In Canada, you can find large areas that appear to be missing gravity, especially around the Hudson Bay region. Gravity in this part of the world appears to be less than anywhere else, it is a phenomenon that was first identified and published in the 1960s. And for the last 50 years, scientists have been struggling to figure out why this happens.
When Earth’s global gravity fields were charted several anomalies were discovered, one of them being that gravity was lower in Canada. Essentially people in Canada will weigh less because a part of the gravitational pull is missing there. At its most basic level gravity is always proportional to mass, so when the mass of an area is smaller then gravity is also made smaller. This means that gravity can vary at a different part of the Earth because even though we think of Earth as a perfect ball, it is quite bulgy. For example at the Equator the Earth becomes flatter, this means that the Earth’s mass is not spread our proportionally. And you also have to factor in that everything is always moving, which means the Earth’s mass can shift position.
Scientists have put forth two theories to explain why the Hudson Bay area has a big reduction in gravity. One of the theories is due to the active magma under the Earth’s mantle, as it is extremely hot. The Earth’s plates under Canada are constantly moving which can change the mass and as a result lower gravity.
Another potential answer is due to the melting of ice, the Laurentide Ice Sheet is melting, it used to be 2 miles thick and weighed down the Earth being so heavy. Now it is melting it is not as heavy, which means the areas around Hudson Bay in Canada have become lighter. This is simply due to how physics works: less mass means less gravity. Which theory is correct has been the plight of many researchers.
As it turns out, the belief today is that both theories are correct, they are both causing the gravity to change in Canada which is why if you ever visit certain areas around Quebec or Hudson Bay, you will find that you are lighter.