fracture

Glacier Crevasses: A Review

Posted by William Colgan on February 29, 2016
New Research / No Comments

We have a new review paper on glacier crevasses in the current issue of Reviews of Geophysics1. We survey sixty years of crevasse studies, from field observations to numerical modeling to remote sensing of crevasses, and also provide a synthesis of ten distinct mechanisms via which crevasses influence glacier mass balance.

Two years ago, our team embarked on what was supposed to be a brief review of crevasse science to help interpret maps of Greenland crevasse extent that we are generating from laser altimetry data as part of a NASA project entitled “Assessing Greenland Crevasse Extent and Characteristics Using Historical ICESat and Airborne Laser Altimetry Data”. The final review ended up containing 250 references and being 43 typeset pages in length. Evidently we found the crevasse life cycle contained more nuances than we had initially assumed! Here are some of the highlights that have shifted our paradigm:

Field observations – Although crevasses are conventionally conceptualized to initiate at the surface and propagate downwards, we were surprised to find compelling evidence that at least some crevasses initiate at several metres depth, before propagating upwards to appear at the glacier surface. For example, observations that new crevasses can intersect relict crevasses at angles as low as 5 ° indicates that the stresses governing fracture are below the depth of relict crevasses (as relict crevasses do not serve as stress foci). This has implications for interpreting “buried” crevasses as relict or active.

Crevasse_Field_Sample

Figure 1 – Measured principal strain rates and crevasse locations observed circa 1995 at Worthington Glacier, USA2. The cross-cutting of relict crevasses by active crevasses indicates relative crevasse chronologies can exist at a single point on a glacier.

Numerical modeling – While crevasses have conventionally been assumed to form perpendicular to principal extending stresses on glaciers, we were intrigued to find strong model evidence that non-trivial crevasse curvature and rotation can result when there is substantial shearing (Mode III fracture) acting in addition to the more the common opening (Mode I fracture). The role of such mixed-mode fracture in shaping crevasse geometry has implications for interpreting curved / rotated crevasses as either deformed following opening or in equilibrium with local shear.

Crevasse_Modes

Figure 2 – Schematic illustrating the three modes of fracture: Mode I (opening), Mode II (sliding), and Mode III (tearing).

Remote Sensing – Remote sensing technologies for crevasse detection exhibited remarkable growth over the past 60 years. Real-time crevasse detection for traverse vehicles advanced from Cold War era rudimentary push-broom “dishpans”, which measured bulk electric current density of surrounding ice, to modern fully autonomous rovers capable of executing ground penetrating radar grids. In terms of satellite imagery, crevasses went from being manually delineated in the coarse resolution visible imagery that became available in the 1970s to now being automatically detected by feature tracking algorithms in higher resolution visible and synthetic aperture radar imagery.

CrevassePastPresent

Figure 3 – Left: Cold War era “dishpan” detection system that inferred crevasses from changes in bulk electric current density3. Right: An autonomous ground-penetrating radar unit (Yeti) being used to map near-surface buried crevasses at White Island, Antarctica. (Photo: Jim Lever)

Mass Balance Implications – While many studies have described individual mechanisms by which crevasses can influence glacier mass balance, we wanted to provide an overview of all the possible mechanisms, and we were fortunate enough to have a graphic artist help us do it in a single schematic. The mass balance implications of crevasses contain several counter-intuitive nuances. For example, crevasses can enhance basal sliding in the accumulation area and suppress basal sliding in the ablation area. Given their myriad mass balance implications, however, crevasses may serve as both indicators and agents of changing glacier form and flow.

Crevasse_Summary

Figure 4 – Schematic overview of the various processes through which crevassed surfaces influence glacier mass balance relative to non-crevassed surfaces: (1) increased solar energy collection and enhanced surface ablation, (2) increased turbulent heat fluxes and enhanced surface ablation, (3) decreased buried crevasse air temperatures and suppressed ice deformation, (4) increased bulk glacier porosity and enhanced ablation area water retention, (5) increased supraglacial lake drainage and suppressed accumulation area water retention, (6) increased supraglacial lake drainage and enhanced ice deformation, (7) attenuated transmission of hydrologic variability (relative to moulins) and suppressed basal sliding velocities, (8) increased cryo-hydrologic warming of ice temperatures and enhanced ice deformation, (9) increased water content / hydraulic weakening and enhanced ice deformation, and (10) iceberg calving.

1Colgan, W., H. Rajaram, W. Abdalati, C. McCutchan, R. Mottram, M. S. Moussavi and S. Grigsby. 2016. Glacier crevasses: Observations, models, and mass balance implications. Reviews of Geophysics. 54: doi:10.1002/
2015RG000504.

2Harper, J., N. Humphrey and W. Pfeffer. 1998. Crevasse patterns and the strain-rate tensor: A high-resolution comparison. Journal of Glaciology. 44: 68–76.

3Mellor, M. 1963. Oversnow Transport. Cold Regions Science and Engineering. Monograph III-A4. 104 pages.

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Glacier Crevasses: Searching for Curious Factoids

Posted by William Colgan on June 01, 2015
Glaciology History / 1 Comment

Along with some co-authors, with whom I am preparing a review paper about glacier crevasses, I am currently searching for a citation for the “deepest air-filled crevasse depth” measured to date. Although there seems to be some anecdotal assertions of 50 m deep crevasses in popular literature, presently, the deepest measured air-filled crevasse depth we have come across in the peer-reviewed literature is a third-hand account of a crevasse rescue in Palmer Land, Antarctica, in 1947, where crevasse depth is noted as “110 feet” (or 34 m). The rescue, one of many briefly recounted in Schuster and Rigsby (1954), reads:

Deepest_Crevasse_Report

One of many crevasse rescues recounted in Schuster and Rigsby (1954).

We presume that someone, somewhere, must have measured a deeper air-filled crevasse depth. I should note, we are aware that deeper crevasse depths have been inferred (rather than actually measured). For example, Hambrey (1976) suggests that the advection of crevasse traces c. 40 years down-glacier from their crevasse field of origin, where surface ablation averages c. 2 m/a, would infer that the fracture tips of crevasses reach c. 80 m depth within the crevasse field. Mottram and Benn (2009) recount the obvious challenge in accurately measuring the depth of an almost infinitely tapering fracture! For the purpose of our review paper, we are most interested in bona fide measurements, such as those made by either ranging devices or rappelling personnel, rather than someone just looking into the abyss and estimating “about X m deep”.

We are quite eager to see if anyone can point us in the direction of a deeper air-filled crevasse measurement. Naturally, we would also welcome (and duly attribute!) any other curious crevasse factoids or photographs that might be suitable for spicing up our meandering tour through the past seventy years of glacier crevasse literature. For example, we think we have identified the widest documented regularly spaced crevasse (air-gap width of 33 m!), which was observed in 1955 by Meier et al. (1957) at Blue Ice Valley, Greenland. We must admit, however, that we do most of our learning in the peer-reviewed literature, so we suspect that more adventurous souls (who might actually do some learning in crevasses!) may possess some alternate knowledge!

C3_Meier57_1_small

Thanks to some graphic assistance from Cheryl McCutchan (animediascience.com), we can merge strain rate and surface morphology maps in older studies, like this depiction of a 33 m wide crevasse at Blue Ice Valley, Northwest Greenland, from Meier et al. (1957).

Hambrey, M. 1976. Structure of the glacier Charles Rabots Bre, Norway. Geological Society of America Bulletin. 87: 1629-1637.

Meier, M., J. Conel, J. Hoerni, W. Melbourne, C. Pings and P. Walker. 1957. Preliminary Study of Crevasse Formation: Blue Ice Valley, Greenland, 1955. Snow, Ice and Permafrost Research Establishment. Report 38.

Mottram, R. and D. Benn. 2009. Testing crevasse-depth models: a field study at Breiðamerkurjokull, Iceland. Journal of Glaciology. 55: 746-752.

Schuster, R. and G. Rigsby. 1954. Preliminary Report on Crevasses. Snow, Ice and Permafrost Research Establishment. Special Report 11.

Twitter: @GlacierBytes

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