The North Island of New Zealand is chock full of volcanoes—and big volcanoes at that. No less than four big calderas stay in the Taupo Volcanic Zone (TVZ) that stretches from White Island in the north to Ruapehu in the south. Some of the very violent volcanic eruptions in human history occurred in ~186 AD. over the caldera, what we call an “ultraplinian” eruption (and whatever is “ultra” has to be enormous). More recently, Tarawera in the Okataina caldera erupted in 1886 in one of the most explosive basaltic (low silica) eruptions on record that had ash plume that reached 10 kilometers (~32,000 feet) and buried a number of towns along with blasting a new valley out of the landscape. The North Island is definitely a place where volcanic unrest is taken.
Thus, the somewhat surprising steam explosions that took place this week have caught a lot of people’s focus. A sizable steam explosion occurred near the coastlines. Afterward, on December 30, the caldera had a second, smaller explosion. The function was brief and around by morning, so very few people really saw the explosion occur. This came as a surprise the volcano observation agency of New Zealand, to GNS Science and was the first hydrothermal explosion at Rotorua in 15 years. Folks close to the explosion reported hearing low, rumbling noises before the blast, which probably were brought on by the movement of hot water and steam subsurface.
The 2nd explosion came on Wednesday (November 30) and this one was caught on video. In that occasion, you may begin to see the lake surface begin to churn with hefty steam and dark material (likely sediment from the lake bottom). This blast appears considerably smaller, with water and debris just reaching several meters (10 feet) over the lake surface.
Hydrothermal explosions such as all these are common in calderas. But, the trigger of those explosions is not clear. GNS Science has implied they might be associated with the recent spate of earthquakes New Zealand has experienced since the M7.8 on November 13. This could make sense: Hydrothermal systems under volcanoes are proven to readjust after earthquakes because of movement on the faults that riddle the earth beneath the TVZ. Brad Scott from GNS Science implied that it’s likely linked to what triggered the first blast, and the next blast might have also been related to recent weather.
Nonetheless, Scott has said that these new steam explosions do not imply that the Rotorua caldera is heading towards any kind of volcanic activity; GNS Science hasn’t increased the alert status for the volcanic system. Steam explosions are driven by building up of steam before the cap of rock above can’t hold the pressure— . These steam explosions, although relatively infrequent at Rotorua, do occur and no eruption has occurred near the lake in 25,000 years. The explosions that occurred during the 1980s- drilling probably helped along 2001 during efforts to exploit the hydrothermal system.
Anybody who has been to Rotorua can attest to its volcanic nature (see previously). There are geysers and mudpots on one side of town, including the Pohutu geyser that is remarkable, buildings happen to be closed due to volcanic gases seeping into their cellars, and several sewer grates steam from heat and the gases of the magma deep below the road. It is actually a remarkable place, as if a small city was built by you right in addition to the hydrothermal aspects of Yellowstone Caldera. Although steam explosions such as these two might look spectacular, they’re not signals of impending destruction at Rotorua, but instead simply continuing evidence of the volcanic nature of New Zealand.