Journal
JOURNAL OF GLACIOLOGY
Volume 52, Issue 177, Pages 175-182Publisher
CAMBRIDGE UNIV PRESS
DOI: 10.3189/172756506781828700
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A newly developed hammer was used to insert two autonomous probes 0.8 m and 2.1 m into clast-rich subglacial till under Black Rapids Glacier, Alaska, USA. Both probes were instrumented with a dual-axis tilt sensor and a pore-water pressure transducer. The data are compared to a 75 day record of surface velocities. Till deformation at depth was found to be highly seasonal: it is significant during an early-season speed-up event, but during long periods thereafter measured till deformation rates are negligible. Both tilt records show rotation around the probe axis, which indicates a change in tilt direction of about 30 degrees. The tilt records are very similar, suggesting spatial homogeneity on the scale of the probe separation (4 m horizontal and 3.3 m vertical). There is evidence that during much of the year sliding of ice over till or deformation of a thin till layer (<20 cm) accounts for at least two-thirds of total basal motion. Basal motion accounts for 50-70% of the total surface motion. The inferred amount of ice-till sliding is larger than that found at the same location in a previous study, when surface velocities were about 10% lower. We suggest that variations in ice-till coupling account for the observed variations in mean annual speed.
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