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The Pamphlet Collection of Sir Robert Stout: Volume 67

Description of Tarawera and Rotomahana Prior to the Eruption

page 41

Description of Tarawera and Rotomahana Prior to the Eruption.

In the preceding description of the Taupo Zone some reference has been made to Tarawera Mountain and Lake Rotomahana, but only incidentally so: in order to a clear understanding of the changes which have taken place in their immediate vicinity some further particulars are necessary.

Rotomahana (or Warm Lake) occupied nearly—but not quite—the lowest position on the central axis of the volcanic zone, between Wahanga and Ruapehu Mountains. Its height above sea-level was 1,080ft., and in area it covered 185 acres. The whole of this was warm water of varying temperature, in some places little more than tepid, in others approaching boiling-point. Around its margins the steam escaped from innumerable hot and boiling springs, fumaroles, and solfataras, more particularly so at the northern end, in the neighbourhood of the celebrated White Terraces (or Te Tarata), the most beautiful objects of the kind in the world. On the western shore, the Pink Terraces (or Otukapuarangi), almost equally beautiful, were surrounded by hot springs and fumaroles. The waters of the lake were of a somewhat dirty greenish hue, reflecting the sombre- coloured fern and tea-tree covered hills which surrounded it, whilst its sedgy margins afforded shelter to vast numbers of duck, teal, toreas, and other aquatic birds, which were attracted there by the warmth of the water and the strict preserve enforced by the Maori owners of the country. Several small streams of cold water found their way into the lake, bringing down the drainage of a watershed of thirty-six square miles. The outlet was from the northern end, where a strong stream of hot water formed the Kaiwaka River, which, after a course of a mile, and a descent of 40ft., fell into Tarawera Lake near the Maori Village called Te Ariki. A short distance from its mouth it was joined by the Awaporohe Stream, which drained Lake Rotoma-kariri.

It has often been a matter of discussion as to the source of the powerful stream which drained the lake, for it was very apparent that the few small streams running into it, added to the water flowing from the terraces and other hot springs, were quite inadequate to account for it. The eruption has, however, disclosed the fact that the supply was mainly subterranean. The watershed was bounded on the north by the Tarawera Mountain, on the cast by the Kaingaroa Plain, on the south by the Maungakakaramea Mountain (which stands on the water-parting of the North Island), and on the west by the page 42 Pareheru Range. All over this area—which had a gentle slope to the lake—the surface was formed of a porous deposit of pumice and the accompanying rust-coloured loam before referred to; whilst here and there portions of the underlying trachytic or rhyolitic rocks protruded. The surface was a "graven" or "sculptured" one, due to the action of water, which had cut out numerous shallow gullies, all leading into the streams running into Rotomahana; but in very few of them could water be found. Indeed, it is one of the remarkable features of the whole of the volcanic zone that, whilst there are on every side the evident signs of a surface deeply scored by the action of water, the water itself, even after the heaviest rains, is rarely seen, and then only for an hour or so, standing in little pools or forming tiny runlets which have no power as excavating agents. Everywhere a thick growth of sombre bracken and tea-tree, or the yellow tussock-grass, covered the surface, relieved at long intervals by clumps of bush. That portion of the watershed which extends eastward from Rotomahana, and forms the gently-sloping plain at the foot of Tarawera Mountain, shows indications of having formed the bed of a former lake, in which deep deposits of the loamy earth lie with a nearly horizontal stratification, intercalated here and there with sonic fragmentary bands of lignite. In parts this plain was thickly strewn with rocks which had fallen from the rhyolitic cliffs of Tarawera.

Lying between Rotomahana and the base of Tarawera Mountain and about a quarter of a mile from the White Terraces, was Rotoma-kariri, the cold-water lake, with an area of about twenty or twenty-five acres, between which and the mountain was a shallow valley called Waingongongongo, in which were two or three swampy ponds; but this valley was of no great depth below the bordering hills.

On the north-cast water-parting of this basin is Rerewhakaitu, a cold-water lake, without visible outlet, but the overflow of which, doubtless, percolated through the loose strata both to Rotomahana and eastward to the Rangitaiki River.

A noticeable feature in this part is the long straight line of escarpment forming the northern edge of the Kaingaroa Plain, the rocks forming which are tufaceous.

The name Tarawera will, no doubt, in future be applied as a general one to the plateau-like mountains which stand immediately to the east of the lake of the same name, though there are three separate and distinct ones given to it by the Maoris. The most northerly is Wahanga, the central and highest part is Ruawahia, and the southern end is Tarawera. The outlines given in Plate No. 3 and No. 3A, carefully traced from photographs, show the general shapes page break
Drawn from Photographs

Drawn from Photographs

Mt. Tarawera Before and after the Eruption.

The dotted line shews the former summit.

page break
Tarawera, as seen from the South West.

Tarawera, as seen from the South West.

Before the eruption

page 43 of these hills, in one of which the alteration caused by the eruption is plainly seen. Map No. 2 also shows the ground-plan, and it will be noticed that they form together two plateaux of a mean height of about 3,500ft. above sea-level, or 2,500ft. above Lake Tarawera, and have an area on top of three square miles. Wahanga is divided from Ruawahia by a gap or defile about 500ft. lower than the plateau, and there is a smaller indentation in the cliffs which divide Ruawahia from Tarawera, though this is not continued across the plateau. The nearly perpendicular "mural crown" of rhyolitic and trachytic rocks which encircles the mountain is only to be scaled with difficulty in a few places, and from it a steep talus (inclination 33°) slopes down to the more undulating country below, on which several forests prior to the eruption were to be found. It has been stated that Tarawera is an ancient volcano on which was a well-defined crater; but this is not so. The writer made the ascent of the mountain three times in 1873, and spent several hours engaged in topographical work on the top, and consequently is able to state positively that no trace of such was to be seen.*

The surface was formed by a confused mass of rhyolitic rocks, broken and jagged, sometimes thrown up into hillocks, at others having the appearance of a lava stream, fractured as it cooled into innumerable large splinters, lying indiscriminately in all directions and positions, generally quite bare of vegetation, but here and there a little bracken or moss, or occasionally a weather-beaten shrub, relieved the sombre grey of the rocks. A central ridge, elevated somewhat above the general level, caused the surface to slope a little towards the cliff-formed edge of the plateau; but, excepting the gap between Wahanga and Ruawahia, it presented few deviations from a broken, rugged plain, over which it was very difficult to travel. The mountains have always been held by the Maoris to be very tapu, as here was situated one of their great tribal burial-grounds; and it was only with difficulty that Europeans obtained permission to ascend. The use of food or tobacco on top was considered especially wicked, and as likely to entail misfortune on the offender, the anger of the atua or guardian spirit being generally manifested by the sudden envelopment of the hill in mist and fog. Such was the fortune of the page 44 writer when attempting to take angles at the trig, station on two occasions; but whether the tobacco in his pocket had any connection therewith is left to the reader's discernment.

The origin of the mountains is undoubtedly volcanic, hut the immediate process by which they have received their present form is somewhat obscure. The shape is remarkable, and like no other mountain in the district. The accompanying section (see Map No. 2) shows this quite clearly—the plateau-like summit, the steep "mural crown," the sharply-inclined talus, and the beautiful easy slope below and this is the section it presents with little variation on every side excepting the east, where a range of much less elevation joins it and thereby spoils its symmetry. The mountain is clearly built up of several lava-flows of probably distinct ages, which have a rude parallelism and a slight westerly dip. The lower one, which forms the cliff above the plain to the south, is composed of rhyolitic lava in very fine laminae, through which are scattered isolated portions of a similar rock, exceedingly hard and close-grained. This flow comes out on the shores of Tarawera Lake, and there forms the picturesque cliffs which, prior to the eruption, were clothed with beautiful pohutukawa, now, alas! all destroyed. A higher stream can be traced in the rounded hill lying immediately to the south of the mountain, whilst above it is the one which forms the "mural crown," and whose surface occupies the higher parts of the mountain. It is difficult to say whether we have not here the remains of a series of great lava-streams whose former extension has disappeared by atmospheric denudation, or, on the other hand, the remains of one of those viscid flows of lava common to the acidic class of volcanic rocks, which never form craters, but, welling up from a fissure, spread out, as it were, in the shape of a great bubble, like many to be seen in the district of Puy de Dome, Central France. The recent formation of the deep crater at the head of the chasm in the south-west of Tarawera should throw much light on the origin of the mountain when the poisonous fumes shall have subsided sufficiently to allow of an examination; whilst the vast number of rocks scattered all over the surface, brought up from great depths, will enable a mineralogist to tell its composition. The mountain as a whole is doubtless rhyolitic; but the varieties of that rock scattered about are very numerous, and sometimes beautiful, especially those containing red spherulites.

The Maori names of this mountain-group would appear to have a strange significance in view of the recent eruption, did we not know that each one has another meaning. Wahanga means the "bursting open," Ruawahia, "the split or cloven hole or cave," Tarawera, "the page break
Tararua Eruption Map Shewing the Great Fissure and Points of Eruption

Tararua Eruption Map Shewing the Great Fissure and Points of Eruption

page 45 burnt cliff or peaks." No weight can be attached to the meaning of these names as indicating that the Maoris ever saw any previous eruption or volcanic action displayed here. The very evident signs of great age, and denudation of the lavas, is against any such hypothesis, even if the size of some of the trees growing on them did not contradict it. Totara trees over 8ft. in diameter, growing in that stony soil, would scarcely reach such dimensions during the time—five or six hundred years—the Maoris have been in occupation of the country.

* Mr. Charles Clayton, of Rotorua, who was surveying in the neighbourhood of the mountain in 1884, states that on one occasion he passed from Ruawahia over the top of Wahanga, and that he then saw a crater-like hollow about 200ft. deep, from the centre of which rose two or three small cones: he believes this to have been a crater. It is strange that, although the whole of the top of Wahanga is visible from Ruawahia, the writer saw nothing of the kind in 1873; and now, if any such crater exists, it is filled up by the late eruption and cannot be traced.