Fig. 6

Observation of lake colour and correlation with eruptive activity. a Observations of lake colour from 1963 to 2021. b Frequency of each lake colour during non-eruptive periods (defined as more than a month before an eruption). c Ruapehu eruptive activity from Scott (2013). The activity scale (which is different to Geonet’s volcanic alert levels) ranges from 1 for small phreatic eruptions confined to the Crater Lake to 5 for large scale explosive eruptions displacing moderate volumes of the lake. d Average lake colour prior to eruption for all eruptions with images within a one-month period prior to the eruption