M S Ramsey and J H Fink (Both at: Department of Geology, Arizona State University, Box 871404, Tempe, AZ 85287-1404, USA; 602-965-5507; email: ramsey@elwood.la.asu.edu)

Remote monitoring of silicic lava dome growth following initial emergence provides insights into further lava extrusion and the potenial hazards of future eruptions. Changes in surface texture of the dome are of interest because they can be used to derive emplacement time, volatile content, and internal structure. In addition, pumiceous textures produce significant variations in thermal emission spectra that are clearly distinguishable using remote sensing techniques.

The surface texture of typical obsidian domes can vary significantly from dense glass to highly vesicular pumice. With regard to thermal emission spectroscopy and remote sensing, these textural variations can be described as a surface consisting of two spectral endmembers, glass and vesicles. The distinct spectral feature of obsidian is commonly muted due to the overprinting of the vesicles which act as blackbody radiators. Assuming that the featureless blackbody radiation combines linearly in direct proportion to the percentage of vesicles present, the spectra can be deconvolved in order to estimate the areal abundances of the glass and vesicles.

A linear retrieval, deconvolution model was developed and applied to airborne, thermal infrared data of the Big Glass Mountain flow in northern California. The flow varies both in composition and texture, and thus serves as a test site for studying these potential complicating spectral effects. Using the obsidian and a blackbody as spectral endmembers, the retrieval algorithm was applied to produce an image of the vesicle percentage. Average values were extracted from regions across the dome's surface and compared with the previously reported values of ~30% for the the finely vesicular pumice (FVP) and ~50% for the coarsely vesicular pumice (CVP). The result was a model-predicted percentage of 25.0% and 59.6% for the FVP and CVP, respectively. The next phase of this study will involve a similar methodology applied to high resolution laboratory spectra of field samples, comparing the results to values derived from thin section analysis. The development of a rapid, remote analysis technique for assessing lava vesicularity will provide useful insight into the degassing state of an active flow or dome.

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Presented at: American Geophysical Union Fall Meeting
Date: 1996