NZ Geology Sites

Places to visit geology in New Zealand

Where to explore the Wellington Fault

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Wellington Fault at Thorndon

The Wellington Fault is one of several large active faults in the lower North Island of New Zealand. From the Tararua Mountains and Kaitoke it runs the length of the Hutt Valley, the edge of Wellington Harbour, through Tinakori in the City and across the hills to Cook Strait. Earthquakes occur on the Wellington Fault approximately every 700 to 1000 years on average, with the last between 170 and 370 years ago. The probability of a rupture in the next 100 years is calculated to be about 10%. Because it runs along the highly populated Hutt Valley and right through the Capital City via its transport bottleneck, it is regarded as one of the country’s highest risk faults. You can find out information about all of New Zealand’s known active faults on the GNS Science Active Faults Database, but in this image you can see a screen grab of those known in the Wellington area, some of them labelled: As you can see there are many other faults in the region, each of which is capable of rupturing, so that the real possibility of a large earthquake occurring at some point from one or other of the faults is something that should inspire everyone to be prepared. (Make some time to go to https://getready.govt.nz/ ) As you can see there are many other faults in the region, each of which is capable of rupturing, so that the real possibility of a large earthquake occurring at some point from one or other of the faults is something that should inspire everyone to be prepared. (Make some time to go to https://getready.govt.nz/ to get the best information on how to do this.) Here is an aerial view of the Wellington Fault trace (bottom right to centre top of the photo) passing through California Park in Upper Hutt and along the centre of  California Drive beyond In neighbouring Harcourt Park, the fault crosses a flight of river terraces at a right angle. This allows us to see clearly that the slip (movements) on the fault are mostly horizontal with some vertical movement as well. Looking across the fault the opposite side moves to the right. This means that the fault is a “dextral oblique slip fault”.   This diagram shows how the Harcourt Park River Terraces are offset by the Wellington Fault The fault can be followed along the Hutt River. In Lower Hutt it runs right along the side of Hutt Road, and into Petone. This photo shows the fault scarp at the end of Te Mome Road where it meets Hutt Road at a T junction:   The entrance to Wellington City at Thorndon is a bottleneck, where the Wellington Fault passes underneath the railway, State Highway and Ferry Terminal, as well as the water supply. This makes Wellington vulnerable to being cut off by a rupture of the Wellington Fault. You can learn more by visiting the Wellington Fault at several points from Upper Hutt to Wellington. Check out this video for details:  

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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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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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Opunake

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Another great geological venue on the South Taranaki coast is at the Opunake boat ramp, where we took our Geocamp participants recently. On two sides of the car park there are cliffs showing a spectacular sequence of strata. information for you to visit this spot is on our GeoTrips website here: www.geotrips.org.nz/trip.html?id=56 It’s a perfect spot to practice drawing a geological section and making detailed observations. Drawing is very valuable as it forces you to be careful and attentive to details. The observations lead to the next question. What can these rocks tell us about the processes that put them in place? These colourful orange, grey and yellow bands contain numerous volcanic clasts (pebbles and boulders) suggesting that they have originated from the Taranaki / Egmont Volcano, an obvious source about 25 kms north-east of Opunake. Generally when you see stratification in a rock it suggests that it has been laid down in moving water (or from the air in some cases such as volcanic ash layers or sand dunes).  This layer shows well developed graded bedding – the larger particles were laid down first, followed by finer and finer material. The very coarse unit above indicates another very high energy phase of deposition. In some places you can find very large boulders that have been deposited and left in scoured out hollows that have then been infilled with finer material. In  this image you can see a channel on the right of centre that has cross cut the horizontal layers and been infilled with gravel. This also shows that the sedimentation process was occurring in a high energy environment. So a fair interpretion of the Opunake sequences is that of volcanic material that has been eroded off Mount Taranaki and deposited in a fluvial (river) environment, possibly as reworked lahars or debris flows that have been mobilised by floods. If you take a look at the landforms on the slopes of Mount Taranaki, you can see numerous gullies and larger valleys where the rocks have been eroded away, either by rivers, lahars or rock slides. Occasionally there have also been the massive debris avalanches such as the one that covered the forest at Airedale Reef)   Over time the mountain has produced the material that blankets hundreds of square kilometres of the surrounding plain. Here is the geological Qmap for Taranaki. The red rocks are volcanic lavas and related rocks centred on Taranaki / Egmont Volcano, whilst the pink rocks are pyroclastic and debris flow deposits. Opunake is on the coast  half way up the map on the left. You can see the profound effect of the volcano on the landscape, as it is at the centre of a radial arrangement of volcaniclastic deposits.  The volcanic rocks have been  spread across the landscape for large distances by the power of gravity and water.

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Waihi Beach Taranaki

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The South Coast of Taranaki near Hawera has extensive rocky beaches lined by high crumbling cliffs. It is a great place for geology, but you should be wary of the potential for cliff falls, especially after rain. This is the view east from the Ohawe beach access point. A first look at the cliff from a safe distance shows that it is made of two main rock types; massive grey muddy sandstone at the bottom, with darker brown soft stratified layers above. The boundary between the two layers is very distinct and can be seen for many kilometres along the coastline. If you take a closer look at some of the muddy sandstone boulders lying on the beach, you can find some nice fossils such as this scallop. In places these rocks are very bioturbated. In other words they have been churned up by organisms that burrowed through them when they were part of the sea floor. For a close up look at the boundary between the two layers that form these cliffs, a good place to go is the beach access track 4.5 kilometres east at Waihi Beach (end of Denby Road see www.geotrips.org.nz/trip.html?id=55 for location and geological info). There, right before you reach the beach, is an easily accessible outcrop where you approach the boundary safely.   Here is a slightly closer view – you can see the change from the lower grey unit containing oysters and scallops with the shell rich layers above. The fossils in the lower unit indicate an environment of deposition about 20 to 50 metres deep. This layer is approximately 3.5 million years old. Here is another image where can see the incredibly abrupt change from the lower muddy sandstone to a much looser sandstone packed with shells. Just below the boundary there are some vertically positioned shells in a line. These have burrowed down into the sediment from above and have been preserved in life position. Although they are found within the 3.5 million year old sandstone, they are actually only as old as the overlying shelly layer, which is about 125 000 years old . This means that the 3.5 million year sea floor sediment has been uplifted, eroded down to sea level, and then covered with shelly beach or estuarine deposits of much younger age. Nearly three and a half million years are missing from the sequence. Interestingly the same unconformity is widespread across Taranaki. Here you can see it at Wai-iti Beach on the north coast. Here the time gap is even greater, as the underlying grey sediments are about 8 million years old and represent deposition at about 500 metres water depth. This shows that there has been greater uplift and erosion in the north compared to the south Taranaki coast.Here is a video of Kyle Bland explaining the Waihi outcrop and the story revealed by fossils:

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Airedale Reef

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Earlier this week I was up in Taranaki, exploring the geology of the area  with two GNS Science researchers Kyle Bland and Richard Levy. One of the sites we visited was Airedale Reef, a short walk east along the coast from the mouth of the Waitara River. There are spectacular remnants of an ancient forest on the shore platform at low tide, with tree stumps in growth position and large logs sitting in a black layer of peaty soil. The forest layer reappears at the base of the nearby sea cliffs, with the roots and tree stumps gradually being eroded out. Just below the dark layer is an olive green bed of dune sands. The carbon rich forest layer is thickest in the depressions between the dunes. This  is one of the tree stumps emerging  from the cliff. But how long ago was this forest still living? How did it die and why was it preserved like this? The answers are in the layer just above it. This overlying layer is made of an unsorted mixture of different boulders and less coarse particles of rock. You can also find chunks of carbonaceous material scattered within the layer that must have been ripped up into it as it was emplaced. This 4 metre  thick layer of material has been mapped  over a minimum area of 255 km2 around north Taranaki, and has a total volume of at least 3.6 km3. It is believed that Mount Egmont (Taranaki) volcano is the source of this layer. Like some of our other andesitic volcanoes, Mount Egmont is made up of layers of unconsolidated volcanic deposits interbedded with more massive lava flows. Because the slope angle of the volcano is very steep, the cone is inherently unstable, resulting in occasional enormous avalanches of debris launching down the mountainside, spreading across the surrounding countryside and out into the sea for distances of up to 40 km from the source. For this reason Egmont is a significant geological hazard that is monitored by GeoNet. On our visit up the  mountain the following day, amidst the lava flows and ash layers we could see deposits such as these – not too different from the bouldery layer at Airedale Reef, although likely to be much younger. Back at Airedale Reef this photo shows a good view of  the layer that buried and destroyed the fossil forest. It is known as the Okawa debris avalanche deposit and has been dated at about 100 000 years old. This means the forest was growing during the last interglacial period. Pollen analyses shows a dense podocarp forest, but lacking specifically coastal plants. It seems that when the forest was alive, the coast was further out than its present position. Rimu Pollen  (Dacrydium cupressum) 43 microns across There is a lot of pollen preserved in the Airedale Reef cliff section. Scientists found over 10,000 pollen grains per cm3 in places.They were analysed to study the plant communities from the period of time represented by these layers. Cyathea treefern spore, diameter 30 microns  This allows research into climate variations through time, as different species appear and disappear up through the cliff section from the base to the top. The Rimu and tree fern species in these two images indicate a lush podocarp forest that grew in warm, wet conditions. In the next layer above, the species found represent a sub-alpine shrubland community that grew in a cooler climate. In this photo you can see two pale coloured tephra (volcanic ash) layers near the top of the carbon rich layer, showing periodic eruptive activity from the volcano. In the last image you can see that another carbon rich layer formed in a depression at the top of the Okawa Formation (centre left). Above that the rest of the section is made up of orange and pale brown soils.

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Tongariro North Crater

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Earlier this week I decided to spend the night camping up on the North Crater of Tongariro. This is the large flat crater that is off to the north and west of the main track of the Tongariro Crossing. For access information have a look at the GeoTrip page here: www.geotrips.org.nz/trip.html?id=279Here is a view across to it from near to the Red Crater: The crater is about 1 km across, and is believed to have once been a lava lake several thousand years ago. In the distance you can see the crater rim to the right of the cone of Ngauruhoe. The surface of the ground is uniformly covered with scattered blocks of lava. This windswept area feels isolated and rarely visited, even though it is so near to the Tongariro Crossing track. There is a spectacular explosion crater within the North Crater itself, over 300 metres across and about 50 or more metres deep. It has broken through and partly obliterated the surface of the solidified lava lake. A low angle valley cutting across the main crater represents the line of a fault. Debris from the explosion crater to the left of the image has partly filled the valley. This photo taken by Lloyd Homer in 1984 shows two more faults (dark lines) crossing the slopes on the western flank of Tongariro. They are normal faults, indicating extension of the crust that is associated with the volcanism in the North Island. They have been active since the Taupo eruption 1800 years ago The Tongariro Crossing passes just below and east of North Crater. There is a barrier prohibiting closer access to the Te Maari Crater / Ketetahi area  . This is the 2 km exclusion zone due to continued volcanic eruption hazard. From the edge of North Crater, there is a view down to Ketetahi Hut and across to Upper Te Maari. It is sobering to think that the hut was damaged by large flying rocks erupted from Te Maari about 2 km away during the August eruption. If you click on the image to enlarge it you can see the hut near the left side of the photo. This was the view at sunrise, looking down on to the steam plume coming from Upper Te Maari.Crater.

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Titahi Bay Geology

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Titahi Bay is a great place to visit if you are interested to see some of the geology near Wellington. There are a number of very interesting features to look at and explore. The first thing to check out is the coastal landforms caused by a combination of the atmosphere and  the sea, as well as the variable resistance of the rock, and a history of earthquakes (uplift). The first image is taken from the Pa site, a few hundred metres north of Titahi Bay beach. If you are a teacher, this is an excellent place to encourage your students to observe some of these natural features, such as sea caves, sea stacks, arches, marine terraces and wave-cut platforms. There is more information about how these features form on coastlines generally on the GNS Science websiteYou can also have a look at this GeoTrips page for specific information if you would like to visit this area. This sea cave marks the line of weakness of a fault. It is no longer at sea level, having been uplifted out of range of the water by earthquakes. It is also a useful way through the rocks between two small embayments. A striking feature of some of the rocks at Titahi Bay is this type of weathering out of the spaces between joints to form distinctive criss cross box structures Having looked at the erosion and weathering features along the coast, the next thing to do is have a look at the structures and the rocks themselves. A good place for this is just south of the Pa site, accessed down a short very steep track from Terrace Road. www.geotrips.org.nz/trip.html?id=69 In this photo you can see that the rocks are made up of alternating bands of massive sandstone, with in-between layers of dark mudstone. These rocks were formed from sands and muds eroded from the margin of Gondwanaland, long before New Zealand existed. The material flowed down into the deep sea and settled over wide areas. The coarser sediment, at the base of each of these submarine landslides, is represented by the sandstone, whilst the mudstone gradually settled on top.After deposition, the sediments were squeezed and deformed by the bulldozing effect of plate collision along the edge of Gondwanaland. You can see how the originally horizontal layers are now  almost vertical at Titahi Bay. Many faults are easy to spot, as they displace the clearly defined rock layers.As well as faults there are also folds in the rocks such as the anticline (upfold) shown here. An interesting challenge is to look for sedimentary features such as graded bedding or cross bedding, in order to tell the direction of younging of the steeply tilted rocks.  In this photo you can see some cross bedding, showing where the rock above my finger cuts across some fine layers that must have been layed down first. If you have time whilst at Titahi Bay, and if the tide is out, you should have a look at the tree stumps of the fossil forest which are sometimes exposed, usually at the south end of the beach. It seems almost unbelievable that these wooden stumps date from a time before the last ice age, about 100 000 years ago. The fossil forest does actually extend right along the beach, but is mostly covered with sand. On rare occasions, about once a decade, storms clear the sand away to expose much more of the forest than you can see here.Look carefully and you can see the growth lines of these ancient tree stumps. Check out the GeoTrip location here: www.geotrips.org.nz/trip.html?id=32https://youtu.be/A2Jed7P-pQ0

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