Scientists from the New Zealand and the United States are planning to use two torpedo-like unmanned underwater vehicles to map the bottom of Lake Rotomahana, near Rotorua, and look for hydrothermal activity on the lake bed.
The project, scheduled to start in late January, will be the first time in New Zealand that a lake bed has been mapped by autonomous underwater vehicles, or AUVs.
As well as producing a detailed three-dimensional map of the lake bed, the sophisticated sensors on the AUVs will enable the scientists to locate hydrothermal vents at the bottom of the lake.
The present-day chemistry of the lake indicates that geothermal fluids are pouring into it from below. Scientists are keen to find out the number of vents on the lake bed, their locations, and the intensity of venting.
The project is a collaboration involving GNS Science, the Woods Hole Oceanographic Institute (WHOI) in the US, the University of Waikato, and the Te Arawa Lakes Trust Board.
Lake Rotomahana, roughly 3km by 6km in size, enlarged in size after the eruption of Mt Tarawera in 1886, which is thought to have destroyed and drowned the famed Pink and White Terraces. The Terraces, or their remnants, are believed to be buried at the bottom of the lake.
The battery-powered AUVs will motor through the lake at about walking pace on a pre-programmed grid pattern to produce a highly accurate three-dimensional view of the lake bed.
Other measurements they will take include temperature, pH, conductivity, depth, optical clarity of the water, and the electrical potential, or Eh, of the lake water. The vehicles will also be able to map the magnetic signature of the volcanic rocks beneath the lake floor and are equipped with side-scan sonar, which helps to distinguish different rock types.
Scientists will compile the data into a layered map of the lake bed which will elucidate the nature of the geology and hydrothermal activity at Rotomahana.
Project leader, Cornel de Ronde of GNS Science, said there were very few examples of hydrothermal activity in freshwater lakes in the world, and even fewer had been studied in detail.
"Our aim is to determine what happened to the Pink and White Terraces hydrothermal system when it was drowned in the enlarged Lake Rotamahana soon after the 1886 eruption," Dr de Ronde said.
"We also want to know what links there are between the drowned geothermal systems of Lake Rotomahana and the adjacent geothermal system at Waimangu.
"This is a rare opportunity to document the death of a land-based geothermal system and its rebirth at the bottom of a lake."
The accuracy of the mapping will enable the scientists to see features on the lake bed about the size of a suitcase.
When vents are located and given a GPS coordinate, scientists will lower instruments from a boat to determine temperature, flow rate, and chemical composition of the vent fluids. This will help build a computer model of the hydrothermal and magma systems beneath the lake.
Prior to 1886, the Pink and White Terraces were considered an international marvel and were sometimes referred to the eighth natural wonder of the world. They were the largest silica terraces in the world and represented an enormous outflow of geothermal fluid.
Scientists believe the Terraces, or their remnants, are today covered by at least 50m of lake water plus an additional unknown thickness of sediment. Lake Rotomahana is 115m deep at its deepest point.
GNS Science, with the support of the Royal Society, will be offering activities to Rotorua primary and intermediate schools in connection with this project. Information on the activities will be sent teachers in the Rotorua area this week.
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