Energy budget analysis for Poás crater lake: implications for predicting volcanic activity

Abstract

AWORKING model for many active stratovolcanoes involves a magma column with a frozen cap, cooled by a meteoric-water hydrothermal system. Systems with such high latent and specific heat capacities may easily buffer internal temperatures and apparent surface activity during short-term changes in power output. The surface manifestation of volcanic hydrothermal systems takes the form of boiling mud pools, hot springs, fumaroles, and in about 20–30 cases worldwide, hot crater lakes^1–7. The latter are rare because they require special conditions to exist: high water supply, confined fumarole discharge, low permeability substratum and effective sub-surface heat transport. Crater lakes at active volcanoes are in a state of dynamic equilibrium whereby annual water losses through evaporation and infiltration are balanced by additions due to, for example, rainfall and runoff. Any change in volcano power output will directly affect the internal energy and surface heat loss of the lake. Vaporization of water within the hydrothermal system, leading to enhanced steam discharge from fumaroles, can also absorb increased power output. For long-term (months to years) power changes, we propose that crater-lake and fumarole discharge variations may well occur before significant signals on seismic and tilt networks are detected. As an illustration of these ideas, we consider here the recent activity at Poas volcano, Costa Rica.