Internal and external processes driving heat transfer at volcanic hydrothermal systems

Abstract

At active volcanic hydrothermal systems, enhanced heat flow results in surface heating and elevated heat flux densities. We consider heat transfer models for two types of system: (1) “dry” (soil heating and fumarolic activity), and (2) “wet” (crater flooded with an acidic lake). Our aim is to collate and review methods to calculate heat flux at these two sysyem types, while assessing the role of external atmospheric processes. To do this, we completed thermal mapping and collected meteorological data in June 2022 at Nisyros (Greece) and in February 2002 at Poás (Costa Rica). Measurements coincided with a thermal infrared satellite (ASTER) overpass. At both systems, the low magnitude of the thermal anomaly means that atmospheric factors, in particular vapor pressure, drive high degrees of variation in heat flux during a diurnal cycle. These variations overprint the volcanic heat flux component. At Nisyros, heat flux densities varied by 200 W m−2 in 24 h. Thus, to characterize heat fluxes at a hydrothermal system, it is best to integrate through a whole diurnal cycle. At both systems, agreement between ground and satellite-based values of heat flux (17.9 ± 5.2 MW versus 22.0 ± 6.7 MW and 20.6 ± 14 MW versus 21.9 ± 4.8 MW, respectively) gives us confidence in our calculations. The calculated heat flux was relatively low revealing a baseline state of activity at both systems. Our intention is that heat flux calculation methodologies detailed and validated here can be used at other similar systems. The key is to adequately account for atmospheric parameters in the thermal boundary layer, i.e. within 10 cm of the surface. This allows robust and comparable heat flux inventories to be applied to better understand activity at hydrothermal systems.