The state of gravitational balance in which the lighter crust (sial) floats on the denser, semi-fluid mantle below (sima), so that high landmasses are compensated by deep "roots" much as an iceberg floats with most of its mass underwater.
- The term was coined by the American geologist Clarence Dutton in 1889; the idea explains why tall mountains do not simply sink and why the crust seeks equilibrium.
- Two classic models: Pratt held that columns of differing density rise to a common depth of compensation, while Airy held that columns of equal density float at differing depths, so mountains have deep roots.
- Removal of load (such as melting of ice sheets) causes the crust to rise, called isostatic rebound; addition of load (sediment, ice) causes it to sink.
- Scandinavia and parts of Canada are still rising slowly because the weight of Ice Age glaciers was removed.
- Isostatic adjustment is one process, alongside plate movement, that shapes the long-term level of the land.
The Pratt versus Airy distinction, the iceberg analogy, and isostatic rebound after deglaciation are standard physical-geography facts that appear as one-mark items.
Pratt (different densities, same depth) versus Airy (same density, different depths with mountain roots); isostasy (vertical balance) is separate from plate tectonics (horizontal movement).
Gravitational floating balance of the lighter crust on the denser mantle; tall mountains have deep roots.