Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size. [This is the broad definition of tephra (Greek "tephra", "ash") proposed by the Icelandic volcanologist 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).] Tephra is typically rhyolitic in composition, as most explosive volcanoes are the product of the more viscous felsic or high silica magmas.

Volcanologists also refer to airborne fragments as "pyroclasts" or sometimes just "clasts". Once clasts have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff. 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 several weeks. 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 climate change: "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 less than 2 mm in diameter
* Lapilli or volcanic cinders - between 2 and 64 mm in diameter
* Volcanic bombs or volcanic blocks - greater than 64 mm in diameter

The words "tephra" and "pyroclast" both derive from Greek. "Tephra" means "ash". "Pyro" means "fire" and "klastos" means "broken"; thus pyroclasts carry the connotation of "broken by fire".

The use of tephra layers, which bear their own unique chemistry and character, as temporal marker horizons in archaeological and geological sites is known as tephrochronology.


External links

* [http://www.geology.sdsu.edu/how_volcanoes_work/Tephra.html How Volcanoes Work]
* [http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/VolcanicMaterials/ Volcanic Materials Identification]

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