While we were sampling at the hydrothermal field at Furnas, workshop participant Matteo Masotta (Istituto Nazionale di Geofisica e Vulcanologia, Italy), in collaboration with Vittorio Zanon (University of Azores) analyzed the stratigraphy of the area.
The fumarole field of Furnas Lake Volcano is imposed on a sequence of lacustrine sediments deposited in recent times. The stratigraphy of this lacustrine deposit has been investigated in detail in an outcrop occurring at the north side of the fumarole field (Figure 1a). The deposit consists mostly of muds deposited either by direct sedimentation from the lake water table variations or by lahars (Figure 1b).
The sequence opens at the base, with a ~60 cm-thick, massive layer of hydrothermally altered consolidated silt. The surface temperature of the basal layer ranges between 40˚C at the base and 30˚C at the top. The basal layer fades upward into a softer, equally altered, ~20 cm-thick layer of consolidated silt. A ~20 cm-thick layer of less altered brown silt extends on top of these two layers. This layer contains cm-sized, altered white pumices and lithic clasts. Above this layer, three thin (<10 cm) layers of finely stratified ash lay over the basal sequence and contain rare, mm-sized lithic clasts. The stratigraphic sequence closes at the top with another massive, 30-50 cm-thick, partially pedogenized silty layer, overlaid by a 20-40 cm-thick vegetated soil.