Diffuse heating of volcanoes prior to eruption

Identifying the observables that warn of volcanic unrest and impending eruptions is one of the greatest challenges in the management of natural disasters. An important but scarcely explored observable is diffuse heating, that is, the heat released passively through the ground. Diffuse heating represents one of the major energy sources in active volcanoes during inter-eruptive periods, and can dominate over the elastic energy released during seismic and deformation events. However, many questions remain open: Is there a direct correlation between diffuse heating and the subsurface processes that precede volcanic eruptions? To what extent are volcanic eruptions preceded by an enhancement of the diffuse emissions of heat? We address these questions by analyzing 16.5 years of long-wavelength (10.780 - 11.280 μm) thermal infrared radiance data recorded over five volcanoes by the moderate-resolution image spectroradiometers (MODIS instruments) aboard NASA's Terra and Aqua satellites; this amounts to >25 TB of data and >159,000 MODIS scenes. Our statistical analysis reveals that volcanic edifices get warm for several years before magmatic, phreatic and hydrothermal eruptions. This pre-eruptive warming, which we attribute to an increase of diffuse heating, has been observed at Ontake (Japan), Ruapehu (New Zealand), Domuyo (Argentina), Calbuco (Chile), and Redoubt (USA) volcanoes. In particular, we found pre-eruptive increases of up to ~1 K in the median brightness temperature of the volcanic edifices; this, based on energy balance, reflects increases of thermal power on the order of 10s-100s MW. We theorize that pre-eruptive diffuse heating is the surface manifestation of the latent heat released by the subsurface condensation of water vapor, and find through a new transport model the link between diffuse heating and the pressure of shallow magma reservoirs. Our retrospective analysis is especially relevant, since seven of the eight eruptions analyzed did occur with little or no warning (e.g., the 2014 phreatic eruption of Ontake and the 2015 magmatic eruption of Calbuco). The possibility of tracking temporal changes of diffuse heating using satellite data opens new horizons to study the thermal reactivation of magma reservoirs and improve the forecasting of volcanic eruptions.