AEROGEOPHYSICAL MEASUREMENTS OF HYDROTHERMALLY ALTERED ZONES AT MOUNT ADAMS, WASHINGTON

Hydrothermally altered rocks can weaken volcanoes, increasing the potential for catastrophic sector collapses that can lead to destructive lahars. At Mount Adams, some of the largest lahars contain abundant hydrothermal clay minerals, an indication that that some of them derived from collapse of altered edifice flanks. As shown at Mount Rainier, helicopter electromagnetic and magnetic detection of the 3-dimensional distribution of highly altered rock, aids evaluation of the hazards associated with such alteration. Intense hydrothermal alteration significantly reduces the resistivity and magnetization of volcanic rock. Ice-penetrating sensors in a recent high-resolution helicopter magnetic and electromagnetic survey of Mount Adams were flown about 45-60 meters above the ground along east- and west-trending lines spaced 250 meters apart. Transmitter and receiver coils in the electromagnetic sensor detected the electromagnetic response of the exposed and ice-covered ground at different frequencies to obtain information from different depths. The deepest that the lowest frequency electromagnetic data could penetrate the low resistivity, altered zones was ~100 m; outside the zones penetration was several hundred meters. Total-field magnetic data can detect magnetization variations to several thousand meters depth. Limited outcrop of altered rock occurs primarily in headwalls of the Avalanche and Adams Glaciers and in Roosevelt Cliff. Cores through the ice cap north of the summit suggest an additional area of 0.5 to 1 km2 of variably altered rock concealed beneath glaciers. The part of this region north of the summit correlates with low magnetizations and resistivities that extend more than 250 m below the surface. The lowest resistivities just north of the summit suggest the presence of water-saturated, clay-rich hydrothermally altered rock. Higher resistivities and magnetizations over the Roosevelt Cliff area indicate that alteration there is not as intense. As at Mount Rainier, the geophysically-derived, 3-dimensional geometry of altered rocks will help to quantify the stability of Mount Adams.