INFRARED IMAGE-ANALYSIS OF VOLCANIC THERMAL FEATURES - LASCAR VOLCANO, CHILE, 1984-1992

Fifteen Landsat Thematic Mapper (TM) images of Lascar volcano (Chile), recorded between December 1984 and April 1992, document the evolution of a lava dome within the summit crater. Four of the scenes were acquired at night. In every image, the two short-wavelength infrared bands, 5 and 7, have detected thermal radiation from the volcano. As a consequence of the Planck distribution function, the relative response of these two channels depends on the proportions of very hot (> 600-degrees-C) surfaces occupying tiny pixel areas and broader regions at moderate temperatures (< 280-degrees-C). Intercomparison of bands 5 and 7 thereby provides a means for interpreting TM thermal anomalies even in the absence of ground observations. Pronounced changes in the configuration and intensity of the Lascar anomaly suggest that the volcano has experienced at least two cycles of lava dome activity since 1984. The first of these progressed through a ''cooling'' period, possibly reflecting a reduced flux of magmatic volatiles at the surface, and culminated in an explosive eruption on September 16, 1986, which appears to have completely destroyed the inferred lava dome. The TM data indicate that a new dome had been emplaced by November 1987, more than 15 months before it was first discovered by local observers. Lascar's style of cyclical effusive and explosive activity is typical of many volcanoes, and die remote sensing techniques presented herein could be applied elsewhere.