Formation of Metamorphic Rocks

1. METAMORPHICMETAMORPHIC
ROCKSROCKS
Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D.
Department of Geology
Acharya Nagarjuna University

2. What type of rock is formed
when change occurs from heat
and pressure in the Earth?
Characteristics
Hardened sediment with layers (strata) of sandy or
clayey stone; mostly brown to gray; may have
fossils and water or wind marks

3. Metamorphic Rocks
• Metamorphic rocks are formed by the effect of heat and
pressure on existing rocks.
• This can greatly affect the hardness, texture or layer patterns of
the rocks.
Magma
metamorphic
rock
forming
here
heat
Pressure from surface
rocks

5. Metamorphic Rocks

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

8. Uniform Stress
Differential Stress
min
inter
max

9. Granite
Granite-Gneiss

10. Type of Metamorphism
• 1. Cataclastic Metamorphism
This type of metamorphism occurs mainly
due to direct pressure
• Cataclastic metamorphism is mere
mechanical breakdown of rocks without any
new mineral formation, however, sometime
due to intense shearing few new minerals are
formed.

11. 2.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
 The rocks formed are non-foliated fine-grained
rocks called as HORNFELS.

12. 3.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.

13. Cataclastic Contact
Regional

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

15. 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). Foliation: planar fabric
element
Random
orientation
Of minerals
Preferred
orientation
Of minerals

16. • 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)
Lineation: any linear fabric elements

17. 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.

18. Shale Slate

19. 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.

20. 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.

21. 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

22. 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

23.  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.

24.  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

25. Index Minerals

26. Metamorphic rocks
1.GNEISS (Granite gneiss)
Grain size : coarse grained
Usual Colour : Grey
Structure : Gneissose
Texture : Banded texture.
Composition : K-feldspar, Quartz are formed as light
colour bands, maffic minerals formed as dark bands
and biotite,hornblende,pyroxene are occur in minor
amounts
Occurrence: Its occurs as a product of high grade
dynamothermal metamorphism and present of gneissos
structure represent high grade metamorphism

27. GNEISS

28. Metamorphic rocks
3. GNEISS
Grain size : coarse grained
Usual Colour : Grey
Structure : Gneissose
Texture : Banded texture.
Composition : K-feldspar, Quartz are formed as light
colour bands, maffic minerals formed as dark bands
and biotite,hornblende,pyroxene are occur in minor
amounts
Occurrence: Its occurs as a product of medium-
high grade metamorphism, gneiss are formed at low
grade metamorphism (originated from regional
metamorphic rocks)

29. Metamorphic rocks
2.SCHIST (Mica Schist)
Grain size : Fine grained
Usual Colour : Light white, Green
Structure : Schistose, flaky
Texture : fine grained with flakes
Composition : Muscovite
Occurrence: Its occurs as dynamothermal
metamorphism and present of muscovite indicate the
low grade metamorphism
Schistose structure indicate that formed under
dynamothermal metamorphism

30. schist

31. Metamorphic rocks
6.SLATE
Grain size : fine grained
Usual Colour : Bluish black (melano)
Structure : sheet like structure (laminated)
Texture : fine grained with bedding planes.
Composition : Mica(muscovite,biotite), chlorite,
Hornblend, qtz, feldspar
Occurrence: It’s a product of cataclastic metamorphism
by directed pressure and its originated from dynamic
metamorphism with the presence of hydroxyli minerals
indicate low grade metamorphism.

32. 1. Slate

33. Regional – composition change is gradational – texture is foliated
index minerals
chlorite
muscovite
biotite
garnet
sillimanite
Slate
Phyllite
Schist
Gneiss
Increasing
metamorphism
Increasing
m
etam
orphism

34. Metamorphic Rocks (7)
Contact Metamorphic
Marble
Quartzite
Regional Metamorphic
Slate (L)
Phyllite (R)
Schist
GneissCataclastic Metamorphic
Mylonite

35. 4.QUARTZITE
Grain size : coarse grained
Usual Colour : Brown
Structure : Granulose
Texture : Granoblastic.
Composition : SANDSTONE (composed of
quartz/feldspars/feldspathoid minerals) when under go
metamorphism result into Quartzite. Because
metamorphism of Sst. Result disappearance of
cementing material, bedding planes.
Occurrence: Its occurs as a product of thermal
metamorphism.
Non-foliated metamorphic rocks

36. Quartzite

37. Quartzite

38. Metamorphic rocks
5.MARBLE
Grain size : coarse grained
Usual Colour : white, pink (leucocratic)
Structure : Granulose
Texture : Granoblastic.
Composition : Calcite
Occurrence: Its occurs as a product of thermal
metamorphism (low grade-high grade) of calcareous
sediments.

39. MARBLE

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