“Te Wai ā-moe has a clear temperature cycle that we have observed since 2003,” GNS Science Duty Volcanologist Geoff Kilgour wrote in GeoNet’s Volcanic Alert Bulletin released June 6.
“During these cycles, the temperature ranges between ~12 and 40 °C (53 – 104 °F) over a period of ~12 months,” Kilgour said.
In February 2018, volcanologist Natalia Deligne described how the relatively long-period of elevated temperature of Te Wai ā-moe was coming to an end. During that elevated temperature period, the lake reached ~ 38 °C (100.4 °F) and scientists expected the lake to then cool as it has done many times before.
Over the past 2 months, the lake has indeed cooled to ~ 20 °C (68 °F) where it remained at this low temperature until Tuesday, May 29, when the lake starting heating again at a rate of ~ 1 °C per day.
Following previous heating cycles, scientists expect the lake to continue heating for the coming weeks.
Coincident with the increasing lake temperature, they have also noticed that the level of volcanic tremor has increased. “This is a characteristic feature of a heating cycle and represents the increased flow of hydrothermal fluids into the lake,” Kilgour said, adding that previous heating cycles at Ruapehu have shown this increased tremor to last for days to weeks.
“The Crater Lake has undergone many heating and cooling cycles in the past and we don’t see any unusual signs of increased unrest,” he said.
Therefore, current observations are consistent with minor unrest behavior and because of this, the volcano remains at Volcanic Alert Level 1 and the Aviation Colour Code stays at Green.
The last eruption of this volcano took place on September 25, 2007, and had Volcanic Explosivity Index of 3. Its last major eruptive phase started on June 16, 1996, and lasted until September 1 of the same year (VEI 3).
Ruapehu is a complex stratovolcano constructed during at least 4 cone-building episodes dating back to about 200 000 years ago. It is 110 km3 (26 mi3) dominantly andesitic volcanic massive and one of New Zealand’s most active volcanoes. It is surrounded by another 100 km3 (24 mi3) ring plain of volcaniclastic debris, including the Murimoto debris avalanche deposit on the NW flank.
Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3 000 years ago.
Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.
Featured image credit: GNS Science / GeoNet