In 1960, when for the first time earth’s global gravity was charted then it was noted that certain regions of earth have lower gravity than others. Regions of Canada, particularly Hudson Bay area have remarkably low gravity compared to other parts of the world. Initially, it appeared to be a mystery but, now there are more than one explanation for the presence of low gravity in that region.
How is gravity created?
Gravity is directly promotional to mass of a body so, in other words mass creates gravity. The shape of earth is not even, it bulges out near equator and gets flatter near poles. Regions of earth with lower masses have lower gravity compared to regions having higher masses. So, do the Hudson Bay area have lower land mass compared to other parts of Earth? Yes, but why? In case of Hudson Bay area, it could be explained by two factors, which are explained below:
Convection in Earth’s mantle
The mantle of earth consists of molten rocks known as magma, magma is in constant movement creating convection currents. These convection currents pull the continental land masses toward them and there by decreases the land mass of the area that is getting pulled. Convection currents are not always same at an area and keep on shifting from place to place bringing on various geographical changes too along with its movements.
Rebound effect of ice sheet
Around two miles thick ice sheet known as Laurentide Ice Sheet existed in the area which is now Hudson Bay area. This ice sheet was extremely heavy and had weighed down the area of earth where it existed. It started melting due to rising temperature of earth and disappeared completely around 10,000 years ago but, had left an indentation in that area. The indentation has pushed land masses towards the sides and created a low land mass in the central area which has resulted in lower than usual gravity in this area.