Retorts

"Meteoritic Harbour?"

Ivan Millett's letter under this heading [Retorts, Dec/Jan] deserves more comment than just that brief quote from John Gamble concerning the nature of the Auckland volcanic field.

Firstly, the Hauraki Gulf has been shown from a century of detailed geologic mapping and more recent geophysical probing to be, in its eastern half, the northern end of a down-faulted crustal block (ie, a graben) of which the Firth of Thames and Hauraki Plains form the southern extension. The irregularly shaped and shallower western half of the Gulf is simply the local portion of the New Zealand continental shelf, floored in, so far as we can tell, by essentially the same types of rocks as exposed on the adjacent land. No evidence in either portion, or on the surrounding land, for any meteoritic impact.

The structure of the region is thus basically of linear elements: faults with NNW-SSE and ENE-WSW trends dividing the area up into blocks which have been tilted and raised or dropped to form the present topography.

Secondly, whilst it is correct that Auckland's volcanic cluster lies at the southern edge of the Hauraki Gulf, there are other volcanoes and volcanic deposits elsewhere around the Gulf: Little Barrier in the north is an andesite volcano; Coromandel Peninsula and its drowned northward extension, Great Barrier Island, have an extensive volcanic cover of acid type (rhyolite, dacite, ignimbrite) with minor andesites; there are scattered basalts and andesites of small volume along eastern Northland between Auckland and Whangarei. Not only are these volcanics of different sorts but also none of them is the same age as the Auckland basalts, and it is clear that they are not genetically related; they cannot be the product of some single event, extra-terrestrial impact or other.

Thirdly, the structural elements and volcanics can be dated. The faults must be younger than the youngest rocks they cut, and lavas and ash can be dated radiometrically, especially by the potassium/argon method. Different volcanic groups were active at different times (the Auckland field, the youngest, going back no more than about 60,000 years) and the same can be said for the faults, but all are younger than 25 million years (ie Early Miocene). None anywhere in or around the Hauraki Gulf can be correlated with the time of the demise of the dinosaur. If their extinction was aided by a cometary impact, as seems possible, even likely, that impact is certainly not represented by the Hauraki Gulf.

Jack Grant-Mackie, Geology Department, University of Auckland