Volcano Structure & Eruptive Style



Volcano Architecture

  • magma chamber
    • magma doesn't always make it to the surface
    • it may accumulate underground in a large irregularly shaped zone
    • the magma may fill cracks between blocks of solid rock
    • it may solidify to form a pluton
    • magma may inject into cracks or between layers to forms sills and dikes
    • the magma may rise along a pathway, called a conduit, to the surface and erupt from a vent
  • conduit
    • may resemble a vertical pipe or chimney
    • may take the form of a crack called a fissure






  • volcano
    • products of an eruption, or series of eruptions, build into a hill or mountain
  • crater
    • a circular, bowl-like depression occurs at the top of the volcano
    • can be a s large as 500 m across and 200 m deep
    • forms either during eruption, as material accumulates around the summit vent
    • can form just after eruption as the summit collapses into the drained conduit
  • caldera
    • a large magma chamber beneath a volcano may drain suddenly
    • leaves a circular depression that is much larger than a crater
    • calderas typically have steep walls and a fairly flat floor
    • may be partially filled with ash or solidified lava
    • examples range from a few kilometers to tens of kilometers across and may be several hundred meters deep
    • later eruptions may build new volcanoes within a caldera





This caldera in Oregon formed about 7,700 years ago. Afterward, it filled with water to become Crater Lake. Wizard Island is a small volcano that grew on top of the crater floor.















Volcano Shapes

  • shield volcanoes
    • broad, gentle domes that resemble a soldier's shield lying on the ground
    • form with low viscosity magma
    • the lave flows easily and can't build into a mound at the vent
  • cinder (scoria) cones
    • cone-shaped piles of basaltic lapilli and blocks
    • the steep slopes represent the angle of repose for lapilli
  • stratovolcanoes (composite volcanoes)
    • large volcanoes (up to 4 km high and 25 km across)
    • interleaved layers of lava, tephra and volcanoclastic debris





A cinder cone on the flank of a larger volcano in Arizona.
Note lava flow to the left in the distance.





Stratovolcano/Composite Volcano











Volcano Eruptive Style: Will It Flow or Will It Blow?


  • Kilauea, a volcano on Hawaii, produces rivers of lava
  • Mt. St. Helens, a volcano near the Washington-Oregon border, exploded catastrophically in 1980
  • volcanoes erupt in different ways and even the same volcano can have different eruptions that differ in characteristics


  • Effusive Eruptions
    • lava pours out from a vent or fissure
    • can create a lava lake in a crater
    • magma is very hot and mafic
    • mafic magma has low viscosity


  • Explosive Eruptions (basaltic: low viscosity)
    • small explosions take place during basaltic (low viscosity) eruptions
    • gas builds up and suddenly escapes
    • splatters lava drops and blobs upward
    • basaltic volcano explosions can occur if the eruption is under water
    • water comes in contact with magma and flashes into steam


  • Explosive Eruptions (felsic: high viscosity)
    • very large explosions can occur when rising magma is very viscous and rich in gas
    • magma may partially solidify trapping gas bubbles
    • as mage reaches the surface, pressure decreases and the bubbles may crack the solid magma and rush out
    • pyroclastic debris shoots out of the volcano like a shotgun blast
    • cubic kilometers of debris can be ejected into the atmosphere
    • sometimes the volcano is destroyed



  • Eruptive Clouds
    • debris (ash, lapilli and blocks) shoots skyward in a vertical column
    • the heated debris becomes a convective cloud and the ash can rise into the stratosphere
    • resembles a nuclear bomb mushroom cloud
    • coarse-grained ash settles close to the volcano
      • column collapse occurs when gravity pulls down substantial amounts of ejected debris and forms a pyroclastic flow
    • fine ash can circle the globe



1986 effusive eruption on Hawaii







Explosive eruption: just beneath the sea surface near Tonga






1991 eruption of Mt. Pinatubo in the Phillipines





Large explosive eruptive cloud





The eruptive jet of an eruption on Mt. Etna, Italy






Redoubt Volcano, Alaska, 1989







Major Volcanic Eruptions of the Past