Geologic Settings of Volcanism

  • different styles of volcanism occur at different locations on Earth
  • most occur along plate boundaries
  • major eruptions can also occur at hot spots and in rifts





Mid-Ocean Ridges

  • products of mid-ocean ridge volcanism cover 70% of the Earth's surface
  • we don't generally see this activity because it occurs under the ocean
  • mid-ocean ridge volcanoes are not all continuously active
  • each one turns on and off in a time scale measured in tens to hundreds of years
  • they erupt basalt which forms pillow-lava mounds
  • water that heats up as it circulates through the crust near the magma chamber bursts out of hydrothermal vents, known as black smokers





Pillow lava




Hydrothermal Vent










Convergent Boundaries

  • most above sea level volcanoes on Earth lie on the edge of an overriding plate along a convergent plate boundary (subduction zone)
  • convergent boundaries border over 60% of the Pacific Ocean
  • a 20,000 km long chain of volcanoes, know as the ring of fire, has developed along the ring of the ocean
  • typically, individual volcanoes in volcanic arcs lie about 50 to 100 km apart
  • some of these volcanoes grow on oceanic crust and become volcanic island arcs, such as the Marianas of the western Pacific
  • others grow on continental crust, building continental volcanic arcs such as:
    • the Cascade volcanic chain of Washington and Oregon
    • the Andes chain of South America


The Aleutian volcanic arc of Alaska







Continental Rifts

  • due to the diversity of magmas that can form beneath rifts, rifts can host:
    • basaltic fissure eruptions
      • in which curtains of lava fountain up
      • linear chains of cinder cones can develop
    • explosive felsic volcanoes can develop
  • the stratovolcanos can also form
    • an example is Mt. Kilimanjaro in Africa




Oceanic Hot-Spot Volcanoes

  • when hot-spot volcanoes form on oceanic lithosphere:
    • basaltic magma erupts at the surface of the seafloor
    • first, pillow lavas form
    • after the volcano has emerged from the sea, effusive eruptions of basalt yield a broad shield shape with gentle slopes
    • as the volcano grows, gravity creates large submarine slumps





  • the Hawaiian Islands are examples of oceanic hot-spot volcanoes














Hot-Spot on Mid-Ocean Ridge: Iceland