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The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest, 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". Tephra mixed in with precipitation can also be acidic and cause acid rain and snowfall.
Tephra fragments are classified by size:
- Ash – particles smaller than 2 mm (0.08 inches) in diameter
- Lapilli or volcanic cinders – between 2 and 64 mm (0.08 and 2.5 inches) in diameter
- Volcanic bombs or volcanic blocks – larger than 64 mm (2.5 inches) in diameter
The word "tephra" and "pyroclast" both derive from Greek: τέφρα tephra means "ash", while the word pyroclast is derived from the Greek πῦρ (pyr), meaning "fire", and κλαστός (klastos), meaning "broken in pieces".
- 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 Íslendinga (1954:1-3).