Thermal monitoring of Lascar Volcano, Chile, using infrared data from the along-track scanning radiometer: a 1992–1995 time series |
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Authors: | M J Wooster D A Rothery |
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Institution: | (1) Department of Earth Sciences, The Open University, Milton Keynes, UK, GB;(2) The Natural Resources Institute, University of Greenwich, Chatham, Kent, UK, GB |
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Abstract: | Lascar Volcano (22°22'S, 67°44'W) is the most active volcano of the central Andes of northern Chile. Activity since 1984
has been characterised by periods of lava dome growth and decay within the active crater, punctuated by explosive eruptions.
We present herein a technique for monitoring the high-temperature activity within the active crater using frequent measurements
of emitted shortwave infrared (SWIR) radiation made by the spaceborne along-track scanning radiometer (ATSR). The ATSR is
an instrument of low spatial resolution (pixels 1 km across) that shares certain characteristics with the MODIS instrument,
planned for use as a volcano monitoring tool in the NASA EOS Volcanology Project. We present a comprehensive time series of
over 60 cloud- and plume-free nighttime ATSR observations for 1992–1995, a period during which Lascar experienced its largest
historical eruption. Variations in short wavelength infrared flux relate directly to changes in high-temperature surfaces
within the active crater. From these data, interpretations can be made that supplement published field reports and that can
document the presence and status of the lava dome during periods where direct, ground-based, observations are lacking. Our
data agree with less frequent information collected from sensors with high spatial resolution, such as the Landsat thematic
mapper (Oppenheimer et al. 1993) and are consistent with field observations and models that relate subsidence of the dome
to subsequent explosive eruptions (Matthews et al., 1997). Most obviously, Lascar's major April 1993 eruption follows a period
in which the magnitude of emitted shortwave infrared radiation fell by 90%. At this time subsidence of the 1991–1992 lava
dome was reported by field observers and this subsidence is believed to have impeded the escape of hot volatiles and ultimately
triggered the eruption (Smithsonian Institution 1993a). Extrapolating beyond the period for which field observations of the
summit are available, our data show that the vulcanian eruption of 20 July 1995 occurred after a period of gradual increase
in short wavelength infrared flux throughout 1994 and a more rapid flux decline during 1995. We attribute this additional,
otherwise undocumented, cycle of increasing and decreasing SWIR radiance as most likely representing variations in degassing
through fumaroles contained within the summit crater. Alternatively, it may reflect a cycle of dome growth and decay. The
explosive eruption of 17 December 1993 appears to have followed a similar, but shorter, variation in SWIR flux, and we conclude
that large explosive eruptions are more likely when the 1.6-μm signal has fallen from a high to a low level. The ATSR instrument
offers low-cost data at high temporal resolution. Despite the low spatial detail of the measurements, ATSR-type instruments
can provide data that relate directly to the status of Lascar's lava dome and other high-temperature surfaces. We suggest
that such data can therefore assist with predictions of eruptive behaviour, deduced from application of physical models of
lava dome development at this and similar volcanoes.
Received: 1 October 1996 / Accepted: 13 January 1997 |
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Keywords: | Lascar Volcano ATSR (Along Track Scanning Radiometer) Remote sensing Infrared |
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