Volcanoes III: Volcano Distribution and Characteristics

Eruption Styles

• two main styles:

  1. effusive: outpouring of molten lava from vent
  2. explosive: gas-driven violent eruption producing pyroclastic deposits

Effusive Eruptions

• low gas content and low viscosity magma = effusive volcanism
  • “tame” volcano with lots of lava flows
  • fewer hazards

Explosive Eruptions

• high gas content and vicious magma = explosive volcanism
  • explosive volcano with lots of pyroclastic (“fire fragments”) material
  • several associated hazards

Global Distribution of Volcanoes

• volcanoes occur at:
  • plate boundaries
  • hotspots

Plate Boundaries: 3 Main Types

• divergent, convergent, transform
  • at transform there isn’t any volcanoes
• crust composition and melt origin will strongly influence the type of volcanism

Oceanic Divergent Boundary

• two oceanic plates spread apart across a rift zone
  • rift zone: a long, linear volcano
• new oceanic crust is created as older crust is pulled away
• volcanism is extensive (lots of lava produced), but not explosive
• when the two plates have spread far enough apart that an ocean has formed between two continents, the divergent boundary is called a mid-ocean ridge
  • most voluminous volcanism on Earth
  • not exposed on Earth’s surface (so we don’t see the eruptions), except in Iceland because there is a hotspot there
• initial source of melting: partial melting of the mantle (due to decompression melting)
• secondary source of magma: oceanic crust
• type of magma formed: mafic (hot, low viscosity/SiO2, low gas)
• hazard: low viscosity lava, low gas content, effusive eruptions, and underwater = no hazard

Continental Divergent Boundary

• two continental plates spread apart to form a rift valley
  • ex: east african rift valley
• initial source of melting: partial melting of the mantle
• secondary source of magma: continental crust
• type of magma formed: mostly intermediate (cooler, high viscosity/SiO2, lots of gas)
• hazard: vicious lava with lots of trapped gas = high hazard (lower hazard in places with more mafic lava)

Subduction Zones

• oceanic plate sub-ducts under another oceanic plate or continental plate
  • releases water
  • water causes partial melting of the mantle (ultramafic = more extreme mafic composition than mafic; super hot, super low silica)
  • magma composition becomes more silica rich as it rises and incorporates oceanic (mafic)/continental (felsic) crust
• at the surface:
  • volcanic arcs: arc-shaped lines of volcanoes
  • 75% of active volcanoes happen in the Pacific Ring of Fire

Oceanic Oceanic Convergent Boundary

• initial source of melting: partial melting of the mantle (due to water released from sub-ducting plate)
• secondary source of magma: (modified) oceanic crust- mafic to intermediate
• type of magma formed: mostly mafic (hot, low viscosity/SiO2, low gas)
• hazard: generally low

A “Local” Continental Volcano

• produced by the Cascadia Subduction Zone
  • subduction of the Juan de Fuca plate (oceanic) under North American plate (continental)
• active volcanoes (last 2 million years)

Oceanic-Continental Convergent Boundary

• initial source of melting: partial melting of the mantle (due to H2O released from sub-ducting plate)
• secondary source of magma: continental crust- felsic (lots of cooler, high viscosity/SiO2 magma with lots of trapped gas)
• hazard: YES