GeoLOGY 101 Lab Lecture Outline

Igneous Rocks and Magmas (Lab 4)

 

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

 

C.  The Zone of Melting

1.The conditions for partial melting (Anatexis)

·       Temperature

·       Pressure

·       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:

1. Texture (Grain size/Rock fabric)

 

2.  Composition (Chemistry/Mineralogy)

 

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