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(ī sos’ t∂ sē) The gravitational balance of large blocks of the Earth’s crust as though they were floating on a denser underlying layer (the asthenosphere). Major topographic features are in isostatic balance. (a) The Airy hypothesis varies the thickness of crustal blocks of constant density so that the thicker parts ride higher. Thus mountainous areas are compensated by deep crustal roots extending to 50–60 km and deep ocean basins by antiroots at 6–8 km. See also Airy-Heiskanen system. (b) The Pratt hypothesis, assumes that the blocks vary primarily in density, less-dense crust rising topographically above areas of denser crust (see Figure I-7). (c) The crustal flexure hypothesis (Vening Meinesz hypothesis) allows some of the balance to be accommodated laterally by the surrounding region rather than only in the vertical direction. The radius of regionality specifies the size of the region over which compensation is distributed (of the order of 200 km). The Hayford modification has the pressure balanced at the ‘‘depth of compensation.’’