Fault found in Terraces scepticism

In his article refuting the likelihood that scientists rediscovered parts of the Pink (and to a lesser extent White) Terraces, Bill Keir uses as the basis of his criticism of the survey done by GNS Science and collaborators, the present depth of the aforementioned terraces and their supposed height above sea level, before the 1886 eruption of Mt Tarawera.

Mr Keir quotes our press releases of 50-60m for the depth to the re-discovered terraces as being 10-20m "too deep" to be the Pink Terraces of pre-1886. Surprisingly Mr Keir is happy to preface his whole argument on surveying done before 1886. Then, in his own words, he says, "... There are no accurate data for the elevations of the two lakes [Tarawera and Rotomahana] before the 1886 eruption ...", and then proceeds to ignore this very pertinent statement, going on to say the terraces we rediscovered are simply too deep.

During our surveys of early 2011 and again this year we used differential global positioning system (GPS) that utilised up to 11 satellites to ensure we had accuracy to within a few vertical centimetres, and tens of centimetres in a horizontal sense. This ensured the absolute location of our vessel was known at any one time. Moreover, the autonomous underwater vehicles (AUVs) equipped with numerous sensors used in 2011 to survey the lake, including the ability to map the lake bathymetry, could equally navigate around the lake to a very high degree of accuracy with respect to location, as we used transponders to triangulate the position of the vehicle which were themselves surveyed by the surface vessel using the above-mentioned GPS system. Thus, unlike before 1886, we knew exactly where the AUV was during its survey of the lake floor to within tens of centimetres. This was repeated this year when we resurveyed the lake floor bathymetry to a resolution of better than 0.5m. The point here is that various lakefloor features we surveyed/mapped/photographed are very accurately located with respect to position (ie, latitude and longitude) and depth in the lake. Any reference to accurate lake level(s) above sea level before 1886 is fraught with potential for large errors.

I would suggest our measurements for the base of the Pink Terraces are much more likely to be the real values as supposed to supposition of inferred heights above sea level by Mr Keir.

Mr Keir then lists possibilities for why the terraces could be 10-20m deeper (in his mind) than they "should" be, including: "... lower sections of the Terraces that were below lake level prior to 1886 and never seen by humans ... remnants of other silica terraces which formed before earlier eruptions in prehistoric times and were buried in the lake long before humans arrived ... and fragments of the Terraces displaced into the crater by the explosion ...". Again this assumes any surveying done before 1886 being accurate. Yet several of the ideas listed above are reasonable.

Certainly nobody knows if terraces existed beneath the lake level of Lake Rotomahana before 1886. Moreover, considering the impact on the general area made by the rift that went through (to the south of) the old Lake Rotomahana, slumping of slopes around the old lake is conceivable.

However, this discussion on terrace depth, even given the misgivings of comparing surveying of pre-1886 (or lack thereof) to today's sophisticated means, is a red herring. Unfortunately Mr Keir has not followed closely what has been reported about our survey results and, moreover, he appears happy to ignore, or perhaps does not understand, the enormous amount of information supplied by the recent surveys of Lake Rotomahana, all of which has been made accessible to the public on various YouTube videos (eg. see "How we found the Pink Terraces"), documentaries (two) and numerous press releases and blogs.

This includes high-resolution bathymetry of the lakefloor that recognised specific pre-1886 land features so that the location of the Pink Terraces, if they had survived, could be made with some certainty. A side-scan sonar survey revealed remarkable images of very hard, convex terraces that juxtaposed against each other and which were stacked two and three high. They have relief of up to 1.5m with round, circular (pool-like) structures on their flat, upper surfaces.

A camera able to capture high-resolution images was lowered down on these structures and found their facades to have the candle wax-like textures so commonly seen in photographs of the terraces, before 1886. Moreover, these partial terraces were exactly where they should have been, using the detailed bathymetric maps then comparing to pre-1886 photographs as a guide.

We have always said the terraces we found mark the bottom-most (in photographs) buttresses to the main Pink Terraces "staircase", immediately south of the latter. They were not found anywhere else (with the exception of lesser examples in the vicinity of the White Terraces location; see YouTube "White Terraces rediscovered") in the lake. A magnetic survey of the lake also shows the Pink Terraces remnants to occur in a distinct area that demarcates long-term geothermal fluid upflow. Finally, if the remnants we discovered in 2011 were what we believed them to be, then the Pink Terraces proper should be slightly to the north. If they did survive the eruption, then they must be buried underneath mud, as no terrace structures were detected by side-scan sonar in this (adjacent) area.

Thus, earlier this year we also conducted a high-resolution seismic survey throughout the lake, including where we thought the Pink Terraces "staircase" might be. What we found was a stack of very hard reflectors under a metre or more of mud that resembled the main body of the terraces, exactly where they were meant to be if the previously discovered terraces were the buttresses we thought they were (see YouTube "Lake Rotomahana stripped bare").

In his own words, Mr Keir says, "... The sites of the Terraces were identified under metres of mud ...". Taken together, we believe that we have found vestiges of the lower-most terraces located immediately south of the main terrace "staircase" and indeed quite possibly the latter buried beneath the mud.

It is ironic that Mr Keir writes, "... It would not be the first time scientists have made the error of seeing what they want to see in their results ..." as nothing could be further from the truth.

This project was entitled "Lake Rotomahana Project" and did not have as its focus the Pink and White Terraces. Rather, it was a survey to understand what became of a geothermal system after being drowned by a lake.

Finding vestiges of the terraces was serendipity, though we knew with the state-of-the-art equipment we were using that there was always the possibility we might.

I suggest Mr Keir had already made up his mind before he wrote his article, as given by his second sentence, and his readiness to argue presupposed surveying before 1886 being accurate, never mind dismissing the wealth of evidence that suggests that indeed some terraces survived the eruption.