Landform

Precarious Boulders and Earthquakes

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The National Seismic Hazard Model is the result of lots of work by scientists to indicate the likelihood of earthquakes happening in different parts of New Zealand. It is made with reference to the historic record of earthquakes that have happened across the country, combined with research into the rupture histories of many individual active faults. Work is done to continuously ‘ground truth’ and improve the Hazard Model through ongoing research and addition of data. Mark Stirling has developed a way of testing the model at particular locations using ancient landforms known as tors that occur in places around the country. These isolated boulders stand like statues. There are many of them near Clyde in Otago, occurring on the flat, uplifted surfaces of nearby ranges, such as the Old Man Range, shown here. You can see that some of these features are quite imposing and have a lot of character. Although some of them are solid looking, there are others that are very delicate.These are the ones that Mark is interested in. The basic idea is to use the beryllium 10 exposure dating method to find out how old these fragile features are, and then to work out the amount of earthquake shaking it would take to knock them down. This tells Mark the minimum amount of time that has lapsed since the occurrence of an earthquake capable of knocking down the feature. This information is then matched with the National Seismic Hazard Model to see if the calculations give similar hazard estimates. Making a numerical calculation of the fragility of the precarious feature is a matter of working out the angles between the centre of mass and the rocking points at the neck (narrowest point) of the tor. For making these calculations with maximum precision, Mark makes a 3D computer model of the tor, by first taping key points on its surface, and then taking many photos from all angles, which are later stitched together. This is what the model of the above tor looks like on the computer screen once completed . During fieldwork with Mark last month, we were able to use a quadcopter drone to get good images of some of the more inaccessible fragile landforms. Here is our video of the project:

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Wairarapa Fault – the Biggest Rupture on Earth

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The Wairarapa Fault is one of New Zealand’s large active faults running along the eastern edge of the Rimutaka range from Palliser Bay north into the Wairarapa. It was responsible for the massive magnitude 8.2 earthquake that violently shook the lower North Island in 1855 in New Zealand’s largest historically recorded ‘quake. This Google Earth view shows the surface trace of the fault, with the Rimutaka Range to the west and the Tararuas in the distance. An interesting location called Pigeon Bush is indicated by the red circle. It is about 50 kilometres north-east of Wellington City. Photo Andrew Boyes / GNS Science The second photo is a view of the Pigeon Bush locality from the nearby road, showing a steep scarp uplifted by earthquake ruptures of the fault. The fault itself runs along the base of the scarp, which is the product of several earthquakes over the last few thousand years. A close up view shows some interesting features beside the fault scarp. Two stream  channels (middle and foreground of image)  appear out of the scarp, with no sign of any catchment gully above them. Meanwhile a bit further along (where the trees are) you can see that there is a deep cut gully in the scarp itself. Geologists have long recognised that the stream that created the two small ‘beheaded’ channels has been shunted along horizontally by the last two ruptures of the fault. In this photo, Rob Langridge, an earthquake geologist from GNS Science, is standing between the first (most recently beheaded) stream channel on the left, and the vegetated gully that was originally connected with it on the right. Some idea of the amount of offset that occurred in the 1855 earthquake can be appreciated from the image. There would also have been some uplift during that earthquake of perhaps one or two metres at this location.  We used a tape measure and recorded the distance along the fault between the centre of the now separated stream gullies, and came up with a figure of about 18 metres. This huge displacement is the largest offset to have been caused by a single earthquake on a land based fault known from anywhere in the world. (It is now known that subduction earthquakes such as the great 2011 Tohoku Earthquake of Japan can produce even greater displacements of the ocean floor) We also measured the offset of the older stream channel which was about 15 metres away from the first beheaded channel.This previous earthquake is thought to have occurred about 1000 years ago. The average repeat interval for ruptures of the Wairarapa Fault is thought to be about 1200 years. Offset stream channels at Pigeon Bush, A Boyes / GNS Science Here is an image taken using a drone and annotated by Andrew Boyes at GNS Science: About 45 kilometres north of Pigeon Bush it is possible to see a view of the fault itself in a cutting of the Ruamahanga River near Masterton. In the photo you can see how older grey rock on the right (west) have been pushed up relative to the younger gravels on the left (east) in a reverse fault. The substantial horizontal movement may also have caused this juxtaposition of older rocks against younger ones. Here is another view of the fault where it is known as the Wharekauhau Thrust in a cliff section at Thrust Creek on the Palliser Bay coast. Royal Society Teacher Fellow Phillip Robinson is inspecting the older shattered greywacke rocks that have been thrust over the gravels from the west (left), tilting the relatively young 50 000 year old gravel layers from a horizontal to a vertical orientation. This is the view looking south from Thrust Creek along to the southern tip of the Rimutaka Range, with Turakirae Head in the far distance. During the 1855 earthquake, a maximum of 6 metres of uplift occurred along this coast. A 10 metre high tsunami also swept along this coastline. Check out this previous post to learn about the amazing uplifted beaches at Turakirae Head. Note that you can now find out how to visit Thrust Creek (and many other geology locations) on our GeoTrips website here: https://geotrips.org.nz/trip.html?id=255

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Stepping Over the Boundary

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This is a classic view of the Southern Alps from Lake Matheson on a still morning, showing the high peaks of Mount Tasman and Mount Cook.The Alpine Fault runs along the foot of the steep rangefront, extending right up the West Coast of the South Island. The mountains are therefore part of the Pacific Plate and all the flat land in front, made up of glacial outwash gravels, is on the Australian Plate. This graphic shows the Alpine Fault as a very distinct line dividing the high mountain topography to the East and from the coastal lowlands along the West Coast. Arrows show the horizontal directions of fault ruptures along the fault, but there is also a vertical component that is pushing up the Southern Alps. At Gaunt Creek near Whataroa, you can get right up close to a cliff exposure of the Alpine Fault.  The pale green rocks in the foreground have endured being crushed and uplifted along the  fault line. They have been altered into what is known as cataclasite, consisting of clay and lots of crushed rock fragments.You can visit this location by checking out our GeoTrips website here: www.geotrips.org.nz/trip.html?id=57 The low angled line of the Alpine Fault is very distinct on the right side of the photo, with the metamorphosed cataclastic rocks that have been uplifted from kilometres down in the crust being pushed over the much younger gravels to the West (right). You really can put your finger on New Zealand’s plate boundary here! The Pacific Plate is on the upper left, thrust over ice age gravels of the Australian Plate on the right hand side of the image. The photo gives a good impression of the nature of the crushed rocks. A more distant view of the cliff section from the creek shows how the uplifted rocks have over-ridden the gravels which are about 15 to 16 thousand years old. The two white arrows show the line of the fault. A short distance away is the Deep Fault Drilling Project (DFDP1) Observatory that was set up after two boreholes were drilled here in 2011. The fault is dipping at about a 40 degree angle, and the boreholes were positioned to intercept it at around 100m depth. Instruments down the boreholes include seismometers and other sensors that have been installed to better understand the physical conditions along the fault as it extends down below the surface. For a bit more background to the DFDP have a look at this previous post from 2011

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The Dart Landslide

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Simon Cox   GNS Science M. McSaveney GNS Science Slip Stream is a tributary to the Dart River in the South Island of New Zealand. There has been an active landslide here for several thousand years, periodically sending down lobes of debris to gradually build up a large fan in the Dart Valley. There was vegetation established right across the fan, but over the last few years the widespread cover of trees has been largely buried and killed off by a very active phase of erosion and deposition. Debris volumes of the order of 100 000 cubic metres have been coming down during heavy rains in the spring and summer periods. Simon Cox  GNS Science The debris gets mobilised into a wet mix of mud and boulders.  The latest large event occurred early in this month (4th January 2014), and the flows continued to build up over several days. M. McSaveney GNS Science The debris flows crossed right over the valley, blocking the Dart River with a low angled, shallow pile of soft sediment. M. McSaveney GNS Science A lake formed in the valley above the slip, becoming about 4 kilometres long. The river is cutting down into the debris, and it is expected that the depth of the lake will fluctuate during landslide activity. The Department of Conservation is diverting the affected part of the Dart Valley track so that trampers can continue to visit the area. Photo DoC/Vladka Kennett This image gives a good overview of the affected area.  It shows the fan with the darker coloured triangle of recent debris, as well as the length of the lake. This is a graph from the Otago Regional Council website showing 7 days’ rainfall recorded from the 9th to 16th January at the Hillocks, about 24 kilometres down the Dart Valley from Slip Stream. The second graph shows how the river flow responded to the rain, with a sharp peak and a gradual tailing off after the rain stopped falling. The tail is not entirely smooth with a dip when the flow gets below 100 cubic metres per second. This suggests that when the river level drops, the continuing input of debris at the slip impedes the flow for a while, until the blockage is overcome and the flow rate increases again. Mark Rattenbury (left), Simon Cox (right) and Mauri McSaveney (behind the camera) visited the area to assess the impact and any possible downstream hazard. Note that a special DOC permit is required to visit Slip Stream as it is in the sacred Te Koroka topuni area of Mount Aspiring National Park.  The slip is in a state of continual instability and the area is hazardous. In this video Simon explains some of the interesting features of the slip, including some very strange bubbles that release dry dust when they burst:

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Turakirae

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Photo: J.Thomson The windswept coastline between Wellington Harbour and Palliser Bay forms the southern tip of the Rimutaka Ranges. These hills themselves are an extension of the axial ranges that stretch the length of the North Island. This is the view east from Baring Head towards Turakirae Head. In good weather, this rugged environment isthe best place in the Wellington area for bouldering (low level rock climbing), but it is also a spectacular place to witness the effects of tectonic uplift on a coastline. From the end of the Wainuiomata Coast Road, follow the track to Turakirae Head seal colony,  which is about 40 minute’s walk. On the way you may notice that there are gentle steps in the landscape running parallel to the shoreline. Photo L.Homer / GNS Science The lines that you can see in this aerial photo are ridges of washed up rocks that have been gradually piled up during many southerly storms. The reason that there are several storm ridges is that the coast has been uplifted by successive earthquakes, thus pushing the shoreline further out  and causing the creation of a new ridge after each event. Photo L.Homer / GNS Science At least 5 ridges have been identified. Carbon dating of shells and plant material in the ridges shows that the oldest one (furthest from the sea) is over 7000 years old, with others (shown in the image) dating back to 5000, 2300, 158 years and present day. These are not thought to represent all the uplift events that have affected the area over that time, but simply the ones that have been well preserved. The most recent uplift was during the 1855 earthquake. This involved  a massive rupture along the Wairarapa Fault that passes very close to Turakirae Head. It was New Zealand’s largest historic ‘quake, with an estimated magnitude of 8.2. It caused widespread damage, such as numerous massive slips in the Rimutakas, but fortunately few fatalities. A similar magnitude earthquake in Wellington nowadays might be a different matter simply because of the denser population and more developed infrastructure.. For more information on the Wellington earthquake hazard check out the GNS website here Turakirae Head featured on the Coasters programme recently, hosted by Steve Logan who met me there with his film crew from Fisheye Films on his way along to Palliser Bay. The flat path like line extending into the middle distance is the top of  the pre 1855 storm ridge. Although very rocky it makes for a reasonable 4WD or walking track. Steve came along on his pushbike and interviewed me about the geological features of Turakirae Head, as well as about its rock climbing attractions.In case you missed the programme on TV1 on Saturday 22nd June, you can watch it online here. As well as Steve and the crew from Fisheye Films, we were accompanied by Sophie and Frank (right in  photo) – two local Lower Hutt climbers who were part of the support team. Here is the GeoTrip information for you if you would like to visit Turakirae: www.geotrips.org.nz/trip.html?id=249

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Saddle Cone

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On my way back to civilisation from Tama Lakes, I decided to take a detour to visit Saddle Cone, ( GeoTrip page here: www.geotrips.org.nz/trip.html?id=53 ) a small isolated crater on the northern slopes of Ruapehu. You can see the tilted rim of the cone in the centre of the photo: The second image is looking into the crater of Saddle Cone, which is about 100 metres across.In spite of its small dimensions, Saddle Cone produced a huge lava field that spreads out over an area of several square kilometres. These lava flows are visible in the distance. On the right side of this photo you can see a moraine ridge, showing that this valley was glaciated until about 10 000 years ago. This provides a maximum age for these lava flows, and many others in Tongariro National Park’s glaciated valleys. Hot arid summers, and freezing blizzards in winter are not too much for hardy alpine plants such as these: After several hours of wandering the semi-desert of the Tama Saddle, I descended to a river less than an hour from the road – a perfect oasis to end my hike on the mountain.

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Ngauruhoe’s Far Side

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Climbing Ngauruhoe from the South is well off the tourist route, and involves scrambling up unstable blocks of lava for about 700 vertical metres up the face of the cone. I chose to go up more or less up the centre of the view you can see here, and it took me about an hour and a half of steady plodding to the top. The crater of Ngauruhoe was last erupting from 1973 to 1975, during which time it occasionally threw out blocks of lava to a distance of about 3 kilometres. If you click on the image to enlarge it you will see people on the crater rim that give an idea of the scale of the image. Ngauruhoe’s crater rim provides what to me is one of New Zealand’s finest landscape views. On the far left is Tongariro peak, then the flat top of North Crater and the Blue Lake (with steam from Te Maari just behind it). Just below the Blue Lake is the top of Red Crater and on the right side are old lava flows in the Oturere Valley. The Tongariro crossing track passes through South Crater as a white line in the centre of the photo. Descending the northern slope of Ngauruhoe, I then climbed a rocky ridge up to Tongariro peak, seen running from the centre to the right side of this photo: Next on my route was Red Crater, followed by a swift run down grey coloured soft scree just visible on the right of the photo. This took me into the Oturere Valley from where I turned back in the direction of my campsite. In the area to the east of Ngauruhoe I cut across country around the base of the volcano. This is a relatively rarely explored area. It took me a few more hours tramping across a variety of moraine ridges and blocky lava flows to reach my tent after a very satisfying day.

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Tama Lakes

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Last weekend I went to camp and tramp in the Tama Lakes area on the saddle between Ruapehu and Ngauruhoe. These lakes were created by several explosion craters within the last ten thousand years  giving them a circular or crescent form. The landscape is covered with blocks of lava and scoria as well as some fine ash  remaining from Ruapehu’s 1995 – 1996 eruptions. There are also some layers of pumice from the huge Taupo eruption about 1800 years ago. This photo shows some charcoal fragments – remains of some of the vegetation that was scorched during the most violent eruption on earth in the last 5000 years. The lower Tama lake is being slowly filled up by a river bringing in eroded ash and other volcanic debris from the surrounding area. You can see this delta on the far side of the lake in the image. Beyond it is a similar adjacent (sediment filled) crater of about the same size. The water is very clean and drinkable, and yes – it really was that blue! I set up my tent in a little hollow, sheltered from the wind and on a nice flat spot. The view north from my campsite shows the Upper Tama lake and the south face of Ngauruhoe, my planned hike for the next day.

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A dynamic landscape in Hawkes Bay

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Last week I was in Hawkes Bay with geologist Kyle Bland, who led a field trip for teachers, students and parents of Crownthorpe School. Hawkes Bay geology is a story of uplift along fault lines, combined with rapid erosion and deposition by rivers flowing from the inland mountain ranges. This story is etched into the geomorphology of the landscape. The Mohaka fault last ruptured between AD 1600 and 1850, and forms an amazingly straight scar across the landscape. Like many faults in New Zealand, it is an oblique strike slip fault, including both sideways and vertical movement.  If you click on the image to enlarge it you can see how streams crossing the fault have been offset by sideways movement from the last rupture. Combined sedimentation, uplift and erosion have produced stepped terraces alongside the Ngaruroro river flowing from the Ruahine range out towards the coast. There are many fossils to be found in the sedimentary rocks that have been uplifted and exposed. Fossil hunting Hawkes Bay style involves using a digger to get access to your specimens! Ancient greywacke sediments are exposed in the Ruahine Range, having been uplifted by tectonic movements of the North Island fault system (Mohaka and Ruahine faults). These rocks were deposited in a trough at the edge of Gondwanaland, long before New Zealand ever existed. In the video below, Kyle gives us a Hawkes Bay case study of landscape evolution.

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Filming on Ruapehu

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Last week I spent some time on Ruapehu with Bruno Cedat, a french documentary maker who is making a film about the geology and landscapes of New Zealand in collaboration with GNS Science. During the making of his film he has participated in challenging outdoor adventures such as climbing, caving and kayaking in a variety of wild places across New Zealand, Here he is climbing the Pinnacles next to Whakapapa ski field. We also tramped up the mountain to the summit plateau, with great views across to Ngauruhoe volcano further north. In this next picture you can see Bruno approaching the Dome, along the edge of the summit plateau.   The Dome Shelter was covered in rime ice. Inside the shelter there is a seismometer that is used to monitor volcanic earthquakes. Here is the GeoTrip page for you to climb up to the Dome: www.geotrips.org.nz/trip.html?id=646 Here is a view of the crater lake, surrounded by a winter blanket of snow. It is currently at Alert Level 1 as you can see on the GeoNet website. For  lots more information on Ruapehu have a look at our website here Here is a preview of Bruno’s Film: New Zealand, Land of Adventure:

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