Thermal infrared image analysis of a quiescent cone on Piton de la Fournaise volcano: Evidence of convective air flow within an unconsolidated soil

摘要

We report on the detection of air convection with infrared thermal images for two quasi-circular craters, 20 m and 40 m wide, forming the volcanically inactive cone of Formica Leo (Reunion Island). The thermal images have been acquired from an infrared camera at regular time intervals during a complete diurnal cycle. During the night and at dawn, we observe that the rims are warmer than the centers of the craters. The conductivity contrast of the highly porous soils filling the craters and their 30° slopes are unable to explain the systematic temperature drop from rim to centers. We suggest that this signal could be attributed to air convection with gas entering the highly permeable soil at the center of each crater, then flowing upslope along the bottom of the soil layer, before exiting it along the crater rims. To quantify this process, we present a two-dimensional numerical modelling of air convection in a sloped volcanic soil with a surface temperature evolving between day and night. This convection depends on a unique dimensionless equivalent Rayleigh number Ra_(eq) which is the product of the standard Rayleigh number with the volumetric heat capacity ratio of the air and the soil. The convective flow is unsteady: during some periods, the convective flow is entirely confined within the soil, and at other times air enters the crater at its center and exits it at the rim crests. When Ra_(eq) = 6000, a value likely compatible with the soil permeability and the geothermal heat flux, a very strong transient cold air plume occasionally develops along the center of the crater. The interval of time between two plumes only depends on the thermal fluctuations within the top boundary layer of the convective cell, and thus is not contrasted by the diurnal cycle. The detachment of a cold plume can occur at any time, after few days of quiescence, and lasts several hours. During the whole convective cycle, the rim to center temperature drop persists and has an amplitude and a shape having an excellent agreement with that found in the IR-images. This work constitutes a preliminary step to explore the deep thermal structure of the active caldera of Bory-Dolomieu and could help to improve the understanding of volcanic hazards of the Reunion volcano.