Subglacial Clast/Bed Contact Forces, 2017 Iowa State University
Subglacial Clast/Bed Contact Forces, John Byers, Denis Cohen, Neal R. Iverson
A laboratory device was built to measure the forces that ice exerts on a 0.05 m diameter rigid plastic sphere in two different configurations: in contact with a flat bed or isolated from the bed. Measurements indicated that bed-normal contact forces were 1.8 times larger than drag forces due to creeping flow past a slippery sphere isolated from the bed. Measurements of forces as a function of the bed-normal ice velocity, estimations of the ice viscosity parameter and observations of markers in the ice indicate ice is Newtonian with a viscosity of ∼1.3 × 1011 Pa s. Newtonian ...
Role Of Transient Water Pressure In Quarrying: A Subglacial Experiment Using Acoustic Emissions, 2017 Iowa State University
Role Of Transient Water Pressure In Quarrying: A Subglacial Experiment Using Acoustic Emissions, Denis Cohen, Thomas S. Hooyer, Neal R. Iverson, Jason Thomason, M. Jackson
Probably the most important mechanism of glacial erosion is quarrying: the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments. Although evidence indicates that erosion rates depend on sliding speed, rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice-bed separation. To study quarrying in real time, a granite step, 12 cm high with a crack in its stoss surface, was installed at the bed of Engabreen, Norway. Acoustic emission sensors monitored crack growth events in the step as ice ...
Glacier-Bed Geomorphic Processes And Hydrologic Conditions Relevant To Nuclear Waste Disposal, 2017 Iowa State University
Glacier-Bed Geomorphic Processes And Hydrologic Conditions Relevant To Nuclear Waste Disposal, Neal R. Iverson, Mark Person
Characterizing glaciotectonic deformation, glacial erosion and sedimentation, and basal hydrologic conditions of ice sheets is vital for selecting sites for nuclear waste repositories at high latitudes. Glaciotectonic deformation is enhanced by excess pore pressures that commonly persist near ice sheet margins. Depths of such deformation can extend locally to a few tens of meters, with depths up to approximately 300 m in exceptional cases. Rates of glacial erosion are highly variable (0.05–15 mm a−1), but ratesa−1 are expected in tectonically quiescent regions. Total erosion probably not exceeding several tens of meters is expected during a glacial ...
Rate-Weakening Drag During Glacier Sliding, 2017 University of Wisconsin-Madison
Rate-Weakening Drag During Glacier Sliding, Lucas K. Zoet, Neal R. Iverson
Accurately specifying the relationship between basal drag on a hard, rough glacier bed and sliding speed is a long-standing and central challenge in glaciology. Drag on a rigid bed consisting of steps with linear treads inclined upglacier—a good idealization for the bedrock morphology of some hard-bedded glaciers—has been considered in sliding theories but never studied empirically. Balancing forces parallel to step treads indicates that drag should be independent of sliding speed and cavity size and set by the limit-equilibrium condition sometimes called Iken's bound. In this study we used a large ring-shear device to slide ice at ...
Sliding Of Ice Past An Obstacle At Engabreen, Norway, 2017 University of Minnesota-Twin Cities
Sliding Of Ice Past An Obstacle At Engabreen, Norway, Denis Cohen, Roger Leb. Hooke, Neal R. Iverson, Jack Kohler
At Engabreen, Norway, an instrumented panel containing a decimetric obstacle was mounted flush with the bed surface beneath 210 m of ice. Simultaneous measurements of normaland shear stresses, ice velocity and temperature were obtained as dirty basal ice flowed past the obstacle. Our measurements were broadly consistent with ice thickness, flow conditions and bedrock topography near the site of the experiment. Ice speed 0.45 m above the bed was about 130 mm d–1, much less than the surface velocity of 800 mm d–1. Average normalstress on the panelwas 1.0–1.6 MPa, smaller than the expected ...
Preexisting Fractures And The Formation Of An Iconic American Landscape: Tuolumne Meadows, Yosemite National Park, Usa, 2017 University of Wisconsin-Twin Cities
Preexisting Fractures And The Formation Of An Iconic American Landscape: Tuolumne Meadows, Yosemite National Park, Usa, Richard A. Becker, Basil Tikoff, Paul R. Riley, Neal R. Iverson
Tuolumne Meadows, in Yosemite National Park (USA), is a large sub-alpine meadow in the Sierra Nevada Mountains. Immediately adjacent to Tuolumne Meadows—and underlain by the same bedrock lithology (Cathedral Peak Granodiorite)—are vertical rock faces that provide exceptional opportunities to climbers. While the presence of a broad meadow suggests bedrock erodibility, the vertical rock walls indicate bedrock durability. We propose that the Tuolumne Meadows’s landscape is the result of variable glacial erosion due to the presence or absence of pre-existing bedrock fractures. The meadows and valleys formed because of concentrated tabular fracture clusters—a distinctive and locally pervasive ...
Glacial Landscape Evolution By Subglacial Quarrying: A Multiscale Computational Approach, 2017 Aarhus University
Glacial Landscape Evolution By Subglacial Quarrying: A Multiscale Computational Approach, Sofie V. Ugelvig, David L. Egholm, Neal R. Iverson
Quarrying of bedrock is a primary agent of subglacial erosion. Although the mechanical theory behind the process has been studied for decades, it has proven difficult to formulate the governing principles so that large-scale landscape evolution models can be used to integrate erosion over time. The existing mechanical theory thus stands largely untested in its ability to explain postglacial topography. In this study we relate the physics of quarrying to long-term landscape evolution with a multiscale approach that connects meter-scale cavities to kilometer-scale glacial landscapes. By averaging the quarrying rate across many small-scale bedrock steps, we quantify how regional trends ...
Magnetic Fabric Of Sheared Till: A Strain Indicator For Evaluating The Bed Deformation Model Of Glacier Flow, 2017 University of Wisconsin-Madison
Magnetic Fabric Of Sheared Till: A Strain Indicator For Evaluating The Bed Deformation Model Of Glacier Flow, Thomas S. Hooyer, Neal R. Iverson, France Lagroix, Jason F. Thomason
 Wet-based portions of ice sheets may move primarily by shearing their till beds, resulting in high sediment fluxes and the development of subglacial landforms. This model of glacier movement, which requires high bed shear strains, can be tested using till microstructural characteristics that evolve during till deformation. Here we examine the development of magnetic fabric using a ring shear device to deform two Wisconsin-age basal tills to shear strains as high as 70. Hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility (AMS) develops during shear due to the ...
Reply To "Comment On ‘Preexisting Fractures And The Formation Of An Iconic American Landscape: Tuolumne Meadows, Yosemite National Park, Usa,’ By Jeffrey P. Schaffer”, 2017 University of Wisconsin-Madison
Reply To "Comment On ‘Preexisting Fractures And The Formation Of An Iconic American Landscape: Tuolumne Meadows, Yosemite National Park, Usa,’ By Jeffrey P. Schaffer”, Richard A. Becker, Basil Tikoff, Paul R. Riley, Neal R. Iverson
No abstract provided.
Estimating The Sliding Velocity Of A Pleistocene Ice Sheet From Plowing Structures In The Geologic Record, 2017 Iowa State University
Estimating The Sliding Velocity Of A Pleistocene Ice Sheet From Plowing Structures In The Geologic Record, Neal R. Iverson, Thomas S. Hooyer
As an ice sheet slides over its sediment bed, some clasts partly embedded in the glacier sole plow through the bed surface. The size distribution of such clasts, if it can be characterized from structures in the geologic record, can be used to estimate the sliding velocity of a past ice sheet. By combining a theory of glacier sliding with a geotechnical theory of cone penetration, sliding velocity can be calculated in terms of clast-size parameters, a fluidity parameter for ice, and the thermodynamic properties of ice and clasts. If frictional properties of the bed are measured, the effective normal ...
Experimental Determination Of A Double-Valued Drag Relationship For Glacier Sliding, 2017 Iowa State University
Experimental Determination Of A Double-Valued Drag Relationship For Glacier Sliding, Lucas K. Zoet, Neal R. Iverson
The contribution of glaciers to sea-level rise and their effects on landscape evolution depend on the poorly known relationship between sliding speed and drag at the ice/bed interface. Results from experiments with a new rotary laboratory device demonstrate empirically for the first time a double-valued drag relationship like that suggested by some sliding theories: steady drag on a rigid, sinusoidal bed increases, peaks and declines at progressively higher sliding speeds due to growth of cavities in the lee sides of bed undulations. Drag decreases with increased sliding speed if cavities extend beyond the inflection points of up-glacier facing surfaces ...
Flow Mechanism Of The Des Moines Lobe Of The Laurentide Ice Sheet, 2017 University of Wisconsin-Madison
Flow Mechanism Of The Des Moines Lobe Of The Laurentide Ice Sheet, Thomas S. Hooyer, Neal R. Iverson
Rapid 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 ...
Distributed Shear Of Subglacial Till Due To Coulomb Slip, 2017 Iowa State University
Distributed Shear Of Subglacial Till Due To Coulomb Slip, Neal R. Iverson, Richard M. Iverson
In most models of the flow of glaciers on till beds, it has been assumed that till behaves as a viscoplastic fluid, despite contradictory evidence from laboratory studies. In accord with this assumption, displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution. Here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by Coulomb friction. Motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress. These fluctuations result from chains of ...
Coupling Between A Glacier And A Soft Bed: I. A Relation Between Effective Pressure And Local Shear Stress Determined From Till Elasticity, Neal R. Iverson, Robert W. Baker, Roger Leb. Hooke, Brian Hanson, Peter Jansson
To predict the distribution of motion beneath glaciers on soft beds, the strength of the coupling between the ice and the bed and its variation with effective pressure must be known. A record of shear strain, acquired with a tiltmeter emplaced in till beneath Storglaciären, Sweden, indicates that fluctuations in water pressure cause variations in the local shear stress on the bed and that the bed deforms elastically in response to these variations. To estimate the shear stress from the elastic component of the total shear strain, the shear modulus of the till was measured in relaxation tests conducted in ...
A Laboratory Study Of Particle Ploughing And Pore-Pressure Feedback: A Velocity-Weakening Mechanism For Soft Glacier Beds, Jason F. Thomason, Neal R. Iverson
If basal-water discharge and pressure are sufficiently high, a soft-bedded glacier will slip over its bed by ploughing, the process in which particles that span the ice–bed interface are dragged across the bed surface. Results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity (velocity weakening). During ploughing at various velocities (15–400 m a−1), till was compacted in front of idealized particles, causing pore pressures there that were orders of magnitude higher than the ambient value. This excess pore pressure locally weakened the till in shear, thereby decreasing ploughing resistance by a ...
Diffusive Mixing Between Shearing Granular Materials: Constraints On Bed Deformation From Till Contacts, 2017 Iowa State University
Diffusive Mixing Between Shearing Granular Materials: Constraints On Bed Deformation From Till Contacts, Thomas S. Hooyer, Neal R. Iverson
Shearing of subglacial till has been invoked widely as a mechanism of glacier motion and sediment transport, but standard indicators for determining shear strain from the geologic record are not adequate for estimating the very high strains required of the bed-deformation model. Here we describe a laboratory study of mixing between shearing granular layers that allows an upper limit to be placed on bed shear strain in the vicinity of till contacts. Owing to random vertical motions of particles induced by shearing, mixing can be modeled as a linearly diffusive process, and so can be characterized with a single mixing ...
Coupling Between A Glacier And A Soft Bed: Ii Model Results, 2017 Iowa State University
Coupling Between A Glacier And A Soft Bed: Ii Model Results, Neal R. Iverson
The relation between the local effective pressure and shear stress on till beneath Storglaciären, Sweden, discussed in Iverson and others (1999), provides an empirical basis for studying the processes that control the strength of the ice/bed coupling. Particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms. Model calculations, based on studies of cone penetration through fine-grained sediment and sliding theory, agree with the observed relation between shear stress and effective pressure if the water layer at the ice/bed interface is assumed to thicken ...
A New Laboratory Device For Study Of Subglacial Processes: First Results On Ice–Bed Separation During Sliding, 2017 Iowa State University
A New Laboratory Device For Study Of Subglacial Processes: First Results On Ice–Bed Separation During Sliding, Neal R. Iverson, Ben B. Petersen
A new ring-shear device allows basal slip and related processes to be studied in laboratory experiments for the cases of hard or soft beds. The device rotates a confined ring of ice (0.9 m outside diameter) across a horizontal bed at a constant velocity or drag, while a vertical stress is applied and basal water pressure is controlled. A bath with circulating fluid regulated to ∼0.01°C surrounds the ice chamber and keeps the ice at its pressure-melting temperature. In a first experiment with a stepped rigid bed and zero basal water pressure, steady lengths of step cavities ...
Ice Flow Across A Warm-Based/Cold-Based Transition At A Glacier Margin, 2017 Iowa State University
Ice Flow Across A Warm-Based/Cold-Based Transition At A Glacier Margin, Peter Lindsay Moore, Neal R. Iverson, Denis Cohen
Where polythermal glaciers have frozen margins that buttress otherwise temperate-based sliding ice, longitudinal compression can strongly influence ice flow trajectory, and consequently sediment transport paths. Past efforts to model flow in the vicinity of a basal thermal transition (BTT) have generally relied on simplified boundary conditions or rheological idealizations, making these model results difficult to apply to real glacier termini. Herein, we present results of numerical simulations using a power-law rheology and with boundary conditions that better represent the frozen margin. Model results indicate that a transition to a non-sliding frozen margin causes a decline in surface velocity made possible ...
Conditions For Thrust Faulting In A Glacier, 2017 Iowa State University
Conditions For Thrust Faulting In A Glacier, Peter Lindsay Moore, Neal R. Iverson, Denis Cohen
Dipping, arcuate bands of debris-rich ice outcropping near the margins of glaciers are often interpreted as thrust faults, assumed to originate in zones of longitudinal compression. Identification of thrusts is typically based either on the geometry and sedimentology of the debris bands or on the crystal fabric of surrounding ice, but the physical processes necessary to generate thrusts are rarely evaluated. Herein, we combine a numerical model of compressive ice flow near a glacier margin with theoretical stress and strain rate criteria for ice fracture and stress criteria for frictional slip to determine the conditions necessary for thrust faulting in ...