Rocks can be divided into three main classes based on their mode of formation: 1) Igneous or primary rocks such as basalt and granite form by the cooling and solidification of magma either underground or on the surface. 2) Sedimentary or secondary rocks like sandstone and limestone form by the compaction and cementation of sediment. 3) Metamorphic rocks such as gneiss, schist, and marble form by the alteration of pre-existing rocks due to heat, pressure, and chemical changes.

1. ROCKS
Presented by seema prajapat

2. GENERALITIES:
• Peterology : science of rocks which form the units of
the earth’s crust.
• Peterography : that deals with the description of rocks.
• Peterogenesis : that deals with the study of the
origin of rocks.
• Rock : defines as a hard mass of mineral
matter comprising of two or more rock
forming minerals.

3. FORMATION:
• Cooling and consolidation of magma:
primary rocks , e.g. Igneous rock.
• Transportation and cemrntation of fragmentary material:
, disintegration and decomposition lead to the
breaking down of pre exiting rocks by wind, water and ice to low lying
areas. E.g. Sedimentary or secondary rocks.
• Alteration or pre- existing rocks:
metamorphic rocks.

4. A SCHEMETIC DIAGRAM OF ROCK
CYCLE:

5. CLASSIFICATION
• According to the mode of formation, the rocks are divided
into the three main classes:
○ Igneous or primary rocks .
○ Sedimentary or secondary rocks.
○ Metamorphic rocks.

6. ROCKS IN THE EARTH’S CRUST
• The composition of the upper 5km of the earth crust :
● SEDIMENTRY ROCKS –
•shales 52%
•sandstone 15%
•limestone and dolomite 7%
●IGNEOUS ROCKS -
•granite 15%
•basalt 3%
●OTHERS - 8%
[100]
74%
18%

7. IN THE WHOLE EARTH CRUST
○IGNEOUS ROCKS - 95%
○SEDIMENTARY ROCKS - 5%
•SHALES 4.0%
•SANDSTONE 0.75%
•LIMESTONE 0.025%.

8. IGNEOUS ROCKS:
• The igneous rocks are formed by cooling and crystallization of
molten material viz,magma, on or beneath the surface of the
earth.
• Some important igneous rocks-
- basalt
- pumice
- granite.

9. FORMATION OF IGNEOUS ROCKS:
• During volcanic eruption ,that are brought about by increased
pressure increases in the earth due to certain geological
changes,the molten mass of fluid lava ,termed as magma,is forced
out to the surface of the earth,where it loses its volatile substances
and cools down suddenly and solidifies to form rocks of fine size
crystals.thus rocks formed a glassy structure.such rocks are called
extrusive or volcanic rocks
I.e. Basalt ,rhyolite,trachyte.

10. INSTRUCTIVE OR PLUTONIC ROCKS:
• When the pressure subsides within the earth’s surface,the magma
starts moving back and,on its way,cools down to form rocks.
the rocks formed by the solidiication of the magma
within the earth’s crust are called instructive or plutonic rocks.
I.e. Gabbro,granite.

11. CLASSIFICATION OF IGNEOUS
ROCKS:
• Based on mode of origin:
extrusive or volcanic rocks - Basalt
intrusive or plutonic rocks - granite.
• Based on chemical composition:
▪acid or over saturated rocks -
>60% silica(granite)
▪basic or under saturated rocks -
< 50% silica.

12. BASALT:

13. GRANITE :

14. SEDIMENTRY ROCKS:
• The sedimentary rocks are formed from sediments,derived from
the breaking down of pre-existing rocks.the sediments are
transported to new places and deposited in new arrangements
and cemented to from secondary rocks.
• Stratification is the most common feature of these rocks and such
as these are also termed as stratified rocks.
• For instance – conglomerate, sandstone, shale limestone etc.

15. FORMATION OF SEDIMENTARY
ROCKS:
• In the formation of sedimentary rocks, four stages are
recognised.these are –
▪weathering
▪ transportation
▪ deposition
▪ diagenesis.

16. CLASSIFICATION OF SEDIMENTARY
ROCKS:
• Inorganically – formed rocks –
are formed by evaporation or precipitation of
material dissolved in sea or lake water, gypsum, halite .
- formed by precipitation and flocculation, lImestone
,dolomite.
• Organically or biochemically formed rocks –
are formed by accumulation and partial
decomposition of organic remains under anaerobic conditions
peat,lignite, voluminous and semi voluminous coal .

17. SANSTONE:

18. LIMESTONE:

19. METAMORPHIC ROCKS:
• The world metamorphic means change in form and
metamorphism is used as a general term for all those changes
that alter more or less completely the original characters of the
rocks.
• Gneiss, Schist,marble ,slate etc.

20. FORMATION OF METAMORPHIC
ROCKS:
• Thermal metamorphism: the thermal (by heat) metamorphism is
also known as contact is additive metamorphism because there is
an addition of magmatic material to the metamorphosis rocks.
• Dynamo – thermal metamorphism: Here the combination of
pressure and heat .

21. GNEISS