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The Pamphlet Collection of Sir Robert Stout: Volume 67

Cause of the Eruption

Cause of the Eruption.

Eruption of the Mountain.—The eruption of Mount Tarawera was a true volcanic explosion of a paroxysmal character, throwing up large quantities of red-hot scoria and pumice, although no lava is known to have been emitted. It may also be looked upon as the opening of a new volcanic vent; for, although it is probable that further investigation may prove Mount Tarawera to be an old volcano, still it is evident that the new fissures are not in the same position as the old crater. The great interest, therefore, from a scientific point of view, is that we see in it the method of establishment of a new volcanic vent, while the eruption was so short and so violent that the openings have not been buried, but can be examined at leisure, which is not the case with either Jorullo or Monte Nuovo.

An examination of the ejectamenta from the craters shows that—(1) A complete series can be made, from compact andesite with decomposed rhyolite to the same rock highly vesicular, but still showing fragments of the rhyolite and quartz; (2) a similar series can be made from decomposed rhyolite to quartzose pumice; (3) the Black Crater has thrown out blocks of an old andesitic lava, which has overflowed a still older rhyolitic lava, the two being intimately connected at the line of junction. The same intimate connection between scoria and pumice is seen in fragments thrown out by Mount Tarawera, and it is very improbable that this similar intimate connection between two similar rocks should have been brought about in two different ways.

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General View from Lake Rotorua

General View from Lake Rotorua

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From this it follows that the scoria and pumice emitted by Mount Tarawera are old surface-rocks that have been lately heated up to the point of fusion, for if this were not so they could not have become vesicular. Evidently, therefore, we have in this reheating of surface-rocks, which had been previously saturated with water, the immediate cause of the eruption of the mountain.

If now we further inquire, How came these rocks to be reheated? we see at once that the cause was local, in the mountain, for the reheated rocks have all previously undergone atmospheric decomposition. Consequently the heat could not have been caused by upward conduction, through the solid crust, of the internal heat of the earth. If such had been the case the heat would have been widespread, and Rotomahana would have been heated as well as the mountain. No difference in the conducting power of rocks can explain the fact that Mount Tarawera was much more strongly heated than Rotomahana. No chemical changes, at all competent to do the work, suggest themselves as the cause of reheating.

The crushing of rock, due to strains in the mountain, and the conversion of these mechanical movements into heat, seems at first sight very plausible, as mechanical movements might be expected to occur along old lines of weakness, such as the line from Tongariro to White Island may be conceived to be. But when closely examined this theory is found beset with difficulties. It has been calculated that ten volumes of rock must be rapidly crushed in order to fuse one volume, and even then the heat of the ten crushed volumes must, in some way, be focussed into the one fused volume. Leaving, however, the localization of the heat out of the question, we find that in the case of Mount Tarawera several millions of tons have been fused and ejected, and therefore several tens of millions of tons must have been crushed but not fused. But very nearly all the material ejected by the mountain has been fused : where, therefore, is the immense amount of crushed but unfused material? and why was none of it ejected? Instead of the mountain having been crushed, there are now two deep gaping fissures in it; and photographs of one of them, taken by Mr. C. Spencer, show that it has solid rock on each side. Again, several tens of millions of tons of rock could hardly have been rapidly crushed without producing violent earthquakes; but the earthquakes which preceded the eruption were not violent, but so slight that many people, even in Wairoa, were not awakened page 16 until after the eruption had begun, and none of the earthquakes were so violent as those due to subsequent explosions of steam from Rotomahana. The heaviest earthquakes were after the eruption had commenced, not before it, and probably therefore all were the effects, not the causes, of explosions. Also many greater earthquakes have occurred in the district without causing an eruption; consequently the local production of heat by crushing appears to be very improbable.

The only other explanation left to us is that the local increase of temperature was due to molten rock having been forced upwards from below into the mountain. We know that molten rocks have, at some former time, been erupted all along the Taupo zone; and the fact that heat has also been lately developed in Ruapehu, without any accompanying earthquakes, points also to this movement of molten rock as the true explanation of the case. When we remember the slow conducting-power of rocks and the great thickness of surface that has been heated, it seems evident that this intrusion of melted rock into Mount Tarawera must have taken place a long time before the eruption; and this is in accordance with the facts that the heat had been developed in Ruapehu in April, and that for several months earthquakes had occurred occasionally in the Tarawera district.

Two hypotheses have been put forward to explain the upward rise of molten rock in the earth. The first is that incandescent rock lying deep down in the earth gradually absorbs condensed water-vapour by capillary action under great pressure, and the quantity gradually increases until the pressure of the overlying rock is overcome, and the water-vapour flashes into steam. The two principal objections to this hypothesis are—(a) that it is very doubtful if the necessary pressure exists in the interior of deeply-seated rocks to force the condensed water-vapour into the incandescent rock; and (6) this condensed water-vapour must have had the same temperature and pressure as the rock when it was absorbed; it is difficult therefore to see why after absorption it should have more expansive force than before it was absorbed. The second hypothesis is that the upward rise of molten rock is clue to pressure arising from movements of the earth's crust over a liquid or viscid interior. This hypothesis is, of course, open to all the objections that have been urged against a viscid interior of the earth. It will not, however, be necessary for me to discuss these rival hypotheses, for I do not sec that the present eruption throws any new light on the subject.

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Part of the Black Crater Throwing up Stones

Part of the Black Crater Throwing up Stones

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Eruptions on the Plains.—The eruptions from the Rotomahana and Okaro craters were very different in character from those of Mount Tarawera. No great heat was developed. They ejected no red-hot stones, nor scoria nor pumice, but up to the last threw out angular slightly-heated fragments of surface-rock. They were therefore not truly volcanic, but only hydro-thermal in action, although they ejected a large amount of fine ash, a phenomenon which was previously supposed to accompany true volcanic action only. It must be remembered, however, that the rocks through which these eruptions burst were already decomposed and soft, with water probably all through them.

The eruptions on the plains followed those from the mountains, and were, no doubt, caused by them. Their position shows that they are in some way connected with the fissure on the mountain; but they can never have been directly connected with that fissure, because, if they had been, (a) scoria and pumice would have been ejected from them as from the mountain, and (b) the eruption of Rotomahana would probably have preceded that of the Okaro craters instead of having followed them. Although following in a general way the direction of the fissure, the craters are by no means arranged in a straight line; indeed, those in and about Rotomahana are scattered over a belt at least half a mile broad. Also no fissure can be seen at the surface as it can on Mount Tarawera; the cliffs now surrounding Rotomahana being due merely to landslips of the much-decomposed rock. Each of the Okaro craters is isolated, so that it is possible to walk round any of them without crossing a fissure, and no fissure is visible on either side.

Still, the position of these craters evidently connects them in some way with an underground extension of the Mount Tarawera fissure. Probably molten rock was injected, as a dyke, into the fissure below, and the earthquakes, caused by the explosions at Mount Tarawera, deranged the subterranean water-reservoirs at Okaro, and allowed some of this water to find its way towards the heated dyke. Before reaching this, however, it was violently ejected as steam, forming the so-called Okaro craters. Subsequent earthquakes caused surface-fissures, which drained off the water of Rotomahana towards the incandescent mass, but this water also was flashed into steam at a considerable distance from the dyke, and consequently no direct communication was established between the subterranean fissure and the surface. The Rotomahana craters are, in fact, only colossal fumaroles, page 18 which we know from observation often commence by throwing up mud and stones.

Lessons of the Eruption.—(1.) The most important point on which this eruption gives evidence is, as I have already said, the method of establishment of a new volcanic vent; but there are other very interesting points in the eruption. For example:

(2.) It is proved that surface-rocks are reheated in a volcano.

(3.) Evidence is forthcoming that this reheating is not due to crushing. But on this head much more evidence can be collected by a close survey of the fissures.

(4.) Evidence is produced in favour of an opinion held by many geologists, that surface water cannot come into contact with molten rock by means of an open fissure.

(5.) Fine dust can be formed from soft rocks without much heat, and without true volcanic action.

Theory of Declining Volcanic Action.—Dr. von Hochstetter and all subsequent observers have agreed in the opinion that volcanic energy was dying out in New Zealand. This opinion was based on the theory of Waltershausen and Bunsen that our alkaline siliceous springs belonged to a second stage of decreasing temperature; the first, or hotter stage, giving rise to acid springs and solfataras.

There are several facts in the Rotorua district which this theory does not explain. For example, the decaying solfatara of Sodom and Gomorrah, between Rotorua and Whakarewarewa, has covered old sinter deposits with thick layers of sulphur; and Waikite, the principal geyser at Whakarewarewa, has lately formed a thin coating of sulphur over the upper portion of its sinter terraces, although the water from it gives a decided alkaline reaction at present. Nevertheless, the theory is no doubt correct on the whole, but it applies equally to dormant and to extinct volcanic districts. Whether the Taupo zone is merely in a dormant state and the late eruption indicates a renewal of activity, or whether the eruption was merely a spasmodic death-struggle, time alone can show; but the probabilities are much in favour of the latter supposition.

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Part of the Blank Crater.

Part of the Blank Crater.