Search

Glacial quarrying stems from the fracturing of subglacial bedrock. Much evidence shows that subcritical crack propagation of bedrock is closely related to subglacial water pressure fluctuations. Here we employ a model that assesses the impact of subglacial water pressure fluctuation on cavity length and subcritical crack propagation, while analyzing the effect of a pre-existing crack location using a phase-field model (PFM). Our results indicate that the cavity length is reduced during diurnal fluctuations in water pressure. There are two patterns of subcritical crack propagation on the corner of the step. The first stems from a rapid drop in water pressure. The second occurs after the water pressure recovers from the fluctuation to the initial steady state. This pattern is a consequence of enhanced stress concentration on the step since the modeled cavity length exceeds its steady value and has higher efficiency in promoting subcritical crack propagation. Additionally, based on the PFM results, we speculate that the subcritical crack initiation and propagation happen on a broader scale, including the ice-bed contact region and its adjacent region. Our findings imply that the duration of subcritical crack propagation is short and typically ceases once the cavity length adjusts to reduced water pressure levels. [ABSTRACT FROM AUTHOR]

The Autonomous Pinger Unit (APU) is an electro-thermal drill with acoustic instrumentation developed for the project EnEx-RANGE in view of a future space mission for the sub-surface exploration of Saturn's moon Enceladus. A main goal is the development of navigation technology for an acoustic guidance system allowing maneuvering a probe through glacial ice. In total 13 APUs were built and tested in terrestrial analog scenarios on alpine glaciers. The APUs form a spatially distributed network that defines a system of reference for the navigation of the maneuverable probe to a point of interest. The APUs have a novel melting head, slow control systems, and a modern system-on-chip (SoC) module that controls the probe and processes the recorded data. The APUs use acoustic emitters and receivers to measure the transit time of acoustic signals between them, allowing for the position reconstruction of all APUs by trilateration. Several auxiliary sensors monitor the internal state of the probe and assist the position estimation. With this instrumentation, the APUs have the ability of dynamically optimizing themselves within the network by changing their position. This paper gives an overview of the developed APU hardware and presents performance results from the field tests. [ABSTRACT FROM AUTHOR]

Increased summer ice velocities on the Greenland ice sheet are driven by meltwater input to the subglacial environment. However, spatial patterns of surface input and partitioning of meltwater between different pathways to the base remain poorly understood. To further our understanding of surface drainage, we apply a supraglacial hydrology model to the Paakitsoq region, West Greenland for three contrasting melt seasons. During an average melt season, crevasses drain ~47% of surface runoff, lake hydrofracture drains ~3% during the hydrofracturing events themselves, while the subsequent surface-to-bed connections drain ~21% and moulins outside of lake basins drain ~15%. Lake hydrofracture forms the primary drainage pathway at higher elevations (above ~850 m) while crevasses drain a significant proportion of meltwater at lower elevations. During the two higher intensity melt seasons, model results show an increase (~5 and ~6% of total surface runoff) in the proportion of runoff drained above ~1300 m relative to the melt season of average intensity. The potential for interannual changes in meltwater partitioning could have implications for how the dynamics of the ice sheet respond to ongoing changes in meltwater production. [ABSTRACT FROM AUTHOR]

Detailed studies on temporal changes of crevasses and their linkage with glacier dynamics are scarce in the Himalayan context. Observations of temporally changing surficial crevasse patterns and their orientations are suggestive of the processes that determine seasonal glacier flow characteristics. In the present study, on a Himalayan valley glacier, changing crevasse patterns and orientations were detected and mapped on Landsat 8 images in an automated procedure using the ratio of Thermal Infrared Sensor (TIRS) band 10 to Optical Land Imager (OLI) shortwave infrared (SWIR) band 6. The ratio was capable of mapping even crevasses falling under mountain shadows. Differential GPS observations suggested an average error of 3.65% and root-mean-square error of 6.32m in crevasse lengths. A year-round observation of these crevasses, coupled with field-based surface velocity measurements, provided some interesting interpretations of seasonal glacier dynamics. [ABSTRACT FROM PUBLISHER]

Our objective is to map dynamic provinces and investigate dynamic changes in Jakobshavn Isbræ, Greenland. We use an approach that combines structural glaciology and remote-sensing data analysis, facilitated by mathematical characterization of generalized spatial surface roughness that provides parameters related to ice dynamics, deformation and interaction of the ice with bed topography. The approach is applied to derive time series of elevation and roughness changes and to attribute changes during rapid retreat. Different dynamic types of fast- and slow-moving ice can be mapped from ICESat Geoscience Laser Altimeter System data (2003-09) and Airborne Topographic Mapper data, using spatial roughness characterization, validated with ASTER and bed-topographic data. Results of comparative analysis of elevation changes and roughness changes of Jakobshavn south ice stream indicate (1) surface lowering of 10-15 ma-1 between 2004 and 2009 and (2) no change in surface roughness and dynamic types. These findings are consistent with a front retreat as part of a fjord-glacier cycle or following warming of fjord water and with climatic warming, but not with an internal dynamic acceleration as a cause of the observed changes during rapid retreat. Relationships to changes in basal water pressure are discussed. All glaciodynamic changes appear to have initiated near the front and propagated up-glacier. [ABSTRACT FROM AUTHOR]

The transition from limited-slip conditions at the base of grounded ice to free-slip conditions beneath floating ice occurs across the few-kilometers-wide grounding zone. This region involves either an elastic flexural transition from bedrock to hydrostatically supported elevations (often tidally influenced), a transition from thicker to thinner ice over a flat bed, or some combination of these two processes. In either case, ice must flow across a changing stress field, often resulting in brittle deformation, manifested as basal crevassing. Thus the position and morphology of basal crevasses reveal important information about the stress state across this transition. Our gridded ground-based radar surveys on Whillans Ice Stream, West Antarctica, indicate a complex pattern of basal crevasses, but most are associated with regions where the surface elevation gradient is steepest. Due to the high reflectivity of sea water, we image many off-nadir crevasses from a corner-reflector geometry involving reflections from the ice/sea-water interface and then from the crevasse, producing echoes with an inverted phase that could be misinterpreted as subglacial returns. Our results indicate that basal crevasses offer a rich dataset for diagnosing stress state and salient processes across grounding zones, and that special care is needed when interpreting subglacial returns in radar data. [ABSTRACT FROM AUTHOR]

There remain large regions of scientific interest in the Antarctic that are not instrumented. These include highly dynamic ice streams and glaciers that are difficult or impossible to reach safely because heavy crevassing impedes an overland trek or an aircraft landing. We have developed an alternative strategy for instrumenting these regions: an aerodynamic sensor that can be dropped from an overflying aircraft. During freefall the sensor accelerates to its terminal velocity of 42 m s-1 before impacting with the glacier. On impact it partially buries itself in the snow while leaving an antenna mast protruding high above the surface to ensure a long operating life. In this paper, we describe the design and results of testing this aircraft-deployable sensor. Finally we present the initial results of two campaigns to instrument inaccessible regions of Pine Island Glacier, West Antarctica, and Scar Inlet, Antarctic Peninsula, with GPS receivers. [ABSTRACT FROM AUTHOR]

The article presents a study regarding the method for figuring the heights of basal crevasses formed at the grounding lines of ice shelves and ice streams. It states that a thin-elastic beam (TEB) formulation and tensional yielding criterion to captivate the physics of flexed ice at grounding lines are applied in the study. Results show the TEB formulation applied at upstream of observed basal crevasses has produces order-of-magnitude crevasse heights.

We have conducted a series of experiments designed to investigate elastic and electromagnetic (EM) velocity anisotropy associated with a preferentially aligned fracture system on a temperate valley glacier in south-central Alaska, USA. Measurements include a three-dimensional compressional wave (P-wave) seismic reflection survey conducted over a 300m×300m survey patch, with uniform source grid and static checkerboard receiver pattern. Additionally, we acquired a multiazimuth, multi-offset, polarimetric ground-penetrating radar (GPR) reflection experiment in a wagonwheel geometry with 948 of azimuthal coverage. Results show azimuthal variation in the P-wave normal-moveout velocity of >3% (3765 and 3630ms-1 in the fast and slow directions respectively) and difference of nearly 5% between the fast (0.164mns-1) and slow (0.156mns-1) EM velocities. Fracture orientations estimated from the GPR and seismic velocity data are consistent and indicate a preferred fracture orientation that is 30-45° oblique to glacier flow; these measurements agree with borehole observations. Anisotropic analysis of the polarimetric data gives a single volumetric water content estimate of 0.73±0.11%. We conclude that meaningful estimates of physical properties in glaciers based on EM or seismic velocity measurements require collecting data such that the presence of anisotropy can be evaluated and an anisotropic analysis employed when necessary. [ABSTRACT FROM AUTHOR]

The dynamics of a surge is manifested in the crevasse patterns: literally, deformation state frozen in ice. This basic observation is utilized as the concept of an automated approach to map and analyze deformation stages and progression of surge kinematics. The classification method allows imagery to be used as geophysical data and is applied to aerial observations (photographic and video imagery, GPS data) collected in September 2011 during the surge of the Bering Glacier-Bagley Ice Valley system, Alaska, USA. As the third dimension that complements two-dimensional imagery, icesurface elevation is observed using aerial laser altimetry. The classification method builds on concepts from signal processing, geostatistical data analysis and neural networks. Steps include calculation of generalized directional vario functions from image data and composition into feature vectors. The vario function operates as an information filter that retains spatial characteristics at an intermediate scale that captures crevasse spacing, anisotropy and other generalized roughness properties. Association of feature vectors to crevasse classes and hence deformation types employs a connectionist algorithm. In general, the connectionist-geostatistical classification allows the mapping of kinematic changes in crevassed glaciers. [ABSTRACT FROM AUTHOR]

Crevasse initiation is linked to strain rates that range over three orders of magnitude (0.001 and 0.163 a-1) as a result of the temperature-dependent nonlinear rheological properties of ice and from water and debris inclusions. Here we discuss a small cold glacier that contains buried crevasses at and near an ice divide. Surface-conformable stratigraphy, the glacier's small size, and cold temperatures argue for limited rheological variability at this site. Surface ice-flow velocities of (1.2-15.5) ± 0.472 m a-1 imply classic saddle flow surrounding the ice divide. Numerical models that incorporate field-observed boundary conditions suggest extensional strain rates of 0.003-0.015 a-1, which fall within the published estimates required for crevasse initiation. The occurrence of one crevasse beginning at 50m depth that appears to penetrate close to the bed suggests that it formed at depth. Field data and numerical models indicate that a higher interior stress at this crevasse location may be associated with steep convex bed topography; however, the dynamics that caused its formation are not entirely clear. [ABSTRACT FROM AUTHOR]

The article presents the study which examined the effectiveness of using icequakes and surface displacement data in forecasting glacier break-off. The study was conducted at the Weisshorn Glacier in Valais, Switzerland. It cites the various characteristic seismic waveforms that are linked to different icequake events, including surface crevassing, calving events, and basal sliding. It presents the three stages of the failure process that lead to break-off, namely, the stable phase linked to a self-organizing regime, the transitional stage, and the catastrophic regime.

The article presents a re-analysis of the cause of the outburst flood from Glacier Tête Rousse that devastated the village of Saint-Gervais-Le Fayet, French Alps in 1892. It offers historical observations and investigations about the disaster with detailed descriptions of the glacier surface and the volume of its upper cavity. Results of the new analysis revealed that the meltwater reservoir in the upper cavity could not come from an enlarging crevasse and that its origin was more likely a supgraglacial lake.

The article discusses a study which utilized a linear elastic fracture-mechanics theory and a boundary element numerical method to investigate the propagation of long fractures in the Ronne Ice Shelf in Antarctica. The authors digitized a feature map of the Ronne-Filchner Ice Shelf using the MODIS Mosaic of Antarctica (MOA) image map and analyzed fractures from the outlet of Evans Ice Stream (EIS) leading downstream. Several experiments were conducted to investigate the importance of structural boundaries in crack tip arrest.

The article discusses a study which investigated the effects of several factors, such as debris thickness, diurnally cyclic radiation and rainfall, on melt rates beneath rock-avalanche debris and sand. A background of studies on the role of debris cover in modifying glacier behaviour is presented. Information is also presented on the sources of supraglacial ablation-zone debris. The article also examines diurnal radiation cycles and debris properties and critical thickness and diurnal cyclicity.

The article discusses the study which examines the debris-covered glaciers and large glacial lakes at the terminus of the Himalaya mountains in Nepal. It notes that the lakes have grown since the 1950s and 1960s which present the potential hazard of glacial-lake outburst floods. It adds that the glaciers have recorded a relatively large lowering of their surface and are likely to develop glacial lakes at their terminus. It confirms that the lowering of the glacier surface could result in the formation of supraglacial ponds and the commencement of calving at the glacial lake.

The article presents a study which evaluates the crevassae depth and surface strain rates in Breiðamerkurjökull, Iceland. The study compares the measured values of the data on crevasse depth and surface strain rates with the results of two crevasse depth models. These models are the simple function and the linear elastic fracture mechanics (LEFM) model. It also states that crevasses and associated ice-fracture process are used in recognizing the importance of calving glaciers to the mass balance of the cryosphere. The study concludes that both depth models can be utilized to estimate crevasse depths in calving models.

The article presents a study which examines the structural control of fourteen englacial conduits using specialized techniques in the temperate Matanuska Glacier in Alaska. The study made a detailed three-dimensional maps of conduits over 3 years in order to characterize the relationships of conduits with glacier structural features. It revealed that the first class of conduits were shaped through the application of hydrostatic crevasse penetration. While the second class of conduits were formed in the area where smaller streams dropped into the glacier.

The article discusses a study which aims to examine the surge behaviour and dynamics of the Sermeq Peqippoq glacier in East Greenland. Surge-types of glaciers occur in different areas near the Greenland ice sheet and the regional surge shows the hydrologically controlled surge mechanism. The glacier is one of the smaller glaciers in the region with identified surge. Data from the National Aeronautics and Space Administration Earth Observing System Data Gateway were used in the study. The study found that the glacier's surge activity showed a multi-phase surge and gradual deceleration.

The article discusses a study which explores the englacial drainage systems that are formed by hydrologically driven crevasse propagation. Surface-to-bed drainage systems were developed at the ice sheet and at the High Arctic glaciers in Greenland. Researchers examined and surveyed englacial drainage systems what were formed by hydrofracturing in the glaciers in Svalbard, Nepal and Alaska. They used speleological techniques and found vertical shafts that were followed by cold ice and temperate basal ice to subglacial conduit. The study found that englacial drainages by hydrofracturing in glaciological periods is a common process.

The article discusses the study which observed the different situation and several new surges and geometric changes at the three glaciers in Blomstrandbreen, Comfortlessbreen and Nathorstbreen, Svalbard in Norway. It compares satellite images from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) which mapped the changes and some velocities derived from tracing of crevasses. It adds that the front velocities were high compared with many other Svalbard surges.

The article offers the authors' insight on the study by R. Duddu and H. Waisman on the ice rheology approach in their damage mechanics model, and its application to the prediction of iceberg calving and crevasse opening. They say that Waisman and Duddu have presented numerical aspects, which appeared to be innovative. They mention the main criticisms of how the model was calibrated and formulated such as the model relied on the formalism of Murakami.