HYPERSPECTRAL THERMAL EMISSIONS SPECTROMETER CHARACTERIZATION OF A 12 KA BASALTIC LAVA FLOW AT CIMA VOLCANIC FIELD, CALIFORNIA

The Cima volcanic field has lava flows (mostly `a`ā), scoria and tuff cones, Quaternary deposits, and locally developed desert pavements, with good correlation between geologic map units, aerial photos, and images from the hyperspectral thermal emissions spectrometer (HyTES). HyTES is an airborne imaging spectrometer with 256 continuous spectral channels between 7.5-12 μm; Cima mapping test data has ~ 6.25 m ground resolution. Individual lava flows vary in land surface temperature and imagery using 10.1, 9.2, and 8.5 μm channels for brightness temperature and emissivity. Shadowed versus sun-facing areas on features can have significant spectral differences, so flow morphology, surface roughness, and relief at many scales is important. Correlations of geochemical and physical properties with spectral characteristics are being developed. For lava flows, physical properties include the amounts of minerals and glass, vesicularity, shapes of fragments, and decimeter to decameter flow surface morphology. A 12 ka trachybasalt `a`ā lava flow with well-displayed flow morphology that has rafted parts of the source scoria cone, and only localized aeolian dust deposits, was sampled for laboratory spectrophotometer measurements, including parts of a lava flow, autobrecciated rubble fragments on top of the flow, and clasts from the rafted cone. Natural cooled (including oxidized) surfaces, fractures, and freshly broken surfaces on a sample of sparsely vesiculated lava have different emissivity spectra in laboratory measurements including the depression in the spectral feature between 9.2-10.2 mm. These different surfaces comprising parts of a larger surface have endmember spectra that proportionally contribute to a mean value. Lava fragments range from dark reddish gray to reddish yellow (oxidized) with 20-30% <1 mm to 45-55% <15 mm vesicles, and typically have 9.6 mm emissivity below the mean lava flow. Clasts from the scoria cone range from weak red to light reddish brown (oxidized) with 2-30% <2 mm to 40-60% <20 mm vesicles, and typically have 9.6 mm emissivity above the mean lava flow. The geochemistry of the lava flow varies by <1 wt% in total alkalis and SiO2, so variations in HyTES spectra for each pixel is a combination of surface morphology and relative proportions of lava flow, rubble fragments, and cone parts.