeability. A comparison of the young Valles caldera and older, well-exposed Lake City and Platoro calderas in Colorado

indicates that hydrothermal alteration within those caldera deposits occurred chiefly along faults and over shallow intrusions (Figs. 4.11 and 4.17). However, in calderas formed during phreatomagmatic eruptions, where the underlying rocks have been highly fractured by hydraulic overpressures and the tuff deposits are nonwelded or only partly welded, there may be considerable formation permeability.