Volcanoes and Explosivity Notes

Overview of Volcanoes and Eruptivity

  • Types of Eruptions
    • Effusive: Outpouring of molten magma leading to lavas
    • Explosive: Gas-driven violent eruptions resulting in pyroclastic deposits

Fundamental Concepts of Volcanic Explosivity

  • Volcanic Events:
    • Effusive Processes:
      • Lava flows (commonly mafic to intermediate composition)
      • Lava domes (commonly felsic to intermediate)
      • Gravitational collapse leading to pyroclastic flows
    • Explosive Processes:
      • Erupting columns of ash
      • Pyroclastic falls and flows
      • Composition of gases and viscosity of magma determine behavior

Key Characteristics Influencing Explosivity

  • Gas Content and Melt Viscosity:
    • More gas results in higher explosivity due to increased pressure and fragmentation.
    • Low viscosity allows gas to escape quietly, while high viscosity leads to dangerous build-ups.

Processes and Behaviors of Different Eruptions

  1. Effusive:
    • E.g., Hawaii
    • Characteristics include low gas content causing fluid lava flows and fire fountaining.
  2. Strombolian:
    • Basaltic/andesitic magma with moderate explosivity.
    • Produces bombs, ash plumes, and lava fountains.
  3. Vulcanian:
    • Very explosive with a mix of viscous andesitic to rhyolitic magma.
    • Characterized by sustained ash explosions.
  4. Plinian:
    • Violently explosive, involving sustained columns of ash and significant pyroclastic flows.

The Volcanic Explosivity Index (VEI)

  • VEI Definition:
    • Describes the size of an explosive volcanic eruption based on volume, eruption height, and duration.
    • Scale ranges from 0-8, with each increase of one unit representing a tenfold increase in volume.
  • Examples:
    • VEI 1: Minor eruptions
    • VEI 5: E.g., Pinatubo, significant global climate impact.
    • VEI 8: Massive eruptions like Toba, with extreme effects on climate.

Effects of Water on Eruptions

  • Contact between magma and water can lead to increased explosivity as water rapidly turns to steam, resulting in violent eruptions (e.g., Phreatomagmatic eruptions).

Visualizing Explosivity and Impact

  • Use descriptions, examples, and maps to illustrate the massive scale of violent eruptions.
    • Example of VEI 8: Up to 100 Pinatubos' worth of material.

Eruption Frequency and Size

  • Historical eruptions typically grouped by frequency from yearly events to those occurring every millennia, demonstrating the long-scale impacts of major eruptions on geology and climate.