It's hot under Lake Rotomahana

03:14, Mar 06 2014

Scientists studying the large, active geothermal system under Lake Rotomahana near Rotorua have found two areas of high heat flow on the lake bed.

The researchers made the discovery while measuring the geothermal heat output from about 110 points on the bottom the lake. They are using data from the measurements to calculate the heat being produced by the entire lake.

The two dominant heat flow areas each measured roughly 1 kilometre by 500 metres, project leader Cornel de Ronde, of GNS Science, said.

The amount of heat coming through in those areas was enough to light up a 60 watt light bulb every square metre.

One of the areas of high heat flow was near the western end of where the famed Pink Terraces used to be before the 1886 eruption of nearby Mt Tarawera, de Ronde said. That meant the hydrothermal system there had carried on since the eruption.


The other area was on the margins of sediment which filled a V-shaped valley through the lake and was up to 40m deep.

Work was still being done to establish the source of the heat.

"There has to be some sort of volcanic heat sources underneath," de Ronde said.

"The heat-flow series proves that there is a lot of heat passing through the bottom of the lake."

It would almost certainly be related to some magma body underneath the lake.

GNS said the heat energy output from the areas of high heat flow, measured in watts per square metre, was about five times higher than similar measurements at hot vents on the sea floor of the Pacific.

Results of the study would be another piece in the puzzle to help scientists understand the size and the state of the magma body underneath that part of the Bay of Plenty.

Once the information was available, a comprehensive story could be put together about the evolution of the volcano-geothermal system since the 1886 Mt Tarawera eruption.

Maurice Tivey, a senior scientist with the Woods Hole Oceanographic Institution in the US, which is involved in the research, said some measurements were showing almost 90W per square metre of heat energy.

"In my career as a scientist studying heat flow, I really have not seen anything this strong," Tivey said on a GNS video about the work posted on YouTube.

"This is an incredible amount of heat coming out of the bottom of the lake."

The study also involves the National Oceanic and Atmospheric Administration from the US and the University of Waikato, and has the support of the Te Arawa Lakes Trust.

The lake floor measurements are being made by devices developed in the US, used only twice before and never before in New Zealand. The devices sit on the lake bed for 12 hours at each measuring place, collecting heat measurements, before being moved to the next spot.

Rotomahana, which is up to 125m deep and covers the remains of the Pink and White Terraces, was greatly altered by the eruption of Mt Tarawera on June 10, 1886.

Before then the lake measured only about 2km by 1km, but the deadly eruption opened up a long rift through the lake and blanketed the surrounding area with mud and rocks.

Rotomahana's outlet was blocked, the lake level rose, and it became much larger. Today the lake covers 800 hectares and measures roughly 3km by 6km at its widest and longest points.

It is within the Okataina Volcanic Centre – between Rotorua and Kawerau – where volcanoes tend to erupt every 700 to 3000 years.

According to GNS Science those eruptions can be between 100 to 10,000 times larger than the more common eruptions at White Island or Ngauruhoe. The Tarawera eruption was one of the smallest at Okataina in the past 21,000 years.