Finn, C.M., Deszcz-Pan, M., Horton, R., Breit, G. and John, D., 2007, Airborne Magnetic and Electromagnetic Data map Rock Alteration and Water Content at Mount Adams, Mount Baker and Mount Rainier, Washington: Implications for Lahar Hazards and Hydrothermal Systems: Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract NS34A-05.

Airborne Magnetic and Electromagnetic Data map Rock Alteration and Water Content at Mount Adams, Mount Baker and Mount Rainier, Washington: Implications for Lahar Hazards and Hydrothermal Systems

High resolution helicopter-borne magnetic and electromagnetic (EM) data flown over the rugged, ice-covered, highly magnetic and mostly resistive volcanoes of Mount Rainier, Mount Adams and Mount Baker, along with rock property measurements, reveal the distribution of alteration, water and hydrothermal fluids that are essential to evaluating volcanic landslide hazards and understanding hydrothermal systems. Hydrothermally altered rocks, particularly if water saturated, can weaken stratovolcanoes, thereby increasing the potential for catastrophic sector collapses that can lead to far-traveled, destructive debris flows. Intense hydrothermal alteration significantly reduces the magnetization and resistivity of volcanic rock resulting in clear recognition of altered rock by helicopter magnetic and EM measurements. Magnetic and EM data, combined with geological mapping and rock property measurements, indicate the presence of appreciable thicknesses of hydrothermally altered rock west of the modern summit of Mount Rainier in the Sunset Amphitheater region, in the central core of Mount Adams north of the summit, and in much of the central cone of Mount Baker. We identify the Sunset Amphitheater region and steep cliffs at the western edge of the central altered zone at Mount Adams as likely sources for future debris flows. In addition, the EM data identified water-saturated rocks in the upper 100-200 m of the three volcanoes. The water-saturated zone could extend deeper, but is beyond the detection limits of the EM data. Water in hydrothermal fluids reacts with the volcanic rock to produce clay minerals. The formation of clay minerals and presence of free water reduces the effective stress, thereby increasing the potential for slope failure, and acts, with entrained melting ice, as a lubricant to transform debris avalanches into lahars. Therefore, knowing the distribution of water is also important for hazard assessments. Finally, modeling requires extremely low resistivities (< 20 ohm-m) that laboratory electrical resistivity measurements indicate are most easily explained by sulfuric acid solutions permeating altered rocks. The acid is the result of oxidation of magmatic H2S to sulfuric acid and highlights the continued alteration of volcanoes during periods of relative quiescence. Our results demonstrate that high resolution geophysical and geological observations can yield unprecedented views of the three-dimensional distribution of altered rock and shallow pore water and hydrothermal fluids within active stratovolcanoes.