Igneous Rock

2. Igneous Rock
Prepared BY : Sheraz Ali Jamali

3. Contents
 Igneous Rock & Its Formation
 Classification Of Igneous Rock
 Texture Of Igneous Rock
 Forms Of Igneous Rock
 Description/Examples Of Igneous Rock

4. Igneous Rock:
 The word igneous is derived from latin word ignis means
fire.
 Primary or first formed rocks are called igneous rocks.
 These rocks are formed by the cooling & solidification of
magma.

5. Formation Of Igneous Rock:
 It has been observed in deep wells , mines etc that the
temperature increases with the depth.
 Generally there is an increase of 1 °C for every 40 miles
depth.
 It may be assumed that this temperature gradient remains
the same , As it is near the earth’s surface , There will be
stage in the interior of earth, which will have a
temperature above the melting point of all the rocks.

7. Formation Of Igneous Rock:
 Such A Stage Is Believed To Be At A Depth Of About 40 Km
(25 Miles) Where The Temperature Is Believed To Be About
1000 °C .
 At This Depth , All The Rocks Are Believed To Exist In A
Molten State.
 This Molten Material Or Liquid Rock When Existing Below
The Earth’s Surface Is Called Magma But When Forced Out
On The Earth’s Surface Is Called Lava.

8. Diagram Of Magma & Lava:

9. Classification Of Igneous Rock
 Classification of igneous rock is based upon;
I. Mineral Composition.
II. SI Content ( As main constituent).
III. Mode Of Occurrence.

10. i) Classification based upon mineral
composition:
 The largest oxide component is SiO2 which in common
igneous rock ranges from 40% to 75% by weight.
 The % of Al2O3 generally ranges from about 10-20% & each
of the other major components ( e.g oxides of Ca , Mg &
Fe ) seldom exceed 10% .
 The major mineralogical components of igneous rock may
be grouped in to felsic & mafic varieties.
 It is classified as;
1. Acid Rocks
2. Basic Rocks

11. 1. Acid Rock:
 The composition of igneous rock depends upon
composition of magma from which they are
originated.
 It is subdivided in to;
a) Acid Magma: It contains high proportion of Si , Na
& K But low portion of Ca , Mg & Fe .
b) Basic Magma: It contains high proportion of Ca ,
Mg & Fe & low portion of Si , Na & K.

12. 2.Basic Rock:
Dark in colour.
High in specific gravity (3.2).
Contains higher proportion of
ferromagnesian minerals.

13. ii) Classification based on proportion of
silica as main constituent
 On the basis of silica , igneous rocks are
classified in to four types ;
I. Ultrabasic Rocks
II. Basic Rocks
III.Intermediate Rocks
IV.Acid Rocks

14. 1. Ultrabasic Rocks:
 Consists of very large amount of metallic
minerals like magnesium.
 Contains silica less than 45%.
 Low Viscosity.
 Also called ultramafic rocks.

15. Example Of Ultrabasic Rocks:
 Peridotite

16. 2. Basic Rocks:
 Contains silica between 45% & 55%
 Contains minerals as olivine , feldspar &
quartz among others
 Also Called Mafic Rocks

17. Examples Of Basic Rocks:

18. 3. Intermediate Rocks:
 Contain silica between 55% & 65% .
 Igneous rock whose chemical composition
lies between those of acidic & basic rocks.
 are more viscous than the mafic magmas.

19. Example Of Intermediate Rocks:
 Diorite

20. 4. Acidic Rocks:
 Contain more than 65% silica.
 Light in colour.
 Low in specific gravity.
 Also called felsic rocks.

21. Examples Of Acid Rocks:

22. iii) Classification based upon mode of
occurrence:
 Depending upon mode of occurrence ,
igneous rocks are classified as;
1. Extrusive Rocks
2. Intrusive Rocks

23. 1. Extrusive Rock:
 Those igneous rocks in which hot magma from inside
the earth flows out (extrudes) onto the surface as lava.
 Are also called volcanic rocks.
 The main effect of extrusion is that the magma can cool
much more quickly in the open air.
 Not enough time for growth of crystals

24. Examples Of Extrusive Rock:

25. 2. Intrusive Rock:
 Igneous rocks form when magma cools slowly
below the Earth's surface.
 Slow cooling gives crystals a chance to grow
 Forms large crystals.

26. Examples Of Intrusive Rocks:

27. Intrusive & Extrusive Rock Diagram:

28. 2) Intrusive Rocks:
 Depending upon the depth of
formation/solidication of magma , intrusive rocks
are divided in to two groups;
I. Plutonic Rocks
II. Hypabyssal Rocks

29. I) Plutonic Rocks:
 Igneous rocks formed when magma cools and
solidifies below the earth's surface.
 Cools slowly

30. Examples Of Plutonic Rocks:

31. II) Hypabyssal Rocks:
 These igneous rock are formed at
intermediate depth at about 2km.
 Exhibits mixed character of plutonic &
volcanic rocks.
 Also Called Subvolcanic rock.

32. Examples Of Hypabyssal Rocks:
Dolerite

33. Texture Of Igneous Rock;
 Texture means size , shape & arrangement of mineral
grains in a rock.
 To study texture , Following 4 parts are important;
I. Degree Of Crystallization
II. Size Of Grains
III. Shape Of Crystal

34. 1.Degree Of Crystallization;
 It is divided in to two parts;
1. Holocrystalline Texture : Rocks show entirely crystalline
texture.

35. 1.Degree Of Crystallization;
2) Holohyalline Texture; Rocks show entirely glassy texture.

36. 2.Size Of Grains
 The size of grains in an igneous rock varies considerably.
 The slow cooling gives crystals time to grow to sizes
greater than 5 mm.
 In rapid cooling , the mineral grains crystallize quickly as
mass of tiny crystals.

37. 3.Shape Of Crystals
 Well developed crystals faces of grains called
Euhedral.
 Partly developed crystals faces called Subhedral.
 The crystal faces are absent such grains of rock
are called Unhedral.

38. Description Or Examples Of Igneous
Rock:
 There are many examples of igneous rock , yet a
few of them which are important from the
subject point of view are discussed below ;

39. 1. Granite:
 It is the plutonic rock of light coour , with a white or pink
tint colour.
 It consists mainly of quartz , mica & feldspar.
 Average density of granite is between 2.65 & 2.75 g/cm3.
 It is the most important & strongest building stone.
 It is also extensively used as a road material.
 It is found in Canada , India , Brazil , China etc

40. Diagram Of Granite:

41. 2. Gabbro:
 It is plutonic rock of dark color from grey to black due to
high proportion of mafic minerals.
 Feldspar & mafic minerals are present in almost equal
amount.
 It occurs in the form of intrusive igneous bodies.
 It is used for all types of structural work
 Because of black color & irregular joints , it is not used
for most important buildings.

42. Diagram Of Gabbro:

43. 3. Basalt:
 It is volcanic igneous rock , having dark brown colour.
 Being a volcanic rock , it is fine crystallined.
 Essential minerals are plagioclase & augite.
 It is mainly used for the construction of bridges.
 It is not used for most important building work because of
dark colour.

44. Diagram Of Basalt:

45. Forms Of Igneous Rock:
 As we know that magma has tendency to penetrate in to cracks
& lines of the weakness of the thin solid crust of the earth &
tries to make its way out on the surface of earth.
 Sometimes this magma is heldup below the earth’s surface & in
consolidation there is different shapes known as forms of
igneous rock.
 It is divided in to 2 categories;
a) Concordant Bodies
b) Discordant Bodies

46. a)Concordant Bodies:
 Sometimes the magma during its upward journey does not possess enough
energy to push or drag through the existing rock through which it is rejected.
 In such cases, magma solidifies in the criticize & planes weakness of existing
rock.
 Such instructions which are influenced by the structural features of rocks are
called concordant bodies.
 Following are important forms of concordant bodies.
I. Sill
II. Phacolith
III. Laccolith

47. I) Sill:
 When the magma is pushed in to the bedding planes of the
existing rock and solidifies there in the form of a thin
sheet is called sill.
II. Phacolith: When magma is pushed in to the crests &
trough of a fold in rock and solidifies there in the form of
crescent structure in the available cavities is called
phacolith.
III. Laccolith: When magma is pushed in to the bedding
planes sometimes it forced upon the layers of the crest .
The magma then solidifies in cavity called laccolith

48. Discordant bodies
 Sometimes the magma, during its upward journey possesses a huge
energy to push, drag or cut through the existing rocks, through which
it is injected.
 In such cases the magma solidifies after disturbing the existing rocks.
Such intrusions which are not influenced by the structural features of
the rocks ( into which the magma has been injected ) and
consolidated after disturbing the existing rocks are called discordant
bodies.
 Following are the important forms of discordant bodies.
 1. Dyke
 2. volcanic neck
 3. batholith

49.  1. Dyke : When the magma is forced through the vertical
( or nearly vertical ) cracks of the existing rocks and
consolidated there as a wall like structure , it is called a
dyke.
 2. Volcanic neck: sometimes the magma is forced into the
vents (holes) of the dormant or extinct volcanoes and is
consolidated there in the form of more or less circular
thick pipe.
 3. Batholith : It is an extremely large sized igneous
intrusion.
 This happens when the magma , moving under a high
pressure forces up to the layers to form an arch like
structure .

50. Diagram Of Forms Of Igneous Rock:

51. Thanks