Accretionary lapilli
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Lapilli_large.jpg
Lapilli_large.jpg
These accretionary lapilli on the surface of the Ka`u Desert south of Kilauea caldera. The lapilli formed during explosive eruptions of Kilauea in 1790 A.D.
Rounded tephra balls between 2 and 64 mm in diameter are called accretionary lapilli if they consist of tiny volcanic ash particles. Volcanic ash sometimes form such balls in an eruption column or cloud, owing to moisture or electrostatic forces. Lapilli (singular: lapillus) means "little stones" in Italian. Acretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus.
External link
- USGS Photo glossary of volcano terms (http://volcanoes.usgs.gov/Products/Pglossary/AccretLap.html)