scholarly journals Surface profiles of the laurentide ice sheet in its marginal areas

1974 ◽  
Vol 13 (67) ◽  
pp. 37-43 ◽  
Author(s):  
W. H. Mathews
Keyword(s):  
Late Pleistocene ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Active Movement ◽  
Shear Stresses ◽  
Marginal Areas ◽  
Image Position ◽  
Basal Shear

Surface slopes of ice lobes can be estimated from the gradients of their margins as shown by ice limits, by contemporaneous recessional moraines, or by lateral melt-water channels, with allowance being made for the dip of an ice lobe laterally, as well as forward, toward its extremities. Profiles can be fitted approximately to a parabola with the equation in which h is the height above and x the distance up-stream from the terminus, in the same units, and A is a coefficient which varies from glacier to glacier. The coefficient A has a value of 4.7 m1 for both the Antarctic ice sheet inland from Mirny and the west central Greenland ice sheet. Several examples of late Pleistocene ice lobes within mountainous terrain of North America and New Zealand have values of A ranging from 2.9 ml to about 4.1 m1. For several ice lobes in the south-western part of the late Pleistocene Laurentide ice sheet, however, values are from about 0.3 to 1.0 m1, corresponding to basal shear stress of from about 0.07 to 0.22 bar. A major problem exists in accounting for the active movement of ice here under such low surface gradients and basal shear stresses. Evidence of basal slip, aided by high subglacial water pressure, should be looked for in the field. Alternatively, other possibilities for the explanation of such low surface gradients should be sought.

scholarly journals Surface profiles of the laurentide ice sheet in its marginal areas

1974 ◽  
Vol 13 (67) ◽  
pp. 37-43 ◽  
Author(s):  
W. H. Mathews
Keyword(s):  
Late Pleistocene ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Active Movement ◽  
Shear Stresses ◽  
Marginal Areas ◽  
Image Position ◽  
Basal Shear

Surface slopes of ice lobes can be estimated from the gradients of their margins as shown by ice limits, by contemporaneous recessional moraines, or by lateral melt-water channels, with allowance being made for the dip of an ice lobe laterally, as well as forward, toward its extremities. Profiles can be fitted approximately to a parabola with the equationin whichhis the height above andxthe distance up-stream from the terminus, in the same units, andAis a coefficient which varies from glacier to glacier. The coefficientAhas a value of 4.7 m1for both the Antarctic ice sheet inland from Mirny and the west central Greenland ice sheet. Several examples of late Pleistocene ice lobes within mountainous terrain of North America and New Zealand have values ofAranging from 2.9 mlto about 4.1 m1. For several ice lobes in the south-western part of the late Pleistocene Laurentide ice sheet, however, values are from about 0.3 to 1.0 m1, corresponding to basal shear stress of from about 0.07 to 0.22 bar. A major problem exists in accounting for the active movement of ice here under such low surface gradients and basal shear stresses. Evidence of basal slip, aided by high subglacial water pressure, should be looked for in the field. Alternatively, other possibilities for the explanation of such low surface gradients should be sought.

scholarly journals Flow mechanism of the Des Moines lobe of the Laurentide ice sheet

2002 ◽  
Vol 48 (163) ◽  
pp. 575-586 ◽  
Author(s):  
Thomas S. Hooyer ◽  
Neal R. Iverson
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Shear Stresses ◽  
Flow Mechanism ◽  
Des Moines Lobe ◽  
Des Moines ◽  
Rapid Flow ◽  
Basal Shear ◽  
Basal Till

AbstractRapid flow of the Des Moines lobe of the Laurentide ice sheet may have been related to its unlithified substrate. New reconstructions of the lobe, based on moraine elevations, sediment subsidence during moraine deposition, and flow-direction indicators, indicate that the lobe may have been ∼3 times thicker than in previous reconstructions. Nevertheless, implied basal shear stresses are <15 kPa, so internal ice deformation was not significant. Instead, the lobe likely moved by a combination of sliding, plowing of particles through the bed surface, and bed shear. Consolidation tests on basal till yield preconsolidation stresses of 125–300 kPa, so effective normal stresses on the bed were small. A model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed, resulting in too small a shear traction on the bed to shear it at depth. Consistent with this prediction, measurements of orientations of clasts in basal till yield a weak fabric, implying pervasive bed shear strain less than ∼2, although some stronger fabrics have been reported by others. We infer, tentatively, that movement was principally at the bed surface by plowing.

Basal-flow conditions at the northeastern margin of the Laurentide Ice Sheet, Lancaster Sound

1988 ◽  
Vol 25 (11) ◽  
pp. 1740-1750 ◽  
Author(s):  
R. A. Klassen ◽  
D. A. Fisher
Keyword(s):  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Shear Stresses ◽  
Flow Conditions ◽  
Baffin Bay ◽  
Ice Caps ◽  
Ideal Plastic ◽  
Surface Slopes ◽  
Basal Shear ◽  
Basal Flow

A major moraine system developed by outlet glaciers of the Laurentide Ice Sheet on coastal mountains of Bylot Island provides a basis for reconstructing paleoglacier surfaces and estimating basal shear stresses within the ice. The system, named Eclipse moraine, lies between elevations of 100 and 500 m asl and can be traced almost continuously over distances of 50–70 km. Where the glaciers flowed inland within valleys, the outline of the moraine on valley walls is used to portray the profile of the glacier along the valley median. If ideal plastic flow is assumed, ice thicknesses and surface slopes derived from the reconstructed profiles indicate yield stresses of about 125 ± 46 kPa. Reconstructed profiles of the main outlet glaciers entering Baffin Bay from Lancaster Sound and Pond Inlet indicate lesser yield stresses of about 55–80 kPa, respectively, within the channels; estimates are 35–60 kPa if isostatic depression is assumed. Calculations indicate mat basal shear stresses in the channels were near the low end of the range characteristic of modern ice sheets and ice caps.

Ice-marginal thrusting of drift and bedrock: thermal regime, subglacial aquifers, and glacial surges

1990 ◽  
Vol 27 (6) ◽  
pp. 849-862 ◽  
Author(s):  
H. D. Mooers
Keyword(s):  
Pore Water ◽  
Thermal Regime ◽  
Pore Water Pressure ◽  
System Development ◽  
Water Pressure ◽  
Laurentide Ice Sheet ◽  
Shear Stresses ◽  
Thrust System ◽  
Basal Shear ◽  
Subglacial Meltwater

Glacial thrust systems composed of blocks of drift and bedrock, associated with hummocky stagnation moraine along the margin of the Rainy lobe of the Laurentide Ice Sheet in Minnesota, are used in conjunction with paleoecological studies to constrain a numerical model of the ice-marginal thermal regime. Subglacial meltwater production in the thawed-bed zone was at least two orders of magnitude greater than the amount that could refreeze to the base of the glacier near the margin. The excess water recharged a thick subglacial aquifer, and thrust-system development was enhanced by the presence of a frozen toe and high pore-water pressures beneath the outer 2 km the glacier. The pore-water pressure required for thrusting is calculated from overburden pressures and basal shear stresses determined by numerical modeling. The heat generated by flow of water through the subglacial aquifer substantially affects the ice-marginal thermal regime, making a steady-state frozen toe 1.0–2.0 km in width unstable. Thrusting apparently occurred during multiple oscillations, or surges, when the ice was advancing over permafrost.

Greenland land-terminating glaciers velocity trends during the last two decades

2021 ◽  
Author(s):  
Paul Halas ◽  
Jeremie Mouginot ◽  
Basile de Fleurian ◽  
Petra Langebroek
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Melting ◽  
Limited Area ◽  
Ice Dynamics ◽  
Basal Sliding ◽  
Surface Melt ◽  
Ice Sheet Dynamics ◽  
The Impact

&lt;div&gt; &lt;p&gt;Ice losses from the Greenland Ice Sheet have been increasing in the last two decades, leading to a larger contribution to the global sea level rise.&amp;#160;Roughly 40% of the contribution comes from ice-sheet dynamics, mainly regulated by basal sliding.&amp;#160;The sliding component&amp;#160;of glaciers has been observed to be strongly related to surface melting, as water&amp;#160;can eventually&amp;#160;reach the bed and impact&amp;#160;the subglacial water pressure, affecting the basal sliding.&amp;#160;&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;The link between ice velocities and surface melt on multi-annual time scale is still not totally understood&amp;#160;even though it&amp;#160;is of major importance with expected increasing surface melting.&amp;#160;Several studies showed&amp;#160;some&amp;#160;correlation between an increase in surface melt and a slowdown in&amp;#160;velocities, but&amp;#160;there is no&amp;#160;consensus&amp;#160;on those trends.&amp;#160;Moreover&amp;#160;those&amp;#160;investigations&amp;#160;only&amp;#160;presented results&amp;#160;in a limited area over&amp;#160;Southwest&amp;#160;Greenland.&amp;#160;&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;Here we present the ice motion over many land-terminating glaciers on the Greenland Ice Sheet for the period 2000 - 2020. This type of glacier is ideal for studying processes at the interface between the bed and the ice since they are exempted from interactions with the sea while still being relevant for all glaciers since they share the same basal friction laws. The velocity data was obtained using optical Landsat 7 &amp; 8 imagery and feature-tracking algorithm. We attached importance keeping the starting date of our image pairs similar, and avoided stacking pairs starting before and after melt seasons, resulting in multiple velocity products for each year.&amp;#160;&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;Our results show similar velocity trends for previously studied areas with a slowdown until 2012 followed by an acceleration.&amp;#160;This trend however does not seem to be observed on the whole ice sheet and is probably&amp;#160;specific&amp;#160;to&amp;#160;this region&amp;#8217;s&amp;#160;climate forcing.&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;Moreover comparison between ice velocities from different parts of Greenland allows us to observe the impact of different climatic trends on ice dynamics.&lt;/p&gt; &lt;/div&gt;

scholarly journals Glaciodynamic context of subglacial bedform generation and preservation

1999 ◽  
Vol 28 ◽  
pp. 23-32 ◽  
Author(s):  
Chris D. Clark
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Shear Stresses ◽  
Morphometric Characteristics ◽  
Glacial Cycle ◽  
Sheet Flow ◽  
Ice Flow ◽  
Ice Stream ◽  
Highly Sensitive ◽  
Basal Shear

AbstractSubglacially-produced drift lineations provide spatially extensive evidence of ice flow that can be used to aid reconstructions of the evolution of former ice sheets. Such reconstructions, however, are highly sensitive to assumptions made about the glaciodynamic context of lineament generation; when during the glacial cycle and where within the ice sheet were they produced. A range of glaciodynamic contexts are explored which include: sheet-flow submarginally restricted; sheet-flow pervasive; sheet- flow patch; ice stream; and surge or re-advance. Examples of each are provided. The crux of deciphering the appropriate context is whether lineations were laid down time-trans-gressively or isochronously. It is proposed that spatial and morphometric characteristics of lineations, and their association with other landforms, can be used as objective criteria to help distinguish between these cases.A logically complete ice-sheet reconstruction must also account for the observed patches of older lineations and other relict surfaces and deposits that have survived erasure by subsequent ice flow. A range of potential preservation mechanisms are explored, including: cold- based ice; low basal-shear stresses; shallowing of the deforming layer; and basal uncoupling.

Recent developments in modeling ice sheet deformation

2020 ◽  
Author(s):  
Felicity McCormack ◽  
Roland Warner ◽  
Adam Treverrow ◽  
Helene Seroussi
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Vertical Shear ◽  
Shear Stresses ◽  
Polar Ice ◽  
Ice Flow ◽  
Polar Ice Sheets ◽  
Basal Shear ◽  
The Impact

&lt;p&gt;Viscous deformation is the main process controlling ice flow in ice shelves and in slow-moving regions of polar ice sheets where ice is frozen to the bed. However, the role of deformation in flow in ice streams and fast-flowing regions is typically poorly represented in ice sheet models due to a major limitation in the current standard flow relation used in most large-scale ice sheet models &amp;#8211; the Glen flow relation &amp;#8211; which does not capture the steady-state flow of anisotropic ice that prevails in polar ice sheets. Here, we highlight recent advances in modeling deformation in the Ice Sheet System Model using the ESTAR (empirical, scalar, tertiary, anisotropic regime) flow relation &amp;#8211; a new description of deformation that takes into account the impact of different types of stresses on the deformation rate. We contrast the influence of the ESTAR and Glen flow relations on the role of deformation in the dynamics of Thwaites Glacier, West Antarctica, using diagnostic simulations. We find key differences in: (1) the slow-flowing interior of the catchment where the unenhanced Glen flow relation simulates unphysical basal sliding; (2) over the floating Thwaites Glacier Tongue where the ESTAR flow relation outperforms the Glen flow relation in accounting for tertiary creep and the spatial differences in deformation rates inherent to ice anisotropy; and (3) in the grounded region within 80km of the grounding line where the ESTAR flow relation locally predicts up to three times more vertical shear deformation than the unenhanced Glen flow relation, from a combination of enhanced vertical shear flow and differences in the distribution of basal shear stresses. More broadly on grounded ice, the membrane stresses are found to play a key role in the patterns in basal shear stresses and the balance between basal shear stresses and gravitational forces simulated by each of the ESTAR and Glen flow relations. Our results have implications for the suitability of ice flow relations used to constrain uncertainty in reconstructions and projections of global sea levels, warranting further investigation into using the ESTAR flow relation in transient simulations of glacier and ice sheet dynamics. We conclude by discussing how geophysical data might be used to provide insight into the relationship between ice flow processes as captured by the ESTAR flow relation and ice fabric anisotropy.&lt;/p&gt;

Sign in / Sign up

Export Citation Format

Share Document