Metamorphism and rock cleavage significance

Metamorphism and metamorphic rock

Metamorphism is the change in mineralogy, texture and structure of a pre-existing rock in the solid state, caused by heat, pressure and chemically active fluids. A metamorphic rock is the rock produced by this process, for example marble from limestone, slate from shale.

Types of rock cleavage

Rock cleavage is the property of a metamorphic rock to split along closely spaced parallel planes.

1. Slaty cleavage - very fine, perfectly parallel planes; rock splits into thin sheets (slate). Caused by parallel alignment of microscopic mica and chlorite perpendicular to the maximum stress.
2. Fracture (strain-slip) cleavage - closely spaced fractures along which small movement has taken place; the rock is crossed by parallel cracks.
3. Flow cleavage - arrangement of flattened grains and minerals produced by plastic flow in the rock (schist, phyllite).
4. Schistosity - coarse foliation of platy minerals in schist (sometimes treated as a separate type).

Engineering significance

• Strength: rock is weak along cleavage planes and strong across them, so strength is anisotropic.
• Slopes: cleavage dipping out of a cut slope gives sliding planes (landslides, rock slides).
• Tunnels and excavation: overbreak, roof falls, and squeezing along cleavage planes; tunnels should be aligned across the cleavage strike.
• Seepage: cleavage planes allow water to pass, so leakage under dams and reservoirs may take place.
• Useful aspect: slate cleavage gives roofing slabs and flooring stone that are easily quarried.