3) obtained by burying thermistors just below the surface of a pile of sand. The mean rock surface temperature is based on experimental values of dry-sand temperatures ( Fig. With the exception of the mean rock surface temperature, the meteorological data are based directly on observations on Bersaerkerbrae. The remaining physical values that were used are given in Table I. The equivalent height was taken as the mean thickness of the rock. This method avoids misleading values which large protuberances or incisions might produce. To apply the model to a particular rock the dimensions of the equivalent cylinder are required these were obtained by finding the diameters of the maximum inscribed and minimum circumscribed circles that would fit the rock’s projected upper face, the equivalent diameter was then taken as the mean of these two diameters. During this period meteorological data were collected on this glacier and the rocks were inspected weekly to determine whether they were perching or sinking, to aid this inspection a photographic record of the rocks was kept. The above mathematical model has been applied to twelve large rocks on Bersaerkerbrae which were observed during July and August 1982. To compare the exposed and the rock-shielded ice, diurnal fluctuations in the energy balances can be eliminated by integrating over a 24h period.Īpplication of Model to Rocks on Bersaerkerbrae, Greenland It therefore seems necessary at this similar site to consider the effects of radiation and convection on ablation. Reference Lister and TaylorLister and Taylor (1961) have shown that for Britannia Gletscher (lat.77° 14' N, long.23° 49' W.) in north-east Greenland the relative importance to ablation of the energy fluxes was Heat will be lost due to out-going long-wave radiation and due to eddy conduction if the temperature gradient is reversed. the latent heat of condensation of water vapour and of freezing of rain. Eddy conduction if the temperature gradient above the ice surface is positive and the air is turbulent.ģ. Solar radiation and long-wave radiation.Ģ. To quantify this process the energy exchanges at the glacier surface have to be accounted for. If the rock reduces the nett energy flux to the ice beneath it as compared to the exposed glacier then the rock will perch, because the ice under the rock will melt more slowly than the exposed ice. The effect of pressure melting due to the weight of the rock can be neglected because the rocks are in general too small to produce a significant pressure on the ice. 24° 40' W.) in north-east Greenland.Ī Mathematical Model for the Formation of Perched Blocks This study has aimed to produce a reliable method of predicting whether a rock will sink or perch and to compare it with experimental data from Bersaerkerbrae (lat. To the author’s knowledge no previous work has been done on this subject. This was caused by pedestals growing underneath them and the rock then falling off the pedestal.Ī structure erected on a glacier or ice cap might also be expected to behave in a similar manner to isolated rocks, which may have more serious consequences than the movement of survey markers. Allen (personal communication in 1981) observed during a survey of Roslin Gletscher that rocks used as markers by previous expeditions were moving relative to the glacier surface. This behaviour has been studied as a result of a problem encountered during the 1978 Sheffield University North-East Greenland Expedition. The pedestal may be further shaped by the action of melt water around it. This is due to the difference in ablation between the exposed glacier surface and the ice beneath the rock. It is easily observed that rocks on a glacier either sink into the ice or grow pedestals underneath them. Perched blocks are isolated rocks or boulders which are supported above the glacier surface on pedestals of ice.