Directed pressure during metamorphism reorients minerals into parallel planes. Foliation is that planar fabric in metamorphic rocks. It ranges from the fine cleavage of slate to the coarse banding of gneiss.

Micas and other platy minerals align perpendicular to the maximum compressive stress, giving the rock a strong preferred orientation. When compositional differences are also segregated, the result is compositional banding. Foliation surfaces often become planes of weakness that control how the rock breaks.

Field geologists use foliation to interpret the orientation of past stresses and the geometry of folds. In engineering, foliated rocks may fail along these planes in slopes or tunnels. The presence and type of foliation are key criteria in the classification of metamorphic rocks.

• Planar fabric in metamorphic rock
• Caused by directed pressure
• Ranges from slaty cleavage to gneissic banding
• Records orientation of past stresses