Rocks, which are typically brittle at the Earth's surface, can undergo ductile deformation when buried and subjected to increased confining pressure and temperature for long periods of time. If stress exceeds their strength or if they are not sufficiently ductile, they will fail by fracture--as a crystal, within a bed or rock, on an earthquake fault zone, and so on--whereas with ductility they can flow and fold.
Some strengths for various rock types under different temperatures and confining pressures are listed in Table 38. The plastic yield strength here is the stress at a 2 percent strain; the ultimate strength, as stated above, is the highest point (stress) on the stress-strain curve.
An increase in confining pressure causes brittle fracture to become shear slippage and eventually causes flow (ductile) behaviour. This transition is also aided by higher temperature, decreased internal pore-fluid pressure, and slower strain rate.
Table 39 gives the values
of some elastic constants--bulk modulus (k), Young's
modulus (E), shear modulus (
), and Poisson's ratio (
)--at
room pressure (1 bar) and high confining pressure (3,000 bars).
The values for clastic sedimentary rocks would be particularly
variable.
