The Auckland Volcanic Fields
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THE AUCKLAND VOLCANIC FIELDS Ian Smith – School of Geography, Geology, & Environmental Science University of Auckland My interest is in the volcanoes, and it’s been a rather satisfying theme through my career that I can use what is really academic research in a practical way to try and understand how volcanoes have behaved in the past and therefore how they may behave in the future. This is particularly important in New Zealand because we have very little in the way of history of volcanic eruptions. If you go to Italy or New Guinea, as I have, and Iceland, many places of the world, volcanoes erupt at frequent intervals. So everybody knows what a volcano can do, and therefore there’s not a problem in communicating this sort of thing. In New Zealand we have very few legends from Maori times, this is a mural by New Plymouth Boys’ High School students depicting one such legend – the fight between Taranaki and Tongariro at some stage in the last thousand years, and you probably all know the story, but they were fighting over a maiden volcano and eventually Taranaki lost the fight and was exiled off to the west. Well, that’s a rather cute legend you might think. Actually recent research shows that it does tell us of a time not so very long ago when these two volcanoes were very active. So that’s a nice legend. There are one or two others, but basically our knowledge of past volcanism really comes from geology and from geological interpretation of deposits like this – this is one of your quarries, you can perhaps see a little bit of variety there, some coarser material at the top, and layered material down the bottom, and you would have your own names for these different deposits – I would call this scoria and ash. And it’s by working through such deposits that we can understand the way volcanoes work. Now, there’s a little bit of politics about this, and this is perhaps worth considering. In New Zealand we are very subject to weather events, storm events, and everybody knows about storm events – every one to five years. Most people, not so much in Auckland, know about earthquakes, small earthquakes, one to five years, return period of large earthquakes every 100 years, and we are all waiting for the big one in Wellington. Volcanoes have a different time scale. Small volcanic eruptions, of the kind that are really more a curiosity than a hazard, might occur every 50 years – say, Ruapehu, has a return period of something like 50 years. Or even as long as 100 years. The return period of large eruptions, and we know there have been large eruptions in New Zealand, is 100+ to maybe 1,000 years, and, to bring politics into it, the return period of elected members of government is three years, and so you can see to argue in a political sense for funding for volcanoes, or even for hazards from volcanoes, is a tricky business. These slides show the essential principles, there’s a few words around there, but don’t worry so much about those. The point is that here is a layered sequence of volcanic deposits – the kind of things that you may quarry. And there’s variety in the layers, and possibly the variety is not so good for you, but is good for me because the variety reflects changes in the nature of the eruption. The sequence from bottom to top is time. So by working through in great detail sequences like this we can understand how a volcano has behaved – in this case it was the volcano Okataina, which is just to the east of Rotorua, and individual layers reflect individual explosions of different natures and the whole sequence there represents probably no more than days to a week. So many events within a short time period. And we know quite well when that happened – it was about 5,000 years ago. A lot of work has gone into that central North Island. I’d like to concentrate on Auckland, which is appropriately referred to as “a city of volcanoes”. But in fact, underneath the buildings bits of Albert Park volcano, which is just off there under Albert Park partly under the University, and towards you from there is the Domain volcano. So there are actually something like 49 volcanoes – people, geologists, argue about the number, but 50 is fine. These volcanoes in Auckland are part of a larger system of small volcanoes that extends up to Northland, up to the Bay of Islands, and off southwards in the Bombay hills. AQA/IOQ Combined Conference – Auckland 2008 Page 1 of 4 This is a very old map, it’s an 1865 map of Auckland's volcanoes. I just want to make a point here that the person who constructed this map – a guy called Ferdinand von Hochstetter, an Austrian brought into New Zealand to do some early geological work in the country – when he came to Auckland there weren’t the buildings, there wasn’t the infrastructure. What he saw really was what there was, and the map that he made is as good as any we can make today. The summary there – we think that the oldest, and this is a moveable sort of feast as new data comes in, eruption in Auckland was about a quarter of a million years ago, 250,000 years ago – it was a long time ago. We can name 50 discreet places where eruptions have taken place in Auckland, the youngest is Rangitoto (700 years ago), and the pattern throughout this time (not a uniform pattern but we can tell in ways that I’ll describe in a moment) that the eruptions were small, they erupted the fluid magma that we know as basalt, and the styles of eruptions ranged from slightly explosive to quietly effusive. This is possibly our best example of the different stages in an eruption from an Auckland volcano – it’s Motukorea (or Brown’s Island). The nice thing about this island is that it tells us of all of the stages of an Auckland volcanic eruption, and you may be able to pick out here on the eastern side of the island a formation - this is actually a crater rim - and the deposits (which don’t come out so well in this light) of that rim are in there and we can say that the early part of this eruption was quite an explosive eruption – it involved the interaction of water with magma and it produced quite a large crater which presumably continued under this whole structure. And then the eruption became, as we call it, dry – water was no longer getting into the magma, and the eruption style changed to a scoria eruption. I’ll show you pictures of these in a moment. And there’s the little scoria cone – it’s one of several actually that are found on the island. And so that was the second stage in the eruption. The third stage was when the thing began to run out of steam, driving force – run out of gas really – and produced larva flows. Now this is a wonderful volcano showing very clearly the different stages that we can expect from an eruption in an Auckland volcano. In more detail, and I won’t go into this too much, but in the detail, this is the lovely quarry exposure, we can see variation in deposits – and you are probably familiar with these. There are some light- coloured deposits, but again it’s very layered, signifying a very active period in the volcano. Then there’s scoria, again it’s layered, signifying variations in the eruption, and then there’s another light colour, and then on. A lot of my research is based on very detailed studies of these sorts of deposits, and that’s what’s depicted on the right-hand side of the slide – just chemical analyses from the eruption – but from variations in the chemistry I can say quite a lot about where the magma was coming from (it’s coming from about 80 km down), how quickly it rose, and what happened to it on the way. Three styles of activity in Auckland, and at the terms that go with them and the lower right Phreatomagmatic activity which is more explosive and it’s when water and magma interact explosively, on the lower left scoria producing activity fire fountaining, it’s called Hawaiian style activity because its very characteristic of Hawaii volcanoes, and at the top a larva flow (from Etna). Now if I can go through those three types of activities. The picture here is Crater Hill which was quarried until fairly recently and became one of our “type” examples of an Auckland style of activity, Auckland deposit in the quarry face, there you see on the right the quarry face at the top, a picture from Iceland of the sort of explosions that would produce those deposits, and then Crater Hill itself which is a very subdued volcano, the crater rim is round the outside – this is where you’re actually looking into the crater here – but it doesn’t stand up at all as do some of the other volcanoes. The second kind of activity, the dry explosive activity that generates scoria and builds cones in Mt Eden, the picture I showed you earlier from I forget which quarry, and on the right an example from Hawaii with a helicopter for scale.