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Thursday, 20 July 2017

Volcano City


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 at www.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 that: "Some eruptions flare-up over what is, geologically speaking, a short period of time. For example, there can be 6-10 volcanoes erupting within a 4000 year timeframe. On the other hand, the volcanic field has also gone quiet for up to 10,000 years in the last 60,000, which is quite a long gap."

For more information about the DEVORA research have a look at this media release.


Thursday, 20 April 2017

GeoTrips - visiting New Zealand's geology and landforms


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 find 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 members of the public, 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.

Please sign up to GeoTrips and comment on and rate the locations that you visit!

We are planning to further develop the site and continue to expand the number of locations shown. So... have a look, explore and plan some trips to become a New Zealand geological investigator!

Here it is:

 www.geotrips.org.nz

 

Monday, 6 March 2017

Earth's Magnetism in Antarctica

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!






H├Ąglund 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.

Wednesday, 7 December 2016

Landslide Dam

Seaward Slide, J.Thomson @ GNS science
Rockfalls and landslides were one of the dramatic consequences of the M7.8 Kaikoura Quake. This first photo shows one that is actually so huge that you might not at first recognise it for what it is. The white cliff in the distance is the landslide scarp and the huge green capped pile of grey in the middle distance is the debris that fell away. This landslide was of course made famous on TV by the cows that became trapped on an isolated hummock in the debris pile.



SH1 and Railway, Steve Lawson @ GNS Science
A large number of coastal cliffs collapsed, causing spectacular damage to the coastal transport infrastructure. In this image you can see how the raiway line has been lifted up and dropped across the road and across the beach.




J.Thomson @ GNS Science

Another example of rockfall damage along the coast:






Hapuku Landslide, Steve Lawson @ GNS Science



In the Canterbury ranges, a short distance inland, a number of landslides have blocked river valleys and created landslide dammed lakes that are now filling up. This image shows the massive Hapuku landslide, which has buried the valley in over 150 metres of debris, weighing many millions of tonnes. The grey coloured lake in the centre of the image is a couple of hundred metres long



Hapuku landslide, J. Thomson @ GNS Science

This is a close up view of the lake taken a few days later. The lake is now near to the point of overflowing the dam. The problem with these dams is that they can fail catastrophically, sending a debris flow of water, mud and rock down the valley with potentially very destructive consequences.





Linton landslide survey, J.Thomson @ GNS Science


In this image you can see another landslide, this time in the Linton Valley. It has also dammed a small river. The team here are surveying the debris and the shape of the valley in order to calculate the possible downstream consequences of a breach of the dam.





Linton landslide, J.Thomson @ GNS Science




This photo shows the size of the landslide.  A large section of forest has slid down with it with many trees still standing. The debris has again blocked the valley to form a lake.







Linton landslide dam, J.Thomson @ GNS Science


The lake level is still about 10 metres below the rim of the dam:








Linton landslide dammed lake, J.Thomson @ GNS Science







In order to measure the lake's water level safely, Chris Massey took a GPS reading from the helicopter whilst it hovered just above the water surface.







Linton landslide, J.Thomson @ GNS Science

Meanwhile at the base of the dam, some water is percolating through the debris, although the flow in the stream bed is much less than usual:
Linton landslide, J.Thomson @ GNS Science








This photo shows the toe of the landslide - a mass of rock debris and damaged trees.







Linton landslide, J.Thomson @ GNS Science




By the end of a few hours, we had lots of data in the form of laser scans of the slip from different locations, as well as hundreds of drone and aerial photos, which are combined to make a 3D digital image that can be used to model the possible consequences of the dam breaching in different ways.




This video made by Steve Lawson is a virtual 'fly through' of the digital model:


video

And here is a short video about these landslide dams:



Finally, there is more information about landslides on the GeoNet website here

Thursday, 24 November 2016

The Kekerengu Fault

Photo Tim Little @ VUW
Whilst there were many faults that ruptured during the recent M7.8 Kaikoura Earthquake, the Kekerengu Fault is perhaps the most awe inspiring in terms of its effect on the landscape and infrastructure. As it ripped through the countryside, it displaced the land to either side by an astonishing 8 to 10 metres sideways and about 2 metres vertically over many kilometres of its length.
Kekerengu Fault offset, J.Thomson @ GNS Science
In places this horizontal offset is even more - up to a whopping 12 m. This is impressive on a global scale. In the first two images here you can see what this looks like where farm tracks have been sliced through at a right angle.

Here is a drone's eye view from above:




Kekerengu Fault,   J.Thomson @ GNS Science
As the trace of the fault passes through different locations, it expresses itself in a number of ways.

Across the river from Bluff Station, it has opened up an enormous crevasse, not unlike the sort of thing that mountaineers often see on a glacier. This will be due to either a slight bend in the fault trace, and/or slumping of the downhill side of the fault where there is a slope.
Kekerengu Fault,   J.Thomson @ GNS Science
Slickensides is the name given to the scrape marks  on the surface of the wall of a fault. Here you can see that they are dipping down at about 28 degrees from the horizontal (towards the south-west). This is useful information to help understand the direction of movement of the rupture, and tells us that this fault moved obliquely (sideways and up).  When we looked across the fault we could see that the land on the far side had moved to the right. It is therefore a 'dextral' or 'right lateral' oblique slip fault.


Kekerengu Fault,   J.Thomson @ GNS Science
Fences are really useful markers to allow measurement of the fault offset, especially when they cross the fault at close to 90 degrees.as in this photo. Yes - those two lines of fencing used to join up!


Kekerengu Fault,   J.Thomson @ GNS Science





The hillside here appears scarred by a simple knife cut...
Kekerengu Fault,   J.Thomson @ GNS Science
...whereas in other places, the slip is distributed over a broad area of surface deformation. In this case it is likely that the groundshaking helped the hillside follow the call of gravity to spread the deformation over a large area.
Kekerengu Fault,   J.Thomson @ GNS Science
Near to the coast, the Kekerengu Fault tracks across this field towards the main state highway and the railway. Here the fault trace is a mound of huge clods of earth and ripped turf. We call this a "mole track", and it results from some compression rather than extension along this part of the fault trace.






Kekerengu Fault,   J.Thomson @ GNS Science
Not far away, State Highway 1 has been pushed sideways in several pieces...
Kekerengu Fault,   J.Thomson @ GNS Science
and the nearby railway has been pulled so hard that it snapped.
Kekerengu Fault,   J.Thomson @ GNS Science











The fault runs right under this small bridge which is totally destroyed.
Kekerengu Fault,   J.Thomson @ GNS Science










Lots of food for thought and plenty of work ahead for earthquake scientist Russ van Dissen and his colleagues.