Rotomahana

The Hot Bed of Rotomahana

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This week I have been with Cornel de Ronde and a group of ocean floor researchers applying more of their methods to expand the large amount of research of Lake Rotomahana done over recent years. This is the lake that used to be decorated by the famous Pink and White Terraces. It was excavated by the extreme violence of the Mount Tarawera eruption in June 1886. This photo of a cliff section in the nearby Waimangu Valley, shows a black horizontal soil layer that was buried by volcanic mud during the eruption. The area still has a lot of geothermal activity. One of the tasks for this expedition was to measure the heat flow coming up through the lake floor. Scientists from Woods Hole Oceanographic Institution (WHOI), the National Oceanic and Atmospheric Administration (NOAA) and the University of Waikato collaborated with the project. Maurice Tivey of WHOI provided the special blankets for measuring heat flow in the ocean. This was the first time they had ever been used on a freshwater lake. The blankets have a thermistor (thermometer) on the top and the bottom. They measure the temperature on the surface of the lake floor sediment and also of the water layer just above. The difference between the two measurements allows the amount of heat flow to be calculated in watts / square metre (w/m2). The heat blankets are lowered on to the lake floor in a pre-determined grid pattern and left for 24 hours to equilibrate with the prevailing temperatures. Then they are pulled up to the surface and re-deployed in a new position. Gradually the whole lake floor gets coverage in this way with the 10 available blankets. The thermistors take readings of the temperature every minute and store the data until they are eventually plugged in to a computer for it to be downloaded. In the image you can see the temperature curves for a blanket that has been deployed at 4 different locations over 4 days. The upper curve shows the data from the lake sediment recorded by thermistor under the blanket. The lower, darker curve is the (cooler) water temperature recorded by the top thermistor. You can see that it takes several hours for the readings to adjust to the lake floor temperature conditions. The last recording on the right hand side is very hot, so the thermistor records a rising temperature. The dots on this map of Rotomahana show the locations of the measurements. Maurice has outlined the hot areas identified initially, although the data had still to be fully processed. You can see how the areas of high heat flow in the map above correlate well with the map of gas bubbles recorded on the surface of the lake in 2012. This may seem obvious for a hydrothermal system, but gas plumes are not necessarily accompanied by heat. This is a map of a heat survey that was undertaken in the 1990s. This week’s survey is more detailed and uses a new method,  but it will be interesting to see how the results compare. In the earlier survey, areas of heat flow of up to 10 w/m2 were outlined. Some of Maurice’s recordings are several times hotter than these. In this video. Maurice describes the new heat flow survey method:

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Rotomahana’s lake floor prompts many questions

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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.

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Lake Rotomahana Seismic

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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.

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Rotomahana multibeam survey

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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.

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Scientists to return to Lake Rotomahana

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Cornel de Ronde is leading another science team to further investigate the remains of the Pink and White Terraces on Lake Rotomahana in March 2012. Last year, a sidescan sonar mounted on an autonomous underwater vehicle or AUV, produced images of parts of the Pink Terraces emerging from the thick mud on the lake floor. Have a look at my previous blog post showing the images, and this video about the discoveries: This year, Cornel and his team hope to find out whether more of the terraces remain concealed under the mud. If you are a teacher of Intermediate or Lower Secondary students you may be interested to engage your classes with an activity related to this year’s project. How would you go about a further exploration of the lake floor? Download the activity via this link. More information about the plans for this year’s investigation are in the GNS Science media release.

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Posters of Lake Rotomahana

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Here is a last look at the posters of Lake Rotomahana created by Rotorua Primary and Intermediate school children.The first image is the winning poster created by Kaya Henderson-Corporaal, Georgia Perham and Breannah Wakefield of Galatea School. We were particularly impressed by the careful detail and realism of features on the lake floor in this poster.  The second prize was shared by two students from Rerewhakaaitu School: Mark Sincalir and Paige Skipper. They each created a very colourful and artistic poster. These portray the whole atmosphere of the lake as well as a variety of hydrothermal features and rocks on the lake floor.  This is Westbrook School student Cassandra Kiff’s picture of what she thought may be found by the Rotomahana scientists. “Maybe the Pink and White Terraces?” She has also shown colourful bubbles and vents in the lake. Traci Benson of Owhata School created this small poster. She clearly related the present lake floor to the history of the Tarawera Eruption. A very colourful and eyecatching picture. There were many other posters that impressed us. Here is another look at some of the display in Rotorua Central Library. The posters have now been moved to the GNS Science building in Avalon, Lower Hutt where they are on display in the foyer. A big thank you to all the teachers, students and others involved in bringing this poster exhibition about, including Sue Heke and other staff at Rotorua Library, Jessie McKenzie of the Royal Society of New Zealand and Harvey James of Waimangu Volcanic Valley.

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Poster Prizewinners

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 Last week I toured a number of Rotorua Schools to show slides and to describe the rediscovery of the Pink Terraces as well as other results of the Rotomahana Project. Over three days I talked to over 1100 children. I also took the opportunity to present prizes to the winners of the Rotomahana Project Poster Competition. The Prizes were awarded to those students who had produced the  most eye catching posters, that also most accurately predicted what the scientists would discover in the lake.  It was very difficult to choose the winners because of the high standard of the posters that had been entered. A lot of effort had gone into the children’s creations. This photo shows the engagement and interest of the children in learning about the Rotomahana Project.    The youngest prizewinning scientists each won a Rotomahana Project baseball cap, a certificate and a family pass to a guided tour of the Waimangu Volcanic Valley as well as a boat cruise on Lake Rotomahana. In this picture, Traci Benson, age six, of Owhata School is getting her prize.   Cassandra Kiff of Westbrook School was also commended for her poster and won an award.   Second Overall Prize went to Rerewhakaaitu School students Paige Skipper and Mark Sinclair who produced visually outstanding  posters. They won a guided tour of the Waimangu Volcanic Valley for 30 students and teachers, as well as hats and certificates for themselves. There were many very high quality posters from this school.     First Prize for best poster went to a trio of Galatea School students Kaya, Georgia and Breannah. Their teacher Marylou has been very enthusiastic about this project and was delighted at the success of her students. Part of the first prize is a guided tour of Waimangu Volcanic Valley and also a boat cruise on Lake Rotomahana for 30 students and adults. This class produced a number of outstanding posters and it is fitting that they will all share in the prize. Well done!    

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Pink Terraces presentations for Rotorua Schools

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From Wednesday 16th February to Friday 18th, I will be touring Rotorua schools with a presentation about the Rotomahana Project. (If you haven’t caught up about this yet you can read an earlier blog post about it or watch this video). Contact me soon if you would like your school included – there are a couple of time slots still available as I write this. This is an opportunity for local school children to learn something of how scientists go about their work as well as the inside story of the Pink Terraces rediscovery. I will also be announcing the winners of the Poster Competition. – If you haven’t seen the posters yet, the exhibition in Rotorua Central Library is still open until Friday the 18th February. The variety and detail of the posters makes for a colourful collection. Some of them have also proved to be very accurate predictions of what the scientists actually found on the lake bed of Rotomahana! Picking winners has not been easy. Here is a photo of the project leader Cornel de Ronde having a careful look at the entries. Watch this space after next week to see which ones were selected and earned the fantastic prizes offered by Waimangu Volcanic Valley

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