The thickness and internal structure of Fireweed rock glacier, Alaska, USA, as determined by geophysical methods

Geophysical investigations on rock glaciers are often difficult because rock glaciers are covered by an unconsolidated debris mantle a few meters thick, are typically <50 m thick and are composed of an ice-rock mixture of unknown composition. Transient electromagnetics (TEM) is a method that allows some of these difficulties to be minimized, and data collection is relatively efficient. TEM, with calibration from terminus exposure, was used to determine the thickness (similar to60 m) of Fireweed rock glacier, Alaska, U.S.A., under complex valley geometry. A conductive layer beneath the rock glacier was identified, and its distribution is consistent with a till-like layer. Seismic refraction, used to resolve the debris-mantle thickness (2-4 m), suggests the presence of a discontinuity at 18-28 m depth within the rock glacier. The discontinuity is also indicated in the radio-echo sounding and the TEM data, but to a lesser extent. This discontinuity is important because the motion of the rock glacier may occur across this as a shear plane.