The behaviour of subaerially produced pyroclastic flows in a subaqueous environment: evidence from the Roseau Eruption, Dominica, West Indies

The Roseau tephra (30,000 yrs B.P.) with its source on the island of Dominica represents the largest eruption in the Caribbean in the last 200,000 years. Pyroclastic flows generated by the eruption flowed offshore into the Grenada Basin giving rise to a widespread layer of sediment gravity-flow deposits. On land the flow deposits are valley-ponded and their granulometric characteristics allow them to be classified as weakly fines-depleted ignimbrites. Offshore, the characteristics of the sediment gravity-flow deposits are consistent with deposition by numerous small-scale events rather than a single large-scale event. A comparison of the on-land and offshore deposits reveals an increase in the crystal:lithic ratio in the latter. This change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water. Mixing and fragmentation occurred not at the air-water interface, but as flows descended the steep submarine slopes of the island of Dominica.