Landform

Whakataki

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Whakataki is a spectacular spot on the Wairarapa coast just north of Castlepoint. It features as one of the localities on our GeoTrips websiteThe shoreline is a large area of tilted rock strata that have been eroded into a broad, flat platform extending for hundreds of metres along the coast.  The rock layers are alternating sandstone and mudstone layers that stand out as distinct lines. It is believed that they formed as cyclically repeated turbulent flows of sand and mud that avalanched down and over the sides of an underwater channel about 500 to 1000 metres below the ocean surface. These sorts of deposits are know as turbidites. Here they are of early Miocene age (roughly 20 million years old) As the sediment laden water surged across the sea floor it laid down a deposit of sand and mud with several distinct layers. The base layer typically has very flat laminations, followed by a more convoluted and rippled layer above it. Above that the particles get finer as the remaining cloud of mud slowly settled on top of the coarser sandy layers below. It is interesting to look at the different structures and imagine how they formed in the dark depths of the sea so long ago. Here I am pointing at some climbing ripples in the upper sandstone layer, above a more regularly laminated base layer of the flow. They show that the current was moving from the left (south). Exploring the area shows up many interesting geological features. Here you can see that the beds are not only tilted up, but they have been dislocated by faults.   In this image you can see joints cutting across the beds at a right angle. They develop as the pressure on the sequence decreases due to erosion of overlying material. You can see how the spacing between the joints is wider for the thicker beds, and closer together on the thinner ones.   The rock layers are of interest to geologists because similar thin bedded fine grained deep sea sediments are often found to be important reservoirs for hydrocarbons which penetrate into the tiny pore spaces between the individual grains of sand.  By studying these beds where they are exposed at ground level, we can gain important information about similar but more inaccessible  sequences deep below the surface that may actually contain trapped oil or gas. During our visit, Garth Archibald was making a laser scan of the surface of the shore platform. This will be translated into a 3 dimensional computer image of the platform which can then be used for detailed analysis of the different layers in the sequence. Garth has used his laser scanner in a wide variety of settings, including a number of Christchurch cliffs that were seriously shattered by recent earthquakes, as you can see in this video.

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Lake Tutira – tectonic uplift, ice ages, landslides and cyclones

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Lake Tutira is a scenic spot on the route between Napier and Wairoa in northern Hawkes Bay. It is in a very rural setting, surrounded by steep hillsides and farmland. The landscape around the lake contains several powerful geological stories. The first is that the hills themselves, made up of rocks that are about 1.8 million years old, reach a height of up to 800 metres above sea level. Before being uplifted and exposed by erosion, the rocks may have been buried to depths of 500 to 1000 metres. This means that they have been rising at an average rate of about a metre every 1000 years. When you look at this steep hillside, you can see lines of cliffs running almost horizontally across the slopes. These bluffs are made of relatively hard limestone, with softer muds and sandstones hidden beneath the grassy slopes separating the cliffs.The top limestone band that you can see correlates with the one on the top of Waipatiki Beach that I showed in a recent post. These cliff lines therefore represent the cycles of global change that repeated every 40000 years. The hard limestones were deposited as sea levels were slowly rising, while ice caps melted at the end of each glacial period, as shown also in my previous posts from Waipatiki and Darkeys Spur. The next landscape feature of interest is this area of grassy hummocks just beyond the pine plantation. These are the debris pile from a massive landslide that slid down from the nearby hills about 7200 years ago. It blocked the stream that flowed down the valley, thus forming the present day Lake Tutira.  Similar huge rock slides occurred in other parts of the region at the same time. Scientists believe that they may have all been triggered by a single massive earthquake. One of our activities on the recent ‘Dinosaurs and Disasters Geocamp” with Hawkes Bay schools was to drill a sediment core from Lake Tutira. Kyle and Richard used a PVC drainpipe which they pushed into a shallow part of the lake bed. Although only about half a metre in length of core was extracted, you can clearly see a number of layers. The top of the core is to the left of the photo, with several organic rich layers visible. The lower half consists of varying amounts of pumice that will have been washed into the lake from where they accumulated after the Taupo eruption 1800 years ago. The lakefloor sediment has in fact been studied in detail by researchers from GNS Science and other institutions . In 2003 a drill rig was set up that retrieved a 27 metre core right through to the base of the sediments. It revealed a detailed history of the environment around Lake Tutira over the last 7200 years: Almost 1400 storms were intense enough to leave their traces in the form of layers of mud washed down from the surrounding hills. Periods when storms were more common started abruptly and could last for several decades. Volcanic eruptions from the Taupo Volcanic Zone (including the well known ‘Taupo Eruption’ of 1800 years ago)  have left layers of ash that can be dated. Changes in land use from native forest to pasture due to human occupation, have increased the sedimentation rates tenfold.. During Cyclone Bola which passed over Hawkes Bay in 1988, over 750 mm of rain fell over four days.  A huge number of mudslides came off the hillsides over the whole region. In this photo, Richard Levy and I have exposed a buried soil layer next to Lake Tutira. It is beneath about metre of pale brown ‘Cyclone Bola Mud’  (top half of image). The dark soil layer below contained branches of wood. Further down there was another pale coloured mud layer from an earlier rainstorm.

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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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Moa Hunting

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Takaka limestone country Whilst on holiday in the Nelson area last week, I went for a look around the limestone plateau of Takaka Hill, not far from the huge natural shaft of Harwood’s Hole. I managed to persuade a couple of friends to come along for the adventure. The area is very rugged, covered with rock outcrops and tangled vegetation. There are many caves (see my earlier blog post from January 2010) and my particular interest was to look for small vertical shafts that might have acted as lethal traps to the moa that once roamed the area. The delights of moa hunting With some careful searching, it did not take long to find some cave entrances. Some of the shafts are very deep and obviously care is needed in this environment to avoid the fate of becoming entombed and fossilised just like the moa that we were hoping to discover. As you can see, some of these caves are very small. With a bit of wiggling and squirming, we were able to push down into them. Moa bones lie scattered at the bottom of a cave Sure enough, a couple of them contained parts of moa skeletons lying at the bottom. In this image you can see a variety of bones, including leg bones and a pelvis. The number of different bones that we saw in this cave indicated that at least three or four moa individuals had been caught there. Moa pelvis  This is a close up of the pelvic bones of a moa Moa bones in narrow fissure At the very bottom of this cave, there were more bones visible, but the fissure was too tight to get close to. We were very satisfied with our discoveries, and happy to leave the bones in place for future rediscovery and study.

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Romanian Ice Cave

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On a recent trip to Europe, I spent time in Romania, France and Italy. Here are some of the geological highlights I visited: This photo is an underground glacier in the Apuseni Park. This is an area of the Carpathian Mountains with over 200 limestone caves. The Focul Viu ice cave that we visited is one of several in Romania. There is about 25 000 cubic metres of ice which has accumulated as snowfall from a large hole in the ceiling of the cave ( along with branches and leaves from the surrounding forest). Due to the poor circulation, cold air sinks into the cave and maintains very low temperatures even in the hot summer months. An 8 metre ice core has been retrieved from this cave. A piece of wood from 7 metres down was dated at about 1700 years old.

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Tararua Range

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With Uwe Morgenstern, also from GNS Science I hiked the length of the Tararuas just north of Wellington over 4 days. The Tararua mountains are a section of the ‘backbone’ of New Zealand. They are made of greywacke rock, pushed up by the tectonic forces of plate collision. Like most of New Zealand’s mountains, they form an obstruction at right angles to the prevailing westerly winds and are renowned for high winds and high rainfall – often making for tough tramping. Luckily for our trip we had very stable and clear conditions, a rare treat! We set out from the Putara road end near to Ekatahuna, at about 9am. After following a stream and hiking upwards in the forest, we came out into the open tussock after a couple of hours. It was quite cloudy through the day. This made for an amazing sunset as we continued along the ridge to Arete Hut which we reached after dark. For the final hour and a half we had to navigate by map and compass, with a bit of searching to finally reach our refuge for the night. Day 2 saw us following the ridge for hour after hour. the distant point on the horizon was our objective for the night and again we reached Jumbo Hut after dark. We passed by this possum that seemed to have lost its way and was hiding under a rock well above the tree line. Day 3 was our longest day. Leaving Jumbo hut at about 7am, we traversed to Mount Holdworth, then dropped down through the forest to the Mid Waohine Hut which we reached at about 11am. After a very brief dip in the river, a long and exhausting climb saw us back on the tops further to the West. From Aokaporangi Peak we headed to Maungahuka Hut which we reached about 5pm. After a short break we decided to keep going in order to be able to complete our traverse in time for work on Monday. As fast as possible we dashed over the Tararua Peaks – very steep ground which we didn’t fancy attempting in the dark. Soon we were beyond them and darkness was upon us. Four hours of arduous tramping by headtorch we arrived at Kime Hut and the end of a fifteen hour day. All that was left was to complete the Southern Crossing of the Tararuas, over Mount Hector and down the less travelled Quoine Ridge. The views in all directions were spectacular, including Taranaki and Ruapehu volcanoes, our own route through the Tararuas, the Wairarapa Plains, Palliser Bay, the Rimutaka Ranges, Wellington Harbour and the Kaikoura Ranges of the South Island. Just above the tree line we came to this GNS Science continuous GPS station cemented into the bedrock. This device is part of a nationwide network that continuously monitors the horizontal and vertical displacement of New Zealand on its plate boundary. As we descended, we entered the Goblin Forest – a wierd world of beech trees covered with mosses and lichens. Finally by 5pm we arrived at our awaiting vehicle and the prospect of a very satisfying rest.

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Marlborough and Kaikoura from the air

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Yesterday I was offered a flight from Paraparaumu Airport to Kaikoura by Felix Morgenstern. His father Uwe is the water and ice dating specialist here at GNS Science. This was a great chance to admire some fantastic landforms created by the uplift of the leading edge of the Australian Tcctonic Plate right up close to the collision boundary, just east of the north end of the South Island. We flew directly south across Cook Strait, away from Wellington’s cloudy skies to a clear day over the South Island. This picture shows the coast of the Wellington Peninsula fringed by marine terraces that have been uplifted and tilted by successive earthquakes. Cook Strait is in the foreground and the white wind generator towers are making use of windy Wellington’s prime natural resource! On the other side of the Strait, the skies were clear. This is a view across the coastline to the south-east of Blenheim. The rocks that underly this landscape include several locations where the famous K-T boundary layer (Cretaceous – Tertiary or Cretaceous Paleocene boundary) is exposed. This thin dark layer of clay is found at different places worldwide. It marks the point in time when a massive asteroid impact in Mexico caused the extinction of the dinosaurs and many other life forms. One such place is the rocky hillside near to the true left of the river in the foreground. Soon we were passing alongside the inland Kaikoura Range, whose highest peak is Tapuae o Uenuku at 2,885 metres. It is made up of a complex of resistant igneous rocks, thought to be of Cretaceous age. Faults active for the last 20 million years are lined up along each side of the range, parallel to the plate boundary just off the coast. These are mainly strike slip (sideways moving) faults but there is also significant vertical movement pushing the mountains up by up to 10 mm a year. I took this picture as we were approaching the Kaikoura Peninsula. Suspended sediment in the sea from river outwash has developed into a nice spiral shaped eddy. Beyond the narrow peninsula is the area where the Hikurangi Trough meets the coast of the South Island. The Kaikoura Canyon – a deep slice in the ocean floor just off the coast, has created a haven for wildlife there including whales and other marine mammals. After landing for a short while at the Kaikoura airstrip we took a short detour out to sea. We didn’t see any whales but there was a crowd of dolphins surrounding a boat. A nice way to leave Kaikoura behind.

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Palliser Bay

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Palliser Bay is an isolated sweep of coastline about 2 hours drive from Wellington. It is separated from New Zealand’s busy capital city by the Rimutaka Range. Yesterday I visited the area with a group of Lower Hutt school children as part of their Year Seven geology camp. Fully armed with the “Kiwi Fossils Hunter’s Guide” as well as another excellent book by Lloyd Homer and Phil Moore that describes the geological features of the Wairarapa Coast called “Reading the Rocks“, we visited several great geology hotspots along the coastline. A striking feature that we noticed straight away was the flat topped escarpment that runs along much of the coast. This is a raised marine terrace that was at sea level about 80 000 years ago. It indicates that the whole area has been undergoing an enormous amount of uplift which continues to this day. First stop was Hurupi Stream. (This is described in detail in the “Kiwi Fossils Hunter’s Guide“). The soft mudstones at the sides of the stream were deposited under the sea in the Miocene Epoch (sometime between 11 and 7 million years ago) , when the Aorangi Range just to the North was an island, separated from other parts of the North Island by a shallow sea. We found quite a few marine molluscs that are very well preserved and easily spotted. Not far along the coast road are the Putangirua Pinnacles. These spectacular features have been eroded out of a thick sequence of conglomerate. Hard layers or large individual boulders within the conglomerate form a protective cap at the tip of each pinnacle. The ground is strewn with loose rubble – testament to the fact that the erosion here is still very active. This might not be the best place to visit in a rainstorm! A few kilometers along the coast road, there is a dramatic example of coastal erosion where a whole section of the original road itself has disappeared! We followed the coast past the small settlement of Ngawi, and a huge tilted slab of fossiliferous sandsone called Kupe’s Sail, to the Cape Palliser lighthouse. This is built on a cliff of volcanic rock that was erupted under the sea as pillow lavas about 100 million years ago. The long staircase up to the lighthouse leads up to a great viewpoint. This is the Southeastern tip of the North Island of New Zealand, with nothing but ocean between here and Antarctica or South America. Just a few kilometres out to sea is the Hikurangi Trench, the collision boundary between the Pacific and Australian tectonic plates. The connection between uplifted terraces, fossils, erosion, earthquakes and volcanoes gave us all something to think about to round off our geological excursion.

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