scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

AbstractThe basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

Abstract The basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

scholarly journals Water-pressure Coupling of Sliding and Bed Deformation: III. Application to Ice Stream B, Antarctica

1989 ◽  
Vol 35 (119) ◽  
pp. 130-139 ◽  
Author(s):  
R.B. Alley ◽  
D.D. Blankenship ◽  
S.T. Rooney ◽  
C.R. Bentley
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Water Pressure ◽  
Effective Pressure ◽  
West Antarctica ◽  
Bed Deformation ◽  
Ice Stream ◽  
Basal Sliding ◽  
Ice Stream B ◽  
Basal Shear

AbstractGeophysical studies and glaciological analyses suggest strongly that Ice Stream B, West Antarctica, moves primarily by pervasive deformation of a meters thick subglacial till. Analysis of the longitudinal profile of the ice stream up-stream of the ice plain suggests that basal sliding is slow everywhere, that effective pressure decreases slowly down-stream, and that the strain-rate of pervasive shear is proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure. Discrete shearing may occur beneath the pervasively deforming zone. These and other hypotheses, which build on the analyses of the first two papers in this series, can be tested in the field.

scholarly journals Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

2002 ◽  
Vol 48 (163) ◽  
pp. 552-558 ◽  
Author(s):  
Marjorie Schmeltz ◽  
Eric Rignot ◽  
Todd K. Dupont ◽  
Douglas R. MacAyeal
Keyword(s):  
Shear Stress ◽  
Element Model ◽  
Shelf Area ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Stream ◽  
Model Study ◽  
Grounding Line ◽  
Glacier Velocity ◽  
Basal Shear

AbstractWe use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

scholarly journals The role of bed separation and friction in sliding over an undeformable bed

1992 ◽  
Vol 38 (128) ◽  
pp. 77-92 ◽  
Author(s):  
Jürg Schweizer ◽  
Almut Iken
Keyword(s):  
Shear Stress ◽  
Functional Relationship ◽  
Water Pressure ◽  
Important Variable ◽  
Sliding Velocity ◽  
Sliding Motion ◽  
Basal Ice ◽  
Bed Separation ◽  
Basal Shear

AbstractThe classic sliding theories usually assume that the sliding motion occurs frictionlessly. However, basal ice is debris-laden and friction exists between the substratum and rock particles embedded in the basal ice. The influence of debris concentration on the sliding process is investigated. The actual conditions where certain types of friction apply are defined, the effect for the case of bed separation due to a subglacial water pressure is studied and consequences for the sliding law are formulated. The numerical modelling of the sliding of an ice mass over an undulating bed, including the effect of both the subglacial water pressure and the friction, is done by using the finite-clement method. Friction, seen as a reduction of the driving shear stress due to gravity, can be included in existing sliding laws which should contain the critical pressure as an important variable. An approximate functional relationship between the sliding velocity, the effective basal shear stress and the subglacial water pressure is given.

scholarly journals Interpretation of radar-detected internal layer folding in West Antarctic ice streams

1993 ◽  
Vol 39 (133) ◽  
pp. 528-537 ◽  
Author(s):  
W. Jacobel Robert ◽  
M. Gades Anthony ◽  
L. Gottschling David ◽  
M. Hodge Steven ◽  
L. Wright David
Keyword(s):  
Flow Direction ◽  
Low Frequency ◽  
Internal Layers ◽  
West Antarctica ◽  
Ice Streams ◽  
West Antarctic ◽  
Ice Stream ◽  
High Resolution Images ◽  
Basal Shear ◽  
Inland Ice

AbstractLow-frequency surface-based radar-profiling experiments on Ice Streams Β and C, West Antarctica, have yielded high-resolution images which depict folding of the internal layers that can aid in the interpretation of ice-stream dynamics. Unlike folding seen in most earlier radar studies of ice sheets, the present structures have no relationship to bedrock topography and show tilting of their axial fold planes in the flow direction. Rather than being standing waves created by topography or local variations in basal shear stress, the data show that these folds originate upstream of the region of streaming flow and are advected into the ice streams. The mechanism for producing folds is hypothesized to be changes in the basal boundary conditions as the ice makes the transition from inland ice to ice-stream flow. Migration of this transition zone headward can then cause folds in the internal layering to be propagated down the ice streams.

scholarly journals Ice Stream C, Antarctica, sticky spots detected by microearthquake monitoring

1994 ◽  
Vol 20 ◽  
pp. 183-186 ◽  
Author(s):  
S. Anandakrishnan ◽  
R. B. Alley
Keyword(s):  
Shear Stress ◽  
Stress Drop ◽  
Fast Moving ◽  
Fault Plane ◽  
Spatial Density ◽  
Ice Stream ◽  
Slow Moving ◽  
Ice Stream B ◽  
Basal Shear ◽  
Plane Area

Microearthquakes at the base of slow-moving Ice Stream C occur many times more frequently than at the base of fast-moving Ice Stream B. We suggest that the microearthquake source sites are so-called “sticky spots”, defined as limited zones of stronger Subglacial material interspersed within a weaker matrix. The fault-plane area of the microearthquakes (O(102m2)) is therefore a measure of the size of the sticky spots. The spatial density of the microearthquakes (O(10 km-2)) is a measure of the distribution of sticky spots.The average stress drop associated with these microearthquakes is consistent with an ice-stream bed model of weak subglacial till interspersed with stronger zones that support much or all of the basal shear stress. We infer a weak inter-sticky-spot material by the large distances (O(103m)), relative to fault radius, to which the microearthquake stress change is transmitted.

scholarly journals Basal Sliding and Conditions at the Glacier Bed as Revealed by Bore-hole Photography

1978 ◽  
Vol 20 (84) ◽  
pp. 469-508 ◽  
Author(s):  
H. F. Engelhardt ◽  
W. D. Harrison ◽  
Barclay Kamb
Keyword(s):  
Flow Direction ◽  
Water Pressure ◽  
Surface Flow ◽  
Water Levels ◽  
Surface Velocity ◽  
Internal Motions ◽  
Basal Sliding ◽  
Bed Roughness ◽  
Atmospheric Factors ◽  
Basal Shear

AbstractBore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer ofactive subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

scholarly journals Ribbed bedforms on palaeo-ice stream beds resemble regular patterns of basal shear stress (‘traction ribs’) inferred from modern ice streams

2016 ◽  
Vol 62 (234) ◽  
pp. 696-713 ◽  
Author(s):  
CHRIS R. STOKES ◽  
MARTIN MARGOLD ◽  
TIMOTHY T. CREYTS
Keyword(s):  
Shear Stress ◽  
Ice Sheets ◽  
Western Canada ◽  
Coupled Flow ◽  
Ice Streams ◽  
Ice Stream ◽  
Stream Beds ◽  
Basal Shear ◽  
Subglacial Meltwater ◽  
Regular Patterns

Rapidly-flowing ice streams are an important mechanism through which ice sheets lose mass, and much work has been focussed on elucidating the processes that increase or decrease their velocity. Recent work using standard inverse methods has inferred previously-unrecognised regular patterns of high basal shear stress (‘sticky spots’ >200 kPa) beneath a number of ice streams in Antarctica and Greenland, termed ‘traction ribs’. They appear at a scale intermediate between smaller ribbed moraines and much larger mega-ribs observed on palaeo-ice sheet beds, but it is unclear whether they have a topographic expression at the bed. Here, we report observations of rib-like bedforms from DEMs along palaeo-ice stream beds in western Canada that resemble both the pattern and dimensions of traction ribs. Their identification suggests that traction ribs may have a topographic expression that lies between, and partly overlaps with, ribbed moraines and much larger mega-ribs. These intermediate-sized bedforms support the notion of a ribbed bedform continuum. Their formation remains conjectural, but our observations from palaeo-ice streams, coupled with those from modern ice masses, suggest they are related to wave-like instabilities occurring in the coupled flow of ice and till and modulated by subglacial meltwater drainage. Their form and pattern may also involve glaciotectonism of subglacial sediments.

scholarly journals Cyclic Surging of Glaciers

1973 ◽  
Vol 12 (64) ◽  
pp. 3-18 ◽  
Author(s):  
G. de Q. Robin ◽  
J. Weertman
Keyword(s):  
Shear Stress ◽  
Pressure Gradient ◽  
Water Pressure ◽  
Stress Gradient ◽  
Longitudinal Direction ◽  
Phenomenological Theory ◽  
Trigger Zone ◽  
Basal Shear ◽  
Velocity Zone ◽  
Theory And Model

AbstractA partly phenomenological theory and model are constructed of cyclically surging glaciers. During the after-surge portion of a surge cycle the lower portion of a glacier becomes increasingly stagnant. The upper part of the glacier gradually becomes more active as both its thickness and the magnitude of its basal shear stress increase. In the region between these two parts, called by us the trigger zone, the value of the derivative of the basal shear stress in the longitudinal direction of the glacier gradually increases with time. The pressure gradient in the water at the base of a glacier is related to the derivative of the basal shear stress. The pressure gradient decreases as the basal shear-stress gradient increases. The pressure gradient actually can take on negative values, a condition which produces “up-hill” water flow at the base of a glacier. A surge is started in the trigger zone when water is dammed there by a zero water-pressure gradient. The zone of fast-sliding velocities propagates up the glacier from the trigger zone with a velocity of the order of a surge velocity. The fast-sliding velocity zone also propagates down the glacier because of increased melt-water production.

scholarly journals Delineation of a catchment boundary using velocity and elevation measurements

1998 ◽  
Vol 27 ◽  
pp. 140-144 ◽  
Author(s):  
S. F. Price ◽  
I. M. Whillans
Keyword(s):  
Field Measurements ◽  
Surface Velocity ◽  
Data Sampling ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Elevation Data ◽  
Boundary Location ◽  
Ice Stream B

The determination of catchment boundaries is a major source of uncertainty in net balance studies on large ice sheets. Here, a method for defining a catchment boundary is developed using new measurements of ice-surface velocity and elevation near the Ice Stream B/C boundary in West Antarctica. An objective method for estimating confidence in the catchment boundary is proposed. Using elevation data, the resulting mean standard deviation in boundary location is 13 km in position or 6000 km2 in area. Applying a similar uncertainty to both sides of the Ice Stream Β catchment results in a catchment-area uncertainty of 9%. Much larger uncertainties arise when the method is applied to velocity data. The uncertainty in both cases is primarily determined by the density of field measurements and is proportionally similar for larger catchment basins. Differences in the position of the velocity-determined boundary and the elevation-determined boundary probably result from data sampling. The boundary positions determined here do not support the hypothesis that Ice Stream Β captured parts of the Ice Stream C catchment.

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