Jemez Mountains & Valles Caldera

Although young and morphologically better developed than the Yellowstone caldera, hydrothermal activity in the Valles Caldera is limited. In part this is due to the relatively dryer environment relative to Yellowstone. It is also a function of the slighter greater age of the Valles system. Numerical models of heat transfer by fluid convection around large rhyolite magma chambers shows that the rate of heat loss is great. Unless continued magma injections occur to replace the heat loss, the thermal system will decline. The Valles caldera has been cooling for about 1.1 Ma since the caldera-forming eruption, and 0.5 Ma since the last rhyolite dome eruptions inside the caldera.

The following table summarizes the estimated thermal output of the three youngest, large ash-flow calderas in the continental Unites States:

• Hydrothermal activity initiated about 1 Ma
• Concentrated on SW resurgent dome and western ring-fracture areas
• Mostly dilute groundwater heat by high-temperatures at shallow depths
• Result of convective circulation of water over deep,hot central caldera rocks
• Meteoric water recharges the system; 300°C at depth of 2 to 3 km
• Convection in deep reservoir with "vapor cap"
• Liquid-dominated system at depth ascends convectively to 500 to 600 m depth
• Top of system is "vapor capped" by subsurface boiling at about 200°C
• Heats shallow levels that are diluted with groundwater flowing laterally
• Springs are acid-sulfate waters
• Result from condensation of steam and oxidation of H2S to form sulfuric acid
• Mixes with near-surface groundwater flowing from northern and eastern moat basins
• Water flows laterally from top of system down hydrologic gradient
• Thermal waters flow down gradient outside of ring fracture in San Diego Canyon
• Vapor cap formed after breaching of SW caldera margin by ancestral Jemez River
• Resulted in draining of intracaldera lakes
• Lowered hydraulic head resulted in drop of maximum elevation of liquid in reservoir