Fig. 1

a Aso caldera and Nakadake volcano in north-central Kyushu. b Google Earth snapshot of Nakadake crater; the star represents the site from where ground-based measurements were collected. The Yudamari crater lake and the fumarole field are also shown by cyan and red contours, respectively. c Schematic diagram of the cyclic volcanic processes at Nakadake crater (modified after Sudo 2001). The three representative Landsat-class images (false color RGB; R: 2.2 µm; G: 1.6 µm; B: 0.8 µm) at the vertexes of the triangle illustrate the thermal pattern during the principal phases of activity (crater lake → high fumarolic → strombolian). Landsat images courtesy of ESA Copernicus Service Data Hub. See the text for details