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.

What does the term tephra refer to?

Individual eruptive fragments are called pyroclasts ("fire fragments"). Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation. Whereas tephra is unconsolidated, a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type.

What is ash and tephra?

Volcanic ash is a term for fine-grained material that is ejected during explosive volcanic eruptions. Volcanic ash particles are smaller than 2 mm (0.08 inches) in diameter. Volcanologists use the word 'tephra' as general term for volcanic rock fragments irrespective of grain size produced during an explosive eruption.