VIII. Summary and Conclusion.

It will be well to state here that in many important respects the conclusions arrived at in the present report differ from those expressed other writers in papers published shortly after the eruption. As these, however, with the exception of the report by Mr. Percy Smith, profess to have the character of preliminary reports, and were page 70 written before it was possible to examine in detail the scene of the eruption or even to ascend the Tarawera Mountain, it has not been thought desirable, except in a few instances, to make special reference to their divergent views.

In some respects the reopening of the Tarawera vent after a long period of rest, which must date back to a time long previous to the arrival of the Maoris in New Zealand, reminds one of the famous eruption of Vesuvius in A.D. 79, when the mountain, which showed no signs of volcanic life and was scarcely suspected to be of volcanic origin, suddenly burst into eruption. The immense showers of ashes which attended it form a further point of resemblance; whilst the destruction of Pompeii and Herculaneum finds its parallel in the burial of the Native settlements at the Ariki and the Moura beneath heavy deposits of ash. The comparison, however, does not hold further, for, whereas in the eruption of Vesuvius half of the old cone was blown away, with the formation of a circular crater, in the present instance we have a very different result—viz., the formation of a fissure nine miles in length.

Much interest has of recent years been taken in a type of volcanic eruptions (never actually witnessed) which have been distinguished from the eruptions from cones, and termed "fissure eruption." Some of the most remarkable examples of this type of volcanic structure are seen in the Western States of North America. For instance the Snake River plain in Idaho is a vast plain of sand and hare sheets of basalt. The river has cut its gorge through the plain, and in the page 71 walls of this gorge the lava is seen to be formed of successive beds reaching a thickness of hundreds of feet. No great volcanic cones are to be seen on the surface whence the floods of lava can have been derived, and it is commonly supposed that the lava has issued from a system of fissures now hidden by the floods of lava which poured from them. Mr. Clarence King states that the lava probably flowed for great distances, for it can be shown in certain cases that the lava received no addition by means of dykes for fifty or a hundred miles.

Strictly speaking, all volcanic eruptions are eruptions from figures, for the subterranean matter finds its way to the surface through great cracks in the rocks of the earth's crust. But, as a rule, these cracks get choked up except at particular points where the volcanic energy is concentrated, leading to the formation of cones.

The chief difference between a fissure-and a cone-eruption would therefore seem to be that in the former case the eruption of lava takes place continuously from a long line of fissure, whereas in the letter case action is concentrated at one or more points along the fissure. It is to be noticed that the lava of the typical fissure-eruption is usually of the most fluid kind—namely, the basaltic—and that only small quantities of scoriae appear to have been formed. With a very fluid lava the choking of a great part of the fissure is less likely to occur.

In the rise of lava to the surface along a considerable length (four miles of the fissure, the present eruption reminds us of a fissure-eruption; but in the dispersal of that lava as scoriæ, and the deposit of these around the margin of the fissure, we are reminded only of eruption from cones. It must be remembered that fissures are frequently formed on the sides of volcanoes : for instance, in the eruption of Etna in 1669 a fissure 12 miles long and 6ft. wide was formed on the flanks of the mountain. The peculiarity of the present instance lies in the formation of the fissure across the whole of the mountain, the practically continuous eruption from so great a length of fissure, and its extension far beyond the mountain. Mr. Clarence King^* has described a very instructive type of volcanic structure, which it will be interesting to cite here. The basalt lava to the south of the Truckee Range occurs in spurs or ridges, which show an arched structure of beds of lava. The beds slope down on each side from the top of the ridge, and have apparently been formed by the overflow of long vertical dykes: that is to say, fissures have been opened, and lava issuing from

^* "Geological Survey of the Fortieth Parallel," I., p. 673.

page 72 them has, by continuous eruption, piled up the centres of the ridge while the sheet of molten material poured down on each side.

The eruption of Tarawera shows a similar fissure, and the lava rising in it formed a dyke. No lava overflowed the fissure, for the explosive force of the contained steam caused its dispersal as scoriæ and dust. No such scoriæ were ejected from the extension of the fissure through Rotomahana, but it seems probable that the lava injected into the depths of that part of the fissure also, being perhaps prevented from rising by the letting-down of surface and subterranean water.

The Cause of the Eruption.—The immediate cause of the eruption lay, of course, in the existence below the district of a mass of molten material highly charged with water and gases. Whilst it was the expansive force of this imprisoned vapour which produced the eruption, it is possible that its occurrence was determined or hastened by the progress of subterranean events affecting a wider area of earth's surface. This is rendered probable by a certain degree of sympathetic disturbance shown at other centres in the Taupo zone, and perhaps also by the greater degree of activity displayed during the last two or three years in the volcanic belt of the earth's surface upon which New Zealand lies.

The eruption must therefore be looked upon as a normal event in the progress and development of the volcanic forces in the Taupo zone. The question then naturally arises, whether another eruption of Tarawera is likely to happen. Such an event is not improbable, but the present state of our knowledge affords us no clue as to when it may occur. It is, however, satisfactory to reflect that the history of other volcanoes shows us that, if the recent eruption be succeeded by another within a moderate number of years, the intensity of the eruption will probably be much less, and it is not likely to be attended by the production of such immense quantities of ash, so that the disturbance to the surrounding district will be comparatively slight. The intensity of a volcanic paroxysm is generally greatest after a long period of rest, and the severity of the late eruption may be ascribed to the long interval since Tarawera had been active so that the tension of the vapours in the heated rock below had to accumulate to a high degree before it could overcome the resistance of the cooled lava above.

Dr. von Hochstetter appears to have been of the opinion that volcanic activity was dying out in the Taupo zone, basing his opinion on the fact that so many alkaline siliceous springs are to be found there. According to the theory of Waltershausen and Bunsen, the page 73 first or earlier stage in the hot springs of volcanic districts gives rise to acid springs and sulphur deposits, whereas the later stage of decreasing temperature is characterized by alkaline springs from which siliceous sinter is deposited. But, though the theory may be correct enough when applied to individual springs or even groups of springs, it affords no safe indication of the state of the volcanic forces beneath a district. At Rotomahana alkaline siliceous springs and acid springs were to be found within the same area, whilst the same association of springs is to be found now in the Waiotapu Valley. Again, it has been shown that in the volcanic districts of Hungary, the western islands of Scotland, and elsewhere, deposits of sinter took place before the eruption of lava.^* The presence of the alkaline siliceous springs therefore cannot be appealed to as a proof that the volcanic forces are dying out in the Taupo zone, and the recent eruption of Tarawera, on the other hand, is an indication that such is not the case.

In concluding, the writer desires to thank the numerous gentlemen who have assisted him in the course of his investigation, and especially Mr. Lundius and other officers of the Survey Department. His thanks are also due to Messrs. Valentine,^† Spencer, and Martin for permission to copy photographs of the scene of the eruption. He is specially indebted to the courtesy and kindness of Mr. S. Percy Smith, Assistant Surveyor-General, for permission to copy the map of the Fissure. The writer has ventured to make various alterations and additions to this, founded on his own explorations made in part prior to the date of the survey, but chiefly at a more favourable season of the year. Of these, the most important is a part of the fissure half-a-mile in length, here described for the first time, which includes its termination on the north-cast side of Wahanga, the most remote and least accessible part of the mountain. For the accuracy of these additions he is, of course, alone responsible.