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We analyzed four distinct effusive episodes at Piton de la Fournaise during the May–July 2003 eruption. We estimated a total erupted volume of lava of ∼2.2 ± 0.3 Mm3, by means of portable Differential Global Positioning System (DGPS) equipment (Ashtech Zextrem) and an infrared handheld camera (ThermaCAM PM695 PAL). The evolution of the lava field in space and time has been reconstructed by cross-checking the infrared and optical images with field observations. These data allowed us to infer the evolution of effusion rates during the dynamic development of the effusive episodes, hereby named Phases I, II, III, and IV (ranging from 21 ± 3 m3/s during Phase I to 0.5 ± 0.1 m3/s during Phase IV, with an average eruption rate of 1.2 ± 0.3 m3/s).

Lavas effused during the first three phases were shelly pahoehoe, slabby pahoehoe, spiney pahoehoe, clinkery 'a'a, and blocky 'a'a. Additionally, we observed direct and “inverse” transition from pahoehoe to 'a'a. This process was not observed during the last phase (Phase IV). This phase was characterized by lower effusion rates associated with the emplacement of a pahoehoe sheet flow. We analyzed the advance of this pahoehoe sheet flow (∼4.2 m/h) by means of longitudinal thermal profiles that exhibited an exponential increase in surface temperature toward the front. Temperature fluctuations at the front were coeval with the advancement of the frontal lobes; they in turn also reflect the onset of minor magma pulses at the vent (ascribed to a gas-piston mechanism).

Thermal analysis revealed that the temperature distribution of the lava field is composed of multiple thermal components related to different cooling histories of the exposed lava surfaces. The acquisition of thermal data on the whole lava field, integrated with GPS leveling, is a powerful tool to detect and constrain changes in the effusion rate. Further developments of this methodology may be used in remote monitoring, including satellite infrared image analysis.

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