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[1] In Jan. 2003 we monitored explosions at Santiaguito Volcano (Guatemala) with thermal, infrasonic, and seismic sensors. Thermal data from 2 infrared thermometers allowed computation of plume rise speeds, which ranged from 8 to 20 m/s. Rise rates correlated with cumulative thermal radiance, indicating that faster rising plumes correspond to explosions with greater thermal flux. The relationship between rise speeds and elastic energy is less clear. Seismic radiation may not scale well with thermal output and/or rise speed because some of the thermal component may be associated with passive degassing, which does not induce significant seismicity. But non-impulsive gas release is still able to produce a high thermal flux, which is the primary control on buoyant rise speed.

Vertical ash plumes were imaged at Santiaguito (Guatemala) using a thermal camera to capture plume ascent dynamics. The plumes comprised a convecting plume front fed by a steady feeder plume. Of the 25 plumes imaged, 24 had a gas thrust region within which ascent velocities were 15-50 m s −1 . A transition to buoyant ascent occurred 20 to 50 m above the vent, where ascent velocities declined to 4-15 m s −1 . Plumes that attained greater heights had higher heat contents, wider feeder plumes and higher buoyant ascent velocities.

Forward-Looking Infrared (FLIR) nighttime thermal images were used to extract the thermal and morphological properties for the surface of a blocky-to-rubbley lava mass active within the summit crater of the Caliente vent at Santiaguito lava dome (Guatemala). Thermally the crater was characterized by three concentric regions: a hot outer annulus of loose fine material at 150-400°C, an inner cold annulus of blocky lava at 40-80°C, and a warm central core at 100-200°C comprising younger, hotter lava. Intermittent explosions resulted in thermal renewal of some surfaces, mostly across the outer annulus where loose, fine, fill material was ejected to expose hotter, underlying, material. Surface heat flux densities (radiative + free convection) were dominated by losses from the outer annulus (0.3-1.5×10 4 s −1 m −2 ), followed by the hot central core (0.1-0.4×10 4 J s −1 m −2 ) and cold annulus (0.04-0.1× 10 4 J s −1 m −2 ). Overall surface power output was also dominated by the outer annulus region (31-176 MJ s −1 ), but the cold annulus contributed equal power (2.41-7.07 MJ s −1 ) as the hot central core (2.68-6.92 MJ s −1 ) due to its greater area. Cooled surfaces (i.e. the upper thermal boundary layer separating surface temperatures from underlying material at magmatic temperatures) across the central core and cold annulus had estimated thicknesses, based on simple conductive model, of 0.3-2.2 and 1.5-4.3 m. The stability of the thermal structure through time and between explosions indicates that it is linked to a deeper structural control likely comprising a central massive plug, feeding lava flow from the SW rim of the crater, surrounded by an arcuate, marginal fracture zone through which heat and mass can preferentially flow.

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[1] An infrared thermometer, spectroradiometer and digital video camera were used to observe and document shortterm evolution of surface brightness temperature and morphology at Santiaguito lava dome, Guatemala. The thermometer dataset shows 40 -70 minute-long cooling cycles, each defined by a cooling curve that is both initiated and terminated by rapid increases in temperature due to regular ash venting. The average cooling rate calculated for each cycle range from 0.9 to 1.6°C/min. We applied a twocomponent thermal mixture model to the spectroradiometer (0.4 -2.5 mm) dataset. The results suggest that the observed surface morphology changed from a cool (120-250°C) crust-dominated surface with high temperature fractures (>900°C) in the first segment of the measurement period to an isothermal surface at moderately high temperature (350 -500°C) during the second segment. We attribute the change in the thermal state of the surface to the physical rearrangement of the dome's surface during the most energetic of the ash eruptions.