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HowStuffWorks
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It's true that a certain part of the Great White North is a little lighter on its feet than surrounding areas. It's not that people are floating off into outer space -- by "little" we mean gravity is decreased about 1/10th of an ounce for a person weighing 150 pounds (68 kilograms) -- but scientists have indeed identified an area of lesser gravity in northern Canada that has vexed them for years. It was thanks to a satellite survey from two highly sensitive NASA satellites called GRACE that the reason for the decreased gravity has been deduced [source: Lovett].
There were essentially two theories for why the gravity might be lessening. The first theory suggested that mantle plates were being tugged, slowly flowing downward. A more recent theory put forth said that during the last Ice Age, glaciers pressed down so much on the Earth's crust so much that they left a lasting impression. That ice covering (called the Laurentide ice sheet, it ranged from the Arctic through eastern Canada and then arching into the northern half of the U.S. Midwest) has long since melted away, but the impression is still rebounding back upward from the impression that was left by the weighty ice [source: Bryner].
The GRACE satellites, working in tandem more than 100 miles above Earth, were able to make a map of our planet's gravity field and also observe changes in that field from one year to the next. The latter was crucial to solving the gravity mystery. The scientists who performed the gravity study with the GRACE telescopes reasoned that if the first theory -- that mantle plates were flowing downward -- was correct then the satellites wouldn't see much in the way of changes, because mantle convection changes occur over enormously long time scales. The second theory -- Ice Age glaciers left an impression in the Earth's crust that is still rebounding -- gained traction when the satellites observed that the area was indeed rebounding at a rate of about half an inch (1 centimeter) each year. Researchers think this rebounding theory can account for about half of the observed gravity loss [source: Lovett].
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