<P> Since the mantle is primarily composed of olivine ((Mg, Fe) 2SiO4), the rheological characteristics of the mantle are largely those of olivine . Additionally, due to the varying temperatures and pressures between the lower and upper mantle, a variety of creep processes can occur with dislocation creep dominating in the lower mantle and diffusional creep occasionally dominating in the upper mantle . However, there is a large transition region in creep processes between the upper and lower mantle and even within each section, creep properties can change strongly with location and thus temperature and pressure . In the power law creep regions, the creep equation fitted to data with n = 3--4 is standard . </P> <P> The strength of olivine not only scales with its melting temperature, but also is very sensitive to water and silica content . The solidus depression by impurities, primarily Ca, Al, and Na, and pressure affects creep behavior and thus contributes to the change in creep mechanisms with location . While creep behavior is generally plotted as homologous temperature versus stress, in the case of the mantle it is often more useful to look at the pressure dependence of stress . Though stress is simple force over area, defining the area is difficult in geology . Equation 1 demonstrates the pressure dependence of stress . Since it is very difficult to simulate the high pressures in the mantle (1MPa at 300--400 km), the low pressure laboratory data is usually extrapolated to high pressures by applying creep concepts from metallurgy . </P> <Dl> <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> </Dl> <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>

What layer of the earth does convection occur