Rotorua Lakes Council geothermal safety inspector Peter Brownbridge talks about the eruption at Ohinemutu this morning.
The spectacular eruption of a geyser near the shores of Lake Rotorua comes as scientists are discovering stunning new insights into the city's famous geothermal backdrop.
Scientists say there isn't any evidence to link the eruption - which sent water gushing up to 30m into the air near Rotorua's Ohinemutu village early on Monday - with this month's earthquake activity.
GNS Science volcanologist Brad Scott said it was also too early to draw any connection with the gradual return of surface features within the Rotorua Geothermal Field that has followed the closure of bores in the 1980s.
Researchers are making fresh observations of the lively natural systems that create Rotorua's bubbling landscape. Photo / Alan Gibson
But it occurred at a time when researchers are making fresh observations of the lively natural systems that create Rotorua's bubbling landscape.
Over the past two months, geologists have teamed up with the New Zealand Defence Force for a bathymetric and magnetic survey of Lake Rotorua, which has yielded significant evidence of hydrothermal activity throughout the lake.
This included pockmarks on the lake floor, which indicated that gas was being discharged through the lake floor, and hydrothermal vents that showed the release of gas and hot water.
"Examples of hydrothermal activity seen on the lake floor include smaller hydrothermal eruption craters that are likely expelling hot water, and pockmarks or circular features that are several metres in diameter and are formed as a result of gas being discharged through the lake floor," GNS marine geologist Dr Cornel de Ronde said.
"Many of them appear in a linear pattern, suggesting they may be related to underlying faults."
The survey, which has so far covered 40 per cent of the lake floor but not the site of this week's eruption, would ultimately provide a base map showing the location of all underwater hydrothermal eruption craters and areas that were hazardous to vessels and sailors.
It was also the first step in a series of surveys to determine how much heat was being discharged through the lake floor from an underlying magma source.
Scott said this source was likely to be just a few kilometres below the field and large enough to be in the order of cubic kilometres.
"There are also a couple of electromagnetic surveys being done at the moment and the data we get from them is going to give us the best imaging we can get of that."
The Rotorua Geothermal Field, believed to have been active for tens of thousands of years, underlies much of the city and the southern fringe of Lake Rotorua, which itself was formed within a large caldera volcano that erupted 240,000 years ago.
The field is part of the wider Taupo Volcanic Zone, whose stretched and fractured crust has been permeated with magma that, in some places, has brought temperatures of at least 350C at depths of less than 5km.
A graphic of one of the vent holes found on the lake floor of Lake Rotorua by a New Zealand Defence Force survey team. The vent hole is 60 metres wide and six metres deep. Source / Supplied
Scott likened Rotorua's geological environment to that of a stovetop: the hot rock above the magma acting like an element, and the geothermal system closer to the surface being a pot of water boiling upon it.
While one of the biggest in recent times, Monday's 4am geyser eruption was consistent with what had happened in the area in the past.
"It's kind of what we expect," Scott said.
Rotorua Lakes Council geothermal inspector Peter Brownbridge said hydrothermal upwelling beneath the lake had created a geyser that was noisy and spectacular, but also said there was little reason for locals to be worried.
"It must have been quite powerful to throw up a big column of water as it did but it's nothing for people to be concerned about."
Hydrothermal eruptions happened "reasonably regularly" in that part of the lake edge - but more often than not they only resulted in the bubbling up of water and mud from the lake bottom.
Whakarewarewa Village. Photo / Stephen Parker
"We don't see many bigger ones these days although eruptions like this were quite common about eight years ago."
Brownbridge explained the eruption was like a cap blowing off a well-shaken bottle of fizzy drink, where the upwards pressure exceeded the capping weight.
"It is a build-up of pressure (steam) and not necessarily an increase in geothermal activity," he said.
"There's nothing we can do but we'll keep an eye on things and people can contact us if they have any concerns."
