A low grassy coastal terrace below an older cliff near Northbank.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Load and vertical displacement ×5. local Airy equilibrium.

Ice 1650 m → equilibrium depression 459 m. Mantle / crust / ice densities: 3300 / 2800 / 917 kg/m³.

How to read these results

The rock column keeps its thickness and mass. Unloading raises the equilibrium position; this static experiment does not predict rebound timing or global sea level.

Isostasy

Explain how the crust responds to loading and unloading, using an ice sheet to explore subsidence and rebound.

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Why it happens

Adding an ice load changes the equilibrium height of the crust. In the local Airy model, a denser mantle supports a less dense crustal column, and unloading raises its equilibrium position.

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A raised beach near Northbank. Former shorelines record changes in relative sea level, which can include land uplift after unloading of ice.

Try a comparison

Calculate 3000 × 917/3300 and compare the displayed depression. Switch to physical scale to see the vertical and horizontal dimensions at the same scale.

Reading the result

The experiment calculates an equilibrium displacement; actual rebound takes time as the solid mantle deforms.

A common misunderstanding

Postglacial rebound shows that the mantle beneath the crust is liquid lava.

The mantle is predominantly solid rock but can deform slowly over long timescales. Removing an ice load changes the equilibrium supported by the underlying material, allowing the land to rise over time. An equilibrium-height model does not by itself calculate how fast that rebound occurs.

Cases

Exam language

Isostasy

The gravitational balance in which the lithosphere is supported by denser material beneath it. Adding or removing a large load causes deformation and adjustment over time. The mantle flows slowly as solid rock rather than as a simple liquid ocean.

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A raised beach at Northbank records former relative sea level. Relative sea level can change through land motion, including glacial rebound, and through eustatic change in global ocean level; the photograph alone cannot separate their contributions.

Glacial rebound

The gradual rise of land following the removal of a large ice load. The solid Earth adjusts as mantle material flows slowly over time. Rebound changes relative sea level locally, even while global mean sea level may rise.

Raised beach

A former beach now above present sea level because of land uplift, a fall in sea level, or both.

Lithosphere

Earth’s rigid outer mechanical layer, comprising the crust and the uppermost mantle. It is divided into tectonic plates and overlies the weaker asthenosphere. The lithosphere is therefore thicker than the crust alone.

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Mantle peridotite carried to the surface as a xenolith within volcanic rock. It provides a sample of solid upper mantle. The lithosphere includes crust and rigid upper mantle; the green xenolith is not a piece of crust.

Asthenosphere

A mechanically weak part of the upper mantle beneath the lithosphere. It is predominantly solid rock that deforms slowly over geological time.

Root

In isostasy, the thickened lower part of continental crust beneath a mountain belt. Less dense crust extends deeper into the mantle and helps support high topography. The term describes crustal structure, not a literal plant-like shape.

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The Himalaya stand above thickened continental crust. Their deep crustal root is inferred from geophysical evidence beneath the mountains visible here.

Eustatic

Describing a change in global mean sea level, for example from changing ocean-water volume or ocean-basin capacity.

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The Aletsch Glacier stores water on land. Melting land ice adds water to the ocean and contributes to eustatic sea-level rise; this photograph alone does not quantify a global sea-level change.

Adjust parameters

Ice thickness1650 m