Mapping Hydrothermal Alteration Products On Mt Tongariro Using Hyperion Data
The threat of a major collapse/landslide from a volcanic cone is a primary hazard of concern in the Tongariro National Park. Volcanic edifice susceptibility to this process is exacerbated by hydrothermal alteration and consequent weakening of the rock. The geologic record at Ruapehu and Tongariro volcanoes shows that past debris flows (lahars) have often contained a high proportion of alteration products, indicating the importance of altered flank areas of the volcano in generating these hazards. The present flanks of both volcanoes have several hydrothermally altered zones that could be sources of future collapses, with those on Mount Tongariro being particularly obvious. The purpose of this study was to interpret Hyperion data using field spectra collected from a portable spectroradiometer to assess the overall extent of alteration products and map altered zones on Mount Tongariro. A Hyperion image of the study area was acquired on June 4, 2003. The image was corrected for atmospheric and topographic effects prior to analysis. A range of different alteration-product zones were identified visually on the image. Accessible alteration zones were visited, samples were collected and reflectance spectra recorded using a portable spectroradiometer, with GPS locations stored. X-Ray diffraction analysis was conducted on the field samples to identify the type of alteration products at each location. The XRD identification and the field spectra provided the ground truth for the Hyperion image, allowing the selection of accurate end-members. The derived end-members were used to carry out a supervised classification, which defined the extent of the alteration products within the study area. The alteration products form some of the least stable zones on the volcano. Identifying these zones provides crucial information in the production of a new lahar hazard map for the Tongariro National Park.