Introduction to Earth Sciences I
1.3.3. The Earth Bends When A Weight Is Placed On It, Continued
Massive Ice Sheets -
Above we noted that the weight of volcanoes can be used to give a measure of the Earth's strength which is an elastic property. Removing the weight would cause the Earth to return to its original shape.
The Earth has also done this experiment. During the last ice age huge ice sheets covered much of Europe and North America. Manhattan was under about a mile of ice. Their weight depressed the crust beneath just as Hawaii depresses the crust. As the current inter-glacial period began the ice sheets melted and retreated back to their present position. In doing so they released a massive weight from the Earth. That weight had depressed the layers beneath as it presently does in Greenland and Antarctica where the rock surface beneath the ice is actually well below sea level.
When the ice sheets were present the Earth was depressed; as they left the Earth was uplifted. But they did not rebound instantaneously. They rebounded slowly as the deep layers flowed beneath. The rate of uplift - fast or slow - is a measure of the ability of the deep layers to flow.