[1] Taku Glacier in southeast Alaska has advanced 7.5 km over the last 115 years, overriding its own glaciomarine and outwash sediments. We have documented rapid erosion of these sediments by comparing radio echo soundings (RES) along five transects (2003)(2004)(2005) to earlier RES surveys (1989 and 1994) and to early bathymetric surveys of the proglacial fjord. Erosion rates, _ E, reached 3.9 ± 0.8 m a

[1] Taku Glacier, an advancing former tidewater glacier in Alaska, has been actively pushing its proglacial sediments along part of its terminus over the last 50 years, producing so-called push moraines. The mobilization of these sediments, which were locally lifted more than 20 m above sea level by 2004, has happened episodically rather than steadily. The last major event of proglacial sediment deformation occurred in 2001, presumably caused by sliding along a basal detachment layer. Since then, most deformation has been localized within a few meters of the terminus, including impressive deformational features of the terminal ice, where slabs of ice, tens of centimeters in thickness, have undercut proglacial sediments and vegetation and lifted them up. Between 2002 and 2004, surface velocities and horizontal displacements were measured across the terminus and in the proglacial push moraine area. Sediment displacement was highest between the end of March and mid-June. A decrease in displacement with distance from the terminus revealed that the sediments were deforming internally rather than along a basal décollement. We present a simple model which suggests that, under 2004 conditions, reactivation of major movement along this décollement is unlikely to happen, unless some critical factors change. These factors include (1) a steepening of the glacier surface, (2) an increased surface angle of the sediment wedge, and/or (3) higher water pressure in the system, which decreases the effective frictional resistance. An observed wet clay-rich layer presumably acted as a major fault plane during the 2001 event.

ABSTRACT. 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 ($60 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 theTEM 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'' . INTRODUCTIONEstimates of the thickness and cross-sectional shape of a rock glacier are important for understanding its stress distribution and motion. On ice glaciers this geometry is routinely determined using seismic and ice-radar methods. These methods are difficult to apply to rock glaciers because rock glaciers are thinner, are composed of a mixture of ice and rock of unknown composition and are covered by a 23 m thick layer of unconsolidated rock (the ''debris mantle''). Radio-echo sounding (RES) is difficult because the absorption and scattering of radar waves are stronger in rockglacier ice^rock mixtures than in clean ice and the basal interface may not be distinct. However, Berthling and others (2000), Degenhardt and others (2000), Isaksen and others (2000), Vonder Mu« hll and others (2001) and were successful in using ground-penetrating radar (GPR) to discern the basal interface and/or internal structures of some rock glaciers. Seismic methods are complicated by the debris mantle, which limits the transfer energy from the source (usually an explosion) into the rock glacier and inhibits geophone coupling. Seismic field techniques have been devised to overcome these problems , but are difficult to apply on a routine basis and have shown limited success. Direct-current (d.c.) electrical resistivity has been widely used in rock-glacier soundings (Fisch and others,1977; Evin and others, 1997;), but the debris mantle again poses problems with electrical coupling and requires laborintensive field set-ups. We have used all of these methods, with the exception of GPR, to investigate the geometry of Fireweed rock glacier with limited success. We have found that transient electromagnetic (TEM) methods provide the best means of investigating the internal structure and thickness distribution of the rock glacier. TEM methods do not require a high degree of physical or electrical coupling with the surface substrate. Here we discuss the methods and results for each of the techniq...

Fireweed rock glacier is a large rock glacier in south central Alaska, U.S.A. It flows relatively fast, with velocities up to 3.5 ma–1, and exhibits both seasonal and annual velocity variations, some of which are related to periodic terminus calving and increased rainfall. Our analysis reveals that motion is likely concentrated in a pseudo-rectangular channel within the larger parabolic channel with a “shear plane” at ~27 m depth. There is likely motion along the shear plane as well as internal deformation above it. We estimate that the ice—rock mixture is up to seven times softer than clean glacier ice with a temperature of –2°C. Calving at the terminus is an important component of the mass balance of this rock glacier.