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1. UPSC MAIN FOCUS
HELPING YOU IN UPSC/PSC
NET/JRF

2. CLASSIFICATION OF
ROCKS
1. The hard, solid material that forms part of the surface of
the earth.
2. A rock is any naturally occurring solid mass or
aggregate of minerals.
3. The earth’s crust is made up of various types of rocks.
4. They differ from one another in texture, structure,
colour, permeability, mode of occurrence and degree of
resistance to denudation.
5. Rocks are classified into 3 major groups:
Igneous, sedimentary and metamorphic acc to their
origin and appearance.

4. IGNEOUS ROCKS
1. The igneous rocks (Ignis – in Latin means
‘Fire’) are formed when magma (at mantle)
cools and solidifies.
2. As igneous rocks form out of magma and lava
from the interior of the earth, they are known as
primary rocks.
3. When magma in its upward movement cools
and turns into solid form it is called igneous
rock.
4. The process of cooling and solidification can
happen in the earth’s crust or on the surface of
earth.

5. TYPES OF IGNEOUS
ROCK
Basis of mineral composition:
1. Acid IR–- contain a high proportion of silica i.e. Granite.
Less dense and lighter in colour than basic rocks.
2. Basic IR–- contain a high proportion of basic oxide i.e. Iron,
aluminium or magnesium. Denser and darker in colour.
Basis of origin:
1. Plutonic IR– also intrusive IR— formed at some depth in
earth’s crust. E.g. granite, diorite and gabbro. Exposed at
the surface by the processes of denudation and erosion.
Large crystal.
2. Volcanic IR–- molten rocks poured out of volcanoes as
lavas. Solidify rapidly on earth surface and small crystals.
Also extrusive rocks and forms lava sheets and lava
plateaus E.g. Deccan plateau in India and Columbia

7. SEDIMENTARY ROCKS
1. Word ‘sedimentary’ – Latin word ‘sedimentum’–
means – ‘settling’
2. SR formed from sediment accumulated over long time
periods, usually under water– through compaction
turns into rocks– ‘lithification’ .
3. Layer formation and are termed stratified rocks.
4. Sediments – brought by streams, glaciers, winds or
even animals.
5. Strata (layers) present and also fossils – present. E.g.
Sandstone, shale etc.

8. TYPES OF SEDIMENTARY
ROCKS
 SRs may be classified under three major categories
depending upon mode of formation:
1. Mechanically formed SRs—
 Formed by mechanical agents like running water, wind,
ocean currents, ice, etc.
 Sediments– accumulation– cemented – rocks. E.g.
sandstone. Other e.g. shale or mudstone. Sand and
gravel may occur in uncemented form.
2. Organically formed SRs—
 Formed from the remains of living organisms– such as
coral or shellfish, whose fleshy parts have been
decomposed, leaving behind the hard shells– most
common rocks formed in this way– calcareous type.
E.g. Limestone and chalk.

9. CONTINUED…
Carbonaceous rocks – also organically formed but from
vegetative matter – swamps and forests – under high P &
T– covert into compact masses of carbon. E.g. Peat,
lignite or coal.
3. Chemically formed SRs—
∙ Such rocks are precipitated chemically from solutions of
one kind or another. E.g. Rock salts– derived from
strata which once formed the beds of seas or lakes.
. Gypsum and calcium sulphate is obtained from the
evaporation of salt lakes, such as Dead sea, have a very
high salinity.

11. METAMORPHIC ROCKS
 Metamorphic– means ‘change of form’.
 Formed under the action of PVT changes.
 Metamorphism occurs -- when rocks are formed down
to lower levels by tectonic processes or when molten
magma rising through the crust comes in contact with
the crustal rocks or the underlying rocks are subjected
to great amounts of pressure by overlying rocks.
 Metamorphism is a process by which already
consolidated rocks undergo recrystallisation and
reorganisation of materials within original rocks.
 Dynamic Metamorphism– without any appreciable
chemical changes – mechanical disruption and

12. CONTINUED...
A. Thermal Metamorphism– materials of rocks chemically
changed and recrystallise due to thermal metamorphism–
contact M and regional M.
B. Contact M – rocks come in contact with hot intruding magma
and lava and rock materials recrystallise under high T. New
materials form out of magma or lava are added to the rocks.
C. Regional M– rocks undergo recrystallisation due to
deformation caused by tectonic shearing together with high T
or P or both.

13. ● Foliation or lineation-- an arrangement of
minerals or grains in layers or lines in the process
of metamorphism.
● Banding -- sometimes minerals or materials of
different groups are arranged into alternating thin to
thick layers appearing in light and dark shades.
Such structure – banding and such rocks – banded
rocks.

14. TYPES OF METAMORPHIC
ROCKS
 Depending upon original rocks classified into two major
groups...
 Some example of metamorphism:
 Clay --- slate
 Limestone --- marble
 Sandstone --- quartzite
Foliated
e.g. Schist,
Slate,
Gneiss
Non-
foliated
e.g.
Quartzite,
Marble

16. . Discovered by– James Hutton
. Igneous rocks are primary rocks and other rocks (
sedimentary and metamorphic ) form from these primary
rocks..

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