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Foliation Geology

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Foliation is the planar arrangement found as a part of the structural or textural features present in any rock type, particularly the one that results from the mineral grains that are found in the metamorphic rocks typically forming a part of straight or wavy planes. Thus, the foliation geology can be said to be a characteristic of repetitive layering in metamorphic rocks. Although foliation is seen to occur parallel to the original bedding of the rocks, it may not be specifically directed for a particular structural or textural arrangement. 


Characteristics of Foliation Geology

As mentioned in the foliation definition geology makes it clear that it is a pattern or a planar arrangement that is a repetitive layering in the metamorphic rocks. A clear idea of the foliation arrangement can be obtained by looking at the image given below:

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The thickness of each layer found in each of the foliated metamorphic rocks can vary from that of a thin sheet of paper to beyond 1 meter in some cases. This thickness resembling a sheet of paper or a leaf forms the basis of the meaning of the word folium in Latin, from which the word foliation is derived. 

Some of the important points to be considered while describing the foliation are given below:

  • Minerals present in the rocks, which help in providing information about prevailing conditions while the formation of foliation.

  • The mineral content of intrafolial areas.

  • The spacing present in between the foliation.

  • Formation of crystals of minerals that form part of the foliation and whether they overprint it or are cut by it.

  • Whether it is undulose, planer, vague or very well developed.

  • Spatial orientation of foliation

  • Relationship to other foliations, to the bedding of the rock and any folding if present. 

  • Intersection lineations measurement. 

Following these notes, is an aid in determining the characteristics of any foliation geology.


Formation of Foliation Geology

The typical characteristics of the foliation geology are determined by the shearing forces or the differential pressure forces on different sections of metamorphic rock as explained below:

  • Shearing Forces: The shearing forces apply forces on different sections of the rock in different directions leading to the formation of different foliated metamorphic rocks. 

  • Differential Pressures: In contrast to such forces, differential pressure is the application of higher pressure in one direction as compared to other directions that result in a pattern of planar arrangement in the foliated metamorphic rocks. 

These layers are formed either parallel to the direction of the shearing forces or perpendicular to the direction of the differential pressures. 


Typically foliation geology is a characteristic of regional metamorphic compression which is a common occurrence in areas with the formation of mountain belts. This becomes a guiding factor for the foliation definition geology has provided. Another factor that also plays a role in the formation of foliation arrangements is the preferred orientation of minerals inside a rock. Along with the role of different forces on different sections of the rock, they also affect the growth of minerals which in turn leads to the peculiar planar arrangement. Because of the effect and probable orientation of minerals, any of the penetrative planar arrangements that include both spatial and geometric arrangement of all the elements making up the rock is also included in the foliation definition geology and geologists have to offer. 


A sedimentary rock or igneous rock or another metamorphic rock can exhibit foliation in standard sequence only when subjected to increasing temperature and pressure which causes the metamorphism within the rock. Especially in the case of mudrocks such as slate, phyllite, schist and gneiss, this is a common occurrence and is visible in the image below:

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The slaty cleavage (i.e. the planar arrangement within the metamorphic rock) that is typical of the slate is formed because of the preferred orientation of the microscopic phyllosilicate crystals. Whereas in the case of gneiss the foliation is the result of compositional banding due to the segregation of mineral phases. Other examples include the bands formed due to the preferred orientation of planar large mica flakes in schist, the small mica preferred orientation in phyllite, extreme fine-grained preferred orientation of clay flakes in slate and the layers that are flattened, smeared, pancake-like clasts found in metaconglomerate. 


The foliated sedimentary rocks or any of the foliated igneous rocks are essentially foliated metamorphic rocks because as per the definition of foliation metamorphic rocks, or any of the sedimentary rocks or igneous rocks that have been subjected to increasing temperature and pressure resulting in foliation is said to have undergone a metamorphosis. In foliated sedimentary rocks, the bedding provides a reference for the form of planar arrangement as in most cases of a foliated sedimentary rock the foliation is parallel to the bedding with some angle of orientation. The foliation in igneous rocks can be caused by the alignment of the cumulate crystals during the convection process in the case of large magma chambers as already clarified by the foliated metamorphic rock definition. 


Conclusion

This article explains briefly the foliation geology as a planar rearrangement of structural and texture features present within a rock formed due to different forces acting on different sections of the rocks. The foliated metamorphic rock definition also includes the role of the orientation of minerals present in the rocks in the formation of foliation. This is also applicable and true for foliated sedimentary rocks and foliated igneous rocks. A concluding point to consider is that they might not always be strictly planar or parallel as they tend to bend around rigid, incompressible bodies such as granite. But overall it provides significant information on the formation of the rocks which also largely influences the mechanical behaviour of the rock masses which becomes a problem especially while the construction of a tunnel, foundation or a slope. 

FAQ (Frequently Asked Questions)

1. What Does Foliated Mean?

Ans: Foliation is ornamentation with foils or leaf design. In geology, there are separate layers arranged in planar texture within a rock and the thickness of such layers is mostly the thickness of a leaf or a sheet of paper. Hence, such separated layers within a rock having the thickness of a leaf are said to be foliated in geology.

2. What Causes Foliation?

Ans: Foliation is the planar arrangement of the structure and texture of different layers within a rock. This arrangement is reached largely under the influence of shear forces/differential pressures on different sections of the rock and the orientation of the minerals present within the rock. Thus, these two are the major causes for the formation of foliation within the rocks.