Lapilli
In volcanology, lapilli ( Italian " little stones") refer to pea- to nut-sized (2–64 mm) pyroclasts , which are produced during an explosive volcanic eruption . The lapilli together with the volcanic ashes (<2 mm), the bombs (> 64 mm, rounded, originally melted) and the volcanic blocks (> 64 mm, angular, already solid at the time of ejection) form the pyroclastic sediments (also known as tephra) called if not solidified) or the pyroclastic rocks (if solidified).
Characteristics
The term lapilli only describes one grain size and is not a genetic name. Lapilli can therefore consist of scraps of lava , slag , remains of old chimney fillings or xenolites . Lapilli can form pyroclastic fall or flow deposits . A pyroclastic rock that consists mainly (> 75%) of lapilli is called lapilli stone. A lapilli tuff , however, is a pyroclastic rock that contains less than 25% bombs and blocks and more than 75% lapilli and ash. Overall, however, these deposits must consist of more than 75% pyroclasts .
Accretionary lapilli are lapilli- sized agglomerations of ash fragments that are produced by phreatomagmatic explosions . They arise when ash particles accumulate around a “core”. This nucleus can e.g. B. be a solid particle, but also a drop of water. In the latter way, they arise when raindrops fall through the eruption column and ash builds up around the drops. Accretionary lapilli occur in both pyroclastic fall and pyroclastic flow deposits.
literature
- Roger Walter Le Maitre: Igneous rocks: IUGS classification and glossary; recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks. 2nd ed., 236 pp., New York, Cambridge University Press 2002, ISBN 0-521-66215-X
- Hans Pichler: Italian volcanic areas III, Lipari, Vulcano, Stromboli, Tyrrhenian Sea . In: Collection of geological guides (vol. 69) Gebr. Bornträger, Stuttgart 1981, ISBN 3-443-15028-4
