Roger LeB. Hooke

On the history of humans as geomorphic agents

The human population has been increasing exponentially. Simultaneously, as digging sticks and antlers have given way to wooden plows, iron spades, steam shovels, and today’s huge excavators, our ability and motivation to modify the landscape by moving earth in construction and mining activities have also increased dramatically. As a consequence, we have now become arguably the premier geomorphic agent sculpting the landscape, and the rate at which we are moving earth is increasing exponentially. As hunter-gatherer cultures were replaced by agrarian societies to feed this expanding population, erosion from agricultural fields also, until recently, increased steadily. This constitutes an unintended additional human impact on the landscape.

Rheology of till beneath Storglaciären, Sweden

In order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciaren, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation. Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr-Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form: e˙∞ e kτ where k is a constant. This is consistent with experimental data reported in the geotechnical literature.

Subglacial Water Pressures and the Shape of Subglacial Conduits

Measured subglacial water pressures are frequently higher than theoretical values calculated by assuming that subglacial conduits are straight and either circular or semi-circular in shape. While this may be attributed to errors in the estimates of conduit roughness or ice viscosity, we suggest here an alternative explanation: namely, that the conduits are actually broad and low rather than semi-circular. Good agreement between measured and calculated pressures can be obtained by assuming that the cross-sectional shape of conduits resembles the space between the arc of a circle and its chord. The angle subtended by the arc, a, is treated as an adjustable parameter. In four cases studied, a ranged from 2 0 to 36 0

Quantifying sediment transport on desert piedmonts using 10Be and 26Al

In situ produced 10 Be and 26 Al, measured in 40 sediment samples collected from the Iron and Granite Mountain piedmonts, eastern Mojave Desert, provide a unique view of piedmont modification processes and process rates over the 10 3 to 10 5 year time scale. Cosmogenic nuclide-based models suggest that the Iron and Granite Mountains generate 0.11–0.13 and 0.082–0.097 m 3 of sediment per year per meter of rangefront, respectively. The sediment moves down the piedmont in an active transport layer (ATL), which is 20 to 30 cm thick (based on visual observations, measurements of depth to a buried B-horizon, cosmogenic nuclide data, and maximum ephemeral channel depths). Sediment in this layer is well-mixed vertically and horizontally on the 10 2 year time scale, indicating that the small ephemeral channels, which dominate the piedmont surface migrate quickly. Interpretive models of increasing nuclide activities at depth in two pits suggest steady sediment deposition on the piedmont (at rates between 17–21 and 38–45 m Ma � 1 ) until the late Pleistocene epoch, when a discontinuity to markedly lower nuclide activities in the isotopically well-mixed active transport layer suggests that deposition stopped, a significant change in piedmont behavior. Nuclide activities in 10 amalgamated surface samples, each collected along a different 4-km-long transect, increase steadily away from the mountain front. Thus, we infer that sediment is uniformly dosed by cosmic rays as it is transported down the Iron and Granite Mountain piedmonts. Interpretive models suggest that long-term average sediment speeds down the Iron and Granite Mountain piedmonts are a few decimeters to a meter per year. D 2002 Published by Elsevier Science B.V.

Snow Accumulation Rate on Qomolangma (Mount Everest), Himalaya: Synchroneity With Sites Across the Tibetan Plateau on 50-100 Year Timescales

Annual-layer thickness data, spanning AD 1534-2001, from an ice core from East Rongbuk Col on Qomolangma (Mount Everest, Himalaya) yield an age-depth profile that deviates systematically from a constant accumulation-rate analytical model. The profile clearly shows that the mean accumulation rate has changed every 50-100 years. A numerical model was developed to determine the magnitude of these multi-decadal-scale rates. The model was used to obtain a time series of annual accumulation. The mean annual accumulation rate decreased from ∼0.8m ice equivalent in the 1500s to ∼0.3m in the mid-1800s. From ∼1880 to ∼1970 the rate increased. However, it has decreased since ∼1970. Comparison with six other records from the Himalaya and the Tibetan Plateau shows that the changes in accumulation in East Rongbuk Col are broadly consistent with a regional pattern over much of the Plateau. This suggests that there may be an overarching mechanism controlling precipitation and mass balance over this area. However, a record from Dasuopu, only 125 km northwest of Qomolangma and 700 m higher than East Rongbuk Col, shows a maximum in accumulation during the 1800s, a time during which the East Rongbuk Col and Tibetan Plateau ice-core and tree-ring records show a minimum. This asynchroneity may be due to altitudinal or seasonal differences in monsoon versus westerly moisture sources or complex mountain meteorology.

Physical environment of drumlin formation

Review of published descriptions of drumlin fields suggests that the following conditions are important to drumlin growth: (1) compressive longitudinal and possibly extending transverse strain rates in the ice, (2) thin ice such as occurs near the glacier margin, and (3) high pore-water pressure in the subglacial sediments. Most drumlin fields display all of these, and no fields of well-developed drumlins were found that did not. On the oilier hand, the lithology of drumlin-forming sediment appears not to be important in promoting drumlin growth, since it varied widely, nor are the lithology and large-scale topography of the bed.

Glacier Calving: A Numerical Model of Forces in the Calving-Speed/Water-Depth Relation

Empirical data suggest that the rate of calving of grounded glaciers terminating in water is directly proportional to the water depth. Important controls on calving may be the extent to which a calving face tends to become oversteepened by differential flow within the ice and the extent to which bending moments promote extrusion and bottom crevassing at the base of a calving face; Numerical modelling suggests that the tendency to become oversteepened increases roughly linearly with water depth. In addition, extending longitudinal deviatoric stresses at the base of a calving face increase with water depth. These processes provide a possible physical explanation for the observed calving-rate/water-depth relation.

Thermal conditions at the bed of the Laurentide ice sheet in Maine during deglaciation: implications for esker formation

The University of Maine Ice Sheet Model was used to study basal conditions during retreat of the Laurentide ice sheet in Maine. Within 150 km of the margin, basal melt rates average � 5m m a -1 during retreat. They decline over the next 100 km, so areas of frozen bed develop in northern Maine during retreat. By integrating the melt rate over the drainage area typically subtended by an esker, we obtained a discharge at the margin of � 1.2 m 3 s -1 . While such a discharge could have moved the material in the Katahdin esker, it was likely too low to build the esker in the time available. Additional water from the glacier surface was required. Temperature gradients in the basal ice increase rapidly with distance from the margin. By conducting upward into the ice all of the additional viscous heat produced by any perturbation that increases the depth of flow in a flat conduit in a distributed drainage system, these gradients inhibit the formation of sharply arched conduits in which an esker can form. This may explain why eskers commonly seem to form near the margin and are typically segmented, with later segments overlapping onto earlier ones.

Extrusion flow demonstrated by bore-hole deformation measurements over a riegel, Storglaciären, Sweden

Inclinometry measurements in a 126 m bore hole slightly down-glacier from the crest of a riegel on Storglaciaren showed what appears to be indisputable extrusion flow during the last 3 weeks of July 1984. The velocity at the bed was about five times that at the surface, and lay in a direction deviating 50–60° from that at the surface. The increase in velocity with depth began more than 50 m above the bed. In August and the first half of September the flow returned nearly to “normal” with only a slight tendency toward extrusion. The flow in July appeared to be related to a period of large diurnal water-pressure variation and cavity opening at the bed up-glacier from the riegel. Decoupling of the ice from the bed on the lowest parts of the riegel may have permitted the extrusion. The return to more “normal” conditions in August appears to have been related to a decrease in ice flux over the riegel and a widening of the zone of decoupling.

Short-term Variations in Strain and Surface Tilt on Storglaciären, Kebnekaise, Northern Sweden

Tiltmeters that can detect changes in slope of a glacier surface as small as 0.1 jJ. rad have been used on Storglaciaren. The records obtained to date have been from the upper part of the ablation area, where the bed of the glacier is overdeepened. A total of 82 d of records has been obtained for various time periods between early June and early September. There is generally a gradual change in inclination of the glacier surface over periods of several days, but these changes do not appear to be systematic. In particular, they are not consistent with vertical movements of stakes located 2-3 ice thicknesses away from the tiltmeters. This suggests that the tiltmeters are sensing disturbances over areas with diameters comparable to the local ice thickness . Superimposed on these trends are diurnal signals suggesting rises and falls of the surface just up-glacier from the riegel that bounds the overdeepening on its downglacier end. These may be due to waves of high water pressure origInating in a crevassed area near the equilibrium line. If this interpretation is correct, the waves apparently move down-glacier at speeds of -20-f)0 m h1 , and become sufficiently focused, either by the bed topography or by conduit constrictions, to result in local uplift of the surface. Also observed are abrupt tilts towards the glacier center line shortly after the beginning of heavy rainstorms. These appear to be due to longitudinal stretching as the part of the glacier below the riegel accelerates faster than that above. Water entering the glacier by way of a series of crevasses over the riegel is believed to be responsible for this differential acceleration. In June 1987, a dramatic event was registered, probably reflecting the initial summer acceleration of the glacier.