strain

• Stress is a force applied per unit area acting on a rock. 
• Stress is measured in pascals. 
• Strain is a deformation, a change in volume or shape. 
• The ratio of change in dimension to the original value of the dimension. 
• Incompetent rocks - mudstone, shale and clay. 
• The yield strength is when stress exceeds the elastic limit. 
• With Increasing pressure rock strength increases, so they are less likely to fracture. 
• Increasing temperatures creates ductile behaviours. 
• In the earth temperature and pressure increase with depth. Temperature overcomes pressure, this results in ductile behaviour at depths. 
• Water weakens rocks by affecting the bonding of materials, high fluid pressure weakens rocks and reduces friction making ductile deformation more likely. 
• Some rock forming minerals like mica and calcite are usually ductile, other rock forming minerals like quartz, olivine and garnet are more brittle. 
• Applied pressure over a short period f time causes brittle behaviour, applied pressure over a long period of time causes ductile behaviour. 
• Strain rate is the change in strain of a material with respect to time. Rocks are slightly stronger at greater strain rates. 
• Faulting results when applied tectonic stresses exceed the fracture strength of a rock. 
• There are 3 principle stresses at right angles to each other, called sigma, max, min and int. Load pressure is always one of the principle stress so the other two are horizontal. 
• Fault type is determined by the orientation of the principle stress components.
• Structural reactivation is when earlier formed faults can be reactivated during later tectonic events, folds can also be refolded. 
• A Fault breccia is broken fragments of rock either side of the fault plane, the fragments are large, angular and are made of competent rock. 
• Faults gouges are grinding of rock along fault planes that produce fine grained particles, they form due to pressure and heat at thrust faults. Ductile deformation of incompetent roc. 
• Slickensides are fault displacements that scratches the fault surface, the lineations produced show the direction of movement, it can be seen when a mineral is precipitated by a fluid that flowed along the fault plane. The grooves and ridges shoe. The direction of the most recent movement along the fault, they feel rough against the motion and smooth along the motion direction. 
• Vertical beds do not get displaced with vertical faulting.