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==Tephra==
==Tephra==
[[Image:Tephra.jpg|thumb|300px|Tephra erupted by Mount St. Helens on 18 May 1980 ranging in size from ash (left 2 piles) to [[lapilli]] (right 2 piles).]]
[[Image:Tephra.jpg|thumb|300px|left|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]].
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.<ref>http://volcanoes.usgs.gov/Products/Pglossary/tephra.html</ref>
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.<ref>http://volcanoes.usgs.gov/Products/Pglossary/tephra.html</ref>
== Supervolcano ==
[[File:375px-Long Valley Caldera cross section.svg.png|thumbnail|200px|right|Cross-section through Long Valley Caldera. The valley is one of the Earth's largest [[caldera]]s.]]
''See also:'' [[Supervolcano]]
According to the website Internet [[Geography]]:
{{Cquote|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]]).<ref>[http://www.geography.learnontheinternet.co.uk/topics/whatisasupervolcano.html What is a supervolcano?]</ref>}}
==External links==
==External links==