The scientists who found remnants of the famous Pink and White Terraces on the bottom of Lake Rotomahana on 2011 returned to the lake in March 2012 to undertake further sonar and seismic surveying. They’ve recently announced that a seismic survey has shown 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 2 metres of sediment. The fate of the White Terraces is still unknown as they were in a part of the lake that was significantly disturbed by the eruption.
Multibeam sonar equipment used to remap underwater surface
The scientists, led by Dr Cornel de Ronde of GNS Science, have now remapped the entire topography (the present-day underwater surface) of the 6 x 3 km lake using multibeam sonar equipment. This was used to create a much higher resolution map than the one they compiled in 2011 using autonomous underwater vehicles equipped with sonar.
The new detailed map shows objects less than 50 cm across and has revealed a number of volcanic craters, faultlines and pockmarks on the lake floor resulting from the 1886 eruption of Mount Tarawera. It also shows ridges and other features that were originally above water and can be matched to old paintings and photographs of both the pre- and post-eruption landscape. The lake floor lies up to 120 m below the water surface at its deepest point and is approximately five times the size of the pre-eruption lake.
Provides context for the 1886 eruption
“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,” says Dr de Ronde.
“Most of the newly identified lake floor volcanic features have been hidden for over 100 years. 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.”
Used both low- and high-frequency seismic equipment
To look beneath the surface of sediment layers, 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 70 m below the lake floor and was reflected off geological layers back up to hydrophones towed behind the boat.
The scientists first surveyed the 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 m of sediment on the lake floor and see the hard post-eruption surface of the lake and other geological structures at depth,” says Dr de Ronde.
Seismic lines over the location of the Pink Terraces showed hard surfaces at the same depth and in the same location as the 2011 survey indicated the Terraces would be.
“In 2011, we found the two bottom tiers of the buttress adjacent to the main staircase of the Pink Terraces. In 2012, the seismic data is telling us that there is a 40-metre 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, although we were not able to determine their state of preservation.”
The stack of hard material is covered by a 2-metre thick layer of sediment that could not be penetrated by the equipment used in the 2011 survey.
Pink Terraces’ shapes difficult to image
Dr de Ronde says 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.
“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 Mount 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 says 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 New Zealand.
“In fact, very few volcanic lakes in the world could match this level of detail.”
The survey was conducted with assistance from the University of Waikato and Te Arawa Lakes Trust Board.
Read about the earlier expedition in this article, Portion of Pink Terraces revealed underwater.
The eruption of Mount Tarawera in 1886 destroyed the Pink and White Terraces. Your students may like to explore this interactive activity that shows the major volcanoes in New Zealand and when they last erupted.
Watch a short informative video about the project from GNS Science.
GNS Science has created a map of Rotomahana's lake floor. It shows floor features that have not been seen for more than a century, including the likely locations of the remnants of the Pink and White Terraces. This article from Stuff explains how the map was made. The GNS Science webshop is offering a free downloadable PDF version of the map.