Key Concepts in Mineralogy and Volcanology

  • Definition of a Mineral:

    • Naturally occurring solid compound.
    • Definite crystalline form and chemical formula.
    • Not formed by life processes (no carbon chains).
    • Rocks: Composed of a combination of minerals (e.g., granite = quartz + feldspar).

  • Crystal Structure:

    • Highly ordered atomic arrangement in geometric patterns (e.g., apatite, feldspar, diamond, quartz).

  • Earth’s Crust Composition:

    • Major elements: Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, Magnesium making up over 98% of the crust.

  • Mineral Formation:

    • Formed under diverse geologic conditions (from magma, lava, and solutions).
    • Hot underground water leads to mineral deposits in cracks.

  • Silicate Minerals:

    • Most common in Earth's crust:
    • Basic building block: silica–oxygen tetrahedron.
    • Tetrahedra can link to form various structures (rods, sheets, networks).
    • Divided into felsic (rich in silicon and aluminum) and mafic (rich in iron and magnesium) minerals.

  • Felsic vs. Mafic Minerals:

    FelsicMafic
    Light coloredDark colored (green to black)
    Lower density (< 3 g/cm³)Higher density
    Formed at lower temperaturesFormed at higher temperatures
    Common in continental crustCommon in ocean crust
    Examples: quartz, feldsparExamples: olivine, pyroxene

  • Igneous Rock Classification:

    • Classified by composition (felsic, intermediate, mafic, ultramafic) and texture (intrusive vs. extrusive).

  • Volcanic Eruptions:

    • Effusive:
    • Occurs at divergent boundaries.
    • Mafic lava flows steadily with low viscosity (1000-1200°C).
    • Explosive:
    • Occurs at convergent boundaries.
    • High silica content keeps gases trapped, leading to explosive eruptions.
    • Explosivity is determined by gas volume and pressure.

  • Partial Melting in Subduction Zones:

    • Melting can occur by heating, lowering pressure, or adding fluids.
    • Felsic eruptions common several hundred km from ocean trench at subduction zones.

  • Bowen's Reaction Series:

    • Describes the order of isolation of silicate minerals during cooling of magma.
    • First minerals to crystallize: olivine, pyroxene; last: quartz.

  • Minerals Influence:

    • Subducted crust changes chemistry, leading to variability in volcanic eruptions.
    • Felsic magma forms in the continental crust due to extensive melting and increased silica content:
    • High viscosity prevents gas escape, creating large gas and ash clouds during explosive eruptions.