Igneous

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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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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The Hot Bed of Rotomahana

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This week I have been with Cornel de Ronde and a group of ocean floor researchers applying more of their methods to expand the large amount of research of Lake Rotomahana done over recent years. This is the lake that used to be decorated by the famous Pink and White Terraces. It was excavated by the extreme violence of the Mount Tarawera eruption in June 1886. This photo of a cliff section in the nearby Waimangu Valley, shows a black horizontal soil layer that was buried by volcanic mud during the eruption. The area still has a lot of geothermal activity. One of the tasks for this expedition was to measure the heat flow coming up through the lake floor. Scientists from Woods Hole Oceanographic Institution (WHOI), the National Oceanic and Atmospheric Administration (NOAA) and the University of Waikato collaborated with the project. Maurice Tivey of WHOI provided the special blankets for measuring heat flow in the ocean. This was the first time they had ever been used on a freshwater lake. The blankets have a thermistor (thermometer) on the top and the bottom. They measure the temperature on the surface of the lake floor sediment and also of the water layer just above. The difference between the two measurements allows the amount of heat flow to be calculated in watts / square metre (w/m2). The heat blankets are lowered on to the lake floor in a pre-determined grid pattern and left for 24 hours to equilibrate with the prevailing temperatures. Then they are pulled up to the surface and re-deployed in a new position. Gradually the whole lake floor gets coverage in this way with the 10 available blankets. The thermistors take readings of the temperature every minute and store the data until they are eventually plugged in to a computer for it to be downloaded. In the image you can see the temperature curves for a blanket that has been deployed at 4 different locations over 4 days. The upper curve shows the data from the lake sediment recorded by thermistor under the blanket. The lower, darker curve is the (cooler) water temperature recorded by the top thermistor. You can see that it takes several hours for the readings to adjust to the lake floor temperature conditions. The last recording on the right hand side is very hot, so the thermistor records a rising temperature. The dots on this map of Rotomahana show the locations of the measurements. Maurice has outlined the hot areas identified initially, although the data had still to be fully processed. You can see how the areas of high heat flow in the map above correlate well with the map of gas bubbles recorded on the surface of the lake in 2012. This may seem obvious for a hydrothermal system, but gas plumes are not necessarily accompanied by heat. This is a map of a heat survey that was undertaken in the 1990s. This week’s survey is more detailed and uses a new method,  but it will be interesting to see how the results compare. In the earlier survey, areas of heat flow of up to 10 w/m2 were outlined. Some of Maurice’s recordings are several times hotter than these. In this video. Maurice describes the new heat flow survey method:

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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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Cape Kidnappers

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Cape Kidnappers and the Clifton Cliffs make for a spectacular geological site in Hawkes Bay. The cliffs extend for several kilometres southwards from Clifton, on the coast near Hastings. They  are very high and consist of quite loose rocks, so it is important not to go too close where possible. It is also important to start your visit on a falling tide which will give enough time for a return trip without being cut off by high water. At the start, near Clifton, the cliffs are made up of thick river gravels, with thin layers of white pumice (volcanic ash) and occasional dark layers of plant material.  Initially the beds are about 300 000 years old. Because they are dipping gently down to the north, you will pass further and further down the sequence as you walk along the beach to the south.and east. Here you can see the fluted erosion of the unconsolidated gravels caused by rainwater. In this photo, a layer of light coloured volcanic ash separates overlying river gravels from marine mudstones below. Just above the ash is a very thin dark organic layer with plant remains in it. There are many pale coloured ash layers in the sequence. They have been erupted from the Taupo Volcanic Zone in  the Central North Island, at least 150 kms away. The thickness of the layers even at this distance, testifies to the magnitude and violence of these past rhyolitic eruptions. In this photo you can also see how a fault has dislocated the beds by several metres. Further along the beach, towards Black Reef, there is a distinct change in the bedding, seen in this image about half way up the cliff. The lower gently dipping beds have been eroded flat with much younger beds deposited on top of them. This unconformity represents a time gap of about two and a half million years. The lower unit is three and a half million years old – the upper one starts at about 1 million. An exciting find on our visit was this fossil whalebone. It extended through the boulder for about one metre. Out on the reef itself were some well preserved shell fossils as well as another orange coloured whalebone fossil slowly being eroded away. Last but not least I should mention the gannets, for which Cape Kidnappers is most famous. The young birds here will take their first flight soon, and without looking back or touching down will travel all the way to Australia. Cape Kidnappers features on our GeoTrips website where you can also find lots of other locations to explore geology and landforms: www.geotrips.org.nz/trip.html?id=182

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Rock in the Boat

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Along with biological specimens, the sled brings a lot of rock off the sea floor. Christian Timm sorts through all the samples, cuts some of them up with a rock saw, and packs and labels them to be studied in detail back at GNS Science. The different minerals present in the samples will be analysed to give detailed information about the processes occurring deep down in the collision zone where the Pacific and Australian plates meet, as well as about the hydrothermal alteration of the rocks at the sea floor. Here is a selection of examples from off the cone of Rumble 2 West volcano that we have been checking out for the last few days: First up is a piece of volcanic rock (basalt) that comes from the cone of Rumble 2 West. It would have cooled rapidly as it encountered the sea water, which has preserved the flow structure running through the centre of the specimen. Hydrothermal fluid contains a lot of iron that it has dissolved from the basalt it has passed through down in the crust. As it reaches the sea floor and cools, it precipitates out the iron as an oxide called haematite, which has a deep red colour. Silicon is the most common element in the earth’s crust, along with oxygen with which it combines as silica (quartz). There are many other siliceous minerals too, some of which are precipitated around hydrothermal vents in association with microbes. These yellow and orange pieces contain several types of silica with different quantities of trace elements that give a wide colour range. The whitish stripes in this piece of basalt are from small crystals of barium sulphate or barite. As sea water flows down into a seamount and heats up, it loses a lot of calcium sulphate which precipitates out. The hot, sulphate poor fluid then dissolves barium from the surrounding rocks, bringing it back up to the sea floor. The barium then combines with the sulphate in the fresh sea water to give rise to these barite crystals. They tell us that hydrothermal fluids have been cycled through the crust in this area, and can even be dated to give a timescale for the process. This small piece is a jam packed mixture of rock fragments and minerals. It It is part of the debris from an old broken up black smoker chimney. It is loaded with valuable metal rich compounds that have crystallised as the hot hydrothermal fluid gushed out into the surrounding sea water.  Cornel de Ronde is checking out the finds and explaining some of their features to crew member Peter Morrison.

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