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Friday, 16 March 2012

Seeing beneath the water and mud of Lake Rotomahana

Here is our video describing this years Rotomahana explorations - a hidden world come into view!

Rotomahana's lake floor prompts many questions

Following last weeks' multibeam sonar survey, the bed of Lake Rotomahana has now been mapped to a resolution of half a metre, bringing to light a mass of detail hitherto unknown to scientists.

The first photo shows last year's map which was made with the assistance of WHOI (Wood's Hole Oceanographic Institution). The resolution of the map is 15 metres.  This year, with the help of ixSurvey, we have improved that by 30x (second image). In this post I will show you some of the features that have come to light.

The colour scale indicates depths in metres. Red represents the shallowest depths found around the shoreline, down to blue which is deepest in the main central part of the lake. The maximum depth is about 115 metres. The grey area is the land around the lake that is above water level, or very shallow parts of the south side of the lake that were not scanned. Click on the image for a larger view.

The map we now have allows close up study of many fascinating features that we can see for the first time. In the third image showing the northern margin of the lake, you can see two explosion craters right on the very edge. They are about 25 metres deep. In the bottom right part of the image is a newly revealed crater, formed at a late stage in the 1886 eruption. Its rim is about 60 metres below the surface, and its floor is at about 80 metres. All of these craters are approximately 100 metres across.

If you click on this image of the flat, deepest part of the lake (blue area), you might just discern a faint circular feature just below and to the right of centre. This is also about 100 metres across and may be the outline of a crater rim that has been almost totally obscured by mud, or it may be the lobe shape of a debris flow that cascaded down from the north, leaving a smooth gouge  in the slope (upper part of the picture).

In the lower (southern) part of the map there are many erosion features visible on the sloping lake floor. On the left of this image you can see some eroded gullies  extending down from the red area (-20m) into the blue (-100m). We believe these runnels formed in the few years after the Tarawera eruption, before the lake filled up, rather than that they were eroded after the water level rose. On the right hand side of the image, there is another area of radiating features. These have quite a different character, being less smooth, and with intriguing lines of hollows. These may have formed as a result of the wave like flow of debris down the slope, but we are uncertain as to why they are so different to the features just to the left (west).

The southern half of the blue area on the map has a lot of gas activity. This was noticed last year on some of the sidescan images showing plumes of bubbles arising from a pick marked area on the bed of the lake. This activity has increased dramatically in this part of the lake floor since the Tarawera eruption.

Now we can see this area of hydrothermal and gaseous activity in detail, with the 'pock marks' showing up as a mass of small vents scattered over a wide area. These are each up to a few metres across.

A very significant feature that was revealed in last years' bathymetric map was the 'spit' or promontary that is shown on early photographs of the Pink Terraces. It is extending into the lake in the middle distance of this photograph, not far to the east of the Pink Terraces visible in the left foreground. The spit rises several metres above water level. (Image courtesy of the Alexander Turnbull Library, Wellington)
On our new bathymetric map we can clearly see the promontary, now with its crest below 50 or 60 metres of water.

Thursday, 8 March 2012

Lake Rotomahana Seismic

The Seismic Survey of Lake Rotomahana is proceeding well this week. Whilst it is being led by GNS Science, the University of Waikato and NIWA are providing technical assistance with some of the equipment being used.

The first photo shows  the survey boat being loaded with the the cable that contains the hydrophones. These pick up the reflected sound waves that are sent down below the surface by the 'boomer', the white object in the background, at the end of the pier.

In the graphic you can see how the set up works. The boat tows the seismic source (either the low frequency 'boomer' or the higher frequency 'CHIRP'). This sends sound waves down through the water and into the rocks below. These signals get reflected back up from the  different rock  layers and are received by the hydrophones in the cable floating behind the boat. Lower frequency sound waves can penetrate deeper into the rocks, whilst higher frequencies give shallower penetration, but provide more detail.

During our survey we are using the boomer to give an overall view of the lake floor first. We are then using CHIRP to go over specific locations that we want to observe in more detail, such as the sites of any terraces and particular volcanic structures.

On this map of the lake floor, you can see how the seismic lines criss cross the lake back and forth to give  overall coverage. This is the planning map, but sometimes the scientists change their plans during the survey, depending on the time they have available, and how well things are progressing.

Chris Leblanc is set up with all the computer hardware and software to process all the data produced by the survey. He creates graphic cross sections of the lake floor that reveal the sub surface geological features. You can see one of these sections on his computer screen.

There has been a great deal of media interest in our investigation of Lake Rotomahana. In the last photo Cornel de Ronde is being interviewed by John Hudson with cameraman Clint Bruce for TV1's Sunday programme.

Wednesday, 7 March 2012

Scientists hope for picture of Pink and White Terraces soon

By the end of this week, scientists hope to have a clearer picture of how the famous Pink and White Terraces look.  Listen to the National Radio interview: (3′37″)
Download: Ogg Vorbis  MP3 | Embed

New Zealand Herald article on the project here

Thursday, 1 March 2012

Rotomahana multibeam survey

This week I am revisiting Lake Rotomahana with Cornel de Ronde and two surveyors from IXSurvey, Mark Matthews and Dave Mundy. Our first goal in this year's research at the lake is to make the most detailed map possible of the lake floor. Next week we will use this detailed map to help us take a closer look at the areas of the Pink and White Terraces using seismic survey techniques. The mapping survey will also give us a great deal more information about the hydrothermal activity underlying large parts of the lake.

Last year, our improved map of the time helped us to identify the comma shaped submerged landform that led us to the remnants of the Pink Terraces. This year we are using  a multibeam sonar scanner that is improving our map resolution by at least ten times. We have been witnessing the gradual revelation of fascinating details of the lake floor that shed additional light on the violence of the 1886 Tarawera Eruption and its aftermath.

The scanner is housed below the centre of the small motorboat. As we travel over the surface of the lake, sound waves are beamed out in a line downwards and out to each side. The time taken for the soundwaves to return to the on-board sensors from each direction is translated by the computer into a bathymetric map of the lake floor. The initial, 'uncleaned' map shows up in realtime on the onboard computer screen, with colours representing different depths from red (shallow) through to yellow, green and blue as the depth increases. In this image, you can see that the boat is mapping a submerged crater at the edge of the lake.

As we criss cross the lake, the map appears as if it is being gradually 'painted' on the screen. Where the lake is shallow, the width of the scan is narrow, perhaps ten or twenty metres, whereas in the deeper areas it can extend to about 100 metres on each side. It is amazing to be able to watch the lake floor appear in crisp detail before ones eyes, showing many features that were created by the 1886 eruption and then hidden below the water for over a hundred years.

There are numerous explosion craters, mudslides, ridges,  depressions and pock marked gas vents. Vast streams of bubbles are also picked up by the scanner, showing that the lake floor is still actively fizzing. Many of the deeper gas bubbles dissolve in the water column as they rise up, but in some places they vigorously break out at the surface as you can see in the photo.

Here Mark is putting a sound velocity probe into the water to calibrate the sonar survey. The sound velocity depends on the water density, which varies with temperature and dissolved minerals. This is important because the velocity of the sound waves affects the calculation of distances and depths.

Just beside the access road to Lake Rotomahana there is a unique geological horizon. The dark line in this freshly excavated roadside outcrop represents the ground surface up to the day before the Tarawera Eruption, ie June 9th 1886. Above the dark line is the mass of erupted pumice known as the Rotomahana Mud that covered the landscape from the early morning on June 10th. A single, dramatic day in time represented in the geological record around Lake Rotomahana!

Our investigations next week will attempt to answer the question as to whether the 'Eighth Wonder of the Natural World', the Pink and White Terraces still lie largely intact under the mud just like the dark soil horizon, or whether the exposed portions we located last year are all that is left.