Is Canada “Missing” Gravity? How and Why?

Since scientists first began mapping the Earth’s global gravity fields back in the 1960s, it has become clear that parts of Canada are “missing” gravity. What this really means is that certain areas of the country – especially in and around the Hudson Bay area – have less gravitational pull than across the rest of the world.

This occurs primarily because gravity is directly proportional to mass. The larger an object, the more gravitational pull it exerts. Contrary to popular belief, the mass of the Earth is not uniform all over, but actually bulges in the centre and narrows at the Poles – making it more reminiscent of an orb than a ball.

But why is the Hudson Bay region particularly affected by low mass – and, as a result, low gravity? There are two main theories to the conundrum, both of which are believed to partly explain Canada’s missing gravity.

The Laurentide Ice Sheet Theory

The newer of the two theories to explain the Hudson Bay mass problem is the Laurentide Ice Sheet theory, first floated around 2006 after data collected over the previous four years was able to allow us insight into the past of our planet. Using the sister Gravity Recovery and Climate Experiment (GRACE) satellites, the Harvard-Smithsonian Center for Astrophysics team measured minor gravitational variations of the Earth’s surface. With both satellites in play, the team were able to assess the relative distances between the satellite over the Hudson Bay region and the other device and extrapolate the missing mass from this data.

Interestingly, this information also allowed scientists a view of how the Earth looked during the last Ice Age. During this period, the Laurentide Ice Sheet covered much of North America, ranging between 2 and 2.3 miles thick – the thickest areas lay over the Hudson Bay. During the ensuing 10,000 years, the ice slowly melted and completely dissipated 10,000 years prior to modern day. However, much like how a rubber ball will retain the indentation of a pressed finger for a moment or two before it returns to its normal shape, the Earth is slowly “rebounding” back to its previous mass. Due to the extra ice in the Laurentide Ice Sheet, these parts of Canada are showing effects of this more than other areas of the world.

The GRACE data is not the only time that modern technology has shed light on the fascinating past of our planet. Back in 2004, a technique known as energy-dispersive X-ray fluorescence analysis with polarization excitement (EDPXRF) was used to analyse sediment deposits on the sea bed and inform us about the Quaternary period of the Earth’s existence. Meanwhile, Antarctic snow also stores a log-book of the Earth's history in the form of trace heavy metals. Such intriguing insights offer invaluable information about the evolution of our world.

The Convection Theory

Despite going some way to explain the missing gravity over Canada, the Laurentide Ice Sheet theory and the data uncovered by GRACE satellites only account for about 25%-45% of the gravitational discrepancy. Therefore, using a simple method of subtraction, scientists estimate that the remaining 55%-75% of the missing gravity is due to a theory revolving around the process of convection.

Deep below the Earth’s surface, a layer of magma (molten rock) is responsible for creating convection currents due to the rising and falling restless nature of this hot substance. These in turn are responsible for dragging the Earth’s continental plates inward, thus reducing the mass and the gravity of the Hudson Bay area.

The gravity is due to return to Canada – but very slowly. With the Earth “rebounding” at a rate of less than half an inch per year and with a distance of 650ft to cover, it could take around 5,000 years for all of the gravity to be restored.