Airy/Pratt isostasy, glacial isostatic adjustment, and post-glacial uplift
Crust of uniform density (2.7 g/cm³) but varying thickness floats on denser mantle (3.3 g/cm³). Mountains have deep "roots." Compensation: h_root = h × (ρ_c / (ρ_m − ρ_c)) ≈ 4.2h. Proposed by G.B. Airy (1855).
Crust of uniform base depth but varying density — less dense material under topographic highs. Proposed by J.H. Pratt (1855) based on gravity anomalies near the Himalayas. Real Earth: combination of both mechanisms.
Ice sheets depress crust; removal allows slow viscoelastic rebound. Fennoscandia rising +8 mm/yr (GPS); Hudson Bay +10 mm/yr. Relaxation time: 3–10 kyr for upper mantle (η ~10²⁰ Pa·s). Affects sea level, coastlines, and geoid.
GRACE satellites measure gravity changes from GIA. GPS networks track uplift. Relative sea level records (tide gauges) show negative trends where rebound > eustatic rise (e.g., Stockholm: −4 mm/yr net). Future projections include GIA corrections in IPCC sea level models.