Art. XLIII.—The Volcanic Outburst at Te Mari, Tongariro, in November, 1892.
[Read before the Hawke's Bay Philosophical Institute, 12th June, 1893.]
I think it ought to be placed upon record that a very important eruption took place on the Tongariro Mountain about the middle of November last year. At the northern end of the mountain, and outside what is properly the old Tongariro crater, are two centres of activity. One is known as Kehetahi, situated on the north-west side of the mountain, at a height of 4,900ft.; the other is Te Mari, situated at the north-east end, at a height of 5,600ft., and immediately above Lake Roto-aira. Few people have ever visited the latter spot, owing to the difficulties of reaching it; besides, its existence only dates back a few years. In a paper read by me two years ago on Ruapehu and Tongariro,* I pointed out that of all places in the volcanic district Te Mari is the spot for vulcanologists to visit, as it was here that volcanoes in embryo could be seen and studied. Ever since the time of the Tarawera eruption there has been a perceptible growth of volcanic activity in the group of volcanic cones forming the Tongariro Mountain system. Ngauruhoe and Ruapehu have by various explosions shown their growing activity, and the mud-pools, solfataras, and explosion-vents at Kehetahi have not only increased in intensity, but the area of activity has been slowly widening, and new places—like new wounds about an old one—are slowly breaking out near to the main centre of activity. Te Mari, when visited by me in March, 1890, consisted of three shafts of unknown depth, each having a crater like an inverted cone, whose centre was the mouth of the shaft. A gyratory force from below had evidently shaped the craters in this way as a prelude to the flowing of lava. Sulphur, or what seemed to be sulphur, was deposited here and there in the crater, and steam was rising in fair quantity from each shaft. The appearances betokened much activity below, and the loose sands and débris around showed that explosions had taken place at no distant date. To the northwest of the shafts, and separated only by a small ridge of lava, is an immense crater of great depth. The walls of this crater are finely banded, showing lava-streams of slightly different characters, and illustrate the way in which the lava must have
[Footnote] * Trans. N.Z. Inst., vol. xxiv., p. 607.
welled from the shaft. This crater was quite extinct at the time of my first visit.
Having heard that explosions had taken place at Tongariro, and being unable to obtain information as to its extent or exact locality, I determined to visit the district again during the Christmas vacation to see what had actually taken place, and to note any facts that might appear worthy of being recorded. The road to the mountains is now in such fair order that it is possible to drive as far as Lake Rotoaira, which is situated between the volcanic cones of Tongariro and Pihanga. This plan of reaching the district was adopted, and on the last day of 1892 a small party pitched tents on the south shore of the lake, and made preparation for the ascent of Tongariro on the morrow. The evidence of an eruption was very clear from our camping-ground, and on portions of the road running along the side of the lake the deposition of débris and a peculiar sweet earthy odour that arose therefrom showed the location of the spot, and in some measure the character of the eruption that had taken place. I had purposed leading our party up the mountain by way of Kehetahi, but the demands of the old chief living at Otukou compelled me to forbear, and it was decided to attempt an ascent of the mountain byway of the rift, or gut, which had been made from Te Mari down the mountain-side leading into Lake Rotoaira. Bounding the northern end of the range between 4,000ft. and 5,000ft. is a belt of bush and scrub. The eruption had cut a deep channel through this bush, and up this it was arranged to climb.
Our party, six in number, two of them being my own children, started to make the ascent on New Year's morning. A day's rations for each, with one to spare for contingencies, was prepared, and at seven in the morning we were on the march in the direction of the gut on the mountain-side. We were not long in reaching this place, and we found to our great delight that the travelling was comparatively easy; so much so, in fact, that ladies could ascend the mountain by this track without much difficulty. The gut varies in width from 30ft. to 60ft., according to the depth of the sides, and it continues up the side of the mountain to a little beyond the limits of the bush, and within 500ft. of the crater of Te Mari. There it terminates in a great face of black basaltic rock, which is polished and smoothed and pitted, and has the appearance of an old waterfall, although at the time of our visit no water was flowing down the gut from the mountain. It was curious to note that all the exposed rocks in the waterway were finely polished, although they appear to have been exposed since the explosion on the mountain. The gut or channel had evidently been washed out by the water, sand, mud, and stones ejected from Te Mari, and which it seemed had not
only filled the channel, but had in places destroyed the trees which stand on the jagged banks. Most of the trees for a chain or so on either side of the channel had their leaves blackened, or reddened, or browned, as if they had been suddenly scalded by hot water, or the roots of the trees had been killed by hot water, the leaves remaining on the trees to brown and die. The material ejected from Te Mari on this side of the mountain was strongly impregnated with alum, and the water in the small water-holes at the base of every rocky face was very bitter and undrinkable, although as clear as crystal. The discoloration, and thereby the destruction, of the forest leaves in this way was to me of special interest in its suggestiveness, and I felt amply repaid for my trouble by the new light such an objective example threw upon a difficulty which had presented itself to me for some time in connection with the deposition of fossil leaves, &c. For several years I have occupied myself at intervals of leisure in collecting impressions of leaves, fishes, &c., from the Poverty Bay and Kidnapper Post-pliocene deposits. Some of the principal beds of the series are made up of a fine pumice, interbedded with a kind of pumice-mud, in which there are beautiful impressions of the forest flora—leaves, flowers, and ferns—of this country, mixed with impressions of fresh-water fishes (koura?) and other traces of animal life. From the appearances represented by the leaves on the scalded trees, I am of the opinion that this mode of destruction explains and illustrates the manner in which the leaves in the Poverty Bay and similar beds were destroyed and subsequently deposited, in a lake or estuary. The impressions on the pumice-mud are raised like the impress on a coin, and they show each vein and veinlet and all the surface irregularities such as appear on a green leaf. Such impressions, it seems to me, could only be made in the case of leaves whose growth had been suddenly stopped and destroyed without injuring the leaves. No doubt the reddened, browned, and blackened leaves from the dying or scalded trees on Tongariro will be carried into Lake Rotoaira, or, maybe, into Lake Taupo, and there deposited with the fine pumice-mud which is constantly flowing into these lakes from the numerous streams and springs in the vicinity of Tongariro.
Beyond the belt of bush and scrub there is little or no vegetation on the mountain. A few scattered plants were gathered, such as gentian, Sophora, Celmisia spectabilis, Angelica, Ranunculus, a sweet-scented Pimelea, a Claytonia, and a Hectorella.
On reaching the old crater which adjoins Te Mari, and to which reference has already been made, the extent of the eruption could be plainly seen. The mountain, extending
from the old crater through the two shafts at Te Mari and passing along the slope of the mountain in a kind of circular direction, was rent in twain by an enormous fissure. This fissure had broken away a portion of the top of the mountain towards the north-east, and it had taken a direction along the foot of the higher slopes of Tongariro, so that the outer wall of the old original Tongariro crater has become a portion of the inner wall of the active vents at Te Mari. This new crater also includes the old extinct crater already referred to, and which on the south-west was showing signs of activity, it having been fractured in this direction at the time of the explosion. In the vicinity of the old shafts everything was in a state of intense commotion. Vast quantities of poisonous gases were rising from the rift, and the whole area on the north side of the rift was a seething mass of sulphur. Our party endeavoured to get near the central part of the rift, but the fumaroles and rising gases were found to be too dangerous for a venture; and our only means of seeing the centre of greatest activity was to ascend to the top of Tongariro overlooking the rift, and from this vantage-ground view the scene. Although the wind was favourable the depth of the rift could not be seen. Now and again water and mud were observed on the north-east, but no traces of flame or fire were noticed. Form appearances near the rift it seemed that water, sand, and small stones were the only things ejected at the time of the eruption, and these all in the same direction, but the top of Tongariro Mountain seems to tell a different story. I had crossed this mountain several times previously, but no sign of pumice had been observed on its sides or top. Now, however, the mountain tells a different tale. Scattered thinly over the top, and in pieces varying in size from ordinary grit to small pebbles, is a deposit of pumice, and the question arises, From whence did it come? This pumice is sometimes heavier than the ordinary froth pumice, and has a somewhat duller appearance than that seen in the cliffs bordering Lake Taupo; but there is no doubt of the fact that it is pumice. I noticed with some care the extent of its distribution, and found no traces whatever in the direction of the Blue Lake, whilst the deposit increases towards Te Mari. The Red Crater, in the direction of Ngauruhoe, was unusually active on the western side, and it is certain that Ngauruhoe had sent out black smoke and great quantities of dust about the time of the outburst at Te Mari. This is not only stated to be the case by Mr. Chase, an intelligent half-caste of my acquaintance who resides near Wai-o-honu, but Mr. Blake, of Tokaanu, told me that the dust darkened the air for several hours during the day following the eruption. Te Mari showed no signs of pumice on the lower parts of the mountain; and the only explanation that appears
to me satisfactory is to suppose that two eruptions took place at Te Mari, during the first of which, water and sand were thrown out, the wind being at the time from the south-west. This was followed by a second explosion, when pumice was ejected, the wind at the time being from the opposite quarter of the compass. The event is an important one, because it suggests a new period of volcanic activity in the district. Whether Te Mari will become more active than it now is remains to be seen. It may be that the late eruptions represent the solfatara condition in the history of the Tongariro Mountain, but the expulsion of pumice is usually looked upon as the prelude to volcanic activity rather than as the termination of it. We know that these mountains must have thrown out vast quantities of pumice in some period of their history, as a basin a hundred miles long and sixty broad is mostly filled with it, and the slope of that basin is from the Tongariro Range. Our knowledge of volcanic phenomena does not allow of a prediction, and time alone will enable us to determine whether Te Mari is beginning or ending a career of volcanic activity.
Addendum.—I find that weak muriatic acid, if sprinkled over green leaves, destroys the green colouring-matter. Other acids will no doubt act in a similar way. It may be inferred, therefore, that water strongly impregnated with acid was thrown from Te Mari during the eruption, and that the leaves on the trees were destroyed in this way.