In three dimension orthogonal coordinate system, we can define stress p as (pxx pxy pxz pyx pyy pyz pzx pzy pzz).
固体地球物理学概论
第七章
弹性概念——应力 (续)
The stresses are symmetrical(对称的), i.e. only six components of the stress tensor p are independent because
P = - (pxx+ pyy+ pzz)/3 This is a general definition of the “pressure”. In the special case of a liquid at rest, pxx= pyy= pzz = - P, this is the hydrostatic pressure. In geology, lithostatic pressure is often estimated by using
When the material in the mantle is heated, it expands and becomes lighter. In spite of its high viscosity(粘性), it rises more or less vertically in some places, especially under the oceanic ridges. With its losing pressure and heat during traveling upward, the material is forced to travel horizontally. They drag the lithosphere motion.