l>Metamorphic Rocks
EENS 1110

Physical Geology

derekwadsworth.com University Prof. Stephen A. Nelson
Metamorphism and also Metamorphic Rocks

Definition the Metamorphism

The word "Metamorphism" comes from the Greek: meta = after, morph = form, therefore metamorphism way the ~ form. In geology this refers to the alters in mineral assemblage and also texture that result from subjecting a absent to pressures and also temperatures various from those under which the rock initially formed. The original rock that has undergone metamorphism is called the protolith. Protolith have the right to be any form of rock and also sometimes the changes in texture and also mineralogy are so dramatic that is difficult to differentiate what the protolith was. keep in mind that diagenesis and weathering are likewise a transforms in type that take place in rocks. In geology, however, we restrict diagenetic procedures to those which occur at temperatures below 200oC and also pressures below around 300 MPa (MPa represents Mega Pascals), this is identical to around 3,000 atmospheres of pressure.

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Metamorphism thus occurs in ~ temperatures and also pressures greater than 200oC and 300 MPa. Rocks have the right to be based on these higher temperatures and also pressures as they come to be buried deeper in the Earth. Such interment usually takes location as a result of tectonic procedures such as continent collisions or subduction. The top limit that metamorphism wake up at the pressure and temperature of wet partial melt of the rock in question. When melting begins, the process changes to an igneous process rather 보다 a metamorphic process. during metamorphism the protolith undergoes transforms in texture of the rock and also the mineral comprise of the rock. These changes take place mainly in the hard state and also are resulted in by transforms in physics or chemistry conditions, i m sorry in turn have the right to be brought about by such things as burial, tectonic stress, heater by magma or interactions through fluids.

Factors that control Metamorphism

Metamorphism occurs due to the fact that rocks undergo alters in temperature and pressure and also may be based on differential stress and also hydrothermal fluids. Metamorphism occurs because some minerals room stable just under specific conditions of pressure and also temperature. Once pressure and also temperature change, chemistry reactions happen to cause the mineral in the rock to adjust to an assemblage that is steady at the new pressure and temperature conditions. But, the process is complicated by such points as how the pressure is applied, the moment over which the absent is subjected to the greater pressure and also temperature, and whether or not there is a fluid phase current during metamorphism. Temperature Temperature increases with depth in the planet along the Geothermal Gradient. Thus higher temperature can occur by interment of rock. Temperature can also increase due to igneous intrusion. Pressure rises with depth the burial, thus, both pressure and temperature will certainly vary v depth in the Earth. Press is identified as a pressure acting equally from all directions. The is a kind of stress, referred to as hydrostatic stress, or uniform stress. If the tension is no equal from every directions, then the tension is dubbed a differential stress.


There are two kinds of differential stress. Normal stress causes objects to it is in compressed in the direction the maximum major stress and also extended in the direction that minimal stress. If differential stress is current during metamorphism, it have the right to have a profound result on the structure of the rock. Shear stress causes objects to it is in smeared the end in the direction of used stress.

Differential tension if acting on a rocks deserve to have a profound influence on the illustration or texture of the rock.

Rounded grains can become flattened in the direction of maximum stress.


Minerals the crystallize or flourish in the differential stress ar can have a preferred orientation. This is especially true of the sheet silicate mineral (the micas: biotite and also muscovite, chlorite, talc, and also serpentine).
These paper silicates will prosper with their sheets orientated perpendicular come the direction of best stress. Wanted orientation of paper silicates causes rocks to it is in easily damaged along about parallel sheets. Such a framework is called a foliation.
liquid Phase.- any kind of existing open space between mineral seed in a rock deserve to potentially save a fluid. This fluid is greatly H2O, but contains dissolved ions. The fluid phase is important because chemical reactions the involve an altering a heavy mineral into a brand-new solid mineral can be significantly speeded up by having liquified ions transported through the fluid. If chemical modification of the rock takes ar as a result of these fluids, the process is called metasomatism. Time - since metamorphism involves changing the rock while that is solid, metamorphic change is a slow-moving process.During metamorphism, several procedures are in ~ work. Recrystallization reasons changes in minerals size and also shape. Chemical reactions occur in between the mineral to kind new sets of minerals the are much more stable in ~ the pressure and also temperature the the environment, and brand-new minerals type as a an outcome of polymorphic phase revolutions (recall that polymorphs space compounds through the exact same chemical formula, yet different crystal structures. laboratory experiments imply that the the size of the mineral grains produced during metamorphism rises with time. For this reason coarse grained metamorphic rocks involve long times that metamorphism. Experiments imply that the time connected is 10s of numerous years.

class of Metamorphism

Metamorphic grade is a general term for describing the relative temperature and also pressure conditions under i beg your pardon metamorphic rocks form. As the temperature and/or pressure increases on a body of rock us say that the absent undergoes prograde metamorphism or the the class of metamorphism increases.

Low-grade metamorphism takes place at temperatures between about 200 come 320oC, and relatively low pressure. Low grade metamorphic rocks are defined by an abundance of hydrous mineral (minerals that contain water, H2O, in their decision structure).

examples of hydrous minerals that occur in short grade metamorphic rocks: Clay minerals Serpentine Chlorite High-grade metamorphism takes ar at temperatures higher than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals end up being less hydrous, by shedding H2O and also non-hydrous minerals become an ext common.

examples of less hydrous minerals and also non-hydrous minerals the characterize high great metamorphic rocks:

Muscovite - hydrous mineral that eventually disappears in ~ the greatest grade of metamorphism Biotite - a hydrous mineral that is secure to an extremely high qualities of metamorphism. Pyroxene - a non hydrous mineral. Garnet - a non hydrous mineral.

Retrograde Metamorphism

As temperature and also pressure fall because of erosion that overlying rock or because of tectonic uplift, one could expect metamorphism to a follow a reverse path and also eventually return the rocks to their initial unmetamorphosed state. Such a procedure is described as retrograde metamorphism. If retrograde metamorphism to be common, we would not typically see metamorphic rocks in ~ the surface ar of the Earth. Due to the fact that we carry out see metamorphic rocks exposed in ~ the Earth"s surface ar retrograde metamorphism walk not show up to be common. The factors for this include: chemistry reactions take place much more slowly together temperature is decreased during prograde metamorphism, fluids such together H2O and CO2 are propelled off, and also these fluids are essential to kind the hydrous minerals that space stable at the Earth"s surface. chemistry reactions take it place an ext rapidly in the existence of fluids, yet if the fluids are pushed off throughout prograde metamorphism, they will not be easily accessible to speed up reactions during retrograde metamorphism.

Metamorphic absent Types

There are two major subdivisions of metamorphic rocks. Foliated – These have actually a planar foliation brought about by the preferred orientation (alignment) of minerals and also formed under differential stress. They have a significant amount of paper silicate (platy minerals and also are divide by composition, serial size, and foliation type. Non-foliated – These have no apparent planar fabric or foliation, crystallized under conditions where there was no differential stress, and also are made up of equant minerals only. These room classified mostly by the minerals current or the chemistry composition that the protolith. Foliated Metamorphic Rocks

Example - metamorphism of a shale, made up initially that clay minerals and quartz every one of clay or silt size.
Slate- Slates kind at low metamorphic grade by the expansion of good grained chlorite and clay minerals. The preferred orientation of this sheet silicates reasons the rock to quickly break follow me the plane parallel come the sheet silicates, causing a slatey cleavage. Note that in the situation shown here, the maximum tension is used at an angle to the initial bedding planes, so the the slatey cleavage has emerged at an angle to the original bedding. since of the almost perfect breakage follow me planes, slates are useful for blackboards and shingles. Phyllite - good mica-rich rock, created by short – medium grade metamorphism. In a phyllite, the clay minerals have actually recrystallized right into tiny micas (biotite and muscovite i m sorry reflect a satiny luster. Phyllite is between slate and also schist.
Schist - The size of the mineral grains tends to enlarge with boosting grade the metamorphism. At some point the rock develops a near planar foliation resulted in by the desired orientation of sheet silicates (mainly biotite and muscovite). Quartz and also Feldspar grains, however show no desired orientation. The irregular planar foliation at this stage is referred to as schistosity.

Schist regularly has various other minerals as well as micas. These include minerals prefer - Quartz, Feldspars, Kyanite, Garnet, Staurolite, and also Sillimanite. Once these non-mica minerals take place with a serial size better than the remainder of the rock, they room called pophyroblasts.

Gneiss As metamorphic grade increases, the sheet silicates end up being unstable and dark colored minerals choose hornblende and pyroxene begin to grow. This dark colored minerals tend to come to be segregated in unique bands through the rock, providing the rock a gneissic banding. Due to the fact that the dark fancy minerals have tendency to form elongated crystals, rather than sheet- prefer crystals, lock still have actually a desired orientation v their long directions perpendicular come the best differential stress.
Granulite - at the highest grades of metamorphism all of the hydrous minerals and sheet silicates come to be unstable and thus over there are few minerals present that would display a desired orientation. The resulting rock will have actually a granulitic texture that is comparable to a phaneritic structure in igneous rocks. Migmatites – If the temperature reaches the solidus temperature (first melt temperature), the absent may start to melt and start come co-mingle through the solids. Generally these melts are felsic v the mafic product remaining metamorphic.
Non-foliated Metamorphic Rocks

Non-foliated rocks absence a planar fabric . Lack of foliation feasible for numerous reasons:

rock not based on differential stress. prominence of equant minerals (like quartz, feldspar, and garnet). absence of platy minerals (sheet silicates).

Non-foliated rocks space given certain names based on their mineralogy and also composition: Amphibolite - this rocks room dark fancy rocks with amphibole (usually hornblende) as their significant mineral. They are usually poorly foliated and kind at intermediary to high grades of metamorphism the basaltic or gabbroic protoliths. Hornfels - this are very fine grained rocks that usually type as a result of magma intruding right into fined grained igneous rocks or shales. The magma reasons a type of metamorphism called contact metamorphism (to be debated later).

Quartzite - A absent made up nearly entirely of quartz. Lock are formed by metamorphism of quartz arenites (sandstones). Since quartz is secure over a large range the temperatures and also pressures, no new minerals room formed during metamorphism, and the only metamorphic impact that wake up is recrystallization of the quartz causing interlocking crystals that make up a very hard rock.

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Marble - A limestone or dolostone comprised only the calcite or dolomite will certainly metamorphose come a marble which is made mostly recrystallized calcite or dolomite. The Recrystallization generally obliterates all fossils. Marbles have a variety of colors and also are regularly complexly banded. They are generally used as a decorate stone.

Protolith Composition

Although textures and also structures that the protolith are usually destroyed by metamorphism, we deserve to still acquire an idea around the original rock indigenous the minerals present in the metamorphic rock.

Minerals the form, perform so because the chemical elements necessary to kind them are current in the protolith.

General terms offered to define the chemistry composition that both the protolith and also the resulting metamorphic rock are:

Pelitic Alumina affluent rocks, usually shales or mudstones. These begin out originally with clay minerals and as a an outcome of metamorphism, Alumina rich minerals favor micas, chlorite, garnet, kyanite, sillimanite and also andalusite form. Since of the variety of sheet silicates, pelitic rocks commonly type slates, phyllites, schists, and gneisses during metamorphism.

Mafic - These space Mg and also Fe affluent rocks with low amounts of Si. Minerals favor biotite, hornblende and also plagioclase form during metamorphism and also commonly produce amphibolites.

Calcareous - These room calcium-rich rocks usually acquired from limestones or dolostones, and thus contain an abundance of Calcite. Marbles are the form of metamorphic rock that results.

Quartzo-Feldspathic - Rocks the contain an abundance of quartz and feldspar fall into this category. Protoliths space usually granites, rhyolites, or arkose sandstones and also metamorphism outcomes in gneisses containing wealth of quartz, feldspar, and biotite.