IV. Igneous Rocks
Minerals Crystallized from Melts
1. The Rock Cycle
2. Formation of Igneous Rocks (and
Bowen’s Reaction Series)
3. Classification of Igneous Rocks
4. Igneous Rocks and Plate Tectonics
The
Rock Cycle


Geological Materials
Transformation Processes
Igneous Rock
Solidification

Magma
Partial Melting

Mantle Rock
Fig 3.1
Fig 1.15
% of Tot. # of atoms
Fe/Mg: Silicon:
Olivine
29%
14%
Systematic Silicate
Mineralogy


Pyroxene
Fig. 2.9
<20%
Group
From bottom to
top



Decreasing
Silica
Increasing
Fe/Mg/Ca
Increasing
Density

Amphibole
Group
<3%
Increasing
Fe/Mg/Ca
Decreasing silica
Mica
<2%
Group
Increasing density
Darker minerals
Quartz
K and Na Feldspar
Ca Feldspar
(0)
(0)
(0)
20%
23%
25%
33%
23%
15%*
% of Tot. # of atoms
Fe/Mg: Silicon:
Olivine
29%
14%
Melting Points of
Silicate Minerals
>1000oC
Mafic
45-52%
Intermediate
53-65%


Pyroxene
Fig. 2.9
<20%
Group
From bottom to
top


Increasing
Melting
Felsic
Temperature >65% silica
<700oC


Amphibole
Group
<3%
Increasing
Fe/Mg/Ca
Decreasing silica
Mica
<2%
Group
Increasing density
Darker minerals
Quartz
K and Na Feldspar
Ca Feldspar
(0)
(0)
(0)
20%
23%
25%
33%
23%
15%*
Melting of Granite
(Quartz, Na Plagioclase, Biotite)
~600oC
~700o
Quartz Melts
First
~900o
~1000o
Silicic
Melt
Biotite begins
to melt
Magma is enriched
in Si, Na, Al (K)
~800o
Na-Feldspar
Begins to melt
Partial Melting and Magma
E.g., Progressive Silica Enrichment
Results in Silicic Magma
Fig. 3.1, 3.12, 3.13
Fig 3.13
Enriched: O, Si, Al, Na, K,
Depleted: Ca, Fe, Mg
Gasses: (H2O, CO2)
Magma  Partial Melting
Fig 1.15
Poor in: O, Si, Al, Na, K, (<50%)
Rich in: Ca, Fe, Mg (>50% wght)
Solidification of Melts
Fig 3.12, 3.13
• Magma, Intruded or Extruded,
• Solidifies (crystallizes) to form
• Intrusive or Extrusive
• Igneous Rocks
Igneous Rock
E.g., Granite (Silicic, Intrusive Igneous Rock):
See Fig. 3.7a
Quartz
Biotite
Na Plagioclase
Crystallized (Solidified)
Silicic Melt
Poor in: Fe, Mg, Ca, (<20%)
Rich in: Silica (>70%)
Formation of Magma

How are rocks melted? Pg. 61




1. Heating ■ 2. Depressurization
3. Increase water content
4. Increased silica content
Where do rocks melt?


Subduction zones (Silicic
and Intermediate)
Mantle Plumes (“Hot Spots”)
not only at
Divergent
Boundaries
Hot and
Low Pressure
Mafic
Hot and
High Pressure
Intrusive vs.
Extrusive

Silicic (a.k.a, felsic) Magmas




Cool (<700oC)
Viscous (sticky, doesn’t flow easily)
Gaseous (steam of H2O and C02)
Silicic Rocks


Usually intrusive, course-grained,
Silicic (Granite) to Intermediate
(Diorite) rock forms plutons
If extrusive, fine-grained rocks
formed by explosive volcanoes
Batholith made of Plutons
(fig. 4.21)

Rhyolite or Andesite Volcanoes
Also injects surrounding rocks
with silica laden steam
Composite Volcano
Dikes: Intruded near a pluton


Silica rich fluids
are injected into
cracks in all
directions
Discordant:
cutting across
layers
Extrusive vs.
Intrusive

Mafic Magmas




Hot (>1000oC)
Non-Viscous (runny, flows easily)
“Dry” (no H2O or C02)
Mafic Rocks



Usually Extrusive, Fine-grained,
Mafic (Basalt) rock forms oceanic
crust, Shield Volcanoes and Basalt
Floods
If Intrusive, course-grained mafic
rocks are formed Gabbro.
If intrusive, Dikes and Sills more
common. (Plutons don’t form)
Mafic Sill: Intruded between layers




Mafic magma is less
viscous and hotter so
Does not form plutons
but
Cuts along layers (Sills)
or even
across layers (Dikes)
Also Baked Zones
of adjacent country
rock and Chill Zones
within the intrusion
Igneous Rock Classification
Intrusive (Plutonic)
Extrusive (Volcanic)
E.g., Basalt
and Gabbro
have two
minerals
 Pyroxene
 Ca-Feldspar
Volume Percent
of Minerals

Fig. 4.10
Continental Crust
Oceanic Mantle
Crust
The
Igneous Rock
Classification
parallels the
systematic
Silicate Minerals
Classification
Framework
Volume Percent
of Minerals
Igneous Rocks
and Silicates
Fig. 3.11
and Fig. 4.10
Sheet
Double
Chain
Single
Chain
Iso.
Fig. 2.9
Igneous Rock Classification
Intermediate
Mafic
Granite
Rhyolite
Diorite
Andesite
Gabbro
Basalt
Extrusive
Intrusive
Silicic
Fig. 3.7
(Porphyritic)
Bowen’s Reaction Series
Two series of minerals formed during crystallization of magma
Low Silica Magma
1200oC
1000o
Intrus. Extrus.
Gabbro Basalt
Diorite Andesite
750o
Framework
Sheet
Double
Chain
Single
Chain
Isolated
Temperature of
Crystallization
Granite Rhyolite
High Silica Magma
Crystallization of Gabbro and Basalt
~1,450oC
~1,400o
~1,350o
Mafic
Melt
Ca-Feldspar
Forms
~1,300o
Olivine
Forms
The remaining
melt is enriched
in silica (Int-Felsic)
If melt is extracted
Ultramafic Rock
Olivine converts will be formed
to Pyroxene
~1,250o
Gabbro
Remaining silica crystallizes into Ca-Feldspar. Intrusive
Cooling: Slowly
Basalt
Extrusive
Quickly
Crystallization of Diorite and Andesite
~1,400oC
~1,300o
~1,200o
Olivine converts
to Pyroxene
Ca Feldspar
absorbs Na
Intermediate
Melt
Ca Feldspar
Forms
~1,100o
If cooled
slowly in the
magma
chamber then
extruded,
Andesite
Porphyry will
be formed
~1,000o
Pyroxene converts Remaining silica
to Amphibole
forms Na-Ca Fldspr.
Diorite
Intrusive
Slowly Cooled
Andesite
Extrusive
Quickly Cooled
Crystallization of Granite and Rhyolite
~1,200oC
~1,100o
~1,000o
Silicic
Melt
Na-Feldspar
forms and grows
~900o
Amph. dissolves
to form Biotite
Pyroxene
forms
Pyroxene dissolves
to form Amphibole
~750o
Remaining silica
forms Quartz
Granite
Intrusive
Slowly Cooled
Rhyolite
Extrusive
Quickly Cooled
Crystallization of Granite and Rhyolite
Global Distributions of Rock Types
And relationships to plate tectonics
• Dark Blue: Oceanic crust  Basalt Fe,Mg,Ca Rich (Si,Al,Na,K poor)
• Light blue: New oceanic crust  new Basalt being formed along MidOcean Ridges
• Red regions: Mountain ranges  Felsic-Intermediate igneous and
metamorphic (green
Global Distributions of Rock Types
And relationships to plate tectonics
• Dark Blue: Oceanic crust  Basalt Fe,Mg,Ca Rich (Si,Al,Na,K poor)
• Light blue: New oceanic crust  new Basalt being formed along MidOcean Ridges
• Red regions: Mountain ranges  Felsic-Intermediate igneous and
metamorphic (green
Pacific Northwest
Pacific Northwest
Mount Hood
Ship Rock, New Mexico
Volcanic Stock and
Feeder Dikes
Volcanic Stock
Feeder Dike
Devils Tower, Wyoming
Devil’s Post Pile, California