lineation

Lineation is a linear structural fabric in rocks characterized by the alignment of elongated minerals, crystals, or grains along a common direction. It represents a line-like element of rock fabric that can be observed on outcrops, in hand specimens, or in thin sections, and it is distinct from foliation, which records planar layering.

Lineation forms during deformation when rocks are sheared or stretched, causing elongated components to align parallel to a principal direction of shortening or extension. It can arise from the crystallographic alignment of minerals, from the preferred orientation of elongated grains, or from the realignment of crystals during ductile deformation. A related concept is stretching lineation, which specifically records the direction of maximum stretch and is often described by its trend and plunge.

Geologists distinguish several categories of lineation. The most common are mineral or shape lineations, produced by the alignment of elongated minerals or grains, and stretching lineation, which reflects the deformation’s extension direction. In the field, lineation is described by its trend (azimuth) and plunge (dip) when it has a measurable 3D orientation. In thin section, lineation appears as a consistent orientation of elongated grains such as mica or quartz prisms.

Lineation provides crucial information about deformation kinematics and the state of strain, aiding interpretations of tectonic history in metamorphic terranes and shear zones. It is typically analyzed together with foliation to reconstruct the finite strain ellipsoid and the sequence of deformation events.