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Understanding Volcanoes and Tectonic Activity

Apr 23, 2025

Lecture on Volcanoes: Tectonic Settings for Volcanic Activity

Introduction to Volcanic Activity

  • Tectonic Settings: Volcanoes occur mainly at convergent and divergent plate boundaries, and at hot spots within tectonic plates.

Convergent Plate Boundaries

  • Subduction Zones
    • Ocean-Continent Convergence: Leads to volcanic activity and formation of continental volcanic arcs.
    • Ocean-Ocean Convergence: Results in volcanic island arcs.
    • Continental-Continental Convergence: No subduction, hence no volcanic activity.
  • Example: Juan de Fuca plate subducting under North American Plate causes volcanic arcs.

Divergent Plate Boundaries

  • Mid-Ocean Ridges: Volcanoes form on ocean floors where plates pull apart, allowing magma to rise.
  • Volcanic Eruptions: Occur when molten rock rises through oceanic or continental crust.

Hot Spots (Intraplate Volcanism)

  • Definition: Volcanic activity within a tectonic plate, not at plate boundaries.
  • Examples:
    • Hawaii: Formed by a mantle plume under the Pacific Plate.
    • Yellowstone: Located within the North American Plate.

Volcanic Activity in the United States

  • California: Mount Shasta, Lassen Peak, Clear Lake (continental volcanic arcs); Long Valley Caldera, COSO volcano (hot spots).
  • Mount St. Helens: Example of a continental volcanic arc, erupted in 1980 due to subduction of the Juan de Fuca plate.
  • Yellowstone: A supervolcano that last erupted 640,000 years ago.

Geologic Hazards

  • Tsunamis: Caused by subduction-related earthquakes, e.g., 1700 megathrust earthquake along Cascadia subduction zone.
  • Earthquakes: Linked to tectonic movements, e.g., San Andreas Fault is a transform boundary with no subduction.

Mountain Ranges and Volcanism

  • Non-volcanic Mountains: E.g., Himalayas, result from continental convergence without volcanic activity.
  • Volcanic Mountains: Require a magma chamber, e.g., Mount St. Helens.

Hot Spot Volcanism

  • Formation: Stationary mantle plumes create chains of islands/volcanoes as tectonic plates move over them.
  • Hawaiian Islands: Chain formed as Pacific Plate moves over a hot spot.
    • Mantle Plume: Fixed, creates a series of volcanic islands and seamounts.

Conclusion

  • Volcanoes can form at subduction zones, divergent boundaries, and hot spots.
  • Hot spots lead to volcanic activity independent of plate boundaries.
  • Understanding different volcanic settings helps predict and prepare for volcanic hazards.

Next Lecture: Will cover pyroclastic flow and further volcanic processes.

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