Visible and near-infrared reflectance of tuff rings and tuff cones
The visual and near infrared spectral reflectance of the basaltic tuff composing six tuff rings and three tuff cones was studied in order to better understand the origin of tuff rings and tuff cones and the alteration processes of the constituent basaltic tephras. Airborne and spaceborne remotely sensed data, plus laboratory and field spectra composed the spectral database. Further mineralogic and chemical information was gained through petrographic microscope work, X-ray diffraction, electron microprobe analysis and iron Mossbauer spectrometry. Fresh tuff rings, resulting from dry pyroclastic surges, are composed of a variable mix of country rock fragments and unaltered sideromelane which has a dark generally featureless reflectance spectrum. As surges become increasingly wet, and as water condenses on the sideromelane, a process of nearly syn-depositional hydration can occur. The type of wet pyroclastic flow that produces tuff cones results in sideromelane deposits that can alter relatively rapidly to palagonite. Spectral features that characterize hydrated but largely unoxidized tuff rings include a low overall reflectance, weak 1.4 and 1.9 $\mu$m H$\sb2$O and OH bands; a 2.2 $\mu$m Al-OH clay band can also be present. In the highly palagonitized tuff cones, the reflectance is twice as high, H$\sb2$O and OH bands are stronger and a 2.3 $\mu$m Mg-OH band is generally present. Oxidation is evident from the development of spectral features attributable to ferric ion phases. Differences in spectral reflectance are ascribed primarily to differences in the degree of alteration which in turn is influenced by the water/magma ratio extant at the time of eruption.
The remotely sensed data was analyzed using a spectral mixture model. It was found that palagonite tuff constitutes an easily mapped spectral endmember, while the hydrated tuff typical of tuff rings is difficult to distinguish from other dark materials. The best mapping was performed with the systems with high spatial resolution such as the NS001 Thematic Mapper Simulator and the Geoscan Mk II advanced multispectral scanner. Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data, calibrated to reflectance through reference endmember modelling, were used to map the 1-$\mu$m feature in the Pavant Butte palagonite tuff and also revealed a previously undetected 2.2 $\mu$m absorption feature in those tuffs.
0799: Remote sensing