Estimating silicic lava vesicularity with thermal remote sensing: a new technique for volcanic mapping and monitoring

Remote monitoring of active lava domes provides insights into the duration of continued lava extrusion and detection of potentially associated explosi ve activity. On inactive flows, variations in surface texture ranging from dense glass to highly vesicular pumice can be related to emplacement time, volatile content, and internal structure. Pumiceous surface textures also p roduce changes in thermal emission spectra that are clearly distinguishable using remote sensing. Spectrally, the textures describe a continuum consis ting of two pure end members; obsidian and vesicles. The distinct spectral features of obsidian are commonly muted in pumice due to overprinting by th e vesicles, which mimic spectrally neutral blackbody emitters. Assuming tha t this energy combines linearly in direct proportion to the percentage of v esicles, the surface vesicularity can be estimated by modeling the pumice s pectrum as a linear combination of the glass and blackbody spectra. Based o n this discovery, a linear retrieval model using: a least-squares fitting a pproach was applied to airborne thermal infrared data of the Little Glass M ountain and Crater Glass rhyolite flows at Medicine Lake Volcano (Californi a) as a case study. The model produced a vesicularity image of the flow wit h values from 0 to similar to 70%, which can be grouped into three broad te xtural classes: dense obsidian, finely vesicular pumice, and coarsely vesic ular pumice. Values extracted from the image compare well with those derive d from SEM analysis of collected samples as well as with previously reporte d results. This technique provides the means to accurately map the areal di stributions of these textures, resulting in significantly different values from those derived using aerial photographs. If applied to actively deformi ng domes, this technique will provide volcanologists with an opportunity to monitor dome-wide degassing and eruptive potential in near-real-time. In J uly 1999 such an effort will be possible for the first time when repetitive , global, multispectral thermal infrared data become available with the lau nch of the Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) instrument aboard the Earth Observing System satellite.