Crustal columns of equal area but different composition or thickness exert equal weight (pressure) on the mantle—isostatic equilibrium. Light continental crust floats higher than dense oceanic crust. When crustal thickness or composition changes, vertical adjustment maintains equilibrium.
Calculate crustal thickness needed to support topographic loads. Apply Archimedes' principle to crustal blocks with different densities.
From your understanding of Earth's interior density structure, you know that the crust is less dense than the mantle beneath it. Isostasy is the direct consequence of this density contrast: the crust floats on the denser mantle much like an iceberg floats in water, and the height at which it floats depends on its thickness and density. This is not a metaphor — it is a direct application of Archimedes' principle to geology. A block of wood floats higher in water than a block of iron of the same size because it is less dense; similarly, thick continental crust (density ~2.7 g/cm³) floats higher on the mantle (density ~3.3 g/cm³) than thin oceanic crust (density ~3.0 g/cm³), producing the elevation difference between continents and ocean floors.
The quantitative framework comes in two classic models. Airy isostasy explains elevation differences through variations in crustal thickness — mountains have deep crustal roots, like an iceberg with most of its mass below the waterline. The Himalayas stand 8 km above sea level partly because their crustal root extends 60–70 km into the mantle. Pratt isostasy explains elevation through density variations — higher-standing regions are made of less dense rock, even if crustal thickness is roughly uniform. In reality, both mechanisms operate simultaneously. The key prediction of both models is the same: at some depth called the compensation depth, the total mass per unit area of any crustal column must be equal. Columns that are tall but light balance columns that are short but dense.
What makes isostasy dynamic rather than static is that loads on the crust change over time. When a continental ice sheet 3 km thick sits on Scandinavia, its weight pushes the crust down into the mantle — the mantle flows viscously out of the way to accommodate the extra load. When the ice melts, the load is removed, and the crust slowly rebounds upward as mantle material flows back. This process, called isostatic rebound (or glacial isostatic adjustment), is still happening today: Scandinavia is rising at roughly 1 cm per year, thousands of years after the last ice sheet melted. The rate of rebound tells geophysicists about the viscosity of the mantle — how quickly it flows in response to changing loads.
Isostasy also explains why you cannot simply pile material on the crust without consequences. Mountain building thickens the crust, which causes it to sink deeper into the mantle (creating a root) while rising higher at the surface. Erosion removes mass from the top, and the crust rebounds upward in response, exposing deeper rocks — which is why deeply metamorphosed rocks originally formed at 20–30 km depth are now found at the surface in eroded mountain belts. The crust is perpetually adjusting toward equilibrium, driven by the density contrast with the mantle and the mantle's ability to flow on geologic timescales.