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tephra
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism.〔This is the broad definition of tephra (Greek ''tephra'', "ash") proposed by the Icelandic volcanologist Sigurður Þórarinsson (Sigurdur Thorarinsson) in 1954, in connection with the eruption of Hekla (Thorarinsson, "The eruption of Hekla, 1947-48II, 3, The tephra-fall from Hekla, March 29th, 1947", ''Visindafélag ĺslendinga'' (1954:1-3).〕
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff.
==Overview==
The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest and therefore closest to the vent, while smaller fragments travel further — ash can often travel for thousands of miles, even circumglobal, as it can stay in the stratosphere for days to weeks following an eruption.
When large amounts of tephra accumulate in the atmosphere from massive volcanic eruptions (or from a multitude of smaller eruptions occurring simultaneously), they can reflect light and heat from the sun back through the atmosphere, in some cases causing the temperature to drop, resulting in a temporary, "volcanic winter", climate change. Tephra mixed in with precipitation can also be acidic and cause acid rain and snowfall.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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