How Volcanoes Work



There are about 550 volanoes on earth that have erupted in historic times. Such volcanoes are considered to be geologically active (see: What is an "active" volcano?). In addition, there are an equivalent number of dormant volcanoes that have not erupted in historic time, but have erupted in the past 10,000 years. Both dormat and "active" volcanoes have the potential to erupt again. On any given day, there are about ten volcanoes that are actively erupting.

Eruptions are highly variable in size and explosiveness. As the table below demonstrates, small eruptions are more frequent than larger eruptions. It takes a greater amount of time to build up the necessary gas pressures needed for the larger eruptions. The global frequency of small eruptions, producing 0.001 to .01 cubic kilometers of volcanic material, is once every few months, whereas the frequency of very large eruptions, producing thousands of cubic kilometers of ash, is about once every 100,000 years. The last truly giant eruption on earth occurred at the Toba volcano on Sumatra 74,000 years ago. It produced ~2,800 cubic kilometers of ash, more than 2000 times the amount generated by the 1980 eruption of Mt. St. Helens!

 SIZE (cubic km)


FREQUENCY (every . .)


Kilauea, Unzen

several months



5 years


 St. Helens (1980)

 10 years



100 years


 Krakatau (1883), Katmai (1912)

 1000 years


 Tambora (1815)

10,000 years


 Yellowstone, Toba

100,000 years

MEASURING EXPLOSIVENESS: The Volcano Explosivity Index (VEI)

It is extremely difficult to assess the magnitude of an eruption in a truly quantitative way. However, an approximation of the explosiveness of an eruption can be obtained from the degree that the airborne volcanic products (tephra) have been fragmented. The greater the explosivity, the greater the fragmentaion of the tephra deposits. Since most eruptions on earth have not been observed by humankind, the degree of tephra fragmentation is the only criterion we can use to determine the explosivity of ancient (non-observed) eruptions.

However, for historic eruptions, that have been observed, we can use additional criteria. Chris Newhall of the U.S. Geological Survey and Steve Self at the Open University(UK) have developed a simple, semiquantitative scheme for estimating the magnitude of historic eruptions, called the Volcanic Explosivity Index (VEI). Historical eruptions can be assigned a VEI number on a scale of 0 to 8, using one or more of the following criteria:

  • Volume of ejecta
  • Height of the eruptive column
  • Qualitative descriptions ("gentle", "effusive", "explosive", "cataclysmic", etc.)
  • Style of past activity
  • Height of spreading of the eruptive plume head (in troposphere or stratosphere)
  • The volume of ejecta and the plume height of the eruption column are probably the two most reliable criteria to use in giving an eruption a VEI number. The VEI numbers below correspond the the following eruption characteristics:




    (See Eruption Types)



     <100 m

     1000s m3




     100-1000 m

     10,000s m3




     1-5 km

     1,000,000s m3


     Galeras (1992)


     3-15 km

     10,000,000 m3


    Ruiz (1985)


     10-25 km

     100,000,000s m3


     Galunggung (1982)


     >25 km

     1 km3


    St. Helens (1980)


     >25 km

     10s km3


     Krakatau (1883)


     >25 km

     100s km3


     Tambora (1815)


     >25 km

     1000s km3


     Toba (74 ka)

    The VEI is similar to the Richter scale for measuring magnitude, in that each interval on the VEI represents an increse in magnitude of about 10 (i.e., it is logarithmic). The VEI has been used by workers of the Smithsonian Institution to assign magnitude to Holocene (<10,000 years before present) volcanoes in the Catalog of Active Volcanoes of the World. No Holocene volcano has been assigned a VEI of 8, although four eruptions, including Tambora (1815) have been assigned a VEI of 7.

    For more information on the relationship of eruption types to column height and the explosivity of an eruption, see Explosivity and the Eruption Column.

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