Home / News and Events / Media Releases / Multi-pronged investigation lays out the latest word on Lake Rotomahana - 10/04/2016

Multi-pronged investigation lays out the latest word on Lake Rotomahana - 10/04/2016

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 following the Mount 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 Mount Tarawera on 10 June 1886, the most destructive eruption in New Zealand for at least 200 years, half a cubic kilometre of lakefloor material was ejected from the lake and splattered on the surrounding landscape.

Lake Rotomahana

Aerial view of Lake Rotomahana with Mt Tarawera in the background. Photo: Lloyd Homer, GNS Science

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.

With its international connections, project leader GNS Science was able to bring the latest technology and scientific heft to provide the kind of information and detail had not been possible with previous investigations of the lake and surrounding areas.

This included 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.

Many of the findings contained in the papers were revealed progressively as the scientific investigations were being done, but the collection of scientific papers in a peer-reviewed journal provides more detail and more robust interpretation of the results.

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 that 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 lake.

Science leader Cornel de Ronde of GNS Science says the inescapable conclusion is 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 the eruption,” he says.

Before the eruption, Lake Rotomahana was only about 20 percent of its present size. As a result of the eruption the previous outflow of the lake became dammed, and over time it filled with water and formed the new Lake Rotomahana, much larger and deeper than the original.

One of the papers in the special issue shows 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 lakefloor and its deepest extent is less clear, but it might extend down as far as 6km or 7km below the lake.

The White Terraces prior to the eruption in 1886.

The White Terraces prior to the eruption in 1886.

For Dr de Ronde a magma body under the lake would account for the phenomenal amounts of heat and gas coming into the present-day lake. It is unclear, however, if the magma body is related to the magma that rose up through the lakefloor in 1886 and mixed with water to produce the extremely violent eruption that blew the old Lake Rotomahana to bits.

Dr de Ronde says until now no-one has seriously considered that a magma body under the lake might have played a role in the 1886 eruption.

“People have always thought the 1886 magma source was directly under Mount Tarawera. My colleagues have shown there is another possibility.”

Heat flow measurements taken on the lakebed 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.

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 United States.

On the final day of the survey, the AUVs recorded a sudden jump in lake temperature by about 0.03°C throughout the lake, or equivalent to an extra 150 megawatts 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 0.03°C is huge,” says Dr de Ronde.

As if to underline the extremely high levels of hydrothermal activity at the lake, scientists also recorded 750 plumes of gas bubbles rising up from the lakefloor.

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 Science

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 Science

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 that 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 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 lakefloor sediment too.

"

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.

Cornel de Ronde

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“Through the application of seismic surveys, we have reconstructed two large and three smaller V-shaped eruption craters that are 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.

“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 stared up in the southern part of the lake since the eruption.

Dr de Ronde says it is the first time anywhere in the world where an on-land hydrothermal system has survived a volcanic eruption and being ‘drowned’, and has 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.”

He says 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.

Note: This June marks the 130th anniversary of the Tarawera eruption.

Plumes of gas bubble at Lake Rotomahana