The Present State Of Volcanicity Of New Zealand
It is a popular belief that the thermal activity of the Rotorua-Taupo area is waning, that it is less than it was, and that it will become less still, and that the chance of renewed volcanism is correspondingly decreasing. A critical consideration of the evidence available to the geologist flatly denies the popular belief.
During any eruptive period, the actual time occupied by the extrusion of lava on the surface is relatively small, and as there is likely to be considerable time between major eruptions or extrusions, as measured by human standards, quiescence should be the normal state of affairs. Volcanologists have demonstrated that there appears to be a cyclic background to volcanism, and decrease of hydrothermal activity at any one place is just as much in conformity with that theory as with the dying-stage belief. Obviously visual observations of hydrothermal areas do not necessarily give the true picture, and the Doctrine of Uniformitarianism applies only in so far as present earth processes correspond to certain similar past stages of the earth's history. This is notably the case with volcanism, which is merely a surface manifestation of subcrustal changes and adjustments, and not an isolated phenomenon. It is important to gauge the present state of the earth tectonically when dealing with such problems as the present state of volcanicity in any area. The story of the past events is available to the geologist in the record of the rocks, and the aim of the volcanologist is to add to this story the knowledge to be gained by measuring the present earth
processes in volcanically active areas with the resources of physics and chemistry.
Within historic times there have been in many parts of the world earthquakes and volcanic eruptions of disastrous magnitude. These tend to be confined to certain portions of the earth's crust, and there is a certain correspondence geographically between volcanic regions and those liable to certain types of earthquakes. As these are the result of normal earth processes, it should be possible to devise means to measure them and get to understand them better. It should be possible to demonstrate what areas are liable to earthquakes, and to, take precautionary methods in layout and building design, so that needless loss of life may be avoided. The subsurface sequence of events preceding eruption on the surface should be measureable, and warning given in areas thought likely to be dangerous.
At present the investigations are still in the probing stage, when data are being collected and new techniques devised for carrying out the measurements. Volcanological observatories have functioned for varying periods at Hawaii, Kamchatchka. Japan, the Netherlands East Indies, Italy and elsewhere, and recently one has been established at Rotorua, but at present lack of staff and equipment do not enable it to function as an observatory. In New Zealand we have the evidence that has been collected from, the geological record, our seismological, chemical and geophysical investigations, and the background of knowledge from the overseas literature.
New Zealand Volcanic Geology.
Volcanic rocks are important in the Pliocene to Recent rocks of New Zealand. In the North Island they are scattered from north to south in the western half, and in the central region cover exclusively over 5,000 square miles. The central rocks are almost entirely rhyolitic—those to the west and north are andesitic and basaltic.
The basalts of North Auckland are scattered from Whangarei to the north, and include extensive flows and numerous scoria cones that form prominent landmarks. They range from Pliocene to Pleistocene, and at Pakaraka a young soil-forming ash surrounds Poerua cone, though this does not appear to be as young as late showers of the central regions. Hydrothermal activity of a low order exists at Ngawha and at odd localities near Kaikohe in the vicinity of the dacite dome of Putahi, whose extinct steam vents are still open. There are no indications of any great hydrothermal activity having been associated with the extrusions.
At Auckland the landscape is dotted with numerous volcanic cones of Pleistocene to Recent age, but the associated flows do not attain the magnitude of those farther north, and there are no signs of hydrothermal activity, present or past. On Motutapu Island is a young volcanic ash from nearby Rangitoto, and the author found this resting on a shell midden at Islington Bay. It was uncertain whether the ash material was slumped on top of the midden, but the freshness of the ash is consistent with the order of age suggested.
From Auckland to Taranaki there are prominent volcanic cones to which belong Karioi, Pirongia, Maungatautari, Egmont and others. These are composed of andesite and basalt, and range in age from Pliocene to Recent. There is little evidence as to when they ceased their activity, but the general lack of signs of hydrothermal activity suggests that they were built up by a succession of flows and erupted scoria over a lengthy period, with quiescent stages between A Recent ash shower from Egmont, reported by Burrell and named after him, is considered by Oliver (1931) and Grange and Taylor (1933) to be less than 500 years old.
Volcanic rocks of the Christchurch, Timaru and Dunedin areas are also considered to be late Tertiary to Recent, but have no associated hydrothermal activity.
The western portion of the Coromandel Peninsula in the North Island is largely covered with andesites that Henderson (1913) considers date back into the Miocene, and are overlain on the east by rhyolitic rocks of Pliocene and Pleistocene age. These consist of older flow rocks and younger welded tuffs of the ignimbrite type, that probably correspond with the Patetere ignimbrite series mapped by Grange (1937) in the Rotorua-Taupo area. These rhyolitic and andesitic rocks of the Coromandel area extend to the north-west, since they are exposed on volcanic islands lying east of the North Auckland peninsula, and have their distribution parallel to the axis of the peninsula. At Mangakino, the
hydro-electric centre of the middle Waikato, rhyolitic tuffs of this series over 1,000 ft. thick are underlain by sands and fine gravels that rest on ignimbrite of the wilsonite type extending beneath sea-level.
The Haparangi rhyolite flows described by Grange are mostly confined to Rotorua-Taupo graben, and associated with them are loose pumice breccias. These rocks overlie fluviatile and lacustrine beds laid down in depressions on the Patetere rocks formed by the crustal adjustments following their extrusion. There is a well-defined belt of hydrothermal activity from Ruapehu to White Island with which are associated active and recently active volcanoes. The main fractures of the region are parallel to this line, as are many Recent fault traces. At the south end are Ruapehu, Ngauruhoe and Tongariro volcanoes, all active within historic times. Their activity extended back into the Pliocene, and young ash-beds, aggregating 20 ft. close to the mountain, rest locally on water-worn conglomerates, providing evidence of a period of quiescence between the later eruptions.
Tihia and Kakaramea, andesitic cones at Tokaanu, are crossed by recently active faults, and have steam vents and hot springs. Taupo Lake is regarded as formed partly by subsidence following eruption, and partly by explosion. The last great rhyolitic outburst came from Taupo at a date of the order of 10,000 years ago, and scattered pumice ash over more than 10,000 square miles. Grange (1932) has recorded fault traces at the north end of Lake Taupo, where movement occurred in 1922. Between Taupo and Rotorua are numerous faults with associated hydrothermal activity, and a number of rhyolite domes and vents surrounded by loose ejectamenta.
A recent check of precise levels on bench marks established 20 years ago between Earthquake Flat and Waimangu tea-rooms shows a relative uplift of 0.9 ft. in the central portion. Earthquakes have been numerous in this area of recent years, and it is not known if this movement has been a gradual one, or whether it occurred at one time, or is even the mathematical sum of various up-and-down movements.
Tarawera dome is considered by Grange to be very young, actually having been formed after the Taupo pumice eruption. The Kaharoa pumice-ash shower, which he thinks preceded the rising of Tarawera dome, covers the low flats adjoining the Rangitaiki River except where not removed by flood waters. There were no signs of activity prior to the 1886 eruption, which actually commenced on the mountain itself.
Throughout the Rotorua region and to the west, numerous young volcanic-ash showers mantle the surface. Still younger than the Kaharoa shower is one from Rainbow Mountain, which still steams at the eruptive centre. Farther north is the youthful andesitic cone of Edgecumbe, and out to sea are Whale Island and White Island. The former is the remnant of an earlier collapsed mass to the east, and has hot springs, while a few miles to the west are the volcanic Rurima Rocks, also still hot. White Island, 30 miles out in the Bay of Plenty, is New Zealand's most active volcano, and temperatures of 500°C were recently recorded in fumaroles there by the author, suggesting that the lava is close to the surface. This is a potential source of danger to the entire Bay of Plenty district. The present cone is partially built on the remnants of a former one now collapsed, and engulfment associated with future activity would be most likely to cause violent steam-blast eruption, of the Krakatoa type.
Andesites have recently been found in the west Taupo area that are probably the southern extension of the Coromandel andesites. They are in direct line of continuation of the Coromandel rocks, curving southwards, and the author considers that in late times, probably very late Pleistocene time, the north-west-curving structure controlling the volcanism, as outlined by Macpherson (1947), has given way to a north-east trending zone of fracture, to which the latest intense volcanism is related. The geological evidence is the distribution of the youngest volcanic rocks, the most intense hydrothermal activity, and the Recent fault scarps.
In 1886 Mt. Tarawera, a young rhyolite dome, erupted basaltic scoria from a number of craters, and near Taupo there are basalt flows of late age capping earlier rhyolite flows. It has been suggested that this change is a significant one, but it must be pointed out that the eruption of the Taupo and Kaharoa pumice ash showers, and the extrusion of Tarawera rhyolite dome itself, are believed to be later than the Taupo basalts.
Recent advances in New Zealand seismology have included the location of moderate-depth earthquake foci down to 200 miles beneath the volcanic region These extend from White Island in a curved line to Taumarunui, beyond which the line extends through Farewell Spit as totally defining the north-west boundary of the New Zealand seismic area. To the north-west occasional shocks have been recorded in odd years in scattered areas, but south-east of the line is an active belt.
The line of moderate-depth foci coincides with the active volcanic belt, and it has been noted elsewhere that throughout the world there is a geographical relationship between present-day volcanism and areas of moderate-depth earthquake foci. Hayes (1943), has summarised the New Zealand seismic information, and has demonstrated a plane of discontinuity marking the greatest depths from which shocks originate over the area south-east of the line mentioned. He has shown the plane to be curved, and to dip steeply north-west in the north, but to flatten towards the south. Records are not available for a sufficiently long period, nor is the instrumentation sufficient to estimate any possible changes in the seismicity. More intensive seismic work throughout the world over a long period is required to assist in building up the present-day story of earth processes.
It is hoped to make complete precise-level surveys of the volcanic regions and beyond, such that they may be rechecked from time to time and any movements measured. These are liable to be both vertical and lateral.
A small amount of magnetic reconnaissance work done from time to time suggests that there are negative vertical anomalies in thermally active areas. It has been postulated that the rocks beneath may be at temperatures above Curie point. The complete magnetic pattern of the entire region is required to assist in solving the problems of structure. With this is required gravity and chemical surveys. The magnetic work will be likely to pick up the southern extension of the andesites of the Coromandel Range towards the west Taupo area, beneath the younger rhyolites.
Throughout much of the Rotorua-Taupo area drill-holes frequently encounter hot water close to the surface where there is no surface thermal activity. Steam may be obtained by drilling in areas adjacent to the thermal areas themselves. In the Hauraki depression deep wells usually tap warm water at great depth, and locally hot water such as at Te Aroha. The ground water in the deep levels of the Martha Mine at Waihi is quite warm. The isogeotherms appear to be high throughout the entire region, but are highest in the Rotorua-Taupo belt. No comparable temperatures have been obtained in the area to the north-west.
There has been in late times a change in the subsurface structure lines, shown notably in the central part of the North Island. The older volcanic belt, that appears to swing from north-east in the south to north-west in the north, is now apparently cut by a structural feature extending north-west from the centre of the North Island through to White Island. Along this line is an active volcanic belt, with which are associated the youngest rhyolitic eruptions. The line also marks the locations of moderate-depth earthquake foci not found elsewhere in New Zealand.
The area is seismically similar to other parts of the world in which there have been volcanic eruptions and earthquakes of major magnitude within historic times, and there is no evidence of any lessening in activity. Eruptive activity is likely to be confined to the active belt extending from Ruapehu to White Island, and major earthquakes to the same belt and the seismic area to the southeast. Japanese observations support the belief that major earthquakes at the same place occur at intervals of about three centuries. Jaggar considers major volcanic eruptions from the same centre to occur at intervals of a little over 130 years, or twice that amount.
The cases of Paricutin in Mexico, of new volcanoes in the Himalayas, and of others elsewhere, suggest that, in areas of suitable tectonic environment, not only may old volcanoes again become active, but that new ones may also be formed. The evidence in New Zealand is that we possess that environment.
The more recent volcanic history as interpreted from the geology, combined with the seismological evidence, suggests that the volcanic belt of New Zealand is an area in which earthquakes and volcanic eruptions of major magnitude may be expected. It is important that the attention of geologists in New Zealand as a whole be drawn to the need for completing the story of past volcanism, so that its relationship to structural development may be traced, and that all the tools of the volcanologist be brought to bear on the present processes.
Grange, L. I., 1932. Taupo Earthquakes, 1922. N.Z. Jour. Sci. and Tech., 14, pp. 139–41.
Grange, L. I., and Taylor, N. H., 1933. Field-work on Soils of Western Taranaki. Dept. S.I.R. Seventh Annual Report, 1934–35.
Grange, L. I., 1937. The Geology of the Rotorua-Taupo Subdivision. N.Z. Geol. Surv. Bull., no. 37.
Hayes, R. C., 1943. Subcrustal Structure in the New Zealand Region from Seismic Data. Bull. Seis. Soc. Am., 33, no. 2, pp. 76–79.
Henderson, J., 1916. The Geology of the Aroha Subdivision. N.Z. Geol. Surv. Bull., no. 16.
Macpherson, E. O., 1947. An Outline of Late Cretaceous and Tertiary Diastrophism in New Zealand. Dept. S.I.R. Memoir, no. 6.
Oliver, W. R. B., 1931. An Ancient Maori Oven on Mount Egmont. Jour. Polynesian Soc., 20, no. 2, pp. 73–80.