Art. LXXI.—Ruapehu and the Volcanic Zone in 1895: No. IV.
[Read before the Hawke's Bay Philosophical Institute, 15th July, 1895.]
The eruption of Ruapehu on the 10th March of this year has opened up an interesting question as to what is likely to be the future of this mountain: Is it to become dormant and finally extinct, or will it be seen again as an active volcano similar to what it was in times which may be counted by the century? The last time when the mountain displayed signs of increasing activity was on the 1st May, 1889. It is only within the last decade that Ruapehu has come to be looked upon as a mountain whose life is not yet over. Tongariro and Ngauruhoe were known to be active volcanoes, because clouds of steam could be seen rising from them at a distance of fifty miles in the direction of Taupo and the Kaingaroa Plains. When Hochstetter, the famous geologist, was at Tokaanu in 1859 he appears to have made the fullest inquiries from the natives concerning the volcanic group to the southward, although he was forbidden to visit or even to approach any portion of the group. Bidwill and Dyson, in 1839 and 1851 respectively, had secretly visited portions of the group, and their published accounts had no doubt fired Hochstetter with a desire to see what was, at that time, looked upon as the only spot where volcanic phenomena could be studied on a large scale in this country. Of Ruapehu, however, nothing was then known, except that native tradition said it was the abode of an evil spirit, Te Ririo, who caused men and women to wander hither and thither over the mountain until bereft of their reason. The fearsome and solitary surroundings in the eyes of an imaginative people caused this mountain to be viewed with awe and dread, and nothing was remembered concerning it beyond the fact that it was the abode of an energy or a cause which brought about injury to humankind. Does this not suggest that the mountain was a centre of danger owing to the frequency of explosion? Hochstetter says, in his great work on New Zealand, page 378, “No one has ever ascended Ruapehu or explored it. Nevertheless, there can be no doubt as to its volcanic nature; but it seems perfectly extinct—there is no trace of a solfatara to be discovered in the distance either at its sides or at the top, and it is totally unknown whether the broad summit forms a plateau or whether it contains a crater.”
Between 1859 and 1880 no attempt seems to have been made to ascend any of the mountains that were held to be tapu by the Natives, and which included the whole of the volcanic group. In the latter year, however, Messrs. Maxwell and Beetham reached the south peak, known as Parae-te-taitonga. There they built a cairn and left several small coins as mementoes of their visit. Since, the mountain has been crossed on several occasions by Government surveyors and others, and the topography of the mountain is now fairly well known.
As pointed out by Hochstetter, the mountain is an enormous truncated cone, with ridges running for miles to the east, north, and south like the gnarled roots of an ancient forest-tree. Towards the south the mountain presents a much steeper face than to the north, east, or west. On the east the mountain is most broken and denuded, and has much smaller snowfields than are met with on the west and south-west. Long, sloping ridges occur here, and these actually form the water-parting between the rivers flowing to the north and south respectively. The forest-lands run close to the snowline on the west and partly on the south; but only clumps of stunted Fagus panax and scrub are found along the east and north-east portion of the mountain. Ruapehu is separated from Ngauruhoe by a valley varying from a mile to two miles in width. The Tongariro group is situated within an area of desert and desolation, which, from its height in comparison with the surrounding area, may be likened to the top of an immense dome, which flattens in the direction of its rim. Here and there transverse corrugations appear to break the general arrangement, as in the case of the Kaimanawha and other smaller ranges; but these do not alter the dome-like structure which the North Island presents, with Ruapehu and Tongariro as a centre.
The country immediately surrounding the cones has a varying height from 4,800ft. to 3,000ft., and it is made up principally of pumice and lava-flows from the mountains. Here and there along the western side of the group are numerous conical hills made up of volcanic lavas, but having no crateral hollows or basins such as are usual in cones from which lava has issued; and it may be assumed that traces of lava-flows will be found along the whole line of country which separates Ruapehu and Egmont. On the east side of the group no such cones are found, and the country as far as the Kaimanawha Range is composed of a grit and pebble pumice mixed with coarse material of many rock varieties similar to what were found in the vicinity of Rotomahana soon after the eruption. The clinkers found on the Rangipo Desert in clumps here and there differ from all the volcanic material I have seen else-
where, and they suggest a partial fusing of pumice with a greenish-looking rock, but before the fusion was complete they were ejected from an orifice and fell on the desert where they are now to be found. Their partially-rounded surfaces imply cooling during their translation, and their freshness and position suggest their eruption from Ruapehu at a very recent date.
A reference to a map of the North Island will show the line of direction of what is known as the volcanic belt. This belt in a measure takes the form of an ellipse, with its major axis running north-east and south-west. The length of this axis is not less than 140 miles, whilst the minor axis extends about 75 miles in a north-west and south-east direction. Imagine a line drawn from the top of Ruapehu, at the south end of the major axis, at a height of 9,000ft., to the top of Whakaari or White Island, in the Bay of Plenty, 800ft., and you have a plane representing the slope of all the intervening cones between these two extremes. These include Ngauruhoe, Tongariro, Pihanga, Tauhara, Tarawera, and Edgecumbe. Including Ruapehu and Whakaari, five of the cones may be said to show signs of activity. The entire country included within this elliptic belt presents direct evidence of volcanic action. The surface is covered either with pumiceous deposits or with lavas of many grades and characters. The boundary of the belt is made up of mountainous country presenting a somewhat scarped face to the volcanic area, and suggesting either an elevation of the bounding rocks or a depression of the enclosing area. Numerous streams and rivers pass through the volcanic belt, and in most of the exposures I have seen trachytic lavas make their appearance, giving one the idea that the entire area is made up of lavas overlaid by pumice-washings and other kinds of volcanic ejectamenta.
On the Taupo Road, between Tarawera (Hawke's Bay) and Rununga, most of the exposures present trachytic lavas. In the bed of the Waipunga Stream, which crosses the Taupo Road at Rununga, the lavas form the bed, and strike across country in every direction. At the Rangitikei River, some twelve miles further on in the direction of Taupo, the same trachytic lavas are to be seen, and they continue along the Kaingaroa Plains, both in the direction of Galatea and the Waiotapu Valley. They surround Taupo Lake; they form the rocks exposed on the banks of the Waikato River from its exit at the lake as far as Ateamuri; they surround Lake Rotoaira, near Tongariro; and they continue along the west side of the volcanic group on to the Waimarino Plain.
Along the centre of this immense belt of trachytes, which covers the entire country like a large sheet, the volcanic cones
rise in the line representing its major axis. Hochstetter, in his geological map of the Auckland Province, shows the distribution of trachytic lavas to the west and north of Taupo Lake, but he appears to have thought that the lavas which surround Tauhara Mountain did not extend for any distance to the east or north-east; and it is very certain that he had no idea that volcanic rocks were to be found of any extent to the east and north-east, or that they struck through the island in the direction of Hicks Bay.
Hochstetter's opinion as to the sequence of volcanic phenomena is very suggestive. He says, “The first volcanic eruptions were submarine, consisting of vast quantities of trachytic lava, breccia, tuff, obsidian, and pumice-stone, which, flowing over the sea, formed an extensive submarine volcanic plateau. The volcanic action continuing, the whole mass was upheaved above the level of the sea, and new phenomena were developed. The eruptions going on in the air instead of under the sea, lofty cones of trachytic and phonolitic lava, of ashes, and cinders were gradually formed. These eruptions breaking through the original submarine layers of trachytic lava, breccia, and tuff, raised them, and left them as we now find them, forming a more or less regular belt round the central cones, and having a slight inclination from the centre outwards.”
This theory assumes three periods of volcanic activity—First, submarine, with deposition of trachytic lavas; second, elevatory, when the whole of the submarine deposits were raised above sea-level; third, formation of cones. Unfortunately, Hochstetter does not suggest a time when the volcanic phenomena began, or what sedimentary rocks were contemporary with the elevation of volcanic ones. But the three periods agree well with the distribution of pumice through the rocks of this district.
In volume xx. of the Transactions, Article xxxix., there is a paper on “The Geological Distribution of Pumice along the East Coast,” and three separate periods of deposition are shown to have taken place: 1st, a Miocene (?), Cretaceo-Tertiary of the Geological Survey; 2nd, a Pliocene; 3rd, a Recent. If the distribution of the pumice over the Island agrees with periods of activity, as stated by Hochstetter, then the Miocene (?) period was one when the North Island presented a few islands only in place of its present area. It was towards its close that the trachytic lava-flows welled up, as it were, without the formation of cones and spread themselves, as they have done, in such a curious manner over the whole of the volcanic belt. During the Pliocene period the North Island had grown considerably, and the pumice, with much of the débris, was spread over the district both east, north, and south by the
rearrangement of watersheds which then took place owing to differential movements of the land. Finally we arrive at the period of cone-formation; and undoubtedly a fresh period of volcanic activity began as the result of earth-movements almost parallel to and on both sides of the major axis of the volcanic zone. How long the activity of the cones along the major axis has continued it is impossible to say; and it is equally uncertain whether the building-up of these began from the south or the north. The extinct Egmont and the equally extinct Hauhangatahi to the westward of Ruapehu almost imply that these mountains were two of the earlier cones, as from their size compared with all except Ruapehu they would require a longer interval of time to cool. In any case, the third period of activity has been more manifest in the southern portion of the zone than elsewhere, and the activity is still greater there than in any other portion of the Island. Ngauruhoe is no doubt the youngest of the volcanic cones, as its marvellous symmetry has not yet been broken down like Tongariro and Ruapehu by the lapse of time and atmospheric influence. Tongariro and Ruapehu are less active than Ngauruhoe, and there can be little doubt that the two former are in a state of collapse. Ruapehu appears, like Tongariro, to have periods of activity depending on hydrothermal causes. The changes which took place at Te Mari, situated at the north end of Tongariro, in November, 1892, correspond exactly with those which have taken place on Ruapehu—first in May, 1887, and again in March of this year.
At the summit of Ruapehu is a crater-lake, situated between the three highest peaks, known as Parae-te-tai-tonga to the south, Ruapehu to the west, and Te Heuheu to the north. This crater-lake on its western half is surrounded by glacial ice, which, as the terminal portion of a glacier, actually forms the walls of a sometimes boiling crater. It has hitherto been impossible to learn the depth of the water in the crater, but its temperature appears to undergo important changes, depending possibly on the supply of ice from the mountain. Observers who have seen this crateral lake report it either as being partially frozen over, as being cool, as being warm, or as being in a state of ebullition. When visited by me in March, 1890, its waters were boiling rapidly, and there was a wave-like movement from west to east, whilst at intervals of about two minutes steam was suddenly thrown from the surface, and a kind of pulsating movement followed, the waters appearing to subside for a short space. There appeared to be little or no space available whereon to stand, assuming it had been possible to reach the lake, except at the south-east bend; and, as the crater-walls at this point were steaming furiously, and our party had no rope available, no attempt was made to
reach the lake. I am pleased to find that this has now been accomplished by Mr. Walter H. Dunnage, of the Survey Department, with two of his men. Mr. Dunnage has made the ascent of the mountain on several former occasions, and when visited by him at Easter last year (1894) the “lake was of a beautiful green colour, apparently cold, with a sulphurous deposit on the surface here and there.” This had all changed on the 5th of April of the present year, when “the lake was of a milky colour, with steam rising from its surface, and surrounded on all sides but the east by a beach from half a chain to a chain in width.” The lake was 10ft. or 12ft. lower than he had seen it before, and, what was the most fortunate thing of all, the party of three were able, with due precaution, to reach the margin of the lake—the first human beings who had ever done so. The temperature of the lake was found to be 128°. “The margin of the water was covered with blocks of very friable scoria, among which were innumerable small boiling springs causing a dull, weird sound.” This visit was made about a month after the explosion which took place on the 10th of March, when a column of steam was thrown up at least 1,000ft. above the crater; and the same thing was observed on several succeeding days.
It does not appear from Mr. Dunnage's account that there was any trace on the mountain of material, such as mud and boulders, having been ejected from the crater and deposited on the snowfields, but it would seem from the lowness of the water that most of it must have been sent out by the several explosions. Mr. Dunnage suggests that the River Wangaehu issues from the crater, but in this he is mistaken, as I have followed this river to its source, the discoloured stream coming from the ice immediately below the rocks known as “The Pinnacles” to the north of the crater.
The discovery of other warm springs, seemingly large puias, on the western side of the mountain is an event which adds interest to the mountain, and points to “sores,” similar to those found at Ketetahi and other places along the slopes of Tongariro. But both mountains have had their day. They present phenomena which may be termed the condition intermediate between activity and repose. It is a warring between heat and cold; and those who have ever sat down within a volcanic area capped with glacial ice and snow can have no doubt as to the victor. Ruapehu has had, and will have for some time to come, periods of activity due to hydro-thermal causes; but everything testifies that its end, from a geological point of view, is near. Still, it is well to keep records of the changes taking place, for the gathering together of facts connected with volcanic phenomena will enable us to arrive at those generalisations by which volcanic phenomena
can be foretold with as much certainty as a study of meteorology enables us to say what kind of weather to-morrow is likely to be. Ruapehu, although capped with ice for a thousand feet or more, is a festering sore—a solfatara—whose symptoms imply decay and extinction. On various parts of the mountain, as in the case of Tongariro, there are traces of thermal activity, and, when the topography of the mountain comes to be better known, no doubt many hot springs and mud-holes will be found similar to those reported by Mr. Dunnage and party as existing on the west side of the mountain. It may be that a supplementary crater will be found on the western side; but we must wait for further exploration before it can be ascertained with certainty that the mountain “broke out,” like Tongariro, in places outside, and at a much lower elevation than the original crater. A mountain like Ruapehu, with its enormous ridges and spurs, can hardly have been formed by means of its present limited crater, and mighty changes must have taken place since the time when the crater was imbedded within the three peaks which now bound it on three sides.
There are no traces of marine beds in the vicinity of the trachytic lava, nor is there evidence in the tuffs of having been raised from below the sea. The limestones, which present bold scarps to the volcanic area along the south-east portion of the district, and, indeed, are traceable on the borderland in every place visited by me, even to the north-west in the direction of Lichfield, belong to the later Tertiaries, and it would seem that the elevation of the volcanic area did not reach its present dimensions till nearing the close of the Pliocene period. This extension of the trachytic area appears to have given rise to the formation of an immense lake, which included the whole of the Taupo plateau, and this continued to enlarge until the increase of volcanic activity along the present line of weakness caused modifications to take place in the watershed of the Island, such as have brought about the present conditions. It is needless to point out the abundant evidence in support of this statement, as further details must be left for another paper.
What the future of Ruapehu and the other volcanic cones will be cannot be predicted with any degree of certainty. Egmont is extinct, Pihanga and Tauhara are also extinct, and the activity of Ruapehu, Tongariro, Tarawera, and Edgecumbe is comparatively unimportant. Of Ngauruhoe the case is different; and I shall conclude by quoting what Hochstetter says of this beautiful and symmetrical cone: “Although this grand volcano, with its various craters, has, within the last centuries, as far as it is known, not had any eruption of lava, yet I would not venture to assert that such might not suddenly
recur again.” Ngauruhoe has within late years poured forth streams of lava from its crater, and it may be that this is the best sign of diminishing activity in the case of Ruapehu and Tongariro, its elevation being much more recent than either of these great truncated cones.