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

Abstract

Hydrothermally altered rocks can weaken volcanoes, increasing the potential for catastrophic sector collapses that can lead to destructive debris flows. Evaluating the hazards associated with such alteration is difficult because alteration has been mapped on few active volcanoes and the distribution and severity of subsurface alteration is largely unknown on any active volcano. At Mount Rainier volcano (Washington, USA), collapses of hydrothermally altered edifice flanks have generated numerous extensive debris flows and future collapses could threaten areas that are now densely populated. Preliminary geological mapping and remote-sensing data indicated that exposed alteration is contained in a dyke-controlled belt trending east-west that passes through the volcano's summit. But here we present helicopter-borne electromagnetic and magnetic data, combined with detailed geological mapping, to show that appreciable thicknesses of mostly buried hydrothermally altered rock lie mainly in the upper west flank of Mount Rainier. We identify this as the likely source for future large debris flows. But as negligible amounts of highly altered rock lie in the volcano's core, this might impede collapse retrogression and so limit the volumes and inundation areas of future debris flows. Our results demonstrate that high-resolution geophysical and geological observations can yield unprecedented views of the three-dimensional distribution of altered rock.

An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards

“…It may be possible that internal alteration is occurring, but not in areas that drain into the hot springs. In order to assess this, future geophysical surveys such as electrical resistivity/conductivity, self-potential, or magnetotellurics (Fuji-ta et al 1999;Finn et al 2001;Nicollin et al 2006;Bedrosian et al 2007;Finn et al 2007;Komori et al 2010;Lesparre et al 2012) would be necessary. Finally, and of some concern, is that the results found here indicate that the precipitous cliffs near the headwall of the Santa María scarp are additionally compromised by the indication that argillic alteration is prevalent there.…”

Hydrogeological settings of a volcanic island (San Cristóbal, Galapagos) from joint interpretation of airborne electromagnetics and geomorphological observations

“…Several systems have been developed, measuring in the frequency-domain such as Resolve and DigHEM by Fugro Airborne Surveys (e.g. Mullen et al, 2007 andSiemon, 2009), and in the time-domain, such as SkyTEM (Sørensen and Auken, 2004) or VTEM by Geotech Airborne Inc. AEM surveys have been successfully conducted during the past decades for risk assessment and hydrogeological prospection in various geological contexts (Bosch et al, 2009;Mogi et al, 2009;Steuer et al, 2009;Supper et al, 2009;Viezzoli et al, 2010b), including volcanic (Auken et al, 2009b;Finn et al, 2001Finn et al, , 2007d'Ozouville et al, 2008). In most of the above-mentioned studies, geophysical data have been interpreted with a substantial set of drill hole and geological data available prior to the survey.…”

Magnetization intensity mapping on Unzen Volcano, Japan, determined from high-resolution, low-altitude helicopter-borne aeromagnetic survey

“…It is thought that these rock bodies with remanent magnetization were fractured into pieces and then rotated into random directions by landslides (cf. Finn et al, 2001).…”