HYDROTHERMAL ALTERATION AND NON-ERUPTIVE HAZARDS AT SOUTH SISTER VOLCANO, OREGON, USING IMAGING SPECTROMETRY AND DIGITAL ELEVATION DATA

South Sister volcano is a basaltic andesite composite volcano located in the Three Sisters region of central Oregon. South Sister is the youngest of the four Quaternary age cones in the area, with the last known eruption occurring about nineteen hundred years ago. South Sister has been of recent interest because of slight inflation observed over the past few years of approximately 10 cm found to the west of the volcano. While this is may be considered evidence for a possible future eruption, there are also hazards associated with non-eruptive activity, such as debris flows and lahars, that can potentially endanger visitors to the Three Sisters Wilderness area.

Hydrothermal alteration of volcanic rock to iron oxides and expansive clays, like montmorillonite and kaolinite, poses a threat to the stability of the flanks and summit areas of South Sister, as excessive rainfall or snowmelt can trigger these types of non-eruptive hazards. We analyzed 1997AVIRIS spectra to identify exposed summit areas with hydrothermal alteration. Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data produces calibrated hyperspectral images in 224 contiguous bands at wavelengths between 400 and 2500 nm. Spectral endmembers were mapped using a spectral angle mapper (SAM) algorithm that determines the similarity between image and endmember spectra by calculating the angular distance between them in radians. We combined the image processed map result with digital elevation data to produce a 3-D model of potential debris flows and lahars. Some of the hydrothermal targets were confirmed during a brief ground-truthing visit.