The document discusses the formation and classification of rocks. It states that there are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form from the cooling and solidification of magma either below or above the earth's surface. Sedimentary rocks form through the compaction and cementation of sediments. Metamorphic rocks form from alterations to existing igneous and sedimentary rocks caused by heat, pressure, and chemical reactions in the earth. The document provides details on the formation processes and examples of different rock types within each of these three main classifications.

1. SAC 101 FUNDAMENTALS OF
SOIL SCIENCE (2+1)
LECTURE 3
SOIL GENESIS: SOIL FORMING ROCKS-DEFINITION, FORMATION,
CLASSIFICATION OF
ROCKS- IGNEOUS, SEDIMENTARY AND METAMORPHIC ROCKS

2. ROCKS
 A rock may be defined as a hard mass of
mineral matter comprising two or more
rock forming minerals – Eg. Granite and
Gneiss
 Rocks are formed from the molten
material known as magma.
 Magma is a mixture of molten or semi-
molten rock, volatiles and solids that is
found beneath the surface of the Earth.

3. Composition of earth crust – Over all
ii.Sedimentary rocks - 5 %
-Shales 4%
-Sandstone 0.75%
-Limestone 0.025%
i.Igneous rocks - 95 %

4. Composition of upper 5 km of earth crust with rocks
i.Sedimentary rocks- 74%
-Shales 52%
-Sandstone 15%
-Lime stone- and dolomite 7%
ii.Igneous rocks - 18 %
-Granite 15%
-Basalt 3%
iii. Others - 8%
Composition of earth crust – Over all
ii.Sedimentary rocks - 5 %
-Shales 4%
-Sandstone 0.75%
-Limestone 0.025%
I. Igneous rocks - 95 %

5. Petrology
 The study of rocks is called Petrology (in
Greek, petra means rock, logos means science)
 It consists of
 Petrography deals with the description of rocks
- branch of science concerned with the composition
and properties of rocks
 Petrogenesis is the study of the origin /
formation of rocks

6. Formation of Rock- Various Processes
 Cooling and consolidation of Magma
 Within or on the surface of the earth
 Primary rocks or igneous rocks
 Disintegration & Decomposition and
Transportation and cementation of fragmentary
material
 Disintegrated either compacted insitu or transported
by wind, water or ice to low lying areas
 Sedimentary rocks or secondary rocks
 Alteration of pre-existing rocks
 Primary & secondary rocks subjected to earth
movement and to high temp and pressure or altered
to new forms
 Metamorphic rocks

7. Rock Cycle

8. Classification of rocks
 Primary rocks or Igneous rocks
 Sedimentary or Secondary rocks
 Metamorphic rocks

9. a) Igneous or Primary rocks - Cooling and
consolidation of molten material -
magma within or on the surface of earth
b) Sedimentary or Secondary rocks-
Transportation and cementation of
primary rocks
a) Metamorphic rocks-Alteration of the
existing primary and secondary rocks

10. a) Primary / Igneous rocks
A.Extrusive or
volcanic rocks
 consolidation of
magma on the
surface of the
earth
 E.g. Basalt
B.Intrusive or plutonic
rocks
Solidification of magma
below the surface of the
earth.
Plutonic –greater depth
Hypabassal -shallow depth
Eg. Granite, syenite, diorite,
Gabbro etc.
Rocks formed in vertical cracks are called dykes
Rocks formed in horizontal cracks are called sills.
C.Vesicular rocks:
Magma cools on
the surface.
Steam of water is
entrapped into
rocks and forms
vesicles
Eg.Pumice
i.Based on mode of formation

11. Extrusive or volcanic rocks
Intrusive or plutonic rocks

13. 2. Based on the Chemical
Composition / Silica content
1. Acid rocks : >66% SiO2 (Granite, Rhyolite)
2. Intermediate : 56 to 66% SiO2
(Sub acid rocks 60 to 66% SiO2 (Syenite and
Trachyte)
(Sub basic rocks 56 to 60 % SiO2 (Diorite and
Andesite)
3. Basic rocks: 40 to 55% (Gabbro, Basalt)

14. High Silica content forms
light-colored rocks –
Eg.Granite
– Light coloured sandy
soils with low
Sp.gravity
Entisols, Inceptisols
Lower (but only by about
25%) Silica content forms
darker-colored rocks –
Eg.Basalt
- dark coloured heavy
soils with high Sp.
Gravity – Black cotton
soils- Vertisols

15.  These Igneous rocks are found in bulks;
do not have particles and layers;
 compact and massive;
 hard and resistant and have well-
developed joints
 Non laminar massive structure

16. S.No Rocks Origin Essential
minerals
Common
minerals
Average
specific
gravity
Remarks
i. Granite Plutonic
holocrystalline
Quartz (20
to 30%)
Hornblende,
Magnetite, Mica
2.64 Light coloured
white or reddish
ii. Syenite Plutonic
Holocrystalline
Quartz,
Orthoclase
Hornblende,
Magnetite,
Biotite
2.80 Light coloured
white or reddish
iii. Diorite Plutonic
Holocrystalline
Quartz Hornblende,
Magnetite,
Biotite
2.85 Darker
iv Gabbro Plutonic
Holocrystalline
Labradorite,
Augite,
Olivine
Hornblende,
Ilmenite
3.0 Blackish
v. Dolerite Hypabasal Labradorite,
Augite,
Olivine
Hornblende,
Ilmenite
3.0 Blackish
vi. Basalt Volcanic
crystalline with
glassy mass
Labradorite,
Augite,
Olivine
Hornblende,
Ilmenite
3.0
Igneous rocks

17. II.Sedimentary Rock
A sedimentary rock is
a rock that is made of
layers of sediments
(sand, clay, mud)
close to the earth’s
surface.
Sandstone

18. Sedimentary rocks
 Formed from the consolidation of sediments ,
derived from the breaking down of pre-existing rocks
– transported through wind or water action at the
surface of the earth.
 Deposited in layers or formed precipitates from
aqueous solutions.
 Sediments with particles cemented together by
substances like SiO2, Fe2O3 or lime CaCO3.
 Also called as clastic rocks.
 Stratification is the most common feature of these rocks
and so these are also termed as stratified rocks.

19. A.Based on Mode origin
1.Residual 2.Transported
Eg. Laterite a. Deposited as solids in suspension
Eg Sandstone, Shale
b. Deposited by chemical precipitation
Eg Limestone, Ironstone
c.Deposited through agency of organic matter
Eg.Peat, Phosphatic deposits

20. B.Based on mode of formation
1.Fragmental, detrital or mechanically formed
These are formed by the deposition and cementation of erosion products
of pre-existing rocks – Shale, Conglomerate, Sandstone
2.Chemically formed
i)Inorganically formed
Formed by the evaporation or precipitation of material dissolved in sea or
lake water- Halite, Gypsum
Ca (HCO3)2 ---- CaCO3+ CO2 + H2O Limestone , Dolomite
ii).Organically or biochemically formed
Formed by the accumulation and partial decomposition of organic remains
under anaerobic conditions e.g. peat, bituminous and semi bituminous
coal.

21. C.Based on the grain size
1. Rocks with boulder, pebbles sized minerals (Rudaceous): Conglomerate
2. Rocks with sand size particles (Arenaceous) : Sandstone
3. Rocks with silt size particles (Silt rocks) :Siltstone
4. Rocks with clay size particles (Argillaceous) : Shale

22. Formation
 Weathering
 Transportation
 Deposition or Sedimentation
 Diagenesis

23. Formation
1. Weathering
The igneous and other rocks disintegrate owing to
physical, chemical and biological weathering.
2KAlSi3O8 + H2CO3+ H2O Al2Si2O5 (OH)4+K2CO3+ 4SiO2
Orthoclase Feldspar + Carbonic acid + Water
Kaolinite + Potassium carbonate + Silica
The products of these weathering provide the basic
materials (gravels, sand, silt, mud) for the formation of
sedimentary rocks.

24. 2. Transportation
The disintegrated material is transported by the
agencies such as water, wind, glaciers, runoff, and
gravity.
Dust of finer fractions carried by Wind-Saltation
Deposition of colluvium (material which accumulates at the
foot of a steep slope) at foot hillls- gravity - Solifluxion
or Soil creep

25. 3. Deposition or Sedimentation
The detrital materials, comprising
minerals and rock fragments are
deposited when the carrying agent has
no longer energy enough to move
further.
Then
Coarser particles settle first
Finer particles settle later
Deposition is called Graded Bedding

26. 4.Diagenesis
Transformation of unconsolidated
sediments into hard rock
Compaction: compression of layers-
water squeezed out. Clays, Shales
Cementation: lime, silica, iron oxide
Water that percolates carries the
binding materials get deposited in
voids of loose sediments and that on
desiccation, binds to form rocks

27. Conglomerate
–from cemented gravel
Sandstone
from sand
Limestone
Shale –from clay

28. c) Metamorphic rocks
 The primary and the secondary rocks when
subjected to earth’s movement and to high
temperature and pressure are altered to new
rocks called Metamorphic rocks
 These are formed from igneous and
sedimentary rocks under the influence of heat,
pressure, chemically active liquids and gases.
 Change may occur in mineral composition or
texture or both.
 The changes due to water is called hydro
metamorphosim and due to pressure is called
dynamo metamorphosim

29. Metamorphic rocks
Formation
The structure and mineral composition of metamorphic
rocks depend on the composition of the original rock and
the kind of metamorphism
The banded or the laminated character is the most
peculiar feature of metamorphic rocks
Hydro metamorphism
Dynamo metamorphism
In thermal metamorphism, heat is the dominant factor
which bring changes- also known as contact or additive
metamorphism because there is an addition of magmatic
material to the metamorphosed rock.
In the case of dynamo-thermal metamorphism, the
combination of pressure and heat forms one of the most
powerful metamorphic forces leading to more or less
complete recrystallization of minerals with new structures

30. Based on the texture and structure,
MR divided into three groups
1.Foliated – Gneiss, Schist, Slate
2.Non foliated - Anthracite
3.Granular – Marble, Quartzite
Foliation in geology refers to repetitive layering
in metamorphic rocks Each layer can be as thin
as a sheet of paper, or over a meter in
thickness. The word comes from the Latin folium,
meaning "leaf", and refers to the sheet-like planar
structure

31. Gneiss - a foliated metamorphic rock

32. Metamorphic Rocks
Sand stone (SR) : Quartzite
Shale (SR) : Slate, Schist
Lime stone / Dolomite : Marble
(SR)
Granite (IR) : Granite Gneiss
Dolerite (IR) : Hornblende Gneiss

33. Quartz (Mineral) and Quartzite

34. Quartz with Sandstone
Marble

35. how heat and pressure changes
these rocks …
Sedimentary Metamorphic
Shale Slate

36. Check out how heat and
pressure changes these rocks …
Igneous Metamorphic
Granite Gneiss

37. Gneiss Bands