Metamorphism, Common Structures and
Textures of Metamorphic Rocks, Classification
of Metamorphic Rocks, Descriptive Study of
Common Metamorphic Rocks (Quartzite,
Marble, Slate, Gneiss, Schist).
• Methamorphic rocks are geologically classified
as one of the major group of rocks which have
been formed out of meta-morphism of preexisting igneous and sedimentary rocks. The
parametamorphic rocks are applied to those rocks
which have originated from igneous and
sedimentary rocks, respectively. Sometimes
metamorphism again. When a rock undergoes
metamorphism more than once, the process is
• The word metamorphism means change of
form (meta= change; morph= form). In
temperature, pressure and chemically active
solutions over the texture, minerals and
composition of parent rocks. Igneous and
sedimentary rocks which serve as parent rock
as formed under a certain physiochemical
environment, i.e.. at the time of their
formation, they were in equilibrium with the
surrounding in terms of temperature, pressure
and chemically active fluids.
• This means the constituent minerals, texture
and composition of parent rocks by
metamorphism change over to new minerals
or new texture or new composition which are
more stable and suitable under new condition.
For ex, As a result of metamorphism (i)
Granite changes to gneiss, sandstone
changes to quartzite, limestone changes to
• The range of temperature and pressure which
occurs in nature is very wide. The normal surface
temperature and pressure affect the rock by
causing weathering. As accepted by many, the
term metamorphism does not include either
weathering of rocks or magma formation. The
represented by paligenesis or ultramorphism.
In these the intensity of temperature and pressure
will be so high that it involves partial melting and
mixing of rocks. Since the identity of the altered
rock is not totally destroyed, in these states, they
come under metamorphism
11. Metamorphic Agents
• The process of metamorphism occur in
temperature, pressure and chemically active
fluids. These three are known as metamorphic
agents. Generally, all these three act together
and cause metamorphism. But sometimes, any
one or two of them dominate and play an
active role. The following are a few relevant
details about metamorphic agents.
12. Metamorphic Agents
13. Metamorphic Agents
• The source of temperature which is responsible
for metamorphism is either due to depth or due
to the contact with magma. The metamorphic
changes mainly take place in the
temperature range of 350 – 850 0 C. The
temperature rise also increases the chemical
activity in rocks and facilitates reaction during
14. Metamorphic Agents
• The pressure which causes metamorphism is of two
different kind namely, uniform pressure and direct
• Uniform pressure increases with depth. It acts vertically
downwards and affects the volume of both liquids and
solids. Naturally, its effect is significant only at great
depths. But not at or near the surface. This also means
that high temperature will also be associate with high
temperature will also be associated with high uniform
pressure. So, both of them act together and bring
15. Metamorphic Agents
• The direct pressure, which is also called stress, is
due to tectonic forces. Such pressure act in any
direction, i.e. upward or downward or sideward's It acts
only on solids and affect the shape of rocks or minerals.
It is effective in the upper layers of the crust and
increases with depth to some extent. With further
increase in depth, of high temperature and pressure
the rocks will not be rigid and become plastic.
• In this way the regions of influence of uniform pressure
and direct pressure are mutually opposite. The
application of stress gives rise to shearing movements
in the rock and produces new minerals which have
platy habit and arranges approximately parallel to
16. Metamorphic Agents
Chemically Active Fluids
• Chemically active fluids play a key role in different ways in
causing metamorphism. First Since metamorphism of any type
cannot take place for the solid minerals in a perfect dry state, the
presence of a liquid medium of some kind is indispensable. Thus
the liquids act as a carrier of chemical components that actually
take part in chemical reaction. In other words, liquids provide the
necessary medium and facilitates the reaction to take place. The
most common liquid which play such role is water.
• Second The huge quantity of volatiles that are associated with
magmatic bodies ultimately permeate through the surrounding
rocks by means of diffusion and cause compositional changes
even in rocks. Far from magma.
• Third: the magma or the hot juvenile hydrothermal solutions
may react directly with those rocks with which they come in
17. Types of Metamorphism
• All kinds of metamorphism in which heat plays a
predominant role are given the common name “
thermal metamorphism”. The surrounding
region of magma chamber or magma intrusion in
which the heat effect is perceptible is called
metamorphic aureole. Heat becomes an
important factor at great depth and also in the
vicinity of magma. In the latter, the country rocks
are sometimes soaked in fluids emanating from
magma. This results in mineral transformation.
18. Types of Metamorphism
19. Types of Metamorphism
Dynamic Metamorphism (Direct Pressure Predominant)
• The type of metamorphism that occur mainly due to
direct pressure or stress, is called Cataclastic
metamorphism, or dynamic metamorphism. With the
little heat, when directed pressure acts, rocks are forced
to move past one another resulting in their crushing and
granulation. This kind of effect is called catalysis.
Cataclasis is mere mechanical breakdown of rocks
without any new mineral formation.
• Dynamic metamorphism is also sometimes called
dislocation metamorphism. This takes place along great
fracture belts of the earth‟s crust.
20. Types of Metamorphism
• The type of metamorphism in which
uniform pressure and heat are predominant
is called static or load or geothermal
• Usually this will not bring out any change in
silicate rocks. But oceanic salt deposits, known
for the variety of their minerals, have suffered
considerable changes in this way.
21. Types of Metamorphism
Metasomatic Metamorphism (Chemically active Fluids
• The type of metamorphism in which significant
compositional changes occur in the parent rock due to
the predominant role played by chemically active fluids
is called metasomatic metamorphism. This
metamorphism alters the composition of rocks
significantly. Due to this, similar rock may be formed
out of different parent rock or different rocks may be
formed out of the same parent rock.
• The country rocks which are in contact with magma
chamber or those rocks which come in contact with it
during its upward journey as an intrusive body may
react and change over to rocks of different composition.
22. Types of Metamorphism
Pressure and Heat Predominant)
• The type of metamorphism in which direct
pressure and heat plays a dominant role is called
dynamothermal metamorphism. Among the
different kinds of metamorphism, this is the most
common type. Gneisses and schists which are
the most common metamorphic rocks are
produced by dynamothermal metamorphism.
23. Types of Metamorphism
• Platonic Metamorphism (Uniform pressure
and heat predominant)
• The type of metamorphism in which
uniform pressure and heat dominate is
called plutonic metamorphism. As this
metamorphism is related to depth and
overburden, it occurs at great depth which are
also associated with high temperature.
24. • Plutonic metamorphism has the tendency to produce
minerals which can accommodate more matter in less
volume; for example transformation of limestone to
• Generally, this type of pressure –temperature effect
will be insufficient to induce notable metamorphic
effects in silicate rocks.
25. The Mechanism of
• The change in minerals composition that occurs in any
rock during its metamorphism are exclusively due to the
chemical reaction under high temperature. The mechanism
of such reaction is as follow:
• The solid mineral constituents of the rock body cannot
react as such in a dry condition. Therefore, they should
either melt or undergo solution first, so that reaction take
place and produce new minerals. In a view of very high
melting point and latent heat of fusion of common rock
forming minerals, it is natural that their fusion becomes
improbable during metamorphism. In this way, reactions
take place from point to point within the rock and slowly
the whole rock formation is metamorphosed into new
one, exhibiting a new mineral assemblage,
26. Visible Changes Produced by
Metamorphism in Rocks
• The following are some of the significant visible change
that are produced as a result of metamorphism.
• Crystallization of calcareous sedimentary rocks (i.e..
limestone) and re crystallization of some igneous and other
• Formation of new minerals which are diagnostic of
• Development of foliation with or without segregation of
minerals of parent rocks
• Formation of drags folds, joints, etc during the rock
failure under pressure.
• Formation of slaty cleavage in argillaceous rocks. This
factor of course, represents a type of foliation itself that has
developed excellent in a very fine grained rock.
27. Nature of Metamorphism with
• In nature “rocks under the influence of
metamorphic agents incorporating such
changes in themselves, so as to survive under
new physicochemical condition”. The actual
changes that occur in parent rocks under the
influence of metamorphic agents and how such
changes help the rock in its survival under new
conditions can be appreciated by considering a
few typical metamorphic rocks like granite,
gneiss, marble, quartzite and slate.
28. Granite Gneiss
• Granite, on metamorphism, produce granite
gneiss. The difference or change observed in these
two rocks is textural change, i.e. Mutual
arrangement of constituent minerals. In
(feldspar, quartz, biotite, and hornblende) are
uniformly or randomly distributed throughout the
rock. But in granite gneiss, these minerals get aligned
i.e. Oriented in a direction normal to the stress of
metamorphism. Granite Gneiss will be more strong
and competent to withstand the pressure effect in
a direction perpendicular to it.
29. Granite Gneiss
• Limestone, on metamorphism, becomes marble, since
limestone is amorphous it has a random arrangement of
molecules. This lessens the strength of the rock. But during
metamorphism, under the influence of pressure and
temperature, the amorphous matter is formed to reduce in
volume by crystallization, i.e. It making it possible for more
matter to occupy less volume. This compactness and
denseness accompanying the crystallization of limestone
enables the rock to withstand greater load
• Thus, the crystallization character introduce a suitable change
in the parent limestone is advantageous for its survival by
increasing its strength.
• Quartzite is formed from sandstone under the
influence of thermal or dynamic metamorphism.
During metamorphism, the visible change that occurs is
the disappearance of cementing material. This recrystallization of constituent material of the parent rock
also causes the disappearances of bedding planes and
reduction of porosity and permeability. Such a physical
change not only makes the rock denser and stronger but
also eliminates the inherent weakness of the parent rock
caused by its cementing material and bedding planes.
This newely acquired additional strength enables
the rock to withstand the pressure effect that would
have prevailed during metamorphism.
• Slate is the product of metamorphism of shale. Shale
is a sedimentary rock made up of random arranged
clay particles. Under the influences of different
metamorphic agents, an infinite number of very minute
mica flakes not only originate but also align within the
rock as innumerable parallel planes perpendicular to the
direction of pressure or stress that prevailed during the
metamorphism of shale. Thus, the change introduced
in the parent rock during metamorphism is
mineralogical and textural. These changes increase
relatively the competence of the rock in the direction
of pressure and enable it to withstand the stress
effect during metamorphism.
36. Common Structures and Textures of
• The process of metamorphism, which is always
accompanied by pressure, induces alignment of constituent
minerals in a rock. The alignment or orientation of minerals
takes place perpendicular to the direction of the great stress.
When platy, lamellar, blady or flaky minerals occurs in
rocks, they orient themselves parallel to one another and of
course perpendicular to the direction of the (greatest)
pressure of metamorphism, such a texture is called foliation.
When prismatic, columnar or rod like minerals occur in rocks
they orient themselves parallel to one another and
perpendicular to the direction of pressure of metamorphism.
This is called lineation. In a section perpendicular to
lineation, such a rock shows evenly granular texture
38. Common Structures and Textures of
• In other direction, it will appear linear. Talc schists,
mica schists, chlorite schists, etc. shows good foliation,
whereas hornblende schist, tremolite schists, staurolite
schiists etc, shows good foliation, whereas hornblende
schists, tremolite schists, staurolite schists, etc, shows
• Since a majority of natural rocks have minerals with
shapes of the a fore mentioned kind, the alignment of
minerals provides a readily recognizable clue to
identify metamorphic rocks in hands specimens.
Gneisses and schists which represents the bulk of
metamorphic rocks exhibit this phenomenon.
• In metamorphic rocks, the distinction between
texture and structure is very vague; hence they
may be considered synonymous. In some
metamorphic rocks, textures of parent rocks are
retained as relief features. In some other
rocks, textures formed due to recrystallization
occur in still others, both may be present together.
Crystalloblastic and Palimpsest Textures
• The textures which have developed newely
during the process of metamorphism are called
crystalloblastic textures. The other textures
which belong to parent rocks but still retained
in metamorphism rocks are called palimpset
41. Crystalloblastic and Palimpsest Textures
Xenoblastic and Idioblastic Textures
• The crystalloblastic textures are of two kinds,
namely xenoblastic and idioblastic. In the
xenoblastic textures, the constituent minerals
of the rock have no well-developed crystal
faces. If the minerals have well developed
crystal faces and forms, the texture is
known as idioblastic.
43. Xenoblastic and Idioblastic Textures
• In the description of metamorphic rocks, the
crystalloblastic textures are named with the
term “blast” as a suffix. Similar, the
palimipset textures are named with the term
“blasto” as a prefix. On the other hand, if such
a texture were present in the parent rock and
continuous to remain so in it ever
• The most common structures found in
metamorphic rocks are gneissose, schistose
and granulose types, though these are called
structures, they are in fact, textures only
because (i) they represent mutual arrangement
of minerals in the rock, and also (ii) these are
small-scale features which can be studied even
in small hand specimen of rocks.
46. Gneissose Structure
• If the rock consists of equidimensional minerals
along with other (i.e.. platy and prismatic
minerals) first segregation of the minerals occur
and alternating bands are formed, then
foliation and lineation of platy and prismatic
minerals take place. Such a texture or
arrangement of minerals is called gneissose
structure. So, in gneissose structure, both
equidimensional and other minerals occur in
considerable proportions and they appear in
alternating bands along with the alignment of
prismatic and platy minerals.
47. Gneissose Structure
48. Schistose Structure
• If the rock consists of only prismatic or platy
minerals, then no segregation takes place but
only foliation and / or lineation such a texture is
called a schistose structure. So, in a schistose
structure, platy or prismatic minerals will be
dominating and they occur with orientation
(alignment). If a few equidimensional minerals
are present, they appear in between the layers of
other minerals as discontinuous patches or lenses
or bands or streaks.
49. Schistose Structure
50. Granulose Structure
• If the rock is composed predominantly of
equidimensional minerals, then neither
segregation nor foliation takes place. Such a
texture is called granulose structure. So, in
granulose structure-bearing rocks, prismatic or
platy minerals will be negligible or absent; only
equidimensional minerals will be present.
• The preceding relations are seen in rock types as
granite gneiss, mica or chlorite schists and
marble or quartzite.
51. Granulose Structure
52. Cataclastic Structure
• In addition to the aforementioned structures, another
structure called Cataclastic structure also occurs in
metamorphic rocks. As the name indicates, it is
produced under the influence of direct pressure in the
upper zones of earth‟s crust.
• Due to this, soft rocks like shales develop cleavage
and hard rocks are shattered to produce crushed
breccias. In some rocks more resistant minerals may
remain unaffected, while the softer minerals are
powdered to fine material. This result in an
appearance similar to porphyritic texture and is
called porphyroclastic structure.
53. Cataclastic Structure
54. Classification of Metamorphic Rocks
• Metamorphic rocks have been grouped as
orthometamorphic or parametamorphic
rocks based on whether they have been
formed out of igneous or sedimentary rocks.
In a different way, metamorphic rocks can be
continently classified based on their physical
appearance , i.e.. as massive or foliated rocks.
55. Classification of Metamorphic Rocks
Massive or Non-foliated Rocks
• Three important rock types coming under this
category are quartzite, marble and hornfels. Of
these, quartzite and marble are formed out of
metamorphism, while hornfels is always the
product of contact metamorphism. Quartzite and
marble have been described later. Hornfels is
dense, fine grained and, for practical purpose, can
be called impure quartzite.
56. Massive or Non-foliated Rocks
57. Classification of Metamorphic Rocks
• Metamorphic foliation, are already explained,
refers to the parallelism of in equidimensional
minerals, which develops under the influence
of pressure. More or less synonymous with
foliation are the terms flow cleavage,
schistosity and slaty cleavage. The common
foliated rocks in the order of increasing grain
size are; slaty, phyllite, schist and gneiss.
58. Foliated Rocks
59. Descriptive Study of Common
• Among different metamorphic rocks, gneiss is
more widespread and abundant than others.
• Gneiss is a general name given to any
metamorphic rock which shows a gneissose
structure. In many cases, gneisses are derived
from granites. Hence, the minerals composition,
grain size, color, etc, will be more or less
similar both in granite and gneisses. For this
reason, it is commonly referred to as granite
61. Descriptive Study of Common
• A few details of its physical description are as follows
• Diagnostic character: Foliation Present
• Color: different shades of gray and pink, but generally
• Grain size: Medium to coarse grained
• Texture and structure: Generally equigranular, but
sometimes porphyroblastic. Foliation i.e.. alignment
of minerals is characteristically seen.
• Minerals Present: Feldspar and quartz usually make
up the bulk of a gneiss.
63. Descriptive Study of Common
granite, sandstones and conglomerates, etc.
Among these, granite gneiss is far more
abundant than all others.
• Due to close mineralogical and other
resemblance, granite gneisses may be treated as
varieties of granites themselves.
64. Properties and Uses of Civil
• As gneiss is a silica-rich rock, it is durable
granite, gneisses also have pleasing color
impermeable, contributing to its strength.
• The gneissose structure with its alternating bands of
contrasting colors on polishing, produces a very good
• The foliation, to some extent, improves the
workability of gneiss.
• The occurrence of gneiss in plenty, in many
places, makes it an important building stone or material
66. Descripative Study of Common
• Like gneiss schists is also a very common
metamorphic rock and it is a general name
given to all metamorphic rocks bearing a
particular structure. Further, like gneiss it is
also a characteristically foliated rock.
68. Descriptive Study of Common
• Some of the important megascopic description of schists are
• Color: Different schists show different color. For example
mics schists is dazzling silvery white in color; biotite is
jet black in color; chlorite schists is dark green in color
and so on.
• Grain Size: This is variable; some schists are fine
grained, while others are medium grained or coarse grained.
• Texture and structure: Constituent minerals occur with
perfect alignment. Good lineation or foliation
occurs, depending on when prismatic or platy minerals
occur predominantly. As the name indicates, the
characteristic structure shown by schist is always schistose.
70. Descriptive Study of Common
• Schists are normally composed of prismatic or
platy minerals which contribute to the
development of schistose structure. The
prismatic minerals which occur commonly in
hornblende, sillimanite, tourmaline etc.
Chlorite, muscovite, talc, kyanite etc. are
commonly platy minerals occurring in
71. Descriptive Study of Common
• Origin: Schists are formed under the influence of
dynamothermal metamorphism of different kinds
of igneous and sedimentary rocks.
• Mica schists is formed out of shale.
• Mica-quartz schists is formed out of feldspatic
• Schists occur in many places and frequently too
because of the derivation from a large variety of
rocks, especially out of shales which are very
common sedimentary rocks.
73. Descriptive Study of Common
weak, incompetent, harmful, and undesirable rocks from the
civil engineering point of view Biotite, hornblende etc which
occur as the usual dominant minerals make the rocks unpleasantly
• The mineral of schists such as talc, chlorite, biotite are relatively
very soft. Hence, these rocks are not hard, strong and durable.
Thus, the various properties of schists make it unsuitable for any
important civil engineering purpose such as :
• (i) Site rock for foundation
• (ii) as a building stone
• (iii) as an aggregate for concrete making
• (iv) as road metal
• (v) as railway ballast.
74. Descriptive Study of Common
• Quartzite is a typical example of a parametamorphic
rock. It is siliceous in composition and is formed out
of dynamic or thermal metamorphism of sandstone.
The sand grains which make up the bulk of sandstone
are mainly quartz minerals or, in some cases feldspar.
• A quartzite has following properties
• Color : Uniform color throughout the rock.
red, brown, grey , green and other colors also may
• Grain size: Variable, some rocks are fine
grained, while other may be coarse grained.
• Fractures: In fine grained rocks the broken surface is
concoidal or subconchoidal
• Hardness: Very Hard, not scratched by glass or
• Appearance: Looks fresh and shining
• Texture and structure: Granulose structure, it is
crystalline dense and compact.
• Minerals present
• Quartz is essential or predominating constituent.
Some quartzite are more or less exclusive composed of
quartz. The other minerals which may occur in small
quantities are mica, feldspar, kyanite, magnetite. Etc.
79. Properties and Uses of Civil
• Because of the predominance of quartz, the color of
quartzite is generally white or pale colored and
pleasing. The silica composition of the rock makes it
highly durable and resistant to weathering. The
predominance of quartz makes the rock very hard.
The recrystallization process which produces these
rocks makes them denser and stronger.
• Thus the quartzite rock is strong, hard, durable and
has a pleasing color. But by virtue of its very high
hardness, the workability or dressing becomes very
difficult. Quartzite rocks are highly suitable as road
metal, railway ballasts, concrete aggregate, paving
• The term marble is derived from marmore “a
shining stone”. Like quartzite, marble is also a
parametamorphic rock. It is a calcareous
metamorphic rock formed out of the thermal
metamorphism of limestone. Though it is not
very hard or strong it is the most valuable rock
occurring in nature. Its value is due to its pleasant
workability, charming translucency and the
availability to take brilliant polish. It is also
valuable because it is not abundantly available in
• Marble shows the following properties
• Color: Some marbles show uniform color
throughout the rock. Pure marble is milky
white in color. Pleasant shades of green, yellow,
brown, blue or grey color may also occur.
• Grain Size: Fine medium or coarse grained, but
the rock is equigranular.
• Texture and structure: Marble shows a typical
granulose structure. This is because calcite,
which is neither platy nor prismatic, is the most
predominant mineral of the rock.
• Hardness: Marble is relatively soft and is easily scratched
with a glass piece or penknife. For this reason, marble can be
conveniently cut mechanically.
• Appearance: When a hand specimen of polished marble is
turned around, a twinkling appearance is noticed because
the surface of the rock passes through cleavage planes, it is
reflected giving a twinkling appearance.
• Translucency: Pure white marble are reasonably
translucent, which is an additional virtue of marble.
• Mineral Present: Just as quartz is the most predominant
mineral constituent of quartzite, so is calcite in marble.
87. Properties and Uses of Civil
• Occurance: As a geological formation, marble is
found in the form of thick or thin beds in association
with other metamorphic rocks like gneiss, schists and
• „as already mentioned, marbles are well known for
their beautiful colors, pleasing appearance and easy
workability. Being denser and less porous, it is of
course, stronger than limestone. Marbles occurring in
nature are quarried, dressed and used as building
stones for temples or other construction. Marbles
provide aesthetic beauty and a pleasing appearance to
the construction. They are used in innumerable
ornamental, decorative and monumental works
where carving, engraving and polishing etc are
grained, argillaceous, parametamorphic rock. It
has the unique character of slaty cleavage. It is
formed out of dynamic or regional metamorphism
of shale. By virtue of its cleavage character. It is
formed out of dynamic or regional metamorphism
of shale. By virtue of its cleavage character, it
splits easily into very thin sheets or slabs of
considerable size. Extreme fine grained
size, absence of reaction with acid, slaty cleavage
and shining on surfaces are diagnostic character
• Physical properties of slate are as follows
• Color: Slate usually exhibit uniform color.
Generally, they are black or dark grayish black. But
other colors like brown, red, green and grey and yellow
• Grain Size: Slate is very dense looking and extremely
fine grained. Individual grains are too fine to be seen
from the naked eye.
• Texture: Foliation is clearly visible through constituent
minerals are fine and unrecognizable. The layers of
shale which are differently colored appear as ribbons or
bands in slaty formation in fields.
• Bedding and fossil content: Some slate retain
their bedding or lamination character of shale
as a relict feature. Fossil may occur rarely but
such fossil are often distorted and squeezed out of
their original shape.
• Hardness: Slates are relatively soft when
compared with other metamorphic rocks.
• Minerals Present: Slates are mainly made up of
secondary mica and quartz other minerals
which may occur are biotite, talc, feldspar etc.
• The majority of slates are the result of dynamic
metamorphism of argillaceous sediments. A
few slates may also be formed from altered
basic igneous rocks.
96. Properties and Uses of Civil
• Slates are dense, fine grained, impermeable and
relatively resistant to decay. However, since
slates are soft and incompetent, they cannot
withstand great loads. So they are not suitable
for foundation purposes. Due to cleavage
character and softness, they split easily. Hence
they cannot be used as building stone. But since
slates are impermeable and can be split into thin
but big slab of uniform thickness, they can be
conveniently used for roofing, flooring, mantle
and shingles etc. they can also be used as
• Engineering and General Geology :By Parbin
• Textbook of Engineering Geology :N.Chenna