The age of ice

Glacier ice is far from being a homogenous material and  is usually stratified in layers, due to the progressive annual accumulation of layers of snow of various  thicknesses: the older parts can be found at the base and the younger  ones closer to the surface. Usually summer ice has a glassier appearance, often full of dark dusts and it is less thick, while winter ice is white and thicker. In a way similar to that used to measure the yearly growth in trees by the number of rings, it is possible to ‘count’ the different layers and therefore infer the number of years. This method, however, can be used only until the growing pressure within the glacier cancels the ice stratigraphy. For the old  and deep  ice, indirect and more complex methods are used, taking into consideration the air bubbles trapped in ice. The radiocarbon method utilizes carbon 14C contained in trapped carbon dioxide in a way similar to that used to date  handicrafts  and organic material, but this method is rarely utilized due to the great quantity of material  required . The most frequently utilized method analyses the heavy isotopes of oxygen present in the air bubbles. For the older and deeper layers other methods must be used based on mathematical models of the ice flow. The finding of levels rich in dust, particularly volcanic ashes, is important: when it is possible to associate them with a known volcanic eruption , it is possible to give a precise age to the layer level at which they have been found.

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