Faults, Folds, and Joints and The Difference Between Them

Structural geology is concerned with the description, spatial representation, and analysis of structural features ranging from microscopic to megascopic. It controls the migration of groundwater, oil, gas, and natural minerals.

Fault

Faults are the fractures in bedrock along which movement has occurred. They are categorized by type of movement as dip-slip, strike-slip, or oblique-slip fault.

Types of Fault

Dip-slip faults: They have movement parallel to the dip of the fault plane. In normal faults, the hanging-wall block has moved down relative to the footwall block. In reverse faults, the hanging-wall block has moved up relative to the footwall block.
Strike-slip faults: They have a movement that is predominantly horizontal and parallel to the strike of the fault plane. A viewer looking across to the other side of a right-lateral strike-slip fault would observe it to be offset to their right. A viewer looking across to the other side of a left-lateral strike-slip fault would observe it to be offset to their left.
Oblique-slip faults: They have movement with both vertical and horizontal components.

How to recognize faults in the field

Discontinuity of structures
Repetition or omission of strata
Silicification and mineralization
Feature characteristics of the fault plane
Sudden changes in sedimentary facies
Physiographic evidence

Significance of faults on landscape/outcrop formation

Destruction of earth surface through earthquake
Lowers the stability of land
May play role in origin of earthquake and volcanism
May results thermal spring in particular area due to friction
Play role in soil formation through silicification and weathering

Folds

Folds are a wavelike bends in layered rock. They represent rock strained in a ductile manner, usually under compression.

The axial plane divides a fold into its two limbs. The surface trace of an axial plane is called the hinge line(or axis) of the fold.

Parts of folds

Hinge (axis): Axis is the maximum curvature in a folded bed.
Axial plane: It is the surface connecting all the axis. It may be vertical, inclined, and horizontal.
Limb: It is on either side of the fold.
Crest: It is the highest part of the folded bed.
Trough: It is the lowest part of the folded bed.
Crest plane: It is the line that connects all crests.
Trough plane: It is the line that connects all troughs.

Anticlines are upward-arching folds, and synclines are downward-arching folds. Anticlines eroded horizontally at the surface show the oldest ages in the centerline. Synclines eroded horizontally at the surface show the youngest ages in the centerline(bedding ages are symmetrically older going away from the center).

Here are some other types of folds:

Plunging folds: They are folds in which hinge line is not horizontal, where surfaces have been levelled by erosion, plunging folds form V- or horseshoe-shaped patterns of exposed rock layers (beds).
Open folds: They have limbs that dip gently, whereas isoclinal folds have parallel limbs.
‏Overturned folds: They have limbs that dip in the same directions.
Recumbent folds: They are overturned to the point of being horizontal.

Significance of folds on landscapes/outcrop Formation

They cause shortening of the earth crust and their subsequent thickening
Usually stream follows from the axis portion of the anticline ridges
Dome structures occur along the axis of the syncline
Repetition of beds in their occurrence infers the presence of fold
May develop fractures and joints in anticline ridge and result soil formation through weathering and: lowers stability of land.

Joint

A joint is a break(fracture) of natural origin in the continuity of either a layer or body of rock that lacks any visible or measurable movement parallel to the surface(plane) of the fracture.

A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of the orientations, spacing, and physical properties. A joint system consists of two or more interlocking joint sets.

Causes of Joints in Geological strata

Joints results from brittle fracture of a rock body.
As a result of tensile stresses
The rise of pore fluid pressure as the result of either external compression or fluid injection
The result of internal stresses induced by the shrinkage caused by the cooling.

Classification of Joints

Joints are classified on the following bases:

Genetic Classification:

Tensional joint: This is a shrinkage joint
Shear Joint: This joint is developed from the sliding history of geological strata
Tectonic joint: Joints developed from tectonic activities and categorized according to attitude of beds.

Geometric classification

Strike joint: Joints strike in the same direction of the rock formation
Dip joint: Joint strike parallel to the dip of the rock formation.
Oblique/diagonal joint: The strike of the joint makes an inclined angle with the strike of the rock formation.

Significance of Joints

To understand the nature and sequence of deformation in an area.
Joints commonly control the drainage pattern of the area.
May act as aquifers or reservoirs in rocks for oil or natural gas or groundwater.
Help to find out the brittle deformation in an area of construction (dams, bridges, road, building, and power plants).
In mineral exploration, finding out the trend and type of fractures and joints that host mineralization that will help in the exploration.
Joints play a great role in soil formation through chemical (decomposition), physical (disintegration), and biological weathering.
It lowers the stability of the land.

Differences between faults, folds, and Joints

Here are some major differences between fault, fold and joints:

The fold is permanent wavelike deformation in layered rock or sediment, fault is a fracture in bedrock along which rocks on one side have moved relative to the other side whereas a joint is a fracture on a rock without noticeable movement.

Joints and faults are types of fractures. A joint is a fracture along which no movement has taken place, usually caused by tensional forces. A fault is a fracture or breaks in the rock along which movement has taken place. One might expect more earthquakes to occur near faults.
The results of stress are folding and faulting. When a rock has stress put on it and does not break it is called folding. Folds appear as wave-like structures in rock layers. Some folds are small and can be seen in individual rocks and some folds are huge and can only be seen from the air.
Folds typically occur in hotter rocks as this makes them more bendy/ ductile.