<Dd> (∂ ln ⁡ σ ∂ P) T, ε _̇ = (1 T T m) × (∂ ln ⁡ σ ∂ (1 / T)) P, ε _̇ × d T m d P (\ displaystyle \ left ((\ frac (\ partial \ ln \ sigma) (\ partial P)) \ right) _ (T, (\ dot (\ epsilon))) = \ left ((\ frac (1) (TT_ (m))) \ right) \ times \ left ((\ frac (\ partial \ ln \ sigma) (\ partial (1 / T))) \ right) _ (P, (\ dot (\ epsilon))) \ times (\ frac (dT_ (m)) (dP))) </Dd> <P> Most of the mantle has homologous temperatures of 0.65--0.75 and experiences strain rates of 10 − 14 − 10 − 16 (\ displaystyle 10 ^ (- 14) - 10 ^ (- 16)) per second . Stresses in mantle are dependent on density, gravity, thermal expansion coefficients, temperature differences driving convection, and distance convection occurs over, all of which give stresses around a fraction of 3 - 30MPa . Due to the large grain sizes (at low stresses as high as several mm), it is unlikely that Nabarro - Herring (NH) creep truly dominates . Given the large grain sizes, dislocation creep tends to dominate . 14 MPa is the stress below which diffusional creep dominates and above which power law creep dominates at 0.5 Tm of olivine . Thus, even for relatively low temperatures, the stress diffusional creep would operate at is too low for realistic conditions . Though the power law creep rate increases with increasing water content due to weakening, reducing activation energy of diffusion and thus increasing the NH creep rate, NH is generally still not large enough to dominate . Nevertheless, diffusional creep can dominate in very cold or deep parts of the upper mantle . Additional deformation in the mantle can be attributed to transformation enhanced ductility . Below 400 km, the olivine undergoes a pressure induced phase transformation into spinel and can cause more deformation due to the increased ductility . Further evidence for the dominance of power law creep comes from preferred lattice orientations as a result of deformation . Under dislocation creep, crystal structures reorient into lower stress orientations . This does not happen under diffusional creep, thus observation of preferred orientations in samples lends credence to the dominance of dislocation creep . </P>

The process of mantle convection taking place in the mesosphere