Art. XXXIV.—Napier to Runanga and the Taupo Plateau.
[Read before the Hawke's Bay Philosophical Institute, 24th June, 1910.]
Although a good deal has been written about the volcanoes of New Zealand from the time when Hochstetter paid his celebrated visit in 1859, very little is yet known of the Taupo Plateau and the rocks that either bound it on the east or are found upon it here and there in exposed areas. Hochstetter and the late Sir James Hector, F.R.S., issued maps of the so-called volcanic district, but they were theoretical rather than actual, as the larger portions of the district were not visited by either of these careful observers.
Few other places in the world present better opportunities for observation of volcanic phenomena of the present-day type than does the area embraced mainly within the Taupo-Waikato basin; but this constitutes only a small portion of the area that is geologically included within the true volcanic zone.
The paper by Dr. Marshall (Trans. N.Z. Inst., vol. 40, p. 79) on “The Geology of Centre and North of North Island,” and the descriptive account of the volcanic cones south of Lake Taupo by Mr. Speight, B.Sc., F.G.S., in the admirable botanical-survey report of the Tongariro National Park by Dr. Cockayne, have given increased importance to the volcanic district, but much remains to be done before complete information can be made available covering the geological history of this interesting district.
The key to the physical evolution of the North Island is to be found in the true interpretation and history of the volcanic area.
In company with the Director of the Dominion Museum, Christmastide was spent in studying that portion of the Taupo Plateau extending from Runanga to the Rangitaiki River, thence to Loch Inver Station, on the northern slopes of the Kaimanawa Range, finishing at Taupo. This work I had another opportunity of reviewing at Eastertide, being anxious to complete a geological section along the coach-road from Napier to Taupo. This section (fig. 1) shows the beds to be met with between the sea-beach at Petane, where the coach-road strikes inland, and Taupo; but the section is continued to Karangahape Head, on the west side of the lake, as the intention is to give in another paper a similar section from Whakatane to Ruapehu, through Lake Taupo. Fig. 2 gives a map of Lake Taupo to scale, with the most important geological features.
A reference to fig. 1 shows Pliocene beds of Napier upper limestone near the Motor Company's stables at Petane. These in places are overtopped with shingle conglomerate representing the Kidnapper shingle conglomerates. The middle beds of the Pliocene series appear up the hill before reaching an elbow in the road, where again the shingle conglomerates top the limestones. No change takes place in the general characters of the Middle Pliocene series through the whole length of the Esk Valley; but the overlying shingles, sometimes mixed with sand, become thicker and thicker, and on the left bank of the river they are seen dipping to the south-east as strong conglomerates. On the right bank here and there the same beds appear, but there are Recent river-gravels mixed with limestone and
shells, and these may be mistaken for portions of the conglomerate series unless some care is taken to observe that the valley has been choked with shingle washing from the higher country at the back.
Leaving the Esk Valley, a long hill gives exposures of fine sand, which is in places fossiliferous. This sand about half-way up the hill becomes pumiceous, and in several places the whole exposure is a somewhat indurated fine pumice similar to what is met with among the shingle conglomerates at the Kidnappers. No fossil leaves, &c., were found by me, but in several other places the beds form a very good collecting-ground.
Proceeding towards the top of the hill near Eskmount, the conglomerates are again met with topping the highest hills on the one hand, whilst the limestone appears in several places, and along the road up to the twenty-mile stone the middle beds appear. Here the conglomerates top the hills on either side of the valley, whilst huge conglomerate blocks lie about the paddocks, and the whole country hereabouts is covered with conglomerates, alternating at times with fine sands mixed with pumiceous clays.
Up the hill leading to Carmichael's, conglomerates, fine laminated clays, sands, shingle, sands, and shingle cement are seen in descending order in great cuttings on the roadside. From the top of the hill (1,250 ft.) the conglomerates continue towards Pohui, the highest point of the road (1,375 ft.), being occupied by them half a mile or so beyond the roadman's whare.
Descending into the Pohui Valley, the conglomerates are replaced by hungry sands, and these continue as the characteristic rocks until crossing a small bridge leading up to the Pohui Hotel. Here nodular limestone appears in the blue clays, and fragments of broken shells are common, but it is difficult to determine their kind. Near the hotel limestone blocks are scattered about, and they appear to correspond to the lowest beds of the Napier series.
Titiokura Hill is made up of Miocene beds. and these are of extreme thickness,
as they are traceable from 2,250 ft., which is the height of the Titiokura Saddle, to 1,000 ft. at the Mohaka Bridge, where the same kind of rocks appear as in the case of the nodular limestone near Pohui, the Miocenc beds varying somewhat in character as the long hill from Mohaka to the top of Turanga-kuma (2,625 ft.) is reached. Here, at what is known as the Te Kooti Track, the Miocene beds are seen resting unconformably upon the Maitai slates, and from this place onward the slates and sandstones are the prevailing rocks as far as Tarawera.
A mile or two on the Napier side of Tarawera intrusive volcanic rocks are met with in several places. Up the Waipunga Valley intrusive rocks are common, and six miles beyond Tarawera rhyolites constitute the only rock by the roadside, and these continue for many miles in the direction of the Rangitaiki Hotel, or, rather, they disappear near the 68th milestone, where the swamp-area begins.
Runanga is at the 65th milestone. A mile or so from this place is a high hill, 3,200 ft. high, known as Otumakioi. It has a trig. station on the top, and there the sandstones appear very similar in appearance to the Permian sandstone of England. It seems to be connected with the Maitai slates. as these were exposed in one or two places on the hillside. The country, however, is so bush-clad that climbing becomes a difficult matter.
Immediately opposite Otumakioi, but to the north-east, is another hill or mountain, Piki-o-hiko-wera, which is composed entirely of volcanic rocks. From Runanga all the exposed rocks, whether on the roadside, in the bed of the streams, or forming the ranges that run to the north-west and north, are volcanic lavas, mainly trachytes and rhyolites. A mile or two along the valley that extends from Runanga towards the plateau extensive washings have taken place, and pumice terraces with sloping sides leading into the valleys make their appearance. This country of late years has been occupied as a cattle and sheep run, and the owner, in order to drain a large swamp-area that is the true watershed between the Mohaka and Rangitaiki Rivers, had a trench dug, so as to drain the water into the Waipunga Stream by way of a smaller stream that passes the roadman's house at Runanga. This drain was originally made about 4 ft. in depth and 2 ft. wide, but the breaking into the pumice has resulted in the formation of a huge gorgelike area nearly three-quarters of a mile long, 60 ft. or more wide, and very deep. The removal of the surface of rushes and Sphagnum has resulted in the washing-out of a gulch that can hardly be understood without inspection. The slightest increase in the rainfall causes the removal of great quantities of pumice pebbles, which fill up the entire area of what was at one time in the history of the district an extensive crater, the walls of which can be distinctly read.
In several places are volcanic hills or mounds, without craters, resembling similar structures such as may be seen to-day in the district where volcanic phenomena are active, particularly on the west of Ngauruhoe and the north-west of Ruapehu. This swamp extends for a number of miles, and to get to the Rangitaiki it is necessary to pass over the ridge or southern crater-wall, from which there is a gradual descent of about 200 ft. in a distance of two miles.
The Rangitaiki River basin is separated from the Mohaka basin by a range of hills, except towards the north-west, where a small stream from the swamp also drains into the Rangitaiki. A study of the surrounding country between the 73rd and 83rd milestones leads to the opinion that
at one time a series of lakes covered this area. The Rangitaiki River has its source in some old crateral lakes on the Loch Inver Station. Here trachytes are found in close proximity to the sandstones which top the lowlying hills that form the northern end of the Kaimanawa Mountains. The lakes seem to have been centres of explosive action, and at the place of outflow, where the water flows into the open country and takes the name of the Rangitaiki Stream, the rhyolites form a lip which has been cut through by the owners of the station by means of blasting, so as to assist in the dramage of the swamp and lake.
The country has few exposures of rock, but in a creek near the station already named both sandstones and the Maitai slates occur, just as a little lower down the volcanic rocks appear, although no intrusive rocks were met with.
Following down the Rangitaiki, which runs between high banks, there appear exposures of peat-lignites of some 9ft. in thickness. These cover a large area between the upper stream and the Rangitaiki Hotel, a distance of some six miles. Whether the beds reappear below the hotel I do not know, but the exposures show that at one time a very large part of what is now pumice-covered country consisted of swamplike areas similar to what are met with and are now being drained in the vicinity of Runanga. The lignite material is made up of Sphagnum, stems of shrubs such as Veronica and Fagus, and a large variety of fibrous roots matted together into a compact mass. Overlying the lignite is pumice, very fine in structure, and without pebbles. It is not more than a few inches in thickness, and it appears to have been deposited by water. It is difficult to account for this covering of lignite or vegetable beds by a pumice-deposit, because the fact of a lignite-bed implies a long period of undisturbed growth and decay. It may be that the lignite-beds represent the old surface of the plain before the last great outpouring and spread of pumice, when the country as far as the 81st milestone contained a series of swamplike lakes that spread here and there among the hills forming the head-waters of the Rangitaiki, and even flowed at times over a saddle into the swamp-lakes that begin in the vicinity of Runanga. A low saddle from the Napier-Taupo coach-road to Loch Inver forms the dray-road to the station; and when viewing the topography of the country from one of the higher hills of the district it can be seen that the various lake-areas were at one time joined by low saddles that are not more than 300 or 400 yards in width. In some parts of the lignite-beds there is a thin band of pumiceous clay between the two main lignite-beds; but everywhere the pumice-beds overtop the lignites, just as the pumice to-day covers the whole surface of the Taupo Plateau.
Down the Rangitaiki River, ten miles or so from the coach-road, a series of rounded hills occur, and these run north-west as far as the track, which divides, one to Galatea and one to the Waiotapu Valley, across the Kaingaroa Plain. At this place there are the remnants of rocks that must at one time have formed an immense crater, the broken-down walls reminding one of old embattlements. It seems as if the country to the north-east of the 81st milestone must have sloped in the direction of Waipunga, and that the surplus waters, charged with pumice, were carried into the Mohaka River, and thence into the area of Hawke's Bay.
In a former paper (Trans. N.Z. Inst., vol. 37, p. 445) reference is made to the original drainage from Ruapehu and Tongariro at the time when the present Lake Taupo was the crater of an immense volcano. The suggestion is made
that the drainage from the mountains went in the direction of the present Rangitaiki. When this took place the series of lakes were in existence to which reference has been made. The whole of the Taupo country to the north was in process of being formed. It can be readily understood that swamp-areas would likely be formed within an area that had already been subject to volcanic forces.
From the hotel on the left bank of the Rangitaiki, Taupo Road, to Lake Taupo is twenty-three miles. A reference to fig. 1 will show that the plateau between the hotel and the lake presents two distinct areas of drainage. The entire area after passing a range of low bush-clad hills on the left is a mass of pumice of unknown depth, interspersed here and there with volcanic grit, magnetic iron, sand, and angular and rounded pieces of heavy rhyolite-andesite lavas that suggest water carriage without much wearing, because bedding is traceable in the pumice wherever washaways occur.
From the 80th to the 85th milestone the country to the south-east and north-west presents a high ridge, and this constitutes the line of separation
between Taupo Lake as a drainage-area and the drainage of what must be considered as a part of the Rangitaiki drainage-area. Taupo itself is only 1,200ft. above sea-level, and Karangahape, the high headland on the western side, reaches only 2,379ft., whilst thirteen miles from the lake (that is, at the 85th milestone), on the Taupo Road, the height is 2,380ft. —in other words, the top of Karangahape is on a level with the highest portion of the plateau.
Taupo as a volcano was apparently much larger than it is as a lake. It would almost seem as if the long, high ridge between the 80th and the 85th milestones formed the eastern side of a great crater in which were many supplementary craters such as are seen to-day as centres of activity in places already named. In the earlier period of volcanic activity the whole country from the Kaimanawa Range to the ranges bounded in most places by the Main Trunk Railroad from the latitude of Ruapehu consisted of one immense lava-flow of similar volcanic material, and it is interesting to compare the height of some of the conelike hills with the heights of the country along the eastern and north-eastern side of Lakes Taupo and Rotokawa. The western and eastern portions of this once great sea of lava have merely left remnants of what they once were as centres of volcanic movement. Here and there a hot spring remains, but otherwise all active volcanic phenomena have long since disappeared, and the only portion that is really active at the present time is the great central line where so many traces of explosive eruptions are found.
The great explosive eruption at Taupo and Rotokawa, as also that at Pihanga Mountain, mark the largest and most disastrous of all those that have occurred in the North Island. The pumice thrown out spread over the country, mainly to the eastward, as far as Poverty Bay in the north and Hawke's Bay in the south, whilst the middle part of the Island was filled up to a great depth with pumice and other forms of volcanic ejectamenta. Pihanga's explosion was towards the north, where the crater-lip was blown right out, and is the counter of the Tarawera explosion, which was towards the south; in fact, the whole face of the country forming the central portion was altered by the explosions and the materials thrown out. The water that had drained into the Rangitaiki from the volcanoes to the south were diverted into the new Taupo crater, which, like Rotomahana, contained many traces of volcanic activity similar to what was seen in the latter basin after the Tarawera eruption; and, just as that basin has been slowly filling with water, so in the same manner, following what was an eruption at Pihanga, Taupo, and Rotokawa, immense quantities of débris were deposited over the country for many miles around the centres of explosion, so that watersheds were altered, drainage-areas diverted, swamp-areas replaced which before had been streams and tributaries, and finally provided a drainage-area of considerable size for the new crater that extended for thirty miles or more, and in which thousands of steaming places were to be found.
At this time there was no Waikato River such as we know it to-day, for the Waikato, from the spot where it leaves Lake Taupo as far as Orakei-korako, merely flows through a chasm or rift formed at the time of the Taupo explosion, but filled towards the crater-lake with pumice that raised the surrounding country several hundreds of feet in height. When the lake filled in course of time the water found a way through the chasm, and lowered the waters of the lake some 200 ft. or more. A subsequent
earthquake caused the Huka Falls and the attendant depression, the silicacemented pumice in the vicinity being sheared as with a knife.
Lake Taupo at one period of its history was much larger than it is at present. The terraces on the west and east, the surface-characters of the slopes in the vicinity of the lake, and the various valleys that run for long distances towards the east at a gentle slope all suggest an area once covered with water. At the northern and southern ends of the lake hot springs and many other evidences of volcanic phenomena occur, and these extend underneath the waters of the lake from a good distance from the shore. Hot springs, fumaroles, &c., are equally abundant 600 ft. or more up the hills towards Tauhara Mountain at the northern end, and even higher at the southern end, where at Waihi only a few weeks ago a hillside from the fumarole-area broke away and slid into the lake.
The activities of Rotomahana and its vicinity such as have been manifested since the eruption, particularly to the southward, correspond in a remarkable manner to what took place to the north of Taupo following the eruption there. In pursuing one's inquiries into this interesting subject one is struck with the fact that the surface-features of the country must have been modified as much by earthquakes as by volcanic agencies; but the key to the true interpretation of all the phenomena to be met with at Waiotapu, Rotokawa, Wairakei, Taupo North and South, and Te Mari is to be found in the study of the phenomena connected with the Tarawera eruption and the subsequent events that have taken place in the district. fig. 2 shows merely the present size and boundaries of Lake Taupo, with such information as I have been able to gather during my frequent visits to the district.
Earthquakes have caused marked changes in the vicinity of Taupo. Cussen reports that on the 28th August, 1883, a schooner was lying at the small wharf where the Waikato leaves the lake, when a sudden fall of 2 ft. took place. In about twenty minutes the river rose again to its former level. Mr. Enoch Hallet, who was present at the time, informed me that between 1 and 2 o'clock the same afternoon, as the river fell, two members of the Constabulary stationed in the township—namely, Major Smith and Sergeant Miles—were bathing in a warm spring called Waiarika, situated on the bank of the Waikato River, about a mile from the lake. Next the river the bath was fenced about with stones, and it stood about 2 ft. above the level of the water. Suddenly the bath became cold, and the bathers were astonished to find that the river had risen to a level with the bath. It remained in this state for about five minutes, and then suddenly returned to its former position.
In August, 1895, a very severe earthquake took place in East Taupo, and the public road in places was entirely destroyed. The pumice cliffs were much altered, and the isolated hill known as Manguaume, about three miles to the eastward of the lake, was split in parallel lines that ran north-east and south-west. Around the lake, but more particularly towards the east and north-east, are long deep valleys, consisting of main valleys and branching ones. It is difficult to account for the formation of these, unless we suppose that the lake was much higher and, of course, larger than it now is. The Rev. Mr. Fletcher, of Taupo, suggests that a subsidence of a large area in the lake took place, or that the wearing-away of a barrier in the river caused the deep, regularly cut valleys. But the same kind of valley is common over the greater portion of the Taupo Plateau, and some of the valleys slope in the direction of the Bay of Plenty. The more likely
reason is that a sudden uplift took place of the country to the east and north-east of the plateau, which affected the entire area extending as far as Rotorua.
In riding over the district one is impressed with the fact that the surface-features suggest changes of comparatively recent date. Bidwill, in 1839, when travelling between Tauranga and Taupo, refers to traces of dead and burnt timber over a large extent of country. At that time no manuka (Leptospermum scoparium) was to be found between the Rangitaiki and Lake Taupo, and, in fact, there was no manuka between Tauhara Mountain and the lake so recently as 1869. Around the base of Tauhara, on the northern slopes, and reaching for several miles, are scores of charred totaratrunks that must have been of enormous size. No Native has any knowledge of the area being covered with forest, but the logs remain to show that at some former time the country was bush-clad, and extended apparently in the direction of the Opepe Bush. Even at the present time the Taupo Plain is only just beginning to have the semblance of a cap of vegetation.
The entire country has been subject to many disastrous explosions and earthquakes, so that the physical changes have been of frequent occurrence. Some of the old craters are possibly overwhelmed along the line where old crateral walls can be traced along the ridge that now separates the Waikato and the Rangitaiki drainage-areas. As already remarked, the growth of manuka on the plateau only began in the “seventies.” Tamati te Kurupae, the present leading chief at Taupo, recollects well when the manuka first appeared, and so rapidly did it spread that it drove many Natives from their settlements, as they were unable to grow crops as heretofore. Neither Bidwill nor any other writer that I can discover makes reference to the Leptospermum as growing freely, and which is now so abundant in the district. This seems to me to imply that the surface of the plateau is comparatively new, and the limited vegetation seems to support this view. A pumiceous surface such as the Taupo Plateau would take a long time to become the abode of even the lowest forms of vegetation, except within the area of the swamps.
The legends of the Natives are most suggestive in connection with the surface-changes on the plains, implying differential movements that altered the flows of the rivers, and even caused jealousies to spring up between them. A long time ago, so runs the legend as told to me by Tamati te Kurupae, the leading Taupo chief to-day, a serious quarrel arose between Waikato and Rangitaiki as to which had the greater mana. Rangitaiki urged that he was the greater and more important among the rivers, but Waikato replied, “No; I am chief; but if you think yourself stronger and better let us put the matter to the test by seeing which of us will first reach the ocean.” Rangitaiki derided the words of Waikato, and many angry scenes were witnessed, but at last it was decided to put the matter to the test. The race began. Rangitaiki started from the swamps, and Waikato from Lake Taupo. Soon the latter river reached the Aratiatia Rapids, but becoming anxious about the Rangitaiki, his rival, he asked Puato (a small stream entering the Waikato on the right bank) if he had seen Rangitaiki. Puato replied that Rangitaiki was moving rapidly towards Waikato, hoping to destroy him. Waikato made strenuous effort to move away from his rival. At Orakeikorako Waikato sent Tori-patutahi (another stream entering the river on the right bank) to go and see what had become of Rangitaiki. Tori-patutahi returned in haste and said that Rangitaiki
was close at hand. This news caused Waikato to alter his course to the sea, and thus he was able to outpace Rangitaiki, who, wandering through the country, lost his way in the Whakatane Swamp, which showed how inferior he was in his mana to Waikato, whose name means “a great chief or leader of the people.”
This legend is most suggestive as showing the likely changes that tock place along the ridge that separates the drainage-areas of Waikato and Rangitaiki to the north of Mount Toruhara. There is no doubt whatever that the surface of the country extending from Rotokawa to Orakeikoraka and thence on to Waiotapu has undergone changes within recent times. It is possible that the Puato and Tori-patutahi Streams flowed into the Rangitaiki before the great eruption at Taupo, and even up to the time when the rift occurred that now forms the Waikato itself from the place where it leaves Taupo Lake down to Orakeikorako. An imaginative people like the Maori personifies the changes or movements that take place in stream and mountain and bush, and the very fact that Puato and Toripatutahi were sent as messengers to look after Rangitaiki implies a change of flow in these streams, or at least a modification in the conditions of the country.
A study of the surface-features of the Kaingaroa Plain to-day shows that the line of movement was to the north-west, and the exposures of the rocks suggest a marked depression to the northward of the fault that runs across the plateau in a north-west and south-east direction about sixteen miles from the lake. A reference to fig. 1 will explain this point. Tauhara Mountain is only 3,600 ft. above sea-level, and Lake Taupo is 1,200 ft. The highest point on the plateau is about 2,530 ft. There are no exposed lavas nearer than Manganamu and Tauhara. The old water-valleys show nothing more than pumice and volcanic ejectamenta, except at King's, near Opepe, where traces of volcanic rocks in situ are said to occur. Thus the lavas towards the Rangitaiki are much higher than are found between Opepe and Lake Taupo, so that unless there has been a big depression between Opepe and the lake, of 1,000 ft. or more, it is difficult to see how there could have been a connection between the lavas from Tauhara and those found at Loch Inver and Runanga.
A survey of the whole of the volcanic area is badly wanted. Every volcanic orifice should be located, and the country mapped to show that the present line of volcanic activity is only one aspect of the phenomena that have taken place over a long period of time to bring about the present surface-features of the country. Isolated inspection is insufficient to formulate opinions as to the history of the surface-changes of the volcanic area and the probable duration represented by the volcanic rocks to be found in the district; but the events at Tarawera in 1886 suggest a line for observation. What took place at Rotomahana has likely taken place in a score of other places, and by placing the facts side by side, and studying the growth of vegetable-life within areas that appear to have been acted on by outbursts, as exemplified in lakes of depression, it may be possible to read far into the past, and even to suggest something as to the future.