Field thermal monitoring during the August 2003 eruption at Piton de la Fournaise (La Réunion )

International audience[1] A detailed set of thermal images collected during the last day of the August 2003 eruption of Piton de la Fournaise (La Réunion), clearly revealed several dynamic processes associated with a spatter cone containing a lava pond and feeding a channelized lava flow. Periods of steady effusion were interrupted by brief pulses of lava effusion that closely coincide with peaks in seismic tremor amplitude. The thermal measurements show that roofing of a lava channel during steady effusion and cooling of surface flows decrease thermal radiance in two different ways. Here we show that the decrease in thermal radiance because of channel roofing is not related to a decrease in volcanic activity, as might be interpreted from satellite data. In addition, we introduce a new method of representing thermal data (hereby named ''Radiative Thermogramme'') that successfully describes thermal patterns produced by distinct eruptive processes within the same span of time. This graphic solution can be directly correlated with volcanic field processes and provides a useful tool for interpreting a high number of thermal data in a wide range of volcanic activities

Morphology and dynamics of inflated subaqueous basaltic lava flows

International audienceDuring eruptions onto low slopes, basaltic Pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness, due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow. Inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments, such as at contemporary subaerial Hawaiian-type volcanoes in Hawaii, La Réunion and Iceland, in continental environments (states of Oregon, Idaho, Washington), and in the deep sea at Juan de Fuca Ridge, the Galapagos spreading center, and at the East Pacific Rise (this study). These lava have all undergone inflation processes, yet they display highly contrasting morphologies that correlate with their depositional environment, the most striking difference being the presence of water. Lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea, lakes, and rivers. Their height is 2-3 times smaller and their length being 10-15 times shorter. Based on heat diffusion equation, we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker (by 25%) cooled layer at the flow surface, which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments, thus limiting lava breakouts to form new lobes, hence promoting inflation. Buoyancy also increases the ability of a lava to inflate by 60%. Together, these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows