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The Pamphlet Collection of Sir Robert Stout: Volume 68

II. Geology of Tarawera and Rotomahana

II. Geology of Tarawera and Rotomahana.

The Tarawera Mountain.——Mount Tarawera is situated on the eastern shore of the lake of the same name, its sides sloping down to the level of the waters of the lake (1,040ft.), or ending abruptly in high steep cliffs. The top of the mountain before the eruption presented an uneven table-land of an elongated rectangular shape, about two and a half miles in length and one mile in breath (PI. p. 22), This was interrupted towards the northern end by a distinct notch or gap 700ft. deep, marking off a portion of the top, about one-fourth of the whole area, which was known as Wahanga (see Map II.).

About the middle of the western border of the remaining and larger part a deep gully could be seen descending from the table-land. An indistinct division into two parts was thus produced, the part to the north known as Ruawahia, whilst that to the south of the gully was the only part to which the name Tarawera was applied by the Natives. Europeans, however, usually found one Native name sufficient to remember, and therefore used Tarawera as the name of the whole of the mountain.

The highest part of the mountain was at the northern end of Ruawahia, and rose to 3,606ft. above the sea, the highest level attained by any mountain in the Hot-lake district.

Viewed from a distance the mountain appeared to have a nearly flat top: strictly speaking, however, it was not level, but was higher towards the middle, and had a very rugged surface. This was covered with hummocks strewn with angular blocks of rock, and occasionally rose into peaked hillocks'. There was no sign of any crater. Around the flat top of the mountain the sides descended at first in a steep page break
From a photograpn by J. Marun, E.Q.S. Before the eruption Survey Deat Liypeo 1886.

Tarawera as seen from the South West.

page 23 face of coarsely jointed rock, only scalable in a few places: this was some 400ft. in height; and at its foot stretched a débris slope having an angle of about 30°, and descending to the level of about 1,000ft. above Lake Tarawera. From this height downwards to the lake the lower slopes had a much more gradual fall. This characteristic form of the mountain, with the table-like top, and the steep wall-like side surmounting a débris slope, is well marked around the greater part of the mountain's circumference. Along a portion of the south-east side, however, the symmetry of its form is a little disturbed by a broad shoulder which abuts against the mountain-side below its upper wall-like face.

It was doubtless the flat appearance of the summit of Tarawera and its wall-like face of reddish-grey jointed rock which led Yon Hochstetter to compare it with Horohoro. He says,* "Tarawera Mountain, however, and Horohoro Mountain, are excellent examples of the remains of a plateau: their upper surface marks the original level of the district."

The two mountains, however, are of very different structure. Horohoro is a portion of the Patetere plateau of the older tuff, whilst Tarawera, as will be seen below, is a true volcanic cone. Von Hochstetter did not ascend the mountain, and possibly any proposition to do so would at that time have been opposed by the Natives, as the mountain had been used as a burial-ground for long generations, and was therefore tapu. It must be remembered, also, that Hochstetter spent less than five weeks in the Taupo zone. Indeed, remembering the time he was in the district and the difficulty of travelling in it, the extent and accuracy of his observations are very remarkable.

The formation of the series of craters along the mountain-top daring the recent eruption has afforded an admirable opportunity of examining its structure. The whole of the flat top of the mountain is composed of a porous rhyolite lava of a light-grey colour, or, where weathered, of a slightly-reddish grey tint. It is always somewhat porous, and towards the margin of the plateau is often coarsely vesicular. Most of the recent craters on the summit have very steep sides where they pass through this rock, but it is just possible to descend into the last two craters on the top of Tarawera proper, and portions knocked off the rock in situ in the wall of the fissure at the depth of 450ft. below the present level of the crater-edge show a very finely porous structure, which may be best described as slightly pumiceous. Thin sections for the microscope show that the rock has

* "Voyage of the Novara,'" Vol. I., p. 106.

page 24 a glassy ground-mass, in which are imbedded numerous crystals of quartz (in double hexagonal pyramids), felspars (most of which are sanidines, though a few are plagioclases), hexagonal plates of black mica, and a few magnetites. The glass of the ground-mass is nearly pure, but shows a few microliths in places, whilst it contains vast numbers of narrow elongated steam-pores, which are arranged parallel to one another in streams, which show the most pronounced fluidal structure, so that, viewed with the microscope, the rock almost seems to flow. The specific gravity of the rock is 2.39.

It is in the craters between Ruawahia and Wahanga, however, : that the true structure of the mountain is most instructively shown. We sec here a section through the side of a scoria-cone composed of inclined beds of pumice, &c. On each side the beds dip outwards; in the middle of the exposed section they dip to the south-west under the mass of rhyolite lava forming the cap of Ruawahia, whilst in the opposite direction they dip towards and under Wahanga. This evidence alone is sufficient to show that a volcanic vent existed on the site of Tarawera long before the eruption of last year. During the eruption preceding that which forms the subject of this report, a fissure must have been opened across the pumice-cone in a north-east and south-west direction, and up this fissure welled a mass of lava, very viscid, as the acid lavas usually are, which filled up the crater and spread out on each side, solidifying so as to form the flat cap of porous but massive rhyolite lava which we now see. The lava on the summit of Wahanga, it is true, forms a distinct mass from that on Ruawahia and Tarawera, but it agrees so closely in its form and structure, and even in the microscopic details of its finer structure, that there can be but little doubt that the two masses were formed at the same time. The lava, therefore, must have risen to the surface along portions of the fissure only, just as in the eruption of 1886.

The summit of the mountain has doubtless suffered a good deal from denudation since, as is shown by the slope of débris underneath the steep rampart-like face of the upper part of the mountain, and by the deposits of fragments and boulders spread on the lower slopes and the broad stony beds of the numerous watercourses which stretch downwards in all directions from the mountain.

The lower slopes are composed in many places of lava-streams, which have often come down as far as the lake, and form bluffs rising high above its waters. The lava has usually a glassy base; it is sometimes a porphyritic obsidian, which may be coarsely vesicular or almost free from pores. One variety collceted by me showed under the microscope a glass free from microliths, with a few small but page 25 perfect crystals of quartz in double hexagonal pyramids, sanidines, and hexagonal plates of black mica. Other specimens, however, were crowded with slender microliths. Sometimes the rook is full of red mil-like spherulites, as in the bluff between Ngawhiri and the next I bay to the north. Near Tapahoro, at the outlet of Lake Tarawera, the lavas show the most extraordinarily twisted and contorted flow-structure. The lava is laminated, and the laminæ are twisted, folded, and bent upon one another, like half-opened bales of cloth confusedly thrown together. Some of these lavas have a felsitie ground-mass; others are partly or purely glassy.

It will be seen, therefore, that the Tarawera cone had a mode of structure analogous to that of the domitic Puys of the Auvergne district of Central France, and of some of the volcanoes of Hungary and Bohemia (e.g., the Chodi-berg). Where the lava issuing from a vent is very viscid, and is not so highly charged with steam or gas as to be scattered by powerful explosions of expanding gases, it tends to heap itself over the vent in a rounded mass which shows no sign of a crater, the original crater or vent being, in fact, overwhelmed and hidden by the accumulation of lava. The first-formed shell is injected with fresh lava from below, and so the mass grows larger. The outer shell, if still viscid, will stretch; if it has cooled, it will fissure. Mr. Percy Smith informs me that the top of Tarawera, before the eruption, presented an imperfect network of fissures deep enough to conceal a man; between these fissures the surface was formed by rough broken masses of rock, occasionally rising into hillocks. The fissures may be best explained as due to the cracking of the outermost portion of the extruded lava.

A rounded hill formed by such an outwelling of viscid lava usually forms a steep-sided mass, which, when its internal structure can be examined, generally shows a distinct concentric arrangement, which has been compared with that of the leaves in the bulb of an onion. This is well shown in the Chodi-berg of Hungary, a great bulbous mass of andesitic rock, and also in the excavation of the hill of the Grand Sarcoui, a similar mass composed of altered trachyte in the Auvergne* The rhyolites too of Hungary and Transylvania show a marked tendency to form isolated summits, which are regarded as the summits of masses formed by the outpouring of a viscid lava from a fissure. Something of this concentric structure can be traced in the section of the lava-cap of Tarawera, where it is exposed in the crater on the south-west side of Tarawera proper.

* Judd : "Volcanoes," p. 161.

Lapparent : "Traité do Géologie," p. 1163.

page 26

Rotomahana.—Some two miles to the south-west of Tarawera was the celebrated warm lake of Rotomahana. The lake was about one mile in length in a north-and-south direction, and on the average about one-third of this in breadth. At its southern end it received the waters of three small streams, whilst at the northern end the overflow of the lake formed the Kaiwaka Stream, which flowed with rapid current towards the Ariki arm of Lake Tarawera, and joined it after a course of about one mile in length. Most of the ground around Rotomahana was warm at a few inches below the surface, and at innumerable points hot springs or jets of steam escaped from the ground. The largest of the boiling springs were those on the top of the White and Pink Terraces, and the siliceous sinter deposited from their waters had in the course of ages built up the famous terraces on the ground which sloped down to the shores of Rotomahana (see Pl. p. 28). The water which almost constantly flowed over the terraces, together with the waters of all the other hot-springs about the shores, fell into the lake. Below the surface of the water of the lake were other hot-springs, and these must have contributed largely to the considerable body of water which poured by the Kaiwaka into Lake Tarawera.

About a quarter of a mile to the east of the White Terrace was Rotomakariri, a lake of cold water, as the Maori name indicates, This was only one-third of a mile in length from west to east, and about half as much in width. Its shores showed numerous circular cones, which, Hochstetter states, reminded him of the volcanic tuff-craters of the neighbourhood of Auckland, and he thought that they were probably due to the former action of hot-springs or explosions of mud-volcanoes around the lake. Around Rotomakariri the ground was swampy, and several smaller pools of water occurred here. The overflow of Rotomakariri formed the creek Awapurohe, which joined the Kaiwaka on its way to Lake Tarawera.

To the east of Rotomakariri was an alluvial plain, a few square miles in area, formed by the numerous watercourses which came down from the south-east and south slopes of Tarawera. The plain had a gentle slope to the west. But this neighbourhood must have been occupied at one time by a lake, for in the strata exposed in the craters due to the recent eruption may be seen a succession horizontal lake-beds, with beds of lignite at intervals, one of the latter being l½ft, in thickness. The strata are composed of pumice sands and gravels, and fragments of various rhyolites, together with volcanic ash, which apparently fell in water, for there is a marked separation of the finest ash from the coarser particles, as if the former had page 27 remained in suspension in water for some time, whilst the coarser part had settled at once. It may be noted here that similar deposits of volcanic ash, in which the finer material rests upon the coarser, may be seen amongst the lake-beds which form the cliffs around Rotorua. Here too the ash seems to have fallen in still water, which I held the finer material in suspension for some time, until the coarser particles had settled to the bottom,

The lake in which the above deposits accumulated may have been but a shallow one, and may have been drained at times, for under one of the bands of lignite may be seen stumps of trees growing erect, with their roots in the underlying sandy layer. The plain is bounded to the south by a kind of escarpment, which here forms the abrupt termination of the Kaingaroa Plains.

Much of the higher ground around Rotomahana was formed of massive porphyritic rhyolites, as in the hill between Rotomahana and Rotomakariri, though the nature of the rocks was much disguised by the intense hydrothermal action to which they had been so long exposed. To the west of Rotomahana the ground rose to the height of over 1,900ft., forming the hill known as Te Hape-o-Toroa, which had an irregular but somewhat flat top. The summit of this hill is composed of a porous light-grey rhyolite, differing but little from that which forms the summit of Tarawera.