We review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses, as well as the three broad classes of numerical models now employed to simulate crevasse fracture. The relatively new insight that mixed-mode fracture in local stress equilibrium, rather than downstream advection alone, can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies. In the past three decades, there have been tremendous advances in the spatial resolution of satellite imagery, as well as fully automated algorithms capable of tracking crevasse displacements between repeat images. Despite considerable advances in developing fully transient three-dimensional ice flow models over the past two decades, both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time. In the past decade, however, multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged. The combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large-scale ice sheet models. Finally, given the broad interest in the sea level rise implications of recent and future cryospheric change, we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance.

Glacier crevasses: Observations, models, and mass balance implications

4 – iceberg drift and deterioration

Origin of washboard moraines of the Des Moines Lobe: Spatial analyses with LiDAR data

There are several cases in which large overthrusts and sub-horizontal faults appear to have occurred in temperate or cold glaciers. As a contribution to solving the problem of their origin, the stress field when there is an abrupt change in the bottom boundary conditions is determined, assuming ice to be isotropic, third-power-law viscous. Deviatoric stresses vary with the distance r to the singularity as r -1/4 , and strain rates as r -3/4 . They are computed numerically to a multiplicative factor, which is determined by the conditions at a large distance, but not computed here. Although the apparent viscosity varies as r 1/2 times a function of the polar angle, the stress field around the singularity is not essentially different from that obtained assuming a constant viscosity. Some considerations on the apparition of faults follow, but at the scale of the microrelief the adopted model becomes oversimplified.

/pdf/overthrusts-due-to-easy-slip-poor-slip-transitions-at-the-hvm5dkmpzt.pdf

Overthrusts due to easy-slip/poor-slip transitions at the bed: the mathematical singularity with non-linear isotropic viscosity

Washboard moraines are a characteristic landform of the Des Moines Lobe. Primarily composed of late Wisconsinan till and oriented parallel to and up-glacier from more conspicuous end moraines, washboard moraines are of low relief (1-2m) and appear to be regularly spaced. These moraines have been assumed in Des Moines Lobe reconstructions to have formed subglacially as basal crevasse fills, with crests developing perpendicular to the ice-flow direction when the lobe was at its maximum extent. Alternatively, the moraines are push moraines formed by seasonal advances of the lobe during its overall recession. The veracity of geomorphic reconstructions of the lobe‘s thickness relies upon the former hypothesis being correct. The morphologies and spatial patterns of the moraines may help reveal their origin but are poorly characterized, owing to the topographic subtlety of the moraines. The acquisition of 1 m LiDAR over the Des Moines Lobe‘s footprint allows washboard moraines to be spatially characterized over broad areas for the first time. After mapping LIDAR-derived elevation data and identifying suitable tracts of moraines, spatial analysis techniques are applied to study moraine spacing and cross-sectional profiles. Using a chi-square test with a significance level of 95%, Fourier analysis of 400 topographic profiles constructed perpendicular to moraine trends reveals a dominant, statistically significant wavelength of 90-110 m. Uniform moraine spacing is expected for crevasse-fill ridges but not for end moraines. The cross-sectional profiles of individual, de-trended moraines are ―stacked‖ to characterize the average moraine shape. This exercise indicates that they lack the systematic asymmetry typically displayed by push-style end moraines, which are generally steeper on their down-glacier sides. Analysis of abrupt changes in moraine trend, called ―cusps,‖ indicates that they occur preferentially in the vicinity of outwash trains of sand and gravel: all cusps point up-glacier, and 62.7% are coincident with surface

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1454&context=etd

Spatial analysis of Des Moines Lobe washboard moraines using LiDAR data

Surface deformations in the neighborhood of a crevasse field were measured over short (3 m) gage lengths in order to study flow conditions associated with crevasse formation. The results obtained were unusual in that they were inconsistent with large-scale results found by previous workers. It was concluded that the presence of small-scale surface effects, such as fractures, pot-holes and healed crevasses give rise to small-scale deformation fields with large spatial and temporal variations and that there is a lower limit of gage length below which deformation measurements pertinent to regional (low phenomena cannot be made. This lower limit is apparently an order of magnitude greater than the spacing of the features which give rise to localized effects.

/pdf/small-scale-strain-measurements-on-a-glacier-surface-232cvxiuzq.pdf

Small-Scale Strain Measurements on a Glacier Surface

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Small-Scale Strain Measurements on a Glacier Surface