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19/3/2012 — Environment and Society
Rotomahana lake floor provides more valuable information

Scientists have backed up findings from last year to show that a substantial portion of the Pink Terraces appears to have survived the 1886 eruption of Mount Tarawera and is sitting at the bottom of Lake Rotomahana covered by two metres of sediment.

The fate of the White Terraces is less certain as they were in a part of the lake that was significantly disturbed by the eruption.

The results come from sonar and seismic surveys of the lake floor.

The scientists started this month’s investigation by remapping the topography of lake-floor with sonar equipment at much higher resolution than a map they compiled last year using free-swimming robotic vehicles.

The new map shows objects less than 50cm across and has revealed a number of volcanic craters, faultlines and pockmarks on the lake floor resulting from the 1886 eruption of Mount Tarawera. Many of these features have not been seen before.

“The new lake floor data helps put all existing information about the 1886 eruption and the two geothermal systems under Lake Rotomahana into a much better context,” said project leader Cornel de Ronde of GNS Science.

“Most of the newly identified lake floor volcanic features have been hidden for over 100 years,” Dr de Ronde said.

“Their discovery provides much greater insight into the sequence of events that made up the 1886 eruption. In particular, the data shows the volcanic craters and the deep rift that formed when the lake floor unzipped violently during the eruption.”

Multibeam sonar provided an excellent view of the topography of the lake floor. To delve even deeper, the scientists used seismic equipment towed behind a boat that followed a pre-determined grid pattern.

The seismic equipment released an acoustic vibration that penetrated up to 70m below the lake floor and was reflected off geological layers back up to hydrophones towed behind the boat.

The scientists first surveyed the whole 6 kilometre by 3 km lake with low-frequency seismic equipment that penetrated deep into lake floor structures. This was followed by a higher frequency survey over areas of particular interest. Higher frequency seismic penetrates less, but shows greater detail of the underlying rock layers.

“The seismic data has enabled us to strip off about 40 meters of sediment on the lake floor and see the hard post-eruption surface of the lake and other geological structures at depth.”

Seismic lines over the location of the Pink Terraces showed hard surfaces at the same depth and in the same location as last year’s survey indicated the Terraces would be.

“Last year we found the two bottom tiers of the buttress adjacent to main staircase of the Pink Terraces. This year the seismic data is telling us that there is a 40-meter-wide and three-storey-high stack of very hard material exactly where we estimate the Pink Terraces should be.

“We believe this represents a substantial portion of the Pink Terraces, though we were not able to determine their state of preservation. We were unable image individual terraces, he added.”

The stack of hard material is covered by a 2m-thick layer of sediment that could not be penetrated by the sonar equipment used in last year’s survey.

Dr de Ronde said the cascading and scalloped shapes of the Pink Terraces had proved challenging to image as the seismic signals had been scattered by the many hard surfaces at different angles.

Even so, the large amount of data collected during this year’s survey would pave the way for future studies of the Tarawera eruption and the two active geothermal systems under the lake.

“We’ve only had time to process part of the data so we are learning new things every day.”

Part of the excitement of this project is knowing that this new data will advance the knowledge and understanding of Mt Tarawera-type eruptions not just in New Zealand, but internationally.

“It will also provide greater insight into sub-lake floor geothermal systems.”

Dr de Ronde said the wealth of data meant that the lake floor of Lake Rotomahana had been mapped and investigated in more detail than any other volcanic lake in NZ.

“In fact very few volcanic lakes in the world could match this level of detail.”

The survey was conducted with the support of the Te Arawa Lakes Trust Board.

Source: gns.cri.nz.

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Scientists prepare the seismic reflection gear before heading out to survey the lake floor of Lake Rotomahana. Photo by Julian Thomson.
Scientists Duncan Graham and Fabio Tontini, both of GNS Science, collecting data during this month's seismic survey of Lake Rotomahana, south-east of Rotorua. Photo by Alex Young.