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Scientists probe Lake Rotomahana's volcanic heat engine

6 March 2014

Scientists probe Lake Rotomahana's volcanic heat engine

The scientists who found remnants of the Pink and White Terraces under Lake Rotomahana two years ago are back at the lake this week measuring its geothermal heat output.

For many years scientists have known there is a large active geothermal system under the lake, and this will be the most sophisticated attempt at measuring the heat output of the 800 hectare lake.

To achieve this they are using state-of-the-art heat measuring devices that are being used in New Zealand for the first time. The instruments sit on the lakebed for up to 24 hours collecting heat measurements before being moved to a new spot.

They are also using a camera to take high-resolution photos of geothermal features on the lake floor and are collecting water samples from the lake floor for analysis.

The project is being led by GNS Science in collaboration with Woods Hole Oceanographic Institution and the National Oceanic and Atmospheric Administration - both from the US - and the University of Waikato. The project also has the support of the Te Arawa Lakes Trust.

The scientists are measuring heat being produced at 110 grid points on the floor of the lake to calculate the heat being produced by the entire lake. This is only the third time the US-developed heat-measuring devices have been used, and the first time they have been deployed in a lake.

Preliminary results from the first batch of measurements have shown a couple of hot spots where heat energy output (watts per square metre) is about five times higher than similar measurements at hot vents on the seafloor of the Pacific.

Results for the entire lake will represent another piece in the puzzle to help scientists understand the size and the state of the magma body that underlies this part of the Bay of Plenty.

"Once we have the information about the lake's heat energy output, we will be able to put together a comprehensive story on the evolution of the volcano-geothermal system since the Tarawera eruption of 1886," said project leader Cornel de Ronde of GNS Science.

"The initial results show the amount of heat passing through the lake floor is truly impressive. There's about two square kilometres of the lake floor where there is enough heat energy to power a 60 watt lightbulb every square metre," Dr de Ronde said.

Te Arawa Lakes Trust Chief Executive Roku Mihinui said the survey had given a clearer picture of what the Tarawera eruption did to Lake Rotomahana and the surrounding landscape where some of the Trust's ancestors had lived.

"It has also given some inkling of the latent power of the geothermal activity under the lake, as well as potential development opportunities and possible risks."

The were was still more to learn and the Trust hoped that the New Zealand and American scientists would continue their research, Mr Mihinui said.

Lake Rotomahana is the warmest of the Rotorua lakes and sits at about 11 to 14 degrees Celsius throughout the year.

The Okataina Volcanic Centre gives rise to a substantial amount of volcanic and geothermal activity east of Rotorua, and was the source of the eruption of Mount Tarawera in June 1886.

The Okataina Volcanic Centre lies east of Rotorua and formed between 250,000 and 50,000 years ago. It has erupted six times in the past 10,000 years, most recently with the eruption of Mount Tarawera in 1886. The time between eruptions from this Centre is long, between 700 and 3000 years. Eruptions from this type of volcanic centre are usually many times larger than those from cone volcanoes such as Ruapehu, Ngauruhoe, and Tongariro.


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