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Pratt model A model for the lithosphere that accounts for isostatic anomalies by assuming there is a level of compensation that lies at a constant depth everywhere. Below the level of compensation all rocks have the same density, but above it density decreases as topographic elevation increases. For a column of material anywhere on Earth, the mass lying above the level of compensation will be the same, and ρch a constant, where ρc is the density of the crust and h the topographic elevation. If the rocks are on the sea bed, then ρch + ρwd is a constant, where ρw is the density and d the depth, of sea water. See also AIRY MODEL.