Newtonian Fluid Behavior
The strain rate is proportional to the stress, resulting in a Newtonian fluid behavior. At higher stress levels creep is the result of the movement of dislocations through the crystal lattice. In this case, the strain rate is proportional to the third power of the stress, resulting in a non-Newtonian fluid behavior. Both diffusion creep and dislocation creep are thermally activated; that is, the strain rates are inversely
proportional to the exponential of the inverse absolute temperature. The strain rate in the mantle is so small that it cannot be reproduced in the laboratory. However, extrapolations of laboratory easurements give fluid properties consistent with thermal convection in the mantle. Independent information on the fluid behavior of the mantle comes from studies of glacial loading and unloading. When an ice sheet forms, its weight forces mantle rock to flow from beneath it, and the Earth’s surface subsides. When
the ice sheet melts, the mantle rock returns, and the Earth’s surface rebounds. It takes about 10,000 years for the rebound to take place. Data on the rate of rebound come from dated, elevated beaches. These data have been used to obtain a viscosity for the mantle of about 1021 Pa s. Although this viscosity is large, it allows a fluid behavior of the mantle on geological time scales.
