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Geology
GEOLOGY
Subject
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GEOLOGY
(For under graduate student.)
Paper No.
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Paper – 02
Introduction to Geology – 02
Topic No. & Title
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49
Metamorphic Rocks
Frequently Asked Questions
FAQ’s
Que 01. Distinguish between the processes metamorphism , diagenesis and
metasomatism
Ans. The word "Metamorphism" comes from the Greek: meta = change, morph =
form, so metamorphism means to change form. Metamorphic Rocks are formed by
the transformation of pre-existing rocks under the influence of high temperatures
and pressures and chemically active fluids. Note that Diagenesis is also a change in
form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic
processes to those which occur at temperatures below 200oC and pressures below
about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3 kilobars
of pressure (1kb = 100 MPa).Metamorphism, therefore occurs at temperatures and
pressures higher than 200oC and 300 MPa. Metasomatism is the drastic change in
the bulk chemical composition of a rock that often occurs during the processes of
metamorphism. It is due to the introduction of chemicals from other surrounding
rocks.
Que 02. What are the agents of metamorphism ?
Ans. Metamorphism is a process by which existing rocks are transformed into new
types by the recrystallisation of their constituents. This transformation occurs due to
marked changes in the physical nature of the surroundings such as temperature,
pressure, etc. the changes take place in the solid state. New minerals grow out of old
and the texture entirely different from those of the original rocks may be produced.
These changes are aided by the presence of solvents such as fluids contained in the
pores.
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Metamorphism is mainly due to the operation of three agents of environmental
factors:
1. The chemical activity of the liquids and gases within the rocks
2. The temperature
3. The pressure
Que 03. What is contact metamorphism ?
Ans. Contact or thermal Metamorphism
Contact or thermal metamorphism occurs adjacent to igneous intrusions and results
from high temperatures associated with the igneous intrusion.
Since only a small area surrounding the intrusion is heated by the magma,
metamorphism is restricted to the zone surrounding the intrusion, called a
metamorphic or contact aureole. Outside of the contact aureole, the rocks are not
affected by the intrusive event. The grade of metamorphism increases in all
directions toward the intrusion. Because the temperature contrast between the
surrounding rock and the intruded magma is larger at shallow levels in the crust
where pressure is low, contact metamorphism is often referred to as high
temperature, low pressure metamorphism. The rock produced is often a finegrained rock that shows no foliation, called a hornfels.
Que 04. What is Regional metamorphism ?
Ans. Regional metamorphism occurs over large areas and generally does not show
any relationship to igneous bodies. Most regional metamorphism is accompanied by
deformation under non-hydrostatic or differential stress conditions. Thus, regional
metamorphism usually results in forming metamorphic rocks that are strongly
foliated, such as slates, schists, and gniesses. The differential stress usually results
from tectonic forces that produce compressional stresses in the rocks, such as when
two continental masses collide. Thus, regionally metamorphosed rocks occur in the
cores of fold/thrust mountain belts or in eroded mountain ranges. Compressive
stresses result in folding of rock and thickening of the crust, which tends to push
rocks to deeper levels where they are subjected to higher temperatures and
pressures.
Que 05. Explain the role of predominance of heat during metamorphism
Ans. Heat Predominant- Heat is the dominating factor in the metamorphism which
ensures in the proximity in igneous masses. Nevertheless, the factor of pressure
comes in through the crowding aside of the country rock, its volume expansion due
to heating and the downwardly-acting weight of the superincumbent mass, although
pressure effects are always subordinate to those of heat. In general, the country rock
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is soaked with gaseous and liquid emanation form the magma, and these greatly
facilitate the mineral transformations which take place.The term thermal
metamorphism may be used for all kinds of change in which heat is the dominating
factor. Thermal metamorphism includes pyro-metamorphism and contact
metamorphism. The term pyro-metamorphism is used to denote high temperature
changes which take place along the immediate contacts of magma with country
rocks, with or without interchange of material.The general metamorphism which
occurs around large igneous masses (aureole) takes place at comparatively low
temperatures (as compared to pyro-metamorphism) and is called contact
metamorphism. In normal contact metamorphism there is little or no change in the
bulk composition of the rock. The magmatic emanations in this case merely increase
the molecular mobility of the interstitial solutions and thus facilitate the mineral
changes.Ex. Argillaceous rocks are changed to hornfels.
Que 06. Explain the role of predominance of pressure during metamorphism
Ans. Directed Pressure Predominant - Natural pressure within the crust is resolvable
into (1) hydrostatic pressure and (2) stress, the latter operating in a definite direction.
In co-operation with heat, stress is the principal factor in regional metamorphism.
With little or no heat, directed pressure produces crushing and granulation through
the enforced movement of rock masses upon one another. This action is restricted to
near surface regions since, with increasing depth, heat becomes more and more the
dominating factor and, accompanied by the general softening of the rocks and
increased activity of solution, leads finally to metamorphism by recrystallisation. The
action of direction pressure alone, leads to the mechanical breaking down of rocks
(cataclasis), with little new mineral formation except along planes of intense
shearing, with the frequent production of parallel banded structures. The
metamorphism which ensures through the dominant action of stress may be called
cataclastic metamorphism.
Que 07. What is metamorphic texture ? Give the main textural classification
of metamorphic rocks
Ans. The fabric of metamorphic rocks is a result of mechanical deformation and
mineralogical reconstitution of the pre-existing rocks. Texture is a term that
describes the size, shape and orientation of the grains constituting a rock, as well as
the relationship between these grains.
Metamorphic textures can be grouped into three main groups:
A - Relict textures (palimpsest textures): are textures inherited from the original rock
type, and which have survived metamorphism.
B - Typomorphic textures: textures characteristic of metamorphism
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C - Superimposed textures: textures characteristic of a post- metamorphic event, e.g.
alteration, weathering, ... etc.
Other smaller groups as “reaction textures”, “polydeformational textures”, etc. may
also be typomorphic or replacement, but are grouped separately because they have
some genetic connotation.
Que 08. Give the criterias for the classification of metamorphic rocks
Ans. Classification of metamorphic rocks depends on what is visible in the rock and
its degree of metamorphism. Note that classification is generally loose and practical
such that names can be adapted to describe the rock in the most satisfactory way
that conveys the important characteristics. Three kinds of criteria are normally
employed. These are:
1. Mineralogical - The most distinguishing minerals are used as a prefix to a textural
term. Thus, a schist containing biotite, garnet, quartz, and feldspar, would be called a
biotite-garnet schist. A gneiss containing hornblende, pyroxene, quartz, and feldspar
would be called a hornblende-pyroxene gneiss. A schist containing porphyroblasts of
K-feldspar would be called a K-spar porphyroblastic schist.
2. Chemical - If the general chemical composition can be determined from the
mineral assemblage, then a chemical name can be employed. For example a schist
with a lot of quartz and feldspar and some garnet and muscovite would be called a
garnet-muscovite quartzo-feldspathic schist. A schist consisting mostly of talc would
be called a talc-magnesian schist.
3. Protolithic - If a rock has undergone only slight metamorphism such that its
original texture can still be observed then the rock is given a name based on its
original name, with the prefix meta- applied. For example: metabasalt,
metagraywacke, meta-andesite, metagranite.
Que 09. What are mylonites ?
Ans. Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along
shear zones deep in the crust. They are usually fine-grained, sometimes glassy, that
are streaky or layered, with the layers and streaks having been drawn out by ductile
shear.
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