The land is still rising 1 cm per year from ice sheets that melted 10,000 years ago. GPS networks measure Earth's viscous response to removing a 3-km-thick ice load.
Erik Ivins, JPL., Public domain, via Wikimedia Commons
The Big Squeeze
During the last Ice Age, an immense sheet of ice smothered North America. The Laurentide Ice Sheet, centered over Hudson Bay, was a colossal glacier covering over 13 million square kilometers. At its thickest point above the bay around 21,000 years ago, the ice reached a thickness of 3 to 4 kilometers. The incredible weight of this ice pressed down on the Earth's crust, causing it to sink into the semi-molten mantle below—a process called isostatic depression.. The land directly under the thickest ice was pushed down by as much as a kilometer.
This massive geological dent remained for millennia. As the climate warmed, the Laurentide Ice Sheet began to melt, a process that accelerated about 15,000 years ago. By about 8,000 years ago, the last of the ice over Hudson Bay had vanished, and the immense weight was finally gone. The removal of this load triggered a slow, ongoing reaction from the planet.
A Slow-Motion Rebound
With the ice gone, the compressed land began to rise, or "rebound." The viscous mantle material, which had been pushed aside, started to flow back under the depressed crust, slowly lifting it back toward its original elevation. This process known as glacial isostatic adjustment, continues today. Around Hudson Bay, the land is rising at one of the highest rates in the world, with some coastal areas ascending by 1 to 1.3 centimeters per year. This vertical motion is precisely tracked by networks of Global Positioning System (GPS) stations, which provide continuous data on crustal movement.
The effects of the rebound are visible in the landscape. A series of parallel ridges, known as raised beaches, can be seen for hundreds of kilometers inland from the current coastline. These are ancient shorelines, left behind as the land rose and the sea receded. The oldest and highest of these abandoned beaches, now more than 130 meters above the current sea level, formed around 7,500 years ago. Geologists estimate that the Hudson Bay region has another 100 meters of uplift to go, a process that will take another 10,000 years to complete.
💡Fun Facts
The land around Hudson Bay is rising so fast that it outpaces global sea-level rise, causing the local sea level to fall. Projections show that some parts of the coast could see a relative sea-level drop of nearly 1 meter by the year 2100.
The immense weight of the Laurentide Ice Sheet displaced so much rock in the mantle that the Hudson Bay region now has a slightly lower gravitational field than the global average. This "gravity anomaly" was first mapped in the 1960s.
As the ice sheet melted, the depression it left behind was temporarily filled by a vast inland sea called the Tyrrell Sea, which was the direct ancestor of modern Hudson Bay.
The Laurentide Ice Sheet was so large that the water locked within it lowered global sea levels by as much as 120 meters.
Accessible 24/7. This is a large-scale geological process visible across a vast, remote landscape.
Admission
Free
Accessibility
The region is remote wilderness. Access to specific geological features like raised beaches typically requires chartered flights, boats, or guided expeditions. Terrain is rugged and undeveloped.
