Rotomahana Project Background

Lake Rotomahana View

In January 2011, a team of scientists from New Zealand and America spent two weeks at Lake Rotomahana, aiming to discover more about this fascinating lake. The project involved sending two autonomous underwater vehicles (AUVs) into the lake to make a detailed survey of features on the lake floor.

A number of measurements were made including water depth, temperature, acidity, water clarity, magnetism, and the electrical potential of the water.

These measurements identified areas of continued hydrothermal activity on the lake floor, and will result in one of the most detailed studies of a freshwater hydrothermal system ever undertaken.

Change over Time

Pink Terraces Painting

Before the 1886 eruption of Mount Tarawera, Lake Rotomahana was much smaller than it is at present. Powerful hydrothermal activity around the lake had been going on for a long time and a huge amount of hot silica rich water was flowing out of the ground, forming the famous Pink and White Terraces. These spectacular formations were regarded as the “eighth wonder of the world” for their grandeur and beauty.

The eruption on 10th June 1886 was thought to have destroyed the terraces, and buried any remaining parts of them under ash and mud. The valley was widened and refilled with water again to form the much larger Lake Rotomahana that we can visit today.

Science Expedition

Rotomahana Survey Boat

By surveying the lake for the first time in such detail, the study has showed in what ways the eruption altered the ‘plumbing’ of the previous hydrothermal system. This is particularly interesting to scientists as the submersion of the hydrothermal vents of Rotomahana following the historic 1886 eruption is a globally unique story.

Was any part of the Pink and White Terraces found?

Before the expedition there were some clues that suggested the terraces could be partly preserved: They were several hundred metres from the actual rift, were probably covered with thick mud and ash during the event, and soon afterwards were drowned by rising lake levels. However, it was also likely that the violence of the eruption completely destroyed them. It was therefore a big suprise when the expedition did reveal that at least parts of both the Pink and White Terraces are still intact on the bed of the lake.

The appearance of the Lake Rotomahana and the Pink and White Terraces before 1886:

Lake Rotomahana Pre Eruption

Lake Rotomahana was highly picturesque - surrounded by bush, with many steam vents, geysers and also the Pink and White Silica Terraces. These were the largest silica terraces the world has seen. Water flowed down from hot springs at the top, depositing silica to form steps and basins like “ornamental fountains”. One visitor said “… to convey an idea of its beauty is impossible…”

The terraces were so large because there was a huge amount of mineralised water pouring out of the hot springs for a long period of time.

A brief description of the Tarawera Eruption:

Tarawera Eruption

Soon after midnight on June 10th 1886, a series of earthquakes shook the area around Rotorua. Within a couple of hours, Mount Tarawera was erupting along its crest, with flickers of lightning and columns of ash and smoke. At about 3.30 am the most violent explosions ripped along a 17km line of vents.

including Lake Rotomahana. A surge of hot ash destroyed villages within a 6km radius, including the famous ‘Buried Village”, with the loss of over 100 lives. Over two cubic kilometres of volcanic rock were erupted.

People in Auckland thought that the explosions were the sound of cannons, and in Blenheim, in the South Island, the eruption as heard as a distant rumbling.

An excellent display of the Pink and White Terraces and the story of the Tarawera Eruption is on show at the Rotorua Museum.

So what happened to Lake Rotomahana?

Before the eruption the lake was about two kilometres long by one kilometre wide. It was surrounded by small bush covered hills with numerous steam vents along the lake shore. The eruption opened up a long rift that passed through the lake. The surrounding area was blanketed with mud and tephra (airborne volcanic rocks) and the lake level rose much higher to eventually cover an area about 3 x 6 kilometres across, 115m deep at its deepest point.

Scientists have found that the remnants of the terraces are now covered by at least 50m of lake water.

How did the scientists survey the lake?

Rotomahana Magnetic Map

In late January 2011, two automated underwater vehicles (AUVs), supplied by the Woods Hole Oceanographic Institution in the United States, were lowered into the water and programmed to automatically explore the lake. Sensors in the AUVs were able to take different measurements and match them to the precise location. After the expedition a detailed digital map of the lake was made and calculations made about the location, size and nature of the hydrothermal system.

How do Autonomous Underwater Vehicles work?

AUV

The AUVs look a bit like torpedos that are driven by a propeller at the back. They have a computerised guidance system as well as ‘high tech’ sensors to make recordings. Four positioning beacons are set up at the corners of an area of the lake to be surveyed. The beacons send information to the guidance system of the AUVs which travel up and down within the area in a grid pattern. Once the measurements have been made, the beacons are moved to another part of the lake so that eventually the whole lake has been surveyed.

What did the scientists find?

As well as being able to map changes to the overall hydrothermal system, parts of the Pink and White Terraces were rediscovered. You can read full details of the expedition and the discoveries in Julian's blog or check out our videos about how we found the Pink Terraces and the rediscovery of the White Terraces.

What next?

The sidescan sonar images that revealed some of the Pink and White Terraces can only detect rock types that outcrop on the lake floor. To find out whether more of the terraces are concealed under the lake floor mud, another research investigation at the lake is planned for March 2012.

Who is involved in this scientific project?

Loading the AUV

Several scientists from GNS Science, including the leader of the project, Cornel de Ronde. There is also a team of American scientists from Woods Hole Oceanographic Institution, and support from the Te Arawa Lakes Trust and Waikato University, and school students from the Rotorua area will be using their skills to predict what may be discovered…

GNS Science gratefully acknowledges the support of the Waimangu Volcanic Valley towards this project

Glossary

AUV– an automous underwater vehicle, or unmanned submersible machine

Ash – fine dust or sand produced during a volcanic eruption

Hydrothermal system - literally ‘hot water system’ the features that together result in steam vents and mud pools etc in a given locality.

Geyser - a jet of hot water and steam

Sediment – mud, sand or other material that deposited to form a blanket like layer

Silica – a rock type with the same composition as quartz or sand

Tephra – volcanic rock of different types (such as ash, scoria or pumice) thrown out during an eruption

Web Links:

Julian Thomson’s Blog

http://juliansrockandiceblog.blogspot.com/2011/02/final.html

TVNZ news video (2 mins 40)

http://tvnz.co.nz/national-news/scientists-search-pink-and-white-terraces-treasure-3897400/video

GNS Science Rotomahana Press Release

http://www.gns.cri.nz/Home/News-and-Events/Media-Releases/Rotomahana

Videos: How we found the Pink Terrraces and White Terraces Rediscovered