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.