l>Metamorphic Rocks
EENS 1110

Physical Geology

thedailysplash.tv University Prof. Stephen A. Nelson
Metamorphism and also Metamorphic Rocks

Definition of Metamorphism

The word "Metamorphism" comes from the Greek: meta = after, morph = create, so metamorphism means the after form. In geology this describes the changes in mineral assemblage and texture that outcome from subjecting a rock to pressures and also temperatures different from those under which the rock originally created. The original rock that has undertaken metamorphism is called the protolith. Protolith can be any form of rock and occasionally the alters in texture and also mineralogy are so dramatic that is challenging to identify what the protolith was. Note that diagenesis and also weathering are additionally a changes in form that happen in rocks. In geology, however, we restrict diagenetic procedures to those which take place at temperatures listed below 200oC and also pressures listed below around 300 MPa (MPa means Mega Pascals), this is identical to about 3,000 environments of pressure.

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Metamorphism therefore occurs at temperatures and also pressures better than 200oC and 300 MPa. Rocks deserve to be subjected to these higher temperatures and also pressures as they come to be buried deeper in the Planet. Such interment usually takes location as an outcome of tectonic procedures such as continental collisions or subduction. The upper limit of metamorphism occurs at the press and temperature of wet partial melting of the rock in question. Once melting starts, the process changes to an igneous procedure rather than a metamorphic process. During metamorphism the protolith undergoes transforms in texture of the rock and the mineral make up of the rock. These transforms take location largely in the solid state and also are resulted in by alters in physical or chemical conditions, which consequently can be caused by such points as interment, tectonic stress, heating by magma or interactions with fluids.

Factors that Control Metamorphism

Metamorphism occurs because rocks undergo alters in temperature and push and also may be subjected to differential tension and also hydrothermal fluids. Metamorphism occurs because some minerals are steady only under specific conditions of push and also temperature. When pressure and temperature change, chemical reactions occur to reason the minerals in the rock to adjust to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complex by such points as just how the push is used, the time over which the rock is based on the greater pressure and temperature, and also whether or not tright here is a liquid phase current in the time of metamorphism. Temperature Temperature increases through depth in the Earth alengthy the Geothermal Gradient. Hence better temperature can take place by burial of rock. Temperature can likewise increase because of igneous intrusion. Prescertain boosts through depth of burial, thus, both press and temperature will vary with depth in the Planet. Prescertain is characterized as a force acting equally from all directions. It is a form of stress, dubbed hydrostatic stress, or unidevelop anxiety. If the tension is not equal from all directions, then the stress is called a differential stress.


Tright here are two kinds of differential anxiety. Regular stress reasons objects to be compressed in the direction of maximum primary stress and anxiety and extfinished in the direction of minimal stress and anxiety. If differential stress is current in the time of metamorphism, it can have actually a prodiscovered impact on the texture of the rock. Shear stress causes objects to be smeared out in the direction of applied stress and anxiety.

Differential tension if acting on a rocks deserve to have actually a prouncovered influence on the appearance or texture of the rock.

Rounded grains deserve to end up being flattened in the direction of maximum anxiety.


Minerals that crystallize or prosper in the differential anxiety area deserve to have a wanted orientation. This is especially true of the sheet silicate minerals (the micas: biotite and muscovite, chlorite, talc, and also serpentine).
These sheet silicates will certainly flourish via their sheets orientated perpendicular to the direction of maximum anxiety. Preferred orientation of sheet silicates causes rocks to be conveniently damaged alengthy about parallel sheets. Such a structure is referred to as a foliation.
Fluid Phase.- Any existing open up room between mineral grains in a rock deserve to possibly contain a liquid. This fluid is largely H2O, but includes dissolved ions. The liquid phase is crucial bereason chemical reactions that involve transforming a solid mineral into a new solid mineral deserve to be greatly speeded up by having dissolved ions transported by the fluid. If chemical change of the rock takes area as a result of these fluids, the procedure is called metasomatism. Time - Due to the fact that metamorphism involves transforming the rock while it is solid, metamorphic readjust is a slow procedure.Throughout metamorphism, several processes are at occupational. Recrystallization reasons changes in minerals size and form. Chemical reactions occur in between the minerals to develop new sets of minerals that are even more steady at the push and temperature of the setting, and brand-new minerals form as a result of polymorphic phase revolutions (recall that polymorphs are compounds via the exact same chemical formula, however different crystal frameworks. Laboratory experiments suggest that the the sizes of the mineral grains developed in the time of metamorphism boosts with time. Thus coarse grained metamorphic rocks involve lengthy times of metamorphism. Experiments suggest that the moment involved is tens of millions of years.

Grade of Metamorphism

Metamorphic grade is a general term for describing the loved one temperature and also pressure conditions under which metamorphic rocks develop. As the temperature and/or pressure boosts on a body of rock we say that the rock undergoes prograde metamorphism or that the grade of metamorphism boosts.

Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and also reasonably low push. Low grade metamorphic rocks are characterized by a wealth of hydrous minerals (minerals that contain water, H2O, in their crystal structure).

Instances of hydrous minerals that happen in low grade metamorphic rocks: Clay Minerals Serpentine Chlorite High-grade metamorphism takes place at temperatures better than 320oC and fairly high push. As grade of metamorphism rises, hydrous minerals become much less hydrous, by losing H2O and also non-hydrous minerals come to be more widespread.

Instances of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks:

Muscovite - hydrous mineral that ultimately disshows up at the highest possible grade of metamorphism Biotite - a hydrous mineral that is steady to extremely high qualities of metamorphism. Pyroxene - a non hydrous mineral. Garnet - a non hydrous mineral.

Retrograde Metamorphism

As temperature and pressure fall because of erosion of overlying rock or because of tectonic uplift, one might intend metamorphism to a follow a reverse course and inevitably rerevolve the rocks to their original unmetamorphosed state. Such a procedure is referred to as retrograde metamorphism. If retrograde metamorphism were widespread, we would certainly not frequently see metamorphic rocks at the surface of the Planet. Due to the fact that we execute watch metamorphic rocks exposed at the Earth"s surface retrograde metamorphism does not show up to be common. The reasons for this include: chemical reactions take location more progressively as temperature is decreased in the time of prograde metamorphism, fluids such as H2O and also CO2 are thrust off, and also these fluids are vital to create the hydrous minerals that are stable at the Earth"s surchallenge. chemical reactions take place even more swiftly in the existence of fluids, however if the fluids are pushed off in the time of prograde metamorphism, they will certainly not be easily accessible to speed up reactions in the time of retrograde metamorphism.

Metamorphic Rock Types

There are two major subdivisions of metamorphic rocks. Foliated – These have actually a planar foliation resulted in by the desired orientation (alignment) of minerals and also created under differential stress and anxiety. They have actually a far-reaching amount of sheet silicate (platy minerals and also are classified by complace, grain dimension, and foliation form. Non-foliated – These have no noticeable planar cloth or foliation, crystallized under problems where there was no differential stress and anxiety, and also are comprised of equant minerals just. These are classified mainly by the minerals existing or the chemical complace of the protolith. Foliated Metamorphic Rocks

Example - metamorphism of a shale, made up initially of clay minerals and also quartz all of clay or silt size.
Slate- Slates form at low metamorphic grade by the development of fine grained chlorite and clay minerals. The desired orientation of these sheet silicates reasons the rock to conveniently break along the planes parallel to the sheet silicates, causing a slatey cleavage. Note that in the case presented right here, the maximum stress and anxiety is applied at an angle to the original bedding planes, so that the slatey cleavage has actually arisen at an angle to the original bedding. Since of the virtually perfect breakage alengthy planes, slates are useful for blackboards and also shingles. Phyllite - Fine mica-wealthy rock, developed by low – medium grade metamorphism. In a phyllite, the clay minerals have recrystallized right into tiny micas (biotite and muscovite which reflect a satiny luster. Phyllite is in between slate and schist.
Schist - The dimension of the mineral grains often tends to enhuge via raising grade of metamorphism. Eventually the rock develops a close to planar foliation brought about by the preferred orientation of sheet silicates (mostly biotite and also muscovite). Quartz and Feldspar grains, however present no preferred orientation. The irregular planar foliation at this stage is called schistosity.

Schist frequently has actually various other minerals besides micas. These encompass minerals favor - Quartz, Feldspars, Kyanite, Garnet, Staurolite, and Sillimanite. When these non-mica minerals happen with a grain size higher than the rest of the rock, they are called pophyroblasts.

Gneiss As metamorphic grade increases, the sheet silicates become unsecure and also dark colored minerals prefer hornblende and also pyroxene start to thrive. These dark colored minerals tend to come to be segregated in unique bands via the rock, offering the rock a gneissic banding. Due to the fact that the dark colored minerals tend to develop elongated crystals, quite than sheet- favor crystals, they still have a desired orientation through their lengthy directions perpendicular to the maximum differential anxiety.
Granulite - At the highest possible qualities of metamorphism every one of the hydrous minerals and also sheet silicates come to be unstable and thus there are few minerals present that would show a wanted orientation. The resulting rock will certainly have actually a granulitic texture that is similar to a phaneritic texture in igneous rocks. Migmatites – If the temperature reaches the solidus temperature (initially melting temperature), the rock may start to melt and also start to co-mingle through the solids. Typically these melts are felsic with the mafic material staying metamorphic.
Non-foliated Metamorphic Rocks

Non-foliated rocks absence a planar fabric . Absence of foliation feasible for several reasons:

Rock not based on differential stress. Dominance of equant minerals (choose quartz, feldspar, and also garnet). Absence of platy minerals (sheet silicates).

Non-foliated rocks are provided certain names based on their mineralogy and also composition: Amphibolite - These rocks are dark colored rocks via amphibole (typically hornblende) as their significant mineral. They are generally poorly foliated and also form at intermediate to high grades of metamorphism of basaltic or gabbroic protoliths. Hornfels - These are incredibly fine grained rocks that normally create as an outcome of magma intruding right into fined grained igneous rocks or shales. The magma causes a type of metamorphism called call metamorphism (to be disputed later).

Quartzite - A rock made up virtually completely of quartz. They are developed by metamorphism of quartz arenites (sandstones). Since quartz is stable over a huge selection of temperatures and also pressures, no new minerals are created throughout metamorphism, and also the only metamorphic impact that occurs is recrystallization of the quartz bring about interlocking crystals that consist of a really hard rock.

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Marble - A limestone or dolostone comprised just of calpoint out or dolomite will certainly metamorphose to a marble which is made greatly recrystallized calpoint out or dolomite. The Recrystallization typically obliteprices all fossils. Marbles have actually a selection of colors and also are frequently complexly banded. They are generally supplied as a decorative rock.

Protolith Composition

Although textures and also structures of the protolith are usually damaged by metamorphism, we have the right to still acquire an principle about the original rock from the minerals current in the metamorphic rock.

Minerals that develop, do so because the chemical elements crucial to develop them are existing in the protolith.

General terms offered to describe the chemical complace of both the protolith and the resulting metamorphic rock are:

Pelitic Alumina wealthy rocks, normally shales or mudstones. These begin out initially through clay minerals and as an outcome of metamorphism, Alumina wealthy minerals choose micas, chlorite, garnet, kyanite, sillimanite and andaluwebsite form. Since of the abundance of sheet silicates, pelitic rocks commonly create slates, phyllites, schists, and also gneisses in the time of metamorphism.

Mafic - These are Mg and also Fe affluent rocks via low amounts of Si. Minerals favor biotite, hornblende and also plagioclase create during metamorphism and also generally develop amphibolites.

Calcareous - These are calcium-well-off rocks normally derived from limestones or dolostones, and therefore contain an abundance of Calcite. Marbles are the type of metamorphic rock that results.

Quartzo-Feldspathic - Rocks that contain a wealth of quartz and feldspar loss into this category. Protoliths are typically granites, rhyolites, or arkose sandstones and also metamorphism outcomes in gneisses containing a wealth of quartz, feldspar, and biotite.