Viscosity and Finite Strength of the Mantle as Determined from Water and Ice Loads*

Some recent examples of transient Earth loads (Lake Bonneville, Utah; Glacier Bay, Alaska; northeast Greenland) indicate that both the viscosity and finite strength of the mantle are lower than is commonly presumed. A time constant (1/e) of 4000 years is estimated for Lake Bonneville, and of 1000 years for northeast Greenland. A strain rate of 10−14 is typical. These figures imply viscosities in a homogeneous half space ranging from 1020 to 1021 poises. An upper limit of finite strength is set by Lake Bonneville at a few times 106 dyn/cm2. If mountain ranges like the Sierra Nevada or Himalaya are regarded as dynamically supported rather than static systems, this low value is not incompatible with other geologic observations.