scholarly journals The role of lateral drag in the dynamics of Ice Stream B, Antarctica

1997 ◽  
Vol 43 (144) ◽  
pp. 231-237 ◽  
Author(s):  
I. M. Whillans ◽  
C. J. van der Veen
Keyword(s):  
Mechanical Control ◽  
Resistive Force ◽  
Data Errors ◽  
Aerial Photogrammetry ◽  
Ice Stream ◽  
Lateral Margins ◽  
Ice Strength ◽  
Ice Stream B

AbstractThe partitioning of resistive force between the bed and sides of Ice Stream B, Antarctica, is obtained for three large areas that have been measured using repeat aerial photogrammetry. Problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block. Results indicate that the bed under Ice Stream B must be very weak and unable to provide much resistance. Mechanical control on this ice stream emanates almost entirely from the lateral margins.

scholarly journals The role of lateral drag in the dynamics of Ice Stream B, Antarctica

1997 ◽  
Vol 43 (144) ◽  
pp. 231-237 ◽  
Author(s):  
I. M. Whillans ◽  
C. J. van der Veen
Keyword(s):  
Mechanical Control ◽  
Resistive Force ◽  
Data Errors ◽  
Aerial Photogrammetry ◽  
Ice Stream ◽  
Lateral Margins ◽  
Ice Strength ◽  
Ice Stream B

AbstractThe partitioning of resistive force between the bed and sides of Ice Stream B, Antarctica, is obtained for three large areas that have been measured using repeat aerial photogrammetry. Problems associated with data errors and local variations in ice strength and velocity are reduced by considering the areally averaged budget of forces for each photo block. Results indicate that the bed under Ice Stream B must be very weak and unable to provide much resistance. Mechanical control on this ice stream emanates almost entirely from the lateral margins.

scholarly journals The role of the margins in the dynamics of an active ice stream

1994 ◽  
Vol 40 (136) ◽  
pp. 527-538 ◽  
Author(s):  
K. A. Echelmeyer ◽  
W. D. Harrison ◽  
C. Larsen ◽  
J. E. Mitchell
Keyword(s):  
Numerical Models ◽  
Force Balance ◽  
Shear Stresses ◽  
List Type ◽  
Ice Stream ◽  
Strain Heating ◽  
Ice Stream B ◽  
Basal Shear ◽  
Marginal Zones

AbstractA transverse profile of velocity was measured across Ice Stream B, West Antarctica, in order to determine the role of the margins in the force balance of an active ice stream. The profile extended from near the ice-stream center line, through a marginal shear zone and on to the slow-moving ice sheet. The velocity profile exhibits a high degree of shear deformation within a marginal zone, where intense, chaotic crevassing occurs. Detailed analysis of the profile, using analytical and numerical models of ice flow, leads to the following conclusions regarding the roles of the bed and the margins in ice-stream dynamics:(i)The overall resistive drag on the ice stream is partitioned nearly equally between the margins and the bed and, thus, both are important in the force balance of the ice stream.(ii)The ice within the chaotic zone must be about 10 times softer than the ice in the central part of the ice stream.(iii)The average basal shear stress is 0.06 × 105Pa. This implies that the entire bed cannot be blanketed by the weak, deformable till observed by Engelhardt and others (1990) near the center of the ice stream — there must be regions of increased basal drag.(iv)High strain rates and shear stresses in the marginal zones indicate that strain heating in the margins may be significant.While the exact quantitative values leading to these conclusions are somewhat model and location-dependent, the overall conclusions are robust. As such, they are likely to have importance for ice-stream dynamics in general.

scholarly journals Velocity Patterns and Elevations On Ice Stream B

1990 ◽  
Vol 14 ◽  
pp. 341
Author(s):  
M. Jackson ◽  
I.M. Whillans
Keyword(s):  
Longitudinal Velocity ◽  
Ice Sheet ◽  
Main Body ◽  
The West ◽  
Antarctic Ice Sheet ◽  
West Antarctic Ice Sheet ◽  
Aerial Photogrammetry ◽  
West Antarctic ◽  
Ice Stream ◽  
Ice Stream B

All of the West Antarctic ice sheet draining into the Ross Ice Shelf lies on bedrock which is below sea level. This is thought to make it especially senstitive to rapid decay which could be triggered by an increase in atmospheric CO2 levels, ice stream B, one of the main outlets of the West Antarctic Ice Sheet, is thinning, possibly in response to changes in climate. However, Ice Stream C, its neighbor, is thickening. One of the most effective ways to study ice streams is by repeat aerial photogrammetry. Thousands of velocity values and elevations are available for Ice Stream B using this technique. Two sections of the ice stream have repeat photogrammetry with control. Maps of elevations, velocity components and velocity gradients have been produced following the methods of Brecher (1986). The area considered here is a 40 by 30 km block across a part of the ice stream. The maps show that most of the increase in longitudinal velocity occurs within about 6 km of the ice-stream margin and reaches a maximum of 460 m a-1 in the center of the stream. Strain rates in the shear margins reach 0.12 a-;1 and are an order of magnitude less in the main body of the stream. The elevation maps show ridges and troughs. These features appear to be related to transverse velocities.

scholarly journals The role of the margins in the dynamics of an active ice stream

1994 ◽  
Vol 40 (136) ◽  
pp. 527-538 ◽  
Author(s):  
K. A. Echelmeyer ◽  
W. D. Harrison ◽  
C. Larsen ◽  
J. E. Mitchell
Keyword(s):  
Numerical Models ◽  
Force Balance ◽  
Shear Stresses ◽  
List Type ◽  
Ice Stream ◽  
Strain Heating ◽  
Ice Stream B ◽  
Basal Shear ◽  
Marginal Zones

AbstractA transverse profile of velocity was measured across Ice Stream B, West Antarctica, in order to determine the role of the margins in the force balance of an active ice stream. The profile extended from near the ice-stream center line, through a marginal shear zone and on to the slow-moving ice sheet. The velocity profile exhibits a high degree of shear deformation within a marginal zone, where intense, chaotic crevassing occurs. Detailed analysis of the profile, using analytical and numerical models of ice flow, leads to the following conclusions regarding the roles of the bed and the margins in ice-stream dynamics: (i)The overall resistive drag on the ice stream is partitioned nearly equally between the margins and the bed and, thus, both are important in the force balance of the ice stream.(ii)The ice within the chaotic zone must be about 10 times softer than the ice in the central part of the ice stream.(iii)The average basal shear stress is 0.06 × 105 Pa. This implies that the entire bed cannot be blanketed by the weak, deformable till observed by Engelhardt and others (1990) near the center of the ice stream — there must be regions of increased basal drag.(iv)High strain rates and shear stresses in the marginal zones indicate that strain heating in the margins may be significant.While the exact quantitative values leading to these conclusions are somewhat model and location-dependent, the overall conclusions are robust. As such, they are likely to have importance for ice-stream dynamics in general.

scholarly journals Velocity Patterns and Elevations On Ice Stream B

1990 ◽  
Vol 14 ◽  
pp. 341-341
Author(s):  
M. Jackson ◽  
I.M. Whillans
Keyword(s):  
Longitudinal Velocity ◽  
Ice Sheet ◽  
Main Body ◽  
The West ◽  
Antarctic Ice Sheet ◽  
West Antarctic Ice Sheet ◽  
Aerial Photogrammetry ◽  
West Antarctic ◽  
Ice Stream ◽  
Ice Stream B

All of the West Antarctic ice sheet draining into the Ross Ice Shelf lies on bedrock which is below sea level. This is thought to make it especially senstitive to rapid decay which could be triggered by an increase in atmospheric CO2 levels, ice stream B, one of the main outlets of the West Antarctic Ice Sheet, is thinning, possibly in response to changes in climate. However, Ice Stream C, its neighbor, is thickening.One of the most effective ways to study ice streams is by repeat aerial photogrammetry. Thousands of velocity values and elevations are available for Ice Stream B using this technique. Two sections of the ice stream have repeat photogrammetry with control. Maps of elevations, velocity components and velocity gradients have been produced following the methods of Brecher (1986).The area considered here is a 40 by 30 km block across a part of the ice stream. The maps show that most of the increase in longitudinal velocity occurs within about 6 km of the ice-stream margin and reaches a maximum of 460 m a-1 in the center of the stream. Strain rates in the shear margins reach 0.12 a-;1 and are an order of magnitude less in the main body of the stream.The elevation maps show ridges and troughs. These features appear to be related to transverse velocities.

scholarly journals Surface Features of Ice Stream B, Marie Byrd Land, West Antarctica

1986 ◽  
Vol 8 ◽  
pp. 168-170 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Aerial Photographs ◽  
Relative Importance ◽  
West Antarctica ◽  
Ice Streams ◽  
Longitudinal Compression ◽  
West Antarctic Ice Sheet ◽  
Surface Features ◽  
West Antarctic ◽  
Ice Stream ◽  
Ice Stream B

Aerial photographs have been obtained of Ice Stream B, one of the active ice streams draining the West Antarctic Ice Sheet. A sketch map made from these photographs shows two tributaries. The margin of the active ice is marked by curved crevasses and intense crevassing occurs just inward of them. Transverse crevasses dominate the center of the ice streams and diagonal types appear at the lower end. A “suture zone” originates at the tributary convergence and longitudinal surface ridges occur at the downglacier end. The causes of these surface features are discussed and the relative importance of four stresses in resisting the driving stress is assessed. We conclude that basal drag may be important, longitudinal compression is probably important at the lower end, and longitudinal tension is probably most important near the head of the ice stream. Side drag leads to shearing at the margins, but does not restrain much of the ice stream.

scholarly journals Velocity pattern in a transect across Ice Stream B, Antarctica

1993 ◽  
Vol 39 (133) ◽  
pp. 562-572 ◽  
Author(s):  
I. M. Whillans ◽  
M. Jackson ◽  
Y-H. Tseng
Keyword(s):  
Aerial Photography ◽  
Lateral Shear ◽  
Ice Flow ◽  
Measured Velocity ◽  
Velocity Gradients ◽  
Spreading Ridges ◽  
Ice Stream ◽  
Surface Slopes ◽  
Fast Ice ◽  
Ice Stream B

AbstractRepeat aerial photography is used to obtain closely spaced measurements of velocity and elevation over a complete transect of Ice Stream tributary B2, including the shear margins, the fast ice of the ice stream and several unusual features, as well as the UpB camp. Persistent features, mainly crevasses, are tracked to provide 1541 values of velocity and 1933 values of elevation. These are used to describe ice flow in the ice stream. Within the ice stream, the dominant velocity gradient is lateral shear. Crevasse patterns are studied in relation to measured velocity gradients. Crevasses intersect one another at acute angles, indicating that their origin is deeper than the depth to which crevasses penetrate. One feature within the ice stream seems to be a raft of stiff ice. Others are crevasse trains. Also, there are spreading ridges, perhaps due to upwelling ice. There is no evidence of large sticky spots within the studied transect, i.e. no steep surface slopes with associated surface stretching just up-glacier and surface compression down-glacier.

scholarly journals Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

1998 ◽  
Vol 44 (146) ◽  
pp. 149-156 ◽  
Author(s):  
C. R. Bentley ◽  
N. Lord ◽  
C. Liu
Keyword(s):  
Radar Data ◽  
Ice Thickness ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Stream ◽  
Crossover Analysis ◽  
Automated Processing ◽  
Basal Reflection ◽  
Field Season ◽  
Ice Stream B

AbstractDigital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

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.

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