In the absence of mineralogical phase transformations, rocks exhibit relatively small density variations (see Chapter 2). Therefore, the incompressible fluid approximation (ρ =const,
Dρ
Dt
= 0, see Chapter 1) is generally valid. In this case,
˙εkk = div( ¯ v) = 0, ˙ε
ij
= ˙εij and the law of viscous friction can be simplified to: