Structural geology

Structural geology is the scientific study of the three-dimensional arrangement of rock units and the deformation events that have shaped them through geological time. Its central aim is to interpret present-day rock geometries in order to reconstruct the history of strain, deformation mechanisms and the stress fields that produced them. By linking these deformation histories to regional and global tectonic processes such as orogeny and rifting, structural geology provides a framework for understanding the evolution of the Earth’s crust.

Use and importance

Structural geology is fundamental to several applied fields. In economic geology, folded and faulted strata frequently create natural traps for oil and gas, while structurally complex zones provide pathways for hydrothermal fluids that concentrate metals into ore bodies. Mineral veins rich in gold, silver, copper, lead and zinc often form in fault systems or within fractures associated with igneous intrusions, reef complexes or collapse structures such as ancient sinkholes.
The discipline is equally critical in engineering geology, where knowledge of faults, folds, foliations and joints is essential for assessing the safety of dams, tunnels, open-pit mines and road cuttings. These structural features often act as weaknesses that influence rock stability. Geotechnical hazard assessment—including landslides, rockfalls and earthquake risk—frequently requires an integrated examination of structural geology and geomorphology.
Structural geology also informs environmental geology and hydrogeology. Fractures, faults and other structural fabrics govern groundwater flow, influencing seepage of contaminants, saltwater intrusion and aquifer performance. Regions characterised by karst landscapes or steep slopes pose additional stability challenges that require structural analysis for hazard mitigation.
Plate tectonics provides an overarching framework for structural interpretation at all scales, from global continental movement to regional and local rock deformation.

Methods

Structural geologists employ a range of techniques to measure rock geometries, reconstruct deformation histories and estimate past stress fields.
Geometric measurement lies at the heart of structural analysis. Fieldwork provides primary data, including measurements of:

• planar structures such as bedding, foliation, axial planes, faults and joints
• linear structures such as stretch lineations, fold axes and intersection lineations

Measurement conventions

The inclination of a planar feature is described by strike and dip. Strike is the horizontal line formed by the intersection of the plane with a level surface, while dip is the angle of maximum inclination measured perpendicular to strike. An alternative expression uses dip and dip direction, an absolute orientation measured clockwise from north. Linear features are described by plunge and plunge direction or, when observed on a planar surface, by rake, the angle between the lineation and the horizontal within that plane.

Structural fabrics and deformation sequences

Planar fabrics are assigned numbered labels according to their relative order of formation. The earliest sedimentary layering is denoted S₀, while later cleavages or foliations created during deformation events are labelled S₁, S₂ and so forth. Folds are similarly labelled F₁, F₂, etc., and deformation events are denoted D₁, D₂, D₃. Axial-plane foliations generally correspond to the fold they accompany; an F₂ fold, for example, tends to form an S₂ axial foliation. Metamorphic events may overlap multiple deformation events and can be correlated using mineral assemblages, porphyroblast overgrowth relationships or geochronology.
Intersection lineations arise from the meeting of two planar surfaces and are identified using the labels of those planes, such as L₁₀ for the lineation formed by the intersection of S₁ cleavage and bedding. Stretching lineations reflect ductile elongation and may be labelled in parallel with planar fabrics.

Stereographic projection

Stereographic projection allows structural geologists to plot three-dimensional orientations of planes and lines onto a two-dimensional grid. By representing structural features on a stereonet, clusters, intersections and deformation patterns become easier to visualise and interpret. Digital stereonet tools have become widely used for analysing datasets and constructing deformation models.

Rock macrostructures

At large scales, structural geology examines the geometry and relationships of stratigraphic units within geological terranes. This involves interpreting the orientations and modifications of bedding, including folding, faulting and foliations generated during tectonic events. The analysis is often geometric, relying on cross-sections and structural maps to reconstruct the arrangement of rock units in three dimensions.