[see further information (in German) on volcanism and related topics under:  http://www.uni-muenster.de/MineralogieMuseum/Vulkane/Vulkan-10.htm

• Volcanisms is not randomly distributed over the world but is concentrated near plate boundaries:
  
  1. Where plate subduction or seafloor spreading takes place.
     
  2. At distinct hot spots: where deep mantle plumes reach Earth‘s surface.

     Fig.1 Major volcanic regions

     ( Source: FAO, 2001.)

     
     
• Landforms in volcanic regions are strongly influenced by the chemical and mineralogical composition of the materials that were deposited during eruptive phases. Volcanic rocks and magmas are grouped according to their silica contents in 3 main categories:
  
  1. Rhyolite = 65 – 75 % SiO₂
     
  2. Andesite = 65 – 55 % SiO₂
     
  3. Basalt = 55 – 45 % SiO₂
     
• The division based on the silica content is done because the SiO₂-content correlates with the viscosity of magmas and the type of volcanism.
  
• Rule of thumb: the higher the SiO₂-content, the more acid and viscous the magma and the more explosive the volcanic eruptions.
  
• World Resource Base (WRB) distinguishes 3 landforms of volcanic regions based on the magma composition:
  
  1. Major landforms in regions with basaltic volcanisms.
     
  2. Major landforms in regions with andesitic volcanisms.
     
  3. Major landforms in regions with rhyolitic volcanisms.
     

Major landforms in regions with basaltic volcanisms:

• Basaltic volcanism occurs where basic mantle material reaches the surface:
  a. Divergent plate margins (sea floor spreading): the midocean ridge (for example Island, Canary Islands)
  b. Hot spot areas: Hawaii (is the top of the largest shield volcano in the world) or the continental rift valleys (East African rift valley)
  
• Basaltic magma is little viscous and gases escape easily, i.e. basaltic volcanism produce little ash and pyroclastic rocks (tephra). The low-viscosity lava (fast moving fluid magma) can flow over large distances.
  

  Fig.2 Basaltic magma

  Fig.3 The low-viscosity lava

• The resulting shield volcanoes are comparatively flat.
  

  Fig.4 Shield volcanoes

  ( Source: FAO, 2001.)

• Most eruptions are fissure eruptions and take place along extensional cracks in the Earth‘s crust (Ethiopia, India, Siberia).
  

  Fig.5 Fissure eruption

  ( Source: FAO, 2001.)

• In the past enormous flood basalts ocurred through fissure eruptions which covered hundreds of square kilometers. Examples are the Parana plateau (S-America), Ethiopia, Siberia, India (Deccan Traps are the largest continental flood basalt and cover about 500.000 km²).
  

  Fig.6 Flood basalts worldwide

  ( Source: FAO, 2001.)

Major landforms in regions with andesitic volcanisms:

• Characteristic appearance at convergent plate boundaries where plate subduction takes place:
  

  Fig.7 Plate subduction at convergent plate boundaries

  a. Cordillera-type mountain belts (Andes)
  b. Island arcs (Philippines and Japan)
  
• Major concentrations are found around the Pacific rim (andesite line) and is sometimes called the ring of fire.
  

  Fig.8 Ring of fire

• The classical volcano type associated with andesitic volcanism is the stratovolcano (= stratified volcano).
  

  Fig.9 Stratovolcano, Stromboli, Italy

• Lava flows are more viscous and extend only a few kilometers. Hence, stratovolcanos have steeper slopes than shield volcanoes. They are composed of alternating layers of lava and pyroclastic rock of andesitic composition.
  
• Magma holds an intermediate position between basaltic and rhyolitic magmas (SiO₂-content) and thus produces both pyroclastics (= tephra) and lavas.

  Fig.10 Tephra of 2-64 mm diameter (lapilli), Kilauea Caldera Hawaii

  ( Source: US Geological Survey)

  
  
• Active, large and high stratovolcanoes can produce devastating volcanic mudflows (lahars).
  

  Fig.11 Lahar

Major landforms in regions with rhyolitic volcanisms

• Produced by partial melting of the continental crust and contains acid (SiO₂-rich) magma.
  
• Magmas are very viscous and withstand very high gas pressures. As a result, the eruptions are very violent. The underlying magma chambers can empty itself completely and huge craters are formed in this way (calderas).
  

  Fig.12 Caldera Lake Toba, Sumatra, Indonesia

• The high viscosity of the lava precludes lava flow; a lava dome is formed instead (e.g. Obsidian Dome, USA).
  
• Main extrusive products of rhyolitic volcanism are:
  
  • Ashes = stone fragments; usually  volcanic glass < 2 mm diamter
    
  • and  ignimbrites
    

    Fig.13 Pyroclastic flows

    ( Source: FAO, 2001.)

    that stem from pyroclastic flows.
    

    Fig.14 Pyroclastic flows

    ( Source: FAO, 2001.)

• These ignimbrites often fill depressions and valleys of tens or hundreds of meters depth. ignimbrite surfaces are flat and featureless (in contrast to surfaces of lava flows).