Only about 10,000 years ago, ice covered all of Canada. Mountains made of ice pushed down on the continent with immense pressure.
For example, the Hudson Bay may have depressed over one kilometer. After the ice melted, Canada began lifting.
We know this because we measure it’s vertical motion with GPS. Imagine squeezing a sponge and watching it return to its shape. The term that describes land returning to its pre-glacier shape is isostatic rebound.
The ground rebounds quick at first, then slows down. This process of glacier rebound is due to the asthenosphere behaving in a fluid manner. Because ice had been removed, the compressional force no longer exists.
Glaciers compress the asthenosphere
Ice ages have put our planet into a deep-freeze. These bone-chilling temperatures have stacked kilometers of glacier ice.
Because of the mass, the interior gets pushed in. It compresses the asthenosphere until it pushes it out-of-the-way.
If you were to remove the ice, you’d find the actual surface to be below sea level. For example, Canada is still on the rise from the last ice age. We know this because we use GPS sensors to detect vertical motion.
It’s rising because the asthenosphere rock is lifting again and flowing beneath. It’s flowing back up underneath slowly because ice is no longer compressing it.
So again, rock in the asthenosphere is behaving in a fluid manner. By deforming like silly putty, glaciers gradually push down it. After the ice melts, it slowly rebounds. This process is isostatic rebound.