The active vent-crater at Mt Ruapehu is occupied by a crater lake. Recently the lake has been cooling and we even discussed the possibility of new record low temperatures. The Crater Lake displays temperatures that typically range between about 15 and 40 °C. The lake has cooled to a minimum of 12 °C (15 August) but has now started to heat rapidly. GeoNet obtains temperatures from the lake using a data logger with a temperature sensor in the lake and communications via a satellite link.
In early August we discussed the possibility of the post 1995/1996 Crater Lake reaching a new low temperature as the lake was cooling strongly at that time. The lake reached a new minimum temperature of 12.0 °C on 15 August. For much of August the lake temperature ranged 13-14 °C, occasionally looking like it maybe going to turn and start heating. The temperature was starting to rise, very slowly, in late-August, but with quite a bit a lot of variability. However by 2 September a rising trend was clearly established. The lake temperature is now 17.6 °C.
On May 11 2016 the lake reached a high of 46°C, the highest we have observed since it reformed in 1999-2000. This high temperature was also accompanied by volcanic tremor and an increase in the output of volcanic gas. The Volcanic Alert Level (VAL) was raised to Level 2 at that time, lowering to Level 1 in early July when the gas output and volcanic tremor levels declined. About 2 days after the lake temperature stared to rise on 2 September, the level of volcanic tremor also started to rise and has remained present since 4 September. The heating and cooling cycles are controlled by a mix of volcano and geothermal processes. Further sampling and visits to the Crater Lake are planned as the weather allows, being part of the standard GeoNet monitoring programme for Mt Ruapehu.