Plateness of the oceanic lithosphere and the thermal evolution of the earth's mantle

Compared to [33], the model of the thermal evolution of the Earth's mantle is considerably improved. The temporal development of the radial viscosity profile due to cooling of the Earth could substantially be taken into account by numerical progress using a new variant of the temperature- and pressure-dependence of the shear viscosity of the mantle, namely Eq (5). The laterally averaged heat flow, the Urey number, the Rayleigh number and the volume-averaged temperature as a function of time come up to the expectations that stern from the parameterized evolution models. The mentioned evolution parameters of the present paper better approximate the observational data. Contrary to the parameterized curves, these quantities show temporal variations. This seems to be more realistic for geological reasons. Due to the activation enthalpy, the presented viscosity profile has a highly viscous transition layer (TL) with steep viscosity gradients at the phase boundaries. A low-viscosity zone is situated above and below the TL, each. The lithosphere moves piecewise en bloc. Thin cold sheet-like downwellings have an Earth-like distribution.