Johnny W. Sanders
University of California, Berkeley
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Featured researches published by Johnny W. Sanders.
Science | 2011
David L. Shuster; Kurt M. Cuffey; Johnny W. Sanders; Greg Balco
Glacial troughs in New Zealand mountains developed by propagation of erosion up valleys. Glacial erosion of mountain ranges produces spectacular alpine landscapes and, by linking climate with tectonics, influences a broad array of geophysical phenomena. Although the resultant landforms are easily identified, the timing and spatial pattern of topographic adjustment to Pleistocene glaciations remain poorly known. We investigated topographic evolution in the archetypal glacial landscape of Fiordland, New Zealand, using (U-Th)/He thermochronometry. We find that erosion during the past 2 million years removed the entire pre-Pleistocene landscape and fundamentally reshaped the topography. Erosion focused on steep valley segments and propagated from trunk valleys toward the heads of drainage basins, a behavior expected if subglacial erosion rate depends on ice sliding velocity. The Fiordland landscape illustrates complex effects of climate on Earth’s surface morphology.
Geology | 2012
Johnny W. Sanders; Kurt M. Cuffey; Jeffrey R. Moore; Kelly R. MacGregor; Jeffrey L. Kavanaugh
Glaciers produce cirques by scouring their beds and sapping their headwalls, but evidence to constrain models of these processes has been elusive. We report a suite of environmental measurements from three cirque glacier bergschrunds, including the first temperature series recorded at depth throughout most of an annual cycle. Compared to the ambient air, the bergschrunds were colder in summer and warmer in winter. Freeze-thaw cycles were rare, and relatively stable subfreezing temperatures persisted from November until May. Using a model for rock fracturing driven by ice segregation, we demonstrate that favorable conditions for fracturing occur not only on the headwall above the glacier, but also within the bergschrund, where periglacial weathering and glacial transport can act together to drive cirque headwall retreat. A small (∼3 °C) year-round decrease in temperatures to conditions more typical of the Pleistocene would likely intensify the weathering process. Though so far ignored in all glacial landscape evolution models, the bergschrund likely plays an essential role in the sculpting of alpine landscapes.
Geological Society of America Bulletin | 2013
Johnny W. Sanders; Kurt M. Cuffey; Kelly R. MacGregor; Brian D. Collins
Cirques form and evolve as glaciers attack the bed and subaerial processes dismantle the surrounding walls. Collectively, these processes—which can make a cirque longer, or deeper, or both—profoundly influence near-divide regions of glaciated mountains and yet are rarely studied in a systematic way. Toward this end, we developed a theoretical framework for the sediment budget of a cirque that includes sediment sources, transport pathways, and storage elements. We quantified each component of the sediment budget using field measurements and remote-sensing data of a glaciated alpine cirque in British Columbia, Canada. The cirque has a plan-view area of 1.64 km 2 and relief of ∼780 m. Our budget values, which correspond to a period of substantial glacier retreat, are based on measurements reflecting time intervals ranging from 1 yr to 80 yr. We report errors as a range (enclosed in parentheses), analogous to 95% confidence bounds. On average, 1640 (250–7950) metric tons of rock are released by the headwall each year; nearly 90% of this debris leaves the wall as small rockfalls or in snow avalanches. Our field observations indicated that snow avalanches originating as cornice failures are currently the most important transport process on the headwall. We estimated the mass of debris transported annually by the glacier to the foreland using (1) the volume and age of the foreland ground moraine and (2) the product of rock mass per unit volume of ice and glacier velocity. Over the past several decades, the glacier delivered 6440 (1180–14930) tons/yr to the foreland via forward ice motion and margin retreat (mostly in subglacial till or sediment-rich basal layers). Less than 3% of the glacierborne sediment flux traveled as supraglacial debris (170 [50–320] tons/yr). At present, sediment evacuation from the cirque occurs in a single meltwater stream. We monitored water discharge and suspended sediment concentration in this stream between 29 June and 28 August 2007. By season’s end, 650 (80–1860) tons of sediment had passed our gauging station (equivalent to an erosion rate of 0.2 [0.03–0.70] mm/yr, when averaged over the glacier bed). Approximately one third of the total annual streamborne sediment transport occurred over a 2 d period during the first major melt event of the year. Using our budget relations and flux magnitudes, we estimate the glacier is removing between 1240 and 2470 tons of rock from its bed per year, a rate equivalent to 0.5–0.9 mm/yr of erosion glacierwide. The headwall, by comparison, is being worn away horizontally at ∼1.2 (0.2–5.9) mm/yr. Thus, our results suggest that the headwall is retreating at rates roughly equivalent to vertical incision by the glacier. Our sediment budget results demonstrate that the wide variety of sediment sources and transport processes active in cirques necessitates a holistic view of cirque formation, one that most morphometric, range-scale, and glacial erosion analyses ignore.
American Journal of Science | 2010
Johnny W. Sanders; Kurt M. Cuffey; Kelly R. MacGregor; Jeffrey L. Kavanaugh; C. F. Dow
Alpine cirques are excavated by glacial erosion, a process that depends in turn on the movement of ice by basal sliding. Cirque glacier flow is usually depicted as rotational sliding of a rigid block, but this model is based on little evidence and implies unorthodox glacier behavior given typical cirque dimensions. The small (∼1 km2), temperate West Washmawapta Glacier occupies an archetypal overdeepened and “armchair-shaped” cirque in the Canadian Rockies. We measured (1) the annual surface velocity field, (2) ice thickness, (3) sliding and internal deformation at one borehole, and (4) sliding in a marginal cavity. The glacier moves slowly, with surface velocities of 3 to 10 m/yr. The maximum ice thickness (∼185 m) occurs in the center of the cirque basin and roughly coincides with the position of greatest ice flux. Using our field measurements, a standard constitutive relation for ice, and simplifying assumptions related to the depth distribution of strain rates, we approximated the driving and resisting forces acting on sections of the glacier, and inferred the general pattern of basal sliding. Sliding is minimum in the center of the cirque and increases toward the margins, especially up the stoss side of the riegel. Internal deformation accounts for all motion in the cirque center, even if an unusually low viscosity for temperate ice is assumed. Basal shear stresses tend toward 105 Pa everywhere, a typical value for mountain glaciers. Transverse and longitudinal straining are significant in some parts of the glacier. Although a component of rotational flow must occur internally, the glacier does not conform to the rotational sliding model in any essential respect.
Journal of Glaciology | 2011
C. F. Dow; Jeffrey L. Kavanaugh; Johnny W. Sanders; Kurt M. Cuffey; Kelly R. MacGregor
To investigate the subsurface hydrological characteristics of an overdeepened cirque glacier, nine boreholes were drilled to the bed of West Washmawapta Glacier, British Columbia, Canada, in summer 2007. All holes were surveyed with a video camera, and four were subsequently instrumented with a combination of pressure transducers, thermistors and conductivity sensors. Diurnal pressure and temperature records indicate the presence of a hydraulically connected subglacial drainage system towards the northern glacier margin. Hydraulic jacking in the overdeepening, controlled by changing water volume in the marginal zone, potentially impacts basal ice flow and erosion. The presence of a sediment layer underlying the glacier also likely impacts hydrology and ice dynamics. Influx of warm groundwater into the basal system raises subglacial water temperatures above the pressure-melting point (pmp) and induces diurnal water temperature fluctuations of as much as 0.8◦C; water temperatures above the pmp could affect basal melt rates and the development of subglacial drainage systems. These observations suggest that the characteristics of the subglacial drainage system substantially affect patterns of flow and erosion by this small cirque glacier.
Earth Surface Processes and Landforms | 2009
Jeffrey R. Moore; Johnny W. Sanders; William E. Dietrich; Steven D. Glaser
Journal of Applied Geophysics | 2011
Jeffrey R. Moore; Alexandre Boleve; Johnny W. Sanders; Steven D. Glaser
Journal of Geophysical Research | 2010
Jeffrey L. Kavanaugh; P. L. Moore; C. F. Dow; Johnny W. Sanders
Journal of Glaciology | 2014
C. F. Dow; Jeffrey L. Kavanaugh; Johnny W. Sanders; Kurt M. Cuffey
1st Canada-US Rock Mechanics Symposium - Rock Mechanics Meeting Society's Challenges and Demands | 2007
Jeffrey R. Moore; Johnny W. Sanders; Kurt M. Cuffey; J. R. Haught; Steven D. Glaser