Bedrock sculpting under an active alpine glacier revealed from cosmic-ray muon radiography

Abstract

Mountain glaciers form landscapes with U-shaped valleys, roche moutonées and overdeepenings through bedrock erosion. However, little evidence for active glacial carving has been provided particularly for areas above the Equilibrium Line Altitude (ELA) where glaciers originate. This is mainly due to our lack of information about the shape of the bedrock underneath active glaciers in highly elevated areas. In the past years, the bedrock morphology underneath active glaciers has been studied by geophysical methods in order to infer the subglacial mechanisms of bedrock erosion. However, these comprise surveys on the glaciers’ surface, from where it has been difficult to investigate the lateral boundary between the ice and the bedrock with sufficient resolution. Here we perform a muon-radiographic inspection of the Eiger glacier (Switzerland, European Alps) with the aid of cosmic-ray muon attenuation. We find a reach (600 × 300 m) within the accumulation area where strong lateral glacial erosion has cut nearly vertically into the underlying bedrock. This suggests that the Eiger glacier has profoundly sculpted its bedrock in its accumulation area. This also reveals that the cosmic-ray muon radiography is an ideal technology to reconstruct the shape of the bedrock underneath an active glacier.

Alpine rockwall erosion patterns follow elevation-dependent climate trajectories

“…In general, rockfalls were prevalent in proximity to the extant glacier, with 77 rockfalls or 29% of all measured events, including four >10 m 3 sourced from rockwalls deglaciated between 1964 and 2019. Enhanced rockfall erosion in proximity to a retreating cirque glacier margin has been observed elsewhere 44 for which paraglacial processes such as thermo-mechanical stresses 13 , unloading stresses amplified by cirque glacier erosion 12,82 , or increased frost cracking within a bergschrund 83 may play a role. While we do not know which processes operate at the Hungerli Valley, the rockwall erosion rate patterns, at least partly, appear to reflect paraglacial adjustment 9,84 .…”

The hydrology of glacier‐bed overdeepenings: Sediment transport mechanics, drainage system morphology, and geomorphological implications

“…Hallet et al ., 1996) of c . 5 mm a −1 if assuming a bedrock density of 2.7 g cm −3 (Nishiyama et al ., 2019) and glacier area of 13 km 2 (Feiger et al ., 2018). Such a value greatly exceeds regional rock uplift rates and local rates indicated by sediment yields (e.g., Stutenbecker et al ., 2018; Delaney et al ., 2018).…”

Alpine Rockwall Erosion Patterns Follow Elevation-Dependent Climate Trajectories

“…In general, rockfalls were prevalent in proximity to the extant glacier, with 77 rockfalls or 29% of all measured events, including four >10 m³ sourced from rockwalls deglaciated between 1964 and 2019. Enhanced rockfall erosion in proximity to a retreating cirque glacier margin has been observed elsewhere 44 for which paraglacial processes such as thermo-mechanical stresses 13 , unloading stresses amplified by cirque glacier erosion 12,84 , or increased frost cracking within a bergschrund 85 may play a role. While we do not know which processes operate at the Hungerli Valley, the rockwall erosion rate patterns, at least partly, appear to reflect paraglacial adjustment 9,86 .…”