For more than 40 years, scientists have tried to figure out what’s causing large parts of Canada, particularly the Hudson Bay region, to be “missing” gravity. In other words, gravity in the Hudson Bay area and surrounding regions is lower than it is in other parts of the world, a phenomenon first identified in the 1960s when the Earth’s global gravity fields were being charted.
Two theories have been proposed to account for this anomaly. But before we go over them, it’s important to first consider what creates gravity. At a basic level, gravity is proportional to mass. So when the mass of an area is somehow made smaller, gravity is made smaller. Gravity can vary on different parts of the Earth. Although we usually think of it as a ball, the Earth actually bulges at the Equator and gets flatter at the poles due to its rotation. The Earth’s mass is not spread out proportionally, and it can shift position over time. So scientists proposed two theories to explain how the mass of the Hudson Bay area had decreased and contributed to the area’s lower gravity.
One theory centers on a process known as convection occurring in the Earth’s mantle. The mantle is a layer of molten rock called magma and exists between 60 and 124 miles (100 to 200 km) below the surface of the Earth . Magma is extremely hot and constantly whirling and shifting, rising and falling, to create convection currents. Convection drags the Earth’s continental plates down, which decreases the mass in that area and decreases the gravity.
A new theory to account for the Hudson Bay area’s missing gravity concerns the Laurentide Ice Sheet, which covered much of present-day Canada and the northern United States. This ice sheet was almost 2 miles (3.2 km) thick in most sections, and in two areas of Hudson Bay, it was 2.3 miles (3.7 km) thick. It was also very heavy and weighed down the Earth. Over a period of 10,000 years, the Laurentide Ice Sheet melted, finally disappearing 10,000 years ago. It left a deep indentation in the Earth.
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