View source for Lapilli

{{Short description|Small pyroclast debris thrown in the air by a volcanic eruption}}
[[File:PuuPuaiLapilli large.jpg|thumb|right|340px|Lapilli on [[Kilauea]]]]

'''Lapilli''' ({{singular}}: '''lapillus''') is a size classification of [[tephra]], which is material that falls out of the air during a [[volcano|volcanic eruption]] or during some [[meteorite impact]]s.<ref name="Bron">{{cite book|last=Bron|first=K.A.|editor=Reimold W.U. & Gibson R.L.|title=Large Meteorite Impacts and Planetary Evolution IV|chapter-url=https://books.google.com/books?id=YB6Xw6rYwksC&q=accretionary+lapilli+meteorite+impact+events&pg=PA222|access-date=22 May 2011|series=Special Paper|volume=465|year=2010|publisher=The Geological Society of America|isbn=978-0-8137-2465-2|page=222|chapter=Accretionary and melt impactoclasts from the Tookoonooka impact event, Australia}}</ref> ''Lapilli'' is Latin for "little stones".

By definition lapilli range from {{convert|2|to|64|mm|in|2|abbr=on}} in diameter.<ref>{{cite journal|last1=Fisher|first1= R. V.|date=1961|title= Proposed classification of volcaniclastic sediments and rocks|doi=10.1130/0016-7606(1961)72[1409:PCOVSA]2.0.CO;2|journal=Geological Society of America Bulletin|volume= 72|issue=9|pages= 1409–1414|bibcode=1961GSAB...72.1409F}}</ref> A [[Pyroclastic rock|pyroclastic]] particle greater than 64&nbsp;mm in diameter is known as a [[volcanic bomb]] when molten, or a volcanic block when solid.  Pyroclastic material with particles less than 2&nbsp;mm in diameter is referred to as [[volcanic ash]].<ref>[http://volcanoes.usgs.gov/images/pglossary/lapilli.php VHP Photo Glossary: Laplli] in USGS Photo Glossary of volcano terms]</ref><ref>[http://www.geology.sdsu.edu/how_volcanoes_work/Tephra.html How Volcanoes Work]</ref>

==Formation==
Lapilli are spheroid-, teardrop-, dumbbell- or button-shaped droplets of molten or semi-molten [[lava]] ejected from a volcanic eruption that fall to earth while still at least partially molten. These granules are the direct result of liquid rock cooling as it travels through the air.

Lapilli [[tuff]]s are a very common form of volcanic rock typical of [[rhyolite]], [[andesite]] and [[dacite]] pyroclastic eruptions, where thick layers of lapilli can be deposited during a basal surge eruption.  Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and ''welding'' of semi-molten lapilli into what is known as a [[Tuff#Welded tuff|welded tuff]].

The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out and then become welded. Welded tuff [[rock microstructure|textures]] are distinctive (termed ''[[eutaxitic]]''), with flattened lapilli, [[fiamme]], blocks and bombs forming oblate to discus-shaped forms within layers.  These rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs, which are unconsolidated and easily [[eroded]].

===Accretionary lapilli===
[[File:Accretionary Lapilli - geograph.org.uk - 831917.jpg|thumb|Accretionary lapilli in the [[Mesoproterozoic]] [[Stac Fada Member]] of the [[Torridonian]], of probable impact origin]]
Rounded balls of [[tephra]] are called ''accretionary lapilli'' if they consist of layered [[volcanic ash]] particles.  Accretionary lapilli are formed by a process of wet [[Volcanic ash aggregation|ash aggregation]] due to moisture in volcanic clouds that sticks the particles together, with the volcanic ash nucleating on some object and then accreting to it in layers before the accretionary lapillus falls from the cloud.  Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus.

This texture can be confused with [[spherulite|spherulitic]] and [[axiolitic]] texture.

====Armoured (or cored) lapilli====
These lapilli are a variety of accretionary lapilli, though they contain lithic or crystal cores coated by rinds of coarse to fine ash.  Armoured lapilli only form in hydroclastic eruptions, where significant moisture is present.  The vapour column contains cohesive ash which sticks to particles within it.

== See also ==
* [[Tuff]]
* [[Scoria]]
* [[Rock microstructure]]

==References==
<references />

== External links ==
{{Wiktionary|lapilli}}
* [https://web.archive.org/web/20051216120850/http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/VolcanicMaterials/ Volcanic Materials Identification]
* [http://pubs.usgs.gov/pp/p1563/ Tephra fall from Mt St. Helens]
* {{cite web|author=Zentner, Nick|title=Supervolcanoes in the Pacific Northwest|date=25 April 2019|website=YouTube|url=https://www.youtube.com/watch?v=NcreTTI9Rew&t=2704 |archive-url=https://ghostarchive.org/varchive/youtube/20211212/NcreTTI9Rew| archive-date=2021-12-12 |url-status=live}}{{cbignore}} (segment on accretionary lapilli beginning at 45:04 in video)

{{Volcanoes}}

[[Category:Tephra]]
[[Category:Petrology]]
[[Category:Volcanic rocks]]