IV. Igneous RocksMinerals 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

Igneous Rock

Solidification

Magma

Partial Melting

Mantle Rock Fig 3.1

Geological Materials

Transformation Processes

Fig 1.15

Fig. 2.9 From bottom to

top Increasing

Fe/Mg/Ca Decreasing silica Increasing density Darker minerals Decreasing

Silica

29% 14%

<20% 20%

<3% 23%

<2% 25%

(0) 33% (0) 23% (0) 15%*

% of Tot. # of atoms Fe/Mg: Silicon:

IncreasingFe/Mg/Ca Quartz

K and Na Feldspar

Ca Feldspar

Systematic SilicateMineralogy

IncreasingDensity

Olivine

PyroxeneGroup

AmphiboleGroup

MicaGroup

Fig. 2.9 From bottom to

top Increasing

Fe/Mg/Ca Decreasing silica Increasing density Darker minerals

29% 14%

<20% 20%

<3% 23%

<2% 25%

(0) 33% (0) 23% (0) 15%*

% of Tot. # of atoms Fe/Mg: Silicon:

Quartz

K and Na Feldspar

Ca Feldspar

Melting Points ofSilicate Minerals

IncreasingMelting

Temperature

Olivine

PyroxeneGroup

AmphiboleGroup

MicaGroup

Felsic>65% silica

Intermediate53-65%

Mafic45-52%

<700oC

>1000oC

Melting of Granite (Quartz, Na Plagioclase, Biotite)

SilicicMelt

~600oC ~700o ~800o

Quartz Melts Na-Feldspar First Begins to melt

~900o ~1000o

Biotite begins Magma is enriched to melt in Si, Na, Al (K)

Partial Melting and MagmaE.g., Progressive Silica Enrichment Results in Silicic Magma Fig. 3.1, 3.12, 3.13

Enriched: O, Si, Al, Na, K, Depleted: Ca, Fe, Mg

Gasses: (H2O, CO2) Poor in: O, Si, Al, Na, K, (<50%)

Magma Partial Melting Rich in: Ca, Fe, Mg (>50% wght)

Fig 3.13

Fig 1.15

Solidification of Melts

• Magma, Intruded or Extruded,• Solidifies (crystallizes) to form

• Intrusive or Extrusive • Igneous Rocks

Fig 3.12, 3.13

Igneous RockE.g., Granite (Silicic, Intrusive Igneous Rock):

Crystallized (Solidified) Silicic Melt Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%)

See Fig. 3.7a

Quartz

Na Plagioclase

Biotite

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

Mafic

Hot andHigh Pressure

Hot andLow 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 (fig. 4.21)

Rhyolite or Andesite Volcanoes Also injects surrounding rocks with silica laden steam

Batholith made of Plutons

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 ClassificationIntrusive (Plutonic)Extrusive (Volcanic) E.g., Basalt

and Gabbro have two minerals Pyroxene Ca-Feldspar

Fig. 4.10

Vo

lum

e P

erce

nt

of

Min

eral

s

Continental Crust Oceanic Mantle Crust

Igneous Rocks and Silicates

The

Igneous Rock Classification

parallels the

systematic

Silicate Minerals Classification

Fig. 3.11

and Fig. 4.10

Vo

lum

e P

erce

nt

of

Min

eral

s

Frame- Double Single Iso. work Sheet Chain Chain

Fig. 2.9

Igneous Rock ClassificationIn

tru

sive

Ext

rusi

ve

Silicic Intermediate Mafic

Granite Diorite Gabbro

Rhyolite Andesite Basalt

Fig. 3.7 (Porphyritic)

1200

oC

1000

o

Bowen’s Reaction SeriesTwo series of minerals formed during crystallization of magma

Intrus. Extrus.

Gabbro Basalt

Diorite Andesite

Granite Rhyolite

Intrus. Extrus.

Gabbro Basalt

Diorite Andesite

Granite Rhyolite

Low Silica Magma

High Silica Magma

Fra

me-

D

oubl

e

S

ingl

e

Isol

ated

wor

k

S

heet

Cha

in

Cha

in

Temperature ofCrystallization

750

o