Short note: Rock cleavage

Rock cleavage is the property of a rock to split easily along closely spaced, parallel planes, producing thin sheets or slabs. It is a secondary structure, developed in a rock after its formation, usually in fine-grained metamorphic rocks by strong directional pressure. It should not be confused with mineral cleavage, which depends on the atomic structure of a single mineral.

Formation

Under compression, platy minerals (mica, chlorite) and flattened grains reorient perpendicular to the direction of maximum stress, so the rock splits parallel to these planes, which are unrelated to the original bedding.

Types

• Slaty cleavage: very fine, perfectly parallel planes in slate; slate splits into thin slabs.
• Fracture (shear) cleavage: closely spaced parallel fractures cutting the rock into thin slices.
• Strain-slip (crenulation) cleavage: fine folding of an earlier cleavage.
• Schistosity (coarser, flaky) in schists and gneissic banding are related foliations.

 Bedding   ====== ====== ======
 Cleavage   / / / / / / / /  (cuts across bedding)

Engineering significance

• Cleavage planes are planes of weakness along which the rock splits easily, so the rock's strength depends on the direction of loading.
• It decides stability of slopes, excavations and tunnel roofs (planar failure along cleavage).
• Useful for obtaining roofing slate and flagstone, but poor for aggregate and foundations.
• Allows water to seep, causing leakage and weathering.