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Tales of Banks Peninsula
The Geological Story Of Banks Peninsula
The Geological Story Of Banks Peninsula.
The legends and stories of Banks Peninsula tell but a small" part of its history; the period over which they extend is a mere point of time compared with the preceding ages during which its romantic hills were first of all slowly constructed by volcanic agencies and then carved into their present form by other geological agents. Even casual observers have some idea that they are volcanic in origin, for the layman recognises that they have peculiar features which mark them off from the mountains which form the back country of the province. Their rich soil, the system of radiating valleys with their lower reaches occupied by the sea, the two noble harbours, and the wave cut cliffs, standing erect and showing excellent sections of the flows of lava and beds of fragmentary material of which they are constructed, all tell of another origin from the slaty and frost riven Southern Alps, with their barren hill sides covered in many cases from base to summit with wastes of moving shingle. This difference is clear to all, but to the geologist additional evidence appeals. A mere fragment of rock with its airfilled vesicles and glistening crystals scattered through a dull stony paste, tells of a former molten condition; the dark coloured layers of solid rock exposed on the hillsides and inclining downwards and outwards in all directions owe their arrangement to the outflow of liquid material from some central page 394elevated area, the so-called volcanic crater, and the accumulations of fragments lying between these solid flows point to explosions under whose disruptive influence the solid rocks were torn asunder or dashed together and reduced at times to the finest dust. Evidence of their orign is thus furnished by almost every portion of the hills.
The only exception to this general rule occurs in one small locality near the head of Lyttelton Harbour. In this part of the district we find rock which cannot be attributed to igneous action, but closely resembles that of which the great mass of the Southern Alps is formed, and we may conclude that at one time it formed a part of a great tract which stretched east from the present mountain region, and, perhaps reached as far as the Chathams. On this land, somewhere near Gebbie's Pass, arose a small volcano, which poured out a white rock, called rhyolite by geologists, now readily seen by anyone who crosses the pass, for it caps the ridges in its vicinity, extends down towards Gebbie's Valley for over a mile, and forms the extremities of the long peninsulas which stretch fingerlike into the upper waters of Lyttelton Harbour. This volcano was a small one compared with others of the same age in the province of Canterbury, for the great mass of Mount Somers, Mount Alford, and the Rockwood Hills dates from this period, and marks the commencement of the age of igneous activity in Canterbury, which passed through all its various stages, from youth, through vigorous maturity to decline and death, before the foundations of Ruapehu and Egrnont were firmly established.
Although the results of this period of activity, as far as Banks Peninsula is concerned, are small compared with other outbursts within the pro-page 395vincial. area, the subsequent manifestations of volcanic action have produced changes which rival any in the whole geological history of New Zealand. After a long period of quiescence, activity recommenced from two centres, and two great mountains, which may be called the Lyttelton and the Akaroa cones, were constructed by normal volcanic processes, lava flows alternating with layers of fragmentary material, till they exceeded in height and volume the great volcanoes of the North Island. In all probability Lyttelton was slightly the earlier of the two but smaller in size. They show a remarkable similarity to each other in their mode of formation, in the character of the lavas they discharged, and even in unessential aspects of mountain construction and destruction. The actual centre of the Akaroa volcano was situated somewhere near the end of Onawe Peninsula and the cone extended with a seven mile radius on all sides, and reached north and east beyond the rocky wave-cut headlands which now terminate the spurs stretching in that direction, while to the south the flows ran as far as Birdling's Flat, and to the west they reached beyond Pigeon Bay and Kaituna Valley and overlapped those coming from Lyttelton. The base of this cone must have been twenty miles in diameter, and its height judging from the average angle of the lava streams must have exceeded ten thousand feet.
The Lyttelton Volcano had an analogous origin and form.
After a long period of construction, both of these cones experienced a similar catastrophe. No doubt as their height increased, the eruptions grew fewer and fewer owing to the increasing difficulty of raising the lava to a great height within the cone, and finally they both became dormant, and page 396the throats of the craters communicating with the liquid reservoir underneath, became blocked with solid plugs of lava. The interior pressure was kept up while the solidification of the plug continued slowly downward, and a great trial of strength ensued between the internal volcanic forces and the resistance of the mountain. Finally the latter gave way, and the tops were shattered by paroxysmal explosions similar to that which destroyed the island of Krakatoa in the year 1882, and to that historical eruption which wrecked the cone of Vesuvius and buried the cities of Pompeii and Herculaneum. The great explosions tore chasms in the tops of the two mountains, leaving cavities which have been enlarged to their present size by the action of water and other erosive agents, and are now occupied by the sea.
An interesting feature of these volcanoes is the series of dykes which penetrate the lava flows and ash beds like vertical walls and radiate like the spokes of a wheel from the explosion centre. In both cases they are best seen on the rocky shore platform which fringes both harbours between high and low water mark. In Akaroa the majority point to the neighbourhood of Onawe and in Lvttelton to a centre at the back of Quail Island. In both cases, however, in the immediate vicinity of the centre they depart from the regular arrangement. In some places they appear on the slopes of the hills and stand up like wails above the surrounding country. This is more apparent round Lyttelton Harbour, where they form well-marked landscape features such as Dover Castle, above Heathcote; Witches' Hill above Rapaki; and the two great wall like masses on Dyke Hill near the Kaituna Pass above Teddington. They are chiefly whitish or greenish in colour and are of trachytic page 397character, but dark basaltic dykes occur in numerous places; for example, they form prominent features on the hills above the Hon. R. Heaton Rhodes' house at Tai Tapu. The systems of dykes are due to the injection of molten rock from below through fissures formed by great pressure or explosions within the cones.
On the very end of the Onawe Peninsula there is a most interesting rock of coarse grained texture related to granite, which closely resembles the pale coloured dykes in chemical composition, and probably represents the subterranean parent mass, from which these were offshoots. No such mass has yet been located in connection with Lyttelton, and if existent at all, it is doubtless buried beneath Quail Island or under the floor of the harbour. It is the only occurrence of granitic rock in position known in Canterbury.
After the destruction of the summits of the two cones, volcanic energy appears to have located tself between Kaituna Valley and Mount Herbert. From a centre somewhere here, probably in the upper portion of Kaituna Valley, lava flows were poured, chiefly towards Lyttelton Harbour, and the summit of Mount Herbert and its westerly extension towards Castle Peak were built up by the streams of basalt which ran northwards and now form the long gentle slopes leading down to Diamond Harbour. On the flanks of Castle Peak and fronting Kaituna Valley the lava sheets lie level, and break off with fine basaltic columns round the heads of the Charteris Bay and Teddington Valleys, while to the south they form fine terraced scarps rising steeply above Kaituna Valley. The peaks of Mounts Sinclair and Fitzgerald were formerly assigned to this period, but in my opinion they represent outlying extensions of the crater page 398ring left when the great Akaroa cone was dissected. and are not independent centres. The flows of which they are composed have no doubt come from the direction of Akaroa and owe their preservation to their power of resistance to weather and other disintegrating agencies.
Quail Island is looked on by some as the last spot which gave forth volcanic manifestations, and it probably represents a small secondary cone built up within the Lyttelton crater ring, though its activity may be contemporaneous with the latest flows from Mount Herbert.
It should be mentioned, however, in concluding this brief sketch of the volcanic history of the Peninsula, that Sir Julius von Haast indicated two other centres of activity, viz., Little River and Pigeon Bay, both of which he regarded as having been formed in much the same way as the two great harbours. It appears to me, after a close examination of the localities, that these valleys can with more justice be attributed to stream erosion, since they show none of the general outward inclination of flows from the centres of the valleys, and no independent systems of radiating dykes. Whether this is true or not, the subsequent history of the area is largely one of dissection of the volcanic mass by water action. To this agency must be attributed the formation of the radial valleys and their submerged seaward extensions, the enlargement of the hollows primarily due to paroxysmal explosions, and the cutting of the entrances to these hollows, all of which proceeded apace when the land was higher, and water action had more power. Then ensued a settling, and the sea invaded the lower reaches of the valleys and the floors of the old craters, and at the same time cut back the cliffs which now form such a striking page 399feature of the outer coast, specially in the vicinity of East Head and near the south head of Akaroa Harbour, where the level flows are clearly exposed on the vertical cliffs. At times, too, as at Scenery Nook, the red ash beds as well as the flows are penetrated by dykes. How much of the eastward extension of the cone has been thus removed it is impossible to say, but no doubt a strip some miles in width has gone. And all the while the rocks were slowly breaking down and producing a rich fertile soil which received a quota, adding to its fertility, from the dust-like material swept by strong north-west winds from the stretch of plain to the west, over which great glacial streams once wandered, distributing far and wide the fine rock-flour worn from the solid rocks of the Southern Alps by the great glaciers which then filled the valleys and deployed fanlike on the western margin of the plains. Banks Peninsula has no glacial history but the remnant of a sub-alpine flora on the hill tops shows that they too came within the sphere of influence of those agencies which in former times modified profoundly the land forms of the mountains further west.