BH250-230a

BH250-230a

Accretionary Lapilli

Accretionary lapilli are small, rounded pellets of volcanic ash and dust that form in the atmosphere during explosive volcanic eruptions. They develop when fine ash particles collide and stick together in a moist eruption column or plume. Aggregation begins around a central nucleus—such as a glass shard or phenocryst—and as the particle is lofted by turbulent air, additional ash accretes onto it in concentric layers. Once formed, these lapilli typically fall close to the volcanic vent.

Compositionally, accretionary lapilli consist mostly of fine volcanic ash, rich in glass shards. Their distinctive concentric internal structure, resembling hailstones, makes them easily recognizable in the field. They often occur in layers within pyroclastic deposits, such as tuffs and ash beds.

These features are commonly well preserved and can become lithified in tephra of the same age. In the tuff deposits I studied in the Turkana Basin, many of the Plio-Pleistocene ash layers contain accretionary lapilli, providing evidence of moist eruptive conditions and proximal ash fall.

Location: Green Sand Beach, Hawaii, USA
GPS Coordinates: 18°56'11.41"N, 155°38'47.19"W

2022

Bereket Haileab

From the rock collection of Bereket Haileab. Sample BH250-230a. Housed at Carleton College in Minnesota.

Bereket Haileab

Hand sample