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Rock Cleavage:
Fracture cleavage is the type of rock cleavage due to closely spaced fractures which are not necessarily parallel to the minerals of the rock. Slip cleavage is rock cleavage along which there has been displacement; this type resembles closely spaced faults. Some terms applied to rock cleavage imply origin. Flow cleavage is the result of rock flowage or plastic deformation. Much of slaty cleavage and schistosity are of this type.Some slaty cleavage and schistosity and all fracture and slip cleavage are shear phenomena. Generally, rock cleavage is due to the metamorphism of rocks under great force and the resulting differential pressure. In many areas the cleavage and the larger scale folding into anticlines and synclines result from the same forces; in this case slaty cleavage is often more or less parallel to the axes of the major folds.
ROCK CLEAVAGE, the property of rocks whereby they break along parallel surfaces of secondary origin. It is found in metamorphic rocks and is usually not parallel to the original bedding. One variety, slaty cleavage, is caused by the parallel arrangement of platy minerals such as mica or chlorite, and is found in less intensely metamorphosed rocks such as slate; these rocks split into thin plates parallel to the cleavage. Schistosity is also due to the parallel arrangement of platy minerals but is found in rocks sufficiently recrystallized to be called schist or gneiss.
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