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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 or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') means "little stones" in Latin. By definition lapilli range from in diameter.〔Fisher, R. 1961. Proposed classification of volcaniclastic sediments and rocks. Geol. Soc Amer.Bull., 72: 1409–1414〕 A pyroclastic particle greater than 64 mm in diameter is 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.〔(VHP Photo Glossary: Laplli ) in USGS Photo Glossary of volcano terms]〕〔(How Volcanoes Work )〕 ==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 not accretionary, but instead 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, 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 welded tuff. The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out as it they becomes 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lapilli」の詳細全文を読む スポンサード リンク
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