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The Pamphlet Collection of Sir Robert Stout: Volume 67


page 69


In concluding this report a few remarks on the general character of the eruption may prove of interest. It has been remarked by Judd that nearly every known volcano has originated in the formation of a fissure, in the earth's crust, from which matter has been ejected in various forms, and for a longer or shorter period of time. The matter so ejected, if in the form of the acidic and more viscid lavas, has usually welled up from below and spread out from the vent in streams of great size, but extending to no great distance, and without forming a cone such as is usually associated with the term "volcanic mountain." These lavas tend rather to the formation of rounded dome-like masses, of which central France offers several examples. If, on the other hand, the ejected matter is more fragmentary, a true cone or series of cones is usually the result. It has also been observed in so great a number of eases as almost to have become a recognized law, that there is a regular sequence in the order in which lavas of different compositions have been ejected from vents. This generalization is due to the researches of Richthofen; and the order, in point of time and composition, which he laid down is as follows: (1) Propylites, (2) andesites, (3) trachytes, (4) rhyolites, (5) basalts.

Now, it is a matter of some interest to ascertain, if possible, to which of the above classes the recent eruption belongs, for on its determination (if the law holds good in this case) depends the question of the future history of Tarawera. It is evident that the solution of the problem depends primarily on the chemical analyses of the ejected matter; but these analyses have not yet been made public to any great extent. Such as are available tend to prove that the ejecta belong to the basic class of rocks, or to a transition from the acidic to the basic, and therefore are more basaltic than anything else—or, in other words, that the last phase in the history of the vulcanicity of this particular district has set in.*

Doubtless the report of the Professors who have already visited the district when it appears will clear up this important matter; but in the meantime there are certain points in connection with the subject which are so obvious, and bear such strong testimony to the great change in the character of the rocks ejected, from those forming the surrounding country, that it will be of interest to note them.

The matter ejected from Tarawera and Ruawahia in the greatest quantity is scoria, black and reddish-brown in colour, and highly page 70 vesicular in texture. So much is it like in outward appearance to the basaltic scoria so common to the volcanic districts of Auckland and the Bay of Islands, that only a very close examination would detect any difference. Now, with the exception of a little around Ruapehu and Tongariro, no such thing as scoria is anywhere to be found all over the Taupo volcanic zone. Pumice there is in abundance; but pumice is to the acidic lavas what scoria is to the basaltic lavas—i.e., the froth or scum—and no pumice, except in such small quantities that a careful search by the Survey party only brought to light a quantity which may be held in the hollow of a hand—was ejected from Tarawera. It is true that the sands and mud which cover so large an area of country have the appearance of being formed largely of pumice in minute fragments; but these, it is believed, are not recent products, nor did they to any great extent emanate from Tarawera, but from the basin of Rotomahana and Rotomakariri: they are merely the comminuted fragments of pre-existing rocks of the acidic class.

No one who has carefully studied the nature of the molten ejecta can doubt for a moment that an entire change has taken place in the character of the volcanic rocks now brought to the surface, from those formerly to be found there. We know that those forming Tarawera are the most acidic of all—trachyte and rhyolite—and no one has ever shown that these lavas produce vesicular black scoria.

We are thus again brought face to face with evidence which tends to prove what has already been attempted from other lines of reasoning, that the late eruption was deep-seated. If Richthofen's law is in any sense true, the deductions from it are equally true. The prevailing belief of geologists is, that the heavier and more basic lavas occupy the lowest position in that fluid belt which Sir William Dawson so ably described in his presidential address to the British Association at its late meeting, and, as a consequence, are the latest to be brought to the surface, and come from the greatest depths.

Since in the recent eruption the ejecta on top of Tarawera have been of that fragmentary character which tends usually to build up a true volcanic cone, we may ask why they have not done so. Before an answer can be attempted to this question, some consideration must be given to circumstances connected with the eruption which have a wider range.

It has been shown in previous pages that there are strong reasons for believing that the volcanic forces in the Taupo Zone had been gradually dying out for ages past; but there were also reasons shown for thinking that latterly, and not very long before the eruption, an increase page 71 in activity had taken place, although this increase was not altogether of such a character as to cause any alarm or apprehension of what was about to occur. We may suppose that during the period of decadence, and in the earlier stages of increasing activity, Ngauruhoe would act as the safety-valve at the southern end of the volcanic zone; and in its eruptions, which have been very minor ones, it would serve as the channel by which the superfluous energy of the clastic forces below would be carried off. But if, through any causes acting on the reservoirs of fluid matter below, a change in the locus of pressure were to occur, another vent would be sought. Whilst Ngauruhoe has remained unchanged in the degree of activity for many years past, the recently-proved activity of Ruapehu tends to show that such a change has occurred, and that the elastic forces endeavoured to find in the old channel of communication of that mountain a vent for their escape. But the effort was futile: possibly the elastic force of the gases was unequal to forcing solid matter to the surface at an elevation of over 9,000ft., and as a consequence another vent at a lower line would be sought. Recent events have shown that such a vent was found at Rotomahana and Tarawera, where the imprisoned forces seem to have gathered strength sufficient to overcome the resistance of the superincumbent rocks. It would appear as if a wave had passed along the fluid belt beneath the crust, first affecting Ruapehu, and passing thence northwards to Tarawera and on to White Island, and perhaps finally continuing its course to Niuafou, in the Tonga Islands, where it burst out in eruption on the 30th August.

When we come to consider the circumstances attending on the recent eruption, one of the things which strikes us as almost inexplicable is this: Why should the outburst have occurred first in the solid mass of rock of which Tarawera is formed rather than in the basin of Rotomahana, where hydrothermal action has been displayed on a larger scale than in any part of the Taupo Zone, and that for ages past, and where it would appear as if a communication pre-existing was already established with the heated interior? To account for it by assuming an origin for the outburst of so great a depth below the crust that the difference in height between the top of Tarawera and the bottom of Rotomahana is, by comparison, so infinitesimal as to cause no difference in the amount of resistance to be overcome, does not remove the difficulty entirely. It is, perhaps, more reasonable to suppose that a communication existed within the mountain, forming the old channel by which the lavas of which it is formed first flowed up to the surface, and the rocks in which had not quite cooled down or consolidated, and that this was the immediate source of the heat which gave rise to the page 72 action of the Rotomahana hot springs and others farther to the south. If this is so, then it is hut natural to suppose that the imprisoned gases would most readily find a way to force the solid matter upwards along these old channels to the surface. Further evidence of this will he found in the actual order of the occurrences at the outburst. The eruption was first seen to occur on top of Ruawahia, Wahanga, and Tarawera. After a time, as the vents became more open, the violence of the outburst would increase and spread along the line of fissure to the south. As the fissure opened up towards the first "bridge," already described, the forces would find less resistance towards the edge of the steep plateau than on top, and consequently they burst through the steep hillside, and formed the deep crater called the "Tarawera chasm," and then, continued the process by opening up the fissure further and further to the south till Rotomahana was reached: there, the lowest point along the line having been arrived at, the outburst was mostly concentrated.

This, as far as we know, is the actual order of the phenomena at the outburst. If it is correct, then we have an explanation as to why a cone has not been built up on top of Tarawera. It is simply this: that the gases which would have continued to eject solid scoria (to be followed probably by lava-streams) from vents confined within, comparatively speaking, narrow limits, such as those on top of the mountain, until a cone was built up, escaped at a lower level and from a larger orifice, where the resistance to be overcome was less, and in its nature different. It has already been pointed out that the change in the width of the great fissure occurs directly the softer rocks are met with. As soon as the outburst reached this part it spread and found more room to act, and gradually caused a cessation of action on top of the mountain by providing an easier outlet for itself. It might be supposed that when the crater at the top of the Tarawera chasm was formed, as this is at a considerably lower level than the fissure above, the lava would here find its way to the surface in the form of a solid stream: the reason why it has not done so is probably to be found in the fact of the lava being at a low level within the fissure, and that it does not anywhere approach the surface. The size of the fissure formed was sufficient apparently to carry off the imprisoned steam and to relieve the pressure, so that it had not power to force the lava up to anywhere near the surface. Some further evidence of the fact that Rotomahana derived its heat from a source within the Tarawera Mountain is found in the "boiling river" which now runs into the crater. As has been described, this stream bursts out suddenly from amongst the débris page 73 of rock and mud within the crater. It is nearly at boiling heat when it first appears, and evidently comes from underneath the cliffs forming the crater-walls. As it passes through only about 300yd. to 350yd. of ejected matter, it cannot derive its heat from that due to the outburst, but rather to subterranean sources.

It appears to the writer that all the evidence we have tends to prove the existence of a reservoir of molten heated matter within that part of the great fissure which underlies the mountains of Tarawera and Ruawahia, connected, of course, with the heated interior, and with other parts of the great fissure. What the immediate cause of the eruption was is a matter of pure speculation. Captain C. E. Dutton, of the United States Geological Survey, in his description of the Hawaiian volcanoes, says: "Volcanic action and regional uplifting are really associated phenomena, and the results of the same cause. When we come to inquire into the cause of elevatory movements of the earth's surface we only encounter a mystery. No solution has ever been given which does not go to pieces under criticism."

The Tarawera eruption appears to offer an example of the first stage in the formation of a volcanic mountain. It is, in fact, an incomplete effort to form a volcano. Whether the energy has exhausted itself in this effort, and matters will remain in a state of quiescence for long ages until it shall have again gathered strength to advance another stage, remains to be seen. As far as can be judged from the present state of affairs, the activity is fast dying out, and a return to the normal state appears imminent. Changes are for ever occurring in Nature, and these will be watched with great interest in this locality. The crater of Rotomahana cannot remain for long in its present state: as the activity decreases the equilibrium at present existing cannot continue; the drainage of the country which falls into the basin will no longer be carried off in steam, but begin to fill it with water and form a lake, probably of hot water. A time will come when this lake will increase to a much larger size than that which formerly occupied its site; but an end to that will come: after rising for some 80ft. above the former level, it will then force its way over the barrier now thrown across the outlet, and when that time arrives a very few hours will see the greater part of this barrier removed, and then the waters of Rotomahana must form a devastating torrent as they pour down into Lake Tarawera. Luckily that lake is of considerable extent, or otherwise the flood which would rise in the Tarawera River would prove more disastrous than any we have page 74 record of in this country. The face of the ash-covered country will change too. Vegetation will soon appear and prepare it for the uses of man. But never, alas! can this compensate for the loss this colony—nay, the world—has suffered in the destruction of those masterpieces of Nature's handiwork, the Pink and White Terraces.

* Vide Appendix No. 1.