Earth Science

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Earthquakes in Southern Cook Strait

This is a screen shot of the Wellington seismometer from very early on Monday morning 22 July Following the earthquakes in Cook Strait over the weekend, it was impressive to arrive at work on Monday morning, and watch how the GeoNet team, many of whom had been busy right through the weekend, were in full action mode again. Ken Gledhill, the head of GeoNet, co-ordinated two meetings of the scientists and technicians during the day. There are lots of different things involved with understanding earthquakes. These include getting accurate locations and magnitudes, modelling the position and orientation of the fault and the type of fault rupture from the seismic wave patterns of the aftershocks, working out the adjusted stress on nearby active faults and then trying to calculate probabilities of future quakes to inform an ‘awakened’ public… Here is the aftershock map from Sunday evening that shows the relative magnitudes of the quakes and their initial locations in 2 dimensions. Up to today there have been nearly one thousand aftershocks already. As more and more occur and get analysed, a more precise 3D image of the fault(s) involved will be built up. To help get more precise seismic data, the GeoNet fast response team are already in Marlborough, setting up some extra temporary seismometers at carefully chosen locations to ‘fill in the gaps’ between existing permanent stations. Here is a photo of one of the team yesterday, packing one of the seismometers for the trip. To see more photos of what these guys do, have a look here. This is a computer simulation of the seismic waves from the M6.5 ‘quake propagating across the North Island and the adjacent sea floor: New Zealand’s background risk of earthquake probabilities has been calculated for the whole country. Once a reasonably large earthquake has occurred, these background risks of a larger quake increase for a while in the local area, and  a sequence of aftershocks follows that typically fits into a fairly predictable pattern of decreasing intensity over time. In the video below Matt Gerstenberger explains how these calculations are made to produce probability tables and maps for future aftershocks. For the latest information about the number and magnitude of aftershocks that have occurred in Cook Strait, as well as forecast probabilities for future quakes, have a look at this GeoNet page

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Turakirae

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

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

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

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

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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Fossil Favourites from GNS Scientists

GNS Science recently produced a new “Photographic Guide to Fossils of New Zealand”. It is a small, pocket sized booklet, packed with photos and information about many of our characteristic fossils. It also contains a very readable introduction to New Zealand geology, the fossilisation process and the geological history of New Zealand. You can find more information about the book, including how to get hold of a copy; here. It is published by New Holland Publishers. Here is the team that created the guide, from left to right: James Crampton, Marianna Terezow, Alan Beu, Liz Kennedy and Hamish Campbell. I asked each of them to choose their favourite image from the book and say a few words about it. Here are their comments: James Crampton: Cretaceous ammonite, scalebar is 1cm My favourite fossil is the small Cretaceous ammonite on the bottom of page 62.  Ammonites are very rare in New Zealand Cretaceous rocks, although they are extremely common in other parts of the world.  This one came from the coastline of Raukumara Peninsula, north of Gisborne, from a lovely, wind-swept, wild, and pohutakawa-lined rocky shore.  This specimen is well preserved and shows how some species deviated from the typical, simple, flat spiral shell form of most ammonites – in this case, as the animal grew, the shell became partially uncoiled to end up looking like a hook.  In life, the animal had many tentacles and extended out of an opening in the shell at the point where the label is fixed (this opening is now filled with rock).  This ammonite was found with many fossils of strange clams that were specialised to live on deep-sea seeps – places where methane was naturally bubbling out of the sea-floor during the Cretaceous Period.  These clams probably ate bacteria that, in turn, survived by using chemical reactions to ‘feed’ on the methane. Fossil shells from Hakateramea, New Zealand Marianna Terezow My favourite photo is the one that resides on the title page. It’s an image of a late Oligocene-Miocene limestone block from Hakateramea, South Canterbury. I love this photograph because it showcases some of the great variety of marine life-forms found throughout New Zealand’s fossil record. From small filter-feeders like the clam Limopsis to the large, carnivorous snails such as Magnatica, this image is a snap-shot of a once-living, thriving marine community. I find these life stories of community dynamics that fossils tell us very fascinating. Struthiolaria frazeri – scalebar is 1cm Alan Beu My favourite image is Struthiolaria frazeri, on page 122. This is the largest, most spectacular and most elaborately sculptured of the “ostrich foot shells”, family Struthiolariidae, which are almost entirely limited to New Zealand, and are one of the really characteristic elements of our fauna. They display a long, complicated history of evolution and extinction, with more than 35 species occurring as fossils over more than 40 million years, and yet only two species still live here now – Struthiolaria papulosa (top of p. 123) and Pelicaria vermis (p. 123, lower on the page). Struthiolaria frazeri provides a clear example of extinction, as it is a key fossil for identifying the end of the Nukumaruan Stage, becoming extinct suddenly 1.6 million years ago, at a cooling spell.  Presumably it was a warm-water species, as it is mainly found in shallow-water rocks in central and northern Hawke’s Bay, with a few specimens found near Whanganui. The very obvious sculpture of prominent, square-section spiral ribs, the tall spire, and the short, oval aperture with thick, smooth lips and a deep sinus in the top of the outer lip make it easy to identify. Cretaceous broadleaf – scalebar is 5cm Liz Kennedy My favourite image is of an undescribed Cretaceous broadleaf angiosperm leaf base and podocarp foliage on page 54. These beautiful leaf impressions, along with many other leaf specimens, came from very hard grey sandstone overlying a thick coal seam which was mined at the Strongman Mine opencast near Greymouth. They are a glimpse of the vegetation which made up New Zealand’s Late Cretaceous forests which were very different to those of today, a time when dinosaurs still wandered about, perhaps even eating this kind of angiosperm leaf. These leaf impressions are generally well-preserved, with the ridges of prominent veins providing texture to the impressions. An assemblage of leaves such as these can provide us with a fascinating picture of the past including what kinds of plants covered the Late Cretaceous New Zealand landscape, how diverse the vegetation was and what the climate was like when the plants were growing. Historic fossil locality in the Chatham Islands Hamish Campbell: “My favourite image is on p.25. It captures not only fossils but also a ‘fossil moment’. After all, photographs are fossils of a kind…preserving a record of things that happened long ago. This beach on the north coast of Chatham Island is famous because this locality, with fossil oysters that are 50 to 55 million years old, is the very first fossil locality to be formally recorded in the scientific literature from New Zealand. It was collected by Ernst Dieffenbach in 1839 and he sent the fossils to the Natural History Museum in London. Oddly enough, this locality was ‘lost’ for more than a century because it was buried beneath a sand dune. It was only rediscovered by us paleontologists at the time of this photograph in 1995.”

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Fossil Whale Hunting

Last weekend I returned  to our fossil whale locality in Palliser Bay with John Simes, the paleontology collections manager at GNS Science. This is where I had found three large jaw bone fragments of a fossil baleen whale last November. We decided to have a good look through some of the loose debris in the area where I had already made some finds.After some time. John spotted another large piece of mandible, very similar to the ones that we had from the previous visit. I decided to try the direct route up the cliff, to get closer to the source of the bones. The ice axe proved to be quite useful for making progress up   the very crumbly mudstone. This got me about half way up the gully, to a point that I had reached last time and where I had found one of the three original bones. On this first re-inspection I didn’t come up with any more fossils. The next plan was to abseil down into the gully from the top, in order to have a very close look at the steep headwall which seems to be the actual source of the fossil whale. I had to take care not to dislodge any large rocks with the rope. Unfortunately this inspection of the cliff didn’t reveal anything even with careful scrutiny. Back in the bed of the gully, I dug around with my ice axe some more and did at last come up with three smaller pieces of bone. Here you can see one of them – we think it is the end of a jaw bone, although it is much thinner than the other pieces. Back in the macropaleontology lab at GNS Science, the thin layer of mudstone coating the bones was quite easily cleaned away with the help of a pneumatic air scribe. The 30 cm long piece shown here turns out to fit perfectly with the previously found  segments of the mandible, giving us a combined total length of 1.5 metres for it. The missing link puts it all together. The latest piece in the puzzle is second from right. The rest were found on the previous trip. Here are the smaller pieces after a bit of cleaning. It is tantalising to think that there must be a lot more of them waiting to be discovered in the mudstone of Palliser Bay.

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Titahi Bay Geology

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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Volcano Gas Flights Video

If you had to work out the daily quantities of different gases coming out of a volcano and spreading across the sky in a huge, mostly invisible plume, where would you begin? This video gives a brief introduction to how New Zealand’s GeoNet scientists go about it: The information is combined with other evidence such as seismic monitoring to judge the risk of future volcanic eruptions.

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