GeoLOGY Lecture Outline

Volcanoes and Eruptions (Ch 5)


I. Lecture Content


       Properties of Magmas

       Formation of Magmas

       Magma Transport – Means of Ascension

       Crystallization of Magmas

       Differentiation Processes in Magma Systems

       Volcanic Rocks and The Rock Cycle

       Classification of Volcanic Rocks

       Volcano Anatomy 101

       Volcanic Eruptions

       Tectonic Setting of Active Volcanic Belts

       Volcanic-related Ore Deposits

       Concluding Points



I. Introduction

A.    Direct connection between magma generation, volcanism, plate tectonics

 and crustal growth.

              1. Oceanic crust building factory at spreading centers


   2. Continental crust building & recycling factory at subduction zones


B.   The great importance of volcanism in shaping the Earth’s surface, and

 the influence upon the climate and biosphere.

              1. Influence on atmospheric chemistry and transparency


              2. Influence on communities of plants and animals


   3. Serious volcanic hazards on ecosystems and societies


III. Properties of Magmas - Definition and characteristics

A.  What is a magma?   Defined:  A magma consists of:

1.   Molten (liquid) rock

2.   Residual crystals

3.   Newly formed crystals

4.   Dissolved gases

5.   Xenoliths (exotic rock material)


B.    Physical Properties

1.   Temperature

2.   Viscosity

3.   Dissolved gas partial pressures


IV. Properties of Magmas - Definition and characteristics

C. Magma Chemistry – Know these Four major types


1. Ultramafic (ultramafic) – Very Low SiO4, V. High MgO, FeO, CaO 

                      Very Hot, Very low viscosity, Dry


2. Mafic (basaltic) Low SiO4, High MgO, FeO, CaO; Hot;

                             Low Viscocity, Wet or Dry


3. Intermediate (andesitic) Mod SiO4, MgO, FeO, CaO,

          NaO, Low K2O; Mod.T; Mod/High Viscocity, Wet


4. Felsic (rhyolitic/granitic) High SiO4, NaO, K2O, Mod/Low CaO,

      Low FeO and MgO; Low T; Low Viscocity, Wet/dry


V. Formation of Magmas – The Zone of Melting

A. Major Source Regions = Tectonic Plate Boundaries


1. Mantle rocks beneath spreading centers.


2. Subducting oceanic slab, mantle wedge, and crustal

 root of a subduction zone.


                      3. Mantle rocks at the top of mantle plumes (Hot spots)


                      4. Deep crustal rocks in continental collision zones.


B. Conditions for Melting


1. Increasing Temperature


2. Decreasing Pressure


3. Increasing Water Pressure


4. Source Rock Composition


C. The Melting Process – Anatexis


1.  Partial melting of rock

      a. Source rock composition


      b. Degree of partial melting (temp/heat-controlled)


      c. Amount of water in rock


      d. Melting of low temperature minerals first


                  2. Major chemical differences between parent source-rock

              and resultant daughter-melt (magma) are mainly due

              to partial melting mechanism (selective mineral melting) 


IV. Magma Transport - How Magma Ascends thru the Earth

A.     Mass Transfer Mechanisms

1.  Fault/fracture-controlled ascent of magma (typically through cold crust)


2.  Diapirism (balloon-like rising of magma bodies through hot crust)


B. Illustrations of Mechanisms


V.  Crystallization of Magma --- A Complex Process

           Controlled by Temperature, Pressure, and Chemistry

             A.  Bowen’s Reaction Series


              B.   Cooling History of a Magma

1.   Compositional changes


2.  Textural changes


VI.  Differentiation -- Igneous Processes that Can

               Change the Composition of Magmas 

1.   Crystal settling


2.   Fractionation


3.   Assimilation


4.   Magma Mixing


VII. Defining a Volcanic Rock


A. What is volcanic rock?  Defined:  An aggregate of

interlocking crystals formed from the crystallization of a

magma that has reached the Earth's surface (lava).


B.  Extrusive rock: Equivalent to the term, volcanic.


C.  Intrusive rock – Form deep beneath the surface (these

           rocks are termed plutonic).


VIII. Volcanic Rocks and the Rock Cycle

A. Volcanism’s Roll in the Endless Rock Cycle


1. Creation of primary surface crustal rocks


2. Creation of major mountain chains


3. Generation of nutrients, water, and gases to the surface


IX. Classification of Volcanic Rocks

A. Based on Texture and Composition


       B.  The Major Volcanic Rock Types (composition-based)

1.  Komatiites – ultramafic mineralogy


2. Basalts – mafic mineralogy


3. Andesites – intermediate mineralogy


4. Rhyolites – felsic mineralogy


Questions: 1) Mineralogy of each type?


 2) Plutonic (intrusive) equivalents of above?


C.  Other Volcanic Rock Types (textural quality-based)

  1.  Porphyry


  2.  Scoria


  3.  Pumice


  4. Obsidian


  5. Tuffs, Welded tuffs


  6. Pyroclastic Material – Loose particles (called tephra)

              a. Ash < 2 mm

              b. Lapilli  > 2mm, < 64mm

              c. Bombs and Blocks > 64 mm


D. Identifying the major Volcanic Rocks in Hand samples

       1. In-class examination of samples


       2. Internet images link -


X. Volcano Anatomy - Styles, Structures, and Hazards
A.  The Five Major Types of Volcanoes

1. Composite (strato)volcano


2. Shield Volcano


3. Cinder Cone


4. Caldera


5. Lava Dome


      B. Volcanic Structures – Necks, Dikes, Columnar Jointing


      C. Types of Volcanic Eruption Hazards

1.  Lava Fountains and Flows


2.  Ash Flows


3.  Ash Falls


4.     Mud Flows


5.     Harmonic Tremors


6.     Eruption Blast


7.  Tsunami


      D.  Active Volcanoes of the United States

1. Washington


2.   Oregon


3.   California


4.   Continental Interior


5.   Alaska and Hawaii


XI. Volcanic Eruptions

A. Types of Volcanic Eruptions


Classification based on Volcanic Explosivity Index - VEI


 0      Hawaiian = Lava Flows; Little explosiveness


              1/2     Strombolian = Jetting lava; some explosiveness


               3/4    Vulcanian = Explosively ejected molten lava and tephra


          5/6  Plinian = High velocity eruption of steam, tephra, & gases


              7/8    Ultra Plinian = Extra massive Plinian eruption


B.    Worldwide Examples of the Major Volcanic Eruptions

1. Mt. Pinatubo – 1990


2. Kilauea – 1983 - Present


3. Mt. Saint Helens – 1980


4.  Krakatau - 1816


5. Mt. Mazama – 6,600 ya


6. Toba, Sumatra, 75,000 ya


       C.  Volcanism Effects on World Climate   

1. Cooling effect of Dust in the Stratosphere


2. Warming Effect of Carbon Dioxide Gases


                3. Cooling/Precipitation Effects of Sulphate particles


XII.  Tectonic Setting for Volcanism

A.  The Distribution of Volcanoes on Earth

1.     Subduction Zones


2.     Spreading Centers


3.     Continental Rifts


4.     Hot Spots


B.     The Distinct Nature of Volcanoes and Volcanic Rock

at each Type of Tectonic Plate Boundary


1.    Subduction Zones

      --- All types of Volcanoes


      ---- Mostly Andesites, Some Basalt and Rhyolite


   2. Spreading Centers

--- Fissure eruptions and Shield volcanoes


--  All Basalt (pillow variety common in undersea)


         3. Continental Rifts

           --- Fissure eruptions, All Major Types


            – Predominant Mix of Basalts and Rhyolites


4. Hot Spots

     -- Shield volcanoes, Cinder cones, and Fissures


     -- Predominantly Basalts (rhyolites too in continents)


C.     San Diego County is a World-Class Example of a Ancient

       Subduction-related Volcanic Arc


              1. Mesozoic-age magmatism (160-90 MYA; "heyday" of the dinosaurs)


              2. Tectonic setting was combination of island arc and continental-margin arc


              3. Wide range of magma compositions (basaltic to granitic)


4. Average magma composition changed over time


5. Active volcanic chain probably had a crest elevation of 8,000 meters


XIII.  Volcanic-Related Ore Development

A. Porphyry copper deposits and related gold/silver veins

1. Subduction-related mineralization


2. Form from hydrothermal fluids circulating over an active

   magma chamber


3. Major source of world's copper


4. Western U.S. has numerous porphyry mines


B. “Black smoker” copper sulfide/metal deposits


       1. Spreading center-related mineralization


       2. Form from hydrothermal fluids circulating over an active

              oceanic magma chamber


       3. Actively mined deposits are part of bodies called "ophiolites"


XIV.  Global Influence of Volcanism

           A. Importance of Volcanism in Building & Shaping the Earth's Crust

              1. Oceanic crust building factory at spreading centers


              2. Continental crust building & recycling factory at subduction zones


           B. Impact of Volcanism to our Global Climate and Ecosystem

              1. Influence on atmospheric chemistry and transparency


              2. Influence on communities of plants and animals


              3. Serious volcanic hazards on ecosystems and societies


XV. Magma & Volcano Vocabulary -


Internet Image Glossary:







Bowen's reaction series


cinder cone

circum-Pacific belt

composite (strato)volcano



fissure eruption                    

harmonic tremor     

Hawaiian-style eruption


lava dome                

lava tube


magma differentiation

mantle plume (hot spot) 

nuee ardente



pillow lava

Plinian-style eruption          

phreatic-style eruption

porphyry system

pyroclastic sheet layers


shield volcano

Strombolian-style eruption          


volcanic explosivity index (VEI)



Vulcanian-style eruption