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 ...
Shear Resistance And Continuity Of Subglacial Till: Hydrology Rules, 2017 Iowa State University
Shear Resistance And Continuity Of Subglacial Till: Hydrology Rules, Neal R. Iverson
The field observations of G.S. Boulton stimulated widespread interest in deformable beds. Shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure. During unsteady deformation, pseudo-viscous shear resistance can be caused by dilation of consolidated tills and resultant pore-pressure decline. This effect is probably uncommon, however, because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective-pressure transients. Stick–slip motion at Whillans Ice Stream, Antarctica, indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip ...
Debris-Bed Friction Of Hard-Bedded Glaciers, 2017 Iowa State University
Debris-Bed Friction Of Hard-Bedded Glaciers, Denis Cohen, Neal R. Iverson, T. S. Hooyer, U. H. Fischer, M. Jackson, Peter Lindsay Moore
Field measurements of debris-bed friction on a smooth rock tablet at the bed of Engabreen, a hard-bedded, temperate glacier in northern Norway, indicated that basal ice containing 10% debris by volume exerted local shear traction of up to 500 kPa. The corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0.05 and 0.08. A model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed-normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1 ...
Glacier Slip And Seismicity Induced By Surface Melt, 2017 Iowa State University
Glacier Slip And Seismicity Induced By Surface Melt, Peter Lindsay Moore, J. Paul Winberry, Neal R. Iverson, Knut A. Christianson, Sridhar Anandakrishnan, Mark E. Mathison, Denis Cohen
Many of the key processes governing fast glacier flow involve interaction between a glacier and its basal hydrological system, which is hidden from direct observation. Passive seismic monitoring has shown promise as a tool for remotely monitoring basal processes, but lack of glacier-bed access prevents clear understanding of the relationships between subglacial processes and corresponding seismic emissions. Here we describe direct measurements of basal hydrology, sliding, and broadband seismicity made in a unique subglacial facility in Norway during the onset of two summer melt seasons. In the most pronounced of these episodes, rapid delivery of surface meltwater to the bed ...
Permafrost, Ice Sheets, And Sea Level, 2017 Iowa State University
Permafrost, Ice Sheets, And Sea Level, Beth Caissie, Julie Brigham-Grette
Sea level rise and inhabited coastlines. Ice shelves and sea ice do not contribute to sea level but they can buttress the land ice sheets from rapid retreat. Causes of Sea Level Rise: •Melting of glaciers, ice caps, ice sheets •Thermal expansion of sea water ••Small scale changes due to anthropogenic land water storage (damming rivers, over-pumping of water and fuels, wetland and forest destruction) •Relative changes in sea level due to tectonic movements (land subsidence or rebound)
Sea Ice Food Webs—Hands On Sampler Teacher Guide, 2017 Iowa State University
Sea Ice Food Webs—Hands On Sampler Teacher Guide, Beth Caissie
This activity is a variation on a food web game that I’ve seen played many times before, but it is adapted to reflect a sea ice food web and show the many organisms that are intimately connected to polar bears.
Brine Rejection Activity, 2017 Iowa State University
Brine Rejection Activity, Beth Caissie, Rob Snyder
As salt water freezes, the salt is pushed out of solution through channels in the ice. This process is called brine rejection or brine exclusion. These channels are often used as microhabitats by ice algae, zooplankton, and even tiny fish. You can easily demonstrate what these channels look like.
Polar Remote Sensing, 2017 Iowa State University
Polar Remote Sensing, Beth Caissie
•Satellite sensors specialize in collecting data about specific wavelengths The Geostationary Operational Environment Satellites (GOES) operated by NASA, NOAA, and the U.S. Department of Commerce provide continuous monitoring of weather conditions. Orbiting the Earth’s equatorial plane at a speed exactly matching the planet’s rotation, satellites in the GOES network seem to hover over fixed spots. They monitor atmospheric conditions that lead to hurricanes, flash floods, tornadoes, and hail storms.
Glacier Goo Activity, 2017 Iowa State University
Glacier Goo Activity, Beth Caissie
We provided the students with background information about what a glacier is, where they are, how they move. Then split the students into four groups each tasked with a question to answer through experimentation Group s 1 and 2: How does temperature change the way a glacier flows? (we provided frozen, and room temperature goo, and a microwave for heating the goo) Groups 3 and 4: How does friction or obstacles change the way a glacier flows? (we provided different pvc tubes—tubes with nothing done to them, tubes with paintable sand applied to them, and tubes with rocks glued ...
Atmospheric Co2 And Temperature. What Is Normal?, 2017 Iowa State University
Atmospheric Co2 And Temperature. What Is Normal?, Beth Caissie, Julie Brigham-Grette
–How much of a change in CO2 concentration and other GHGs is natural? –What is the normal range of CO2 and temperature variability? How is normal defined in this context? –What is the relationship between CO2 and global temperatures?