[Delivered before the Wellington Philosophical Society, 30th June, 1886.]
After thanking the members for re-electing him to office, Dr. Hector said that the Society had been very successful during the last year, and the papers read before the Society formed by no means an inconsiderable portion of the contents of the volume. Several very important events which had taken place during the past year had received notice on the part of the members of the Society. One of these was the eclipse of the sun, on the 9th September, 1885, a phenomenon only rarely witnessed from any one particular spot of the earth's surface. A very full account of that eclipse was embodied in the “Transactions.” The outcome of all the observations tended to show that the sun had only a moderate degree of activity at the time of the eclipse, that the scarlet prominences were only moderately developed, and that they were clustered and combined in a very irregular outline. No laminated structures—that is to say, no structures parallel with the sun's surface—were observed; but there were several other minor phenomena. He stated that he had been very fortunate in securing a number of photographic negatives of this eclipse, all of which he had sent Home and lodged with the Royal Society, where they will be preserved and compared with views obtained on future occasions. Another interesting phenomenon was the late occultation of Jupiter and its satellites, under very favourable circumstances for determining whether anything of the nature of an atmosphere surrounds the moon.
An event of considerable importance was the visit of the United States exploring ship “Enterprise,” under Commander Barker. When she left New Zealand she was to make for the coast of South America, and Captain Barker was good enough to consult the speaker as to whether there was any particular course that could be of more use than others for the purpose of taking soundings. He informed the meeting that he had lately
received from Captain Barker the results of the soundings taken, and was able to lay before them a sketch-map showing the form of the Pacific, tinted according to the soundings he had obtained. The first result was that the sea area between New Zealand and the Chatham Islands had a depth of about 1,300 fathoms. The water gradually shoaled, until when close to the Chatham Islands the depth is found to be 160 fathoms, or about the same as in Cook Straits. Immediately beyond the Chathams there was a sudden plunge, just the same as to the west of New Zealand. Deep soundings of 3,000 fathoms were carried to long. 118° W., when the water suddenly shoaled and a great bank was found, on which the depth was determined by a number of soundings shown in the return sent by Captain Barker. Another plunge reached 3,000 fathoms; then the depth shoals to about 1,500 fathoms, at 500 or 600 miles from South America. Near the coast 1,200 fathoms was found, which appears to be the ruling depth of the South Pacific, subject to these great depressions. From New Zealand to the Chatham Islands the bottom is found to be level. From the Chatham Islands, Captain Barker first met with a grey mud, passing into brown mud with minute white shell-sand. Yellowish sand was found in the South Pacific “pot-hole,” after which a brown mud, followed again by grey mud with shells. This information confirmed the views of Professor Hutton, in opposition to the views of Wallace, that in the South Pacific there is a submarine plateau, indicating the former existence of a great continental land connection between South America and Australia.
Dr. Hector next referred to a report which he had made to the Hon. the Minister of Mines, dated the 23rd inst., for full particulars of the observations he had been able to make regarding the recent eruption of Mount Tarawera. He pointed out that the eruption of Ngauruhoe in 1870 was really much more important than the late outburst, though less advertised by sad incidents, for then great lava and steam eruptions continued for a considerable period. Referring to a large geological map, he gave a general sketch of the geology of the district. Omitting the Post-pliocene and newer coastal formations, the whole country, from the sea-level in the neighbourhood of Wanganui, was originally covered by a crust of limestone of Older Pliocene age, that rose up to 4,000 feet on the slopes of the Kaimanawa Range. Under this there was the middle tertiary, or blue papa (or marlstone); but the whole series did not exceed 2,000 or 3,000 feet in thickness, and rested on much older rocks wherever its base had been observed. The crust of limestone presents the same character throughout, from the sea-level to the greatest altitudes. It did not now constitute a continuous sheet, but occurs only in isolated masses that have remained perched on the hill tops. As it is largely composed of shells of huge
oysters of the same species throughout, this limestone must have been deposited in the same depth of water marking the period of the close of the deposit of the great blue papa formation. Since that period this limestone has been inclined by the gradual dome-like upheaval of the central area of the North Island of New Zealand. During this period of upheaval there was no trace whatever of the contemporaneous existence of any of the volcanic rocks that played such an important part in the later history of the district. Thus, it was not until we got on the top of the limestone that we found, near Ruapehu, on its south side, outliers of conglomerate and gravel, showing water-carried material derived from these volcanic rocks; but in the opposite direction, towards the Bay of Plenty, and towards the Thames and Waikato Valleys, or any part of the northern half of the dome, we nowhere find any trace of the marine tertiary rocks. If present, they had been completely smothered by subsequent volcanic deposits. In explanation of this, it may be suggested that all the ejected volcanic matter has, in past times, by a prevailing southerly direction of the wind, been carried to the north, and so smothered the country as to completely obscure the tertiary rocks. Be that as it might, what was found was that the southern flank of this dome was composed of marine tertiary rocks, while the northern is a sloping plateau, superficially composed of volcanic detritus.
Dr. v. Hochstetter, who first examined and gave an account of this district, long ago pointed out that all over this sloping plateau great valleys have been eroded and then filled up again by the products of eruptions, cones have been built up by volcanic matter, and great flows of lava have taken place of an extremely siliceous type, so siliceous that they are barely fusible, along with others which set in a glossy mass called obsidian, or in a vesicular form as pumice stone, which is nothing but glassy lava blown out by steam.
Now, wherever this kind of lava has been accumulated so as to form great volcanic cones, of which you find many instances at Tauhara, Tarawera, Mount Edgecumbe, Ruapehu itself, there has been sooner or later formed a corresponding depression, simply, as v. Hochstetter pointed out long ago, by the local subsidence of the surface over the vacuity from which some part of the ejected matter had been abstracted; and for long after a mild generation of steam was kept up round the basins enclosing the lakes by the expiring energies of the former great volcanic activity. That applies to every part of the country except Ngauruhoe, where there still remains part of the primitive form of volcanic activity, as evidenced in the ejection of actual masses of lava, which forms on cooling into stony rock. Such was the lava exuded in 1870. With that exception, all volcanic action in this district has always
been considered to belong to the solfatara type, as distinguished from the more active form of volcano. The formation of the terraces at the Hot Lakes was due to the action of steam, derived from water heated at a great depth from the earth's surface, forcing its way through the siliceous rocks at a high pressure and temperature, and carrying with it an extract, as it were, of everything soluble in water of very high temperature and pressure. These matters were immediately deposited at the surface, on the water escaping as steam, slowly, and film after film. Extensive deposits of almost pure silica formed in this manner constituted the magnificent terraces at Rotomahana. Now that action, although not always so beautifully exemplified as at Rotomahana, is going on at other points, such as Whakarewarewa, Rotorua, Orakeikorako, Karapiti, Wairakei, Tokano; and no one ever expected that action would suddenly become more violent. It was now the question: were we dealing with any new force, or was this merely a gigantic display of the same force which up to the present time had been exercised in a moderate manner, and, he believed, due to the influence of lavas still retaining their original heat, upon deep-seated strata saturated with underground waters? He was inclined to the latter view, or, in other words, that the phenomenon was due to a sudden accession of hydro-thermal activity, and not a renewed volcanic force.
Tarawera Mountain was a most conspicuous land-mark all over this country, standing up boldly above the other hills in the district, and very similar in appearance to Horohoro, which was the southern termination of the plateau-topped ridge extending from Cape Colville to this point. It was composed of a rock called trachyte breccia, a volcanic rock consisting of mudstone, cementing huge masses of trachyte and porphyry rock, and there was but little doubt that it was of submarine formation, and altogether antecedent to the superficial volcanic rock. v. Hochstetter considered that the Tarawera Mountain also belonged to this older formation. This is not quite correct, because its composition is of different origin. Seen from Tarawera Lake it showed great precipices of columns like those at the Giant's Causeway in Ireland, but composed of obsidian, or unannealed natural glass; and in every respect Tarawera resembles in structure the island called Tuhua, or Mayor's Island, in the Bay of Plenty. Now, it would have been a very different thing if Tarawera had belonged to the older formation, which must have been quiescent for an incalculably greater period than the recent. If we considered it a mountain of the recent formations, it would be much easier to understand how the energy may have given one last dying kick; and that, in the opinion of the speaker, when the matter is fully investigated, would prove to be the source of the whole disaster.
Tarawera Mountain, he continued, stood a little way back from Rotomahana, and consists of three tops—Wahanga, separated from the others by a deep chasm, and two others, Ruawahia and Tarawera proper; but he could not say how the latter came to have separate names. It was at the south foot of Tarawera that Rotomahana was situated, and around this lake a continual outpouring of boiling water was going on, throwing up huge geysers; and enormous deposits had been accumulating round the lake of siliceous matter, which completely sealed any escape other than the immediate geyser vents. He had already stated to the Society many years ago that all these terraces seemed to have the power of building up to about 70 feet; and he recommended it to engineers to think that question out, and see whether any relation can be established between the temperature and an hydraulic head of 70 feet. This was very obvious at Whakarewarewa. The result remains that this action ended in cementing over the surface with 70 feet of hard and heavy siliceous rock. The continual outpouring of this matter from beneath, and the continual action of hot water on the rock, must have absorbed a great deal of heat. That heat was really derived from a remnant of the uncooled lava in the core of the Tarawera Range.
On the 9th of this month (June) we had very stormy weather, and on that day there set in all over this country a complete change of weather. For nine months previous, he had been informed, they had hardly had a shower of rain in the Tarawera District; but on the 9th down came an enormous flood of rain, and a change set in to very wet and cold weather. Another circumstance deserves attention. This year we had had a most unusual arrangement of atmospheric pressure in the Southern Hemisphere. When passing the Equator, on the whole, barometric pressure gradually declines towards the poles. It declines much more rapidly towards the South than towards the North Pole, so that while the barometer averages over 30 at the Equator, in New Zealand it is about 29.8, and a very little way south the usual reading is only 29.2 or 29.3. But for some reason an area of high pressure is generally situated to the N.E., which this year has passed down much nearer to the South Pole. When at the Antipodes Islands, in March, Captain Fairchild found the unprecedented high reading of 30.8, the barometer being in perfectly good order. In consequence of that, there had been a great deal of continued easterly weather in New Zealand. In any case, there were exceptional conditions of weather and atmospheric pressure; and, in connection with this, he mentioned that a difference of an inch in the barometer meant a difference of pressure of nearly one million tons to the square mile.
The earliest trace that he had been able to discover of any
symptoms was reported by Mr. Godfrey, who resides at Tawaite, a whaling-station at the entrance to Tory Channel. At that place the whalers were, as early as 6 p.m. on the evening of the 9th, disturbed by booming noises coming through the earth. They were accustomed to hear the booming of the sea outside Tory Channel, but they were clear that these noises were not of that description. According to their account, these noises did not continue through the night. They afterwards heard the noises like the firing of guns, such as were heard at Nelson, Kaikoura, and even Christchurch and Auckland, at a distance of not less than 200 or 300 miles from the focus at the time, which agrees with what he would have to mention as the first stage of the eruption. A very distinct statement of the event was given to the speaker on the 13th by the Native Interpreter, Mr. Edwards, who resides on Pukeroa Hill, at Ohinemutu, and was fully confirmed by the account given on the same date by Mr. Roche, the Railway Engineer, who was encamped at the edge of the bush above Rotorua, and by the account of Mr. Macdonald, given on the 16th, who witnessed the whole eruption from the Kaingaroa Plains.
At 10 minutes past 2 a.m. the eruption began by the blowing off of the cap of Wahanga. The top seemed to go up as a great mass in fragments, and must have been illuminated, otherwise the spectators could not have seen what they did see. Then followed an up-throw of stones, accompanied by noises, and, about five minutes later, the top blew off Ruawahia, immediately followed by an out-throw of a vast column of steam, charged with stones and dust. Then came an outburst, obliquely, from the south end of Tarawera Mountain proper, right over Rotomahana. Noises and eruptions continued steadily for about two hours, when a most terrific earthquake was experienced. Some pretty severe ones had been previously felt at Wairoa, but, even at as short a distance as Rotorua, no damage was done by earthquakes. But at 4 a.m., or shortly before, there was a vastly heavier shock, the sensation of which was described as just as if you had been running fast and come against a fence. At the same moment a terrific sound rent the air, but it was not of the same clear report-like character as those accompanying the first eruptions, and suddenly an immense cloud, composed of steam and dust, was thrown up, from which lightning issued in all directions. This gigantic cloud sprang up, and then was seized by a kind of gigantic pantings or throbbings, each one accompanied by a fresh access to the volume of steam. The steam formed a flat-topped cloud, which drove right in the direction of Rotorua; and the people of that place who saw it advancing on them, and throwing out lightnings that seemed to touch the water's edge, thought their destruction inevitable. Just at that time a violent hurricane from the S.W. caught the
cloud on its edge, and seemed to make it rear up on end, and arrested its progress. That time agrees with the fall of mud at Wairoa; and no doubt the condensation of the steam, and consequent collapse of the edge, led to the deposit of mud in a moist condition, but as far as known quite cold, upon the unfortunate inhabitants of Wairoa and the surrounding country. After this deposit of mud, a cloud of higher stratum appeared to reach all over the country. The heavier and more damaging kind of dust was driven towards the N.E., the edge being condensed so as to throw down the dust in the form of mud, from Wairoa to Te Puke, in a narrow strip. Away to the east the country was covered in dust, causing darkness from Gisborne to Tauranga until 11 a.m. Above that there was a higher cloud, unaffected by the wind, that appears to have spread out to a distance of 120 miles, depositing a light thin grey dust known as the Tauranga dust. Other kinds of dust were deposited at Opotiki, Hicks Bay, and the East Cape, the latter containing organic matter; and a grey dust also fell inland from Poverty Bay. A very distinct form of the ejected material is the black vesicular mud-stones resembling scoria, thrown out by Mount Hazard, and the great sandy deposit that completely covered the country in the neighbourhood of the focus of eruption.
He then described how, on Sunday, the third day after the eruption, he got to Rotorua, and, immediately on emerging from the bush from Tauranga, came in full view of the eruptions. He was able to count seven distinct points of eruption, while every now and then from three more marked points great outbursts of a reddish-brown character took place. During two clear nights he carefully watched these eruptions from Rotorua with a powerful glass, and never saw any evidence of a reflected glare, or sign of cracks or fissures in the mountain through which molten lava could be seen. The detritus had almost completely smothered the outline of the range, and had nearly filled up the valley between Ruawahia and Wahanga. He obtained a subsequent observation of the range from the opposite side from the Kaingaroa Plains on the seventh day after the eruption. On the top of Ruawahia the cone had greatly increased, with the addition of an outer ring. The most curious thing was that right along the back of the mountain could be seen with the glass a large crack or fissure, running obliquely to a great height on the southern end of the mountain. All along this line little wreaths of steam were escaping. In front of that crack could be seen an enormous terrace of a clear white colour, all the rest being of a mouse-grey colour, except at the extreme top which was brown. No doubt the terrace consisted of pumice sand, that was thrown up and fell perfectly dry and hot.
Another view of Tarawera was obtained from Te Hape-o-Toroa, a hill close to Rotomahana; and here a fissure was seen, as in the above woodcut, to the south end of Tarawera Mountain, ruuning in a S.W. direction. The eastern side of the fissure was tolerably straight, but the view was much obscured by steam. It has the appearance as if part of the mountain, 2,000 feet by 500 by 200 feet, had been blown out. There is quite sound ground between the south end of the fissure and Okaro Lake. The direction of the fissure passes to the west of that lake; and Mr. Park, who examined that part of the field most closely, estimated that not more than three chains of ground separates the original edge of the lake from the point to which the fissure has reached. The fissure is not of the nature of a fault by a downthrow, but is really a row of pit-like craters, having two sides pretty much on a level, the material that occupied the intervening space having been simply blown out. The whole country in the vicinity is covered with the dazzling white sand, which creaks like starch under the foot. It was still quite hot on the fourth or fifth day, and where it has covered old forest trees they were smoking and burning. The valleys were all partially filled up, and the hill-tops covered, as if with terrific snow-drifts. This white sand must have been matter thrown out of the rent that intersected Rotomahana.
At one point the fissure was building a cone of stones thrown out by a volcano. Several craters were throwing stones 800 to 1,000 feet high. One crater, Mount Hazard, was double-
barrelled, having two orifices which alternately spouted out showers of hot water and dark-coloured stones to a height of 700 to 800 feet. These showers sometimes were oblique, and, clashing together, the stones fell on the outside slope of the crater, so that they are building up a miniature cone. This fissure, with its vents, will become an important feature in the district. Of course we have lost the lake and the terraces; and all that was gained was this hideous fissure and the active cones on Tarawera, which, he believed, will soon become dormant, and probably the only marked new feature resulting from the outbreak will be Mount Hazard, and the fissure, that will fill with water and become a lake.
He again repeated that, so far as he was able to see, up to the time when he left, there was no development of lava; and, therefore, if that were the essential feature of a volcanic eruption, there had been no proper eruption, merely a much more gigantic development than usual of great hydro-thermal forces, the conversion of heat and water into steam, and the dispersal, by its agency, through the atmosphere of an immense volume of rock fragments derived from superficial strata.
The study of this wonderful phenomenon fully explained how the rock terraces of the Waikato, which extend into its lateral valleys, have been smothered by pumice and re-excavated. This was formerly difficult to understand; but the whole mystery disappears in the light thrown on the subject by this eruption. The valleys were excavated by running water; but, instead of being filled and protected by great shingle flats, as in the Southern Alps, there had been in former times sudden eruptions of pumice sand, which had filled the valleys, and then the water had, with extraordinary rapidity, re-excavated the terraces down to the original bed-rock. The same applied to the valleys towards the East Coast; so that the cutting of the pumice terraces had nothing to do with the original cutting-out of the rock terraces themselves.
The conclusion to be arrived at was that this eruption was on a very gigantic scale, but was yet a very simple one as far as we know. He had a clear view from every point accessible. The party passed so close to White Island that we were able to see it quite active; in a like manner, Rotorua and other hot springs all showed extra activity, but there is nothing more in this than is usual after a great change in the weather or in barometric pressure. At Tokano there was no change whatever. It is mentioned as if new that there is a lake on Ruapehu, but if they looked at the model of that mountain in the Museum that was made years ago they would observe the very lake. This lake was first described by Messrs. Maxwell and Beetham, and steam is often seen rising from it, as if from a warm pool in cold weather. The fact of its being a lake surrounded by permanent
snow and ice-cliffs, proves that it must be affected by some local hot spring; but Mr. Park, who observed it in January last from the top of Ruapehu, reports that it was then frozen over.
On the 15th of June Ngauruhoe showed no unusual activity, but appeared to be giving out rather less steam than usual. As for an underground connection between the different sources of volcanic energy, the eruption completely disproves it: because had there been any such connection with the outburst of steam from Rotomahana, what would have been the effect on Whakarewarewa? It would have drained the springs there. As it was, there were no signs of any effect on springs a short distance off. On the whole, he saw no ground for anticipating that we should have any renewal of the volcanic energy. It must have been a very long time since the last outbreak, for it is very plain that the terraces would have been destroyed by it. Now, it has been shown that such terraces in America, but of very much smaller dimensions, must have taken at least 1,000 years to form; and it may have taken perhaps ten times as long for the White Terrace. What the exact circumstance may be that has broken in on this period of rest, has still to be suggested.