Presentation 6 Igneous rocks 2025

Introduction to Rocks

  • Rocks are categorized into three main groups:
    • Igneous Rocks: Formed from magma.
    • Sedimentary Rocks: Formed from deposition due to water, wind, or ice.
    • Metamorphic Rocks: Altered from existing rocks under high temperature and pressure.

Igneous Rocks

  • Focus: Formation, environments, and characteristics of igneous rocks.

Formation Environments

  • Two Primary Environments for Igneous Rock Formation:
    • Divergent Margins: Mid-ocean ridges, continental rifts.
    • Rocks melt via decompression melting (lowering pressure).
    • Subduction Zones: Water addition leads to melting.
    • Rocks melt through hydration melting (addition of water).

Melting Phase Diagrams

  • Phase Diagrams: Show stability fields of different phases of a material.
    • Example with Water (H2O)
    • Triple Point: Where solid, liquid, and vapor phase coexist.
    • Water uniquely has a negative slope between solid and liquid phases.
    • The pressure affects states, leading to potential transformations (e.g., ice to water).

Melting Mechanisms

  1. Decompression Melting

    • Occurs as mantle rock rises and pressure decreases without temperature increase.
    • Important in:
      • Mid-Ocean Ridges: Create new oceanic crust (basaltic magma).
      • Continental Rifts: Similar melting process occurring within continents (e.g., East African Rift).
      • Mantle Plumes: Hotspots where rock rises from deep within the Earth (e.g., Hawaii).

  2. Hydration Melting

    • Takes place in subduction zones: Water lowers melting points.
    • Leads to higher silica magmas (e.g., Andes).
    • Results in diverse magma compositions and eruptive styles.

Bowen's Reaction Series

  • Classification based on mineral crystallization as temperature decreases.
  • Basaltic Rocks (
  • Intermediate Rocks (55-65% Silica):
    • Fine-grained andesite (erupted) and coarse-grained diorite (underground).
  • Felsic Rocks (>65% Silica):
    • Fine-grained rhyolite and coarse-grained granite.
    • High viscosity, leading to explosive eruptions (e.g., Mount St. Helens).

Volcanic Landforms

  • Shield Volcanoes (Decompression Melting)

    • Broad, gently sloping; formed from fluid basaltic lavas (e.g., Hawaiian Islands).

  • Stratovolcanoes (Hydration Melting)

    • Steep-sided with layers; formed from viscous silica-rich lavas (e.g., Mount Shasta).
    • Characterized by explosive eruptions, pyroclastic flows, and lahars.

Conclusion

  • Understanding igneous rocks involves exploring their formation, environments, and the influence of tectonic settings on their characteristics and volcanic landforms.