How and how fast glaciers slide: decisive factors are the thickness of the ice and the unevenness of the rock bed

The 79 N glacier, Greenland. The image shows an area of fast glacier flow with a high percentage of sliding friction. Photo: Julia Christmann, AWI

Computer modell of Jülich physicist Bo Persson shows that ice with more than 1000 m tickness flows on a lubricating film of water

Jülich physicist Bo Persson has presented a theory on the sliding of glacier ice over rocky ground. Among other things, it describes the influence that water-filled cavities between ice and subsurface have on the sliding speed. Glaciologists could use Persson’s theory to improve the computer models they use to predict the flow rate and melting of glaciers.

In the past, there had been some unanswered questions. For example, it was not known exactly how the unevenness of the rock bed affects the friction of the ice layers above it and, with that, the speed of the flow of the glacier.

Bo Persson from the Peter Grünberg Institute in Jülich, Germany has now contributed to a fundamental understanding of the sliding process. He used a theory of contact mechanics, which he developed himself and has already applied successfully. At the same time, he also took into account the work of other scientists from the 1970s who had, for example, dealt with the phenomenon of regelation: Due to its special chemical structure, ice can melt when the pressure on it increases and freeze again when the pressure decreases.

What is particularly significant in the new mathematical model: „It shows that cavities form when glaciers glide over the ground at a typical speed,“ says Persson. „For glaciers 1,000 m thick and more, such as the polar caps, the temperature of the ice on the glacier floor is at the melting point. The reason for this is the geothermal heat from the earth’s interior, which cannot be dissipated upwards – the ice is too thick. The cavities thus fill with water. This not only forms a kind of lubricating film between the glacier and the ground, which reduces friction. It presses against the layers of ice above and thus carries part of the glacier mass along with it: the glacier slides faster.“

The sliding speed calculated by the researcher by means of his model corresponds to the values observed in nature. This is an important indication that Persson’s model reflects reality well. In principle, the interface between ice and rocky ground is difficult to access for research because the glacier lies above it.

In glaciers the roughness of the bedrock is responsible for the phenomenon of regelation: the alternating melting and freezing of the ice is due to local pressure fluctuations, which in turn are caused by the unevenness of the ground. Persson explains the phenomenon by means of a bump: „The moving glacier pushes the ice against this bump from one side. At higher pressure, however, a lower temperature than usual is sufficient to break up the crystal structure of the ice and melt it.” On the other side of the bump, the pressure of the glacier is reduced and the melting point of the ice increased.

The Journal of Chemical Physics: „Ice friction: Glacier sliding on hard randomly rough bed surface, B.N.J. Persson, J. Chem. Phys. 149, 234701 (2018)

(07.02.2019, USA: 02.07.2019)