Floating Continents
Just as an iceberg floats with most of its mass below water, Earth's crust floats on the denser underlying mantle. This principle of isostasy, recognized by Airy and Pratt in the 1850s, explains why mountains have deep roots, why removing ice sheets causes land to rise, and why the ocean floor sits several kilometers below the continents.
Airy Isostasy
In Airy's model, the crust has uniform density but varies in thickness. Mountains are supported by deep crustal roots displacing dense mantle material, much like a tall wooden block floating deeper in water. Seismic surveys confirm this: the Moho beneath the Himalayas reaches 70 km depth compared to 35 km beneath stable platforms, and 6-7 km beneath ocean basins.
Pratt Isostasy
Pratt's alternative model proposes that all crustal columns extend to the same compensation depth but differ in density. Higher topography has lower density. This model applies well to mid-ocean ridges, where thermal expansion reduces density and elevates the seafloor, and to some continental regions where lateral density variations dominate over thickness changes.
Isostatic Adjustment
When the load on the lithosphere changes — through ice sheet growth or melting, erosion, or sedimentation — the crust adjusts toward a new equilibrium. This glacial isostatic adjustment occurs over thousands of years, limited by the viscous flow rate of the mantle asthenosphere. Current uplift rates in Scandinavia and Canada, measured by GPS and tide gauges, constrain mantle viscosity to approximately 10^21 Pa·s.