Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Glacier tables are structures frequently encountered on temperate glaciers. They consist of a rock supported by a narrow ice foot which forms through differential melting of the ice. In this article, we investigate their formation by following their dynamics on the Mer de Glace (the Alps, France). We report field measurements of four specific glacier tables over the course of several days, as well as snapshot measurements of a field of 80 tables performed on a given day. We develop a simple model accounting for the various mechanisms of the heat transfer on the glacier using local meteorological data, which displays a quantitative agreement with the field measurements. We show that the formation of glacier tables is controlled by the global heat flux received by the rocks, which causes the ice underneath to melt at a rate proportional to the one of the surrounding ice. Under large rocks the ice ablation rate is reduced compared to bare ice, leading to the formation of glacier tables. This thermal insulation effect is due to the warmer surface temperature of rocks compared to the ice, which affects the net long-wave and turbulent fluxes. While the short-wave radiation, which is the main source of heat, is slightly more absorbed by the rocks than the ice, it plays an indirect role in the insulation by inducing a thermal gradient across the rocks which warms them. Under a critical size, however, rocks can enhance ice melting and consequently sink into the ice surface. This happens when the insulation effect is too weak to compensate for a geometrical amplification effect: the external heat fluxes are received on a larger surface than the contact area with the ice. We identified the main parameters controlling the ability of a rock to form a glacier table: the rock thickness, its aspect ratio, and the ratio between the averaged turbulent and short-wave heat fluxes.
Highlights
We develop a simple analytical 1-D heat conduction model accounting for the various mechanisms of the heat transfer on the glacier using local meteoreorological data, and which displays excellent agreement with the field measurements
We develop a 1D conduction model taking into account the effect of the solar irradiation as well as sensible and longwave heat fluxes, which is in excellent agreement with the field measurements and which illustrates 55 the synergistic effect between solar irradiation and sensible flux responsible for glacier table formation
As this seems to be well verified in the case of rock 4 whose temporal changes in surface temperature are well captured by the 170 model, this is justified only if the rocks are not too thick compared to the thermal skin depth δdiff = TdayDrock/π = 0.2 m where Drock = 1.4×10−6 m2·s−1 is the thermal diffusivity of the rock and Tday = 24 h is the period of variation of the external heat flux (i.e. Φ(t))
Summary
A wide variety of spectacular shapes and patterns formed through differential ablation can be found in Nature: surface patterns 15 known as rillenkarren result from the dissolution of soluble rocks (Cohen et al, 2016; Claudin et al, 2017; Cohen et al, 2020; Guérin et al, 2020), mushroom rocks undergo erosion of their base by strong particle-laden winds (Mashaal et al, 2020), and hoodoos which consist of a hard stone protecting a narrow column of sedimentary rock from rain-induced erosion (Young and Young, 1992; Bruthans et al, 2014; Turkington and Paradise, 2005). The study focused on the initial behavior of pattern 45 formation using cylindrical "rocks" of various sizes, aspect ratio and materials, initially resting on a flat ice surface This small scale study under controlled conditions allowed one to understand the physical mechanisms that could play a role glacier table formation, it did not encompass the complexity of the energy balance on a natural glacier, in particular the effects of the direct solar irradiation and of the wind. 60 The lower part of the Mer de Glace (below a 2000 m altitude) is largely covered with granitic debris, with sizes ranging from submillimetric up to several meters In the following, their dimensions are characterized by their thickness e and their larger and smaller widths d1 and d2
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
