STRESS SUMMARY Stress= amount of force per unit area (units Pa) Force= mass * acceleration (units N)

Lithostatic stress  * g * h  = density of rock g = acceleration of gravity h = height of rock column (or depth in crust)

Surface Stress Stress acting on a plane is a VECTOR quantity (has both magnitude and direction). This vector can be divided into normal and shear components.

Fy= F. SinA F A Fx= F. CosA y x Normal Shear

Principal Stress Components On the surface of the earth, TYPICALLY one principal stress component will be vertical. Principal stresses are NORMAL stresses (NOT SHEAR) The three principal stresses are always 90 degrees from each other  1 >  2 >  3

Lithostatic Stress Similar to hydrostatic pressure Magnitude of stress components is the same in all directions

Deviatoric Stress This is the amount of stress in each direction that is NOT lithostatic

Magnitude of Normal and Shear Stresses 11 33 Normal Shear

Strain Rate   = e/t what are the units?

Strain vs. Time = “Creep Curve”

Mechanical behavior depends on material properties Solids: strong, rigid Liquids: weak, deforms easily Plastic: can flow, but isn’t liquid (play-doh?) Elastic: Deforms, but returns to original shape when stress is released.

Stress vs. Strain diagrams Elastic Yield Point Plastic

Increasing Confining Pressure results in greater strain before failure: easier to flow Resists fracture formation

Rock type response to confining pressure

Effect of T: At low T, failure occurs quickly. High T, flow is easy

Effect of Strain Rate: slow=flow Fast Slow

Effect of Composition Slow Halite Gypsum Calcite Quartz Plag. Pyroxene Olivine

Effect of Composition 30% rule: Weakest mineral that makes up >30% of the rock will control the deformational style Examples…

Brittle vs. Ductile Deformation Brittle deformation: Fractures and Faults Ductile deformation: Folds, Foliation, lineation, shear zones

x-axis is differential stress or “STRENGTH” COLD HOT