Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism.[1]
Tephra horizons in south-central
Iceland. The thick and light coloured layer at the centre of the photo is
rhyolitic tephra from
Hekla.
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.