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

Abstract

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 and richer in gas and fine particles of vitric ash. A third or more of the material discharged was dispersed in the light, gas-rich clouds which accompanied the avalanches during their descent. Glowing avalanches discharged by prehistoric eruptions which reached a climax 4,000 years ago deposited materials essentially similar to those of 1902. The prehistoric avalanche deposits, together with mudflows and stream-laid materials, filled valleys on the western side of the Soufrière and formed a large fan at its eastern foot. The St. Vincent glowing avalanche deposits are chemically, mineralogically, and textura Jly rather similar to a number of pumice- and scoria-flow deposits. Crystals have been concentrated in the ash fraction of pumice-flow deposits of Crater Lake and Asama beyond their proportion of the magma, probably by the same mechanism as they were concentrated on St. Vincent. A scarcity of bombs in the St. Vincent deposits is apparently the only major difference between these and the scoria- and pumice-flow deposits of other volcanoes.