The Povoaçمo Ignimbrite, Furnas Volcano, Sمo Miguel, Azores

The Povoaçمo Ignimbrite Formation (PIF) was emplaced by one of the larger explosive trachytic eruptions of Furnas Volcano, Sمo Miguel, Azores. Trachytic ignimbrites are common in the products of Furnas Volcano and examples of welding occur in at least three ignimbrites of which the Povoaçمo Ignimbrite is the most extensive. The PIF may correlate with the formation of the main caldera of Furnas. In the Povoaçمo Ignimbrite, the welded horizons thicken, without evidence of rheomorphism, into palaeovalleys and can be seen to 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 horizons. On interfluves, the ignimbrite is stratified and some of the welded horizons show pinch and swell and occasional cross-bedding. The welding is interpreted as a primary depositional feature with the clasts sintering on emplacement. It is argued that this ignimbrite was emplaced from a turbulent pulsatory pyroclastic flow. Some pulses were hotter which enabled more extensive development of welding. The flows became more concentrated and denser down valleys favouring the emplacement of thicker welded units.