Large volumes of altered rock located high within a steep volcano are potential sources of landslides. Such landslides typically transform into lahars that travel many tens of kilometers downstream from a volcano. The deepest that the lowest frequency EM data can penetrate the surface at Mount Rainier is about 150 m beneath the ice. The techniques used to identify altered volcanic rocks within a volcano are effective because altered rocks have low magnetization and electrical resistivities. In contrast, fresh volcanic rocks have relatively high magnetic signatures and high resistivities.
Ridges generally produce magnetic highs, while valleys produce lows.
Because intense hydrothermal alteration reduces the high magnetization of fresh volcanic rocks, the aeromagnetic data shows a magnetic low over an area that is altered. The location and approximate volume of altered rock is determined from the data. The resistivity of ice is 1M ohm-m based on analogs with other temperate glaciers and modeling. Measured resistivities of a few saturated altered rock samples from Mount Rainier and from magnetotelluric data range from about 10 to 500 ohm-m. These results show that the resistivities expected from the altered rocks are at least an order of magnitude lower than those of the unaltered rock.