2. Igneous Rocks
Igneous Rocks
 “Ignis” means fire
 Rocks that are formed from
cystallization of magma
 Magma is molten rock
 Lava
is magma that is on the Earth’s
Surface

3. Igneous Rocks
 Two
Types of Igneous Rocks
 Extrusive
(Exterior)
 Intrusive (Interior)
Igneous
rocks that solidify into rock
beneath Earth’s surface
Coarse Grained
Cool Slowly
Granite

4. Extrusive Igneous Rocks
Extrusive (Exterior)
 Igneous rocks
that solidify into
rock on Earth’s
surface
 Fine Grained
 Cool Quickly

5. Intrusive Igneous Rocks
Intrusive (Interior)
 Igneous rocks
that solidify into
rock beneath
Earth’s surface
 Coarse Grained
 Cool Slowly
 Granite

6. Igneous Rock Formation

7. Igneous rocks: Igneous textures
Texture
a. size
b. shape
c. arrangement
of interlocking crystals

8. cooling rate
crystal size
Slow cooling
larger crystals
Fast cooling
small or no crystals

9. Fine-grained
Large crystals in matrix
Coarse-grained
Glassy

10. fast cooling magma/lava
forms at or near surface
sometimes holes present
can’t see individual crystals
Fine-grained
forms far
below surface
slow cooling
intergrown
crystals
Coarse-grained
magma cooled slowly for a
while then erupted
minerals crystallize at
different temperatures and
or rates Large crystals in matrix
Glassy
very rapid
cooling
ions unable to
unite in orderly
crystalline
structure

11. Magmas with high silica content tend to form
long chainlike structures
- impedes ion transport
- increases resistance to flow
viscosity

12. In violent volcanic eruptions,
rock fragments
ash
consolidate
molten blobs
large angular blocks
Pyroclastic rock

13. Igneous textures
 Texture
is used to describe the overall
appearance of a rock based on the size,
shape, and arrangement of interlocking
minerals
 Factors affecting crystal size
Rate
of cooling
Slow
rate promotes the growth of fewer but
larger crystals

14. Igneous textures
 Factors
affecting crystal size
Rate
of cooling
Fast
rate forms many small crystals
Very fast rate forms glass
Amount
of silica (SiO2) present
Amount
of dissolved gases

15. Igneous textures
 Types
of igneous textures
Aphanitic
(fine-grained) texture
Rapid
rate of cooling of lava or magma
Microscopic crystals
May contain vesicles (holes from gas bubbles)
Phaneritic
Slow
(coarse-grained) texture
cooling
Crystals can be identified without a microscope

16. Aphanitic texture

17. Phaneritic texture

18. Igneous textures
 Types
of igneous textures
Porphyritic
texture
Minerals
form at different temperatures as well
as differing rates
Large crystals, called phenocrysts, are
embedded in a matrix of smaller crystals, called
the groundmass
Glassy
Very
texture
rapid cooling of molten rock
Resulting rock is called obsidian

19. An obsidian flow in Oregon

20. Igneous textures
 Types
of igneous textures
Pyroclastic
texture
Various
fragments ejected during a violent
volcanic eruption
Textures often appear to more similar to
sedimentary rocks
Pegmatitic
texture
Exceptionally
coarse grained
Form in late stages of crystallization of granitic
magmas

21. Porphyritic texture

22. Glassy texture

23. Igneous Compositions
 Igneous
rocks are composed primarily of
silicate minerals
Dark
(or ferromagnesian) silicates
Olivine
Pyroxene
Amphibole
Biotite
mica

24. Igneous Compositions
 Igneous
rocks are composed primarily of
silicate minerals
Light
(or nonferromagnesian) silicates
Quartz
Muscovite
Feldspars
mica

25. Igneous compositions
 Granitic
versus basaltic compositions
Granitic
composition
Composed
of light-colored silicates
Designated as being felsic (feldspar and
silica) in composition
Contains high amounts of silica (SiO2)
Major
constituents of continental crust

26. Igneous compositions
 Granitic
versus basaltic compositions
Basaltic
composition
Composed
of dark silicates and calcium-rich
feldspar
Designated as being mafic (magnesium and
ferrum, for iron) in composition
More dense than granitic rocks
Comprise the ocean floor as well as many
volcanic islands

27. Igneous compositions
 Other
compositional groups
Intermediate
(or andesitic) composition
Contain
at least 25 percent dark silicate
minerals
Associated with explosive volcanic activity
Ultramafic
Rare
composition
composition that is high in magnesium
and iron
Composed entirely of ferromagnesian
silicates

28. Classifying Igneous Rocks
 Igneous
 Crystal
rock textures:
size is dictated by the rate of cooling
of the magma body. A slower cooling rate
results in larger mineral crystals being
formed in the rocks as they cool.
 Porphyritic texture - Large crystals
surrounded by fine grained rock. The rock
initially cools slowly to form some large
crystals and then cools quickly to form the
fine grained rock surrounding them.

29. Mineralogy of common
igneous rocks

30. Igneous compositions
 Silica
content as an indicator of
composition
Silica
content in crustal rocks exhibits a
considerable range
A
low of 45 percent in ultramafic rocks
Over 70 percent in felsic rocks

31. Igneous compositions
 Silica
content influences a magma’s
behavior
Granitic
High
magma
silica content
Extremely viscous
Liquid exists at temperatures as low as 700oC

32. Igneous compositions
 Silica
content influences a magma’s
behavior
Basaltic
Much
magma
lower silica content
Fluid-like behavior
Crystallizes at higher temperatures

33. Igneous compositions
 Naming
igneous rocks – granitic (felsic) rocks
Granite
 Phaneritic
 Over
25 percent quartz, about 65 percent or more
feldspar
 May exhibit a porphyritic texture
 Very abundant as it is often associated with mountain
building
 The term granite covers a wide range of mineral
compositions

34. Granite

35. Igneous compositions
 Naming
igneous rocks – granitic (felsic)
rocks
Rhyolite
Extrusive
equivalent of granite
May contain glass fragments and vesicles
Aphanitic texture
Less common and less voluminous than
granite

36. Rhyolite

37. Igneous compositions
 Naming
rocks
igneous rocks – granitic (felsic)
Obsidian
Dark
colored
Glassy texture
Pumice
Volcanic
Glassy
texture
Frothy appearance with numerous voids

38. Obsidian

39. Pumice

40. Igneous compositions
 Naming
igneous rocks – intermediate
rocks
Andesite
Volcanic
origin
Aphanitic texture
Often resembles rhyolite

41. Andesite porphyry

42. Igneous compositions
 Naming
igneous rocks – intermediate
rocks
Diorite
Plutonic
equivalent of andesite
Coarse grained
Intrusive
Composed mainly of intermediate feldspar
and amphibole

43. Diorite

44. Igneous compositions
 Naming
igneous rocks – basaltic (mafic)
rocks
Basalt
Volcanic
origin
Aphanitic texture
Composed mainly of pyroxene and calciumrich plagioclase feldspar
Most common extrusive igneous rock

45. Basalt

46. Igneous compositions
 Naming
igneous rocks – basaltic (mafic)
rocks
Gabbro
Intrusive
equivalent of basalt
Phaneritic texture consisting of pyroxene and
calcium-rich plagioclase
Makes up a significant percentage of the
oceanic crust

47. Gabbro

48. Igneous compositions
 Naming
igneous rocks – pyroclastic rocks
Composed
of fragments ejected during a
volcanic eruption
Varieties
Tuff
– ash-sized fragments
Volcanic breccia – particles larger than ash

50. The Scheme for
Igneous Rock
Identification

51. Mineralogy of common
igneous rocks

52. A Brief Tour

53. Crystal size
Grain Size
Description

54. Where it was formed
Outside the volcano: Extrusive
Inside the Earth: Intrusive

55. Bubbles
?
Yes= Vesicular

56. Bubbles
?
No=Non-vesicular

57. Dark
Very Light
NotVery Light nor Dark Very
Color LightNeitherLightNot Very Dark Dark
Darkest
Lightest

58. Densit Very Light Medium Density
Light
DenseVery Dense
y

59. Finding T he
Miner Unless you have other information,
Identify the rock als
work in the middle of the rock’s box.

60. This is the amount of Potassium Feldspar in the rock.

61. This is the amount of Quartz in the rock.

62. This is the amount of Plagioclase Feldspar in the rock.

63. Use tick marks on a scrap paper to measure the percentage.
Potassium Feldspar 25%
Quartz 40%

64. Pr actic
e
Rhyolite
Name a light-colored, fine-grained
rock with no bubbles.

65. Name a coarse-grained, dense rock.
Gabbro

66. suwun

67. Igneous rocks have
“Intergrown Crystals”

68. Intergrown

69. Intergrown

70. Intergrown

71. Not Intergrown