Furnas volcano is the easternmost of the three active central volcanoes of São Miguel and corresponds to a nested caldera complex (7x6 km wide). Like the other two active central volcanoes of the island, Furnas caldera is partially occupied by a lake. The recent eruptive history of Furnas (last 5 ky) is marked by, at least, 10 intra-caldera trachytic Sub-Plinian eruptions, which shifted eruptive style between magmatic and hydromagmatic phases. Three of these eruptions are known to have produced lava domes in their final stages.
Here, we present the detailed stratigraphy and physical parameters of Furnas E (1520±30 yr BP). Based on its internal characteristics, it was possible to divide the Furnas E deposit into three main units: The basal ash unit (BA) is mostly composed of fine-grained ash, with some stratifications of coarser-grained ash and finegrained pumice lapilli. The P1 unit corresponds to a pumice lapilli bed, occasionally with a coarse-grained ash matrix. The top unit of the deposit is characterised by a fine-grained ash layer (TA), which is frequently altered into a paleosol.
The Furnas E eruption was sourced from the area of Pico das Marconas or Pico das Caldeiras and its pyroclastic products were dispersed predominantly along a NNE to NE direction. A trachytic dome was extruded in the last stage of the eruption. A total tephra volume of 0.24 km3 was estimated for the Furnas E eruption, corresponding to a VEI 4 event, which yields a DRE volume of 0.11 km3 . A maximum eruption column height was estimated at 19 km, which corresponds to a mass eruption rate of 4.2 x107 kg/s.
The stratified nature of the Furnas E deposit suggests that the eruption alternated between magmatic and hydromagmatic pulses. This is a common feature to other recent Sub-Plinian eruptions of the three active central volcanoes of São Miguel, indicating that this is a recurrent pattern due to the existence of important hydrologic systems on the island.