LECTURE AT TILE AUCKLAND INSTITUTE.

New Zealand Herald, Volume XXIII, Issue 7693, 19 July 1886, Page 12

LECTURE AT TILE AUCKLAND INSTITUTE.

On the 12th July a meeting of the members of tho Auckland Institute was held in tho Museum Buildings, Princes-street, to hear papers read by Mr. S. Percy Smith, F.R.G.S., and Mr. J. A. Pond, on " Observations on the Recent Eruption of Tarawera." There was a large attendance, and in tho absence of tho president of the Institute, Dr. Purehas presided. The first part was read by Mr. Percy Smith and the second by Mr. Pond. Tho paper commenced by a description of the volcanic zones : the first from Tongariro to White lslaud ; tho second, the isthmus of Auckland ; tho third, that of tho Bay of Islands. It then proceeded:—"There are many very essential differences in the general character of the results of volcanic action in these three zones, tho firatnamed being that in which any extent of vitality appears to havo remained unto the present day, though tho Bay of Island district has still its group of hot springs ; whilst that of Auckland, so intimately known to all of us, has ceased to show any sign of life at all, though exhibiting to the observer some of the most perfect examples of extinct volcanic action in its several stages known to the world. Of these essential dillerences the most prominent, and those winch alone require notice on the present occasion, are the character of the rock masses and materials winch go to build up the vast accumulation of volcanic remains forming the mountains and ejected matter.. The rocks of tho central or Taupo zone are composed of materials known generally under the name of acidic rocks, whilst those of the other two zones arein their latest manifestation at ail events—entirely formed of basic rocks. Wo may take as a general name descriptive of these two classes of rocks—traehy tic rocks for the acidic areas, basaltic rocks for those of the basic areas—the distinction being in tin n ituro of the constituents and their forms of aggregation. Tho researches of modern science tend to confirm tho idea that there is a regular equence in the order in which these two, classes of rocks are ejccted from volcanoes, the acidic or traehy tic denoting the carlierf the basic or basaltic the later stages of volcanic life. There aro well known exceptions to this general rule, but taken as a whole, the evidence tends to show that such is the life history of most volcanic districts. It may be that some volcanoes commence their career by the ejection of acidic matter, and continue throughout the whole course up to their final extinction, terminating in the ejection of basaltic matter, without material interruption of their activity, whilst others, after making a commencement, are quiescent, or only partially active for ages, remaining in tho acidic stages for such lengthened period.*, that volcanoes, which can be shown to be far younger in actual age, have had their day, and become extinct. Such seems to be the case with the Taup-> or central zone. It is still in the acidic stage, while the younger volcanoes of the isthmus a; par to havo run their full course and b -come extinct." The paper then ive a detailed description of the central volcanic district of tho island, and then follows aomo notes as to PREMONITORY SIGNS. " New Zealand has been colonised so short a time compared with the geologic ages of the past that observation has not yet been continued sufficiently long to record any great changes in the volcanic regions alluded to. It is true that from time to time slight eruptions of Tongariro (or rather Ngauruhoe) have been noted ; earthquakes have occurred on a larger or smaller scale, the hot springs have been occasionally more or less active, floods and landslips, involving loss of life, and due more or less directly to volcanic agency, have occurred, but no great catastropne has been recorded to bring home to us the fact that any great changes are going on. If anything a general opinion has been current to the effect that the forces have been decreasing in activity rather than the contrary, and Maori tradition lends weight to this impression. They have many stories of the greater activity of the hot springs ; indeed, Europeans have seen many fine geysers in play which arc now quiet or extinct, but none of their legends speak of any great calamity having befallen their ancestors through volcanic agency, and we may be sure, that amongst a people who arc so scrupulously careful in handing down their history, any great catastrophe would have certainly been noted. A consideration of some few occurrences in that district during the twelve months, and immediately preceding tho eruption, ought at least to have warned us that some changes were going on, a few of which will be noted." Some note 3 of symptoms of greater activity, seen by Mr. Josiah Martin when visiting the Terraces last summer, were then read, and also by. Mr. Laurence (Jussen, as to the greater activity lately observed at Tongaiiro and Ruapehu. The paper then proceeded : — "It will be remembered that on the evening previous to the eruption an occultation of Mars by the moon occurred at twenty minutes past ten p.m., tho moon being just then entering her second quarter. It would be high water oil the coast near Maketu that evening at about ten p.m. We do not give much importance to these facts, but it is worthy of note that the well-known theory of the tides assumes that the waters of the ocean are, at high water, piled up, as it were, on that particular portion of the earth's surface which is just under the moon, but through friction and the counter j attraction of the sun that tho tidal wave lags ] after the time of passing of the moon over any particular meridian. It is equally a part of this theory that the solid materials of the earth are at the same moment subject to a wave, much more limited in extent, but still appreciable ; and it is well known that an atmospheric wavo passes round the earth at two o'clock each day. Hence, the crust of the earth being in a state of tension, if there is any predisposing cause tending to a fracture about the period of this earth wave, it is a natural inference that the conditions are then most favourable for the production of such a fracture. The attraction of the planet Mars added to that of the moon may be, and no doubt is, very slight ; but the fact remains, that whatever influence the moon may exert at any particular moment, it happened to be greater by the sum of her own and the planets very shortly before the eruption" The barometrical records were then given, and the phenomena observed at the outburst were noted in considerable detail. SYMPATHETIC ACTION. The paper then discussed the sympathetic action at other points as follows:—"It has been stated that the eruption is quite local in its action, and goes to prove that the aeries of hot springs in different places, and other signs of volcanic action in the central zone, are separated and have no connection or sympathy with one or another. A consideration of tho following facts relating to events which occurred at the time of eruption or soon after, go to prove that such a conclusion has been drawn from insufficient data. Allusion has already been made to the activity of the crater on Ruapehu. News has reached us within the last few days which confirms a report brought to Rotorua by Te Heuheu, the chief of Tokaanu, at the south end of Taupo Lake, that Tongariro, or more properly Ngaruruhoe, was much disturbed on the night of the eruption. A road party under the charge of Mr, A. B. Wright was

encamped on the Poutu river, just a few mile 3 north of Tongariro, they were awakened by heaving a great rumbling, and a large number of sharp detonations like the reports of cannon proceeding from Tongariro, vivid flashes of lightning played around the summit all night and early tho following morning, and every minute an eruption was expected. Strange to say, however, no earthquake was felt. Tho springs at Tokaanu were not apparently affected. Next we call attention to the greatly increased activity of the springs at Kotorua. A small steam fumarole, which in its ordinary state was only occasionally visible, near the Government agent's house, becamo a largo boiling spring, about 10 feet in diameter, from which a good-sized stream of hot water ran away towards tha lake. Further north, at tho base of the Pukeroa hill, and in the direction of the Maori village of Ohinemutu, steam came forth from innumerable cracks in the earth, sometimes accompanied by hot water, which formed streams, ruuning alongside the road from the old to the new township, and in the pa itself a spring burst out in the great meetinghouse of Tamato-kapua; another in the path leading down to it, and yet another juat behind the building. All of these outbursts occurred on the night of the eruption ; they all follow, however, tho old deposits of sinter at tho base of the Pukeroa hill, the last remaining signs of former great activity in that locality. The activity of the vast number of fumaroles and springs in and around Ohinemutu was certainly greater than usual a few days after the lObh. I ho level of Rotorua lake oscillated somewhat on the 10th June, but to no great extent. At seven a.m. it fell one inch ; at nine a.m. it rose six inches, and fell again at noon three inches, and remained so all day, falling on tho night of the 10th five inches, since when tho oscillation has been continuous, but to no very great extent. 'liie temperature of Rachel's epring at the Sanatorium on the 11th Juno was lTOdeg., and from that date to Ist July it gradually rose t<> IDUJeg., with a greater flow than before. For these exact data wo are indebted to Mr. Boscawen, who obtained them from Mr. Hall, the observer. In tho far north of tho central zone at White Island it was reported by the s.s. Jane Douglas that the crater was showing unusual iiignu of activity at nine p.m. on the 13th, whilst the Hinemoa reported it to bo in its usual state on the 14th. To Puke settlers saw a ' violent eruption of steam on tho morning of tho 10.h;' Te Anau reported that nothing but an unusual amount of steam was rising on the 13th. On tha Ht,h vast columns of steam were reported as being seen all day from Tauranga, and the same on the 15th. At Wairake/, near Taupo, the springs and geysers are reported to be ' in an extraordinary state of activity ' on the 10th. We may add that we saw much more steam than usual arising from the large group of sorings south of Maungakakarainea on the 14 th ; but these being in the direct line of the great fissure, it is only natural to expect this." A detailed description of the points eruption was then given, and also of the appearance of the road to tho Wairoa, TIIB EJECTED MATTER. The paper then described tho material composing the ejected matter as follows : —" We have first the dry ash laid in the vicinity of Rotomahana, and extending in a gradually reducing thickness 10 Galatea. Then the mud precipitate I over the Wairoa, Rotoiti, Okareka, and Okatuina. The dry ash carried in tho shape of tine powder over Tauranga, and an course sand at Whakatano and otiki. Then we have a secondary coating of" mud overlying tho dry ash in tho immediate vicinity o. the geysers at Rotomuhaua, and the varying degrees of fineness of the ash deposited at long distances, notably at Whakatano, where a coarse sand fell for tho first few hours, followed by a very line dust for some hours afterwards. Tho same circumstance, but in a less conspicuous degree, was noted at Tauranga. In the order aa arranged we find the mud to be chiefly composed of quartz in tho form of fragmentary rock crystal, and as sinter, both white and coloured pink by peroxide of iron, together with a large amount of volcanic scoria in fine fragments, and exceedingly vesicular. This fragmentary scoria we shall find to be in very different proportion as we proceed, and the greatest interest will be felt in this fact, together with its bearing on tho future fertility of the soil on which it lias fallen, or will itself have replaced. Wo have not, however, found pumice to any large extent. In some of the older fragmentary rocks isolated patches were attacned, but the tine deposits are singularly free from it. In addition to these varieties of ash, we have also the solid portior.3 of stone which have fallen, not merely in the vicinity, but also at long distances from the scenes of eruption, Trio materials thrown out vary considerably. In tho immediate neighbourhood of the geysers are tobe found stones from a few ounces to over a ton in weight. These vary considerably in formation, but are all portions of the rhyolitic rocks adjacent — a fine-grained tuff and coarse-grained brccciated trachytic rocks being plentiful. In the Wairoa, however, we find both scoria and the coarsegrained trachyte just alluded to, while on the eastern end the principal solid material is composed of a basic scoria, in the form of lapilli. Returning now to the examination of the mud and ash, wo find that the deposits at Okaro, Wairoa, Tikitapu, and Tauranga are very similar in appearance, being composed very largely of silica, both in tne glassy solid crystalline form and as suiter, together with a small but varying proportion of scoria. Coming nexc to the deposit at Matata and Whakatane we find the silica in the same forms, but the scoria has increased considerably iu proportion. Advancing still further eastward to Upotiki, wo find the same characteristics, but the scoria has still further increased iu its proportion to the uncombined silica. Mow, if wo turn to the analyses wo have made of tho materials obtained from the places mentioned, wo find that they bear out the results of our optical examination. Clearly the a3h from Okaro, Wairoa, and Tauranga are of the acidic group, while those trom Whakataue and bpotiki are more nearly approaching the basic form. Again, the scoria obtained from Wairoa, aud also from the southern end of the eruption, are undoubtedly basic, and have been thrown oat in exceedingly large quantities, viewed from the amount and composition of the eastern deposits. Now, hitherto wo have had the whole of the rocks of this region placed in the acidic group, and certainly no large mountain masses of a basaltic character could well ese:ipa tho practised eyes of Von Hoehstetter or the numbers of the Geological stall of our colony. We are therefore forced to the conclusion that large quantities of basaltic scoria were ejected from the Tarawera volcano or mountain at the earlier stages of the eruption on the morning of tne 10th of June. Tins is fully borne out by the numerous eyewitnesses, who unanimously speak of columns of fire rushing up irom tue newlyformed crater, aud masses of lire bursting and falling back and around the sides of the mountain. That there was no outflow of molten lava actually discernible after tho night in question is accountable by the enormous rush of high pressure steam carrying off the molten mass in a fine state into the air, where it was carried away by the strong S. W. wind, which had now commenced to blow, or by being covered up by the subsequent deposit of ashes. We see from the foregoing that we have had two distinct eruptions, the one hydrothermal, the other volcanic, throwing out differently rocks, acidic and basic, tho physical characters of these rocks being as different as their chemical composition. Thrown to a great height, they were caught by the wind storm, and borne along by it in parallel lines, from whence they emanated, the acidic to tho westward, and the basic to the eastward, and but slightly commingled. In this order they advanced, and in tins order were precipitated on the lands over which they passed. Coarse sand, finer particles, oust—thus it was laid, in tho order most to be desired by the agriculturist. So fine, indeed, is a large portion of the deposit, that the elements of nutrition in it are available for vegetation almost aa soon us tho first rains have carried it into the soil, while the particles not so exceedingly fine are already being attacked by that wonderful disintegrator, carbonic acid. For a moment let us glance at the basaltic lava cones in the vicinity of Auckland ; and here we fiud the richest land, capable of growing extensive crops. The more decomposed, tne finer the particles, the greater the amount of disintegration ; tho richer the ground, tho greater the profusion of the elements of fertility. Aud this is the material which haa been so lavishly spread over the land on the eastern portion of tho district, and which is so largely intermixed with the acidic matter which has fallen over the western. Thai this rock in its unbroken, undecomposed form is nearly valueless for plant life, we can learn by turning to the basaltic floes aud

cinder deposits of Rangitoto; but even there, in the few gullies where rain has washed the dust and given depth of friable soil for plants to live in, where will we see a richer profusion of vegetation. The result of this downpour over so large an area need not dismay us, but rather give cause for rejoicing that in the majority of instances a richer soil has been added than formerly existed ; and so lightly and finely has it fallen that the winter rains will not have passed before it will have been washed into the Boil to invigorate the new vegetation and improve the pastures, except in close proximity to the scene of the eruption. Even here we have shown that these deposits aro capable of supporting vegetation." The next subject discussed was PROBABLE CAUSE OF THE ERUPTION. "To hazard a theory for so stupendous a cataclysm ■ without first obtaining the most complete data on which to build, would appear reckless and unscientific, but the amount of data already accumulated and the certainty that many months must elapse before a complete investigation of Tarawera and Rotomahana can bo made, prompts us to advance a theory based on known laws, the working of which has been a source of Wonder and attraction and of world-wide interest centering in Rotomahana. Hero, a3 we aro well aware, rose the beautiful terraces of Te Tarata and Otukapuaraugi ; here, also, were geysors, ngawhaa, mud springs, steam holes, solfataras, and fumaroles, each and all pouring out, iu larger or smaller quantities, it 3 volume of heated water until the lake itself was fully deserving of its name ' Kotomanana' (warm lake), and its effluent Kuivvaka vva3 worthy of a similar distinction. Now, the body of water debouching from this lake was large and continuous, anil many millions of gallons were daily discharged into the Tarawera lake. It, now, we turn to Rotomahana and witness the effects of these hot springs and geysers, we find an amount of sinter deposited which is surprising, for though we have been used to speak of the two Terraces, there were several others in a state of decadence or fragmentary condition, while lavishly around us were the evidences of sinter deposit. Year after year, probably for centuries, had this deposition gone on, though only a tithe of the silica which rose in solution had been arrested. Fortunately these waters have been analysed, tho results of Mr. Skey's examination, showing the water from the White Terrace to be charged with mineral matter to the extent of 111 grains t'> the gallon; and from the Fink Terrace, 15-1 grains. Accepting this as equal for all the springs so constantly at work, wo shall have 111 the course of years a very large amountof rock material withdrawn from the earth, most probably leaving cavernous spaces and a weakening of the earth crusts locally. If then we bear in mind the proximity of Mount Tarawera towering to a height of over 3GUO feet, we have a source of danger that must make itself felt sooner or later. At length we may presume that the weakened honeycombed rocks were unable to resist tho enormous pressure of the mountain, a portion of the base of which fell in at a place nearest to the source that had sapped it. Possibly this effect may have been precipitated by some slow moving earth wave, evidences of which we have already adduced. We liavo now ample cause for the immense rift on tho S. W. side of Tarawera, as the effect of the waters from Rotomahana rushing into the chasm and coming into contact with a large suiface of molten rock j would be followed by a terrible convulsion, the escaping steam ripping up the side of the mountain in the manner already described. The sudden bursting of the overlying rocks would be also followed by the ejectment of volcanic matter, which had been molten but dormant at some distance beneath its base. Again and again would the water rush down on the heated roaks, only to be driven back and dissipated into the surrounding space, together with the frag-, mentary matter and dust resulting from the shock. I'he water from the lake would thus bo driven up, together with the steam and debritic mass, to fall over long distances in tho form of mud, as we now see it, until the water had been expelled from the lake, and with it all the solid material resting on its bed. By this action the bed of the lake has been lowered, and its sides greatly exteuded, while there can be but little doubt that the whole of the Terrace formation has been swept away. That this long-dormant molten mass of lava underlying the mountain extended no further is very questionable, and the evidences of further extension in a southwest direction are shown by the length of the rift extending to the Rotomahana tneuce by its entire length, and finally proceeding in the direction of the Okaro lake for about a mile. Here we find its effects very violent, the live active craters already described not being buiit up, but blown directly out of the otitic rock. In reviewing the weakening of the rocks by soluable matter withdrawn in the waters, it is difficult to arrive at anything like accuracy in regard to the amount of water discharged. If, however, wo tako tho tlllieut, the Kaiwaka stream, as of a mean depth of 2 feet, and a width of 25 feet, flowing at the rate of three miles per hour, we have 19 million cubic feet, of 80 million gallons in 24 hours. If we now estimate the solid contents of one million gallons of water, containing 154 grains per gallon, we obtain nearly 10 tons as tho result, and this, multiplied by SO million callous, will give a total of SOO tons of solid water. If, however, wo only accept one half this amount of water as being discharged from tho springs, the final result gives -400 tons of rock material withdrawn in each day. That the assumption of half the water being discharged from the springs is below the mark, the heat of tho lake and its diluent gave fair proof, as well as tho small amount of water which entered by its tributaries." The paper concluded with the following remarks on tho PROBABLE EFFECTS OF THS ERUPTION. " From the results which wo have been considering, if viewed in connection with the theory advanced as to the cause of the eruption, we can venture to surmise the action which will take place in the future in this and tho surrounding districts. As we have already shown the eastern part of the Rotomohana, valhy has been overlaid with a deep deposit of stones, s:md, and mud, until the effluent of the lake has been entirely filled up and considerable mound 3 of the deposit stand above this plain, beneath which the river ran into the Ariki. At the same time almost all tho water, previously in the lake, has been dissipated. At present the water lying in the Okaro lake is not being added to, owing to the immense amount of dry ash in its vicinity which absorbs the rain as it falls, and the same remark applies to the entire basins of the Rotomahana. Until the point of the saturation of this material has been arrived at the present state of geyser action will be very slightly altered, but, after this period the gradual inflow of water into the basin will be followed by increased activity of geyser action until the maximum will bo reached by the waters coveyng tho present steam outlets, owing to the barrier at tho outlet of the lake. We may anticipate now that large masses of water coming into contact with extensive surfaces of highly-heated rocks will cause a succession of intermittent geysers grander and more extensive than anything of the kind known in this part of the world. It is probable that this will continue for a long time, unless tho barrier at Te Ariki is broken down by the pent-up water, when tho present state will be resumed. Under any circumstances, we feel confident that with the gradual rise of waters over the floor of the lake, the sealing of tho present outlets which will take place, will give rise to greatly increased activity in the immediate vicinity. At the same time there is every reason to anticipate that the effects will be extremely local, beyond earth tremors and small shocks, which may last for a long period and extendi beyond the district. Already we have stated our conviction that Tarawera and the thermal springs in its neighbourhood are associated with and in actual connection with those of Kakaramea, l'aeroa, and probably with Ohinemutu. Assuming this to be tho case, that from the same mass of molten matter all these springs emanate, the late eruption will have so relieved any pressure in other directions that there is hardly any danger for a very long period of serious manifestations of a like character in any of these localities, in fact the reverse may bo anticipated—the gradual decadence of the hot springs of tho surrounding districts and their concentration at Tarawera." After the reading of the paper, the Chairman drew attention to a number of maps showing the district covered by the volcanic ejeeta, the craters, &c., and also a series of photographic views of the eruption taken from many points of view, which were laid upon the table. There were also a dumber ot specimens of the volcanic deposit, samples of tnc dust and rock being also exhibited under the microscope. Much interest was displayed by those present in the inspection

of the specimens shown, which included grasses growing in mud deposited by the eruption, exhibited by Mr. Pond. Tne Chairman' said they would all give hearty thanks to the gentlemen who had prepared the papers, though it was not their custom to accord formal votes of thanks. The facts had been ascertained almost at the risk of their lives, auci were most interesting to those who had heard the papers read, and would be to those who, he did not doubt, would havo the opportunity of perusing them by-and-by. (Applause.) The reading of the paper occupied so long a time that any discussion had to be ad« journed till next meeting.