stress and strain

• Competent rock stays the same thickness when deformed and reacts by fracturing. They are brittle e.g. igneous rocks, limestone. 
• Incompetent rock varies in thickness when deformed, they behave plasticly and do not fracture. E.g. mudstone, shale and clay. 
• A stress is a force that is applied to a rock. 
•  

  A strain is the change in shape or volume to the applied force. 
• $Strain =$$△ in length of line / original length of line.$
• competent rock

  A) fracture point

  B) elastic

  
• incompetent rock

  A) elastic limit

  B) plastic

  C) elastic

  
• Higher temperature causes rock to act mor plastically, so rocks fold and don’t fault/fracture at high temperature. Cold rock fractures 
• Strength of rock increases with confining pressure, deeper rocks are harder to deform. 
• Short periods of stress mean brittle behaviour, a longer periods of stress cause rock to be more plastic.