Geological strain is how structural geologists quantify how rocks change shape and volume in response to tectonic forces. This study is essential for understanding mountain building, the formation of continental crust, and the distribution of natural resources. By analyzing the permanent changes preserved in rock layers, geologists reconstruct the history of deformation across vast regions of the Earth’s surface. Strain is simply the measure of this change from an original state to a final, deformed state.
Defining Strain and Its Relationship to Stress
Geological strain represents the change in a rock’s size, shape, or orientation. It is a kinematic measure, describing the geometry of the deformation without regard to the forces that caused it. Strain is a dimensionless quantity, often expressed as a ratio of final length to original length, or as an angular change.
This change is directly caused by stress, which is the force applied to a rock body per unit area. Stress is a dynamic measure and shares the same units as pressure, such as Pascals or bars. The relationship is causal: stress is the agent, and strain is the resulting reaction.
When a rock is subjected to compressional, tensional, or shear stress, it accumulates strain. The rock’s final, permanent shape records the total, or finite, strain that occurred after the stress was applied over geological time. Understanding this distinction between the applied force (stress) and the resulting change (strain) is crucial for interpreting the Earth’s deformed crust.
How Geologists Classify Strain Geometry
Geologists classify strain geometry by considering how the principal axes of deformation behave during the process. These principal strain axes represent the three mutually perpendicular directions of maximum shortening, intermediate extension/shortening, and maximum extension within the deformed rock.
One classification is coaxial strain, often called pure shear or flattening. In this scenario, the principal strain axes maintain their orientation relative to the rock material as deformation progresses. The rock deforms along these fixed directions without any bulk rotation.
Conversely, non-coaxial strain, such as simple shear, involves the rotation of the principal strain axes relative to the rock material during deformation. This occurs when one part of the rock mass slides past another, similar to shuffling a deck of cards. Strain is also classified as homogeneous (uniform deformation) or inhomogeneous (strain intensity varies from point to point).
Measuring Strain in Deformed Rocks
Quantifying geological strain involves finite strain analysis, which determines the three-dimensional shape of the strain ellipsoid. This imaginary ellipsoid represents the deformed shape of an originally perfect sphere within the rock. Its three principal axes, X, Y, and Z, correspond to the maximum, intermediate, and minimum stretches.
Geologists rely on strain markers to measure this deformation. These are objects within the rock whose original, undeformed shape can be reasonably assumed, often spherical or circular. Common strain markers include:
- Fossils like trilobites
- Originally spherical ooids
- Lapilli fragments
- Chemical reduction spots
By measuring the now-elliptical shape of these markers in a rock section, the two-dimensional strain ellipse can be calculated.
The change in length of a line is quantified by elongation (\(e\)), which is the fractional change in length, or by the stretch (\(S\)), which is the ratio of the final length to the original length. The degree of angular distortion, or shear strain (\(\gamma\)), is measured from the change in angle between two originally perpendicular lines. By taking measurements on multiple, differently oriented surfaces of the rock, the full three-dimensional strain ellipsoid is reconstructed.
Physical Structures Caused by Strain
The accumulation of strain results in recognizable geological structures. When strain occurs under low temperature and pressure, the rock responds in a brittle fashion by fracturing. This brittle strain leads to the formation of faults, where the rock has broken and one side has moved relative to the other.
When rocks are subjected to high temperatures and pressures deep within the crust, they undergo ductile strain, changing shape permanently without fracturing. This flow-like deformation creates folds, where rock layers are visibly bent and warped. Folds range in size from microscopic corrugations to mountain-scale arches and troughs.
Intense shortening or flattening, a pervasive type of ductile strain, creates foliation or cleavage. This fabric is a planar arrangement of mineral grains, such as the layers seen in slate or schist, which develops perpendicular to the maximum shortening direction. These structures—faults, folds, and foliation—are the physical evidence geologists use to reverse-engineer the stress and strain history of a region.