Borehole‐Based Characterization of Deep Mixed‐Mode Crevasses at a Greenlandic Outlet Glacier

Abstract

Surface crevassing contributes to bulk ice motion and enables the transfer of water and its thermal energy from the surface of an ice mass to its subsurface (Colgan et al., 2016). Theoretically, creep closure limits the depth of dry crevasses to some tens of meters (see Mottram & Benn, 2009;Nye, 1955), but this can be increased substantially by hydrofracturing of water-filled crevasses (van der Veen, 1998;Weertman, 1973). Once initiated, hydrofracture extends the tip and, if enough water is available, propagation can continue to the glacier bed. This mechanism has been used to explain the rapid transfer of meltwater to the glacier bed during discrete surface lake drainage events (e.g.,

Structures and Deformation in Glaciers and Ice Sheets

“…It is also possible to investigate the internal structure of valley glaciers, ice sheets, and ice shelves, by using a hot-water drill to create a borehole, which is then imaged and logged using an optical televiewer (e.g., Ashmore et al, 2017;Hubbard & Malone, 2013;Hubbard et al, 2008Hubbard et al, , 2021Roberson & Hubbard, 2010). The scanned image of the borehole is transformed to appear as a core; in this way structures such as primary stratification, crevasse traces, folds, and foliations can be measured.…”

“…However, this mechanism does not explain the apparently great depths to which crevasse traces penetrate. Borehole logging by optical televiewer of a crevassed zone of Store Glacier, a fast-flowing outlet glacier draining the Greenland Ice Sheet, revealed the presence of crevasse traces to a depth of 265 m (Hubbard et al, 2021). Furthermore, surface fracturing was inferred to extend to c. 400 m below the surface of the glacier, as indicated by excess ice temperatures.…”

Controls on Water Storage and Drainage in Crevasses on the Greenland Ice Sheet

“…However, the net effect of this on ice dynamics has yet to be identified. Additionally, crevasses that remain ponded and then refreeze at the end of the season will release latent heat and facilitate ice warming (Lüthi et al, 2015) at depths of up to hundreds of meters (Hubbard et al, 2021).…”

“…Indeed, the use of simple stress thresholds to predict hydrological behavior is common across studies of GrIS supraglacial hydrology (Clason et al, 2015;Everett et al, 2016;Koziol et al, 2017;Poinar et al, 2015;Williamson, Banwell, et al, 2018;Williamson, Willis, et al, 2018). However, these thresholds are identified from observational studies performed with the aim of identifying suitable predictors of crevasse presence, not of crevasse hydrology (Hambrey & Müller, 1978;Harper et al, 1998;van der Veen, 1998;Vaughan, 1993); they may not even be suitable for that purpose (Mottram & Benn, 2009), as ice fracture is increasingly understood to be complex and multi-dimensional (Colgan et al, 2016;Hubbard et al, 2021;van der Veen, 1999). To date, no observational studies exist to support the use of any such controls, stress or otherwise, on the hydrological behavior of crevasses.…”

Controls on water storage and drainage in crevasses on the Greenland Ice Sheet

“…Indeed, the use of simple stress thresholds to predict hydrological behavior is common across studies of GrIS supraglacial hydrology (Clason et al, 2015;Everett et al, 2016;Koziol et al, 2017;Poinar et al, 2015;Williamson, Banwell, et al, 2018;Williamson, Willis, et al, 2018). However, these thresholds are identified from observational studies performed with the aim of identifying suitable predictors of crevasse presence, not of crevasse hydrology (Hambrey & Müller, 1978;Harper et al, 1998;van der Veen, 1998;Vaughan, 1993); they may not even be suitable for that purpose (Mottram & Benn, 2009), as ice fracture is increasingly understood to be complex and multi-dimensional (Colgan et al, 2016;Hubbard et al, 2021;van der Veen, 1999). To date, no observational studies exist to support the use of any such controls, stress or otherwise, on the hydrological behavior of crevasses.…”