THE 3-D DISTRIBUTION OF ALTERATION, ICE AND WATER FROM HELICOPTER MAGNETIC AND ELECTROMAGNETIC SURVEYS AT MTS. ADAMS, BAKER AND RAINIER, WASHINGTON: IMPLICATIONS FOR DEBRIS FLOWS HAZARDS

Large debris flows from Mts. Adams, Baker and Rainier can originate in water-saturated, highly altered rock. Helicopter magnetic and electromagnetic (EM) data flown over these rugged, ice-covered volcanoes reveal the distribution of hydrothermal alteration, water and ice thickness essential to evaluating volcanic landslide hazards. Hydrothermally altered rocks, particularly if water saturated, can weaken volcanoes, thereby increasing the potential for catastrophic sector collapses that can lead to far-traveled, destructive debris flows. Such alteration significantly reduces the magnetization and electrical resistivity of volcanic rock, resulting in a clear distinction between altered and unaltered rock in helicopter magnetic and EM measurements. These geophysical data, combined with geological mapping and rock property measurements, indicate the presence of appreciable thicknesses (>500 m) of altered rock west of the modern summit of Mt. Rainier in the Sunset Amphitheater region and in the central core of Mt. Adams north of the summit. Alteration at Mt. Baker is restricted to thinner (<300 m) zones beneath Sherman Crater and the Dorr Fumerole Fields. The presence of water not only helps form clay minerals that weaken the edifice but also can reduce the effective stress, increasing the potential for slope failure. In addition, the water, with entrained melting ice, acts as a lubricant to transform debris avalanches into lahars. The EM data identified water-saturated rocks from the surface to the detection limit (100 – 200 m) in discreet zones at Mt. Rainier and Mt Adams and over the entire summit region at Mt. Baker. The best estimates for ice thickness are obtained over relatively low-resistivity (<800 ohm-m) ground for the main ice cap on Mt. Adams and over most of the summit of Mt. Baker. Calculation of ice thickness, hydrologic potential, and sub-ice slope will help identify regions with high potential for debris flows and floods. The modeled distribution of alteration, pore fluids, and partial ice volumes helps identify likely sources for future alteration-related debris flows and clearly shows that debris flow hazard studies on altered volcanoes are greatly enhanced by helicopter magnetic and EM data.