THERMAL CONVECTION IN THE INTERIOR OF THE EARTH

Abstract

The paper deals with thermal convection in the shell of the earth, caused by various assumed zonal temperature perturbations. One temperature perturbation here treated is that due to the difference in the temperature distribution under a continental crust made up of 10 km. of granite on top of 20 km. of basaltic material and a sub-oceanic crust consisting of 25 km. of basalt. The kinematic viscosity v was assumed to be 3 × 1021, as estimated recently by N. A. Haskell from a study of the uplift of Fennoscandia after the ice load. It is found that when g, v, p and a, the coefficient of volume expansion, are constant, the velocities and their gradients are proportional to the amplitude of the temperature perturbation and to (ag/v), while the stresses are independent of the viscosity. The velocities are found to be of the order of i cm./year. The shearing stress exerted by the convective substratum on the crust is of the order of 107 dyn./cm.2, while the normal stresses are about 10 times larger. The crust is pushed upwards under the warmer (continental) regions and pulled downwards under colder (oceanic) regions. The maximum stress-difference occurs at the bottom of the crust over the centre of the oceans or continents. The surface inequalities are nearly compensated.