The Part of Canada That Doesn’t Want You

If you’ve taken a high school science class, you’ve probably learned that the gravitational constant here on Earth is about 9.8 meters per second squared. In other words, things on the planet are constantly falling back to the ground, accelerating at that rate. We don’t tend to think about it much — unless you’re going to the International Space Station or on a Vomit Comet, gravity is the same wherever you are — hence the name.

Except that, it actually isn’t.

Gravity is a function of mass — the more massive an object, the stronger its gravitational pull. And that pull depends not just on how much mass there is, but how that mass is distributed around an object’s center. From afar, the Earth looks like a smooth, round ball, but in reality, it isn’t. Our planet isn’t a uniform sphere, and therefore, doesn’t actually have uniform density, so gravity varies slightly from place to place. In most cases, the difference is so small that it’s basically undetectable. But in one part of Canada, the difference is noticeable enough — with the right instruments — that scientists spent decades trying to figure out why.

That region is near the area around the Nastapoka Arc in Canada’s Hudson Bay. (Here’s a map.) If you were to stand there, you’d weigh about four-thousandths of a percent less than you would at the global average. That’s not enough to notice on a normal bathroom scale — you’d need extremely precise instruments to detect it — but it’s real, and it’s measurable by scientists who have better tools than we regular people do. Until recently, no one was sure why.

When scientists first discovered this anomaly in the 1960s, they had two theories. The first involved ice — specifically, the Laurentide Ice Sheet, a massive glacier that covered most of Canada during the last Ice Age. The ice was over three kilometers thick in places, and its weight pushed down the continental crust like a finger pressing into a memory foam mattress. When the ice melted around 10,000 years ago, the crust began to rebound — but slowly. According to IFL Science, the Laurentide Ice Sheet “pushed through the country and shifted dense rock out of its way, compressing it down below.” The result was a dent in the Earth, with less mass underneath — and less mass means less gravity.

The second theory pointed (literally) deeper, to the mantle beneath the crust. The mantle is made of molten rock heated by the Earth’s core, and it moves in slow convection currents over geological timescales. These currents can drag continental plates downward, reducing the mass in certain areas. Scientists suspected that something like this was happening beneath Hudson Bay, pulling the region down and further reducing its gravitational pull.

For decades, no one knew which theory was correct — or if both were. Then, in 2002, NASA launched a pair of satellites called GRACE, the Gravity Recovery and Climate Experiment. By measuring tiny variations in the Earth’s gravitational field, GRACE gave scientists the data they needed. As CBC News reported, the satellites allowed researchers to “measure tiny, minuscule ripples or changes in that gravity field, and that has never been possible before.”

And it turned out that both theories were right. According to BBC Science Focus, “Hudson Bay’s gravitational anomaly is caused by a combination of mantle convection currents and the legacy of the Laurentide Ice Sheet.” The ice accounts for somewhere between 25 and 45 percent of the missing gravity; the rest is due to what’s happening deep underground. The crust is still rebounding at about 12 millimeters per year, which means it’ll take another 300,000 years or so for the region to fully recover from the Ice Age. But even then, the convection currents will still be there — meaning Hudson Bay will always be a little lighter than the rest of the world. Just not as light as it is today.

So if you’re looking to lose weight, moving to northern Canada won’t help you fit into your old jeans. But it will, technically, make the number on your scale slightly smaller. Just don’t expect anyone to notice.

Bonus fact: Trees have a theoretical maximum height on Earth, and gravity is the reason why. BBC Science Focus explains: “Trees grow very tall when water and nutrients are plentiful and there is intense competition for sunlight. But as they grow, gravity gets stronger. Plants and trees carry water to their leaves for photosynthesis in a tube called the xylem. If there isn’t enough water, or gravity is very strong, this water column can break, creating potentially deadly air bubbles. This places an upper limit on tree height, which theoretically lies somewhere between 122 meters and 130 meters (400 to 426 feet).” That theoretical limit hasn’t been reached, though — the tallest known tree, a sequoia known as Hyperion, is only (only?) 116.22 meters (381.3 feet) tall.

From the Archives: Just Say No to Gravity: Meet the people who tried to harness the power of gravity — if not ending it.