Tephra block (also called a ballistic) on the crater floor of Mount St. Helens about 100 m from the lava dome. This lava fragment was blasted from the dome by a short-lived explosion caused by either (1) a vigorous release of gas from magma within or beneath the dome; or (2) by superheated groundwater "flashing" to steam. The rock broke apart when it landed on the crater floor. Note black camera lense cap for scale.
The hazard from the impact of large tephra fragments is greatest close to a volcanic vent (within 5-10 km) and decreases with increasing distance. For example, a few explosions at Mount St. Helens during a period of dome growth between 1981-1986 hurled dense lava blocks as large as 1 m in diameter as far as 2 to 3 km from the dome.
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Last modified: October 14, 1998