Igneous rocks

1.  Textural
 Compositional
 Depth of emplacement of magma
 Modal Mineralogy
 Pyroclastic material

2. Phaneritic
 Crystals visible with naked eye
Plutonic or intrusive rocks

3. Aphanitic
 Crystal are too small for naked eye
Volcanic or extrusive rocks

4. Porphyritic
Two different, dominant grain sizes
Large crystals = phenocrysts;
Small crystals = groundmass

5. Pegmatitic
 Very large crystals (cm to 10s of cm); i.e.,
slowly cooled and forms at last stage of
crystallization.

6. Glassy
Non-crystalline; cools very fast (e.g., obsidian)
Volcanic rocks

7. Vesicular
Vesicles (holes, pores, cavities) form as
gases expands when lava cools at the
surface. (Basalts)
Volcanic rocks

8. Felsic: feldspar + silica
~55-70% silica, K-feldspar > 1/3 of feldspars present
light-colored silicate minerals — Continental crust
Intermediate: between felsic and mafic
~55-65% silica, plag > 2/3 of feldspars present
Na-rich plag predominates over Ca-rich plag
Mafic: magnesium + ferric iron
~45-50% silica; Ca-rich plag dominant feldspar
dark silicate minerals — Oceanic crust
Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars
Mantle-derived

9. Composition Phaneritic Aphanitic Color index
(% dark minerals)
Felsic Granite
Syenite
Monzonite
Rhyolite
Trachyte
Latite
10
15
20
Intermediate Granodiorite
Diorite
Dacite
Andesite
20
25
Mafic Gabbro Basalt 50
Ultramafic Peridotite Komatiite 95

10. Composition of Igneous Rocks

11.  Plutonic rocks
 Hypabyssal rocks
 Volcanic rocks

12. Plutonic Rocks
 Crystallizes at great depth beneath the Earth’s
surface.
 Due to slow cooling at such depth large crystals
are formed.
 e.g. Granite, Gabbro, Dunite etc.

13. Hypabyssal Rocks
 When magma crystallizes near the Earth’s
surface.
 Grains are finer than plutonic and coarser than
volcanic and the typical texture formed is
porphyritic.
 e.g. Dolerite

14. Volcanic Rocks
 When magma reaches the Earth’s surface it
solidifies and forms fine grained rocks.
 Vesicles are common in volcanic rocks as during
cooling of lava volatiles escape leaving cavities
in the rock.
 e.g. Basalt, Rhyolite, Trachyte etc.

15. Mode
 It is the mineralogical composition of the rock
calculated from the volume percentage of
minerals by point counting in the thin section.
Norm
 Calculated mineral composition based on
conversion of chemical analysis of the rock
into mineralogical formula.

16. Method for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An
Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

17. A classification of the phaneritic igneous rocks.
Phaneritic rocks with more than 10% (quartz + feldspar +
feldspathoids). After IUGS.
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Quartz-rich
Granitoid
90
90
60
60
20
20
Alkali Fs.
Quartz Syenite
Quartz
Syenite
Quartz
Monzonite
Quartz
Monzodiorite
Syenite Monzonite Monzodiorite
(Foid)-bearing
Syenite
5
10 35 65
(Foid)-bearing
Monzonite
(Foid)-bearing
Monzodiorite
90
Alkali Fs.
Syenite
(Foid)-bearing
Alkali Fs. Syenite
10
(Foid)
Monzosyenite
(Foid)
Monzodiorite
Qtz. Diorite/
Qtz. Gabbro
5
10
Diorite/Gabbro/
Anorthosite
(Foid)-bearing
Diorite/Gabbro
60
(Foid)olites
Quartzolite
Granite Grano-
diorite
Q
A P
F
60
Plutonic rocks

18. A classification of the phaneritic igneous rocks.
a. Gabbroic rocks. b. Ultramafic rocks. After
IUGS.
Plagioclase
Olivine
Pyroxene
G
a
b
b
r
o
T
r
o
c
t
o
l
i
t
e
Olivine
gabbro
Plagioclase-bearing ultramafic rocks
90
(b)
Anorthosite
Olivine
Clinopyroxene
Orthopyroxene
Lherzolite
Websterite
Orthopyroxenite
Clinopyroxenite
Olivine Websterite
Peridotites
Pyroxenites
90
40
10
10
Dunite
(c)
Gabbroic
rocks
Ultramafic
rocks

19. A classification and nomenclature of volcanic
rocks. After IUGS.
(foid)-bearing
Trachyte
(foid)-bearing
Latite
(foid)-bearing
Andesite/Basalt
(Foid)ites
10
60 60
35 65
10
20 20
60 60
F
A P
Q
Rhyolite Dacite
Trachyte Latite Andesite/Basalt
Phonolite Tephrite
Volcanic rocks

20. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975)
Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of
the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
Pyroclastic rocks