Data from remote sensing satellites operating in the short-wavelength infrared can be used to measure temperatures of about 150øC and above. The gain settings of the Landsat Thematic Mapper (TM) sensors are such that to record pixel-integrated temperatures from 150øC to over 1000øC requires the use of several spectral bands. If the radiant source occupies less than a whole pixel, as is the case for fumaroles or a crusted lava surface with incandescent cracks, then the pixel-integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength. By determining pixel-integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel. In some cases our observations provide the only available data on the state of activity of a particular volcano; in others they support and refine field observations. Our Landsat TM data reveal otherwise unobserved precursor and subsequent activity related to the September 1986 eruption of Lascar volcano (Chile) and supplement other data on the March 1986 eruption of Augustine volcano (Alaska). They also indicate the continued presence of lava lakes at Erta 'Ale (Ethiopia) in the absence of any ground-based observations and document minor eruptive activity at Mount Erebus (Antarctica). Landsat Multispectral Scanner data have been used to measure the variation in temperature along a lava flow on Sierra Negra (Galapagos Islands) which occurred in 1979.

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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...