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Metamorphic rocks
One of the three major groups of
rocks that make up the crust of the Earth. The other two groups are igneous
rocks and sedimentary rocks. Metamorphic rocks are preexisting rock masses in
which new minerals, or textures, or structures are formed at higher temperatures
and greater pressures than those normally present at the Earth's surface. See
also: Igneous rocks; Sedimentary rocks
Two groups of metamorphic rocks may
be distinguished; cataclastic rocks, formed by the operation of purely
mechanical forces; and recrystallized rocks, or the metamorphic rocks properly
so called, formed under the influence of metamorphic pressures and temperatures.
Cataclastic rocks are mechanically
sheared and crushed. They represent products of dynamometamorphism, or kinetic
metamorphism. Chemical and mineralogical changes generally are negligible. The
rocks are characterized by their minute mineral grain size. Each mineral grain
is broken up along the edges and is surrounded by a corona of debris or strewn
fragments (mortar structure, Fig. 1a). During the early stages of this
alteration process the metamorphosed product is known as flaser rock (Fig. 1b).
Eventually the original mineral grains are entirely gone, as in the mylonites.
When seen through the microscope, the comminuted particles consist of a mixture
of finely powdered quartz, feldspar, and other minerals with an incipient
recrystallization of sericite or chlorite. Pseudotachylite is an extreme end
product of this crushing process. See also: Metamorphism; Mylonite
Fig. 1 Fabrics of metamorphic rocks under
microscope. (a) Mortar fabric; (b) flaser or mylonitic fabric; (c) granoblastic
fabric (after E. E. Wahlstrom, Petrographic Mineralogy, John Wiley and Sons,
1955). (d) Lepidoblastic fabric; (e) nematoblastic fabric; (f) porphyroblast
with reaction rim (after T. F. W. Barth, Theoretical Petrology, John Wiley and
Sons, 1952).
Structural
relations
Metamorphic rocks, properly so
called, are recrystallized rocks. The laws of recrystallization are not the same
as those of simple crystallization from a liquid, because the crystals can
develop freely in a liquid, but during recrystallization the new crystals are
encumbered in their growth by the old minerals. Consequently, the structures
which develop in metamorphic rocks are distinctive and of great importance,
because they reflect the physiochemical environment of recrystallization and
thereby the genesis and history of the metamorphic rock.
Crystalloblastic
structure
A crystalloblast is a crystal that
has grown during the metamorphism of a rock. The majority of minerals are
frequently bounded by their own crystal faces (idioblasts). Larger crystals are
often packed with small inclusions of other minerals exhibiting the so-called
sieve structure (poikilitic or diablastic structure).
Granoblastic refers to a nondirected
rock fabric, with minerals forming grains without any preferred shape or
dimensional orientation (Fig. 1c). Lepidoblastic (Fig. 1d), nematoblastic (Fig.
1e), and fibroblastic refer to rocks of scaly, rodlike, and fibrous minerals,
respectively.
The metamorphic minerals may be
arranged in an idioblastic series (crystalloblastic series) in their order of
decreasing force of crystallization as follows: (1) sphene, rutile, garnet,
tourmaline, staurolite, kyanite; (2) epidote, zoisite; (3) pyroxene, hornblende;
(4) ferromagnesite, dolomite, albite; (5) muscovite, biotite, chlorite; (6)
calcite; (7) quartz, plagioclase; and (8) orthoclase, microcline.
Preferred
orientation
Certain minerals have a tendency to
assume parallel or partially parallel crystallographic orientation. The shape
and spatial arrangement of minerals such as mica, hornblende, or augite show a
definite relation to the foliation in the schist or gneiss; that is, both
foliation and fissility of a metamorphic rock are directly related to the
preferred position assumed by the so-called schist-forming minerals, such as
mica, hornblende, and chlorite. See also: Gneiss; Schist
Students of structural petrology
distinguish between preferred orientation of inequidimensional grains according
to their external crystal form, and preferred orientation of equidimensional
grains according to their internal or atomic structure. See also: Petrofabric
analysis; Structural petrology
A special microscope technique
(universal stage technique) is necessary in most cases to demonstrate in detail
the preferred orientation of the mineral grains according to their atomic
structure. See also: Petrography
Relic structures
Mineral relics often indicate the
temperature and pressure that obtained in the preexisting rock. If a mineral,
say quartz, is stable in the earlier rock and is also stable in the later rock,
it will be preserved (unless stress action sets in) in its original form as a
stable relic. However, when a mineral or a definite association of minerals
becomes unstable, it may still escape alteration and appear as an unstable
relic. These relics are proterogenic, that is, representative of an earlier,
premetamorphic rock, or of an earlier stage of the metamorphism. Hysterogenic
products are of later date, and are formed in consequence of changed conditions
after the formation of the chief metamorphic minerals.
A common phenomenon, fairly
illustrative of the tendency toward equilibria, is the formation of armors or
reaction rims around minerals (Fig. 1f) which have become unstable in their
association but have not been brought beyond their fields of existence in
general (the armored relics). Thereby the associations of minerals in actual
contact with one another become stable. If, however, the constituent minerals of
a rock containing armored relics are named without noting this phenomenon, it
may be taken as an unstable association. See also: Porphyroblast
Structure relics are perhaps of
still more importance, directly indicating the nature of the preexisting rock
and the mechanism of the metamorphic deformation. The interpretation of relics
has been compared to the reading of palimpsests, parchments used for the second
time after original writing was nearly erased. Every trace of original structure
is important in attempting to reconstruct the history of the rock and in
analyzing the causes of its metamorphism.
In sedimentary rocks the most
important structure is bedding (stratification or layering) which originally was
approximately horizontal. In metamorphic rocks deformed by folding, faulting, or
other dislocations, the sum of all deformations can be referred to the original
horizontal plane, and the deformations can be analyzed.
Fissility and
schistosity
One of the earliest secondary
structures to develop in sediments of low metamorphic grade is that of slaty
cleavage (also referred to as flow cleavage or fissility), which grades into
schistosity which is different from fracture cleavage, or strain-slip cleavage.
Slaty cleavage is developed normal to the direction of greatest shortening of
the rock mass, and cuts the original bedding at various angles. Tectonic forces
acting on a book of sediments of heterogeneous layers will throw them into a
series of folds, and slaty cleavage develops in response to the stresses imposed
on the rock system as a whole, because of the differential resistance of the
several layers. Consequently, folding and slaty cleavage have a common
parentage, as illustrated in Fig. 2.
Fig. 2 Diagram showing the general relationship
between deformation folding and slaty cleavage caused by pressure PP or the
couple ScSc. Heavy black lines denote the original bedding deformed into folds.
Thin lines indicate slaty cleavage which may grow into schistosity (false
bedding). (After G. Wilson, Proceedings of the Geological Association,
1946)
In the rock series
slate-phyllite-schist the slaty cleavage will grade into schistosity. It is a
chemical and recrystallization phenomenon, as well as a mechanical one, and the
directions of the schistosity become the main avenues of chemical transport. See
also: Rock cleavage; Slate
Contact-metamorphic
rocks
Igneous magma at high temperature
may penetrate into sedimentary rocks, it may reach the surface, or it may
solidify in the form of intrusive bodies (plutons). Heat from such bodies
spreads into the surrounding sediments, and because the mineral assemblages of
the sediments are adjusted to low temperatures, the heating-up will result in a
mineralogical and textural reconstruction known as contact metamorphism. See
also: Pluton
The width of the thermal aureole of
contact metamorphism surrounding igneous bodies varies from almost complete
absence in the case of small intrusions (basalt dikes or diabase sills) to
several kilometers in the case of large bodies. See also: Contact aureole
The effects produced do not depend
only upon the size of the intrusive. Other factors are amount of cover and the
closure of the system, composition and texture of the country rock, and the
abundance of gaseous and hydrothermal magmatic emanations. The heat conductivity
of rocks is so low that gases and vaporous emanations become chiefly responsible
for the transportation and transfer of heat into the country rock.
Alteration of stratified
rocks
Stratified rocks are altered in the
contact zone to what is commonly called hornfels or hornstone. They are
hardened, often flinty rocks, usable for road material, and so fine-grained that
the mineral components can be discerned only with the microscope. Hornfels used
to be regarded as “silicified” sediments. However, T. Kjerulf, in the later half
of the nineteenth century, analyzed sedimentary shale and “silicified” shale of
the Oslo region and found that, chemically, they were identical (except for
water and carbon dioxide content). Then geologists realized that the “hardening”
of the shale took place without appreciable change in the chemical composition.
Kjerulf summarized his results by saying that the composition (the shale) was
independent of the kind of adjacent igneous rock.
Later H. Rosenbusch arrived at the
same conclusion and pronounced that no chemical alterations accompany the
formation of hornfelses except for the removal of fugitive constituents. The
KjerulfRosenbusch rule is useful but needs modification, because chemical
changes may ensue from hydrothermal and pneumatolytic action.
The next problem then is to see how
the mineral assemblages of the hornfelses depend upon the chemical composition
of the original sediments. The chief types of sedimentary rocks are sandstone
(sand), shale (clay), and limestone. Among the varieties of hornfelses which may
develop from different mixtures of these components, the continuous series from
shale to limestone is the most interesting.
Most shales contain some iron- and
magnesia-bearing constituents in addition to feldspar and clay minerals. Quartz,
SiO2, is always admixed. Consequently, sufficient SiO2 is often present in the
hornfelses to form highly silicified minerals. Other than SiO2, the four chief
chemical constituents are Al2O3, CaO, FeO, and MgO. The last two constituents
are grouped together to define a system of three components: alumina, lime, and
ferromagnesia.
By applying the mineralogical phase
rule, which states that the number of stable minerals in a rock shall not be
larger than the number of components, it follows that, except for quartz and
some alkali-bearing minerals listed below, no more than three additional
minerals should occur in any one (variety) of these hornfelses. Observations
have verified this. Thus from alumina, lime, and ferromagnesia, seven minerals
will form that are stable under the conditions of contact metamorphism:
andalusite, Al2SiO5; cordierite, Mg2Al4Si5O18; anorthite, CaAl2Si2O8;
hypersthene, (Mg,Fe)SiO3; diopside, Ca(Mg,Fe)Si2O6; grossularite, Ca3Al2Si3O12;
and wollastonite, CaSiO3. Only three (or fewer) of these minerals can occur
together. In this way different mineral combinations develop, each combination
(plus quartz and an alkali-bearing mineral) representing a natural hornfels.
There are 10 such combinations, corresponding to hornfelses of classes 1–10 of
V. M. Goldschmidt's terminology. See also: Hornfels
Variations from the above scheme are
easily explained. Usually enough water and potash are present to produce mica;
muscovite may form instead of, or together with, andalusite, and in the
hornfelses of classes 4 and 5, biotite is usually present inducing a
characteristic chocolate-brown color into the rocks. In hornfelses of class 10
some lime-rich hydrous silicates may develop, for example, vesuvianite
(idocrase). The presence of ferric iron may produce andradite, Ca3Fe2Si3O12, a
yellow to dark-green garnet which will form mixed crystals with grossularite.
Pneumatolysis and
metasomatism
Other factors of importance in
contact metamorphism are chemical changes that ensue from pneumatolytic and
hydrothermal action. These changes are brought about by the magmatic gases and
high temperatures that accompany igneous intrusions. The surrounding rocks are
deeply penetrated not only by the heat but also by water and other volatile
compounds. Because chemical alterations take place in this so-called
pneumatolytic or hydrothermal contact zone, the Kjerulf-Rosenbusch rule is not
applicable. The width of the affected zone varies from nil to thousands of feet.
See also: Metasomatism; Pneumatolysis
The primary magmatic gases are acid
and in consequence show high reactivity. If the contact rock is basic,
especially limestone, the acid gases will react effectively with it. Limestone
acts as a filter, capturing the escaping gases. As a result, a great variety of
reaction minerals is formed. The corresponding rocks are known as skarns. If the
reaction rocks are limestones composed of lime silicates, the reaction minerals
are mainly garnet and pyroxene, often accompanied by phlogopite and fluorite.
Sulfides of iron, zinc, lead, or copper may be present, and in some occurrences
magnetite is formed. See also: Skarn
Summary
Contact metamorphism caused by
deep-seated magma intrusions is very common, and the products (disregarding the
pneumatolytic action) vary regularly in accordance with the chemical
compositions of the preexisting contact rock. Another factor of equal importance
is the variation in temperature as influenced by the nature of the intruding
rock and the distance from the contact. Thus it is possible to distinguish
between an inner and an outer contact zone. The zones grade into each other by
imperceptible transitions, but the mineral associations in the typical inner
contact zone, the only zone considered so far, are markedly different from the
associations in the outer contact zone.
These problems involve a
consideration of the general relationships between minerals and mineral
associations, on the one hand, and the temperature and pressure, on the other.
They are discussed further in connection with the facies principle and the
general process of regional metamorphism. However, it is important to realize
that contact metamorphism, although it appears to be well defined and seems to
stand out as an isolated natural phenomenon, is complex and variegated and
passes by gradual transitions into other kinds of metamorphism. Geologically,
contact metamorphism should be considered in connection with, and as a part of,
the general system of rock metamorphism and metasomatism.
Regional metamorphic
rocks
Crystalline schists, gneisses, and
magmatities are typical products of regional metamorphism and mountain building.
If sediments accumulate in a slowly subsiding geosynclinal basin, they are
subject to down-warping and deep burial, and thus to gradually increasing
temperature and pressure. They become sheared and deformed, and a general
recrystallization results. However, subsidence into deeper parts of the crust is
not the only reason for increasing temperature. It is not known what happens at
the deeper levels of a live geosyncline, but obviously heat from the interior of
the Earth is introduced regionally and locally, partly associated with magmas,
partly in the form of “emanations” following certain main avenues, determined by
a variety of factors. From this milieu rose the lofty mountain ranges of the
world, with their altered beds of thick sediments intercalated with tuffs, lava,
and intrusives, all thrown into enormous series of folds and elevated to
thousands of meters. Thus were born the crystalline schists with their variants
of gneisses and migmatites. See also: Earth, heat flow in; Orogeny
A. Michel-Lévy (1888) distinguished
three main étages in the formation of the crystalline schists; F. Becke and U.
Grubenmann (1910) demonstrated that the same original material may produce
radically different metamorphic rocks according to the effective temperature and
pressure during the metamorphism. Grubenmann distinguished three successive
depth zones, epizone, mesozone, and katazone, corresponding to three consecutive
steps of progressive metamorphism. In eroded mountain ranges, rocks of the
katazone are, generally speaking, encountered in the central parts; toward the
marginal parts are found rocks of the mesozone and epizone.
It is of paramount importance to
obtain better information about the temperature-pressure conditions of the
recrystallization, and thus to show the relation between the chemical and
mineralogical composition of all varieties of rocks. A large-scale attempt in
this direction was the development of the facies classification of rocks.
Mineral
facies
As defined by P. Eskola (1921), a
mineral facies “comprises all the rocks that have originated under temperature
and pressure conditions so similar that a definite chemical composition has
resulted in the same set of minerals, quite regardless of their mode of
crystallization, whether from magma or aqueous solution or gas, and whether by
direct crystallization from solution … or by gradual change of earlier
minerals.…” To learn which mineral associations were characteristic of high
temperature or of low temperature, and to determine which associations combined
with high pressure and with low pressure, Eskola studied the mineral
associations in the rocks.
It has long been known that in an
area of progressive metamorphism each successive stage, or each new zone of
metamorphism, is reflected in the appearance of characteristic rock types (G.
Barrow, 1893). Rocks within the same zone may be called isofacial, or isograde
as proposed by C. E. Tilley (1924) who, furthermore, proposed the term “isograd”
for a line of similar degree of metamorphism.
In going from an area of
unmetamorphosed sedimentary rocks into an area of progressively more highly
metamorphic rocks, new minerals appear in orderly succession. Thus, in a series
of argillaceous rocks subjected to progressive metamorphism, the first index
mineral to appear is usually chlorite, followed successively by biotite, garnet
(almandite), and sillimanite. A line can be drawn on the map indicating where
biotite first appears. This line is the biotite isograd. The less metamorphosed
argillites on one side of this line lack bioite, whereas the more metamorphosed
rocks on the other side contain biotite. An isograd can be drawn for each
mineral. Actually the isograds are surfaces, and the lines drawn on the map are
the intersections of these surfaces with the surface of the Earth.
Further work along these lines
resulted in the conclusion that it was possible to single out a well-defined
series of mineral facies. Sedimentary rocks of the lowest metamorphic grade
recrystallized to give rocks of the zeolite facies. At slightly higher
temperatures the greenschist facies develops—chlorite, albite, and epidote being
characteristic minerals. A higher degree of metamorphism produces the
epidote-amphibolite facies, and a still higher degree the true amphibolite
facies in which hornblende and plagioclase mainly take the place of chlorite and
epidote. Representative of the highest regional metamorphic grade is the
granulite facies, in which most of the stable minerals are water-free, such as
pyroxenes and garnets. Any sedimentary unit will recrystallize according to the
rules of the several mineral facies, the complete sequence of events being a
progressive change of the sediment by deformation, recrystallization, and
alteration in the successive stages: greenschist facies → epidote-amphibolite
facies → amphibolite facies → granulite facies. The mineral associations of
these rocks are summarized in the next section.
During regional metamorphism a
stationary temperature gradient is supposed to be established in the mountain
masses. Usually, the outer parts of a geosynclinal region are less affected, and
in the ideal case the marginal parts contain unmetamorphosed sediments, clay,
sand, and limestone, which gradually change into metamorphic rocks of
successively higher facies as they extend into the central and deeper parts.
The table summarizes the metamorphic
series of rocks that develop from the several types of common sediments and
usually converge toward a granitic composition regardless of the nature of the
original material. Basic igneous rocks (gabbros, basalts) show a composition
related to that of marl and yield analogous metamorphic products. Not listed are
ultrabasites (peridotites, and others) which by metamorphism become serpentine,
chlorite or talc schist, soapstone, hornblende schist, pyroxene, or olivine
masses. Original acid igneous rocks (granite, diorite, rhyolite) show a
composition related to that of arkose and yield analogous products. Leptite is
primarily fine-grained, usually showing tufaceous or blastoporphyric relic
structures; or it is derived from argillaceous sediments. Hälleflintas are dense
rocks of conchoidal fracture, genetically related to leptites. Kinzigites,
characterized by containing aluminum silicates and usually also rich in
magnesia, are metasomatic gneisses, but probably argillites also enter into
their constitution. Granulite is a gneiss recrystallized in the high-temperature
mineral facies group. See also: Granulite
Chemical
alterations
The chemical changes in the
progressive series are complicated. When a sediment is heated, it obviously
loses water and other volatile components, carbon dioxide, halogens, and so
forth. These vapors act as carriers of several of the nonvolatile elements, for
example, Si, Fe, and Mn. As heat emanates from the central parts of a
geosynclinal region, it is heralded by a cloud of these vapors migrating
centrifugally through the surrounding sediments. Usually, however, no major
chemical alterations take place. Petrographers used to believe that the vapors
were rich in alkalis and that large quantities, particularly of sodium,
gradually would be deposited and fixed in the sediments. The argument was that
the sediments, concomitant with a progressive change by increasing metamorphism,
seem to exhibit an increase in their sodium content. However, this is not a real
increase, but due to an erroneous sampling of the sedimentary material. Normally
the lower stages of metamorphism tend to conserve the original composition.
Mineral associations of the
facies
Rocks of the greenschist facies
recrystallized at low temperatures and often under high shearing stress. These
include chlorite schists, epidote-albite schists, and actinolite schists, all of
which are green, hence the name. Other common rocks in this group are
serpentinites, talc schists, phyllites, and muscovite (sericite) schists.
Plagioclase is not stable in greenschists but breaks up into epidote and albite.
Glaucophane schists are rare and probably they represent greenschists formed
under conditions of high stress. Rocks of the epidote-amphibolite facies
recrystallized in a somewhat higher temperature range. Amphibolites with epidote
and either albite or oligoclase are typical. Mica schists (garnet), biotite
schists, staurolite schists, and kyanite schists are common.
Cordierite-antophyllite (gedrite) schists and chloritoid schists also occur.
Sodic plagioclase is stable in these rocks.
Rocks of the amphibolite facies
recrystallized at about 1100°F (600°C). Amphibolites of hornblende and
plagioclase are typical and often carry quartz or biotite or both, or garnet.
Sillimanite-muscovite schists (gneisses) are common. Andalusite and staurolite
occur at low pressure; kyanite and staurolite at intermediate and high pressure.
Rocks of the granulite facies have
their greatest extension in old Precambrian areas, but are also found in younger
deeply eroded mountain chains. Diagnostic association is ortho- and
clinopyroxene; hence the alternative name, two-pyroxene facies group.
Hypersthene and augite (garnet) plagioclase gneisses, usually with quartz but
occasionally with olivine or spinel, are typical. Amphibolites with hypersthene
or diopside or both, sillimanite gneisses, and at high pressure kyanite gneisses
are found. The temperature range is probably around 1500°F (800°C). The
experiments by H. S. Yoder (1952) in the system MgO–Al2O3–SiO2–H2O indicated
that at approximately 1100°F (600°C) and 1200 atm (120 megapascals) it is
possible to have different mineral assemblages suggestive of every one of the
now accepted metamorphic facies in stable equilibrium. These different mineral
assemblages (artificial facies) observed by Yoder are the result of differences
in the water content, and are not related to variation in temperature and
pressure.
As an example, the mineral
clinochlore, corresponding to one of the most common rock-making chlorites,
shows an upper limit of stability, either alone or in association with talc, of
1300°F (680°C). This is, indeed, a high temperature for any kind of
metamorphism, almost a magmatic temperature. But according to the tenets of the
mineral facies, chlorite is strictly limited to the greenschist facies, about
390°F (200°C).
Although Yoder has proved that there
is no absolute relation between temperature and facies, it appears likely that,
to the field geologist and to the laboratory worker as well, the facies will
still remain the best system of classification of metamorphic rocks; and in a
majority of cases the facies will indicate the temperature-pressure conditions
under which the several rocks recrystallized. Generally speaking, there is a
regular relation between the chemical activity of water and the facies of the
metamorphic rock.
Water content and mineral
facies
The role of water in metamorphism is
determined by at least four variable, geologically related parameters: rock
pressure, temperature, water pressure, and the amount of water present. During a
normal progressive regional metamorphism, rock pressure and temperature are
interdependent. The amount of water and the pressure of water are related to the
encasing sediments and to the degree of metamorphism in such a way that,
generally speaking, the low-grade metamorphic facies are characterized by the
presence of an excess of water, the medium-grade by some deficiency in water,
and the high-grade by virtual absence of water.
In the usual diagrammatic
illustration of the mineral facies of rocks, temperature and pressure (depth)
are taken as coordinates; in regional-metamorphic rocks a third, dependent
coordinate may be added, the activity of water running upward approximately
along the geothermal gradient.
Fig. 3 Mineral facies groups of regional
metamorphic rocks showing the temperature and pressure of metamorphism. °F = (°C
× 1.8) + 32.1 km = 0.6 mi.
Facies series and
groups
Metamorphic facies may be divided
into facies series depending mainly on pressure, and facies groups depending
mainly on temperature.
Three facies series have been
proposed: low-pressure, intermediate-pressure, and high-pressure series. The
low-pressure series dominates the Hercynian and Svecofennian of Europe, the
Paleozoic of Australia, and part of the paired belts in
Four facies groups are recognized
from low to high temperature: laumontite and prehnite-pumpellyite; greenschist,
including glaucophane schist; amphibolite, including epidote-amphibolite; and
the two-pyroxene (granulite) facies group.
This scheme is presented in Fig. 3.
The “normal” geothermal gradient is in the range 45–70°F per mile (15–23°C per
kilometer) depth. The intermediate-pressure facies series is found in areas with
this gradient; the high-pressure series (the Alpine series) in areas with lower
thermal gradients or with high over-pressure (orogenic pressure); and the
low-pressure facies series in areas with steep thermal gradients. The phase
boundaries of the polymorphic forms of Al2SiO5 (andalusite, sillimanite,
kyanite) have a central position in this scheme, the triple point being located
approximately at 840°F (450°C) and 6 kilobars (60 MPa). The “minimum” melting of
granite under water pressure occurs approximately along the boundary between the
amphibolite and the two-pyroxene facies group. A separate eclogite facies is not
recognized by this scheme. See also: Eclogite
Figure 3 illustrates the distributions and interrelations of the various metamorphic rocks. It also represents a schematic profile through the continental crust down to 40-mi (60-km) depth, that is, down to the Moho discontinuity. Thus the normal continental crust is entirely made up of metamorphic rocks; where thermal, mechanical, and geochemical equilibrium prevails, there are only metamorphic rocks. Border cases of this normal situation occur in the depths where ultrametamorphism brings about differential melting and local formation of magmas. When equilibrium is restored, these magmas congeal and recrystallize to (metamorphic) rocks. At the surface, weathering processes oxidize and disintegrate the rocks superficially and produce sediments as transient products. Thus the cycle is closed; petrology is without a break. All rocks that are found in the continental crust were once metamorphites.
- M. G. Best, Igneous and Metamorphic Petrology, 1982
- K. Bucher and M. Frey, Petrogenesis of Metamorphic Rocks, 6th ed., 1994
- R. Mason, Petrology of the Metamorphic Rocks, 2d ed., 1990
- A. Miyashiro, Metamorphic Petrology, 1994
- A. Nicolas and J. P. Poirier, Crystalline Plasticity and Solid-State Flow in Metamorphic Rocks, 1976
- H. Ramberg, Gravity, Deformation and the Earth's Crust in Theory, Experiments and Geological Application, 2d ed., 1981
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