the world's no.1 science & technology news service
canada's on the up - us is going down
17:37 20 may 04
newscientist.com news service
the melting of a huge ice sheet has caused canada to bounce back (image: sella et al)
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.
to compile the map, giovanni sella of northwestern university in evanston, illinois, and colleagues added 10 years of gps (global positioning satellite) data obtained in 2003 from natural resources canada to the data they already had from the us national geodetic survey.
the data document how the height of more than 200 locations scattered across both countries is changing. "we have a signal that's really quite dramatic" says sella.
previous surveys of north america have measured the up-and-down movements of the plate. but they were far less extensive, with only a handful of data points to cover the vast landmass of canada.
the researchers also believe the new map could help explain some of the most enigmatic earthquakes that strike north america.
earthquakes usually occur at plate boundaries or faults, where two masses of rock grind past each other. however, there are seismically active regions towards the eastern edge of the us and canada that are far from plate boundaries and appear to be fault-free.
seismologists speculate that the rebounding of the north american plate may cause these "intra-plate" quakes. "this has been postulated, now we finally have data that we can put into the models" says sella.
ultimately, says team-member seth stein, also from northwestern university, the data may help seismologists to assess the earthquake risk in different regions. "that's one of the prime reasons for getting at this problem" he says.
the map was presented at the joint assembly of the american geophysical union in montreal, canada, on wednesday.
jenny hogan, montreal
bet you thought this was going to be about either economics or politics!!!
May 23, '04
I'd heard that about Chicago.....
Chicago feels chill of the Ice Age as geologists say Windy City is sinking
By David Usborne in New York
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
Last edit by donmurray on May 23, '04