Processes in the upper mantle

Abstract

All current tectonic hypotheses assign the cause of vertical crustal movements to processes within the earth. Some suppose an expansion and contraction of mantle material, probably caused by physical transitions (polymorphic, phase and electronic) and chemical reactions. Pressure and temperature are the principal parameters, which, in combination, give rise to thermodynamic conditions that are functions of depth. They are the basic physical factors defining the state of material in upper mantle and in the earth as a whole. The material of the earth is subject to geodynamic stresses under given thermodynamic conditions. The geodynamic stresses are produced by pressure of the overlying layers combined with centrifugal acceleration by the rotation of the earth. It is likely that this rotation is the primary initiating cause of processes in the upper mantle that cause effects simultaneous throughout the earth, because this alters the distribution of geodynamic stress. One of the main causes of onset of transitions or reactions at depth is variation in geodynamic stresses in the inhomogeneous upper part of the mantle. Another very active cause of conditions giving rise to transitions are thermoelastic stresses, which are associated with deviations from thermal equilibrium, especially in particular regions or local zones in the crust and upper mantle. There are probably other, as yet unknown, sources of stress in the mantle, but those dealt with here are sufficient to cause physico-chemical changes within the earth. It is most likely that transitions and reactions at depth between 80 and 413 km (layer B) are the main causes of crustal movement. The processes in layer C (410–1000 km) which occur simultaneously over regions of large horizontal and vertical dimensions cannot have any marked effect on individual crustal elements.