Seeing beneath the water and mud of Lake Rotomahana
Here is our video describing this years Rotomahana explorations – a hidden world come into view!
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Lake
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.
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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
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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.
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Groundwater dating around Lake Rotorua
“If you want to have an insight into a society, just look at the water in the streams and rivers” Uwe Morgenstern, GNS Science’s specialist in groundwater and ice dating, runs a laboratory that is the most accurate water dating facility in the world. His methods are so precise, that they are four times more accurate than the next best lab, out of a total of about 70 such laboratories worldwide. In a nutshell, groundwater dating works like this: Cosmic rays from outer space interact with our atmosphere and form very small amounts of tritium, a radioactive hydrogen isotope with a half life of 12.3 years. This cosmogenic tritium eventually becomes part of the atmospheric water, with one normal hydrogen atom replaced by a tritium atom. As this water (or snow) precipitates and becomes incorporated into groundwater, it is no longer interacting with the atmospheric tritium, and its tritium concentration starts to deplete due to radioactive decay. Measurement of tritium concentrations in groundwater allows the time since it fell from the sky to be calculated, back to a maximum age of about 100 years. Over the last few days I have been out in the field with Uwe and Mike Toews (a groundwater modeller at GNS Science) sampling the streams and springs around Lake Rotorua. The water quality in Lake Rotorua, and the many other smaller lakes in the area, is very important to the local community, for drinking, agriculture, recreation and tourism, including world famous trout fishing. Farming, especially dairy, beef and sheep farming, is also a very important activity around the region. Farm effluent and fertilisers cause nutrients, particularly nitrates, to enter the groundwater and eventually get transported into the lake. As a result the chemical balance changes, with potential negative impacts such as the growth of toxic algal blooms and other ecological changes such as impacts on fish. To understand the effects of land use on the water quality in the ground, in streams, rivers, and lakes, you need to not only monitor the concentration of pollutants in the water, but also measure the age of the groundwater. For this reason, Uwe has been studying the groundwater around Lake Rotorua for a number of years. With such large groundwater systems, it can take many years or decades for polluted water (for example nitrate from farms) to reappear back on the surface in streams and lakes. Because of this time lag, large groundwater systems can silently become contaminated until the contaminated water reaches the spring discharge. Then it will also take the same long time to flush the contaminated water out of the groundwater system. The data Uwe is coming up with shows a range of time spans for the input of lake water, from very quick (months) to over a hundred years in the case of Hamurana Spring. The map shows coloured dots representing the springs and streams that were on our list for resampling. For a news article about the findings of this research have a look here. Here is a video, describing the research and the findings so far:
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The White Terraces Reappear after 125 years
On 10th June 1886, exactly 125 years ago today, Mount Tarawera erupted briefly and violently, resulting in the disappearance of the Pink and White Terraces of Rotomahana, and devastation of the landscape. The former lake disappeared and was slowly replaced by the much larger and deeper lake which remains to this day. This 1880 Charles Spencer image is courtesy of Te Papa Museum Last January, in a GNS Science led international expedition, Cornel de Ronde and his team rediscovered the Pink Terraces at the bottom of the modern lake, which had been so drastically altered and deepened by the eruption. The Pink Terraces were first spotted in images from a side-scan sonar that was mounted in an autonomous underwater vehicle (AUV) used to survey the lake. Today Cornel de Ronde announced that the White Terraces have also been found using data retrieved on the last day of the expedition, that had not been analysed until recently. When the Pink Terrace side-scans were first seen, they were nothing like anything that had been observed by the team before. An underwater camera was used to confirm that they did indeed represent the Pink Terraces. (For details of the Pink Terrace discovery watch this video). Similar looking side scan images have now been found in the location where the White Terraces are expected to have once existed. A horizontal segment of the formations over 150 metres across may be the remains of the silica terraces along the former shoreline of the lake, now tens of metres below water level. It is not yet known whether more of the terraces lie hidden beneath volcanic mud, or whether the rest of them were forever destroyed in 1886. Future exploration may settle this question. Ron Keam of Auckland University is an expert on the history of the Tarawera Eruption and the Rotomahana landscape. He compiled this map of the former Lake Rotomahana as accurately as possible by detailed study of pre 1886 photographs. The Pink Terraces can be seen on the left (west) side of the lake, with the White Terraces at the top (northern) end, about a kilometre northeast of the Pinks. The image to the right is the compiled side scan of the part of the modern lake under which the remains of the terraces lie. The long straight lines show the path of the AUV as it progressed up and down the lake area. The red circles show the locations of the two sets of terraces, about 1 kilometre apart. Lower left are the Pinks and upper right are the newly refound parts of White Terraces. This close up of the side scan image shows the curved overlapping terrace formation on the lower half below the blank, unscanned area. These features are very similar in general appearance to the photographically verified scans of the Pink Terraces found last summer. (All sidescan images courtesy of our US project partners at the Woods Hole Oceanographic Institution) For more details have a look at our media release, and watch the video of Cornel de Ronde describing how the discovery unfolded step by step, including the crucial hook shaped landform that first led to the location of the Pink Terraces, followed now by the Whites:
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Pink Terraces found!
The Pink Terraces of Rotomahana before the Tarawera Eruption of 1886 (Permission of the Alexander Turnbull Library, Wellington, New Zealand, must be obtained before any re-use of this image). Devastation after the Tarawera eruption Yes – the unbelievable news is, that inspite of being located at the centre of New Zealand’s most violent eruption of 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 re-discovered! Dan Fornari of WHOI Scientists involved in the Rotomahana Project announced their findings last night at a special meeting in the marae at Whakarewarewa, 125 years after the Tarawera Eruption. Thanks to the underwater vehicle and imaging technology and skills developed by the Wood’s Hole Oceanographic Institution (WHOI), and the expertise of marine scientists from Lamont-Doherty Earth Observatory (LDEO) and NOAA-PMEL in the USA, this discovery has been made possible. A key component of the expedition’s field approach was the use of the Remus100 autonomous underwater vehicles (AUVs) developed at WHOI. Dr. Vicki Ferrini of LDEO observed the terrace formations as she was processing images from the sidescan sonar on one of the AUVs on Saturday. After checking details like water depth, location, orientation, shape and size of the features, Cornel de Ronde and the rest of the science team reached the conclusion that these can only be a part of the original Pink Terraces last seen on June the 9th 1886. The features show up as curved step like surfaces that are visible in the sonar images as bright reflectors due to their strong reflectivity, This means that they comprise a hard material, unlike the softer, therefore acoustically darker sediment that surrounds them. It is the steeply sloping or vertical sections that show up most brightly, probably because they are free from any overlying sediment. This early, pre-eruption photo of the lake shows the Pink Terraces on the left. In the middle distance you can see a hooked spit of land extending into the lake as a very distinct feature. This gives the lake shore a very identifiable contour just northeast of the Pink Terraces. In the compiled bathymetric map produced by Vicki from the AUV survey data, the same hooked-shaped peninsular can be seen in the lake bed, now several tens of metres under water. This is in the northern section of Lake Rotomahana, in the area that has been indentified as the probable location of the Pink Terraces by previous researchers. The close-up of the lake floor bathymetry shows this feature clearly (depths in the map at right are color coded- pink is deep and red is shallow). The main cascade of the Pink Terraces would fit in the green embayment in the top centre of the image, and the wide lower part of the terraces should extend down roughly in the centre along the green or pale blue band.When overlaid on top of the bathymetric map, Vicki’s step like rock features lie exactly on top of this location. This means that the lower portion of the Pink Terraces still remain. It is possible that they were covered in debris by the eruption, and that subsequent water erosion has exposed their edges again. The question still remains as to whether the upper section of the terraces is still intact underneath a layer of sediment. The sonar sensors used in this survey are unable to reveal adequate subsurface detail to answer this. However, a future expedition could settle this question using seismic reflection techniques. A further investigation was made by lowering an underwater camera, developed by Dr. Dan Fornari at WHOI, down to the bed of the lake to take a closer look. The following images are a selection from those taken. The first shows a small crater with a hazy cloud of bubbles and coming out of it. This depression is roughly a metre across. Because of disturbance caused by the high level of hydrothermal activity, the ranges of the camera images are only a few metres. * In the next photo you can see some vertical relief. On the right, the dark shadow is one of the terrace steps, whilst further to the left, across the sloping muddy lake floor there are some smaller exposed vertical sections of rock. These shapes are typical of hydrothermal silica deposits seen in other parts of the Rotorua geothermal area.* The last of the underwater photos, taken near the region identified by Vicki as being where the strong reflectors of the Pink Terraces are located, show the vertical edge of a terrace head on. The scaling is not exact but in the region of one or one and a half metres in height and could be an exposure of the lower part of the Pink Terraces. * These initial findings leave many questions that can be followed up in the future. But for now, to know that at least a part of the Pink Terraces of Lake Rotomahana are still there, hidden in the depths of the water, is a fantastic outcome of the Lake Rotomahana Project. Quite apart from this discovery, the analysis of the overall expedition findings will give the scientists plenty to do towards the goal of understanding the whole hydrothermal system of Rotomahana. Dr. Cornel de Ronde, of GNS and leader of the expedition has something to smile about. * “Digital underwater photographs taken by Dr. Dan Fornari – Woods Hole Oceanographic Institution (WHOI), Cape Cod, MA, USA, using equipment developed with funding from the US National Science Foundation and WHOI. Digital underwater camera developed by Mr. Mark Olsson of DeepSea Power & Light, San Diego, CA, USA. Copyright D. Fornari – Woods Hole Oceanographic Institution.”
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