How can parts of Canada be missing gravity

How can parts of Canada be missing gravity. Gravity, a phenomenon which attract things towards each other and it directly proportional to the mass of the body. We stand on the earth due to this phenomenon but for more then 40 years ago, a very strange thing happened in large part of Canada , most particularly in Hudson Bay region. It had noticed that the gravity in these areas were missing. These areas, especially Hudson Bay region, have gravity lower then the other parts of the world. It was first identified when global gravity field of Earth were being charted in the year of 1960. The question arises, why these regions have lower gravity then the other parts of Earth?
So, according to scientists there are two theories that supposed to be the main reason of this lower gravity. We can clarify them one by one. The first one is the basic gravity which depends upon the mass of the body. So, on the Earth, the parts where the mass of an area is lower then the gravity is also lower. Depending on the mass the gravity can differ in different parts of the Earth. We can understand it by considering Earth as a ball and actually the Earth bulges at the Equator side and due to its rotation it gets even at the poles. The mass of the Earth is not even every where, due to which the gravity is also not same every where. It is considered that some geological and environmental changes the mass of Hudson Bay region decreases and lowers its gravity.

Laurentide Ice Sheet, is considered one of the main reason of low mass. It is a sheet which covered most of the part of the present day Canada and the north part of U.S. According to research, in most sections this sheet was 3.2 km thick but in two areas of Hudson bay region, Laurentide Ice Sheet was 3.7 km thick. It was so heavy that it weighed down the Earth. But over a period of 10,000 years, the Laurentide Ice Sheet started to melt, finally disappear and left a deep indentation in that part of the Earth. It was clear by this example that if you lightly press a piece of really springy bread with your finger it creates indentation and some of the bread moves to the side. And after you remove your fingers it bounces back to its normal shape. The same thing happened in case of Laurentide Ice Sheet, but the process of bouncing back or rebounding is very slow that less then half an inch of the mass bounces back per year. This is the reason the Hudson Bay region has less mass because the Laurentide Ice Sheet pushed the part of the Earth to sides. So, Hudson bay region has less mass and less gravity.

It is clear that both these theories are correct and play a major role in decreasing the gravity around Hudson bay region and the area surrounding it. According to satellite, it was observed that the 25 percent to 45 percent gravitational variations occurs due to ice sheet theory and the remaining 55 percent to 75 percent gravitational variation is due to convection. It is said that the area of Hudson Bay region comes back to its normal position should take about 5,000 years and it has to rebound more than 650 feet. The changes occur in nature very slowly so, the rebound effect is still visible but slow. It is a fact that the sea level rises around the world but due to this Laurentide Ice Sheet decreasing and earth rebound process the sea level along Hudson Bay coast is dropping because of recovery of Earth.

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