Icefall

About: Icefall is a research topic. Over the lifetime, 56 publications have been published within this topic receiving 1262 citations.

Ice petrofabric observations from Blue Glacier, Washington, in relation to theory and experiment

Abstract: Ice observed on the surface of Blue Glacier is classified texturally into three types: coarse bubbly ice, coarse clear ice, and fine ice. The three types occur intercalated to form the observed foliated structure of the bulk glacier ice. Petrofabric study of fine ice reveals consistently a broad maximum in the density of c-axis orientations, centered about the pole of the foliation plane. This single-maximum fabric is in some respects similar to fabrics of stressed ice from polar glaciers, and the textures of fine ice and polar ice are similar. The fine-ice layers, also resemble layers that have recently been produced by rapid shearing deformation of glacier ice in the laboratory. It is inferred that the fine-ice layers in the glacier constitute zones that are undergoing (or have recently undergone) rapid mechanical plastic flow, and that the adjacent coarse-ice layers originate by recrystallization from fine ice and are not now deforming rapidly by mechanical plastic flow (basal glide). Whether the fine-ice layers have predominately a tectonic origin or whether they originate predominately as in-fillings of snow in crevasses in the icefall is not known for certain. Coarse bubbly ice fabrics generally show more than one maximum in the density of c-axis orientations. The statistical significance of multiple-maximum fabrics is tested by a comparison of several independent fabrics from given stress situations, and it is shown that the basic four-maximum pattern is reproducible, though subject to unexplained fluctuations in orientation. The ‘diamond-shaped’ four-maximum pattern is characteristic of ice subjected to long-continued shear stress of persistent orientation, and the long axis of the ‘diamond’ is (approximately) parallel to the direction of the stress vector that acts across the persistent plane of maximum shear stress. It is inferred that the basic features of the pattern develop at some depth within the glacier, and that subsequent deformation has affected the pattern to some extent. The results of recent experimental studies of the origin of ice fabrics are in moderately good agreement with the Blue Glacier observations. Recent theoretical treatments are in sufficient disagreement to be ruled out. A new method of presenting orientation data allows statistical inferences to be drawn directly from the fabric diagrams.

Neurobiological degeneracy and affordance perception support functional intra-individual variability of inter-limb coordination during ice climbing.

Abstract: This study investigated the functional intra-individual movement variability of ice climbers differing in skill level to understand how icefall properties were used by participants as affordances to adapt inter-limb coordination patterns during performance. Seven expert climbers and seven beginners were observed as they climbed a 30 m icefall. Movement and positioning of the left and right hand ice tools, crampons and the climber’s pelvis over the first 20 m of the climb were recorded and digitized using video footage from a camera (25 Hz) located perpendicular to the plane of the icefall. Inter-limb coordination, frequency and types of action and vertical axis pelvis displacement exhibited by each climber were analysed for the first five minutes of ascent. Participant perception of climbing affordances was assessed through: (i) calculating the ratio between exploratory movements and performed actions, and (ii), identifying, by self-confrontation interviews, the perceptual variables of environmental properties, which were significant to climbers for their actions. Data revealed that experts used a wider range of upper and lower limb coordination patterns, resulting in the emergence of different types of action and fewer exploratory movements, suggesting that effective holes in the icefall provided affordances to regulate performance. In contrast, beginners displayed lower levels of functional intra-individual variability of motor organization, due to repetitive swinging of ice tools and kicking of crampons to achieve and maintain a deep anchorage, suggesting lack of perceptual attunement and calibration to environmental properties to support climbing performance.

Vertical extension of the subglacial drainage system into basal crevasses

Abstract: Water plays a first-order role in basal sliding of glaciers and ice sheets and is often a key constituent of accelerated glacier motion Subglacial water is known to occupy systems of cavities and conduits at the interface between ice and the underlying bed surface, depending upon the history of water input and the characteristics of the substrate Full understanding of the extent and configuration of basal water is lacking, however, because direct observation is difficult This limits our ability to simulate ice dynamics and the subsequent impacts on sea-level rise realistically Here we show that the subglacial hydrological system can have a large volume of water occupying basal crevasses that extend upward from the bed into the overlying ice Radar and seismic imaging combined with in situ borehole measurements collected on Bench Glacier, Alaska, reveal numerous water-filled basal crevasses with highly transmissive connections to the bed Some crevasses extend many tens of metres above the bed and together they hold a volume of water equivalent to at least a decimetre layer covering the bed Our results demonstrate that the basal hydrologic system can extend high into the overlying ice mass, where basal crevasses increase water-storage capacity and could potentially modulate basal water pressure Because basal crevasses can form under commonly observed glaciological conditions, our findings have implications for interpreting and modelling subglacial hydrologic processes and related sliding accelerations of glaciers and ice sheets

Lithofacies Architecture and Construction of Volcanoes Erupted in Englacial Lakes: Icefall Nunatak, Mount Murphy, Eastern Marie Byrd Land, Antarctica

Abstract: Mount Murphy is a large Miocene shield volcano flanked by several small basaltic satellite centres that were erupted beneath a thick (> 200 m) ice sheet. Three empirical models illustrating the hydraulic evolution of glacio-volcanic systems are deduced from glacier physics, with distinctly different implications in each case for the resultant lithofacies architecture. Glacier hydraulic considerations and facies analysis are used to describe the evolution of one of the satellite centres (Icefall Nunatak). The nunatak was constructed from several vents during three main stages. Each stage demonstrates different aspects of englacial volcano construction, mainly in a flooded vault or lacustrine setting. An initial mainly effusive phase was dominated by lava and cogenetic joint-block breccia, and eruption was probably confined mainly within an englacial vault or lake (stage I). Renewed activity, at a different vent and beneath a re-established ice sheet (stage II), began with coarse sediments flushed away subglacially. A subaqueous tuff cone was then constructed in an englacial lake, from explosively erupted coarse glassy tephra probably produced mainly during sustained eruptions and distributed by high-density turbidity currents. Fine detritus is common only in the basal tuff cone unit, possibly as a result of lower, denser (largely subaqueous?) eruption columns. A spectacular slope failure is represented by numerous large blocks, which were displaced to low elevations on extensively fractured tuff cone flanks, and the failure event may have initiated zones of high pore-water discharge. Stage II culminated with two phases of lava delta progradation, indicating that the volcanic edifice ultimately penetrated the entire ice-sheet thickness and that the vent became emergent. Stage III commenced with lava effusion, probably through a thin re-formed cover of permeable snow and firn. A small cinder cone was also constructed and was partially palagonitized because of its structural position on top of a water-saturated volcanic pile and likely presence of vent intrusions driving hydrothermal circulation.

Sedimentation in arctic proglacial lakes: Mittivakkat Glacier, south‐east Greenland

Abstract: Several sediment cores were collected from two proglacial lakes in the vicinity of Mittivakkat Glacier, southeast Greenland, in order to determine sedimentation rates, estimate sediment yields and identify the dominant sources of the lacustrine sediment. The presence of varves in the ice-dammed Icefall Lake enabled sedimentation rates to be estimated using a combination of X-ray photography and down-core variations in 137 Cs activity. Sedimentation rates for individual cores ranged between 0.52 and 1.06 g cm ˇ2 year ˇ1 , and the average sedimentation rate was estimated to be 0.79 g cm ˇ2 year ˇ1 . Despite considerable down-core variability in annual sedimentation rates, there is no significant trend over the period 1970 to 1994. After correcting for autochthonous organic matter content and trap eAciency, the mean fine-grained minerogenic sediment yield from the 3. 8k m 2 basin contributing to the lake was estimated to be 327 t km ˇ2 year ˇ1 . Cores were also collected from the topset beds of two small deltas in Icefall Lake. The deposition of coarse-grained sediment on the delta surface was estimated to total in excess of 15 cm over the last c. 40 years. In the larger Lake Kuutuaq, which is located about 5 km from the glacier front and for which the glacier represents a smaller proportion of the contributing catchment, sedimentation rates determined for six cores collected from the centre of the lake, based on their 137 Cs depth profiles, were estimated to range between 0.05 and 0.11 g cm ˇ2 year ˇ1 , and the average was 0.08 g cm ˇ2 year ˇ1 . The longer-term (c. 100‐150 years) average sedimentation rate for one of the cores, estimated from its unsupported 210 Pb profile, was 0.10‐0.13 g cm ˇ2 year ˇ1 , suggesting that sedimentation rates in this lake have been essentially constant over the last c. 100‐150 years. The average fine-grained sediment yield from the 32. 4k m 2 catchment contributing to the lake was estimated to be 13 t km ˇ2 year ˇ1 . The 137 Cs depth profiles for cores collected from the topset beds of the delta of Lake Kuutuaq indicate that in excess of 27 cm of coarse-grained sediment had accumulated on the delta surface over the last approximately 40 years. Caesium-137 concentrations associated with the most recently deposited (uppermost) fine-grained sediment in both Icefall Lake and Lake Kuutuaq were similar to those measured in fine-grained sediment collected from steep slopes in the immediate proglacial zone, suggesting that this material, rather than contemporary glacial debris, is the most likely source of the sediment deposited in the lakes. This finding is confirmed by the 137 Cs concentrations associated with suspended sediment collected from the Mittivakkat stream, which are very similar to those for proglacial material. Copyright # 2000 John Wiley & Sons, Ltd.