Canada is on the up and the US is going down, according to the most detailed map ever made of the vertical movements of the North American plate. The height of some places is changing as fast as 10 millimetres a year.

The movements began more than 18,000 years ago, with the melting of a mighty ice sheet that stretched from the North Pole to below the current Canadian-US border.

The weight of the ice had pushed the northern part of the North American plate down, with the southern part tilting up in response. When the ice melted, the plate began to rebound, but only as fast as the mantle beneath it could move. The process continues today because mantle rock is very slow-flowing.

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