Igneous Rocks and Volcanism Overview

Feb 3, 2025

Introduction to Igneous Rocks and Volcanism (Lab 4)

Overview

  • Focus on igneous rocks formed from crystallization of magmatic melt.
  • Classification based on composition and texture.
  • Lab duration: 1 hour and 40 minutes.

Igneous Rocks Formation

  • Above Ground: Rapid cooling results in finer textures (e.g., basaltic lava).
  • Below Ground: Slower cooling results in coarser textures.

Classification of Igneous Rocks

Composition

  • Basaltic:
    • Origin: Partial melt of asthenosphere.
    • Silica content: ~50%.
  • Andesitic:
    • Origin: Partial melting of ocean lithosphere.
    • Silica content: ~65%.
  • Granitic:
    • Origin: Partial melt of continental crust.
    • Silica content: ~70%.
  • Silica Content:
    • Higher silica = lower iron/magnesium content.

Texture

  • Fine-Grained: Rapid cooling above ground.
  • Coarse-Grained: Slow cooling below ground.

Viscosity

  • Definition: Resistance to flow.
  • High viscosity: Thick, sluggish (e.g., rhyolitic magma).
  • Factors:
    • Silica Content: Higher silica increases viscosity.
    • Temperature: Higher temperature lowers viscosity.

Tectonic Settings for Basaltic Magmas

  • Mid-Ocean Ridges: Spread zones.
  • Mantle Hotspots: e.g., Hawaiian Islands.
  • Back-Arc Basins: e.g., Sea of Japan.
  • Subduction Zones: Basalt may leak through cracks.

Basalt Flows

  • Types:
    • Pahoehoe: Low viscosity, ropey texture.
    • Aa: Higher viscosity, blocky texture.

Intermediate Composition Rocks

  • Formation: Subduction zones.
  • Types:
    • Andesite: More mafic.
    • Dacite: More felsic (e.g., Mount St. Helens).

Continental Collisions and Granite Formation

  • Partial melt of continental crust forms granite.
  • Examples: Appalachian Mountains, Yellowstone Park.

Bowens Reaction Series

  • Mineral Crystallization:
    • High Temp Minerals: Olivine, calcium-rich plagioclase.
    • Intermediate Temp: Amphibole, sodium-rich plagioclase.
    • Low Temp: Quartz, potassium-rich feldspar.

Fractionation Processes

  • Separation: Solid phase from melt phase.
  • Methods:
    • Crystallization: Minerals settle or stick to chamber walls.
    • Partial Melting: Melts first, separates from solid.

Igneous Rock Chart Classification

  • Zones:
    • Granite/Rhyolite: High silica.
    • Andesite/Diorite: Intermediate.
    • Basalt/Gabbro: Low silica.

Practical Identification

  • Minerals: Identify using rock ID chart.
  • Compatibility: Use mineral compatibility for identification.

Volcanic Landform Morphology

  • Hawaiian Volcanoes: Low slope angles due to low viscosity.
  • Cascade Volcanoes: High slope angles due to high viscosity.

Volcanic Eruptions

  • Hawaiian Islands: Quiet eruptions due to low viscosity and gas content.
  • Subduction Zone Volcanoes: Explosive due to high viscosity and gas content.

Next week: Introduction to sedimentary rocks and processes.