Fifteen Landsat Thematic Mapper (TM) images of L•scar 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 (> 600øC) surfaces occupying tiny pixel areas and broader regions at moderate temperatures (< 280øC). 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 L•scar 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. Lfiscar's style of cyclical effusive and explosive activity is typical of many volcanoes, and the remote sensing techniques presented herein could be applied elsewhere.
INTRODUCTION
Satellite remote sensing can serve volcanological inquiry in various ways. Preliminary investigations have demonstrated the utility of spaceborne infrared sensors for observing volcanic thermal phenomena such as lava flows and lava lakes[Rothery etal., 1988], fumarole fields [Oppenheimer and Rothery, 1991], and crater lakes [Oppenheimer, 1993]. Detection of new thermal anomalies, or of changes in existing ones, can be of especial value for hazard evaluation. In addition, thermal measurements can illuminate the physical processes that trigger eruptions and influence the behavior of erupted materials. Data from the Landsat Thematic Mapper (TM) enabled Pieri et al. [1990] and Oppenheimer [1991] to infer thermal properties of lava flows. Although orbital remote sensing offers a continuity of observation difficult to achieve by other means at many volcanoes worldwide [Francis, 1979], previous studies have tended to concentrate on techniques for analyzing individual "snapshot" TM images. Thermal anomalies in TM data can be difficult to identify. While a long thermal signature extending radially from a volcano summit is very likely to represent a lava (or possibly pyroclastic) flow, confined lava bodies and high-temperature fumarole fields can be indistinguishable, on spatial grounds, in satellite data. The 30 x 30 m TM pixels often cover substantial portions of such features and the strongest sources of short-wavelength infrared (SWIR) radiation may occupy only tiny fractions (< 0.1%) of pixel areas [Oppenheimer, 1991; Oppenheimer et al., 1993]. Rothery et al. [1988], i...
