THE 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 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 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 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 , 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 .

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 that we know as , 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 (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 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. It’s a very spectacular style of activity but is not one that is particularly hazardous unless you’re really in the way. And then there’s larva activity, you’re familiar with basalt rock, that originates as a river of hot magma and this is probably the least hazardous style of activity.

AQA/IOQ Combined Conference – Auckland 2008 Page 2 of 4 What I’ve described as “what we think we know” what we don’t really know well is how often eruptions occurred in the past, and where the next eruption will be, and how much warning will there be, how long will the eruption last, and even whether there’ll be another eruption. These are the questions that we are asking. Frequency of eruptions in the past, one thing we do know (and it’s a recent discovery) is they are very irregular – the last eruption was 700 years ago, about 10,000 or 9,000 years before that that Mt Wellington erupted, and then in the period before that there was a whole series of eruptions, and in fact if you get up on to one of Auckland’s viewpoints and look out over the city and see the little cones that make up the Auckland , then most of those are young, most of those are younger than say 35,000 years. And its seems like, well, I like to think of the field as being like an adolescent – it’s just getting going. It had a fairly modest childhood, it’s now an adolescent, with the most recent eruption by Rangitoto was the biggest and looking at analogues elsewhere we can say that it’s probably about the quarter of the way through its life. This is a slide plotting as dots the ages of the volcanoes (on the x axis) and the volumes of volcanoes, the amount of stuff that has come out during an eruption (on the vertical axis) a fairly modest childhood producing a few small volcanoes, and then in recent times (the last 35,000 years) the have become bigger and also more frequent, and this I think is geologically interesting, but for a city like Auckland, it’s perhaps important as well because there really is a potential threat from future activity.

A lot of my research is directed towards understanding the magma, where it comes from and what happens to it as it rises, and this really is a summary of that, that it’s forming at about 80 km depth (60-100 is OK), it rises quickly through the crust, and this is actually a bit of a surprise and it’s a bit of a concern in terms of the warning time because once magma gets going, it seems to rise at something like 5km/hour, which sounds very modest until you realise that 80km and 5km/hr doesn’t give you very much time to pick it up as it rises. And each eruption is somewhat different, there isn’t a pool of magma underneath Auckland – there’s lots of little hot pockets, and from time to time these are activated and produce magma, and the magma rises.

This is work from a student from Victoria University – it’s geophysical work - who analysed the way that waves passing through the earth are affected by this source of magma and was able to recognise a geophysical anomaly – a place in the mantle, at about 80km, where the seismic waves are slowed down and the interpretation of that is that is where there is melt (magma).

Rangitoto is the biggest volcano. It is slightly different in its composition. It is possibly from a slightly shallower source. The interest in Rangitoto is that is may be an indication of what is to come. Will there be another eruption in Auckland? What are we doing? Several things are being done. There is the “system research” to understand how are produced and how they rise. There is volcanological research to understand what will happen when the magma reaches the surface in different places. There is a lot of contingency planning, looking at vulnerability of infrastructure, at what areas could be affected, what to do with the people in those areas, and so on. There’s public education and there’s a civil defence booklet that tells people, hopefully in simple language, what may happen and why. There is the phonebook. And this is the question – will it ever erupt again, and what will we do if it does?

I’ll outline what we are doing for monitoring the volcanoes in a moment, but the summary on how often and how likely it is, is on average we probably have a local eruption in the Auckland field once every 3,000 years. But averages don’t mean a thing in natural systems. We have studies planned that may give us more detail so we can be more confident about frequency in the past, but there is strong evidence of clustering of eruptions in the past. It is very hard to know, just because it’s been only 700 years since the last eruption, it would not be safe to say “well, we’re not going to have another one for another 10,000 years or something”. We don’t really have a good idea where the next eruption is going occur, the younger ones in the field tend to be on the north-eastern side, like Rangitoto or Mt Wellington, I’ve often somewhat facetiously said that the next eruption will be in the America’s Cup course, because we don’t really need it any more, and some scenarios are based on that. We don’t really know where it might happen – it could happen anywhere in the Auckland area. We know something about the style of eruptions we could expect, the scale of eruptions (it’s not going to be huge, it won’t require the evacuation of everyone from Auckland) but it will require consideration of evacuating some areas in Auckland.

AQA/IOQ Combined Conference – Auckland 2008 Page 3 of 4 A little bit about the intensity. The activity in an eruption – we don’t know how long an eruption in Auckland might last – but by analogy with places elsewhere in the world, a few hours to up to a decade, but most likely only days to a month. We also know from work being done that a new volcano is likely to have quite complex changes in its activities, although I’m not too sure of the timescale. This is not really a geological assessment, it is an insurance industry assessment of the threat from volcanic eruption, and insurance people like to talk about risk, which is a combination of hazard (geologically what is going to happen), and vulnerability, what are those things going to happen to, and in some ways Auckland's hazard is quite small - we are looking a small eruptions, fairly infrequent eruptions, quite quiet eruptions. Fantastic tourist attractions. However our city forebears chose to build New Zealand’s largest city on this field, which is geologically an active field, so the vulnerability is very high, and actually if you do a very simple sum, multiplying a very small number by a very large number, it still gives you quite a large number (as long as the small number is greater than 1), and so we do have a significant volcanic risk in Auckland, and it is taken seriously. A mitigation strategy is down there monitoring contingency plans, various preparatory exercises.

I’d like to briefly mention the Auckland Volcano Seismic Monitoring Network which is something that was initiated from the university at a time when volcanic risk wasn’t taken so seriously (in Auckland), but it’s now been taken onboard by the Auckland Regional Council, by GNS Science, the Earthquake Commission who fund and manage Geonet, and now we actually have (on a world scale) quite a good system of seismometers – instruments designed to pick up earth tremors – and they are arrayed around the city, there are 6 at the moment and its going to go up to 10, both further out and further in and the intention or expectation is that when the magma begins to move and starts breaking its way towards the surface, then the little earthquakes that will be generated by that process can be picked up and the magma essentially tracked.

(There are all sorts of technical problems and I just thought for entertainment that I’d mention one. These are some records from one of the seismometers, and you’ve got time along there and the little humps here are activity that’s picked up. What’s been picked up there, day and night, during the day it’s a problem during the day with movement of people and vehicles and we get quite a lot of seismic activity/vibration in the city and during the night things are relatively quiet. So we see this and we work out various ways of getting around this, but another interesting and perhaps entertaining aspect of this is that this is just one day here, the recognition that so say here’s the beginning of the day and the beginning of commuter traffic the dip in activity is morning tea time. Morning tea time in seems to be quieter period than lunch, or perhaps lunch is longer or more spread out, but there’s morning tea there, and there’s afternoon tea there. We can pick this up seismically.)

So this is the picture for Auckland somewhere down 80km or so, there’s a spongy sort of zone that’s liable to melt and from that zone melts are likely to rise, they come up through the mantle probably as individual little blobs, it’s a little bit of a concern because we don’t really know that maybe we won’t begin to see proper seismic signals until we reach the crust, which is more brittle and the magma starts breaking its way through, but this is the kind of picture we have of the system and there’s some words around that to give us some idea of the time it takes for magmas to rise. Basically, the message is “magmas will be quick”.

So, what sort of warning will be get? The optimists say 11 days and the pessimists say 14 hours, and we don’t really know until the next time it happens.

I’ll just finish with three pictures to bring home to you the possibility of volcanic eruption in Auckland – these pictures were generated during an exercise last March. The exercise ran for one week, and at the end of that week on the Friday suddenly we got this report that a helicopter flying over Mangere had seem a plume of steam rising from the harbour. Not very long afterwards, the plume turned black and obviously there was ash and then a little while later the Auckland eruption of 2008 began.

I hope that this has been useful and entertaining. I find it very satisfying to be able to do some basic academic research that actually has a practical point to it, and I think we have come a long way towards understanding Auckland – hopefully we will never need this information, but just in case, we have it or are getting it.

AQA/IOQ Combined Conference – Auckland 2008 Page 4 of 4