In certain parts of the world, such as Canada’s Hudson Bay, gravity is slightly lower than elsewhere.
This is because objects with more mass exert a greater gravitational force and Earth’s mass isn’t uniform, so gravity varies from place to place. You’d need some very accurate scales to notice though – gravity in Hudson Bay is about 4/1,000ths of a per cent less than Earth’s average.
When scientists discovered this anomaly in the 1960s, they developed two theories to explain it. The first is that Earth’s crust is still rebounding from the last Ice Age.
Around 20,000 years ago, the huge Laurentide Ice Sheet blanketed North America. The weight of the ice, which was over 3km (almost two miles) thick in some locations, pushed down the continental crust.
When the Ice Age ended, Laurentide rapidly melted, but the crust has been slower to spring back. It’s rebounding by about 12mm (0.5in) a year, so it’ll be another 300,000 years before it returns to its original position. This dent left in the crust means that Hudson Bay has slightly less mass and, therefore, slightly less gravity.
The second theory relates to the movement of mantle beneath the crust.
Mantle is made up of molten rock that’s heated by Earth’s core. Over geological timescales it rises, before sinking again as it cools. This creates convection currents that cause the tectonic plates to move. The currents can also pull continents downwards, slightly decreasing the mass, and gravity, in those areas.
In 2002, satellite measurements revealed that both theories are correct. Hudson Bay’s gravitational anomaly is caused by a combination of mantle convection currents and the legacy of the Laurentide Ice Sheet. So, even after the crust has fully rebounded, this part of the world will still have slightly lower gravity than average.
This article is an answer to the question (asked by Victor Gregory, via email) 'Is it true you weigh less in Canada?'
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