HUDSON BAY: 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 first it’s important to 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 it is considered like 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.
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 US. This ice sheet was almost 3.2km thick in most sections, and in two areas of Hudson Bay, it was 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.
To get a better idea of what happened, think about what happens when you lightly press your finger into the surface of a cake or a piece of really springy bread. Some of it moves to the sides and there’s an indentation. But when you remove your finger, it bounces back to normal. A similar thing happened with the Laurentide Ice Sheet, the theory proposes – except the Earth isn’t so much “bouncing” back as it is rebounding very slowly (less than half an inch per year). In the meantime, the area around Hudson Bay has less mass because some of the Earth has been pushed to the sides by the ice sheet. Less mass means less gravity.
So which theory is correct? It turns out that both of them are. Convection and the ice sheet’s rebound effect are both causing some of the decrease in gravity around Hudson Bay.
It turns out, according to a satellite observed data that the ice sheet theory only accounts for 25 per cent to 45 per cent of the gravitational variation around Hudson Bay and the surrounding area. Subtracting the “rebound effect” from the area’s gravitational signal, scientists have determined that the remaining 55 per cent to 75 per cent of gravitational variation is likely due to convection.
The Hudson Bay area is going to have less gravity for a long time. It’s estimated that the Earth has to rebound more than 650 feet to get back to its original position, which should take about 5,000 years. But the rebound effect is still visible. Although sea levels are rising around the world, the sea level along Hudson Bay’s coast is dropping as the land continues to recover from the weight of the Laurentide Ice Sheet.
While the mystery surrounding Canada’s gravitational anomalies has been put to rest, the study has wider implications. Scientists involved in the Harvard-Smithsonian Center study were amazed that they were able to see how the Earth looked 20,000 years ago. And by isolating the influence of the ice sheet’s rebound effect, researchers better understand how convection affects gravity and how continents change over time. Finally, the satellites have provided scientists with data on many ice sheets and glaciers. By examining climate change that took place thousands of years ago, scientists may gain a better understanding of how global warming and rising sea levels are affecting our planet today and what impact they will have on our future.