Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size. 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 as it can stay in the stratosphere for several weeks.
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 as temporal marker horizons is known as tephrochronology.
