Lapilli
Lapilli(sg.:lapillus) is a size classification oftephra,which is material that falls out of the air during avolcanic eruptionor during somemeteorite impacts.[1]Lapilliis Latin for "little stones".
By definition lapilli range from 2 to 64 mm (0.08 to 2.52 in) in diameter.[2]Apyroclasticparticle greater than 64 mm in diameter is known as avolcanic bombwhen molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to asvolcanic ash.[3][4]
Formation[edit]
Lapilli are spheroid-, teardrop-, dumbbell- or button-shaped droplets of molten or semi-moltenlavaejected 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.
Lapillituffsare a very common form of volcanic rock typical ofrhyolite,andesiteanddacitepyroclastic 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 andweldingof semi-molten lapilli into what is known as awelded tuff.
The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out and then become welded. Welded tufftexturesare distinctive (termedeutaxitic), 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 easilyeroded.
Accretionary lapilli[edit]
Rounded balls oftephraare calledaccretionary lapilliif they consist of layeredvolcanic ashparticles. Accretionary lapilli are formed by a process of wetash aggregationdue 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 withspheruliticandaxiolitictexture.
Armoured (or cored) lapilli[edit]
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[edit]
References[edit]
- ^Bron, K.A. (2010)."Accretionary and melt impactoclasts from the Tookoonooka impact event, Australia".In Reimold W.U. & Gibson R.L. (ed.).Large Meteorite Impacts and Planetary Evolution IV.Special Paper. Vol. 465. The Geological Society of America. p. 222.ISBN978-0-8137-2465-2.Retrieved22 May2011.
- ^Fisher, R. V. (1961). "Proposed classification of volcaniclastic sediments and rocks".Geological Society of America Bulletin.72(9): 1409–1414.Bibcode:1961GSAB...72.1409F.doi:10.1130/0016-7606(1961)72[1409:PCOVSA]2.0.CO;2.
- ^VHP Photo Glossary: Laplliin USGS Photo Glossary of volcano terms]
- ^How Volcanoes Work
External links[edit]
- Volcanic Materials Identification
- Tephra fall from Mt St. Helens
- Zentner, Nick (25 April 2019)."Supervolcanoes in the Pacific Northwest".YouTube.Archivedfrom the original on 2021-12-12.(segment on accretionary lapilli beginning at 45:04 in video)
