22 May 2004

The United States is sinking while Canada is on the rise. This has nothing to do with political rivalry between neighbours and everything to do with geology.

There is a see-saw motion going on across the North American continent and it is the southern superpower that is on the downward tilt. This is the startling finding of a group of scientists at Northwestern University in Chicago, based on global positioning satellite readings taken over 10 years at 200 points across the American continent. The Windy City is just on the wrong side of the hinge. It is dipping at the rate of one millimetre a year.

Explaining this unlikely phenomenon is the history of the Ice Age. The researchers argue that when most of the territory from the Great Lakes northward was under the weight of ice that was about a mile thick the crust of the Earth was being pressed down. When the ice started to melt about 18,000 years ago, a rebound began to occur and is still going on today.

More specifically, it was a layer of semi-molten rock below the crust of the planet that was sent oozing sideways when the ice was at its heaviest. Once pressure from the glaciers started to be relieved, the rock began to slide back into place, pushing the formations above it slowly upwards again.

"All of Canada's going up," said Professor Seth Stein of Northwestern. "The US is going down."

While a millimetre a year seems modest, that translates into a downward motion for Chicago of four inches a century. Moreover, there is a potentially even more significant effect on water levels in Lake Michigan. As the motion continues, more water is gathering at the lake's southern end where Chicago sits.

"Water is moving from the Canadian side, slowly but surely, to the US side," commented Chuck Southam of Environment Canada, which studies water levels in the Great Lakes. "Over a century, it's got quite an effect." Not such a large one that anyone is calling Chicago the Venice of America. Not yet.

Meanwhile the see-saw effect becomes more distinct the further north you travel in Canada away from the hinge. The researchers believe that the northern shores of Lake Superior have reared up by as much as 18 inches in the past 100 years. The shores of Hudson Bay may be up by three feet.

"Basically, everything north of the Great Lakes is going up, with the speed of that uplift increasing the closer you get to Hudson Bay," confirmed Professor Stein. A map showing the degrees of change also suggests that areas of the eastern US south of Washington DC are sinking by 5mm a year.

He credits the newly available GPS data for revealing the phenomenon. "Before, we had no idea what the pattern looked like," he said at a press conference in Montreal last week. Now, there is no mistaking it.

Scientists are suggesting that the tilting of the Earth's crust and the travelling back into old positions of the semi-molten rock, mostly made up of magnesium, iron, aluminium and silicon, may offer an explanation for the occasional, mostly minor, earthquakes and tremors that occur in the east of the continent that cannot be explained by the grinding of tectonic plates.

http://news.independent.co.uk/world/americas/story.jsp?story=523719