Three‐dimensional geophysical mapping of rock alteration and water content at Mount Adams, Washington: Implications for lahar hazards

Abstract

[1] 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. Evaluating the hazards associated with such alteration is difficult because alteration has been mapped on few active volcanoes and the distribution and intensity of subsurface alteration are largely unknown on any active volcano. At Mount Adams, some Holocene debris flows contain abundant hydrothermal minerals derived from collapse of the altered edifice. Intense hydrothermal alteration significantly reduces the resistivity and magnetization of volcanic rock, and therefore hydrothermally altered rocks can be identified with helicopter electromagnetic and magnetic measurements. Electromagnetic and magnetic data, combined with geological mapping and rock property measurements, indicate the presence of appreciable thicknesses of hydrothermally altered rock in the central core of Mount Adams north of the summit. We identify steep cliffs at the western edge of this zone as the likely source for future large debris flows. In addition, the electromagnetic data identified water in the brecciated core of the upper 100-200 m of the volcano. Water helps alter the rocks, 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. 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 aiding evaluation of the debris avalanche hazard.

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.…”

Developing integrated methods to address complex resource and environmental issues

“…These permeable units most likely represent local perched water units. The HEM data identified water-saturated rocks from the surface to the detection limit (100-200 m) in discreet zones at Mount Rainier (Finn and others, 2001) and Mount Adams (Finn, Deszcz-Pan, Anderson, and John, 2007) and over the entire summit region at Mount Baker ).…”

“…Critical to quantifying the hazard from volcanic landslides is knowledge of the three-dimensional (3-D) distribution of pervasively altered rock and shallow groundwater that plays an important role in transforming debris avalanches to far-traveled lahars, which are the debris that flows down the slopes of a volcano (Vallance and Scott, 1997;Vallance, 1999), both of which can be detected by helicopter electromagnetic (HEM) and magnetic data (Deszcz-Pan and others, 1998;Finn and others, 2001;Finn, Descsz-Pan, and others, 2007;Finn, Deszcz-Pan, Anderson, and John, 2007;Finn and Descsz-Pan, 2011). Hydrothermally altered volcanic rocks possess low electrical resistivities because of their water and clay contents, whereas high resistivities characterize fresh, dry, massive volcanic rocks.…”