STRESS, DUE TO OVERBURDEN
If all three of the principal axes are of equal magnitude, then the ellipsoid simplifies to a sphere, and each of the infinite number of stress vectors are equal.
This particular type of stress is termed Hydrostatic stress. Under these conditions, none of the infinite plans feel a shear stress, and the material may undergo volume and/or mineralogical changes, but no deformation occurs. This type of stress is commonly experienced by deeply buried rocks.