GeoLOGY Lecture Outline

The Rock Cycle, Igneous Rocks,

and Magmatism (CH 4 & 5)

I. Lecture Content

Introduction

The Rock Cycle

Definitions

Major Igneous Minerals

Classifying Igneous Rocks

Formation of Magmas

Magma Transport

Magma Emplacement

Magma Crystallization

Magma Differentiation

Tectonic Settings of Magmatism

Igneous mineral resources

Vocabulary Terms

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 rock 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) Partial melting of rock (called anatexis) and the Transport, and

Emplacement of magmas = <magmatic processes>

2) Cooling, Differentiation & Crystallization of magma =

<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 & Igneous Rocks

A. Definitions - Make sure you know these!

1. What is magma?

Defined: Naturally produced molten (liquid) rock that typically

includes mineral crystals, rock fragments, and dissolved gases.

2. What is a rock?

Defined: Any natural material made up of one or more minerals or

mineraloids, and/or broken pieces of rocks. Rocks can also

consist of the shells of once-living animals, or of compressed

pieces of plants.

3. What is an igneous rock?

Defined: A rock formed from the crystallization of a

magma.

4. What is an extrusive (volcanic) rock?

Defined: A rock formed from the rapid crystallization

of a magma that has reached the surface (lava).

5. What is an intrusive (plutonic) rock?

Defined: A rock formed from the slow crystallization

of a magma situated deep within the crust.

V. Review of the Igneous Rock-Forming Minerals

A. Classification based upon two primary criteria:

1. Chemical Composition

2. Crystal Structure

B. Major Igneous Rock Mineral Groups

1. Silicates

2. Oxides

3. Sulfides

C. List of Rock-forming Minerals in Table

D. The Silicate Mineral Group

· Subgroups, based upon chemical composition.

Ø Ferromagnesian minerals

Ø Nonferromagnesian (felsic) minerals

1. The Ferromagnesian (Fe/Mg-rich) Silicate Minerals

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

Garnet -- Fe₃Al₂(SiO₄)₃ (almandine variety)

Chlorite -- (Fe, Mg, Al)₆(Si, Al)₄O₁₀(OH)₈

2. The Nonferromagnesian (Felsic) (Ca/Na/K-rich) Silicates

Plagioclase -- (Ca,Na)AlSi₃O₈

Potassium Feldspar -- KAlSi₃O₈

Muscovite -- KAl₂(ALSi₃)O₁₀(OH)₂

Kaolinite (clay group) -- [Si₄]Al₄O₁₀(OH)₈ ^. 4H₂O

Quartz -- SiO₄

3. The Major Oxide Minerals

Magnetite -- Fe₃O₄

Ilmenite -- FeTiO₃

Hematite -- Fe₂O₃

VI. Classification of the Major Igneous Rock Types

A. Based mainly upon two criteria:

1. Texture (grain size)

2. Composition (mineralogy)

B. The major igneous rock types

1. Volcanic (extrusives) – fine-grained

Compositional types = Komatiite, Basalt, Andesite, Rhyolite

2. Plutonic (intrusive) – coarse-grained

Compositional types = Peridotite, Gabbros, Diorite, Granite

C. Igneous Rock Classification Diagram

Ø Also see Figure

D. Hand Sample Identification Criteria for Igneous Rocks

1) Crystalline "Igneous" Texture

2) Grain Size:

· phaneritic (individual crystals seen by eye)

= coarse/medium-grained = plutonic

· aphanitic (too small to be seen by eye)

= very fine-grained = volcanic

· porphyritic (mixed: some seen, some not)

= mixed-grained = volcanic

3) Mineralogy

· Olivine + pyroxene + Ca-plagioclase

= basaltic/gabbroic

· Pyroxene + Ca/Na-plag + amphibole +biotite

= andesitic/dioritic

· Na-plag + K-feldspar + quartz + biotite

= rhyolitic/granitic

VII. Formation of Magmas

A. The source regions (parent rocks) for magmas

· Upper Mantle

· Oceanic crust

· Continental crust

B. The physical and chemical properties of magmas

· Temperature

· Viscosity

· Magmas consist of Melt, Crystals & Dissolved volatiles

· Magmas are the result of Partial Melting

C. The Zone of Melting

1.The conditions for partial melting (Anatexis)

· Temperature

· Pressure

· Source rock composition

· Amount of water in the rock – IMPORTANT!!!

VIII. Transport of Magmas

A. Mass Transfer Mechanisms

1. Fault/fracture-controlled -capillary-like ascent (upper crust)

2. Diapirism - balloon-like ascent (lower crust)

IX. Emplacement Styles and Structures

A. Intrusions

§ Plutons – Single intrusive bodies

§ Batholith – Mosaic of multiple intrusive bodies

§ Dikes – Vertical-oriented, sheet-like bodies

§ Sills – Horizontal-oriented sheet-like bodies

§ Lacoliths – Mushroom-like intrusive bodies

B. Extrusions

Volcanoes

Cinder Cones

Lava Domes

Lava Flows

Pyroclastic Sheets

C. Illustration of Igneous Rock Structures

X. Crystallization of Magmas --- A Complex Process

A. Cooling History of a Magma

1. Compositional changes

2. Textural changes

B. 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

XI. Processes that Change the Composition of Magmas

A. Crystal settling

B. Fractionation

C. Assimilation (melting of country rock)

D. Magma Mixing

XII. Tectonic Setting of the Igneous Rock Formation

A. The Regions of Active Magmatism on Earth

1. Spreading Centers

2. Subduction Zones

3. Continental Collision Zones

4. Hot Spots

B. The Unique Nature Igneous Rocks at each Boundary

1. Spreading Centers – Gabbros and Basalts

2. Subduction Zones --- Mainly Intermediate

3. Continental Collision Zones – Mainly Granites

4. Hot Spots --- Basalts

C. Igneous Rocks of San Diego County - The Peninsula Ranges Batholith

1. World-Class Example of a Subduction-related Magmatic Arc

2. Mesozoic-age extrusive and intrusive magmatism

3. Wide variety of rock types and ages

· 150-170 MYO andesite volcanics

· 130-90 MYO gabbros, diorites & granites

XIIi. Igneous Activity and Growth of Continents

· Central Geologic Importance of Magmatism in the Evolutionary

History of Earth's Crust

· Igneous Origin for Many Important Mineral Resources

1. Platinum reefs

2. Porphyry copper/silver/gold deposits

3. Spreading Center ‘Black smoker” deposits

XIV. Vocabulary Terms

amphibole

andesite

aphanitic texture

assimilation

basalt

batholith

Bowen's reaction series

biotite

carbonate minerals

compound

country rock

crystal settling

crystallization

differentiation

diorite

dike

element

ferromagnesian silicates

felsic magma

felsic silicates

gabbro

granite

igneous rock

intermediate magma

lava

lava flow

mafic magma

magma

magma chamber

magma mixing

mineral

(mineral) reserve

(mineral) resource

muscovite

native element

nonferromagnesian (felsic) silicates

olivine

peridotite

phaneritic texture

plagioclase

pluton

plutonic rock

porphyritic texture

potassium feldspar

pyroclastic material (tephra)

pyroxene

quartz

rhyolite

rock

rock-forming mineral

silica

silicate

sill

vesicle

viscosity

volcanic

volcanic rock