The Povoacao Ignimbrite, Furnas Volcano, Sao Miguel, Azores

The Povoacao Ignimbrite Formation (PIF) was emplaced by one of the larger e xplosive trachytic eruptions of Furnas Volcano, Sao Miguel, Azores. Trachyt ic ignimbrites are common in the products of Furnas Volcano and examples of welding occur in at least three ignimbrites of which the Povoacao Ignimbri te is the most extensive. The PIF may correlate with the formation of the m ain caldera of Furnas. In the Povoacao Ignimbrite, the welded horizons thic ken, without evidence of rheomorphism, into palaeovalleys and can be seen t o thin and in some places become completely attenuated over old ridges. The welded horizons are intimately associated with non-welded ignimbrites and in some places there is an alternation between welded and non-welded horizo ns. On interfluves, the ignimbrite is stratified and some of the welded hor izons show pinch and swell and occasional cross-bedding. The welding is int erpreted as a primary depositional feature with the clasts sintering on emp lacement. It is argued that this ignimbrite was emplaced from a turbulent p ulsatory pyroclastic flow. Some pulses were hotter which enabled more exten sive development of welding. The flows became more concentrated and denser down valleys favouring the emplacement of thicker welded units. (C) 1999 El sevier Science B.V. All rights reserved.