Scientists probe lake heat

GNS scientist Fabio Caratori Tontini (left) with Maurice Tivey of the US-based Woods Hole Oceanographic Institution. Photo / Alan Gibson

Project aims to boost understanding of the magma body that underlies Rotomahana.

From the steaming banks of Lake Rotomahana, watching a man hanging off the side of a boat and dropping a strange yellow object into the water may seem an odd sight.

But for scientists trying to understand the complex engine that heats the central North Island lake, the work couldn't be more serious.

Over the past few days, US geologist Dr Maurice Tivey has been working alongside the same GNS Science team who discovered remnants of the long-lost Pink and White Terraces beneath the lake two years ago.

This time, the scientists are carrying out a full geothermal survey of the lakebed, using technology never seen before in New Zealand.

For years, scientists have known there are two active geothermal systems under the 800ha lake, considered the warmest of the Rotorua lakes with year-long temperatures of 11C to 14C.

One of the systems is believed to be related to the 1886 Mt Tarawera eruption that destroyed the famed terraces.

The surrounding Okataina Caldera gives rise to a substantial amount of geothermal activity east of Rotorua, and was the source of the eruption.

To gain a detailed picture of these separate systems, scientists are dropping heat-measuring devices at more than 100 different points across the lakebed.

The heavy instruments are dropped from a boat and lie on the lakebed for 12 hours collecting measurements.

Scientists are also using a camera to take high-resolution photos of geothermal features on the lake floor, while collecting water samples.

Preliminary results from the first batch of measurements have shown a couple of hot spots where heat output is about five times higher than measurements at ocean vents in the Pacific.

Results for the entire lake will help scientists understand 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 system since the Tarawera eruption of 1886," said project leader Cornel de Ronde of GNS Science.

"The heat flow survey ... shows the amount of heat passing through the lake floor is truly impressive."

Dr Tivey, whose Massachusetts-based Woods Hole Oceanographic Institution pioneered the technology, said the rate of watts coming out of the lake was "ridiculously high".

The results would be compared against those from a survey in 1992.

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

Taking the temperature

* Scientists are using state-of-the-art measurement devices to gain a comprehensive picture of geothermal heat flow beneath Lake Rotomahana, south of Rotorua.

* The tube-like devices sit on the lakebed and have thermometers on either side, measuring heat coming from the bottom and the lake water temperature above.

* They are being deployed at 110 GPS points around the lake, and are moved on after gathering recordings for 12 hours.

* Preliminary results from the first batch of measurements have shown a couple of hot spots where heat output is about five times higher than similar measurements at ocean vents in the Pacific.

nzherald.co.nz

Watch the video here: tinyurl.com/geothermostudy.