Spontaneous Formation of an Internal Shear Band in Ice Flowing over Topographically Variable Bedrocks
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Key Points: 10 • Ice flowing over a rough basal topography may spontaneously develop an inter-11 nal shear band on topographical highs. 12 • The shear strain rate localization and shear heating in the internal shear band is 13 amplified by a non-linear rheology. 14 • We identify two competing mechanisms that affect the energy balance near the 15 bedrock: vertical advective cooling and internal shear heating. Abstract 17 The dramatic acceleration of ice surface speed from upstream to downstream is a no-18 ticeable feature in many ice streams and glaciers. This speed-up is thought to be asso-19 ciated with a transition from internal, distributed deformation to highly localized defor-20 mation at the ice-bedrock interface, but the physical processes governing this transition 21 remain unclear. Here, we argue that basal topography amplifies the feedback between 22 shear heating and localization, leading to the spontaneous formation of an internal shear 23 band for a non-linear rheology. We model the thermo-mechanical ice flow over a simpli-24 fied basal topography using a high-resolution Stokes solver. To capture the interactions 25 between ice and rock, we implement an Immersed Boundary Method and use a level-set 26 approach to represent the free surface of the ice. Our results suggest that an internal shear 27 band can form on topographical highs, continuously heating the basal ice and may grad-28 ually enable a transition to basal sliding. This effect depends sensitively on rheology, with 29 the composite rheology by Goldsby and Kohlstedt (2001) amplifying shear heating no-30 tably. 31 Plain Language Summary 32 On its way towards the ocean, ice speeds up dramatically from less than one me-33 ter per year inland to up to a kilometer per year downstream. In this paper, we inves-34 tigate the physical processes controlling this speed-up. More specifically, we focus on the 35 role that the bedrock topography underneath the ice might play to facilitate this tran-36 sition. We use a two-dimensional numerical model to simulate the temperature distri-37 bution and deformation within a slab of ice flowing down a ramp over a simplified to-38 pography. We find that including basal topography could lead to the development of in-39 ternal shear band located on top of topographical highs. Around half of the total shear 40 deformation within the ice occurs within this band. We compare our model results to 41 borehole measurements from Greenland and find evidence that supports the existence 42 of a shear band. 43Keywords:
Shear band
Bedrock
the bedrock geologic map portrays the current interpretation of the distribution of various bedrock stratigraphic units present at the bedrock surface the bedrock surface is buried by unconsolidated surficial sediments mostly quaternary over most of its extent but this surface coincides with the modern land surface in areas of bedrock exposure the map is consistent with all available data including drill records and well samples as well as surface bedrock exposures both natural and man made and shallow to bedrock soils units nrcs county soils maps mapped stratigraphic intervals are portrayed primarily at the group level i e a grouping of bedrock formations each characterized by distinctive lithologies rock types summarized in the map key and associated metadata the distribution of bedrock units was mapped to conform to the current map of bedrock topography elevation of the bedrock surface the structural configurations of relevant stratigraphic datums were intercepted with the bedrock topographic surface to produce the map contacts the line style shown on the bedrock geologic map qualitatively reflects both data density and degree of certainty of individual stratigraphic contacts downloadable
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Analysis of topographic and bedrock surface data from 41 sites 23.3 km2 (9 m2) within Midwestern areas glaciated during the Late Wisconsinan identifies an average drift thickness-maximum bedrock relief transition equation for the presence or absence of bedrock surface influences on topographic detail. For sites with average drift thicknesses between 15 and 35 m, the transition occurs when average drift thicknesses exceed 0.5 maximum bedrock relief + 10 m. This equation may provide a practical tool in searching for buried bedrock valleys.
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Skyscrapers in Manhattan need to be anchored to bedrock to prevent (possibly uneven) settling. This can potentially increase construction costs if the bedrock lies deep below the surface. The conventional wisdom holds that Manhattan developed two business centers--downtown and midtown--because the depth to the bedrock is close to the surface in these locations, with a bedrock valley in between. We measure the effects of building costs associated with bedrock depths, relative to other important economic variables in the location of early Manhattan skyscrapers (1890-1915). We find that bedrock depths had very little influence on the skyline; rather its polycentric development was due to residential and manufacturing patterns, and public transportation hubs.
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Significance Hilly landscapes are typically mantled with soil and underlain by a weathered bedrock zone that may extend tens of meters beneath the surface before reaching fresh bedrock. The weathered bedrock zone influences water runoff to channels, the chemistry of that water, the rates and processes of erosion, and atmospheric processes due to plant uptake of moisture and return to the atmosphere. However, the spatial pattern of the underlying fresh-bedrock surface is essentially unknown. We present a testable model that predicts hillslope form and the depth to fresh bedrock. The depth increases upslope and depends strongly on the porosity and permeability of the bedrock and the rate of channel incision at the base of the hillslope.
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The till_bedrock data layer is part of a comprehensive study to produce a statewide digital map of the surficial geology at a 1:24,000-scale. This compilation of surficial geologic materials defines the areas of exposed bedrock, and the boundaries between glacial till, glacial stratified deposits, and overlying postglacial deposits. The till_bedrock layer shows areas of thin till, thick till, bedrock outcrops, and areas of abundant outcrop and shallow bedrock for a 12-quadrangle area in east-central Massachusetts. Thin till and bedrock outcrops polygons are mutually exclusive, thick till polygons overlie areas of thin till, and abundant outcrop/shallow bedrock polygons overlie areas of thin till and bedrock outcrops. This data layer should be used in conjunction with the overlying stratified_deposits and postglacial data layers.
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<p>Active incision of bedrock rivers exerts a vital control on landscape evolution in upland areas. Previous research found that bedrock rivers were typically steeper and sometimes narrower than alluvial rivers. However, most of the literature on partially-exposed bedrock rivers has employed small samples mostly from mountainous regions, so their geomorphological properties remain poorly understood. In contrast with the existing literature, a large-sample analysis of bedrock river channel properties would allow the controls on bedrock river width and slope to be unpicked and reveal whether or not the existing literature is biased towards pristine, mountainous bedrock rivers. Overall, such an analysis could improve the reliability of upland landscape evolution models.</p><p>Here we present an analysis of 1,924 river sites from the EPA National Rivers and Streams Assessment to assess the geomorphological differences between bedrock and alluvial rivers. The influences of lithology and uplift on bedrock channel properties are examined using external datasets. We find bedrock rivers to be significantly steeper and wider than alluvial rivers. Sedimentary bedrock rivers were seen to be significantly wider than igneous/ metamorphic bedrock rivers, consistent with findings from Ferguson et al. (2017). We estimated shear stress and critical shear stress for each river site and assessed correlation with bedrock exposure. We found that exposed bedrock could not always be explained by local sediment transport exceeding local sediment supply, indicating that bedrock exposure may be controlled by other factors in some bedrock rivers. Currently, uplift data are being compiled for further analysis.</p>
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Stream power
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Even small bumps in a river's bedrock can change how sediment grains erode bedrock, a new modeling study shows. Observations have indicated that bumpy bedrock erodes more quickly, but most previous models of bedrock erosion assumed approximately planar beds.
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Bedrock weathering runs to the hills Fractures in bedrock drive the breakdown of rock into soil. Soil makes observations of bedrock processes challenging. St. Clair et al. combined a three-dimensional stress model with geophysical measurements to show that bedrock erosion rates mirror changes in topography (see the Perspective by Anderson). Seismic reflection and electromagnetic profiles allowed mapping of the bedrock fracture density. The profiles mirror changes in surface elevation and thus provide a way to study the critical zone between rock and soil. Science , this issue p. 534 ; see also p. 506
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Elevation (ballistics)
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