Water

Glacier movement

Glacier movement is not uniform throughout its mass and it is not even constant in time. The speed of the movement is slower near the walls and the base, where the glacier is slowed down by friction with the bedrock, and maximum in the central zones, where friction is minimum and ice thickness is maximum. Different speeds may be noted also at the confluence of two ice snouts, usually marked by a “floating” medial moraine, a long strip of debris on the sides of the glacier snouts along the entire length of the ablation area. If there is melted water at the base, the glacier moves more rapidly. Therefore, temperate glaciers are the ones that “shift” more, while those with a cold base may remain “anchored” to the frozen bedrock and move very little, or in “jerks”, as along a fault. Glacier speed varies greatly depending on the characteristics of the ice and the bedrock: from a few metres a year to many hundreds of metres per year. One of the fastest glaciers is the Columbia Glacier in North America that, since the Seventies, has been moving at a speed of 24 metres per day. Intuitively one may think that this unrestrainable movement pushes the glacier front more and more downstrem – the faster the ice is, the more it should move forward. Therefore continuous observation of the position and shape of the glacier front in time should give us reliable indications on its state of advance and retreat. But actually the situation is much more complex. Even when the front is stable, apparently still, the glacier continues to move downstream: the fact that the front does not move means that the ice lost by ablation is continually replaced by new ice coming from the accumulation area, at the same rhythm at which it is being lost. In case of the Columbia Glacier, its high speed would lead us to think of its rapid advancement, however the large losses on the front make it a glacier with an overall retreat - since 1982 its retreat has been 14 km.