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Snow-covered Kanaga Volcano in Alaska erupts a small column of tephra, gas, and steam. Kanaga is a stratovolcano. View is toward the west.

A volcano forms when molten rock, in the form of lava, is able to breach the surface of the Earth. Many volcanoes take the shape of mountains, although some are so large that they form entire islands, as is the case of Mauna Loa. Volcanoes come in three main types, stratovolcano (also composite cone), shield volcano, and cinder cone. Stratovolcanoes are renowned for their explosiveness, which is cause by the high viscosity of the magma they contain. They can grow to be very large and have steep slopes, but in some eruptions can blow themselves apart. In contrast, shield volcanoes contain magma of very low viscosity, resulting in less violent eruptions and shallow slopes. Finally, cinder cones form very fast and consist of amalgamated igneous materials. They have very steep slopes and are susceptible to erosion.


Tephra erupted by Mount St. Helens on 18 May 1980 ranging in size from ash (left 2 piles) to lapilli (right 2 piles).

Tephra is a general term for fragments of volcanic rock and lava that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Tephra includes large dense blocks and bombs, and small light rock debris such as scoria, pumice, reticulite, and volcanic ash.

As tephra falls to the ground with increasing distance from a volcano, the average size of the individual rock particles becomes smaller and thickness of the resulting deposit becomes thinner. Small tephra stays aloft in the eruption cloud for longer periods of time, which allows wind to blow tiny particles farther from an erupting volcano.[1]


Cross-section through Long Valley Caldera. The valley is one of the Earth's largest calderas.

See also: Supervolcano

According to the website Internet Geography:

A supervolcano is any volcano capable of producing a volcanic eruption with an ejecta volume greater than 1,000 km3 (240 cu mi). This is thousands of times larger than normal volcanic eruptions. Supervolcanoes are on a much bigger scale than other volcanoes. Unlike composite volcanoes, with their steep sides, they are difficult to spot. They are typically depresssions in the ground. The calderas are so large they can be seen from space. They have been identified in Indonesia, in New Zealand, in South America and an extinct one in Glen Coe in the UK.

Supervolcanoes can occur when magma in the mantle rises into the crust from a hotspot but is unable to break through the crust, and pressure builds in a large and growing magma pool until the crust is unable to contain the pressure (this is the case for the Yellowstone Caldera).[2]

External links

  • Interactive Volcanoes There are many active volcanoes worldwide. Is there anything we can do to predict how and when they will erupt?


  1. http://volcanoes.usgs.gov/Products/Pglossary/tephra.html
  2. What is a supervolcano?