View source for Earth:Foliation (geology)

[[File:Gneiss.jpg|thumb|Gneiss, a foliated metamorphic rock.]]

[[File:Quartzite Sollières.jpg|thumb|[[Earth:Quartzite|Quartzite]], a non-foliated metamorphic rock.]]

'''Foliation''' in geology refers to repetitive layering in metamorphic rocks.<ref name=EG>Marshak, Stephen, ''Essentials of Geology,'' W. W. Norton 3rd Ed, 2009 {{ISBN|978-0393196566}}</ref> Each layer can be as thin as a sheet of paper, or over a meter in thickness.<ref name=EG/> The word comes from the Latin {{lang|la|folium}}, meaning "leaf", and refers to the sheet-like planar structure.<ref name=EG/> It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others). The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated [[Earth:Metamorphic rock|metamorphic rock]]s are typically formed in the absence of significant differential pressure or shear.<ref name = EG/> Foliation is common in rocks affected by the regional [[Earth:Metamorphism|metamorphic]] compression typical of areas of mountain belt formation ([[Physics:Orogeny|orogenic belts]]).  

More technically, foliation is any penetrative planar [[Earth:Fabric (geology)|fabric]] present in metamorphic rocks. Rocks exhibiting foliation include the standard sequence formed by the [[Earth:Metamorphism#Prograde and retrograde|prograde metamorphism]] of [[Earth:Mudrock|mudrock]]s; [[Chemistry:Slate|slate]], [[Chemistry:Phyllite|phyllite]], [[Chemistry:Schist|schist]] and [[Earth:Gneiss|gneiss]]. The ''slatey [[Earth:Cleavage (geology)|cleavage]]'' typical of slate is due to the preferred orientation of microscopic phyllosilicate [[Earth:Crystal|crystal]]s. In gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as [[Earth:Tectonite|S-tectonite]] in [[Earth:Shear (geology)|sheared]] rock masses. 

Examples include the bands in [[Earth:Gneiss|gneiss]] (gneissic banding), a preferred orientation of planar ''large'' mica flakes in [[Chemistry:Schist|schist]] (schistosity), the preferred orientation of ''small'' mica flakes in [[Chemistry:Phyllite|phyllite]] (with its planes having a silky sheen, called ''phylitic luster'' – the Greek word, ''phyllon'', also means "leaf"), the extremely fine grained preferred orientation of clay flakes in [[Chemistry:Slate|slate]] (called "slaty cleavage"), and the layers of flattened, smeared, pancake-like clasts in [[Earth:Metaconglomerate|metaconglomerate]].<ref name=EG/>

==Formation mechanisms==
Foliation is usually formed by the preferred orientation of [[Earth:Mineral|mineral]]s within a rock.  

Usually, this is the result of some physical force and its effect on the growth of minerals. The planar fabric of a foliation typically forms at [[Right angle|right angle]]s to the maximum principal stress direction. In [[Earth:Shear (geology)|sheared zone]]s, 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 [[Chemistry:Mica|mica]]s and [[Chemistry:Clay mineral|clay]]s via physical rotation of the minerals within the rock. Often this foliation is associated with [[Earth:Diagenesis|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 platy minerals, typically of the mica group, is usually a result of [[Earth:Metamorphism#Prograde and retrograde|prograde metamorphic]] reactions during deformation. Often, retrograde metamorphism will not form a foliation because the unroofing of a metamorphic belt is not accompanied by significant compressive stress. Thermal metamorphism in the [[Earth:Metamorphism#Contact (thermal)|aureole]] of a [[Chemistry:Granite|granite]] is also unlikely to result in the growth of mica in a foliation, although the growth of new minerals may overprint existing foliation(s).

Alignment of tabular minerals in [[Earth:Metamorphic rock|metamorphic rock]]s, [[Earth:Igneous rock|igneous rock]]s and [[Earth:Intrusive rock|intrusive rock]]s may form a foliation. Typical examples of metamorphic rocks include [[Earth:Porphyroblast|porphyroblast]]ic 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 [[Astronomy:Convection|convection]] in large [[Earth:Magma chamber|magma chamber]]s, especially ultramafic intrusions, and typically [[Chemistry:Plagioclase|plagioclase]] laths. Granite may form foliation due to frictional drag on viscous magma by the wall rocks. Lavas may preserve a flow foliation, or even compressed eutaxitic texture, typically in highly viscous [[Earth:Felsic|felsic]] [[Earth:Agglomerate|agglomerate]], [[Earth:Tuff#Welded tuff|welded tuff]] and [[Earth:Pyroclastic rock|pyroclastic]] surge deposits. 

Metamorphic differentiation, typical of [[Earth:Gneiss|gneiss]]es, is caused by chemical and compositional banding within the metamorphic rock mass. Usually, this represents the [[Earth:Protolith|protolith]] chemistry, which forms distinct mineral assemblages. However, compositional banding can be the result of [[Physics:Nucleation|nucleation]] processes 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. 

''[[Earth:Crenulation|Crenulation]] cleavage'' and ''[[Earth:Oblique foliation|oblique foliation]]'' are particular types of foliation.

==Interpretation==
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 [[Earth:Thrust fault|thrust fault]]s, 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.  
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==Description==
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
* any porphyroblasts or 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
* measure intersection lineations 

Following such a methodology allows eventual correlations in style, metamorphic grade, and intensity throughout a region, relationship to [[Earth:Fault (geology)|faults]], [[Earth:Shear (geology)|shears]], structures and mineral assemblages.

==Engineering considerations==
In [[Engineering:Geotechnical engineering|geotechnical engineering]] a foliation plane may form a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, [[Engineering:Tunnel|tunnel]], [[Organization:Foundation (engineering)|foundation]], or [[Engineering:Slope stability analysis|slope]] construction.

==See also==
{{Div col|small=yes}}
* [[Earth:Cleavage (geology)|Cleavage (geology)]]
* [[Engineering:Discontinuity (geotechnical engineering)|Discontinuity (geotechnical engineering)]]
* [[Earth:Exfoliating granite|Exfoliating granite]]
* [[Earth:Fissility (geology)|Fissility (geology)]]
* [[Earth:Fold (geology)|Fold (geology)]]
* [[Earth:List of rock textures|List of rock textures]]
* [[Earth:List of rock types|List of rock types]]
* [[Earth:Rock microstructure|Rock microstructure]]
* [[Earth:Shear (geology)|Shear (geology)]]
{{Div col end}}

==References==

<references />
* 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}}

[[Category:Metamorphic petrology]]
[[Category:Metamorphic rocks]]
[[Category:Petrology]]
[[Category:Structural geology]]

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{{Sourceattribution|Foliation (geology)}}