23.3.b metamarphic rock.ppt

Metamorphic rocks
 When rocks are baked by heat of molten magma or
squeezed by the movements of huge tectonic plates or
by the pressure of overlying thick succession of rocks
 They are altered or changed beyond their recognition
 i.e. change in Chemical composition, texture and
structure
Metamorphic rocks

Metamorphism
Is the process that occur in rocks
due to the effects of
 High temperature
 High pressure
 Chemically active fluids

 The source of temperature is either from magma or due to
the depth factor
 Metamorphism usually result into change in min. comp. and
texture of rocks (Ig. and Sed.) which are subjected to temp.
> 1000 C and pressure > 1000’s Mpa.
 Low-grade metamorphism:
Occurs at about 1000 C to 5000 C.
 High-grade metamorphism:
Occurs at > 5000 C
Temperature

UNIFORM PRESSURE
Pressure
- increases with depth due to
increase in overburden.
- acts vertically downwards and
affects the volume of both liquid
& solids.
- increases with depth upto some
extent, effective in the upper part
of the crust.
DIRECT or Differential PRESSURE
- acts in all direction and affects only
on solids resulting into deformation
of shape and change in mineral
composition
- high temperature is also
associated with (due to depth
factor)
- high temperature is not always
associated. to depth factor)
- Lithostatic pressure- due to
overburden
- Stress- due to tectonic forces

Uniform Stress
Differential Stress
min
inter
max

STRUCTURES IN METAMORPHIC ROCKS
 Foliation: when platy, lamellar or flaky minerals (eg.
sheet silicate minerals the micas: biotite and muscovite,
chlorite, talc, and serpentine), occurring in rock orient
themselves parallel to one another (i.e. perpendicular to the
direction of maximum pressure or stress).
Random
orientation
Of minerals
Preferred
orientation
Of minerals

 Lineation: when prismatic or rod-like
minerals (eg. Hornblende, tourmaline etc.)
occurring in a rock orient themselves parallel to
one another (perpendicular to direction of maxi.
Pressure or stress)

SLATY CLEAVAGE
- usually formed during the early stage of
Low-grade Metamorphism due to lithostatic
stress.
- New sheet-structure minerals tends to be
parallel to the bedding planes during
metamorphism.
- however, further deep burial along
the continental margin;
compressional forces will cause
deformation (folding).
- hence, the sheet minerals as well as
foliation will no longer be parallel to
the bedding planes, such type of
foliation in fine grained rocks is
called slaty cleavage.

PHYLLITES
- usually associated with intermediate grade of metamorphism; where the
mineral grains grows large in size as compare to that seen in slates
-This develops a pronounced foliation where the preferred oriented
minerals are seen.

SCHISTOSE STRUCTURE
- usually formed during intermediate and high grade metamorphism
- Grain size increases and can be seen by naked eye; grains tends to enlarge
with increasing grade of metamorphism; the coarse grained sheet-structure
minerals show preferred orientation
- grain size is the main difference between the slaty structure and schistos
structure.

GNEISSIC STRUCTURE
- usually associated with high-grade regional metamorphism (where
differential stress prevails I.e. tectonic forces)
- where the sheet silicates and other minerals like
quartz/feldspars/hornblende/pyroxene are segregated in distinct bands in
the rocks- known as gneissic banding.

Classification of Metamorphic rocks based on
texture/structures
PHYLLITE
-similar to slate, but slightly coarser phyllosilicate grains
-grains can be seen in hand specimen, giving silk appearance to cleavage
surfaces
-often cleavage planes less perfectly planar than slates
SLATE
-strongly cleaved rock
-cleavage planes are developed due to orientation of fine phyllosilcate grains
eg. Muscovite, biotite, chlorite etc.
-individual grains too fine to be visible with naked eye
-overall dull appearance

SCHIST
-parallel alignment of moderately coarse grains (fabric=schistocity)
-grains are visible by eye
-mainly phyllosilicates and other minerals such as hornblende, kyanite etc.
GNEISS
-coarse grained rock (grain size several millimetres) and
-foliated (planar fabric: either schistosity or compositional layering)
-tendency for different minerals to segregate into layers parallel to foliation
(gneissic layering): typically quartz and feldspar rich layers tend to separate
from micaceous layers.
Varieties:
--Orthogneiss: rocks formed from Igneous rocks
-- paragneiss: rocks formed from Sedimentary rocks -metasedimentary
gneisses

QUARTIZITE
-it comprise equidimensional minerals viz. quartz and feldspars
Non foliated; show GRANULOSE STRUCTURE

Type of Metamorphism
 Cataclastic Metamorphism
 This type of metamorphism occurs mainly due to direct
pressure
 eg. when two bodies of rock slide past one another along a fault
zone. Heat is generated by the friction of sliding along the
zone, and the rocks tend to crushed and pulverized due to the
sliding.
 Cataclastic metamorphism is mere mechanical breakdown of
rocks without any new mineral formation, however, sometime
due to intense shearing few new minerals are formed.

 Contact Metamorphism-
 This type of metamorphism occurs locally adjacent to the igneous intrusion;
with high temp. and low stress
 There is little change in bulk composition of the rock
 Area surrounding the intrusion (Batholith) is heated by the magma;
metamorphism is restricted to a zone surrounding the intrusion, this zone is
know as METAMORPHIC AUREOLE.
 The rocks formed are non-foliated fine-grained rocks called as
HORNFELS.

 Regional Metamorphism-
 metamorphism occurs covering larger area, which is subjected to intense
deformation under direct or differential stress.
 Rocks formed under such environment are usually strongly foliated, such
as slates, schists, and gniesses.
 The differential stresses result from tectonic forces,
 eg. when two continental masses collide with one another resulting into
mountain building activity. Compressive stresses result in folding of the
rock

Types of Metamorphic Rocks
FOLIATED
The common foliated rocks in the order of increasing grain size are
SLATE – PHYLLITE – SCHIST – GNEISS
NON-FOLIATED
Quartzites and hornfels

 Importance of Metamorphic rocks-
 SLATES
 Fine grained impermeable, cleavable and soft
 Incompetent; cannot withstand great loads
 But since they are impermeable and split easily; thin large sized slabs of
uniform thickness can be extracted for roofing purpose.
 Economic importance: Since they are bad conductor of electricity– used in
electrical industries for switch board base
 GNEISS
 Gneissic rocks are rich in SILICA i.e. predominantly Quartz and Feldspars
along with garnet, pyroxene, Hornblende etc.
 Non-porous and impermeable nature increases the strength of the rock
 Foliated character to some extend improves workability
 Load perpendicular to foliated planes gives more stronger foundation

If mineral assemblage is more or less similar to
Granite (with less % mafic minerals) then:
It is used as building stone
As aggregate for making concrete
As road metals etc.
SCHIST
Mainly composed of prismatic or platy minerals, which contributes in
development of Schistose Structure. Eg. Hornblende, tourmaline, sillimanite etc
(prismatic); chlorite, muscovite, biotite, talc, kyanite etc. (platy)
Cleavable nature of Schists is the main reason for their weakness; they are
incompetent

 QUARTZITE
 SANDSTONE (composed of quartz/feldspars/feldspathoid minerals) when
under go metamorphism result into Quartzite.
 Granulose texture/structure (Granoblastic) makes them most competent rock
amongst all other metamorphic rocks.
 Because metamorphism of Sst. Result disappearance of cementing material,
bedding planes, fossil content etc.
 Quartzites are compact, hard and strong; very less porous and less permeable
than the parent Sst.
 Predominance of Quartz makes the rock very hard and suitable for road
metal; can be used as concrete aggregate etc.
 Acts as strong foundation for any CE structure.

 MARBLE
 Latin word “Marmor”– Shining stone.
 Calcareous metamorphic rock
 Though it shows granulose structure it is not as hard as Quartzite because of
its Calcareous composition; but can withstand reasonable load.
 Due to its pleasant colour and brilliant appearance when polished it is
extensively used as building stone.
Calcite

23.3.b metamarphic rock.ppt

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23.3.b metamarphic rock.ppt