The Architecture of the Crust
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. It examines how rocks respond to stress.
Stress: The force applied per unit area on a rock.
Compressional Stress: Squeezing or pushing together (occurs at convergent boundaries).
Tensional Stress: Pulling apart (occurs at divergent boundaries).
Shear Stress: Forces acting parallel to each other but in opposite directions (occurs at transform boundaries).
Strain: The resulting deformation or change in the shape of a rock in response to stress.
How Rocks Deform
Brittle Deformation: The rock breaks or fractures. This occurs at low temperatures and pressures, near the Earth's surface. This process creates faults.
Ductile (or Plastic) Deformation: The rock bends or flows without breaking. This occurs at high temperatures and pressures, deep within the Earth's crust. This process creates folds.
Faults
A fault is a fracture in a rock along which there has been significant movement.
1.Normal Fault: The hanging wall (the block above the fault) moves down relative to the footwall (the block below). Caused by tensional stress.
2.Reverse Fault (or Thrust Fault): The hanging wall moves up relative to the footwall. Caused by compressional stress.
3.Strike-Slip Fault: The blocks move horizontally past one another. Caused by shear stress. (e.g., the San Andreas Fault).
Geologic Maps
A geologic map shows the distribution of different rock types and geologic features at the Earth's surface.
Strike and Dip: A symbol used on a map to indicate the orientation of a rock layer.
Strike: The direction of the line formed by the intersection of a rock layer with a horizontal plane. It indicates the trend of the rock layer on the surface.
Dip: The angle at which the rock layer is inclined from the horizontal, measured perpendicular to the strike. It indicates how steeply the layer is tilted.