How an ice sheet bent the ground beneath it
The Laurentide Ice Sheet did more than cover much of Canada: its immense weight pushed the land downward. The rigid outer shell of Earth, called the lithosphere, flexed under the load. Beneath it, hot rock in the mantle slowly shifted sideways, making room for the depression.
That mantle rock is not a vast underground ocean of liquid. It is mostly solid, but over long periods it can deform and flow, much as very stiff material does under sustained pressure. The ice’s retreat removed the load, but the mantle did not instantly flow back.
A response measured in millennia
With less weight above it, the land began rising. The mantle’s sluggish return continues to lift the lithosphere, while the crust itself also springs back elastically. The two responses operate on different timescales: the elastic part is relatively quick; the deeper adjustment can continue for thousands of years after the ice has gone.
Scientists track this motion with GPS, which can detect changes in a location’s height over time. Rates are especially high around Hudson Bay, where the former ice sheet was thickest. The uplift is gradual, not a sudden or violent rebound, even when its yearly pace is striking by geological standards.
The coastline is moving, too
As the land rises, the sea does not have to rise for the shoreline to shift. In parts of the region, the ground’s upward motion changes relative sea level, altering coastlines and the shape of bays and waterways. The same long recovery can affect how water drains across the landscape.
So a GPS point near Hudson Bay is not fixed against an unchanging planet: it is still climbing as Earth’s deep interior slowly responds to an ice-age load that disappeared thousands of years ago.
