Isostasy is the balance between Earth's crust and the denser mantle beneath it, with the crust floating at a height that depends on its thickness and density. When a load such as an ice sheet is added or removed, the crust sinks or rebounds.
Key takeaways
- Isostasy is the gravitational balance that lets the crust float on the mantle at a height set by its thickness and density.
- Heavy ice sheets pressed the crust down, and it is still rising slowly now that the ice has melted.
- Isostasy is one reason the Ice Age sea level of roughly 120 m down does not translate directly to a coastline.
- Our drain slider ignores isostasy and uses today's seabed unchanged.
How isostasy works
Isostasy is a state of gravitational equilibrium between Earth's crust and the mantle below it. The crust floats, rather like a block of wood in water, and the height at which it floats depends on how thick and dense it is [1]. Thick, light continental crust sits high. Thinner, denser ocean crust sits lower.
This is part of why the planet has two main levels, continents and ocean floors. The floating balance helps explain why different landforms sit at different heights [1].
Isostasy also responds to changes in load. Add weight and the crust sinks. Take it away and the crust rises back towards its balance, but slowly, because the mantle beneath has to flow to make room.
The best-known example is the ice age. The great ice sheets of the Last Glacial Maximum pushed the crust beneath them down [1][2]. When the ice melted, the land began to rebound. Regions once under ice, such as the Baltic Sea and Hudson Bay areas, are still rising, and ancient sea cliffs now stand hundreds of metres above the sea [1].
The effect on coasts is that sea level and land height change together, so the shoreline at any one time is a product of both the water and the ground.
An example from our data
Our Ice Age map lowers the sea by 120 m and exposes 21.7 million km² of seabed. It does this on today's seabed, with no allowance for crust that has moved since. In areas that were under ice or near its edge, the ground was at a different height then, and some of it has risen since.
Look at the North Sea: the model shows 84.5% of it dry at 120 m lower. The real result would differ because the crust around the old ice sheets was not where it is now.
The thing people get wrong
Many people expect the land to be fixed and only the sea to move. In reality the ground moves too, just far more slowly. Land can rise or sink by amounts that matter over thousands of years.
For this reason our slider is only a bathtub model. It treats the seabed as a fixed bowl and the sea as the only thing that changes. Isostasy is one of the main things it leaves out, and it is the reason to treat the Ice Age map as a guide, not a reconstruction.
Sources
- Isostasy, Wikipedia.
- Last Glacial Maximum, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).