Wednesday, May 5, 2010

Tephra is airborne fragmental material ejected by a volcanic eruption regardless of composition, fragment size or emplacement mechanism. Tephra is typically rhyolitic in composition, as most explosive volcanoes are the product of the more viscous felsic or high silica magmas. The term "tephra" refers to particles that were erupted into the air and then fell back to the ground or to deposits of those particles. The term was introduced by Thorarinsson (1944, 1954) to describe volcanic ash and coarser detritus that were projected through the air.

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. When large amounts of tephra accumulate in the atmosphere from massive volcanic eruptions, they can reflect light and heat from the sun back through the atmosphere, causing the temperature to drop, resulting in a climate change known as volcanic winter.

Tephra fragments are classified by size: 1) ash, which is particles smaller than 2 mm in diameter; 2)lapilli, which is between 2 and 64 mm (0.08 and 2.5 inches) in diameter; 3) volcanic bombs or volcanic blocks, which are larger than 64 mm in diameter. The words "tephra" and "pyroclast" both derive from Greek and means "ash".