Reduced gravity in North a legacy of Ice Age

Last Updated: Friday, May 11, 2007 | 3:57 PM CT

CBC News

Moving to northern Canada won't make it any easier to fit into your old swimsuit, but it will help fool your bathroom scale into thinking you've lost a gram or two, thanks to a lower force of gravity in some regions.

That's according to researchers studying the impact of a massive glacial sheet that covered Canada during the last Ice Age.

According to a study published Friday in the journal Science, the crushing weight of a massive glacier more than three kilometres thick caused rock beneath to compress and sink, displacing some of the semi-fluid mantle underneath the Earth's crust.

This created two areas in northern Canada — on either side of Hudson Bay — where the mass of the Earth underfoot was less than elsewhere on Earth, which translates into a reduced pull of gravity.

Naturally, the mass of any object wouldn't change, but its weight — defined as its mass times the acceleration of gravity — would be affected.

The difference in gravity is minuscule, about four-thousandths of one per cent in the regions where the effect is most pronounced.

Scientists from the University of Toronto and Harvard University looked at data taken from the Gravity Recovery and Climate Experiment (or GRACE), twin satellites launched in 2002 that take detailed measurements of the Earth's gravitational field.

"What these satellites allow us to do is measure tiny, minuscule ripples or changes in that gravity field, and that has never been possible before," said Jerry Mitrovica of the University of Toronto and co-author of the research.

"Now we have whole maps over Canada that say here's how the gravity field over Canada is changing," he said. "I think we've now nailed the Ice Age, at least what the Ice Age looked like over Canada."

The results give new insight into the shape and size of the glaciers that helped carve the Canadian landscape, and also add new information about the movement of the planet's mantle, the thick region that lies beneath the Earth's crust and above its core.

The scientists said the shifting movements of the crust and mantle will eventually cause the unique gravitational conditions in the North to disappear, but not for a long time yet.

The pull of gravity is increasing slightly in the areas as the crust slowly rises and the mantle refills the cavity underneath, a process that is expected to take up to 300,000 years.