Deformation: Unconformities, Folds and Faults
Deformation of Rocks: Stress-Strain Graphs
Stress – forces applied to a rock
• Tensional Stress = pull apart
• Compressional Stress = move together
Strain – the deformation caused by the stress
Stress-Strain Graphs Key Words:
Elastic Deformation – no permanent deformation, will recover after
stress is removed
Plastic/Ductile Deformation – permanent change in shape
Fracture Point – when the rock starts to break
Elastic Limit – marks the start of permanent deformation
Brittle Response – yield strength of the material has been exceeded and the material fails in a brittle way
Factors Affecting Stress and Strain
• Temperature
• Composition of rock/strength
• Confining pressure
• Strain rate
Taking Dips and Strikes
Strike 1 Angle/direction of no dip
True Dip (dip amount) 4 The maximum angle of dip a rock can have
Dip Direction 2 Direction in which the rock is tilted
Apparent Dip Direction 3 Any angle between 0 (strike) and maximum tilt
Unconformities
Unconformity – a boundary that marks a hiatus (break) in deposition, a period of erosion. It represents a time
gap
Development of an Unconformity
1) Horizontal deposition (oldest at the bottom)
2) Tilting due to tectonic compression
3) Erosion of surface
4) Deposition of new materials
The unconformity is the boundary between the new material and eroded
Deformation of Rocks: Stress-Strain Graphs
Stress – forces applied to a rock
• Tensional Stress = pull apart
• Compressional Stress = move together
Strain – the deformation caused by the stress
Stress-Strain Graphs Key Words:
Elastic Deformation – no permanent deformation, will recover after
stress is removed
Plastic/Ductile Deformation – permanent change in shape
Fracture Point – when the rock starts to break
Elastic Limit – marks the start of permanent deformation
Brittle Response – yield strength of the material has been exceeded and the material fails in a brittle way
Factors Affecting Stress and Strain
• Temperature
• Composition of rock/strength
• Confining pressure
• Strain rate
Taking Dips and Strikes
Strike 1 Angle/direction of no dip
True Dip (dip amount) 4 The maximum angle of dip a rock can have
Dip Direction 2 Direction in which the rock is tilted
Apparent Dip Direction 3 Any angle between 0 (strike) and maximum tilt
Unconformities
Unconformity – a boundary that marks a hiatus (break) in deposition, a period of erosion. It represents a time
gap
Development of an Unconformity
1) Horizontal deposition (oldest at the bottom)
2) Tilting due to tectonic compression
3) Erosion of surface
4) Deposition of new materials
The unconformity is the boundary between the new material and eroded
