Water

How much does ice weigh?

Like an object that floats on water, analogously the Earth’s crust “floats” in equilibrium on the viscous plastic rocks of the underlying mantle. A decrease in the weight of the crust, caused, for example by the removal of rocks due to erosion, makes the rocks lighter and the crust rises, while an increase in the weight makes the crust sink even deeper into the “soft” and viscous mantle, by a process called isostasy. The formation of thick layers of ice, (as in the glaciations of the past), causes an overload on the ice covered crust, and the result is that it sinks into the mantle, various hundreds of metres, in some cases even below sea level. Knowing the average density of ice and its thickness, it is easy to calculate its weight at the base. At present, due to the weight of the ice-sheet, which reaches 4.5 km in some parts, the Antarctic has sunk over 900 m. Radar measurements carried out in Greenland show that one third of the rock base is below sea level and the weight of the accumulated ice has pushed the rock downwards by over 600 m in some parts. As the large ice-sheets retreat after the last glaciation, the territories that are freed of the weight of the ice have started to rise. For example, the region around the Hudson Bay has risen over 300 m in a little over 10,000 years after the Laurentide ice-sheet retreat. This rise is not over as the territory still has not reached its height before the last glaciation. Also the Scandinavian peninsula is still rising at a rhythm that reaches 9 mm per year in the middle of the Gulf of Bothnia. The delay in the response to the removal of the load is due to the viscosity of the material of the mantle, which has a certain amount of inertia. The rise after the end of the last glaciation is greatly camouflaged by the rise in the sea level as a consequence of the melting of large quantities of continental ice.