Volcanoes IV: Volcano Characteristics and Types

Global Distribution of Volcanoes

  • volcanoes occur at:   * plate boundaries   * hot spots

Hot Spots

  • mantleplume: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: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

\ \