Formation of the Crystal-Rich Glowing Avalanche Deposits of St. Vincent, B.W.I.

Andesitic glowing avalanches discharged from the Soufriere of St. Vincent on May 7, 1902, deposited ash which is noted for its fine-grained texture and richness in crystals. The largest avalanche deposit of this eruption contains several per cent of blocks and bombs and 15-30 per cent of lapilli. The median diameter in a representative sample is 0.6 mm., and the sorting coefficient is 2.9. Crystals form 45 per cent of the bombs and have been concentrated to form almost 58 per cent of all juvenile material in the deposit and 73 per cent of the juvenile ash finer than 2 mm. The glowing avalanches were discharged from the outer part of a vertical ash column which carried a large volume of ash to considerable elevations. When the column became denser than the surrounding atmosphere, it discharged clouds of pyroclastic material laterally. The clouds quickly became stratified into a lower, denser fraction (the glowing avalanches) containing the coarse ejecta and bulk of the crystals, and an upper part lighter a...