"I THINK I'LL P U T SOME MOUNTAINS ... H E R E " :
V O L C A N I C RISK IN A U C K L A N D
By Reg Nichol
Anthropology Department, U n i v e r s i t y of A u c k l a n d , P r i v a t e B a g , A u c k l a n d .
Whenever I admit to carrying out research on the history of Rangitoto
the first question I am asked is usually 'When will the volcano erupt
next'. In part this question reflects the strong local tradition that the
island has already erupted on a number of separate occasions (Atkinson
and Bell 1973, Searle and Davidson 1873, Heming 1976, 1980, Millener
1979, Williams and Cochrane 1979, Simkin et al 1981, Cranwell 1981,
Molloy and Enting 1982). Having examined the evidence for this in some
detail in the course of research on the fossil footprints on Motutapu
(Nichol 1981), my conclusion is that this tradition is seriously lacking in
support (Nichol unpublished, Nichol in preparation), so to some extent it
is possible to reassure people, and I usually reply that it is extremely
unlikely that Rangitoto will ever erupt again. The bad news, however, is
that what will happen instead will probably be very much worse.
The question of the timing and location of the next eruption in the
Auckland area has received a good deal of attention in the last few years,
not surprisingly given the drama and publicity that attend on major
eruptions. This is far from saying that the next Auckland eruption need
be major, but there can be little doubt that there really will be another
eruption. A n idea that does not seem to have registered in the public
mind, however, is that there will probably be another after that, and
another, and who knows how many more. The fact is, the Auckland field
has seen at least 50 eruptions in the last 60 000 years (Searle 1964, p95),
and this activity may well persist for another 60 000 years. There seems
to be some confusion over the way to interpret these figures, however,
and Searle goes on to deduce that the chance of a volcanic eruption
during the next century is approximately 2%. I think this estimate is far
too low. Searle arrives at this figure by assuming that the activity of the
Auckland field comes in "spasms" of three or four separate eruptions,
and that the spasms are on average about 4000 years apart. Hence the
chance of an outbreak in any century is 1 in 40, if each outbreak is an
independent event.
But this chance of 1 in 40 is actually the chance of a spasm beginning
in any century, and for a couple of reasons this is unlikely to be the
correct way to go about assessing the probabilities. First, it is not
entirely clear that these spasms really exist, as the dates do not
necessarily coincide. Searle (1964) lists dates for several eruptions that
fall around 30 000 years ago including Panmure at 28 000 ± 1500, Wiri
at 28 300 ± 690, Ihumatao at 29 000 ± 1500, and Mt Albert at 30 000.
T A N E 31, 1985-86
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But the Mt Albert date was only a minimum (Fergusson and Rafte
1959), and more recent dates from A . N . U . include 25 370 ± 350 for Wh
and a pooled age of 41 740 ± 700 for Ihumatao (McDougall et al. 1969;
The tight group that Searle identified has therefore almost complete!
dispersed.
Second, if the spasms do exist, it would not be unreasonable to assume
that we are actually within one at the moment, as Rangitoto is only
around 600 years old (Law 1975, Nichol n.d.). But while it might be
better to work on a "worst case" assumption, the straightforward
approach is to calculate on the basis that the 50+ eruptions within the
Auckland field were scattered at random over the last 60 000 years.
Assuming that this rate will persist, the chance of an eruption during
the next century is greater than 1 in 12, or more than 8%. This can be
interpreted as almost 1% per decade! Yodelayeehoo. And of course, if
spasms exist, and if we are within one, then the chance of another
eruption within the next decade might be considerably higher than even
that alarming figure.
Another point worth thinking about concerns the likely location of the
next outbreak, and Searle (1964) has made some important comments
about this:
'If we again assume that future patterns will accord with those of the past we may
predict that locality will largely determine the nature of a future outburst. If it
occurs in the Waitemata terrains of the isthmus or on the southern or eastern
borders of the field it is likely to be dominantly effusive. If in the low-lying areas, the
probability is that it will be dominantly explosive. The areas most likely not to be
sites of eruption are the main Waitemata strata highs — e.g. the Hillsborough and
Remuera-St Heliers Bay districts' (Searle 1964, p99).
Lauder (1965) has also made an interesting point, that the distribution of eruptive centres is far from random, and there appears to be a
major concentration of relatively young eruptions, which he takes to be
those less than 20,000 years old, in a band running down the middle of
the field. A slightly modified version of his illustration (ibid: fig. 1) is
reproduced here (Fig. 1).
My suggestion accommodates both these ideas, i.e. that the area of the
next eruption is likely to be in low-lying terrain, and centrally placed in
the field. A location that would satisfy both requirements would be in the
entrance to the Waitemata Harbour, say at a point equidistant from
Rangitoto, North Head, and St Heliers. The present depth of water there
is about 12 m (RNZN Hydrographic Chart 5324 — Tamaki Strait and
Approaches, 1977) so this would certainly be one of the most low-lying
areas left within the field, and a volcano at that point would go some way
toward filling in the gap in the series of relatively young eruptive
centres.
A n eruption beginning beneath the sea would probably commence as
an explosive event, as did Rangitoto, and the harbour itself would be
134
Fig. 1. Volcanic eruptions in the Auckland area (after Lauder, 1965, fig 1). 0 =
eruptions older than 20 000 years; X = eruptions younger than 20 000 years.
very exposed to the effects of the seismic waves that are likely to result
(Searle 1964). Even if the eruption actually misses the harbour it could
still commence with an extremely destructive explosion, and over 50% of
135
the eruptions in the Auckland area began explosively (Searle 1981)
Studies of heat-flow rates suggest that these must have been caused by
the interaction of magma with surface water rather than ground water
(Nunns 1975), and as the majority of the Auckland volcanoes erupted at
a time of low sea-level (Searle 1981, p43-45), the chance that the next
eruption will begin explosively must be considerably greater than
50%. This could cause many casualties.
In the longer term, however, the greater threat will come from any
lava flows that follow the explosion. Though unlikely to kill anyone other
than unwary sightseers, these flows could come to form a very
substantial cone. The volume of Rangitoto, for instance, is over two cubic
kilometres (Milligan 1977, p9), and though there is no reason to believe
that the next eruption will be as voluminous, neither is there any
particular reason to think that Rangitoto is going to remain by far the
biggest volcano in the field. In fact a cone could be rather smaller than
Rangitoto and still represent an economic catastrophe for Auckland and
for New Zealand as a whole.
There are some enormous concentrations of invested wealth in the
Auckland area, immovably fixed in the form of buildings and other large
structures, and though these concentrations are relatively small targets
within the field as a whole, the effect is still no better than playing
Russian roulette with none of the chambers really empty. And what look
like some of the emptiest areas, at present covered by sea, have great
recreational value and on reflection great commercial value too. In 1981,
for example, 46% of New Zealand's imports and 35% of our exports
passed through the Port of Auckland (Anon 1983, p624, 628). This trade
would stop almost instantly, and if not permanently then for a very long
time, if a volcano even a fraction of the size of Rangitoto should suddenly
block the entrance to the Waitemata Harbour. Felton Mathew may yet
prove correct in preferring the Tamaki River as the site for Auckland
City (Rutherford 1940, p l 5 , 169).
Inevitably this discussion has been highly speculative, but this is still a
very serious subject. Many people have expressed astonishment that the
prehistoric people on Motutapu could have busied themselves with
garden making when a major eruption was taking place a mere five
kilometres away (Nichol 1981). My reply is always that at least the
people whose footprints I have found knew that they could survive the
ash-showers, because after all were they not still there to prove it? So are
modern Aucklanders in the same situation? In fact they are much less
well informed. 'Hazard awareness declines with time elapsed since
impact', as Blong (1984, p386) puts it, and as there have been no impacts
at all so far as Europeans are concerned, the level of awareness is very
low indeed. Questions about "The next eruption of Rangitoto" point this
up very well. As I said earlier Rangitoto is not going to erupt again:
136
'There is no doubt that volcanic risk exists . . . It is, however, virtually certain that
the next eruption will be in a new place, not from one of the existing volcanoes'
(Ballance 1968, p44).
This is actually a great pity, because Rangitoto is practically the only
place in the region where a major eruption could be fitted in without
doing enormous harm. So really the question "When will Rangitoto erupt
next?" is expressing a vain hope, that the eruption to come will be well
out of the way, (but not that far out of the way that we will miss the
spectacle?). Unfortunately Aucklanders may get a better view than they
would like. In the meantime, it is to be hoped that much more research
will help to clarify the many obscure aspects of the history of the
Auckland field, and Rangitoto in particular.
One obvious line for this research to take is the accurate dating of
most of the eruptions. It is unlikely that the existence of Searle's (1964)
spasms can be usefully tested, as reasonable error margins on early
dates are as great as or greater than the average interval between
eruptions, but more and better dates might allow the testing of Lauder's
(1965) suggestion of time trends in the location of eruptive centres.
Really precise dates might even indicate sub-groupings of eruptions on a
number of finer time trend lines than shown in figure 1. Ultimately, this
could make a major contribution to the accurate prediction of the
locations of eruptions. Given the virtual certainty of further eruptions
(plural), the moderate return period of around a thousand years, and the
value of the target in terms of life and property, some detailed research
along these lines could have an immense practical pay-off. As Searle
argues
'We must get to know our volcanic areas better, for their patterns of behaviour in
the past may present keys to the better prediction of their future' (Searle 1970, p69).
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