Geology

What’s on our Plates?

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Researching tsunami deposits on the East Coast

New Zealand has thousands of active faults each of which will produce an earthquake of some magnitude when it ruptures. However the two giants are the Alpine Fault and the Hikurangi Subduction Fault. They each form a segment of the plate boundary – the Alpine Fault can be traced across land, the length of the South Island, whilst the Hikurangi Subduction Fault is lying in wait under the Eastern part of the North Island, with its surface trace hidden deep underwater along the bed of the Hikurangi Trough and Kermadec Trench. Kekerengu Fault rupture in Nov 2016 (J.Thomson / GNS) Each of these plate boundary faults is capable of causing a massive earthquake greater than magnitude 8, thereby wreaking major destruction and disruption across New Zealand. It makes sense then that a lot of research effort is going in to understanding the past history of these faults. This allows us to gain insight into the probabilities of future ruptures and the sorts of impacts that could occur when one or the other of them next produces a big ‘quake. It also makes sense that if you are living in New Zealand, you should be interested in learning about how the scientists go about their research and what they have been discovering! What’s On Our Plates? is a set of free multimedia learning modules designed to enable anyone to explore Aotearoa New Zealand’s active plate boundary online, including the Alpine Fault and Hikurangi Subduction Zone. The modules are for any interested non specialist who would like to know more about out Plate Boundary research, but they also include notes for teachers who would like to use them as an educational resource. So get ready to dig in to the fascinating story of our two colliding tectonic plates. You can access the modules here.   The resource has been created by a collaboration of AF8 (Alpine Fault Magnitude 8) and East Coast LAB (Life At the Boundary). AF8 is undertaking a comprehensive study of the impacts a rupture of the Alpine Fault would have on infrastructure and the people living in communities across the South Island. It is a collaboration between the South Island Civil Defence Emergency Management (CDEM) groups and scientists from six universities and Crown Research Institutes, emergency services, lifelines, iwi, health authorities and many other partner agencies. The programme is managed by Emergency Management Southland. East Coast LAB (Life at the Boundary) is also a collaborative programme. It brings together scientists, emergency managers, experts and stakeholders across the East Coast to help us better understand and prepare for the natural hazards such as earthquakes and tsunami that may affect us. https://youtu.be/L8UXkQmbHZw

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Geology of Bitou, Lailai and Beiguan, Taiwan

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Bitou – this small fishing village is about 70km north of Yilan City. Right next to it is the Bitou Geopark. Here you can take a clifftop walk above steep sandstone cliffs, or descend to the shore platform to see some strange mushroom like features at close quarters Here the shore platform is festooned with these strange mushroom shaped concretions. They really are unusual, and make this a famous geological location in Taiwan. As you can see it is also a popular spot for fishing. Due to storms and occasional freak waves there are many accidents all along this coast where people get swept into the sea. Our next stop was the well known shore platform at Lailai. Here the gently dipping sedimentary beds have been folded and faulted. with hard layers of sandstone being less easily eroded (and therefore sticking out more) than the more easily etched out softer mudstones. The shore platform is impressive, with the tilted sedimentary rocks folded into gentle curves, and a lot of faults cutting through the layers. It was a perfect area to use my drone to get these aerial images. A short distance away there is a dyke (an igneous intrusion that originally pushed into the sedimentary rocks as hot magma)  that can be soon cutting through the sedimentary layers of the shore platform. It stands out because it is made of harder rock than the surrounding sediments, and is therefore more resistant to being eroded. Here you can see the dyke is offset – sometimes by faults but also simply by the magma pushing up through slightly different pathways in the original country rock. You can see here how the dyke has baked the adjacent mudstone – giving it a darker colour for about 40cm  to either side of the once hot dyke. A closer view of the dyke standing up like a man-made wall on the shore platform. The baked sediments right next to it have also been hardened by the heating process, so they have also resisted erosion more than the softer surrounding rocks. This video shows a bit more detail of the rocks of Lailai ,which I think is an ideal place to run a geological field trip: Finally on our way back to Dongshan, we stopped in the small Beiguan Tidal Park where you can see these rocks with impressive joints forming a diamond checkerboard pattern. In the background is Turtle Island, another well known local feature.

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Geology on the Yilan Coast, Taiwan

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To find worthwhile locations that offer great learning opportunities in geology, you have to spend time exploring outcrops, trying to make sense of the geological features that are exposed and then think of ways that students can explore and make sense of them out of their own activity. This inquiry learning process can work well via guided questions that encourage careful exploration and observation and then the unfolding of ideas and understanding. however it doesn’t usually just happen by magic – it takes some working out to frame interesting learning activities at a given unique location. With a small group of teachers from CiXin School, we explored several locations along the coast north and south of Yilan. Heading South we went to a coastal fishing settlement called Feniaolin. Here there were some amphibolites (metamorphic rocks) that are part of a long outcrop extending further south. These are amongst the oldest rocks in Taiwan and have been exhumed from many kilometres deep in the earth’s crust. Just past the fishing wharf there is an area of sea stacks – classic coastal erosion features: We continued further south to the Nanao Valley where there is a mixture of rocks on the river bed including many huge boulders. Some of the boulders were granites (that were once molten magma deep in the earth). They had lumps of schist included in them – fragments of the crustal rocks (xenoliths) that must have been incorporated into the molten magma before it crystallised. – given them a very striking apprearence. All in all there is plenty here to discover – rocks and minerals that have been metamorphosed by intense pressure and heat a long way down in the earth’s crust. Here is a video I made about our trip:

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Jinguashi – Gold and Copper mining in Taiwan

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During my stay in Taiwan I have been invited by the teachers to visit several areas that they consider to have educational potential for school camps. Iron and Copper minerals have stained the famous “Golden Waterfall” The first area was on the NE coast around Keelung Mountain which is an old dacite volcano. This area is rich in minerals including copper and gold. Gold was first discovered here by some chinese workers who were washing their food bowls in a local stream. They happened to be experienced gold panners, having moved to Taiwan from California where they had been part of the famous 19th century gold rush some years earlier. Part of the Jinguashi Mine complex, now abandoned. We spent a couple of days exploring the area, including several the rock outcrops, a museum, and the Jinguashi mining buildings. Memorial, Jinguashi Mine There is a memorial at the site of the prison camp where prisoners of World War 2 were held by the Japanese and made to work in the gold mine in slave conditions. Gold miner at Jinguashi, Taiwan This local old timer has a huge collection of minerals and a practical knowledge of the geology of the area as well as methods for mining gold and other precious metals (see video below). Keelung Mountain We decided that the area had great potential for a camp for the year 11 students, with lots of opportunity to explore chemistry, mineralogy and mining methods along with the social, environmental, economic and historical aspects of how resources are used in an area.

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Natural Hazards Science for East Coast Schools

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Natural Hazards Activities for Schools This article is about a science education project that I was involved in that was supported by MBIE’s Unlocking Curious Minds fund in 2018. It involved four three-day natural hazards science camps for intermediate level students in New Zealand’s rural Tairawhiti (Gisborne – East Cape) region. A total of 109 year 7 and 8 students and 16 teachers, from 19 schools were represented. The science camps were based at four different locations (Gisborne, Te Karaka, Tolaga Bay and Ruatoria / Te Araroa) with the activities and field trips tailored to suit each area. The events included lots of field trips and hands-on problem solving tasks. Here for the record is a summary of some of the activities and also a few of the places we visited: Introductory activities included observation and thinking exercises around the theme of science and natural hazards. These included demonstrations such as: earthquake P (longitudinal) and S (transverse) waves using slinky springs. A TC1 seismometer was used to demonstrate how ground shaking (created by participants jumping on the floor) can be captured as a wave trace on a projector screen, giving a record of the magnitude (energy) and duration of the vibrations. The TC1 can be used by schools and individuals who wish to detect earthquakes and interact with other enthusiasts online as part of the Ru programme in New Zealand Rock deformation: Alternating layers of flour and sand in a transparent container show how rocks can be faulted and folded by compression of the earth’s crust. For information about this demonstration have a look this earthlearningidea.com page. For a bit of a hands-on problem solving challenge, participants were invited to create some model structures to protect an area fromcoastal erosion. The models were set up in shallow storage containers. Once the ‘seawalls’ were built, the area behind them was backfilled with sand, the containers tilted at a shallow angle, and water added to within about 15cm of the ‘sea wall’. Here is one example: Testing involved using a plastic lid to push the water in waves up against the seawall, first gently, then with increasing energy. Different designs could then be compared and strengths and weaknesses discussed.   Following this exercise we travelled to Wainui Beach, Gisborne where there has been a variety of attempts to protect the foredunes which have properties built on them. Interestingly, many of the methods that had been used were similar to those that the students thought of with their model making. Here you can see the remains of a concrete wall that has been undermined by wave erosion    Another similar model making exercise, this time including a slope of cardboard at one end, was to design rockfall barriers.  These were also tested to destruction using varying quantities and sizes of rocks rolled down the slope. We were also able to do another activity associated with flooding which has been a big issue this year in the Gisborne area.The photo shows the sediment covering some of the farmland near Te Karaka following the floods. For this activity, participants had to design a stop-bank, and test how long it could retain water, by recording any pooling of water on the ‘dry’ side , every 30 seconds. If you are an educator wanting information sheets to run these activities they can be found on the GNS Science website learning pages here. Here are some of the field locations that we visited, and what we investigated at them: At Pouawa Beach, north of Gisborne, we made careful drawings of some deposits that are thought to have been laid down by a tsunami. Shells in these layers have been radiocarbon dated at about 2000 years old. The layers include gravel and shells that would have been transported from the sea floor. Using a  drone we could get a good view the top of a marine terrace (the flat surface upper left of pic) at the north end of the beach. The terrace was formed at sea level as a wave-cut platform during the last interglacial (about 80 thousand years ago), and has been uplifted since its formation by tectonic activity. There is a wide shore platform which you can see just covered by water in the photo. Another great example is nearby at Tatapouri –  for more information check out the GeoTrip here – these surfaces will also be uplifted eventually to form another step in the landscape. These marine terraces show that earthquakes and tsunamis have a long history on the East Coast! At the north end of the beach. we passed a landslide that had occurred during the very wet weather in June 2018. From the ground we could see the toe of the slip which included tree trunks, boulders and lots of muddy sediment. With our drone, we were able to get a much more complete view of the slip, including the source area, which was not visible from where we were standing. This shows clearly the value of drone technology as a tool to extend our view of this active landscape: Next stop Tolaga Bay, where the beach has been covered by logs, brought down from the forestry plantations by the recent heavy rainfall. The logs caused a lot of damage to properties, bridges and land as they travelled down the flooded rivers. Here we spent some time analysing the types of logs scattered on the beach, by counting the different species (pine, poplar, willow or other) within 10m square quadrats. The results showed that by far the majority came from pine forestry. We were able to visit a forestry area inland of Tolaga Bay, which showed that following harvesting of the pine trees, there is a period of time where the land is vulnerable to erosion before the next generation forest grows large enough to stabilise the soil. Following clear-felling, the slash (abandoned logs and branches) can get washed into rivers during heavy rainfall. Further North still we did a day trip

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Volcano City

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Mangere Mountain, L. Homer / GNS Science Volcanic cones, explosion craters and lava flows form much of Auckland’s natural topography. All of these, apart from one (Rangitoto Island) are from vents that erupted once only (monogenetic), with eruptions lasting a few weeks or months and then ceasing completely.  There are many accessible and beautiful locations that can be visited to uncover the geological history of the area. Auckland volcanoes, GNS Science Although there are about 50 volcanoes within a 20km radius of the city, there is a similar eruption process that generated them, with three main possible styles of eruption. Knowing the difference between these eruption styles allows you to interpret the different features and rock types of each of the volcanoes that you might wish to explore. The magma that erupts in the Auckland Volcanic Field (AVF) is generated in a ‘hotspot’ about 80 to 100 kilometres below the surface. It is a very fluid type of basalt that is known to rise quickly to the surface (at up to 5km / hour) from the magma source. Tuff outcrop at North Head, J..Thomson / GNS Science Once at the surface, the style of eruption depends largely on the amount of groundwater or sea water present. If there is a lot of water near the vent, its interaction with the hot magma (1000 plus deg C) causes it to instantly vaporise.  This, along with the expansion of gases within the lava itself, creates extremely violent eruptions that fragment the lava into small particles and blasts them upwards and sideways from a wide, flat explosion crater. This becomes surrounded by a ring of ash. Such deposits are known as tuff (pronounced ‘toof’ as in ‘woof’). You can see outcrops of this in Auckland, for example around the shoreline at North Head. Each individual layer represents an explosion from the vent. Surtsey eruption, courtesy NOAA This type of eruption is known as a phreatomagmatic or wet eruption, and a classic example occurred off the coast of Iceland from 1963-67 when the island of Surtsey was born. Mount Eden Crater, J.Thomson / GNS Science Scoria outcrop, Mount Wellington, J.Thomson / GNS If the magma reaches the surface where there is little interaction with water there is a different type of eruption. This includes eruptions in areas of dry land, as well as those that start off as wet eruptions, but where the water supply near the vent gets used up before the supply of erupting magma runs out. The magma then erupts in a fountain of lava, driven up by gases within it that are expanding as the pressure is reduced.The lava fountains might be several hundreds of metres high, with blobs of lava partially solidifying in mid-flight, and landing as scoria in a ring around the vent. This is a bit like the froth coming out of a soda bottle once the lid has been removed.  The scoria pieces and lava bombs are relatively sticky and can build the steep sided cones that are very recognisable in the Auckland landscape. The reddish colour comes from the oxidation of iron in the magma as it cools during its flight through the air. Lava bomb approx 1/2 m in length, Mangere Mountain  If you look at the rock that makes up these cones, you will see that it is made of bombs and fragments that may be partially glued together or more or less loose and rubbly. Takapuna lava flow, J.Thomson / GNS Science If one of these eruptions gets to the stage where the gas has mostly been expelled, then there is less energy available and the fire-fountaining stage ends. Should the eruption continue (which is not always the case) then the third eruption style starts to dominate. Lava pours out of the vent and pushes through the sides of the scoria cone to spread out around the volcano. Because it is such a fluid type of lava, a  variety of flow structures are preserved when it finally solidifies. Lava tree mould with bark impression, J.Thomson / GNS A great example of such a lava flow can be found along Takapuna Beach. About 200,000 years ago lava poured out of the nearby Pupuki crater and flowed through a forest. The tree trunks and branches were surrounded by the lava which cooled around them. The trees then burnt, leaving tree shaped holes within the lava. Takapuna Fossil Forest and Rangitoto, J.Thomson / GNS For more information about where to go in Auckland to see some of these geological localities, have a look at our new online map of geological locations atwww.geotrips.org.nz Could a volcanic eruption occur in Auckland in the future? What are the probabilities in the short to medium term and what would the impacts be? The short answer to the first question is ‘Yes,  definitely!’ There is no reason to think that eruptions won’t occur again. In order to answer the last two questions (‘When?’ and ‘What?’) it is important to get as clear a picture as possible of the history of past events, their timing, duration and magnitude, and their geographic relationship to the housing and infrastructure in the wider Auckland area. Auckland Museum Volcanic Eruption Auckland City and Mount Victoria, J.Thomson / GNS These questions are the focus of a long term scientific programme called DEVORA (Determining  Volcanic Risk in Auckland). DEVORA is led by GNS Science and the University of Auckland, and is core-funded by the EQC and Auckland Council. The first part of this programme has been to further our knowledge of the eruption history of the Auckland Volcanic Field volcanoes. What this work has shown is that there is no simple pattern that we can project to help easily forecast the likelihood of eruptions in the future. The timeline of eruptions shows them to be clustered, with large gaps between phases of relatively high activity.  Graham Leonard, photo by Brad Scott / GNS Graham Leonard of GNS Science is a co-leader of the project. He comments

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GeoTrips – visiting New Zealand’s geology and landforms

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The Tasman Glacier Lake from the air

Tasman Glacier Lake,  J.Thomson / GNS Science New Zealand is an isolated country with a very active plate boundary running right through it. For a relatively small landmass it has an astonishing variety of landscapes and is being continuously subject to dramatic physical occurrences that include earthquakes, volcanic eruptions, floods, landslides, rapid erosion and sedimentation. The geology of New Zealand can be explored in innumerable individual localities that each give individual insights into the geological story, like pieces of a jig saw puzzle. In order to visit these locations, a non specialist normally has to seek information in widely scattered sources such as specialist papers, local guidebooks, various websites or visitor centres. Many of these are out of print or out of date, and hard to get hold of. To overcome this issue, GNS Science has created a New Zealand geological locations map that allows interested people (eg members of the public, researchers, teachers and students) to have the information they need to explore our geology first hand. The content is provided by geoscientists and is aimed to encourage you to go to these localities and make your own observations, just like scientists do. As well as some geological background, there are images, directions, and some basic safety and accessibility information too. You can search the map using filters to focus on specific topics, rating scales or accessibility.  So… have a look, explore and plan some trips to become a New Zealand geological investigator!  www.geotrips.org.nz  

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Earth’s Magnetism in Antarctica

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A blog post by Tanja Petersen and Neville Palmer from their recent GNS Science trip to Antarctica to measure the Earth’s Magnetic Field. It took 8 hours for the Hercules aircraft to fly from Christchurch to Williams airfield, a runway on the Ross ice shelf close to Scott Base. Both of us had never been to Antarctica before; we had a big smile on our faces when we stepped out from the airplane onto the ice being greeted by dry crisp cold air and what seemed like a never ending blanket of snow.  Read up on the Hercules – it is a quite fascinating aircraft and has been around since the 50s! The view from Crater Hill, a volcanic cinder cone on the foot hills of Mt Erebus, provides a fantastic overview of the settings of Scott Base. You can see Williams airfield (upper left corner); the boundary between the thick ice shelf and the thin sea ice meanders diagonally through the photo towards White Island in the distance. The pressure ridges on the sea ice are semi-circling the green painted buildings of Scott Base. 10pm at Scott Base. 24-hour sunlight! Looking out from the back towards the two geomag huts (left). We are here to measure the strength and direction of the Earth’s magnetic field at two locations in the Ross Sea area, Lake Vanda & Cape Evans, where people have been repeatedly measuring it since 1974 and 1911, respectively. And we also want to check up on our equipment inside the two little green huts outside the back of Scott Base, which is continuously recording the local variations of the Earth’s magnetic field.  The accommodation for the night at Scott Base: One of the many corridors inside Scott Base connecting the buildings of different sizes and shapes. Corner, stairs up, another corner, stairs down … a bit of a labyrinth for a newbie! Häglund snow vehicle to the left, Mt Erebus in the background, a toilet tent, two sleeping tents, some shelters built into the snow and a flag marking a safe route.The inside of Scott Base is being kept warm & cosy at T-shirt temperature, but outside it is more like -6 to -12 degrees C (including wind chill – important factor!). The Antarctic Field Training is giving us a good practice run on how to keep warm outside, before heading into the field. Antarctica New Zealand provided us with heaps of layers of warm clothes to wear. We then were ready to load up the helicopter that flies us from Ross Island to Lake Vanda, in the Dry Valleys, 125 km away on the Antarctic mainland. The Wright Valley with Lake Vanda in the distance. Our fieldwork in the Dry Valleys, Antarctica, begins. First thing is to set up the fluxgate magnetometer near the Lake Vanda camp, before we walk to the nearby repeat measurement sites to get readings of the strength and directions of the magnetic field. Neville is measuring the directions of the Earth’s magnetic field at Lake Vanda. In 1767 the South Magnetic Pole was located around here; now it is about 1720 km away. We are repeating these measurements several times over the course of four days. Tanja on a special mission – the “P bottle” is part of keeping the environment as we found it. After those four days working at Lake Vanda we continue to Cape Evans, Ross Island, Antarctica for a day. The historic magnetic hut there was constructed in 1911 as part of Scott’s Terra Nova expedition. It has asbestos in its wall panels; its structure is protected by a plywood construction around it. Inside that hut is the wooden pillar that Captain Robert Falcon Scott and his team of explorers used to take magnetic measurements before heading into their ill-fated expedition to the South Pole. Over 100 years later Neville performs the same type of measurements, but in a slightly different outfit. The Terra Nova Hut nearby. Captain Scott’s base for his explorations of the frozen continent, in the early 1900s. It was also used by Shackletons’s Ross Sea party. After completing our work successfully our flight back gets delayed and we have a bit of time for some recreational activities on the ice shelf close to Scott Base before heading home to New Zealand.

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Tracking Dinosaurs in NW Nelson

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Greg Browne. Image Julian Thomson @ GNS Science In New Zealand there is only one area (with six individual locations not far from each other) in which dinosaur footprints have been identified. This is in NW Nelson in the South Island. They were discovered and researched by Greg Browne, a sedimentologist at GNS Science who has spent many years doing geological fieldwork in the area. The first announcement of their discovery was in 2009 as shown in this video. Dinosaur footprints near Rovereto, Italy. Image J Thomson When compared to the easily recognisable dinosaur trails that are found in other parts of the world, the structures that have been classified as footprints in New Zealand are not initially obvious.  The photo shows an example from near Rovereto in northern Italy where each footprint is about 30 cm across. Image Julian Thomson @ GNS Science In comparison, the New Zealand examples are irregular in shape and position. It took a lot of research and a process of elimination to be certain that these structures are indeed trace fossils of dinosaurs, rather than originating from another biological or mechanical cause..  In order to be able to point at a dinosaur origin for these impressions, there are several factors that have to be considered. As a starting point we can look at horses on a modern beach: Image: Van der Lingen, G.J. & Andrews, P.B This photo was taken by researchers who investigated horse hoof marks that were imprinted on a beach sand in New Zealand (from van der Lingen, G.J. & Andrews, P.B. 1979, Journal of Sedimentary Petrology). They carefully cut a vertical slice through the imprint to study the details of how the horizontal layers of sand were deformed by the weight of the passing animal. The hand lens shows the scale: Base image: Van der Lingen, G.J. & Andrews, P.B There are essentially three ways in which the original sediment has been affected:(A) – Jumbled particles and blocks of sand have  fallen into the depression made by the footprint.(B) The footprint has a clear vertical margin on either side(C) The sediment underlying the footprint has been compressed downwards.   It is most likely that these horse footprints were soon eroded after their formation in the late seventies, due to tides, storms, wind or even the action of shore creatures such as crabs, worms or shellfish. On the other hand, there is a small possibility that they were  preserved quickly beneath a new layer of sand and are still intact beneath this protective covering. Base image: Van der Lingen, G.J. & Andrews, P.B Over geological time, sediments such as these can become buried deeply, compressed into solid rock and later revealed by uplift and erosion at the modern land surface. In the case of the horse footprint, its appearence on the surface (in 2 dimensions)  would then depend on the amount and angle of erosion. For example, if it is were eroded near to the top of the footprint (the level of line 1 in the photo) it would appear relatively large compared to if the erosion had removed most of the material, and only the lower part of the footprint were showing (line 2). Similarly if a vertical section of the footprint were to  be exposed, its size and appearance would differ depending on whether the section that was revealed represented the centre of the footprint (3) or its edge (4). Image Greg Browne @ GNS Science Here is an example of one of the footprints that Greg identified in the Upper Cretaceous rocks of Nelson. It shows similar features in cross section to the horse footprint (at approximately the same scale)- the infilling (A), the distinct margin (B) and the compressed underlying layers (C). Image Greg Browne @ GNS Science Here is another example of a vertical slice through a footprint, with the dotted line highlighting the distinct margin of the structure: Julian Thomson @ GNS Science This photo shows a footprint eroded horizontally. The heel has cut a sharp edge into the sediment at the back end of the feature (lower left), while the front has been compressed into ridges as the foot tipped forwards during locomotion (near finger).   Having confirmed these features as footprints being preserved in sediment from an intertidal environment, the question then arises as to whether animals other than dinosaurs could have made them. Having tackled this question over many years, Greg Browne worked through the following possible examples and discounted them for the reasons given:  Fish feeding or resting traces: depth of penetration and lack of deformed strata below. Amphibian foot prints: unlikely to have an amphibian large enough. Bird foot prints: bird would have to be large and heavy. Mammals: the only pre-Pleistocene mammals known from New Zealand are Early Miocene mouse-like fossils. Evidence throughout the world indicates that Cretaceous mammals were small, and did not develop into large animals until after the end of the Cretaceous extinction event and the demise of the dinosaurs. Reptile foot prints: dinosaurs: only dinosaurs would be of sufficient size and weight to have generated these deformed point source compression structures. Recently, with funding from the Unlocking Curious Minds Fund of the Ministry for Business, Innovation and Employment (MBIE), a team from GNS Science were assisted by teachers and students of Collingwood Area School, to clean up a large rock slab in the search for more dinosaur footprints. With a lot of hard work, involving cleaning mudoff the rocks with buckets of water, brooms and shovels, some hitherto unseen dinosaur footprints were revealed for the first time since the Cretaceous Period, about 70 million years ago. Here are some quotes from our assistants:“It was a wonderful once-in-a-lifetime opportunity to work with a team of scientists and look at a real dinosaur footprints.” “It was an honor and very humbling knowing that we were the first people to see these footprints in 70,000,000 years.” “It was an incredible opportunity. We were able to work alongside

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Digging into the Alpine Fault

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The Alpine Fault has been the focus of a lot of research over recent years, including the Deep Fault Drilling Project, Alpine Lake Sediment Research and the Earthquake Records at Hokuri Creek amongst them. These are building a much clearer picture of the history of previous fault ruptures, and allowing better estimates of the size and likelihood of future earthquakes. The Alpine Fault is a long, straight, geologically  fast moving fault that typically produces very large earthquakes rupturing along large segments of its total length. At its northern end the Alpine Fault branches into a number of different faults that cross Marlborough and are known as the Marlborough Fault System. This means that here the Alpine Fault only takes up a proportion of the total displacements in this region and is likely to have a different earthquake history compared to thecentral and southern parts of the fault. Recently a GNS Science expedition to the northern part of the Alpine Fault near Springs Junction, involved digging two trenches across it to better understand its local earthquake history through some careful investigation. This location features as one of our GeoTrips that you can visit. www.geotrips.org.nz/trip.html?id=59 . This image shows the trenches (left of centre foreground) from the air. The trace of the Alpine Fault passes through the trenches and into the distance between the hills. Once the trench has been dug out,  the walls need to be cleaned up carefully so that the fine detail of the different sediments and structures can be observed and recorded. A string grid is pinned against the walls of each trench to help map them out, and markers are placed to highlight significant features that can sometimes be very hard to discern. The leader of this project is Rob Langridge, shown here having a close look at the detail. Many hours are spent drawing accurate maps of the trench walls as well as taking high resolution panoramic images of them. These are taken in order to document the excavation so that later interpretation of the data can continue once the trench has been filled up and the team has returned to the office. This image shows the Alpine Fault in section with the line of the fault shown.  The scarp or slope at the ground surface has been produced by earthquakes uplifting the left hand (eastern) side.You can also see the effect of fault movements on the river sediments below the ground. The grey clay layer on the left has been cut off at the fault and the overlying gravel layer has been dragged out of shape by repeated fault movements. This is a close up view showing the complexity of the sediments and structures close to the fault. When earthquakes uplift ground on the left side of the fault, loose material at the surface collapses across the fault and forms a wedge shaped pile of sediment on the ground called a colluvial wedge.These earthquake associated layers later get buried by younger material. They can be very hard to identify, but are a critical record of past ruptures. They can form repeatedly, so that wedges from earlier earthquakes may have more recent colluvium laid over the top of them. Once the colluvial wedges have been identified, the next step is to look for plant or animal material that has been trapped in them at the time they were created. These carbon rich specimens are carefully collected for dating in the lab using the radiocarbon dating method. (See below for a video that explains carbon dating) When a major fault ruptures during an earthquake, it can branch out near the ground surface to produce a number of smaller faults close to the main fracture. Here is an example that showed up in the trench wall a few metres from the main fault. The layers on the right have been pushed up  relative to those on the left. By carefully observing which layers have, or have not been affected by these secondary faults, the earthquake record can be further clarified. Once all the data and specimens have been gathered and logged, the trenches are filled in once more so that the surface can revegetate back to its original state. Here is a 3 minute video of the project: And this video explains radiocarbon dating: Finally click here for the TVNZ news report on the trenching.

Digging into the Alpine Fault Read More »