Water

The “material” ice

Ice has singular physical properties that condition all the processes that take place on the surface and within a glacier. At ambient pressure, ice is a very fragile material. If it is subjected to mechanical stress such as compression or distortions it reacts and it forms fractures and breaks in a fragile manner (to verify this try dropping an ice cube : it will break into a myriad of splinters that will melt rapidly on your kitchen floor). In high pressure conditions, as for example within a glacier, or as a result of stress applied very slowly, on the contrary, ice is plastic, and deforms and twists continuously, without forming any fractures (as with a packet of plasticine), in order to prove this, try the classic experiment of stretching a thread with two weights on each end, on an ice cube – slowly the thread will penetrate the ice cube, and the borders where the thread passes will weld together as the thread progresses downward, crossing the ice cube completely without leaving any traces of its passage. Therefore ice in a glacier behaves very differently on the surface and deeper under. This may seem of little importance, but it is fundamental for water to circulate and for storage of water reserves within the glacier. Ice, in fact, is an impermeable material, that does not allow water to pass : however it becomes permeable, enabling water circulation, when it is fractured. The fragility of the superficial layers is also responsible for the better known morphologies of the surface of a glacier : crevasses and seracs, immense fractures that at times make it very difficult and dangerous to cross glaciers. Often described as “bottomless abysses”, actually due to the physical characteristics described above they rarely reach a depth over 40-50 m (not much in a glacier where the ice thickness is over 800 m, as in the case of the Aletsch Glacier, but sufficient to cause reverent fear). Direction of crevasses and fractures depends on tensions originated in the ice in response to irregularities in the bedrock and friction along the walls, and can be useful to reconstruct the trend of the underlying rock and to evaluate the ice thickness. For example, fields of seracs, large “cascades” of blocks of ice that are intensely crevassed may indicate a sudden variation in the inclination of the bedrock, or a rocky “threshold”, that may indicate the presence of subglacier lakes, which could be extremely dangerous due to their instability.