It is a type of stress , called hydrostatic stress or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress.
Normally geologists talk about stress as compressional stress. Note that extensional stress would act along the direction of minimum principal stress. Since most phyllosilicates are aluminous minerals, aluminous pelitic rocks like shales, generally develop a foliation as the result of metamorphism in a differential stress field.
Example - metamorphism of a shale made up initially of clay minerals and quartz. Metamorphic petrologists and structural geologists refer to the original bedding surface as S 0. Note that in the case shown here, the maximum principle stress is oriented at an angle to the original bedding planes so that the slatey cleavage develops at an angle to the original bedding. The foliation or surface produced by this deformation is referred to S 1. These dark colored minerals tend to become segregated into distinct bands through the rock this process is called metamorphic differentiation , giving the rock a gneissic banding.
Because the dark colored minerals tend to form elongated crystals, rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress. In general, the grain size of metamorphic rocks tends to increase with increasing grade of metamorphism, as seen in the progression form fine grained shales to coarser but still fine grained slates, to coarser grained schists and gneisses.
Most regionally metamorphosed rocks at least those that eventually get exposed at the Earth's surface are metamorphosed during deformational events. Since deformation involves the application of differential stress, the textures that develop in metamorphic rocks reflect the mode of deformation, and foliations or slatey cleavage that develop during metamorphism reflect the deformational mode and are part of the deformational structures.
The deformation involved in the formation of fold-thrust mountain belts generally involves compressional stresses. The result of compressional stress acting on rocks that behave in a ductile manner ductile behavior is favored by higher temperature, higher confining stress [pressure] and low strain rates is the folding of rocks. Original bedding is folded into a series of anticlines and synclines with fold axes perpendicular to the direction of maximum compressional stress.
These folds can vary in their scale from centimeters to several kilometers between hinges. Note that since the axial planes are oriented perpendicular to the maximum compressional stress direction, slatey cleavage or foliation should also develop along these directions. Thus, slatey cleavage or foliation is often seen to be parallel to the axial planes of folds, and is sometimes referred to axial plane cleavage or foliation.
As discussed above, gneisses, and to some extent schists, show compositional banding or layering, usually evident as alternating somewhat discontinuous bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic layers.
The development of such compositional layering or banding is referred to as metamorphic differentiation. Throughout the history of metamorphic petrology, several mechanisms have been proposed to explain metamorphic differentiation. Preservation of Original Compositional Layering. In some rocks the compositional layering may not represent metamorphic differentiation at all, but instead could simply be the result of original bedding.
For example, during the early stages of metamorphism and deformation of interbedded sandstones and shales the compositional layering could be preserved even if the maximum compressional stress direction were at an angle to the original bedding. In such a case, a foliation might develop in the shale layers due to the recrystallization of clay minerals or the crystallization of other sheet silicates with a preferred orientation controlled by the maximum stress direction. Skip to main content. Module 8 — Metamorphism and Metamorphic Rock.
Search for:. There are three basic rock types: Igneous, sedimentary, and metamorphic. Metamorphic Rocks Metamorphic rocks changed rocks are made when existing rocks are subjected to high temperatures and high pressures for long periods of time. How Metamorphic Rocks are Made Pressure from the weight of overlying rocks or from stresses of mountain building rearranges the minerals in rocks into bands or rearranges the atoms of the minerals into new minerals.
Heat from the intrusion of a large igneous mass can metamorphose a large area. Heat from the intrusion of a dike or sill or flow can bake the adjoining rocks in a contact metamorphic zone.
Composition The mineral composition of the rock can be determined based on observations with a hand lens and if needed, physical or chemical tests. Texture The term texture refers to the size, shape, and boundary relationships of the minerals, particles, and other substances that make up a rock. Examples of Metamorphic Rock Textures Texture Characteristics Rock Name foliated banded very thin layers Slate foliated banded wavy layers with sheen Phyllite foliated banded thin layers of mica Schist foliated banded thick layers of quartz, feldspar, and mica Gneiss non-foliated massive welded quartz sandstone Quartzite non-foliated massive sugary to course crystals, fizzes in HCl acid Marble non-foliated massive dense, black, fine grained, flint-like fracture Hornfels Foliated Banded Metamorphic Rocks In this texture, the mineral crystals in the rock are aligned with each other.
Non-Foliated Metamorphic Rock Table Characteristics Former Rock Rock Name Very hard, smooth Stretched and welded cobbles and pebbles — Fractures through grains, not around them as in rougher conglomerate Composed of rock fragments, quartz, chert Conglomerate Meta- conglomerate Very hard, smooth Welded sand grains — Fractures through grains, not around them as in rougher sandstone Sandstone Quartzite Fizzes in dilute acid Medium to coarse-grained Sugary to crystalline Composed of calcite CaCO 3 Limestone Marble Very hard, flint-like fracture Smooth, very fine-grained Dark colored to black Very dense, compact Claystone Slate Mudstone Shale Hornfels Black to brown Dense, highly altered plant remains Carbon, opaque, non-crystalline Peat Coal.
Licenses and Attributions. CC licensed content, Original. Privacy Policy. Very thin layers, like blackboards Very fine-grained Smooth, flat surfaces, from slatey cleavage Separate grains not visible Dense, brittle, clinking sound. Very, very thin, irregular layers of mica Usually pale gray green Satin sheen to rock rather than individual flakes Fine to medium-grained Uneven surfaces Grains visible.
Thick bands, wavy, semi-continuous layers of white quartz, feldspar, and mica Medium to coarse-grained Banded, coarsely crystalline Large, crystalline grains. Very hard, smooth Stretched and welded cobbles and pebbles — Fractures through grains, not around them as in rougher conglomerate Composed of rock fragments, quartz, chert. Very hard, smooth Welded sand grains — Fractures through grains, not around them as in rougher sandstone.
Very hard, flint-like fracture Smooth, very fine-grained Dark colored to black Very dense, compact. In other words, sedimentary rocks with layering will have thin layers of coarse and fine sediments or fragments. Upon close observation, one will be able to notice marks, trace fossils and soft sediment deformation. To differentiate foliation and layering, let us begin with how they are formed.
Foliation is based on the principle of stress while layering is caused by small mica fragments embedding on the rocks. Foliation is formed by fire and stress; layering is caused by thin embedding of both coarse and fine deposits.
Also, foliation is due to the alteration of minerals from heat and pressure. Layering, on the other hand, is seasonal or event based. In terms of physical aspect, foliation has layers or striations while layering has marks on them.
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