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 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 Crevasse Deformation and Examples From Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 211-211
Author(s):  
P. L. Vorriberger ◽  
I. M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Velocity Fields ◽  
Normal Strain ◽  
Lateral Shear ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Wide Range ◽  
Ice Stream B

Crevasses are subject to rotation and bending according to the velocity field through which they travel. The objective of this study is to determine to what extent the velocity field can be inferred from measurements of the resulting shapes of crevasses.A quantitative model of crevasse deformation is developed, based on the following assumptions: (1) each crevasse is assumed to open perpendicularly to the principal extensional regional strain-rate, (2) the crevasse forms when the principal extensional strain-rate exceeds some specified critical value, and (3) velocity gradients are constant over the area of interest. The first two assumptions are reasonable and the third is necessary for an analytic solution of flow trajectories. The crevasse is carried along, rotated, and bent, and may continue to increase in length. Calculations are made for different velocity fields, and velocity fields are sought that produce crevasses similar to those found in three different areas of Ice Stream B.Hook-shaped crevasses occur just outside the chaotic zone at the ice-stream margin. These are similar to the curved marginal crevasses often found in the accumulation zone of valley glaciers. They are successfully modelled by combining strong lateral shear with slow flow of ice from the ice ridge into the ice stream. The curvature at the most sharply bent part of the crevasse is found to be a useful measure and, together with measurements of ice flow from the ridge, can be used to infer the rate of lateral shear. This rate compares favorably with the single measurement obtained so far (Bindschadler and others 1987).A pattern of splaying crevasses develops on the ice stream down-glacier of its narrowest part. These crevasses are similar to longitudinal crevasses found in the ablation zone of many valley glaciers. Models with linear variation in velocity cannot reproduce the observed pattern. However, we have been able to simulate higher-order variations by joining together successive linear models. The observed crevasse pattern is successfully produced if the side shearing varies as the third power of distance from the center of symmetry of the crevasse pattern. Such a variation is expected for a linear gradient in side-drag stress and a third-power constitutive relation for ice. The observed crevasse pattern is thus consistent with side drag varying linearly across the ice stream.The third example is the rotation of transverse crevasses, which occur in trains on the main part of the ice stream. This rotation is due to side shearing but its magnitude is also affected by turning of the flow line and by normal strain-rates. It is therefore possible to reproduce the observed pattern for a wide range of velocity fields, and so measurements of the orientation of transverse crevasses provide only an upper limit on side shearing within the main body of the ice stream.There are many other examples of crevasse patterns on Ice Stream Β and on other glaciers that can be studied in this way. We propose that important constraints can be placed on velocity gradients and on the flow dynamics by using quantitative modelling of crevasse shapes.

scholarly journals New and improved determinations of velocity of Ice Streams B and C, West Antarctica

1993 ◽  
Vol 39 (133) ◽  
pp. 483-590 ◽  
Author(s):  
I. M. Whillans ◽  
C.J. Van Der Veen
Keyword(s):  
Stream Flow ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Stream ◽  
Fast Ice ◽  
Ice Stream B ◽  
Shelf Flow ◽  
Streaming Flow

AbstractMeasurements of velocity have been made on and next to Ice Streams Β and C, West Antarctica. The results are more precise than previous work and constitute a 93% increase in the number of values. These velocities are used to describe the confluence of flow into the ice streams and the development of fast ice-stream flow. The onset of fast-streaming flow occurs in many separate tributaries that coalesce down-glacier into the major ice streams. For those inter-stream ridges that have been studied, the flow is consistent with steady state. Along Ice Stream B, gradients in longitudinal stress offer little resistance to the ice flow. The transition from basal-drag control to ice-shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag. Velocities in the trunk of Ice Stream C are nearly zero but those at the up-glacial head are similar to those at the head of Ice Stream B.

scholarly journals New and improved determinations of velocity of Ice Streams B and C, West Antarctica

1993 ◽  
Vol 39 (133) ◽  
pp. 483-590 ◽  
Author(s):  
I. M. Whillans ◽  
C.J. Van Der Veen
Keyword(s):  
Stream Flow ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Stream ◽  
Fast Ice ◽  
Ice Stream B ◽  
Shelf Flow ◽  
Streaming Flow

Abstract Measurements of velocity have been made on and next to Ice Streams Β and C, West Antarctica. The results are more precise than previous work and constitute a 93% increase in the number of values. These velocities are used to describe the confluence of flow into the ice streams and the development of fast ice-stream flow. The onset of fast-streaming flow occurs in many separate tributaries that coalesce down-glacier into the major ice streams. For those inter-stream ridges that have been studied, the flow is consistent with steady state. Along Ice Stream B, gradients in longitudinal stress offer little resistance to the ice flow. The transition from basal-drag control to ice-shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag. Velocities in the trunk of Ice Stream C are nearly zero but those at the up-glacial head are similar to those at the head of Ice Stream B.

scholarly journals Ice-flow features on Ice Stream B, Antarctica, revealed by SPOT HRV imagery

1993 ◽  
Vol 39 (133) ◽  
pp. 515-527 ◽  
Author(s):  
Carolyn J. Merry ◽  
Ian M. Whillans
Keyword(s):  
High Resolution ◽  
Satellite Imagery ◽  
Aerial Photography ◽  
Large Scale ◽  
Small Scale ◽  
Ice Flow ◽  
Ice Stream ◽  
Flow Features ◽  
Ice Stream B

AbstractNumerous features of glaciological significance appear on two adjoining SPOT High Resolution Visible (HRV) images that cover the onset region of Ice Stream B, Antarctica. Many small-scale features, such as crevasses and drift plumes, have been observed in aerial photography. Subtle large-scale features, such as long flow traces that have not been mapped previously, are clear in the satellite imagery. Newly discovered features include ladder-like runners and rungs within certain shear margins, flow traces that are parallel to ice flow, unusual crevasse patterns and flow traces originating within shear margins.

scholarly journals Ice-flow features on Ice Stream B, Antarctica, revealed by SPOT HRV imagery

1993 ◽  
Vol 39 (133) ◽  
pp. 515-527 ◽  
Author(s):  
Carolyn J. Merry ◽  
Ian M. Whillans
Keyword(s):  
High Resolution ◽  
Satellite Imagery ◽  
Aerial Photography ◽  
Large Scale ◽  
Small Scale ◽  
Ice Flow ◽  
Ice Stream ◽  
Flow Features ◽  
Ice Stream B

AbstractNumerous features of glaciological significance appear on two adjoining SPOT High Resolution Visible (HRV) images that cover the onset region of Ice Stream B, Antarctica. Many small-scale features, such as crevasses and drift plumes, have been observed in aerial photography. Subtle large-scale features, such as long flow traces that have not been mapped previously, are clear in the satellite imagery. Newly discovered features include ladder-like runners and rungs within certain shear margins, flow traces that are parallel to ice flow, unusual crevasse patterns and flow traces originating within shear margins.

scholarly journals Crevasse Deformation and Examples From Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 211
Author(s):  
P. L. Vorriberger ◽  
I. M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Velocity Fields ◽  
Normal Strain ◽  
Lateral Shear ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Wide Range ◽  
Ice Stream B

Crevasses are subject to rotation and bending according to the velocity field through which they travel. The objective of this study is to determine to what extent the velocity field can be inferred from measurements of the resulting shapes of crevasses. A quantitative model of crevasse deformation is developed, based on the following assumptions: (1) each crevasse is assumed to open perpendicularly to the principal extensional regional strain-rate, (2) the crevasse forms when the principal extensional strain-rate exceeds some specified critical value, and (3) velocity gradients are constant over the area of interest. The first two assumptions are reasonable and the third is necessary for an analytic solution of flow trajectories. The crevasse is carried along, rotated, and bent, and may continue to increase in length. Calculations are made for different velocity fields, and velocity fields are sought that produce crevasses similar to those found in three different areas of Ice Stream B. Hook-shaped crevasses occur just outside the chaotic zone at the ice-stream margin. These are similar to the curved marginal crevasses often found in the accumulation zone of valley glaciers. They are successfully modelled by combining strong lateral shear with slow flow of ice from the ice ridge into the ice stream. The curvature at the most sharply bent part of the crevasse is found to be a useful measure and, together with measurements of ice flow from the ridge, can be used to infer the rate of lateral shear. This rate compares favorably with the single measurement obtained so far (Bindschadler and others 1987). A pattern of splaying crevasses develops on the ice stream down-glacier of its narrowest part. These crevasses are similar to longitudinal crevasses found in the ablation zone of many valley glaciers. Models with linear variation in velocity cannot reproduce the observed pattern. However, we have been able to simulate higher-order variations by joining together successive linear models. The observed crevasse pattern is successfully produced if the side shearing varies as the third power of distance from the center of symmetry of the crevasse pattern. Such a variation is expected for a linear gradient in side-drag stress and a third-power constitutive relation for ice. The observed crevasse pattern is thus consistent with side drag varying linearly across the ice stream. The third example is the rotation of transverse crevasses, which occur in trains on the main part of the ice stream. This rotation is due to side shearing but its magnitude is also affected by turning of the flow line and by normal strain-rates. It is therefore possible to reproduce the observed pattern for a wide range of velocity fields, and so measurements of the orientation of transverse crevasses provide only an upper limit on side shearing within the main body of the ice stream. There are many other examples of crevasse patterns on Ice Stream Β and on other glaciers that can be studied in this way. We propose that important constraints can be placed on velocity gradients and on the flow dynamics by using quantitative modelling of crevasse shapes.

scholarly journals Of isbræ and ice streams

2003 ◽  
Vol 36 ◽  
pp. 66-72 ◽  
Author(s):  
Martin Truffer ◽  
Keith A. Echelmeyer
Keyword(s):  
Ice Sheet ◽  
Shear Zones ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Bed Topography ◽  
Ice Stream ◽  
Fast Ice ◽  
Fast Flow ◽  
End Members

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

scholarly journals Seismic-Reflection Profiling of a Widespread Till Beneath Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 210 ◽  
Author(s):  
Sean T. Rooney ◽  
D. D. Blankenship ◽  
R. B. Alley ◽  
C. R. Bentley
Keyword(s):  
Seismic Reflection ◽  
Ross Sea ◽  
High Porosity ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
West Antarctic ◽  
Ice Stream ◽  
Seismic Reflection Profiling ◽  
Ice Stream B

Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams. Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

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