1. ROCK MECHANICS The study of rock behavior in the solid state under varying environmental and internal conditions Main focus is how rocks respond to applied stresses, especially those that naturally occur due to: –Gravity –Mantle Convection –Plate Tectonics –Diapiric Movements: magma, salt diapirs

2. Stress Same as Pressure = Force/Area Units in Earth Sciences are MPa = 10 6 Pa = 10 bars =10 atm = 147 psi

3. STRAIN Response to or result of applied stress Linear strain is defined as (L 1 – L 0 )/L 0 Units are ???? (unitless) Volumetric strain = (V 1 -V 0 )/V 0 Units are ????? Areal Strain = (A 1 -A 0 )/A 0 Strain is usually expressed as a percent change

4. ENVIRONMENTAL CONDITIONS affecting rock behavior Stress Factors: –Applied Stress –Stress History Duration How stress applied over time –Temperature –Pore Fluids and Pressure

5. Lithospheric Pressure Aka Confining Pressure The vertical component can be calculated as Pc =  gz, Where  = average density in g/cc, g= grav. constant, z = depth In the Lithosphere Pc increases at about 25-30 MPa/km In general Pc increases at  *10 MPa/km

6. Pressure vs. Depth in Earth

7. Geothermal Gradient T increases in the Lithosphere at 25 0 C/km T continues to increase all the way to the core, but at a lower rate The melting T of Quartz is 573-870 0 C The melting T of Feldspar >1100 The melting T of calcite is 910

8. Temperature vs. Depth in the Earth

10. Horizontal Stresses in the Earth do not equal vertical load,  gz, but are about 0.25-0.5 of  v depending on composition, porosity, pore fluids

11. INTERNAL CONDITIONS affecting rock behavior Composition ( silicates and oxides most resistant to strain ) Texture/Fabric ( phaneritic most resistant ) Pores, cracks, flaws Previous stresses stored in grains Degree of Weathering ( minerals that weather from oxides and silicates to other compounds are weakened ) Presence of fluids in pores or cracks

12. Rock Texture/Fabric Grain size Interlocking or grains in cement? Sorting Foliation/ non-foliated Sedimentary layering

13. Textures Note: Most Strain is not due to breaking bonds, but is due to slip of grains against each other, or between grains and cement

14. Porosity vs. Depth

15. Rigid and Brittle Behaviors Rigid: No strain, no response Most rocks show rigid behavior at room T and no Pc, and at stresses below Pa – Brittle: No strain before failure In ideal brittle behavior, if stress is removed before rock fails, there will be no internal signs of strain