MAPPING THE DISTRIBUTION OF VESICULAR TEXTURES ON SILICIC LAVAS USINGTHE THERMAL INFRARED MULTISPECTRAL SCANNER

The Thermal Infrared Multispectral Scanner (TIMS) is a potentially pow erful tool for mapping detailed chemical variations in silicic lava fl ows which in turn could expand knowledge of pre-eruption compositional gradients and mixing processes in silicic magma bodies. However, ther mal infrared image data may be greatly influenced by the range of vesi cular textures which occur on silicic flows. To investigate the effect of vesicularity on TIMS imagery independent of chemical variations, w e studied Little Glass Mountain at the Medicine Lake Volcano of northe rn California, a large rhyolitic flow of uniform composition but textu ral heterogeneity. The imagery was recalibrated so that the digital nu mber values for a lake in the scene matched a calculated ideal spectru m for water. TIMS spectra for the lava show useful differences in coar sely and finely vesicular pumice data, particularly in TIMS bands 3 an d 4. Images generated by ratioing these bands accurately map out those areas known from field studies to be coarsely vesicular pumice. These texture-related emissivity variations are probably due to the larger vesicles being relatively deeper and separated by smaller septa leavin g less smooth glass available to give the characteristic emission of t he lava. In studies of inaccessible lava flows (as on Mars) areas of c oarsely vesicular pumice must be identified and avoided before chemica l variations can be interpreted. Remotely determined distributions of vesicular and glassy textures can also be related to the volatile cont ents and potential hazards associated with the emplacement of silicic lava flows on Earth.