Volcanoes IV: Volcano Characteristics and Types

Global Distribution of Volcanoes

  • volcanoes occur at:
    • plate boundaries
    • hot spots

Hot Spots

  • mantle plume: hot material that rises from the core/mantle boundary (very very deep)
    • creates magma near the surface
    • melting (magma) caused by lower pressure near the surface (decompression melting)

Global Distribution of Hot Spots

  • if hot spot is under oceanic crust, it’s mafic magma
  • if hot spot is under continental crust, it’s intermediate magma

Hot Spot Under Oceanic Crust

  • 85 million years of volcanic activity due to one plume
  • initial source of melting: partial melting of mantle
  • secondary source of magma: oceanic crust
  • type of magma formed: mafic (hot, low viscosity,/SiO2, low gas)
  • hazard: mostly no (unless you walk too close to the lava)

Hot Spot Under Continental Crust

  • initial source of melting: partial melting of mantle
  • secondary source of magma: continental crust
  • type of magma formed: intermediate felsic (lots of cooler, high viscosity/SiO2 magma with lots of trapped gas)
  • hazard: Yes, erupt very infrequently, but the biggest hazard when they do

Type Of Volcanoes

  • 4 main types:

    1. shield volcanoes
    2. stratovolcanoes
    3. calderas
    4. cindercones

Shield Volcanoes

  • lava erupts from a fissure (large crack), runs down gentle slopes and cools
  • erupt often
  • mafic lava
  • effusive eruption

Shield Volcanoes-Lava Flows

  • lava flows downslope and ponds in topographic lows
  • outer crust cools and solidifies, insulating molten interior

Shield Volcanoes-Fire Fountain

  • if mafic lava is gas rich, small explosive eruptions from fire fountains
  • liquid lava falls back to ground and may form a lava flow

Stratovolcanoes

  • aka a composite cone
    • interbedded lava flows, pyroclastic flows, and lahars
  • mafic to intermediate to felsic
  • can be explosive (dependent on magma type)
  • may erupt many times and stay active for 100,000 years
  • explosive eruptions
  • associated with many hazards
    • ash cloud and fall of ash to ground
    • pyroclastic flows
    • larger pyroclastic material close to vent

Calderas

  • created when the roof of a magma chamber collapses after a large, explosive eruption of felsic pyroclastic material (up to miles across)
  • caldera created by “supervolcanoes”
  • different from a crater
    • crater: depression in ground caused by an eruption
  • features:
    • high silica, high gas magmas
    • intermediate felsic
    • massive explosions
    • the most explosive of all volcano types
    • collapse produces an “inverse volcano”, or “caldera”
    • spanish for cauldron

Cinder Cones

  • form in various tectonic settings, associated with other volcano types
  • layers of ejected pyroclastic material (from fire fountaining)
  • mafic
  • steep sides (30-40 degrees)
  • small volcanoes that never grow up
    • usually erupt for few years then never again
  • ex: Mount Paricutin in Mexico
    • created in 1943
    • most of the explosive activity that grew the volcano happened in the first year
    • 1,100 ft high
    • continued to erupt to 1952 and hasn’t erupted since
    • another 290 ft of material added