Geological Structures

Geological Structures

Fold Structures

  • Folds are wavy, bend-like structures in rocks due to deformation under stress.

  • This deformation can be compressional, causing rocks to fold, or tensional, causing rocks to break and displace.

  • Describing a fold includes its antiform shape (upward arch) and synform shape (downward trough).

  • The axial plane of a fold refers to the imaginary flat surface that divides the fold as symmetrically as possible.

  • The fold axis is the line of maximum curvature on a fold layer.

Fault Structures

  • Faults occur when stress on rock surpasses their internal strength, causing a failure in the form of crack, along which movement occurs.

  • Normal faults are a result of tensional stress, where hanging wall blocks drop down relative to the standing wall, mainly due to gravity.

  • Reverse faults occur due to compressive stress, pushing hanging wall blocks upwards.

  • Strike-slip faults show horizontal movement due to shear stress.

Fractures and Joints

  • Fractures happen when rocks fail due to stress, but unlike faults, there’s no appreciable movement along the cracks in the rocks.

  • Joints are a form of fracture where no shear displacement occurs.

Unconformity

  • An unconformity is a ‘missing time’ gap in rock sequences signifying erosion or lack of deposition for a period.

  • Disconformity is a type of unconformity, a break between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

  • Angular unconformity happens when flat layers of sedimentary rocks are deposited on tilted and eroded layers.

  • Nonconformity is when sedimentary rock is deposited on eroded, older igneous or metamorphic rock.

Stratification and Bedding

  • Stratification refers to a system of stacked layers (beds) of sedimentary rock or soil. Each stratum (singular of strata) is typically different in colour, physical properties, fossil content, and reveals the kinds of environment during the time of deposition.

  • Cross-bedding is inclined sedimentary structures within a layer of rock that can indicate water currents or wind direction in the past.

Please note that these points represent only an overview of geological structures. Additional reading and practice will provide more detailed understanding and familiarity.