Ice aprons on steep high-alpine slopes: insights from the Mont-Blanc massif, Western Alps

Abstract

Ice aprons are defined as very small ice bodies covering steep rock slopes. They have only been the subject of increased scientific interest for a few years, despite the fact that they are a condition for mountaineering and obvious elements in the high-alpine landscapes. However, very little is known about their distribution, evolution and physical characteristics. In this paper, we review the existing knowledge on ice aprons, which have almost exclusively been investigated in the Mont-Blanc massif, Western Alps. We supplement this review with novel results from recent surveys of ice aprons. We used a wide array of methodologies, from remote sensing (multi-source imagery) to in situ (stakes and thermal monitoring) and laboratory (radiocarbon dating and texture analysis) glaciological investigations. In the Mont-Blanc massif, ice aprons occupy 4.2 km2 within the alpine permafrost zone. Temperature measured at the ice–rock interface is indeed largely negative. Thinness of ice aprons coupled with the cold context implies a quasi-stationary shear regime without basal Sliding. Only ice at the surface can possibly melt in warm periods. After a shrinking period from the end of the Little Ice Age to the mid-to-late-1960s, ice aprons experienced a short period of expansion, followed by an accelerated shrinkage since the beginning of the 21st century. This shrinkage now favours rockfall triggering and poses a serious threat to a glaciological heritage since ice aprons host several-thousand-year-old ice. Finally, we synthesize this information to assess the existing definition of ice aprons, and propose some future research directions.

18-years of high-Alpine rock wall monitoring using terrestrial laser scanning at the Tour Ronde east face, Mont-Blanc massif

“…The center of the rock wall can be divided in two parts: the upper part and lower part. The upper part, formerly covered by an ice apron (IA hereafter, figure 2(b)), defined as small ice bodies lying on slopes >40 • (Guillet and Ravanel 2020, Kaushik et al 2022, Ravanel et al 2023, depicts a complex pattern formed by a block chaos lying on a complex substratum. The lower part shows at least three fracture sets: one very steep with a N-trend, one parallel to the sub-vertical N70 • E major faults, and one almost horizontal.…”

“…When shrinking, glaciers and IAs free boulders and rock masses become unstable (Krautblatter et al 2013). Ravanel et al (2023) found that after the 2003 and 2015 summer heatwaves, rockfall deposits heavily accumulated at the foot of recently deglaciated areas in the MBM due to retreat of IAs. Permafrost degradation at TREF was enhanced by the reduction of the IA surface area.…”

“…ory and sometimes triggering factor to the rockfall occurrence(Gruber and Haeberli 2007) Legay et al (2021). andRavanel et al (2023) concluded that shallow rockfall sources (<4-6 m depth) would result from daily and seasonal freeze/thaw cycles within the active layer, whereas active layer thickening due to permafrost degradation would involve deeper source areas. The increase in the mean rockfall thicknessfrom 1.5 to 2.1 m at TREF suggests a response to the 1 • C increase of the mean annual air temperature (MAAT) from 2006-2008-2021-2022.…”

Using Borehole Temperatures for Knowledge Transfer about Mountain Permafrost: The Example of the 35-year Time Series at Murtèl-Corvatsch (Swiss Alps)

“…As part of this important ongoing activity and progress, Philip Deline has built up an internationally recognised research unit carrying out pioneering work on characteristics, processes and climate-change impacts related to the coldest, steepest, highest and logistically most challenging mountain belts. The state-of-the-art review by world-leading experts on "Ice loss from glaciers and permafrost and related slope instability in high-mountain regions" (Deline et al, 2021) as well as a continued series of unique specific publications on the topic (e.g., Ravanel and Deline, 2010;Bodin et al, 2015;Magnin et al, 2015Magnin et al, , 2017Ravanel et al, 2017Ravanel et al, , 2023 document the precious contribution accomplished by the "Chambéry team". The following thoughts are devoted to Philip in recognition of, and gratefulness for this admirable achievement.…”

Climbing Mont Blanc (4808 m a.s.l.): What Are the Risks in the Grand Couloir Du Goûter and On the North Face of Mont Blanc Du Tacul?

“…The eastern margin of the north face is occupied by a cold-based hanging glacier (Pralong and Funk, 2006). It is surmounted by an ice apron (Ravanel et al, 2023). Immediately to the West is the Triangle du Tacul (3970 m a.s.l.)…”