Environmental Geology & Natural Hazards: Volcanism Notes

Types of Volcanism

  • Ocean-Ocean Convergent Margin
    • Formation of stratovolcanoes and shear zones from two oceanic plates converging.
  • Ocean-Continent Convergent Margin
    • Subduction of an oceanic plate under a continental plate, leading to stratovolcano formation.
  • Ocean Spreading Centre
    • Formation of new oceanic crust via mantle upwelling and volcanic activity in mid-ocean ridges.
  • Continental Rift Zone
    • Volcanic activity associated with tectonic plates pulling apart, leading to the formation of rift valleys and sometimes new ocean basins.
  • Hot Spot Volcanism
    • Formed by a stationary mantle plume, creating volcanoes like the Hawaiian Islands as tectonic plates move over them.

Volcano Types

1. Shield Volcanoes

  • Shape: Gentle arch, broad shape.
  • Magma Composition: Low silica; basalt.
  • Viscosity: Low.
  • Eruption Type: Primarily effusive; produces lava flows and occasional tephra ejections.
  • Example: Mauna Loa, Hawaii.

2. Stratovolcanoes (Composite Volcanoes)

  • Shape: Cone-shaped with steep sides.
  • Magma Composition: Intermediate; andesite.
  • Viscosity: Intermediate.
  • Eruption Type: Combination of effusive lava flows and explosive activity.
  • Example: Mt. Fuji, Japan.

Eruptive Styles

  • Effusive Eruptions: Produce lava flows that stream away from vents, creating lava lakes or fountains.
  • Explosive Eruptions: Generate pyroclastic flows due to gas pressure overcoming magma viscosity.
    • Can result in significant hazards due to rapid ejection of material and ash clouds.

Pyroclastic Flows

  • Highly hazardous with a typical high temperature (>800 °C) mixture of rock fragments, gas, and ash.
  • Can travel rapidly, with small flows averaging 10-30 m/s and larger ones up to 200 m/s.
  • Form through several mechanisms:
    • Directed blasts
    • Dome collapses
    • Flank collapses
    • Gravitational release
  • Notable Events:
    • 1991 Mount Unzen eruption resulting in fatalities due to pyroclastic flows.

Lahars

  • Volcanic mudflows composed of pyroclastic material, water, and debris, capable of causing destruction at high speeds.
  • Travel speeds can reach up to 100 km/h.

Volcanic Ash

  • Comprising fragments <2 mm, volcanic ash poses health hazards, and can damage vegetation and contaminate water supplies.
  • Significant aviation hazard as demonstrated in the 1989 Mount Redoubt incident where ash caused engine failure in a commercial flight.

Volcanic Hazards

  1. Pyroclastic Ejecta: Blocks, bombs, and ash can cause destruction upon falling.
  2. Lateral Blasts: Explosive eruptions that devastate large areas.
  3. Volcanic Smog (Vog): Caused by sulfur dioxide emissions, leading to health hazards and visibility issues.
  4. Lava Flows: Slow-moving but can destroy everything in their path; historical examples show both destructiveness and mitigation attempts (e.g., Redirection in Heimaey, Iceland).
  5. Volcanic Gases: Sometimes lethal, causing health effects ranging from irritation to asphyxiation.

Disasters and Mitigation

  • Case Studies:
    • El Chichón, Mexico 1982: A significant eruption leading to widespread fatalities and destruction.
    • Kīlauea, Hawaii: Ongoing eruptions causing extensive damage and economic impacts.

Summary of Volcanic Threats

  • Primary Impacts: From the eruption itself (lava flows, pyroclastic activity).
  • Secondary Impacts: Environmentally wide-ranging effects (volcanic ash, gases).
  • Preparedness: Local agencies monitor volcanoes, providing alerts to mitigate risks effectively, especially in high-risk areas.