The Experts below are selected from a list of 57465 Experts worldwide ranked by ideXlab platform
Neal R. Iverson - One of the best experts on this subject based on the ideXlab platform.
-
Linking bedrock discontinuities to glacial Quarrying
Annals of Glaciology, 2019Co-Authors: Jacob Woodard, Neal R. Iverson, Lucas K. Zoet, Christian HelanowAbstract:AbstractQuarrying and abrasion are the two principal processes responsible for glacial erosion of bedrock. The morphologies of glacier hard beds depend on the relative effectiveness of these two processes, as abrasion tends to smooth bedrock surfaces and Quarrying tends to roughen them. Here we analyze concentrations of bedrock discontinuities in the Tsanfleuron forefield, Switzerland, to help determine the geologic conditions that favor glacial Quarrying over abrasion. Aerial discontinuity concentrations are measured from scaled drone-based photos where fractures and bedding planes in the bedrock are manually mapped. A Tukey honest significant difference test indicates that aerial concentration of bed-normal bedrock discontinuities is not significantly different between quarried and non-quarried areas of the forefield. Thus, an alternative explanation is needed to account for the spatial variability of quarried areas. To investigate the role that bed-parallel discontinuities might play in Quarrying, we use a finite element model to simulate bed-normal fracture propagation within a stepped bed with different step heights. Results indicate that higher steps (larger spacing of bed-parallel discontinuities) propagate bed-normal fractures more readily than smaller steps. Thus, the spacing of bed-parallel discontinuities could exert strong control on Quarrying by determining the rate that blocks can be loosened from the host rock.
-
glacial landscape evolution by subglacial Quarrying a multiscale computational approach
Journal of Geophysical Research, 2016Co-Authors: Sofie V Ugelvig, David L Egholm, Neal R. IversonAbstract: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 post-glacial topography. In this study we relate the physics of Quarrying to long-term landscape evolution with a multi-scale 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 in basal sliding speed, effective pressure, and bed slope affect the rate of erosion. A sensitivity test indicates that a power-law formulated in terms of these three variables provides an acceptable basis for quantifying regional-scale rates of Quarrying. Our results highlight the strong influence of effective pressure, which intensifies Quarrying by increasing the volume of the bed that is stressed by the ice and thereby the probability of rock failure. The resulting pressure dependency points to subglacial hydrology as a primary factor for influencing rates of Quarrying and hence for shaping the bedrock topography under warm-based glaciers. When applied in a landscape evolution model, the erosion law for Quarrying produces recognizable large-scale glacial landforms: U-shaped valleys, hanging valleys and overdeepenings. The landforms produced are very similar to those predicted by more standard sliding-based erosion laws, but overall Quarrying is more focused in valleys, and less effective at higher elevations.
-
a theory of glacial Quarrying for landscape evolution models
Geology, 2012Co-Authors: Neal R. IversonAbstract:Numerical models are used commonly to study the topographic evolution of glacially eroded landscapes. These models are grounded on a power-law rule that relates bedrock erosion rate to either the sliding speed or discharge of glaciers. This rule, however, is poorly linked to the principal process of glacial erosion, Quarrying, in which bedrock blocks are dislodged from the bed by sliding ice. A new model of Quarrying allows this erosion rule to be evaluated. As in past Quarrying models, ice-bed separation during sliding controls deviatoric stresses in the rock that cause crack growth. Unlike past models, bedrock strength heterogeneity resulting from preglacial fractures is included using a Weibull statistical distribution of rock strength. This strength distribution is predicated on the observation that larger rock bodies have lower strengths because they have a greater probability of containing a large fracture. Results can, indeed, be closely fitted with a power-law erosion rule, but its nonlinearity, the range of sliding speed over which it applies, and erosion rates depend sensitively on bedrock strength heterogeneity and effective pressure. This theory anchors large-scale models of glacial erosion to the primary small-scale process that these models hope to simulate and reinforces recent emphasis on the role of bedrock fractures in accelerating geomorphic processes. Moreover, by linking basal water pressure to erosion rate, the theory can improve efforts with numerical models to study feedbacks between subglacial hydrology and landscape evolution.
-
control of glacial Quarrying by bedrock joints
Geomorphology, 2012Co-Authors: Thomas S Hooyer, Denis Cohen, Neal R. IversonAbstract:Abstract A principal assumption of models that describe bedrock Quarrying by glaciers is that cracks in the bed are small and isolated. These cracks are assumed to grow slowly in response to glacial loading to eventually delimit the areas of quarried surfaces. If this assumption is correct, then quarried-surface orientations will be controlled by orientations of principal stresses in the bed associated with sliding and resultant ice-bed separation downglacier from rock ledges and roches moutonnees, rather than by orientations of joints that predate glacial loading. To test this hypothesis, we compared orientations of quarried surfaces and preglacial joints in nine recently deglaciated forefields of glaciers in Canada and Switzerland with different bedrock lithologies and bedforms. In all but one forefield, quarried surfaces and major joint sets are coincident, with orientations of quarried surfaces bearing no systematic relationship to sliding direction other than their obvious tendency to not face upglacier. A difference in sliding direction of 64° between two areas of one glacier forefield had no effect on the orientation of quarried surfaces, with the same joint set exploited in both areas. These observations motivate a new characterization of subglacial bedrock that differs from that of existing Quarrying models. The bed is idealized as a group of rock blocks separated by preglacial joints. Slow crack growth necessary for Quarrying would be confined to small bodies of rock (bridges) that connect the faces of joints. This model of the bed does not allow it to be treated as an intact elastic solid with hydraulically isolated cracks as is usually assumed, which would have a profound influence on characterization of Quarrying mechanics.
-
role of transient water pressure in Quarrying a subglacial experiment using acoustic emissions
Journal of Geophysical Research, 2006Co-Authors: Denis Cohen, Neal R. Iverson, Thomas S Hooyer, Jason F Thomason, Miriam JacksonAbstract:[1] 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 slid over it. Vertical stresses, water pressure, and cavity height in the lee of the step were also measured. Water was pumped to the lee of the step several times over 8 days. Pumping initially caused opening of a leeward cavity, which then closed after pumping was stopped and water pressure decreased. During cavity closure, acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically. With repeated pump tests this crack grew with time until the step's lee surface was quarried. Our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded. Natural basal water pressure fluctuations should also concentrate stresses on rock steps, increasing rates of crack growth. Stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size, which may help explain the effect of sliding speed on erosion rates.
Wolfgang Hofmeister - One of the best experts on this subject based on the ideXlab platform.
-
geochemical provenance analyses of roman lava millstones north of the alps a study of their distribution and implications for the beginning of roman lava Quarrying in the eifel region germany
Journal of Archaeological Science, 2011Co-Authors: Tatjana M Gluhak, Wolfgang HofmeisterAbstract:Abstract In Roman times, rotary querns and different types of millstones, driven either by horse-capstan or water power, were produced in the lava quarries of the quaternary volcanic Eifel region and exported to many parts of the Empire. The geographic distribution of Roman lava millstones from the Eifel region provides important information about trade patterns and, in cases of well dated millstones, also allows an estimate as to when the Roman lava Quarrying in the Eifel region began. Sixty-two millstones from Germany, France and Austria were sampled and analyzed for major and trace elements by X-ray fluorescence. To determine their provenance, the millstone data was evaluated by a combination of geochemical discrimination and cluster and discriminant analyses using an extensive and detailed database of all Roman lava quarries in the Eifel region. An Eifel provenance could be confirmed for forty-four artefacts and, furthermore, determined down to the exact lava flow. The affiliation of the other artefacts to other possible regions where millstones of comparable lava were extracted was carried out on the basis of geochemical data from the literature. However, because of insufficient data, only assumptions about the regional provenance can be made. The origins of the other finds are assumed to be the Vogelsberg region, the Massif Central, Orvieto, and, possibly, the Pannonian Basin. A preliminary map of the distribution of Eifel millstones in Roman times based on these data is presented; the beginning of Roman lava Quarrying can be constrained to 8–7 BC.
Miriam Jackson - One of the best experts on this subject based on the ideXlab platform.
-
role of transient water pressure in Quarrying a subglacial experiment using acoustic emissions
Journal of Geophysical Research, 2006Co-Authors: Denis Cohen, Neal R. Iverson, Thomas S Hooyer, Jason F Thomason, Miriam JacksonAbstract:[1] 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 slid over it. Vertical stresses, water pressure, and cavity height in the lee of the step were also measured. Water was pumped to the lee of the step several times over 8 days. Pumping initially caused opening of a leeward cavity, which then closed after pumping was stopped and water pressure decreased. During cavity closure, acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically. With repeated pump tests this crack grew with time until the step's lee surface was quarried. Our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded. Natural basal water pressure fluctuations should also concentrate stresses on rock steps, increasing rates of crack growth. Stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size, which may help explain the effect of sliding speed on erosion rates.
Denis Cohen - One of the best experts on this subject based on the ideXlab platform.
-
control of glacial Quarrying by bedrock joints
Geomorphology, 2012Co-Authors: Thomas S Hooyer, Denis Cohen, Neal R. IversonAbstract:Abstract A principal assumption of models that describe bedrock Quarrying by glaciers is that cracks in the bed are small and isolated. These cracks are assumed to grow slowly in response to glacial loading to eventually delimit the areas of quarried surfaces. If this assumption is correct, then quarried-surface orientations will be controlled by orientations of principal stresses in the bed associated with sliding and resultant ice-bed separation downglacier from rock ledges and roches moutonnees, rather than by orientations of joints that predate glacial loading. To test this hypothesis, we compared orientations of quarried surfaces and preglacial joints in nine recently deglaciated forefields of glaciers in Canada and Switzerland with different bedrock lithologies and bedforms. In all but one forefield, quarried surfaces and major joint sets are coincident, with orientations of quarried surfaces bearing no systematic relationship to sliding direction other than their obvious tendency to not face upglacier. A difference in sliding direction of 64° between two areas of one glacier forefield had no effect on the orientation of quarried surfaces, with the same joint set exploited in both areas. These observations motivate a new characterization of subglacial bedrock that differs from that of existing Quarrying models. The bed is idealized as a group of rock blocks separated by preglacial joints. Slow crack growth necessary for Quarrying would be confined to small bodies of rock (bridges) that connect the faces of joints. This model of the bed does not allow it to be treated as an intact elastic solid with hydraulically isolated cracks as is usually assumed, which would have a profound influence on characterization of Quarrying mechanics.
-
role of transient water pressure in Quarrying a subglacial experiment using acoustic emissions
Journal of Geophysical Research, 2006Co-Authors: Denis Cohen, Neal R. Iverson, Thomas S Hooyer, Jason F Thomason, Miriam JacksonAbstract:[1] 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 slid over it. Vertical stresses, water pressure, and cavity height in the lee of the step were also measured. Water was pumped to the lee of the step several times over 8 days. Pumping initially caused opening of a leeward cavity, which then closed after pumping was stopped and water pressure decreased. During cavity closure, acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically. With repeated pump tests this crack grew with time until the step's lee surface was quarried. Our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded. Natural basal water pressure fluctuations should also concentrate stresses on rock steps, increasing rates of crack growth. Stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size, which may help explain the effect of sliding speed on erosion rates.
Tatjana M Gluhak - One of the best experts on this subject based on the ideXlab platform.
-
geochemical provenance analyses of roman lava millstones north of the alps a study of their distribution and implications for the beginning of roman lava Quarrying in the eifel region germany
Journal of Archaeological Science, 2011Co-Authors: Tatjana M Gluhak, Wolfgang HofmeisterAbstract:Abstract In Roman times, rotary querns and different types of millstones, driven either by horse-capstan or water power, were produced in the lava quarries of the quaternary volcanic Eifel region and exported to many parts of the Empire. The geographic distribution of Roman lava millstones from the Eifel region provides important information about trade patterns and, in cases of well dated millstones, also allows an estimate as to when the Roman lava Quarrying in the Eifel region began. Sixty-two millstones from Germany, France and Austria were sampled and analyzed for major and trace elements by X-ray fluorescence. To determine their provenance, the millstone data was evaluated by a combination of geochemical discrimination and cluster and discriminant analyses using an extensive and detailed database of all Roman lava quarries in the Eifel region. An Eifel provenance could be confirmed for forty-four artefacts and, furthermore, determined down to the exact lava flow. The affiliation of the other artefacts to other possible regions where millstones of comparable lava were extracted was carried out on the basis of geochemical data from the literature. However, because of insufficient data, only assumptions about the regional provenance can be made. The origins of the other finds are assumed to be the Vogelsberg region, the Massif Central, Orvieto, and, possibly, the Pannonian Basin. A preliminary map of the distribution of Eifel millstones in Roman times based on these data is presented; the beginning of Roman lava Quarrying can be constrained to 8–7 BC.
