J. Ondrusek et al., MAPPING THE DISTRIBUTION OF VESICULAR TEXTURES ON SILICIC LAVAS USINGTHE THERMAL INFRARED MULTISPECTRAL SCANNER, J GEO R-SOL, 98(B9), 1993, pp. 15903-15908
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.