Scientists from New Zealand and the United States who found part of the Pink Terraces in Lake Rotomahana in January today confirmed they have also found remnants of the White Terraces on the lake floor.
The find comes from side-scan sonar data of the lake floor collected on the last day of the 10-day project at Lake Rotomahana last summer.
Using new software, which became available after the data collection phase of the project had finished, the scientists found the sonar data contained images of hard, crescent-shaped structures on the lake bed in a similar location to where the White Terraces were before the eruption of Mt Tarawera in June 1886.
Scientists load an autonomous underwater vehicle (AUV) onto a support boat during the underwater survey of Lake Rotomahana in January 2011. Photo: Julian Thomson, GNS ScienceThe structures are at roughly 60m depth - a similar depth to the Pink Terraces which were found in January. The lake is about 122m deep at its deepest point.
The Pink and White Terraces were buried by the eruption of Mt Tarawera 125 years ago on 10 June 1886. Prior to that, they were New Zealand's premier tourist attraction. The two sets of cascading silica terraces were separated by several hundred metres.
Project Leader Cornel de Ronde, of GNS Science, said the sonar images from Lake Rotomahana showed the lake floor was covered overwhelmingly by soft sediment and mud.
"The two places on the lake floor where we encountered hard, up-standing crescent-shaped features correspond to the locations of the Pink and White Terraces before the Tarawera eruption," Dr de Ronde said.
"The sonar image that appears to show part of the White Terraces came to light after the project had finished. It shows a horizontal segment of terraces over 100 meters long, although we don't know which part of the terraces it is.
"The rounded terrace edges are standing up from the lake floor by about a metre in some places. The sonar images of both sets of terraces are strikingly similar."
Scientists managed to capture several colour photographs of part of the Pink Terraces in January, but they did not lower an underwater camera over the White Terraces location during the project as they were unaware of what the sonar data was showing at the time.
Dr Cornel de Ronde, of GNS Science, who led the New Zealand-US project to find the Pink and White Terraces earlier this year. Photo: Margaret Low, GNS ScienceThe fate of the remaining sections of the Pink and White Terraces is unclear. They might have been destroyed in the eruption. Alternatively, they could be lying under thick sediment which is impenetrable to sonar signals sent out by the two autonomous underwater vehicles (AUVs) used in the survey.
Dr de Ronde said finding remnants of both sets of terraces was a remarkable outcome for the project.
"The project team was absolutely thrilled in January when we realised our AUVs had detected remnants of the Pink Terraces. Finding part of what we believe is the White Terraces as well has been surprising and very satisfying.
"The original aim of the project at Lake Rotomahana was to map the lake floor and investigate the extensive geothermal system under the lake and how it evolved from an on-land geothermal system to a submerged one. Anything else was a bonus," Dr de Ronde said.
"It's gratifying to be part of a science project that can answer a century-old mystery about the fate of the Pink and White Terraces."
Dr de Ronde said the announcement of the find was timed to coincide with the 125th anniversary of Mt Tarawera's eruption. An hour-long television documentary about the project at Lake Rotomahana is scheduled to screen on Prime at 8.30pm this Sunday.
The ten-day project was a collaboration involving GNS Science, Woods Hole Oceanographic Institution in Massachusetts, Lamont-Doherty Earth Observatory at Columbia University in New York, the National Oceanic and Atmospheric Administration in Seattle, and the University of Waikato.
Dr de Ronde also acknowledged the permission and support from the Te Arawa Lakes Trust Board and tourism operator Waimangu Volcanic Valley, which helped with access to Lake Rotomahana.
This side-scan sonar image, collected in January 2011, shows horizontal scallop-shapped features on the bottom of Lake Rotomahana near Rotorua. Scientists believe it is a section of the White Terraces that disappeared during the eruption of Mt Tarawera on 10 June 1886. The white areas in the image represent the hard, reflective edges of the terraces as seen from above at an oblique angle. The dark areas indicate soft sediment and mud. The overall length of this feature is about 180m. This sonar image was collected by a torpedo-like autonomous underwater vehicle (AUV) built and operated by Woods Hole Oceanographic Institution (WHOI) in the United States.