Volcanoes

  • Types of Volcanoes
    • Cinder Cones
      • Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is ejected violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone.
  • Schematic representation of a cinder cone
    Representation of the internal structure of a typical cinder cone.
    • Shield Volcanoes
      • Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. For Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), rises up 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.
Schematic diagram of a typical shield volcano
Internal structure of a typical shield volcano
    • Composite Volcanoes
      • Composite cones are also known as stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows. Volcanic ash cinders, blocks, and pyroclasts(clouds that contain steam and sulphur) may rise as much as 8,000 feet in the air. One of the most conspicuous and beautiful composite mountains is Mount Fuji in Japan.
      • he important feature of a composite volcano is a conduit system through which magma beneath the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., which are added to its slopes.
    Schematic representation of a composite volcano
    The internal structue of a typical composite volcano.

     

    • When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed and is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano.

     

Evolution of a Composite Volcano

A . Magma rising upward through an opening erupts at the Earth's surface to form a volcanic cone. Lava flows spread over the surrounding area.
B . As volcanic activity continues perhaps over several centuries, the cone is built to a great height and lava flows to form an extensive plateau around its base. During this period, streams enlarge and deepen their valleys

C . When volcanic activity stops, erosion begins to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened "volcanic plug" in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas.
D . Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or "volcanic neck," a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau.

Source:
http://pubs.usgs.gov/gip/volc/types.html