Hydrothermal Alteration on Composite Volcanoes: Mineralogy, Hyperspectral Imaging, and Aeromagnetic Study of Mt Ruapehu, New Zealand

Abstract

Groundwater heated by shallow molten rock and volcanic gases beneath a volcano can become acidic. When this acidic groundwater rises to the surface, it chemically interacts with volcanic rocks. This interaction makes the rocks and therefore the volcano weaker, which makes the whole thing more susceptible to collapse causing large-scale landslides. This study integrates geological, remote sensing and geophysical methods to understand the history of this process at Mt Ruapehu, New Zealand. The rock samples collected indicate different minerals associated with alteration processes due to acidic groundwater. Hyperspectral imaging, which measures reflected radiation from the Earth's surface at hundreds of wavelengths, was used to map the distribution of the surface mineralogy using image classifications. The underground distribution of the weak rocks was identified using aeromagnetic surveys. Aeromagnetic data are sensitive to iron-bearing minerals that are often dissolved by hydrothermal fluids, leaving low magnetic anomalies distributed underground at Mt Ruapehu. Based on the combination of these methods, we have come up with a new model for how Mt Ruapehu has changed in the last 200,000 years. This gives us a fuller assessment of natural hazards associated with large-scale landslides.