The Earth’s surface is constantly shifting. One process that illustrates this is isostatic rebound, also referred to as post-glacial rebound or glacial isostatic adjustment. This phenomenon occurs as land that was once compressed under the weight of glaciers slowly rises back up—a process that can take thousands of years.
What causes isostatic rebound?
During the last Ice Age, large ice sheets covered vast areas of the Northern Hemisphere. These ice sheets were so heavy that they pressed down on the Earth’s crust, pushing it into the softer mantle below. When the ice began to melt, the pressure was released, and the crust started to rise back to its original elevation. This upward movement isn’t instantaneous; it takes place at a slow pace, with some areas still rebounding today.
This rate of the Earth’s crust rising back up is called Isostatic rebound, or post-glacial rebound or glacial isostatic adjustment. Regions that experienced heavy glaciation during the Ice Age, such as parts of Scandinavia and Canada, are where isostatic rebound is most noticeable.
Notable examples of isostatic rebound
Scandinavia
An example of isostatic rebound can be found in Scandinavia. Here, the land that was once buried under the Fennoscandian Ice Sheet continues to rise at a rate of several millimeters per year. Over the past 10,000 years, some parts of the region have risen by 286 meters as the Earth’s crust adjusts to the loss of the ice sheet’s weight. This ongoing rebound has reshaped coastlines and created new landforms, providing evidence of the region’s glacial history.
Hudson Bay, Canada
In Canada, the land around Hudson Bay is also rebounding from the effects of the Laurentide Ice Sheet, which covered much of North America during the last Ice Age. The rebound in this region, while slower compared to Scandinavia, continues to shape the area’s landscape.
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Kvarken Archipelago, Finland
The Kvarken Archipelago in Finland also experiences isostatic rebound. According to NASA, this region sees uplift at approximately 9 millimeters per year, with about 700 hectares of new land emerging from the sea annually. This process is transforming the shallow coastline and creating new islands.
The Kvarken Archipelago has features like the De Geer moraines, which are ridges formed by sediment deposited at the edges of retreating glaciers. These moraines are typically 1 to 2 kilometers long, 2 to 5 meters high, and spaced 50 to 200 meters apart. As the land rises, these glacial features become more visible, providing a record of past glacial activity.
Recent advancements in LiDAR technology have uncovered additional moraines in southern and western Finland, giving scientists a more detailed understanding of how glaciers shaped the terrain.
Scientists believe the formation and spacing of De Geer moraines are influenced by the rate of ice retreat, water depth, and underlying terrain. Recent LiDAR-based elevation models have revealed more of these formations in southern and western Finland than previously known, offering new insights into glacial dynamics.
Over the next several thousand years, the land is expected to continue rising until the remaining 100 meters of depression caused by the ice are balanced. Geophysical and climate models help predict how the landscape will evolve, but the rate of apparent uplift will also depend on global sea level changes.
Forebulge
While areas directly under former ice sheets rise, regions at the edges of those ice sheets—known as the forebulge—experience the opposite effect. During glaciation, the weight of the ice caused the crust at the ice sheet’s periphery to bulge upward. As the ice melted and the land beneath it began to rebound, this forebulge started to collapse, causing the surrounding regions to sink.
This process, known as forebulge collapse, significantly impacts local sea levels. For instance, while areas like Scandinavia experience falling relative sea levels due to land uplift, regions affected by forebulge collapse, such as parts of the Eastern United States, face rising relative sea levels.
Around Chesapeake Bay, the land is sinking due to forebulge collapse. According to NOAA, this subsidence could result in land sinking by as much as half a foot over the next century, compounding the effects of rising global sea levels. This makes forebulge collapse a critical factor in understanding the challenges faced by coastal communities.
Isostatic rebound is ongoing
The process of isostatic rebound is ongoing. In places like the Kvarken Archipelago, scientists estimate that the land will continue to rise for thousands of years, with some areas potentially rebounding by another 100 meters before reaching equilibrium. Advances in geophysical and climate modeling are helping researchers predict how landscapes will evolve as this process continues.
At the same time, isostatic rebound helps scientists understand Earth’s glacial past. Features like the De Geer moraines and newly exposed landforms provide insights into the dynamics of glacial retreat and the interactions between ice, land, and sea.
References
Poutanen, M., & Steffen, H. (2014). Land Uplift at Kvarken Archipelago/High Coast UNESCO World Heritage area. Geophysica, 50(2).
