In certain areas of the world, such as Canada’s Hudson Bay, gravity is slightly lower than in other areas.
This is because an object with more mass has more gravity, and since Earth’s mass is not uniform, gravity varies from place to place. However, you need a very accurate scale to notice it. Hudson Bay’s gravity is about four thousandths of a percent lower than Earth’s average.
When scientists discovered this anomaly in the 1960s, they developed two theories to explain it. The first is that the Earth’s crust is still recovering from the last Ice Age.
About 20,000 years ago, the huge Laurentide ice sheet covered North America. The weight of the ice, which was more than 3 kilometers (almost 2 miles) thick in places, pushed down on the continent’s crust.
When the Ice Age ended, the Laurentide melted rapidly, but the Earth’s crust slowly returned to normal. It rebounds by about 12 mm (0.5 inch) each year, meaning it will take another 300,000 years to return to its original position. This indentation left in the Earth’s crust means that Hudson Bay has slightly less mass and therefore slightly less gravity.
The second theory concerns the movement of the mantle beneath the Earth’s crust.
The mantle is made of molten rock heated by the Earth’s core. It rises over geological time scales and then sinks again as it cools. This creates convection currents that move the plates. Ocean currents can also pull continents downward, slightly reducing mass and gravity in the area.
Satellite measurements in 2002 revealed both theories to be correct. The Hudson Bay gravity anomaly is caused by a combination of mantle convection and the legacy of the Laurentide ice sheet. Therefore, even after the Earth’s crust has fully rebounded, gravity in this part of the world will remain slightly lower than average.
This article answers the question (asked by Victor Gregory via email): “Is it true that people in Canada weigh less?”
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