GeoLOGY Lecture Outline

The Rock Cycle, Igneous Rocks,

and Magmatism (CH 4 & 5)


I. Lecture Content


       The Rock Cycle                             


       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



 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



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


       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



aphanitic texture




Bowen's reaction series


carbonate minerals


country rock

crystal settling






ferromagnesian silicates

felsic magma

felsic silicates



igneous rock

intermediate magma


lava flow

mafic magma


magma chamber

magma mixing


(mineral) reserve

(mineral) resource


native element

nonferromagnesian (felsic) silicates



phaneritic texture



plutonic rock

porphyritic texture

potassium feldspar

pyroclastic material (tephra)





rock-forming mineral







volcanic rock