Thermal state and thermal regime of the Earth's interior

Abstract

A simplified version of the thermal state and thermal regime of the Earth's mantle is given. It is supposed that the large-scale convection in the mantle is double-layered. Convection in the upper mantle is determined by heat flux from below, and in the lower mantle by internal heating. It is estimated that the thermal relaxation time of the upper mantle is ∼ 10 8 y, and the time of the thermal relaxation of the lower mantle τ lm is ∼ 1.5 × 10 9 y. The thermal relaxation of the Earth is determined by τ lm. In the convective mantle the superadiabatic temperature drops are formed in the thermal boundary layers: in the upper mantle ΔT 1 is ≤ 600 K, at the top of the lower mantle ΔT 2 is ≲ 650 K, at the bottom of the lower mantle ΔT 3 is ≲ 650 K. The adiabatic temperature at the core-mantle boundary is ∼ 2900 K and the real temperature must be ∼ 3500–4500 K. The thermal boundary layers in the mantle are the super-heated zones and must coincide with low Q μ zones ( Q μ is the mechanical quality for S waves). The first zone of low Q μ is at a depth of ∼ 100–250 km, the second one is at 670–850 km, and the third one is at 2700–2900 km. The source zone for hot spots may be in the second thermal boundary layer. It is interesting that the horizontal velocity of the layer, which is ∼ 1 cm y −1, must determine the limits of precision for absolute velocities in ‘Plate Tectonics’. The boundary between the upper and lower mantle must be the weak chemical boundary because the upper mantle is the source for the Earth's crust. The thermal regime and instability of the third thermal boundary layer (at a depth of 2600–2900 km) may play a very important role in the magnetohydrodynamics of the Earth's core and particularly for the change in polarity of the geomagnetic dipole and for understanding the geomagnetic time scale.