Regional analysis of tertiary volcanic calderas (Western U.S.) using landsat thematic mapper imagery

Abstract

The Basin and Range province of southern Nevada is an excellent setting to study Landsat imagery over volcanic rock assemblages. The area is semiarid, sparsely vegetated, and contains relatively youtful (6–14 Ma) volcanic calderas with compositionally diverse volcanic rock assemblages. Investigation of rock spectra, whole rock geochemistry, desert varnish, and Landsat imagery indicates that the longer wavelength visible and near infrared Thematic Mapper (TM) bands 3, 5, and 7 provide more effective lithologic discimination than the shorter wavelength bands due partly to deeper penetration of lower frequency energy and greater primary rock compositional response. (Spectral contamination of vegetation degrades lithologic information in TM Band 4.) Thus ratios, color composites, and intensity - saturation - hue images using TM Bands 3, 5 and 7 generally lead to superior lithologic contrast. Shorter wavelength, TM Bands 1 and 2 are affected more by surficial weathering products including desert varnish which may or may not provide an indirect link to lithologic identity. These relationships lead to a rock-varnish albedo difference that aids identification of desert varnish on leucocratic rocks, using TM Band 2, 5, and 5 2 ratio images. Principles component color composites provide best overall lithologic contrast but not specific lithologic identification. PC2 tends to contain lithologically dominant variance and combines favorably with other images influenced strongly by lithology. Late magmatic differentiates that are “evolved” with respect to alkalies and the “incompatible” elements exhibit steep spectral curves throughout the TM Bands 5–7 interval, leading generally to low 5 7 ratios and dark contrast on 5 7 ratio images. Guidelines for lithologic analysis of volcanic terrain using Landsat TM imagery are outlined.