Thermal evolution of the earth

Abstract

Models to study the thermal evolution of the earth have been developed based on a calculation of finite-amplitude thermal convection in an incompressible and viscous mantle with temperature and pressure-dependent physical parameters. The lateral dependences of the variables are resolved through their spherical harmonic representations, while the radial and time dependences are calculated using numerical procedures. The major emphasis of this paper is to illustrate the effects of different factors on the thermal evolution of the earth rather than to present a definite earth model. The main conclusions achieved in this study are: 1. (1) A significant portion of the present-day temperature in the mantle and surface heat flux is due to initial heat of the earth resulting from accretion and core formation. 2. (2) Since the effective Rayleigh number of the earth's mantle is high, solid-state convection is found to be sufficiently efficient to maintain an almost adiabatic temperature gradient within the convecting region, while large temperature gradients exist across the thermal boundary layers (core—mantle boundary) and below the lithosphere. 3. (3) The convection is oscillatory with avalanche-type properties. This is due to the slow development of instability in the thermal boundary layer near the surface which causes the episodic behavior. The periods of the oscillations are about 50–250 m.y. The oscillation in the mantle also induces periodic features in the surface heat flux and the thickness of the lithosphere. Mantle viscosity structure plays an important role in the cooling rate and nature of oscillations. 4. (4) The mobility of the lithosphere is an important factor controlling the thermal state of the earth's interior. A mobile lithosphere will cool the interior faster.