l > Metamorphic Rocks
EENS 1110

Physical Geology

pilsadiet.com College Prof. Stephen A. Nelson
Metamorphism and also Metamorphic Rocks

Interpretation of Metamorphism

Words "Metamorphism" originates from the Greek: meta = after, morph = kind, so metamorphism indicates the after type. In geology this describes the modifications in mineral assemblage as well as structure that arise from subjecting a rock to temperature levels and also stress various from those under which the rock initially created. The initial rock that has actually gone through metamorphism is called the protolith. Protolith can be any type of kind of rock and also often the modifications in appearance as well as mineralogy are so remarkable that is challenging to differentiate what the protolith was. Keep in mind that diagenesis as well as weathering are likewise an adjustments in type that happen in rocks. In geology, nonetheless, we limit diagenetic procedures to those which happen at temperature levels listed below 200oC as well as stress listed below concerning 300 MPa (MPa represents Huge Pascals), this amounts around 3,000 ambiences of stress. Metamorphism consequently happens at stress and also temperature levels greater than 200oC as well as 300 MPa. Rocks can be based on these greater temperature levels and also stress as they come to be hidden deeper in the Planet. Such funeral generally happens as an outcome of structural procedures such as continental accidents or subduction. The ceiling of metamorphism takes place at the stress and also temperature level of damp partial melting of the rock concerned. The procedure adjustments to an igneous procedure instead than a metamorphic procedure when melting starts. Throughout metamorphism the protolith undertakes adjustments in structure of the mineral and also the rock compose of the rock. These adjustments occur mainly in the strong state and also are brought on by adjustments in chemical or physical problems, which consequently can be brought on by such points as interment, structural stress and anxiety, home heating by lava or communications with liquids.

Elements that Regulate Metamorphism

Since rocks go through adjustments in temperature level and also stress and also might be subjected to differential anxiety as well as hydrothermal liquids, metamorphism takes place. Due to the fact that some minerals are secure just under specific problems of stress and also temperature level, metamorphism takes place. When stress as well as temperature level modification, chain reactions strike trigger the minerals in the rock to alter to an assemblage that is steady at the brand-new stress and also temperature level problems. Yet, the procedure is made complex by such points as just how the stress is used, the moment over which the rock goes through the greater stress and also temperature level, and also whether there is a liquid stage existing throughout metamorphism. Temperature level Temperature level enhances with deepness in the Planet along the Geothermal Slope. Hence greater temperature level can happen by interment of rock. Temperature level can additionally raise because of igneous invasion. Stress enhances with deepness of interment, hence, both stress and also temperature level will certainly differ with deepness in the Planet. Stress is specified as a pressure acting similarly from all instructions. It is a sort of tension, called hydrostatic tension, or consistent tension. If the anxiety is not equivalent from all instructions, then the anxiety is called a differential anxiety.


There are 2 sort of differential tension. Regular stress and anxiety reasons challenge be pressed towards optimum primary stress and anxiety and also expanded towards marginal anxiety. If differential tension exists throughout metamorphism, it can have an extensive impact on the appearance of the rock. Shear anxiety reasons challenge be smeared out towards used tension.

Differential anxiety if acting upon a rocks can have an extensive affect on the look or structure of the rock.

Spherical grains can come to be squashed towards optimal tension.


Minerals that expand or take shape in the differential tension area can have a favored positioning. This is particularly real of the sheet silicate minerals (the micas: biotite and also muscovite, talc, chlorite, and also serpentine).
These sheet silicates will certainly expand with their sheets related vertical to the instructions of optimal tension. Preferred alignment of sheet silicates triggers rocks to be conveniently damaged along roughly identical sheets. Such a framework is called a foliation.
Liquid Stage.- Any type of existing open room in between mineral grains in a rock can possibly consist of a liquid. This liquid is mainly water, yet has liquified ions. Due to the fact that chemical responses that include altering a strong mineral right into a brand-new strong mineral can be significantly speeded up by having actually liquified ions delivered by the liquid, the liquid stage is crucial. If chemical change of the rock occurs as an outcome of these liquids, the procedure is called metasomatism. Time - Due to the fact that metamorphism entails transforming the rock while it is strong, metamorphic modification is a slow-moving process.During metamorphism, a number of procedures go to job. Recrystallization creates modifications in minerals shapes and size. Chain reaction happen in between the minerals to create brand-new collections of minerals that are much more steady at the stress and also temperature level of the setting, and also brand-new minerals create as an outcome of polymorphic stage improvements (recall that polymorphs are substances with the very same chemical formula, yet various crystal frameworks. Lab experiments recommend that the dimensions of the mineral grains created throughout metamorphism enhances with time. Therefore crude grained metamorphic rocks include very long times of metamorphism. Experiments recommend that the moment entailed is 10s of numerous years.

Quality of Metamorphism

Metamorphic quality is a basic term for explaining the family member temperature level and also stress problems under which metamorphic rocks develop. As the temperature level and/or stress rises on a body of rock we state that the rock goes through prograde metamorphism or that the quality of metamorphism boosts.

Low-grade metamorphism occurs at temperature levels in between around 200 to 320oC, as well as fairly reduced stress. Reduced quality metamorphic rocks are defined by a wealth of hydrous minerals (minerals which contain water, WATER, in their crystal framework).

Instances of hydrous minerals that take place in reduced quality metamorphic rocks: Clay Minerals Serpentine Chlorite Top-quality metamorphism happens at temperature levels higher than 320oC as well as reasonably high stress. As quality of metamorphism rises, hydrous minerals end up being much less hydrous, by shedding water as well as non-hydrous minerals come to be a lot more usual.

Instances of much less hydrous minerals and also non-hydrous minerals that define high quality metamorphic rocks:

Muscovite - hydrous mineral that ultimately goes away at the greatest quality of metamorphism Biotite - a hydrous mineral that is secure to extremely high qualities of metamorphism. Pyroxene - a non hydrous mineral. Garnet - a non hydrous mineral.

Retrograde Metamorphism

As temperature level and also stress autumn because of disintegration of superior rock or as a result of structural uplift, one could anticipate metamorphism to an adhere to a reverse course and also at some point return the rocks to their initial unmetamorphosed state. Such a procedure is described as backward metamorphism. We would certainly not typically see metamorphic rocks at the surface area of the Planet if backward metamorphism were typical. Because we do see metamorphic rocks subjected at the Planet"s surface area backward metamorphism does not seem usual. The factors for this consist of: chain reactions occur extra gradually as temperature level is reduced throughout prograde metamorphism, liquids such as water and also carbon dioxide are repelled, as well as these liquids are required to create the hydrous minerals that are steady at the Planet"s surface area. chain reactions happen much more quickly in the visibility of liquids, yet if the liquids are repelled throughout prograde metamorphism, they will certainly not be readily available to quicken responses throughout backward metamorphism.

Metamorphic Rock Kind

There are 2 significant communities of metamorphic rocks. Foliated –-- These have a planar foliation brought on by the recommended positioning (placement) of minerals and also developed under differential stress and anxiety. They have a considerable quantity of sheet silicate (platy minerals and also are identified by make-up, grain dimension, and also foliation kind. Non-foliated –-- These have no obvious planar material or foliation, taken shape under problems where there was no differential anxiety, and also are included equant minerals just. These are identified mostly by the minerals existing or the chemical structure of the protolith. Foliated Metamorphic Rocks

Instance - metamorphism of a shale, composed originally of clay minerals as well as quartz every one of clay or silt dimension.
Slate- Slates create at reduced metamorphic quality by the development of great grained chlorite and also clay minerals. The recommended positioning of these sheet silicates triggers the rock to quickly damage along the aircrafts alongside the sheet silicates, triggering a slatey bosom. Keep in mind that in the event revealed right here, the optimum stress and anxiety is used at an angle to the initial bed linen aircrafts, to ensure that the slatey bosom has actually established at an angle to the initial bed linen. As a result of the almost ideal damage along aircrafts, slates work for tiles as well as chalkboards. Phyllite - Great mica-rich rock, developed by reduced –-- tool quality metamorphism. In a phyllite, the clay minerals have actually recrystallized right into small micas (biotite and also muscovite which mirror a satiny gloss. Phyllite is in between slate as well as schist.
Schist- The dimension of the mineral grains has a tendency to increase the size of with raising quality of metamorphism. At some point the rock creates a close to planar foliation brought on by the recommended alignment of sheet silicates (generally biotite as well as muscovite). Quartz as well as Feldspar grains, nevertheless reveal no favored alignment. The uneven planar foliation at this phase is called schistosity.

Schist frequently has various other minerals besides micas. These consist of minerals like - Quartz, Feldspars, Kyanite, Garnet, Staurolite, as well as Sillimanite. When these non-mica minerals accompany a grain dimension above the remainder of the rock, they are called pophyroblasts.

Gneiss As metamorphic quality boosts, the sheet silicates end up being dark and also unsteady tinted minerals like hornblende as well as pyroxene begin to expand. These dark tinted minerals have a tendency to end up being set apart in unique bands with the rock, offering the rock a gneissic banding. Due to the fact that the dark tinted minerals often tend to create extended crystals, instead of sheet- like crystals, they still have a recommended alignment with their lengthy instructions vertical to the optimum differential tension.
Granulite - At the greatest qualities of metamorphism every one of the hydrous minerals and also sheet silicates end up being unsteady and also therefore there are couple of minerals offer that would certainly reveal a favored alignment. The resulting rock will certainly have a granulitic structure that resembles a phaneritic structure in igneous rocks.Migmatites –-- If the temperature level gets to the solidus temperature level (very first melting temperature level), the rock might start to begin and also thaw to co-mingle with the solids. Normally these thaws are felsic with the mafic product staying metamorphic.
Non-foliated Metamorphic Rocks

Non-foliated rocks do not have a planar textile. Lack of foliation feasible for numerous factors:

Rock exempt to differential tension. Supremacy of equant minerals (like quartz, garnet, as well as feldspar). Lack of platy minerals (sheet silicates).

Non-foliated rocks are offered certain names based upon their mineralogy as well as structure: Amphibolite - These rocks are dark tinted rocks with amphibole (normally hornblende) as their significant mineral. They are typically badly foliated and also develop at intermediate to high qualities of metamorphism of gabbroic or basaltic protoliths.Hornfels - These are extremely great grained rocks that normally create as an outcome of lava horning in fined grained igneous rocks or shales. The lava creates a sort of metamorphism called call metamorphism (to be gone over later on).

Quartzite - A rock comprised virtually totally of quartz. They are developed by metamorphism of quartz arenites (sandstones). Given that quartz is steady over a huge series of stress as well as temperature levels, no brand-new minerals are created throughout metamorphism, as well as the only metamorphic impact that happens is recrystallization of the quartz causing interlacing crystals that compose an extremely acid rock.

Marble - A sedimentary rock or dolostone comprised just of calcite or dolomite will certainly transfigure to a marble which is made primarily recrystallized calcite or dolomite. The Recrystallization generally eliminates all fossils. Marbles have a selection of shades and also are typically complexly grouped. They are frequently utilized as an ornamental rock.

Protolith Make-up

Although structures as well as frameworks of the protolith are generally ruined by metamorphism, we can still obtain a concept concerning the initial rock from the minerals existing in the metamorphic rock.

Minerals that create, do so due to the fact that the chemical components essential to create them exist in the protolith.

General terms made use of to explain the chemical structure of both the protolith and also the resulting metamorphic rock are:

Pelitic Alumina abundant rocks, generally shales or mudstones. These begin initially with clay minerals and also as an outcome of metamorphism, Alumina abundant minerals like micas, chlorite, garnet, andalusite, kyanite as well as sillimanite kind. Due to the wealth of sheet silicates, pelitic rocks generally develop slates, phyllites, schists, and also gneisses throughout metamorphism.

Mafic - These are Mg and also Fe abundant rocks with reduced quantities of Si. Minerals like plagioclase, hornblende and also biotite kind throughout metamorphism as well as typically generate amphibolites.

Calcareous - These are calcium-rich rocks typically stemmed from dolostones or sedimentary rocks, and also therefore consist of a wealth of Calcite. Marbles are the sort of metamorphic rock that results.

Quartzo-Feldspathic - Rocks which contain a wealth of quartz and also feldspar fall under this classification. Protoliths are typically granites, rhyolites, or arkose sandstones as well as metamorphism cause gneisses having a wealth of quartz, biotite, as well as feldspar.