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Igneous Rocks
With Some Graphics from Press et al.,
Understanding Earth, 4th Ed.
(Copyright © 2004 by W. H. Freeman & Company)
By:
Abboud Suliman Ahmed
Igneous Rocks
Igneous rocks form from molten rock (magma)
crystallizing below earth's surface or from
volcanic activity. They commonly form at plate
boundaries and are commonly exposed in
mountainous
areas.
Igneous
rocks form from molten rock
(magma) crystallizing below earth's
surface or from volcanic activity. They
Igneous rocks form
fromform
crystallization
of magma
commonly
at plate boundaries
and at depth
(within the earth's
crust) orexposed
at the surface
(from volcanic
are commonly
in mountainous
eruptions)
areas.
There are two (2) basic types or forms of igneous rocks:
1. Plutonic rocks = intrusive igneous rocks = igneous rocks
that form from cooling magma at depth
2. Extrusive igneous rocks = igneous rocks that form from
volcanic activity (at or near surface)
Key Terminology
Plutonic
Intrusive
Extrusive
Volcanic
Texture
Phaneritic
Aphanitic
Porphyritic
Glassy
Vesicular
Pyroclastic
Magma
Lava
Bowen’s Reaction Series
Assimilation
Partial melting
Fractional crystallization Discordant
Concordant
Dike
Stock
Batholith
Sill
Laccolith
Lopolith
Table. 5.2
Felsic
Granite
Rhyolite
Intermediate
Granodiorite
Dacite
Mafic
Diorite
Gabbro
Andesite
Basalt
Viscosity
Melting Temperature
Bowen's Reaction SeriesPlagioclase
(Ca-feldspar)
Olivine
Pyroxene
Amphibole
Biotite
Plagioclase
(Na-feldspar)
Orthoclase
(K-feldspar)
Muscovite
Quartz
Common Minerals
General characteristics
of magma
• Igneous rocks form as molten rock cools and
solidifies
• General characteristics of magma:
• Parent material of igneous rocks
• Forms from partial melting of rocks
• Magma at surface is called lava
General characteristics
of magma
• General characteristic of magma
• Rocks formed from lava = extrusive, or
volcanic rocks
• Rocks formed from magma at depth =
intrusive, or plutonic rocks
The Rock CycleMinerals form rocks
All rocks can be transformed into other rock
types
Rocks are divided into 3 categories
Igneous- crystalline- forms as liquid cools
Metamorphic- crystalline-forms as rocks are
heated and squeezed
Sedimentary- non-crystalline- smaller pieces
or chemicals from other rocks
Igneous formed from
Magma and Lava
Magma
• molten rock below Earth's surface.
L ava
• magma on the Earth's surface.
Pyroclastic material
• (pyro = fire, clastic = debris)
• Airborne lava
— cools as it falls
Igneous Rock Chemistry
•
•
■ Major elements:
Approximately 99% of Igneous Rocks are
comprised of only eight elements.








Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
Magnesium
The amount of silica (SiO2)
determines the mineral content
and general color of igneous
rocks.
■ Rare Elements:
■Trace elements are those which occur in very low
concentrations in common rocks (usually < 0.1 % by weight).
■ Their concentrations are therefore commonly expressed in
parts per million (ppm; 1 ppm = 10-4 weight%).
■ Unlike major elements, trace elements tend to concentrate in
fewer minerals, and are therefore more useful in formulating
models for magmatic differentiation, and in some cases, in
predicting the source of a particular magma.
■ Trace elements most commonly used for the interpretation of
the petrogenesis of igneous rocks include: Ni, Cr, Sc, V, Rb, Ba,
Sr, Zr, Y, Nb and the rare earth elements (La to Lu).
‫■عندما تتبلور المعادن االساسية من الصهير تسمح بعض المعادن المكونة‬
‫للعناصر النادرة بدخول بنيتها البلورية‬
‫■حيث وضع العلم ‪ Goldschmidt‬عام ‪ 1937‬قاعدة توضح سلوك‬
‫العناصر النادرة تسمى قاعدة (‪ )Goldschmidt‬و هى كاالتى‪:‬‬
‫‪-1‬عناصر مستترة (‪ :)Camouflage‬ويسمى االحالل بالتخفى‬
‫ ايون اساسى (‪ )A‬و ايون نادر (‪)B‬‬‫‪A++ = B++‬‬
‫‪AR = BR‬‬
‫مثال ‪:‬‬
‫‪ mg++‬و ‪co++‬‬
‫‪R=0.80 A‬‬
‫‪R=0.83 A‬‬
‫ حالتين‬: )Capture( ‫ االسر‬-2
A R = BR
●
A++ ≠ B++++
K+ Ba++
R=1.33 A
R=1.34 A
AR > BR
A+++ = B+++
●
‫ حالتين‬: )Admission( ‫القبول‬-3
AR = BR
●
A+++ = B+
mg++
Li+
:‫مثال‬
R=0.68 A
R=0.66A
AR < BR
A+++ = B+++
●
Sources of Heat for Melting
Heat from below : Heat moves upward
a. __________________
(by conduction and convection) from the very
hot (>5000 ̊ C) core through the mantle and
crust.
Minerals start to crystallize from a cooling
magma between 1200 ̊ C - 600 ̊ C.
Geothermal Gradient
b. ___________________________________
:
3o C /100 m (30o C/km)
At great depth
temperature alone
would melt rock BUT
high pressure may
cause it to remain solid.
iii. Not the same
everywhere (i.e., It’s
higher in volcanic
regions).
i.
ii.
c)
Radioactive Decay
• Heat byproduct during decay.
• High concentration may cause
temperature to increase with depth at a
rate greater than the geothermal gradient.
d)
Friction
• Rock grinding past rock
• Active Mountain building regions. Friction of moving and
shifting rock masses in regions of mountain building may
combine with heat from other sources to melt rock.
Melting due to the Addition of Volatiles
Viscosity of Magma/ Lava
Viscosity- important for volcanic activity
• the resistance of a liquid to flow
— high viscosity = thick and stiff
— low viscosity = thin and "runny".
Convergent Margins- flux melting
Divergent Boundary
Hot Spots (e.g. Hawaii)
Silica Tetrahedron
Silicon
Oxygen
Silicates are classified on the basis of Si-O polymerism •
■The Silicate structures are known from X-rays.
■The complexity of igneous rocks are attributed to
complexity of silicate structures.
■ Decompositions of mineral depends on silicate structures
[SiO4]4-
Independent tetrahedra Nesosilicates
Si : O
1:4
Example:
Olivine group
(Fe,mg)2SiO4
Garnet A3B2Si3O12
Usually B is Aluminum, A divalent
(Mg,Fe,Mn,Ca)3(Fe3+,Cr,Al)2Si3O12
characteristic colors:
Pyrope Mg3Al2Si3O12 – deep red to black
Almandine Fe3Al2Si3O12 – deep brownish red
Spessartine Mn3Al2Si3O12 – brownish red to black
Grossular Ca3Al2Si3O12 – yellow-green to brown
Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown,
black
Uvarovite Ca3Cr2Si3O12 – emerald green
Common Nesosilicates: The Aluminosilicates
Kyanite, Sillimanite, Andalusite
Al2SiO5 = Al2 (SiO4)O
Topaz
Al2SiO4(F,OH)2,
Si2O7]6-
Double tetrahedra
Sorosilicates
Si : O
2:7
• Double silicon tetrahedra linked
• by one bridging oxygen Sorosilicates
commonly also contain independent silica
tetrahedra (SiO4)-4
• Typically monoclinic symmetry
Epidote Group
Zoisite/Clinozoisite – CaAl3O(SiO4)(Si2O7)(OH)
Epidote – Ca2(Fe,Al)Al2O(SiO4)(Si2O7)(OH)
Cyclosilicates
n[SiO3]2- n = 3, 4, 6
Si : O
1:3
N=3, [Si3O9]6N=4, [Si4O12]8N=6, [Si6O18]12-
Beryl, Be3Al2 (Si6O18)
Inosilicates
single chains
[SiO3]2-
Si : O
1:3
Example: Pyroxine group
(Fe,mg) SiO3
Diopside , camg si2o6
Augite , ca (mg, Fe) si2o6
Inosilicates
Double- chains
[Si4O11]6Si : O
4 : 11
Example:
amphibole minerals
Actinolite,
ca2(mg,Fe)5Si8O22(OH)2
Phyllosilicates
Sheets of tetrahedra
[Si2O5]2Si : O
2:5
Mica group:
Muscovite,
KAl2(AlSi3)O10(OH)2
Tectosilicates
[SiO2]
Si : O
1:2
3-D frameworks of tetrahedral: fully polymerized
quartz and the silica minerals feldspars feldspathoids
zeolites
Examples:
Quartz, SiO2.
Orthoclase, KAlSi3O8.
Plagioclase, CaAl2Si2O8
Plate Tectonic - Igneous Genesis
1. Mid-ocean Ridges
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental
Margins
5. Back-arc Basins
6. Ocean Island Basalts
7. Miscellaneous IntraContinental Activity

kimberlites, carbonatites,
anorthosites...
Binary phase diagram for a solid solution of Olivine
Fayallite (Fa)
% Fo (Mg2SiO4)
Forsterite (Fo)
quartz
hornblende
feldspar
Olivine
(Mg,Fe)2SiO4
Single
tetrahedra
No cleavage
Pyroxene
(Mg,Fe)SiO3
Amphibole
Biotite Mica
Ca2(Mg,Fe)5Si8022(OH)2
K(Mg,Fe)3AlSi3O10(OH)2
Single chains
2 cleavages
(90o)
Double chains
2 cleavages
(60o and 120o )
Sheets
1 cleavage
Olivine –
 Found in mafic and ultramafic igneous rocks, can be found as beach sand from
volcanic islands, used in manufacturing of basic refractories, or as magnesium
ore, gem quality peridot
Micas  Found in igneous (pegmatities and granites) and metamorphic rocks (schists
and amphibolites), used for elect and heat insulation, paper products,
fireproof paint and dry lubricants
Hematite –
 Found in extrusive igneous rocks and secondary mineral of sed deposits
(oxidizing environment), most important iron ore, red ochre pigments and
polishing powders
Hornblende – a member of
the amphibole group
Potassium feldspar
Plagioclase feldspar