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

R Nishiyama,F Schlunegger,A Ariga,M Vladymyrov,P Scampoli,D Mair,P G Valla,T Ariga,A Lechmann,A Ereditato,C Pistillo

doi:10.1038/s41598-019-43527-6

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.

Highlights

Glaciers play an important role in limiting the height of mountain ranges and in shaping alpine-type landscapes, which are commonly characterized by U-shaped valleys, cirques and steep-edged ridges along their thalwegs[1,2,3,4]
We explored the morphology of the Eiger glacier, which is the target of our survey (Fig. 1)
The Eiger glacier consists of four morphologic domains I-IV (Fig. 1): (I) a headwall reach forming a concave amphitheatre-shaped cirque with c. 50° steep flanks, (II) a relatively flat domain, and (III) a middle segment where a prominent bedrock ridge forces the ice flow to diverge over c. 250 m distance before converging again farther downslope

Summary

Introduction

Glaciers play an important role in limiting the height of mountain ranges and in shaping alpine-type landscapes, which are commonly characterized by U-shaped valleys, cirques and steep-edged ridges along their thalwegs[1,2,3,4]. In the present work we apply the cosmic-ray muon radiography technology to map the lateral margin of the Eiger glacier (Fig. 1) situated in Switzerland (Central European Alps at 46°34′05′′N latitude and 7°59′56′′E longitude) This method is based on the high penetration power of cosmic-ray muons that hit the Earth’s surface continuously[30], where the attenuation rate of the muon flux mainly depends on the density of the traversed material[31]. In 2015, Nishiyama et al.[31] launched a pilot survey for the application of such a technology in an Alpine environment using emulsion films as muon detectors These authors[31] successfully mapped a small portion of the bedrock underneath the uppermost part of the Aletsch glacier (Central European Alps) over an area of c. The observed density information is quantitatively converted to the location of the boundary separating the low-density ice from the high-density bedrock[31], providing a unique and high-resolution reconstruction of the bedrock topography at the bottom and lateral margins of the Eiger glacier

Methods

Results

Conclusion