Art. XLVI.—Thermal Activity in the Ruapehu Crater.
[Read before the Auckland Institute, 26th July, 1886.]
During my last season's work in the triangulation of the King Country I had occasion to ascend Ruapehu, to include a trig. station on Paretetaitonga, one of its southern peaks, with our system of triangles. It was not my intention to attempt a geological examination of the mountain; but the few notes which I was able to make in the short time I could devote to the
subject will be of interest, chiefly as disclosing the fact that the volcanic forces considered to be long since spent on Ruapehu are still active, in at least so far as the solfatara stage is concerned.
It was not until the 9th April that the snow on the lower slopes of the mountain, where it is usually too soft to walk over, had sufficiently melted off to allow of an ascent. We made a start on the 9th, following up the valley of the Whakapapa River for some three miles to an open plain at the foot of the mountain, covered with native blue-grass (Patiti) and tussock; here we left our horses, and travelled on on foot for about four and a-half hours, crossing over lava ridges and deep ravines in a southerly direction, to reach a long prominent ridge which ran down in a tolerably regular line from the top of the cone to its base. Finding a convenient camping-ground in a deep ravine, where a small tongue of the stunted bush which covers the lower western slopes of the mountain runs up into the gorge, we camped for the night to await the first opportunity of clear weather for our ascent. The next morning a thick heavy fog hung over the mountain, and came down throughout the day in a drizzling rain, which, however, cleared up towards evening, and there was a hard frost during the night. The morning was beautifully clear, with a cloudless sky. We left our camp while the stars were yet bright, as we had 4,000 feet to ascend, our camp being about 5,000 feet above the level of the sea, and very nearly at the limits of vegetation. For about an hour and a half we followed up the gorge, shut in on both sides by high precipitous rocks and ridges of lava, over which it was difficult to get a passage. At the head of the gorge, however, we found a practicable passage, and reached the back of the spur we had selected for our ascent just as day was dawning.
The scene was indeed a magnificent one, as the first rays of the sun lit up the snowy peaks towering high above us, and gradually shone over the snow-fields and great dark ridges and gorges of the mountain. All around us were examples, most varied and instructive, of volcanic phenomena, and the forms and shapes assumed by the cooling lava. The ridge on which we stood was probably built up by the most recent eruptions from the mountain, formed of alternate sheets of lava and layers of ashes; at its base were immense masses of jagged scoria rocks, piled up in irregular heaps and presenting most grotesque shapes and forms. To the south of the ridge, and running down from under the snow, was a well-defined stream of lava, embracing in its course large blocks of half-molten rocks, around which the lava stream had cooled, giving them the appearance of stones standing in a river current. At the base of this flow was a most beautiful example of the columnar form assumed by cooling lava. A large mass of the cooling metal would seem to have become detached, and rolled
down the mountain side in a separate mass, or boulder, from 70 to 100 feet in height; the outside of it presents a slaggy scoria-like appearance, becoming gradually closer and more compressed towards the centre. One side of this boulder has been broken away, probably from the masses of rock moving down the mountain side coming in contact with it, and thus the construction of the interior is exposed. It presents a most remarkable appearance, a number of long prismatic columns, about 9 inches in diameter, extend outward, radiating from a central point at the bottom to the top and sides in a fan-like fashion, somewhat in the form of a peacock's tail, fitting closely together at the centre, the space between them widening towards the outside; they are intersected by transverse cracks, which divide them into various lengths; some of them can be moved and replaced; though being of various lengths the regularity and symmetry of the portion exposed is very striking and wonderful. About half a mile to the northward of the ridge we were ascending, another lava stream appeared to have cooled running down over the ridge, and to dip down on the lower side of it in the same direction as the slope of the underlying rock, giving to the lava-flow the appearance of a waterfall in the distance, at the foot of which great masses of scoria were piled on top of one another in a confused irregular fashion. The effect of frost upon the rocks became more apparent as we went higher up the mountain; masses of trachytic lava lay in heaps and ridges, broken up into fragments as if struck by sledge-hammers; the travelling was difficult, and sometimes accompanied by danger, over these masses, which would give way beneath the hands and feet, and roll down in large quantities.
We reached the perpetual snow-line in about three hours from our camping ground. There was yet about 2,500 feet to ascend, but the remainder of the ascent was all over the frozen snow, and not very difficult. The ridge was rather narrow in places, whilst on both sides of it steep snow-fields sloped away many hundreds of feet, terminating over the rocky precipices which girt the base of the mountain. Our party were five in number, and we travelled over the snow in “single file,” a long rope fast from one to the other to guard against accident, lest either through a caving-in of the snow or by a false step any of us should slide down over the steep snow-fields. It took five hours from our camping-ground to reach the summit.
The weather was still beautifully clear when we got on top, and the view in all directions around us was truly magnificent. To the westward, the snowy cone of Mount Egmont was very conspicuously prominent, its distance from us being 73 miles. We thought we could distinguish the houses at Waitara with our telescopes; and some of our party suggested that a column of smoke which we saw rising up there came from the
chimney of the Auckland Freezing Company's establishment. The sea was visible beyond the east and west coasts, and all the successive mountain ranges and river valleys in both directions could be traced out with our telescopes. The rugged peaks of the Kaimanawa Mountains, extending for many miles away to the eastward, looked rather insignificant beneath us, although their height varies from 4,000 to 6,000 feet above the sea. The crater of Ngauruhoe, nine miles to the north of us, looked like the dilapidated chimney of some vast furnace down into which we were looking. Taupo Lake, which I have seen described as “a vast inland sea,” as seen from some of these mountains, looked quite small from our great height. The distant peaks of Pirongia, Te Aroha, and other prominent features of the Lower Waikato District looked but a short distance away from us, considering they were over 120 miles off; and as all our party hailed from that direction, each took pleasure in recognizing the familiar landmarks which surrounded his own home, and which he had not seen for many months past. The comparatively low country lying between us and the west coast, though intersected by deep valleys and mountain ridges, seemed rather like a level plain, and, as one of our party remarked, “the mountains only looked like potato ridges.”
The exact form and construction of the top of Ruapehu it would be impossible to describe, the whole mountain-top being covered in a deep mantle of snow. The view presented to the eye is as follows: three prominent peaks, one to the extreme north being exactly a mile distant from where we stood, and not quite so high as the peak we were on; another half a mile to the eastward of us, somewhat higher than ours. Paretetaitonga itself, on which our station is, is a very sharp pointed peak, formed of loose masses of probably trachytic lava, broken into all shapes and sizes by the action of the frost. It has an almost perpendicular inner face, so much so that the snow seldom rests against it, and is soon thawed by the heat of the sun on the rocks during the daytime. Between these three principal peaks lies a snow-field of unknown depth. This snow field is intersected by long crevasses running in all directions through it; they are from 10 to 30 feet in width, and run to great depths; some that we saw, I should think, were several hundred feet deep.
Deep down in a crateral hollow of basin-like shape, its steep sides covered with perpetual snow and ice, is a pool of water of a greyish-cream or drab colour. From the trig-station we overlooked this lake, the peak on which we stood being the south-west portion of the old crater-lips which surround the lake. From its peculiar surroundings of snow and ice, it was difficult to estimate with any degree of accuracy the diameter of the lake, and time would not allow of a proper measurement. It appeared
to me to be nearly of a circular form, and 500 feet or more in diameter. It was quite impossible to descend to the lake, except by the aid of a long rope, and even then the descent would be attended with danger and difficulty. When I got on the top of the peak I noticed little clouds of steam rising from the surface of the water. On watching more closely, the water appeared now and again to assume a rotatory movement, eddies and whirlpools passing through it from the centre to the sides, and steam flashing up from those eddies, leaving little doubt in my mind that the water was in a boiling state. Close to the water the sides of the crater are bare of snow, and appear to be formed of loose particles of rock and volcanic ash; above are steep inclines of snow, sloping in all directions towards the water and terminating in icy masses overhanging the lake. The masses of ice show, in the cracks and crevasses which intersect them, and in their fringes of icicles, the effect which the heat from the lake has on them.
We had not very long been engaged in trigonometrical observations at the trig-station before a heavy cloud rolling up the mountain side enveloped the peak and covered us almost in darkness, so that we could not see one another ten yards distant. Whilst we sat on the mountain top waiting for this to clear away and allow us to complete our observations, a portion of the icy mass surrounding the lake, breaking away from its position, crashed down over the precipice into the lake below, sounding with an awful and solemn effect amongst the stillness of all around. As the dense cloud continued to hang on the peak, and the time had arrived for us to start back for camp, we were obliged to leave our work for another day and commence our descent. I may mention as a curious fact that on top of one of the highest peaks of Rua [ unclear: ] ehu we found, on a ledge of ice, the remains of a rat in a good state of preservation, the skin only being devoid of the fur, and a portion of it still remaining on the chest, across which the fore-legs were folded.
As I mentioned before, the snow-fields which fill up the crateral hollows of the mountain prevent the possibility of judging what the shape of the top is; but from the vertical inward faces of the peaks which can be seen, and the outward appearances lower down the mountain on the western side, it would seem rea [ unclear: ] onable to infer that they surrounded a great central hollow, and that the mountain had been a truncated cone, large portions of the sides of which were blown away by eruptions; and that subsequently, inside the remains of the old cone, two or more craters had broken out and built up new cones. The vent in which the thermal action still continues seems to be the last crater which was active, judging from the appearance of the lava streams down the mountain side. Around the lower slopes of the mountain, and underneath some
of the lava flows, are immense beds of consolidated tuff; on the lower slopes the soil of volcanic loam is forming.
The waters of Wangaehu have a sulphurous taste and smell. Its course can be traced down the eastern side of the mountain from nearly the top of the cone in which is the hot lake; it may therefore be inferred that the water receives its character from this lake, through a subterranean passage in the mountain. Since discovering that the crater contained hot water, I have mentioned the fact to the oldest Natives in the district, and they all concur in the belief that it is something new. I am, however, inclined to doubt this, and to believe that a low volcanic heat must have prevailed there throughout. Five years ago, when engaged in triangulation on the Kaimanawa Ranges, I noticed hanging over Ruapehu, in the position of the crater, what seemed to be a cloud mass. This I remarked more than once, but I did not know of the existence of the lake at that time, and I considered that it must be a cloud or fog rising through some of the gorges of the mountain, although it closely resembled a column of steam. I may also mention that some eighteen years ago, (I am informed), an abnormal flood occurred in the Wangaehu River, carrying down with it large blocks of snow and ice. There had been no heavy rains at the time to account for this flood: it is therefore reasonable to infer that it was caused by an escape of the warm water from the lake, passing down through some underground passage below the edge of the water, and thawing the snow and ice on the mountain side. This, however, appears certain, that before or about the beginning of April, a considerable increase of volcanic heat in the Ruapehu crater took place, which continued to increase until towards the end of May, after which time I had no opportunity to observe it.
On the 16th of April I noticed a well-defined column of steam rising from the crater, several hundred feet above the mountain top; it was also visible several days subsequently. I showed it to several of the Natives, who said they never had known of such a thing before. If it were of common recurrence, and in such volume, I think it impossible that it could have escaped the notice of the Natives.
On the 23rd May, the weather being very clear and bright, a larger column than usual ascended from the crater, about 300 feet above the mountain, spreading out horizontally into a cloud-like mass, the outside portions of which descended again and rolled down the mountain side. Towards noon this column began gradually to decrease, until it disappeared altogether by sunset. Since the end of May the dull weather prevented any further observations of the mountain. Should the volcanic heat so increase as to cause a sudden thaw of the ice and snow which fill up the crateral hollows of the mountain and mantle its sides for several thousand feet, the result must be heavy
floods in the Wangaehu, Waikato, and Wanganui Rivers, probably attended with serious consequences to the town of Wanganui. The great boulders in the Whakapa and Wanganui Rivers, some of them weighing over 50 tons, would seem to have been carried down by such floods in the past. That the atmospheric conditions affect the state of the thermal springs and fumaroles in the Tongariro group appears very evident. I had not sufficient opportunity of noting the state and conditions of the steam-vents, under various atmospheric conditions, to make any definite statement on the subject, but I noticed that the discharge of steam was greater in the early morning with southerly winds and frosts; and the Natives always look for bad weather when the steam hangs low on Ngauruhoe in the morning.