Aerogeophysical measurements of collapse-prone hydrothermally altered zones at Mount Rainier volcano

Hydrothermally altered rocks can weaken volcanoes, increasing the potential for catastrophic sector collapses that can lead to destructive debris flow s(1). Evaluating the hazards associated with such alteration is difficult b ecause alteration has been mapped on few active volcanoes(1-4) and the dist ribution and severity of subsurface alteration is largely unknown on any ac tive volcano. At Mount Rainier volcano (Washington, USA), collapses of hydr othermally altered edifice flanks have generated numerous extensive debris flows(5,6) and future collapses could threaten areas that are now densely p opulated(7). Preliminary geological mapping and remote-sensing data indicat ed that exposed alteration is contained in a dyke-controlled belt trending east-west that passes through the volcano's summit(3-5,8). But here we pres ent helicopter-borne electromagnetic and magnetic data, combined with detai led geological mapping, to show that appreciable thicknesses of mostly buri ed hydrothermally altered rock lie mainly in the upper west flank of Mount Rainier. We identify this as the likely source for future large debris flow s. But as negligible amounts of highly altered rock lie in the volcano's co re, this might impede collapse retrogression and so limit the volumes and i nundation areas of future debris flows. Our results demonstrate that high-r esolution geophysical and geological observations can yield unprecedented v iews of the three-dimensional distribution of altered rock.