Cleavage (geology)

The presence of fabric elements such as preferred orientation of platy or elongate minerals, compositional layering, grain size variations, etc. determines what type of cleavage forms. Cleavage is categorized as either continuous or spaced. Continuous cleavage Continuous or penetrative cleavage describes fine grained rocks consisting of platy minerals evenly distributed in a preferred orientation. A common outdated term for disjunctive cleavage is fracture cleavage. It is recommended that this term be avoided because of the tendency to misinterpret the formation of a cleavage feature. Transposition cleavage When an older cleavage foliation is erased and replaced by a younger foliation due to stronger deformation and is evidence for multiple deformation events. ==Formation==

The development of cleavage foliation involves a combination of various mechanisms dependent on the rock's composition, tectonic processes, and metamorphic conditions. The magnitude and orientation of stress coupled with pressure and temperature conditions determine how a mineral is deformed. Cleavages form approximately parallel to the X-Y plane of tectonic strain and are categorized based on the type of strain. The mechanisms currently believed to control cleavage formation are rotation of mineral grains, solution transfer, dynamic recrystallization, and static recrystallization. Mechanical rotation of grains During ductile deformation, mineral grains with a high aspect ratio are likely to rotate so that their mean orientation is in the same direction as the XY plane of finite strain. Mineral grains may fold if oriented perpendicular to shortening direction. Solution transfer Cleavage foliations may result due to stress-induced solution transfer by the redistribution of inequant mineral grains by pressure solution and recrystallization. This would also help to increase rotation of elongate and tabular mineral grains. Mica grains undergoing solution transfer will align in a preferred orientation. If the mineral's grains affected by pressure solution are deformed through plastic crystal processes, the grain will be extended along the XY-plane of finite strain. This process shapes grains into a preferred orientation. Dynamic recrystallization Dynamic recrystallization occurs when a rock undergoes metamorphic conditions and reequilibrium of a mineral's chemical composition. This happens when there is a decrease in free energy stored in deformed grains. Deformed micas can store a sufficient amount of strain energy that can allow recrystallization to occur. This process allows oriented regrowth of both old and new minerals into the damaged crystal lattice during cleavage development. Static recrystallization This process occurs either after deformation or in the absence of dynamic deformation. Depending on the intensity of heat during recrystallization, the foliation will either be strengthened or weakened. If the heat is too intense, foliation will be weakened due to the nucleation and growth of new randomly oriented crystals and the rock will become a hornfels. If minimal heat is applied to a rock with a preexisting foliation and without a change in mineral assemblage, the cleavage will be strengthened by growth of micas parallel to foliation. ==Relationship to folds==

in sandstone shale sequences with axial planar cleavage, refraction of cleavage visible on right-hand limb Cleavages display a measurable geometric relationship with the axial plane of folds developed during deformation and are referred to as axial planar foliations. The foliations are symmetrically arranged with respect to the axial plane, depending on the composition and competency of a rock. For example, when mixed sandstone and mudstone sequences are folded during very-low to low grade metamorphism, cleavage forms parallel to the fold axial plane, particularly in the clay-rich parts of the sequence. In folded alternations of sandstone and mudstone the cleavage has a fan-like arrangement, divergent in the mudstone layers and convergent in the sandstones. This is thought to be because the folding is controlled by buckling of the stronger sandstone beds with the weaker mudstones deforming to fill the intervening gaps. The result is a feature referred to as foliation fanning. ==Engineering considerations==

In geotechnical engineering a cleavage plane forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel, foundation, or slope construction. ==See also==