W. D. Harrison

Glacier motion dominated by processes deep in underlying till

Black Rapids Glacier is a 40 km long surge-type glacier in the central Alaska Range. In spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large. The drilling revealed a layer of subglacial till, up to 7 m thick, that is believed to be water-saturated. At one location a string of instruments, containing three dual-axis tiltmeters and one piezometer, was successfully introduced into the till. The tiltmeters monitored the inclination of the borehole at the ice-till interface and at 1 and 2 m into the till, for 410 days. They showed that no significant deformation occurred in the upper 2 m of the till layer, and no significant amount of the basal motion was due to sliding of the ice over the till. The measured surface velocity at the drill site is about 60 m a -1 , of which 20-30 m a can be accounted for by ice deformation. Almost the entire amount of basal motion, 30-40 m a -1 , was taken up at a depth of > 2 m in the till, possibly in discrete shear layers, or as sliding of till over the underlying bedrock. We propose that the large-scale mobilization of such till layers is a key factor in initiating glacier surges.

Updated estimates of glacier volume changes in the western Chugach Mountains, Alaska, and a comparison of regional extrapolation methods

and 2001/2004. Average net balance rates ranged between � 3.1 to 0.16 m yr � 1 for the tidewater and � 1.5 to � 0.02 m yr � 1 for the nontidewater glaciers. We tested several methods for extrapolating these measurements to all the glaciers of the western Chugach Mountains using a process similar to cross validation. Predictions of individual glacier changes appear to be difficult, probably because of the effects of glacier dynamics, which on long (multidecadal) timescales, complicates the response of glaciers to climate. In contrast, estimates of regional contributions to rising sea level were similar for different methods, mainly because the large glaciers, whose changes dominated the regional total, were among those measured. For instance, the above sea level net balance rate of Columbia glacier (� 3.1 ± 0.08 km 3 yr � 1 water equivalent (weq) or an equivalent rise in sea level (SLE) of 0.0090 ± 0.0002 mm yr � 1 ) was nearly half of the total regional net balance rate of the western Chugach Mountain glaciers (� 7.4 ± 1.1 km 3 yr � 1 weq or 0.020 ± 0.003 mm yr � 1 SLE between 1950/1957 and 2001/2004). Columbia glacier is a rapidly retreating tidewater glacier that has lost mass through processes largely independent of climate. Tidewater glaciers should therefore be treated separately when performing regional extrapolations.

Implications of till deformation on glacier dynamics

The dynamics of glacier motion are governed to a large extent by the properties of the basal interface. In this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of Black Rapids Glacier, Alaska, U.S.A. We also investigate whether or not a till layer has a clear surface-observable signature in the dynamics of the glacier. Towards this end we use a finite-element ice-flow model with a Coulomb failure criterion within the basal till layer. We find that simple till physics can be used to describe decadal, seasonal and short-term (hours to days) velocity variations, and possibly uplift events. Mechanisms for each of these variations involve an increase in the extent of till at failure, a transfer of shear stress across the bed, and a consequent increase in ice deformation. Effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models. Our analyses, however, have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier.

Short-period observations of speed, strain and seismicity on Ice Stream B, Antarctica

The speed ofIce Stream B, Antarctica, was measured twice a day over a 1 month study period, and found to be steady at about the ±3 ~% level, the sensitivity of the measurements. The vertical strain was measured at three sites over a 1 year period at 1 h intervals with sensitivities of 2 or 0.2 ppm. The strain rate varied on all time-scales. Events of high strain rate were observed, but never at more than one site at a time. They can probably be understood in terms oflocal modification of the strain field associated with crevassing. Diurnal variation in strain rate was observed at one and possibly two sites during two summers. The seismicity was measured at all three sites, and diurnal and seasonal variations were prominent at all, the seismicity being much more intense in winter. Several possible causes of the diurnal variations in strain and seismicity are considered: thermal and atmospheric effects, and the effects of tides in the Ross Sea.

Observation of Basal Sliding of Variegated Glacier, Alaska

Variegated Glacier is a surge-type glacier in the St Elias mountain range in Alaska. The interval between surges is about 20 years; the last one occurred in 1964 to 1965. This glacier has been studied extensively since 1973 (Bindschadler and others, 1977). Thus far, measurements of ice velocities have been restricted to the surface. They have been analyzed using geophysically measured ice depths, in order to estimate ice velocities in the ice mass and at the base (Bindschadler and others, 1978). From 1973 to 1977 the distribution of annual ice velocities along most of the length of the glacier can be explained primarily by internal deformation without major contribution from sliding at the base. However, the variation of surface velocity with time gives definite indication that sliding occurs in summer and that the average summer rate is increasing progressively from summer to summer and that in a zone 5 to 7 km below the head of the glacier the summer-to-summer increase in inferred sliding rate is especially rapid. This is a notably distinguishing feature, which is probably indicative of a build-up toward the next surge. In order to obtain direct information about sliding-rates and water pressures at the base in this zone, a bore hole was drilled to the bottom of the glacier about 6 km below the glacier head. Observations in the hole started in June 1978 and were continued until 31 July 1978. The hole connected to an englacial water system at a depth of 204 m whereupon the water level dropped gradually to about 100 m below the surface. The last 6 m above-the base at 356 m could be drilled only by means of a cable tool because of the presence of debris-rich ice. Upon reaching the bottom, the water level increased rapidly to the firn water table at about 8 m below surface. Large variations in water level of about 200 m occurred during the following period of observation of 35 d. Major events such as audible icequakes, heavy rainfalls, and a period of unusually high ablation were associated with abrupt increases of water level up to the firn water table. High water pressure at the bottom drove a flow of muddy and sandy water upward in the hole. Consequently high freezing rates in the lower 150 m of the hole produced a very rough bore-hole wall covered with ledges, coral-reef-like features, grooves, and pockets filled with sand. Near the bottom, embedded rocks stuck out of the bore-hole wall. These features were recognized by bore-hole television. The bore-hole bottom consisted of sand which continuously proliferated and washed into the hole. Attempts to remove this sand by means of a sand pump failed, the bailed-out sand being replaced immediately. From bore-hole inclinometry an internal deformation of the ice mass of 0.22 m d^(−1) was obtained. Together with average surface velocity of 0.47 m d^(−1) we get a sliding velocity of 0.25 m d^(−1), averaged over the time of observation. This result confirms the sliding velocities inferred from surface velocity measurements. It also lies on the exponential trend line of increasing summer-to-summer velocities showing a doubling of sliding velocities about every two years (Bindschadler and others, unpublished). This strongly indicates that the next surge is likely to occur in the early eighties. Input of water from the surface probably will play a role in triggering the surge.

The Ice-Rock Interface and Basal Sliding Process as Revealed by Direct Observation in Bore Holes and Tunnels

The glacier bed, where basal sliding occurs, was reached by cable-tool drilling and sand-pump bailing in seven bore holes in Blue Glacier, Olympic National Park, Washington. Basal sliding velocities measured by bore-hole photography and confirmed by inclinometry are unexpectedly low, ranging from 0.3 to 3.0 cm/day and averaging 1.0 cm/day. This is much less than about half the surface velocity of 15 cm/day, which was the sliding-rate expected from earlier deformation measurements in bore holes made by thermal drilling alone.

Chapter 13 – Glacier Surges

Surge-type glaciers periodically undergo large flow acceleration after extended quiescent phases of slow movement, usually accompanied by terminus advance. Such glaciers are relatively rare but occur in many of the worlds glacierized areas. High water pressures and extreme basal sliding are obvious characteristics but key questions concerning this, usually spectacular phenomenon, remain open. Why are glaciers in some regions surge-type but not in others, what sort of “memory” lets glaciers surge again and again, what is the influence of climate, geology, and topography? Besides their scientific interest, glacier surges can also be a threat to humans, especially in connection with rapidly forming lakes and their sudden outbursts. Cases of hazard- and disaster-related glacier surges are described from the Pamirs, the Andes, the Italian Alps, and Alaska.