Parts of Canada are experiencing a curious phenomenon: a subtle weakness in gravity around Hudson Bay. The common tale attributes this to an ancient ice sheet that once pressed down on the Earth’s crust, but recent findings suggest there’s more to the story.
Geophysicists identified this gravity low back in the 1960s during gravity surveys. While it’s too slight for anyone to notice—standing on the shore won’t make you feel lighter—it covers a vast region and puzzled researchers for decades.
How We Measure Gravity
The Gravity Recovery and Climate Experiment (GRACE) transformed our understanding. Launched in 2002, these satellites orbit Earth and measure tiny changes in gravity by tracking their distance apart. Instead of capturing just a single moment, GRACE reveals trends over time, helping scientists see how gravity shifts across different areas.
The Ice Sheet Theory
The leading explanation for the gravity low involves the concept of glacial isostatic adjustment. During the last ice age, the massive Laurentide Ice Sheet weighed down the crust. Once the ice melted about 10,000 years ago, the crust began to rise again, still moving upward at a rate of about one centimeter per year. While this rebound explains some of the gravity deficit, studies have shown there’s more at play.
In a 1992 study, researchers suggested that traditional models could only account for 15 to 30 percent of the gravity anomaly in Hudson Bay. This indicated that additional factors were influencing the gravity field.
New Insights from 2007
In 2007, a landmark study isolated different elements affecting gravity. Researchers found that the gravity low wasn’t just leftover from the ice age. They concluded that about 25 to 45 percent of the anomaly is due to other factors, likely related to mantle convection—the slow movement of rock beneath the Earth’s surface.
This movement isn’t a pool of molten lava, but mostly solid rock that behaves like very stiff putty over long timescales. In this context, denser material sinking in the mantle pulls the surface downward, leading to lower gravity.
Current Research and Future Directions
As GRACE launched its successor, GRACE Follow-On, in 2018, scientists continue to gather data that separate the ongoing gravitational changes from the static ones. The longer this record, the clearer the understanding becomes of what drives the gravity low in Hudson Bay—decades of ice age history versus millions of years of mantle dynamics.
Understanding these forces not only enhances our knowledge of geological processes but also offers a fascinating glimpse into Earth’s past and present. This case demonstrates how seemingly small shifts in gravity can reveal the intricate dance of geophysical forces beneath our feet.
For more detailed information about the GRACE mission and its findings, you can visit the GRACE website.
