In the 1960s, scientists discovered that the Hudson Bay area of Canada has less gravity than the rest of the world. The weakness is slight-around a tenth of an ounce for a 150-pound (68-kilogram) person. There are two explanations for this phenomenon: convection in the Earth's mantle, and the melting of the Laurentide Ice Sheet. Convection currents in the shifting mantle beneath the Earth's crust have pulled tectonic plates downwards, lowering the mass in these regions and thus lowering the gravity as well. The Laurentide Ice Sheet, which melted away some 10,000 years ago, also decreased the mass of the Hudson Bay area. The massive sheet weighed down the Earth underneath it and pushed the ground upwards on either side, leaving a large indent when it finally disappeared.
Canada's "Missing" Gravity
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Key Facts In This Video
Earth isn't exactly round; it's wider at the equator. 00:08
Hudson Bay in Canada has less gravity than other parts of the world. 01:20
Coral reefs are the largest living structures on Earth, some even visible from space. 03:49