THE ERUPTION OF TARAWERA AND ROTOMAHANA. Report by Professor Thomas, M.A,, F.L.S.

Te Aroha News, Volume VI, Issue 293, 25 August 1888, Page 6

THE ERUPTION OF TARAWERA AND ROTOMAHANA. Report by Professor Thomas, M.A,, F.L.S.

The results of Professor Thomas's extended investigations at the scene of the eruption of Tarawera have been embodied in a book which has lately issued from the Government piess. The report reviews the eruption mainly from it& scientific aspects, but the descriptions of the phenomena which attended this extiaordinary outburst are sriven in a form that maintains the interest of the general reader. The special aim of the work is to place on recoui an account of the eruption which will be of permanent scientific value. In this aim the Professor has been completely successful, and we are sure that his labouis will be welcomed by geologists in all parts of the world. The Tarawera eruption occurred under circumstances that were peculiarly favourable for accurate observation, and its phenomena will throw a good deal of light upon the imperfectly-understood and much-debated subject of volcanic action. The treatise before us deals with the subject so exhaustively that, while it supplies an abundance ol materials for other j lv found theories upon, the author's own opinions are so well-supported as to ensure tor them a wide acceptance. The delay in issuing the book is due to the painstaking way in which its data have been settled bv lepeated visits to the scene of the eiuption, and an extended examination of the suirounding country. The author gives a complete geological outline of the volcanic belt of country upon which the outburst occurred, and this will enable geologists at a distance to obtain almo=t as clear an understanding ot the occurrence as if they had enjoyed the opportunity of examining the ground. A seiies of well-executed maps, diagram*-, and pictures contribute materially to this result. One of the most interesting divisions of the work is that which treats oi the formation of the craters along the line of the great fissure, and the change^ which they have undergone since the first gieat outburst.

Effect of the Eruption on Vegetation. With regard to the effects of the eruption on vegetation and agriculture, the author sajs : — "During the following summer most of the smaller soft-wooded trees in the Tikitapn bush icvived, putting out numerous he-sh shoots. The largei hard-wooded tiees. however, like the rimu and lata, were killed. Even where the fall of the ash was not sufficient, to break down or cover up the vegetation, the green parts, as for instance the fronds of tiee terns and bracken, were scorched and quickly turned brown. Some plants appear to withstand the effects of a volcanic eruption much better than others, but these aie not always the most useful kinds. The clocks near the Ngae were up thioivh four and a half inches of \ oleanic ash within fi\ c weeks of the eruption, although it was midwinter. During the following summer it was noticed that bracken fern was the plant which appeared to be able to penetrate the gieatest thickness of ash. When this was two feet in thickness the fern was able to force its way up to the surface, though it appeared rather later in the season than ib would otherwise have done. We have here an excellent example of the survival of the fittest ; and it is, I think, a matter woithy of consideration whether the prevalence of the fetn over extensive areas of the North Island, where showers of ash have been of frequent occunence in past times, is not partly connected with its power of surviving such adveise conditions. No doubt fern grows abundantly elsewhere, but the power it ha<* of coming up through great thicknesses of volcanic ash must have been of consideiable use to it i-i Now Zealand iji lormer times. With the exception of a few patches at the Wairoa and at the settlements at Lake Taraweia, destroyed by the eruption, none of the country covered with a great thickness of ash was under cultivation, so that little permanent damage was done to agriculture. On the other hand, the fall of the ash will ultimately prove a considerable benefit over large tracts of country. A good deal of the soil that is thickly covered by the deposit was of a light ehaiacter, being formed from the decay of rhyolitic rocks, and often consists largely of pumice. The vhyolites yield soils which, both in their mechanical condition and chemical composition, are too 'light.' The rocks contain on an average 74 per cent, of silica and only 26 per cent, of bases, bub it is the latter which must be regarded in estimating the character of the soil which the locks are likely to form, for, althoug-h the silica is taken up by plants and has other important functions to play in the soil, yet it is always present in sufficient amount, and in the rhyolities ie in excess of any requirements. Part of the silica, too, is free in the form of quartz crystals, v/hich resist decay and remain as sand, valueless as plant food and rendering the soil lighter mechanically. In the basic rocks, on the other hand, the silica and bases are present in nearly equal amounts, so that as the rocks decay, a larger amount of valuable material is available as plant food. The recent ash contains a large proportion of basic rock, and when this decays under the influence of atmosphere and vegetation, it may be expected to yield a distinctly richer soil than a pumiceous one before, Between the mouths of the Tarawera

and Whakatane rivers is a large area of lowlying land, the Te Toko swamp. This has received a coating of ash three inches to six inches in thickness, and much more of the loose ash will doubtlessly be washed down into it from the surrounding hills. Not only ia the level of the swamp raised, but the soil, rich in vegetable matter, will be distinctly improved by tho addition of the ash. In the agricultural settlements along the Bay of Plenty, the, deposit of ash nowhere exceeded three inches, and as a rule was le-s. No doubt oven this thickness of ash meant a good deal of loss to the farmers in the first instance, for the grass was covered up, and the sheep and cattle reduced to starvation. But the benefit from tho deposit was felt within a few months, for the old grass soon came u p through the top-dressing of ash, and when I visited these settlements in the following summer an abundance of grass was to be soon everywhere, and the farmers reported that there had never been so abundant a supply as during that summer. Themaizecropsat Whakatane and Opotiki were also unusually heavy. The ash, as shown by the analysis of Tauvanga ash by Professor Brown, contained an appreciable amount of solublo matter — (3 per cent.), consisting of sulphates and chlorides of lime, soda and potash, the last substance in very small quantities, together with a little free hydrochloric acid. Theso substances we're of value as manure, and were immediately available for plant-food, and no doubt- contributed largely to the increased growth of vegetation where the ash had fallen."