What is tephra associated with?

Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.

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. Tephrochronology is a geochronological technique that uses discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which paleoenvironmental or archaeological records can be placed.

When a volcano explodes, it releases a variety of tephra including ash, cinders, and blocks. These layers settle on the land and, over time, sedimentation occurs incorporating these tephra layers into the fossil record. Often, when a volcano explodes, biological organisms are killed and their remains are buried within the tephra layer. These fossils are later dated by scientists to determine the age of the fossil and its place within the fossil record.

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 words "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".