Hidden wonders discovered in Lake Rotorua’s depths
Scientists have mapped the floor of Lake Rotorua to reveal thousands of pockmarks, an ancient river, and what could be an undiscovered hydrothermal system.
A geologist involved in the yearslong effort says the just-released maps point to the potential for lakes in the famously active region to host “significant” geothermal resources.
The new insights span back to detailed surveys carried out with the New Zealand Defence Force in 2016 and 2017 to map the lake’s floor and the complex geophysical processes playing out beneath it.
GNS Science’s Dr Cornel de Ronde said the mapping, covering nearly 70 per cent of the lake, down to 1m resolution, turned up a range of features deep beneath the surface, some of which hadn’t been observed before.
Among them was evidence of a long-lost river that would have meandered from Rotorua’s Sulphur Point to the west of Motutaiko Island, before curving to the northeast.
It offered a fascinating window to how the landscape might have appeared before the lake was formed within a large caldera volcano that erupted 240,000 years ago.
The lake itself, lying over part of the Rotorua Geothermal Field, is thought to be at least 65,000 years old and has been at its current level for some 22,000 years.
De Ronde and colleagues were similarly surprised at the thousands of pockmarks dotted across the lake floor, some of which spanned more than 50m in diameter.
All these indicated gas was being discharged from below, he said.
The largest was likely releasing methane stemming from decomposing organic matter.
In another intriguing discovery, the team found evidence of a doughnut-shaped ring of negative heat flow on the lake floor surrounding a circular area of positive heat flow, north-west of Mokoia Island, likely linked to a magnetic anomaly in the same area.
“This suggests a possible, separate, geothermal system in the lake associated with a magma body with recharge by lake water.”
Closer to the city, just off Sulphur Point, lay another active system with high heat flows and numerous hydrothermal eruption craters still belching hot water and gas.
De Ronde said this appeared to be a kilometre-long extension of the geothermal system lying beneath the city, making it much larger than once thought.
The surveys happened to be carried out just as a geyser near Rotorua’s Ohinemutu village sent water gushing up to 30m into the air.
They’ve taken scientists closer to determining just how much heat was being discharged through the lake floor from an underlying magma source, likely to sit just a few kilometres beneath the field.
“This work has also shown — as with other lakes in the Rotorua area, such as Rotomahana and Rotoiti — the potential for significant geothermal resources to be hosted by lakes.”
De Ronde said more sampling could help compare hydrothermal processes beneath and offshore of the city, which would better model the whole Rotorua geothermal system.