Bill Keir: Imagination needed to see Terraces

On February 2, 2011, the GNS Science website posted a blog by the organisation's outreach educator, Julian Thomson, which said: "Pink Terraces found! Yes - the unbelievable news is that in spite of being located at the centre of New Zealand's most violent eruption in historic times, shaken by volcanic earthquakes, covered by many metres of mud and ash, and then flooded underneath a large lake, a large area of New Zealand's iconic Pink Terraces of Rotomahana has been rediscovered!"

Subsequent blogs and media announcements said that, "At least parts of both Pink and White Terraces are still intact on the bed of the lake", and gave the depth of these finds as 50-60m. Newspapers and television gave extensive coverage and Julian Thomson's blogs posted in March 2012 gave updates with higher resolution data.

These conclusions came from imagery obtained during the ongoing Lake Rotomahana Survey Project using underwater sonar instruments and cameras. The published images appear to show scallop-shaped, stepped features of solid material similar to the famous silica terraces, which were either buried or destroyed during the eruption of the Tarawera-Rotomahana-Waimangu rift on June 10, 1886.

However, these imaged features are almost certainly not in-situ remains of the Pink and White Terraces. They are too deep in the lake. Simple arithmetic demonstrates this.

My calculation starts with today's known mean elevations of the lakes found on the latest topographical maps: Lake Tarawera 299m above mean sea level; Lake Rotomahana 337m above mean sea level. Therefore, today, Lake Rotomahana is about 38m above Lake Tarawera.

There is no accurate data for the elevations of the two lakes before the 1886 eruption. However, we know that prior to the eruption the outlet of Lake Rotomahana, the Kaiwaka Stream, flowed into Lake Tarawera. Ferdinand von Hochstetter's 1859 map of the area labelled this stream with the words, "Disembogues into Lake Tarawera". Therefore, before the eruption, Lake Rotomahana must have been at least 1m higher than Lake Tarawera to give sufficient fall for the water to flow the 1.5km between the two lakes.

We also know that outlet blockage and other factors caused Lake Tarawera to rise after the eruption, and to fall again when the outlet blockage burst in November 1904 causing a flash flood that affected communities downstream.

The precise change in level to which Lake Tarawera stabilised to today's level is not known, but Ronald F Keam, who published a book about the eruption in 1988, estimated it as "less than ten feet". For my calculation I take today's level of Lake Tarawera to be 3m above the pre-eruption level, which gives a nominal pre-eruption height of 296m above mean sea level for Lake Tarawera. This demands at least 297m above mean sea level for the pre-eruption height of Lake Rotomahana.

We also know that, before the eruption, the Pink and White Terraces descended to the shore level of Lake Rotomahana - many pre-eruption photos verify this.

It follows from these facts that, (a) Lake Rotomahana could not have risen more that 40m from its pre-eruption level to today's level, (b) the base of the Terraces, if still in-situ, could not be more than 40m below the surface of today's Lake Rotomahana. A shallower depth than this would be yielded if the original height of Lake Rotomahana were higher above Lake Tarawera than the 1m I have assumed. A shallower depth would also result if the pre-eruption level of Lake Tarawera were higher than I have assumed. A variation of 1m or 2m in my calculations is possible from the known small variations in mean sea level and the seasonal levels of the lakes.

These calculations rule out the possibility that GNS scientists have discovered in-situ remains of the Terraces at a depth of 50-60m in Lake Rotomahana.

The precise vertical height of the top of the Terraces above Lake Rotomahana was never recorded prior to the eruption. The most commonly quoted approximate figure for the White Terraces is 30m. The White Terraces were probably slightly higher than the Pink Terraces.

Pre-eruption photos of people standing on the Terraces support this estimate. It follows that the top of the Terraces, if still in situ, would not be more than about 10m below today's level of Lake Rotomahana.

So, what have the GNS scientists found at a depth of 50-60m? There are several possibilities. They could be lower sections of the Terraces that were below lake level prior to 1886 and never seen by humans. They could be remnants of other silica terraces which formed before earlier eruptions in prehistoric times and were buried in the lake long before humans arrived.

They could be fragments of the Terraces displaced into the crater by the explosion. They could be other step-shaped objects exposed in the crater by the explosion. Or they could be something else entirely.

One needs quite a lot of imagination to see the Pink and White Terraces in the published images. They are open to a variety of interpretations. It would not be the first time scientists have made the error of seeing what they want to see in their results.

Efforts to determine the fate of the Pink and White Terraces began immediately after the eruption. At that time the site of the original Lake Rotomahana was a huge explosion crater more than 100m deep, and more than a kilometre wide, with a small hot lake at the bottom, and surrounded by walls of ejected mud. It took about 10 years for this hole to fill with water to the level of the lake we know today.

Investigators had plenty of time to find the Terraces if they were in the slightest discernible in the crater. Scientists, surveyors, photographers and newspaper reporters were on-site assessing and recording within a few days of the eruption.

Bill Keir is a freelance researcher living in Hokianga. In tomorrow's Daily Post GNS Science project leader Cornel de Ronde will respond to Bill Keir's article.