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Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption. Lapilli (singular: lapillus) means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.
By definition lapilli range in size from 2 mm to 64 mm in diameter. A pyroclastic particle greater than 64 mm in diameter is correctly known as a volcanic bomb when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash.
Lapilli are spheroid, teardrop, dumbbell, or button shaped droplets of molten or semi-molten lava ejected from a volcanic eruption which fall to earth while still at least partially molten. These granules are not accretionary, but are the direct result of liquid rock cooling as it travels through the air.
Lapilli tuffs are a very common form of volcanic rock typical of rhyolite, andesite and dacite pyroclastic eruptions. Here, large thicknesses 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 welded tuff.The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out as they become welded. Welded tuff 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lapilli". A list of authors is available in Wikipedia.|