Effective viscosity of the Earth derived from isostatic loading of Pleistocene Lake Bonneville

Abstract

An average load of 145 in of water covered an area of about 50,000 km2 in western Utah 15 to 25 thousand years ago and has since been removed by overflow and evaporation. Deformation of the shore lines of this lake indicates that the earth attained about 75 per cent of isostatic compensation during both loading and unloading. This requires that the anomalies created diminished to 1/e of their initial value in about 4000 years and indicates that the effective viscosity of the earth in this area is about 1021 poises. This is an order of magnitude less than the value of 1022 poises generally obtained from ice unloading in Fennoscandia.