How Volcanoes Work

Volcano Shapes
Chile's Villarrica Volcano is a stratovolcano.
Chile's Villarrica Volcano is a stratovolcano.
© Stringer/Chile/Reuters/Corbis

Most land volcanoes have the same basic structure, but volcano shape and size varies considerably. There are several elements that these different volcano types have in common are:

  • a summit crater - the mouth of the volcano, where the lava exists
  • a magma chamber - where the lava wells up underground
  • a central vent - leads from the magma chamber to the summit crater.

The biggest variation in volcano structure is the edifice, the structure surrounding the central vent. The edifice is built up by the volcanic material spewed out when the volcano erupts. Consequently, its composition, shape and structure are all determined by the nature of the volcanic material and the nature of the eruption. The three main volcano shapes are:

  • Stratovolcanoes: These are the most familiar type of volcanoes, and generally have the most destructive history of eruptions. They are characterized by a fairly symmetrical mountain edifice, which curves steeply near the relatively small summit crater at the top. They are usually built by Plinian eruptions that launch a great deal of pyroclastic material. As the lava, ash and other material spews out, it rapidly builds the edifice around the vent. Stratovolcanoes tend to have highly infrequent eruptions -- hundreds of years apart -- and typically form in subduction zones.
Sunset Crater, a scoria cone volcano in Arizona
Photo courtesy USGS
  • Scoria cone volcanoes: These relatively small cones are the most common volcano type. They are characterized by steep slopes on both sides of the edifice, which lead up to a very wide summit crater. This edifice is composed of ashy tephra, usually spewed out by Strombolian eruptions. Unlike stratovolcanoes, many Scoria cone volcanoes have only one eruption event.
Mauna Loa, a shield volcano in Hawaii.
Photo courtesy USGS
  • Shield volcanoes: These wide, relatively short volcanoes occur when low-viscosity lava flows out with minimal explosiveness, such as in Hawaiian eruptions. The lava disperses out over a wide surface area -- sometimes hundreds of kilometers -- building up a shield-shaped dome. Near the summit, the edifice gets a little steeper, giving the volcano a slightly raised center. Many shield volcanoes erupt with great frequency (every few years or so).

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