The creep strength of the Earth's mantle

In this paper we show that it is probable that at stresses greater than about 10 −2 bar creep in the earth's mantle is caused by dislocation motion rather than by the mass transport of atoms through diffusion from one grain boundary to another. The latter process leads to Nabarro‐Herring creep. Only at very low stresses (<10 −2 bar) should Nabarro‐Herring creep be dominant. Dislocation creep equations (which are nonlinear in stress) are used to estimate the ‘effective viscosity,’ at a constant creep rate of 10 −16 sec −1 , of the moon, Mars, and the mantles of Venus and the earth. The effective viscosity of the mantles of Venus and the earth diminishes with increasing depth, goes through a minimum, and then increases continuously up to the mantle‐core boundary. The effective viscosity in the lower mantle is much smaller than estimates derived from the Nabarro‐Herring creep equation. The effective viscosity in the deep interior of the moon is approximately constant and does not increase appreciable with depth. The effective viscosity of Mars is intermediate in behavior between that of the earth and of the moon.