Themes of the Day:

• Stress and Strain
• Strike and Dip

Up Now: Structural Geology - encompasses folding, faulting, and other aspects of deformation of rocks

Stress & Strain

• Stress - force applied per unit area - Three Types
  • compressive
    
  • tensional
    
  • shear
    
• Strain - deformation of a material as a result of applied stress
  • elastic - completely recoverable when stress is removed
  • yield point - boundary between elastic and plastic behavior
  • plastic - permanent (ductile) deformation imparted
  • fracture - brittle failure - permanent and unrecoverable
• Type of behavior is influened by:
  • Rock Composition (e.g., calcite in limestone and marble deforms in a ductile manner under the same conditions that quartz and feldspar in a granite deform in a brittle manner)
  • Temperature and Pressure - High T, P favor ductile behavior (e.g., earthquakes - brittle behavior - are most common in the shallow crust - < 15 km deep)
  • Strain Rate - faster fosters brittle behavior, slower fosters ductile behavior

Strike & Dip

• Strike - the intersection of a horizontal plane with a planar feature in a rock (such as sedimentary bedding or metamorphic foliation) - measured in the horizontal plane as the azimuth or bearing of the strike line
• Dip - oriented perpendicular to strike - the directly downhill direction - measured in the vertical plane as the angle from the horizontal down to the dipping plane
• Strike and dip are measured in the field and used to describe the attitude of geologic features on maps