VOLCANOES:

Mountains formed by pyroclasts and lava.



Click here to learn more about volcanoes!











Infromation included in this page includes:



There are three main types of volcanoes, and they include:


  1. Shield Volcanoes
  2. Cinder Cones
  3. Strato-volcanoes




Shield Volcanoes


  • A shield volcano is widely known to have low angled slopes and is generally less destructive than the other volcanoes, because of its "gentler" ejection of lava. They resemble a warrior's shield with a curved appearance, which is caused by the "fluid" mostly basaltic lava. Shield Volcanoes are considered to be some of the tallest mountains on earth when the portion under the sea is taken into account. They have gradually built-up over many years by the lava flows, and some have a total height of over 30,000 feet.

    Kilauea is one of the most active shield volcanoes in the world.

    Mauna Lao is the largest volcano in the world and, along with four other shield volcanoes, makes up the islands of Hawaii.




    Cinder Cones


  • Cinder Cones are charecterized by slightly steeper slopes than the shield volcanoes, but they are relatively small in size. They are between the shield and the strato-volcano in regards to the explosions and the violence of them. These volcanoes are formed by lava fragments which consist of pyroclastic materials. They can start eruption without any warning, and without anyone even knowing they existed, and then can stop suddenly as quickly as they began.

    Paricutin is a famous cinder cone, because it is one of the only volcanoes that geologists have actually witnessed the formation of, from start to finish.





    Strato-volcanoes


  • Strato-volcanoes, also known as composite cones, are the most violent types of volcanoes, and they can erupt unexpectedly and be very deadly. The pressure builds-up to an extent that there is literally an explosion, and lava and gases and incandescent bombs flow from the cone. Unique to the strato-volcano is the nuee-ardente. This is characterized by the grouping together of both lava and gaseous build-ups of steam, which flow from the cone and race down the mountian at speeds well in excess of 60 miles-per-hour, demolishing everything in its path. Strato-volcanoes are however, some of the most beautiful volcanoes, and in the dormant stage are very pleasing to the eye.

    A recently erupted strato-volcano is the     Pavlof Volcano located in Alaska.

    Another widely known strato-volcano is Mount Mayon located in the Phillipines.











    Viscosity



    Viscosity is basically the measurement of how resistant a fluid is to flow. This viscosity accounts for the shapes of volcanoes and how they are formed.

    The viscosity is influenced by different agents, such as, heat, the type of magma, gases, and a few other smaller agents. All of these have an affect on the viscousness of the lava.

    The explosiveness of a volcano is also determined by viscosity, because the stiffer the viscous magma, the less the gases can escape, and the bigger the explosion will be, due to the gas build-up.

    The lava extruded from the shield volcano(basaltic) is characterized as low in viscosity, and therefore it is "runny" and does not tend to produce steep-sides, but instead flows over a greater area. The strato-volcano, on-the-other-hand, ejects a high viscous lava known as sialic lava, which causes great explosions when built-up and then forced out by gases trapped inside. This lava, being less fluid, does not flow as far from the cone, so it causes the volcano to build steep slopes.











    A volcanic eruption of Crater Peak, showing ash which is reaching 46,000 feet above sea level.









    Materials ejected from volcanoes



    The different materials ejected from volcanoes include, but are not limited to:



    Gases


  • Volcanoes eject different types of gases, the main ones being water vapor, carbon dioxide, nitrogen, sulfur, and other less abundant gases. These gases are the means by which the lava is ejected from the volcano, and they also prepare a passage-way for the magma to flow through inside of the volcano.




    Lava Flows


  • Lava flows are produced by the magma which reaches the earth's surface. This lava then begins to flow down the side of the mountian. One type of a lava flow is a Pahoehoe flow which has a smooth surface with wrinkles in it, which create a silky-apearance.
    Another lava flow is the aa flow which is rough in appearance, and is sharp and blocky. Both of these are basaltic lava flows which flow at a relatively slow pace, generally covering a great distance before they stop.




    Pyroclastic Materials


  • Pyroclastic material is characterized by the rapid expansion of lava, rocks, and glass fragments which blow out in many pieces varying greatly in size. Many times bombs and blocks are ejected from the volcano, but normally they fall close to the cone.




    Nuee-ardente


  • Nuee-ardente is an extremely fast-moving body of lava and gas. The lava actually floats on top of the gas which causes little friction, and thus the two travel down the mountian at speeds well over 60 miles-per-hour. The nuee-ardente is most commonly associated with the strato-volcano, and can be very devastating due to the rapid movement.



    For an interesting study on some of the materials ejected from volcanoes, as well as a look at the dangers of them, visit Volcano Hazards.














    Web Sites of recent eruptions



  • Bardarbunga and Grimsvotn Volcanoes is located in Iceland.

  • Loihi Volcano is located in Hawaii.

  • For a great look at the volcanoes in Hawaii, visit Virtual Hawaii, where you can go and view many of the volcanoes in Hawaii, as well as, see the cities located on them.

  • The Kuju Volcano is located in Japan.




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    Brian Jones: Jonesbr2@gpc.edu
    Dekalb College
    Document last modified on March 12, 1997.