The flexural rigidity of an elastic sheet is defined as ET³/12(1 – σ‎²) where E is Young’s modulus, T is the thickness of the sheet, and σ‎ is Poisson’s ratio. In the Earth, the lithosphere is usually treated as an elastic sheet that responds to loading by ice caps, volcanic piles, etc., from which the thickness of the lithosphere and the viscosity of the mantle can be derived.