HTML University

Science Dept.

Geology and Soils GEO 100 Notes
Volcanoes and Igneous Activity

1. Definitions
   1. Rock: An aggregate of one or more minerals

   2. Igneous Rock: Congealed from a hot silicate melt

   3. Magma: Hot silicate melt, intrusive

   4. Lava: Hot silicate melt, extrusive

2. Magma
   1. Silica and water content largely control the properties of a magma

   2. Principal consituents are: O, Si, Al, Ca, Na, K, Fe and Mg

3. Origin of Magma
   1. Basalt: Spreading Centers - partial melting of upper mantle

   2. Granite: Subduction Zones - partial melting of oceanic crust & metamorphism in mountain belts

4. Volcanism
   1. Fissure Eruptions
      1. Columbia Plateau in NW U.S.

      2. Deccan Plateau basalts in India

   2. Volcanoes

5. Volcanoes
   1. Craters & Calderas

   2. Evolution of Calderas

   3. Evolution of Volcanoes
      1. Start off with Basaltic lava flows (volcanism in folded mountain belts begin with Na rich basalts) then change to andesitic lava flows

      2. Basaltic materials are less viscous, flow readily while...

      3. Andesitic lava flows are more viscous and do not flow as readily and gases excape with difficulity

      4. Oceanic basalts have low viscosity, and flow easily while continental magmas and islands (near continental borders) magmas are more viscous, have more gas and tend to produce more explosive volcanic eruptions

6. Lava and Associated Activity
   1. Composition of Lava
      1. Felsic - acid lavas 70%+ silica

      2. Intermediate lavas 50 - 70% Silica

      3. Mafic Basic lavas 50% or less

   2. Texture of Lavas
      1. Pahoehoe: "ropy" smooth & billowy

      2. Aa : angular, jagged

   3. Temperature
      1. Basic lavas ~ 1100^oC

      2. Min. temp ~ 750^oC

   4. Volcanic Gases
      1. H₂O
      2. CO₂
      3. N₂
      4. CO
      5. Sulfur Gases
      6. H₂
      7. Cl₂
      8. HCl
      9. HF

   5. Pyroclastic Debris (tephra)
      1. Ash
      2. cinders - 0.5 - 2.5 cm
      3. lapilli (size of walnuts)
      4. Blocks

   6. Fiery Clouds - Nuees Ardentes

   7. Fumaroles/Geysers

7. Classification
   1. Shield

   2. Composite

   3. Cinder

8. History of Volcanoes
   1. active

   2. dormant

   3. extinct

9. Examples
   1. Vesuvius, "Mt Somma", Herculaneum & Pompei, Aug 24, 79 AD

   2. Krakatoa, between Java & Sumatra, Aug 26, 1883

   3. Paricutin, 320 km west of Mexico City, Feb 20, 1943 - in 5 days cone was 100 m high; in 1 year = 425 m high

   4. Hawaii

10. Distribution of Active Volcanoes
    1. Divergent Plates

    2. Convergent Plates

11. Earth's Heat
    1. Geothermal Gradient 25^oC/km

    2. Average Heat Flow = 1.2 microcal/cm²-sec or Average = 40 cal/cm²-yr

    3. Causes of Igneous Activity
       1. Radioactive Decay

       2. Frictional Movement of Rock Material

       3. Residual Heat from Planet Formation

12. Masses of Igneous Rocks: Plutons - Intrusives
    1. Tabular Plutons
       1. Sill - concordant

       2. Dike - discordant

       3. Ring Dikes & Dike Swans

       4. Loppoliths

    2. Massive Plutons
       1. Laccoliths

       2. Stock

       3. Batholith
          1. Stoping

          2. Xenoliths

          3. Granitization

13. Formation of Igneous Rocks
    1. Volcanic
       1. Tuff: vol ash hardened into rock

       2. Volcanic Breccia: angular fragments of congealed lava in ash

       3. Volcanic conglomerate: rounded fragments of congealed lava in ash

    2. Crystallization & Bowen's Reaction Series
       1. "reaction" as a magma cools the first formed minerals undergo continuous modification with the remaining silicate melt

       2. In each of the series below, the mineral to first crystalize is listed first and crystalization proceeds down the list
          1. Continuous Plagioclase Feldspar Series
             1. Anorthite - Ca Calcic feldspar

             2. Labradorite - Ca-Na Calcic Feldspar

             3. Andesine - Na-Ca Sodic Feldspar

             4. Albite - Na Sodic Feldspar

          2. Discontunuous Ferromagnesian Series
             1. Olivine

             2. Augite

             3. Hornblende

             4. Biotite

          3. The Last in the Series
             1. K feldspar

             2. Muscovite

             3. Quartz

14. Differentiation
    1. Fractionation by Crystal Settling - eg the Triassic Palisades Sill

    2. Partial Melting

    3. Assimilation

    4. Mixing of Magmas

15. Texture of Igneous Rocks see Link to the Igneous Rock Identification Chart
    1. Rate of Crystallization
       1. Slow Cooling & Large Crystals vs

       2. Quick Cooling & Glassy Texture

16. Mineral Composition of Igneous Rocks see Link to the Igneous Rock Identification Chart