HYDROTHERMAL ALTERATION ON THE CASCADE STRATOVOLCANOES: A REMOTE SENSING SURVEY

Remote sensing imagery collected over the past ten years from several different sensors has been analyzed to study distributions of hydrothermally altered rocks on the major Cascade stratovolcanoes. Data sets include hyperspectral imagery from NASA's Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and Hyperion, the first spaceborne imaging spectrometer, as well as multispectral imagery and digital elevation data obtained from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Together these data sets provide cloud-free coverage of the major volcanic centers at times of minimal snow cover. The remote sensing spectral data enable mapping of hydrothermal clay and sulfate minerals that have distinctive visible and infrared spectral characteristics. The digital elevation data permit analyses of altered rock volumes and slope stability.

Large masses of altered rock are visible in the remote sensing images of Mount Rainier and Mount Adams. In both cases, altered rocks are well exposed on steep failure surfaces that generally remain snow-free during the summer months. Large exposures of altered rock are also visible within the steep walls of Sherman Crater at Mount Baker; however, glacial ice and altered tephra obscure other parts of Mount Baker's upper edifice. Smaller masses of altered rock are visible on Mount Hood, Mount Shasta, and Glacier Peak. On Mount Shasta the oldest areas of the composite edifice appear to be the most altered.

Altered rocks are practically absent on Mount Saint Helens, South Sister, and Lassen Peak volcanoes. This absence presumably reflects the youthful ages of these systems, and perhaps in the case of Lassen Peak and South Sister, a relative lack of glacial dissection. Mount Jefferson, North Sister, and Middle Sister volcanoes also appear to lack extensive alteration, although the images examined to date are partially obscured by snow cover. Structural features and other possible influences on altered rock distributions at each volcano will also be reviewed.