MAPPING HYDROTHERMAL MINERALS AT THREE SISTERS VOLCANOES, OREGON USING HYPERSPECTRAL IMAGERY

Three Sisters volcanoes are three potentially active volcanoes of Quaternary age located in the High Cascades of central Oregon. North Sister and Middle Sister have not been active since the Pleistocene, but South Sister has been active as recently as 2000-1500 years ago. The Three Sisters area has a history of explosive eruptions characterized by basaltic to rhyolitic volcanism. Past events have also included lahars, pyroclastic flows, and ash falls. Current volcanic hazards in the Three Sisters area include landslides, lahars and debris avalanches due to rocks weakened by hydrothermal alteration, as well as eruption-triggered events such as pyroclastic flows.

We used 1996 Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) hyperspectral data to map hydrothermal minerals at Three Sisters. Prior to analysis of the data, we applied atmospheric radiation transfer models to remove water vapor and carbon dioxide data from the spectra because of the 20 km altitude at which the AVIRIS sensors were flown. Field spectra were used reduce the data to surface reflectance. Following image classification, resultant maps were warped to a 30 m U.S. Geological Survey DEM (digital elevation model) to produce a three dimensional image showing the distribution of hydrothermal minerals relative to slope. Field samples were collected and spectrally analyzed in the laboratory to check mapping results.