Geological structures (folds and faults)

Geological Structures: Folds

Folds are bends or wrinkles formed in rocks in response to stress. This stress could be from tectonic forces, which cause rocks to deform plastically.
There are three main types of folds - anticlines, synclines, and monoclines.
Anticlines are symmetric or asymmetric folds with the oldest rocks in the core and the limbs dipping away from the axis. The layers appear to arch upwards.
Synclines are folds in which the layers dip towards the axis and the youngest rocks are in the core. They appear as downward arches.
Monoclines are folds where there is a step-like bend in otherwise horizontal rock layers. They display a gentle dip in one direction.
The degree of folding can range from gentle ripples in the rock strata to complex, tight folding.

Geological Structures: Faults

Faults are fractures in rocks where movement has occurred. They typically result from tectonic forces and can range in size from a few centimetres to hundreds of kilometres.
There are three main types of faults - normal faults, reverse faults (or thrust faults), and strike-slip faults.
Normal faults occur where tectonic forces cause the crust to expand or extend. The hanging wall block moves down relative to the footwall block.
Reverse faults occur when compressive forces cause the crust to shorten. The reverse is true of normal faults, the hanging wall block moves up relative to the footwall block.
Strike-slip faults occur when rocks slide past each other laterally. If you stood and looked along the fault, a right-lateral strike-slip fault would show the other side of the fault moving to the right and vice-versa.
The movement along faults can lead to earthquakes.
The identification of faults and their movement direction helps geologists map areas of crustal movements and contribute to plate tectonic theory.

Important Terminology Involving Folds and Faults

The hanging wall is the rock mass above an inclined fault plane, and the footwall lies beneath.
The dip is the angle that a rock layer or fault makes with the horizontal plane.
Plastic deformation refers to the permanent deformation of a material undergoing stress – like rock being molded or bent.
The axis of a fold refers to a line drawn along the points of greatest curvature in folded strata.
Limbs are the sides of the fold that dip away (anticline) or towards (syncline) the axis.

Understanding these underlying concepts will enhance interpretation of geologic maps and cross-sections and aid in predicting rock formations beneath the surface.