I. Lecture
Content
Introduction Magma Differentiation
The Rock
Cycle Common Igneous Minerals
Definitions Classifying Igneous Rocks
Formation of Magmas Tectonic Settings
Magma Transport San Diego's Igneous Rocks
Magma
Emplacement Igneous mineral resources
Igneous Rock Structures Vocabulary Terms
Crystallization
of Magmas
II. Introduction
A.
The fundamental connection between magmatism, igneous rock formation,
plate tectonics and crustal growth.
B.
The central importance of igneous activity in the rock cycle.
III. The Rock Cycle - (Know this forwards and backwards!)
A. The Earth's endless rock cycle that ceaselessly shapes
the Earth's ever-changing surface.
B. The Rock Cycle Diagram (see inside of front cover
of text)
C. The four material
reservoirs:
1) magma; 2) igneous; 3) sedimentary; and 4)
metamorphic
D. The primary rock-forming
processes paths that inter-
connect the various material reservoirs.
1) Anatexis<partial melting> (magmatic processes);
2) Transport, Emplacement, Cooling & Crystallization of
magmas (igneous processes);
3) Weathering, Erosion, Transport, Sediment Refining,
Deposition, Compaction/Cementation
<lithification>
(sedimentary processes);
4) Heating, Recrystallization, Neocrystallization, Stress-
induced Mineral Reorientation, & Fluid-related
chemical reactions (metamorphic processes).
IV. Definitions of Magmas and Igneous Rocks
A.
Definitions - Make sure you know these!
1.
What is magma?
Defined: Naturally produced molten
rock that occurs
deep beneath the surface.
Magmas typically
contain mineral crystals, rock fragments, and
dissolved gases.
2.
What is a lava?
Defined: Naturally produced molten rock that occurs
on or near the surface. Lavas typically contain
mineral crystals, rock
fragments, and dissolved gases.
3.
What is a rock?
Defined: Any natural material
formed of minerals.
4. What is an igneous rock?
Defined: A rock formed from the crystallization of a
magma.
5. What is an extrusive (volcanic) rock?
Defined: A rock formed from the rapid crystallization
of a magma that has reached the surface (lava).
6. What is an intrusive (plutonic) rock?
Defined: A rock formed from the
slow crystallization
of a magma
situated deep within the crust.
V. Formation of Magmas
A.
The source regions (rocks) of magmas
· Mantle
· Oceanic crust
· Continental crust
B.
The physical and chemical properties of magmas
· Temperature
· Viscosity
· Magmas consist of Melt and Crystals
· Magmas are the result of Partial
Melting
· Source rock composition
· Amount of water in the rock
– IMPORTANT!!!
VI. Transport of Magmas
A.
Mass Transfer Mechanisms
1. Fault/fracture-controlled
-capillary-like ascent (upper crust)
2. Diapirism - balloon-like
ascent (lower crust)
VII. Emplacement Styles and Structures
A. Intrusive Rock Structures
§ Plutons
§ Batholith
§ Concordant intrusions -
§ Discordant intrusions
§ Sheet Dikes
§ Radial Dikes
§ Ring Dikes
§ Sills
§ Laccoliths
B. Extrusive Rock Structures
Shield
volcanoes
Strataform
(composite) volcanoes
Cinder
Cones
Lava
Domes
Lava
Flows
Pyroclastic
Sheets
C. Illustration of Igneous Rock Structures
·
See Figure 5.1, Page 77 in Lab Manual
VIII.
Crystallization of Magmas
A.
Crystallization of a Magma is a Process Controlled by
Several
Independent Environmental Variables
1. Temperature
2.
Pressure
3.
Melt Composition
4. Water & Volatile
Content
B. Cooling History of a Magma
1. Compositional changes
· Term for this is called differentiation
2. Physical changes
· Temperature
· Viscosity
· Density
C. Bowen’s Reaction Series
1) Continuous and Discontinuous
Series
·
Crystal/melt equilibrium reactions during the slow cooling of a
crystallizing magma
·
High Temp minerals crystallize from melt first,
and replaced later by lower Temp minerals.
2)
Bowen's Reaction Series Diagram
IX. Differentiation - Changes in the Composition of Magmas
A. Crystal settling
B. Fractionation
C. Assimilation (melting of
country rock)
D. Magma Mixing
X. Major Igneous
Rock-Forming Minerals
A. Igneous Mineral Groups
· Silicates
· Oxides
B. Common Igneous Minerals and the Rocks They Form
See Figure 5.3, Page 79 in the Lab
Manual
C. The Silicate Mineral Group
·
Subgroups, based upon chemical composition.
Ø Ferromagnesian minerals
Ø Felsic minerals
1. Common Ferromagnesian (Fe/Mg-rich) Silicates
Olivine -- (Fe,Mg)2SiO4
Pyroxene -- Ca(Mg,Fe,Al)(Al,Si)2O6
Amphibole -- NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
Biotite -- K(Mg,Fe)3(Al,Fe)Si3O10(OH)2
2. Common Felsic
(Ca/Na/K-rich) Silicate Minerals
Plagioclase -- (Ca,Na)AlSi3O8
Potassium
Feldspar -- KAlSi3O8
Muscovite
-- KAl2(ALSi3)O10(OH)2
Quartz -- SiO4
3. Common Oxide Minerals
Magnetite -- Fe3O4
Ilmenite
-- FeTiO3
XI. Classification of the Major Igneous
Rock Types
A. Based
Primarily Upon Two Criteria:
B. The Three Major Igneous Rock Types
1. Volcanic
(extrusives) – fine-grained
Komatiite, Basalt, Andesite, Rhyolite
2. Plutonic
(intrusives) – coarse-grained
Peridotite, Gabbros, Diorite, Granite
3. Porphyritic
(intrusive extrusive) – mixed-grained
C. Igneous Rock Classification Diagram
Ø Also see Figure 5.3, page 79 in the Lab
Manual
D. Hand Sample Identification Criteria for Igneous
Rocks
1) "Igneous"
Texture = Crystalline
2) Grain Size Terms:
· phaneritic (individual crystals seen by
eye)
= coarse/medium-grained = plutonic
· aphanitic (too small to be seen by
eye)
= fine-grained = volcanic
· porphyritic (mixed: some
seen, some not)
=
mixed-grained = volcanic
3) Mineralogy Versus Associated Rock Type
·
Olivine + pyroxene + Ca-plagioclase
= basalt/gabbro
·
Pyroxene + Ca/Na-plag + amphibole +/-biotite
= andesite/diorite
·
Na-plag + K-feldspar + quartz +/- biotite +/-muscovite
= rhyolite/granite
4) Igneous Rock "Color
Index"
·
Percentage of mafic minerals (by volume)
·
Classification divisions
ü Granite<15%<Diorite
ü Diorite<45%<Gabbro
ü Gabbro<85%<Peridotite
·
Color Index for Individual Minerals
o
See Figure 5.3, page 78 in Lab Manual
XII.
Igneous Rock Classification Flowchart
A. Step-by-Step Method for Classifying an Igneous
Rock
1. Qualify the rock's
texture
·
Pegmatitic or Phaneritic texture?
·
Aphanitic and/or Vesicular texture?
·
Glassy texture?
·
Pyroclastic (fragmental) texture?
2. Identify the rock's
mineral composition and color index
3. Use an igneous rock
classification flowchart to name the
igneous rock
·
See Figure 5.2, page 78 of Lab Manual
XIII.
Tectonic Settings For Igneous Rock Formation
A.
Regions of Active Magmatism on Earth
1.
Spreading Centers
2.
Subduction Zones
3.
Continental Collision Zones
4.
Hot Spots
B.
Nature of Igneous Rocks at Each Plate Boundary
1.
Spreading Centers
· Gabbros and Basalts
2.
Subduction Zones
·
Mostly Andesites and Diorites
·
Mafic and Felsic rocks types too
·
Proportion depends on degree of involvement
with continental crust
3. Continental Collision
Zones – Mainly Granites
3.
Hot Spots --- Basalts
C. San Diego
County is a World-Class Example of a
Subduction-related Magmatic
Arc
1. World-Class Example of a Subduction-related
Magmatic Arc
2. The Peninsula Ranges batholith
3. Mesozoic-age extrusive and intrusive magmatism
4. Combination of Island and Continental Margin Arc
XIV.
Igneous-related Ore Development
A.
Platinum reefs
B.
Porphyry copper/silver/gold
deposits
C. Spreading Center ‘Black smoker” deposits
XV. Concluding Point
A. Importance of Magmatism in the Evolutionary
History
of
our Planet Earth
XVI. Useful Vocabulary Terms
amphibole
anatexis
andesite
aphanitic texture
ash fall
ash flow
assimilation
basalt
batholith
Bowen's reaction series
biotite
cinder cone
cleavage
color index
compound
country rock
crystal settling
crystalline solid
crystallization
differentiation
diorite
dike
radial dike
ring dike
sheet dike
element
ferromagnesian silicates
felsic magma
felsic silicates
gabbro
granite
igneous rock
intermediate magma
lava
lava dome
lava flow
mafic magma
magma
magma chamber
magma mixing
mineral
(mineral) reserve
(mineral) resource
Mohs hardness
muscovite
native element
nonferromagnesian (felsic)
silicates
obsidian
olivine
peridotite
phaneritic texture
plagioclase
pluton
plutonic rock
porphyritic texture
potassium feldspar
pumice
pyroclastic material
(tephra)
pyroxene
quartz
rhyolite
rock
rock-forming mineral
scoria
silica
silicate
sill
vesicle
vesicular
viscosity
volcanic
volcanic rock