Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the typical metamorphic rock sequence of
slate, phyllite, schistand gneiss. The "slatey cleavage" typical of slate is due to the parallel growth of microscopic mica crystals. In gneissthe foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as S-tectonite in sheared rock masses.
Foliation is usually formed by the preferred orientation of
minerals within a rock.
Typically this is a result of some physical force, and its effect upon the growth of minerals. The planar fabric of a foliation typically forms at
right angles to the minimum principal strain direction. In sheared zones, however, planar fabric within a rock may not be directly perpendicular to the principal stress direction due to rotation, mass transport and shortening.
Foliation may be formed by realignment of micas and
clays via physical rotation of the minerals within the rock. Often this foliation is associated with diagenetic metamorphism and low-grade burial metamorphism. Foliation may parallel original sedimentary bedding, but more often is oriented at some angle to it.
The growth of
platey minerals, typically of the mica group, as a result of prograde metamorphicreactions during deformation. Often, retrograde metamorphismwill not form a foliation because unroofing of a metamorphic belt is not accompanied by significant compressive stress. Thermal metamorphism in the aureoleof a graniteis also unlikely to result in growth of mica in a foliation, although growth of new minerals may overprint existing foliation(s).
tabular minerals in metamorphic rocks, igneous rocks and intrusive rocks may form a foliation. Typical examples of metamorphic rocks include porphyroblastic schists where large, oblate minerals form an alignment either due to growth or rotation in the groundmass.
Igneous rocks can become foliated by alignment of
cumulate crystals during convectionin large magma chambers, especially ultramaficintrusions, and typically plagioclase laths. Granite may form foliation due to frictional drag on viscousmagma by the wall rocks. Lavas may preserve a flow foliation, or even compressed eutaxitictexture, typically in highly viscous felsic agglomerate, welded tuff and pyroclasticsurge deposits.
Metamorphic differentiation, typical of
gneisses, is caused by chemical and compositional bandingwithin the metamorphic rock mass. Usually this represents the protolithchemistry, which forms distinct mineral assemblages. However, compositional banding can be the result of nucleationprocesses which cause chemical and mineralogical differentiation into bands. This typically follows the same principle as mica growth, perpendicular to the principal stress.Metamorphic differentiation can be present at angles to protolith compositional banding. Crenulationcleavage is a particular type of foliation.
Foliation, as it forms generally perpendicular to the direction of principal stress, records the direction of shortening. This is related to the axis of folds, which generally form an "axial-planar" foliation within their axial regions.
Measurement of the intersection between a fold's axial plane and a surface on the fold will provide the
fold plunge. If a foliation does not match the observed plunge of a fold, it is likely associated with a different deformation event.
Foliation in areas of shearing, and within the plane of
thrust faults, can provide information on the transport direction or sense of movement on the thrust or shear. Generally, the acute intersection angle shows the direction of transport. Foliations typically bend or curve into a shear, which provides the same information, if it is of a scale which can be observed.
Foliations, in a regional sense, will tend to curve around rigid, incompressible bodies such as granite. Thus, they are not always 'planar' in the strictest sense and may violate the rule of being perpendicular to the regional stress field, due to local influences. This is a megascopic version of what may occur around porphyroblasts. Often, fine observation of foliations on outcrop, hand specimen and on the microscopic scale complements observations on a map or regional scale.
When describing a foliation it is useful to note
* the mineralogy of the folia; this can provide information on the conditions of formation
* the mineralogy in intrafolial areas
* foliation spacing
porphyroblastsor minerals associated with the foliation and whether they overprint it or are cut by it
* whether it is planar, undulose, vague or well developed
* its orientation in space, as strike and dip, or dip and dip direction
* its relationship to other foliations, to bedding and any folding
List of rock textures
List of rock types
* Blatt, Harvey and Tracy, Robert J.; 1996, "Petrology: Igneous, Sedimentary, and Metamorphic", 2nd ed., p. 359-360, W. H. Freeman, ISBN 0-7167-2438-3
* Vernon, Ron H., 2004, "A Practical Guide to Rock Microstructure", Oxford University Press, Oxford. ISBN 0-521-89133-7
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