The secrets of the Tarawera eruption that buried the Pink and White Terraces
Scientists have finally uncovered secrets left behind by the huge 1886 eruption of Mt Tarawera which destroyed the Pink and White Terraces, as JOHN CALLAN reveals.
A multi-year study of the volcanic plumbing under Lake Rotomahana near Rotorua has provided scientists with the clearest view yet of the impact on the lake and its geothermal systems of the Mt Tarawera eruption in 1886.
The research, representing thousands of hours work by dozens of scientists, has just been published as 11 papers in a special issue of the Journal of Volcanology and Geothermal Research.
It wraps up a five-year effort by scientists from New Zealand and the United States who worked together to get a better understanding of the complex series of events that have resulted in the lake that we see today.
During the eruption of Mt Tarawera on June 10, 1886, the most destructive in New Zealand for at least 200 years, half a cubic kilometre of material was ejected from the lake floor and splattered on to the surrounding landscape.
Many of the findings contained in the papers were revealed progressively as the scientific investigations were carried out, but the collection of scientific papers in a peer-reviewed journal provides more detail and robust interpretation of the results.
The research was driven partly out of scientific curiosity and partly by the desire to clear up long-lasting uncertainties about the eruption and its impact on the lake and the world-famous Pink and White Terraces that sat opposite each other on the edges of the lake.
Project leader GNS Science and international partners were able to bring the latest technology and scientific heft to provide the kind of information and detail not possible with previous investigations of the lake and surrounding areas.
What set this major collaborative investigation apart was the combination of different survey methods used - high-resolution bathymetry, magnetics, measurements of the water column, side-scan sonar, seismic surveys, underwater photography and surveys of gas plumes and of the amount of heat entering the lake.
The combined power of these techniques has enabled scientists to make many interesting interpretations of what actually happened on the day of the eruption and in the weeks and months that followed.
The finding that resonates with most people is that they discovered what appears to be remnants of the Pink and White Terraces on the bottom of the lake.
The destruction of the majority of the terraces is perhaps not surprising given the eruption was so violent it was heard in Auckland and in the South Island. The blast left a 17km-long gash through the mountain and southwestwards beneath the lake.
Science leader Cornel de Ronde of GNS Science said the inescapable conclusion was that most of the Pink and White Terraces were destroyed during the eruption.
"However, we found tantalising evidence from underwater photographs and side-scan sonar that remnants of both sites survived."
Before the eruption, Lake Rotomahana was only about 20 per cent of its present size. As a result of the eruption, the previous outflow of the lake became dammed. Over time it filled with water and formed the new lake, which is much larger and deeper than the original.
One of the papers discusses the find of what could be a sizeable magma body located under the southwestern end of the 17km-long explosion rift, near present-day Waimangu. The interpretation comes from a geophysical technique called magnetotellurics, that measures electrical resistivity of the Earth down to about 10km.
At its shallowest, the likely magma body is within about 2km of the Rotomahana lake floor. Its deepest extent is less clear, but it might extend down as far as 6km or 7km below the lake.
For de Ronde, a magma body under the lake would account for the phenomenal amounts of heat and gas coming into the present-day lake.
"People have always thought the 1886 magma source was directly under Mount Tarawera. My colleagues have shown there is another possibility," he said.
Heat-flow measurements taken on the lake bed during the investigation are among the highest recorded in a lake anywhere in the world and suggest that the total heat output of the lake is between 112 and 132 megawatts (MW).
This is equivalent to the output of a typical geothermal power station in the Taupo region. A power station of this size is capable of producing enough electricity to power between 50,000 and 60,000 households.
One of the most telling measurements was made on a single day during an 11-day survey by torpedo-shaped autonomous underwater vehicles (AUVs) built and operated by Woods Hole Oceanographic Institution in the US.
On the final day of the survey, the AUVs recorded a sudden jump in lake temperature by about 0.03 degrees Celsius throughout the lake, or equivalent to an extra 150MW of heat energy entering the lake during a single day.
"That may not sound like much of a shift in temperature, but the amount of energy needed to heat a large body of water like Lake Rotomahana by 0.03C is huge," de Ronde said.
Scientists also recorded 750 plumes of gas bubbles rising up from the lake floor, as if to underline the extremely high level of hydrothermal activity and large amounts of gas being expelled at the lake.
Coincidentally, analysis of small magnitude, shallow earthquakes that occur periodically in the vicinity of the lake shows they cluster along an almost vertical line, or fault, that connects the likely magma body at depth with the bottom of the lake. This, the scientists conclude, provides easy passage for hot, buoyant fluids and volcanic gases into the lake.
"It's fairly clear there are pulses of heat entering the lake that are associated with earthquake swarms under the lake. We believe have identified the heat source and the pathway for this to happen.
"Pulsing of fluids along faults is the kind of thing that people have been talking about for a long time, but which is not easily verified. It was pure serendipity that we were in the right place at the right time to measure the sudden increase in lake-water temperature it as it happened."
One of the beauties of this investigation is that technology has not only allowed the lake water to be stripped away, but the thick lake-floor sediment too.
"Through the application of seismic surveys, we have reconstructed two large and three smaller V-shaped eruption craters that are today buried under sediment beneath the floor of modern Lake Rotomahana. We have precise dimensions of these craters and have effectively brought them back to life," de Ronde said.
"We attempted to define the ground surface immediately after the 1886 eruption. This has then allowed us to render three-dimensional images of the craters that have since been infilled by sediment once the lake filled with water.
"There were eye-witness accounts of these craters immediately following the eruption, but visibility was hampered by steam rising from the craters. So we arguably now have the most accurate record of the dimensions and shapes of these craters following the eruption."
One of the lasting curiosities for scientists is that the large hydrothermal system that fed the thermal waters to the terraces prior to 1886 has survived the eruption. There is vigorous hydrothermal activity today where the Pink Terraces used to stand, although activity in the vicinity of the former White Terraces has stopped altogether. But an entirely new area of hydrothermal activity has started up in the southern part of the lake since the eruption.
De Ronde said it was the first time anywhere in the world where an on-land hydrothermal system had survived a volcanic eruption and being "drowned", yet still remained active.
"This project has been a unique opportunity to apply a lot of investigative technology in the study of a drowned geothermal system. It was truly a pleasure to do this work and we hope we have left a legacy contributing to the history of this famous landmark."
GNS Science could not have undertaken the project without the assistance of Woods Hole Oceanographic Institution in the United States, the University of Waikato, the Te Arawa Lakes Trust Board, tourism operator Waimangu Volcanic Valley and the Bay of Plenty Regional Council.
John Callan is communications manager for GNS Science.