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 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 Crevasse Deformation and Examples from Ice Stream B, Antarctica

1990 ◽  
Vol 36 (122) ◽  
pp. 3-10 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Critical Value ◽  
Velocity Fields ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Lateral Shearing ◽  
Extensional Strain ◽  
Ice Stream B

AbstractCrevasses, once formed, are subject to rotation and bending according to the velocity field through which they travel. Because of this, crevasse shapes can be used to infer something about the velocity field of a glacier. This is done using a model in which each crevasse opens perpendicularly to the principal extensional strain-rate, when that strain-rate exceeds some specified critical value, and is then deformed according to the same velocity gradients that formed the crevasse. This model describes how crevasses are formed, translated, rotated, bent, and lengthened.Velocity fields are sought for which calculations produce crevasses approximating those found in three example areas on Ice Stream B, Antarctica. The first example is the hook-shaped crevasses that occur just outside the chaotic shear zone at the ice-stream margin. They are used to infer a rate of lateral shearing, and side drag. The second example, a pattern of splaying crevasses, is satisfactorily simulated by a model with side-drag stress varying linearly across the ice stream. This confirms that this region is restrained almost entirely by side drag. The third example is transverse crevasses and their change in orientation, but many different velocity fields can produce the observed pattern. Of these three examples, the shapes of hook-shaped marginal crevasses and splaying crevasses can provide useful information whereas transverse crevasses are less helpful.

scholarly journals Crevasse Deformation and Examples from Ice Stream B, Antarctica

1990 ◽  
Vol 36 (122) ◽  
pp. 3-10 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Critical Value ◽  
Velocity Fields ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Lateral Shearing ◽  
Extensional Strain ◽  
Ice Stream B

AbstractCrevasses, once formed, are subject to rotation and bending according to the velocity field through which they travel. Because of this, crevasse shapes can be used to infer something about the velocity field of a glacier. This is done using a model in which each crevasse opens perpendicularly to the principal extensional strain-rate, when that strain-rate exceeds some specified critical value, and is then deformed according to the same velocity gradients that formed the crevasse. This model describes how crevasses are formed, translated, rotated, bent, and lengthened.Velocity fields are sought for which calculations produce crevasses approximating those found in three example areas on Ice Stream B, Antarctica. The first example is the hook-shaped crevasses that occur just outside the chaotic shear zone at the ice-stream margin. They are used to infer a rate of lateral shearing, and side drag. The second example, a pattern of splaying crevasses, is satisfactorily simulated by a model with side-drag stress varying linearly across the ice stream. This confirms that this region is restrained almost entirely by side drag. The third example is transverse crevasses and their change in orientation, but many different velocity fields can produce the observed pattern. Of these three examples, the shapes of hook-shaped marginal crevasses and splaying crevasses can provide useful information whereas transverse crevasses are less helpful.

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 Kinematic signatures of triaxial stellar systems

1993 ◽  
Vol 153 ◽  
pp. 407-408
Author(s):  
Richard Arnold ◽  
Tim De Zeeuw ◽  
Chris Hunter
Keyword(s):  
Velocity Field ◽  
Dynamic Models ◽  
Elliptical Galaxies ◽  
Velocity Fields ◽  
Stellar Systems ◽  
Viewing Angle ◽  
Upper Limit ◽  
Wide Range ◽  
Triaxial Stellar Systems ◽  
Kinematic Models

Analytic dynamic models of triaxial stellar systems, such as elliptical galaxies and galactic bulges, can be used to calculate the velocity fields of systems in a wide range of potentials without the need for orbit integrations. We present results from a first application of these models, in the form of velocity fields projected onto the sky. The appearance of the velocity field depends strongly on the viewing angle. Thin orbit models provide a theoretical upper limit to streaming in all possible kinematic models in a given potential.

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

1993 ◽  
Vol 39 (133) ◽  
pp. 463-470 ◽  
Author(s):  
W. D. Harrison ◽  
K. A. Echelmeyer ◽  
H. Engelhardt
Keyword(s):  
Strain Rate ◽  
High Strain ◽  
Ross Sea ◽  
Diurnal Variations ◽  
Atmospheric Effects ◽  
Vertical Strain ◽  
Ice Stream ◽  
Short Period ◽  
Local Modification ◽  
Ice Stream B

AbstractThe speed of Ice Stream B, Antarctica, was measured twice a day-over a 1 month study period, and found to be steady at about the ±3½% 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 of local 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.

scholarly journals Complex ice stream flow revealed by sequential satellite imagery

1993 ◽  
Vol 17 ◽  
pp. 177-182 ◽  
Author(s):  
Ted A. Scambos ◽  
Robert Bindschadler
Keyword(s):  
Image Analysis ◽  
Velocity Field ◽  
Strain Rate ◽  
Stream Flow ◽  
Regional Scale ◽  
Satellite Image ◽  
Ice Sheet ◽  
Landsat Images ◽  
Ice Stream ◽  
Satellite Image Analysis

The velocity field of the confluence area of two large ice stream tributaries forming Ice Stream D in West Antarctica is studied using sequential Landsat images. Sequential satellite image analysis allows for a very high density of velocity measurements, based on computer-measured displacements of features such as crevasses, crevasse scars, and ice mounds recognizable in both images. Automated displacement measurement of these features results in a detailed map of surface velocities from which surface-horizontal strain-rate fields can be calculated. Correlations between the surface morphology, the velocity field, and the strain-rate field of Ice Stream D reveal a number of important characteristics of ice stream flow: • the characteristic flowband appearance of streaming ice is present at velocities from below 100 m a−1 to above 350 m a−1; • in the upstream areas, there appears to be no sharp transition between “sheet” flow, typical of the surrounding ice sheet, and “streaming” flow; • the fastest moving portions of the ice stream are nearly devoid of surface topography undulations; • the confluence area is characterized by acceleration of the ice in the slower tributary as it impinges on faster-moving ice, and by highly convergent flow. Velocity in the faster-moving tributary changes little, and there is no persistent evidence of the shear margins of the joined tributaries downstream of the confluence. This study demonstrates that sequential satellite image analysis, coupled with computer-determined displacement measurements, can provide accurate velocity and strain-rate information on a regional scale rapidly and cost-effectively. Such data sets are required for modelling ice sheet evolution, and for monitoring any changes in ice flow within the ice streams.

scholarly journals The marginal shear stress of Ice Stream B, West Antarctica

1997 ◽  
Vol 43 (145) ◽  
pp. 415-426 ◽  
Author(s):  
Miriam Jackson ◽  
Barclay Kamb
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Time Scale ◽  
Shear Zones ◽  
Hot Water ◽  
West Antarctica ◽  
Ice Stream ◽  
Flow Law ◽  
Ice Stream B

AbstractTo ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis ofc-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994) and the corresponding strain-rate enhancement factors differ greatly (E≈ 1–2 vs 10–12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ∼1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

scholarly journals Observations of velocity fields in WN and Of stars

1979 ◽  
Vol 83 ◽  
pp. 475-478
Author(s):  
Virpi S. Niemelä
Keyword(s):  
Velocity Field ◽  
Large Scale ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Emission Lines ◽  
Temperature Structure ◽  
Early Type ◽  
Velocity Gradients ◽  
Early Type Stars

Systematic wavelength shifts of series of spectral line centers observed in many early type stars, generally interpreted as due to large scale motions, can give us information about the velocity gradients in stellar atmospheres. However, it should be borne in mind that the velocity gradients inferred from the observed displacements of spectral lines may not correspond to a unique alternative (e.g. see Karp 1978). Also, and especially when we are dealing with stars which have emission lines in their spectra, the structure of the velocity field depends on the assumed temperature structure of the atmosphere, i.e. in which atmospheric region do the lines originate.

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