3D Geomorphology Masterclass · Internal Earth Forces

Tap the globe to move the earthquake. Press FIRE to send the waves.

Active model

Isostasy — Airy vs Pratt

Analytical brief

Isostasy = gravitational balance between lithospheric columns above a level of compensation.

Airy (1855)Uniform density, variable thickness; mountains have deep roots, ocean has anti-roots.

Root lawR = (ρc / (ρm − ρc)) · h ≈ 4.45 h (ρc 2.67, ρm 3.27 g/cc).

Pratt (1855)Uniform depth of compensation; density varies inversely with column height.

Pratt lawρ_h · (T + h) = ρ₀ · T → higher column = lower density.

CompensationBelow the line of compensation, pressure from every column is equal.

Vening MeineszRegional (flexural) isostasy — crust bends elastically over a wider area.

AIRY: R = ρc·h /(ρm−ρc) ≈ 4.45h PRATT: ρ_h(T+h) = ρ₀T

Answer-sheet drawing guide

Diagram: asthenosphere tank → blocks of varying height. AIRY → equal shading, roots ∝ height (draw downward keels). PRATT → flat common base (line of compensation), shade taller blocks lighter; annotate densities.

PYQ trend (GS-1): 'Distinguish between the views of Airy and Pratt on isostasy'; gravity anomalies & isostatic adjustment.

Crustal Equilibrium Simulator