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EXPLORATION: Earthquakes and Volcanoes

2. Volcanoes

Causes

Almost all volcanoes happen near plate divisions.

  • Destructive (violent) volcanoes usually happen near subducting plates. i.e. The Pacific Ring of Fire
  • Less violent volcanoes usually happen near diverging plates.

Watch the short video from National Geographic below. You will need to respond to this video in your learning guide.

    Types of Volcanoes
    • Cinder Cone
      • basaltic magma ejected under high pressure from a narrow neck cools to form cinder and ash which falls back down to build the sides of the cone
      • cone can build rapidly with steep sides i.e. 30 - 40 degree angle eg the volcano Paricutin in Mexico in 1943 grew 500 m in 8 months
      • if the narrow neck of the cone becomes plugged the volcano can explode violently


      Volcano
      Photograph by J.P. Lockwood on December 1, 1975 (Courtesy: USGS)

    • Shield Cone e.g. Kilauea in Hawaii
      • often formed from basalt lava which has a low viscosity allowing the lava to move for some distance forming a broad gently sloped volcano i.e. 10 -20 degree angle
      • less explosive tendency because the magma flows freely allowing the gases to escape


      Volcano
      Photograph by M. Mangan on April 16, 1992 (Courtesy: USGS)

    • Composite Cone (or sometimes called Stratovolcanoes) e.g. Mt. St. Helens, Mt. Baker, Mt. Rainier, and Mt. Etna in Sicily
      • consists of alternating layers of lava and ash
      • shallow slopes at bottom, steep at top, looks like a "normal" mountain in many ways.
      • lava layers permit the cone to attain considerable width while the cinder/ash layers encourage vertical growth
      • as a result, some of the world's highest volcanic mountains are in this category
      • can be very explosive spewing much material (i.e. cinders, ash, and rock fragments) high into the atmosphere and / or rapidly down the side of the volcano. Such a flow sears everything their path.
      • mud flows, known as lahars, are frequently triggered as heat from the volcano melts snow and ice


      Note: volcanoes often occur as "complexes" with different types of volcanoes together. Cinder cones often occur on the sides of shield or composite volcanoes.