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The Pamphlet Collection of Sir Robert Stout: Volume 67

Earthquake-Cracks

Earthquake-Cracks.

It has been before mentioned that the great fissure is continued in a south-westerly direction by earthquake-cracks. There are many features of interest connected with these as bearing on the recent eruption, and also on the proof they offer of the great disturbance of the crust of the earth in this locality in times long antecedent to the late eruption, when the earthquakes must have far exceeded in violence those recently experienced, and at the same time have extended over a much larger area than the near neighbourhood of the fissure. A glance at the map will show that there is a series of cracks which is directly in the line of fissure, whilst on both sides others exist which have a nearly parallel direction. No doubt, were the covering of ash removed from off the space between these cracks and the Southern Crater others connecting them with that crater would be found. The proof of it is this: The Survey party were in the habit of using one of the little valleys leading from Pareheru to the Black Crater as the road to their work each morning; but on one occasion after a heavy rain they found their way cut off by a crack which had appeared during the night, and this was so large and so deep as to be impassable. It extended across the valley from side to side, a distance of 60yd. by 20ft. wide, and had a varying depth of from 15ft. to 25ft. No sign of this had been seen before. Doubtless the heavy rain had worked its way down under the ash until it came to the crack, and had then excavated its sides till the covering fell in and disclosed its presence.

Now, it is remarkable that all the fresh cracks seen—and they were very numerous, only a few being shown on the map through want of time—followed the lines of old cracks, or subsidences, or perhaps it would be better to call them faults. In nearly every valley in this part of the country, a peculiar feature had been noticed by the writer for many years past, and many an hour had been occupied in endeavouring to assign a cause for it. These valleys are almost invariably dry, and, being generally of gradual easy ascent or descent, the ancient Maori pathways follow along them from one watershed to another. They thus formed the high roads of the country, and, being invariably grassed, whilst the hill-sides were covered with fern, formed excellent places for observing. Every here and there—sometimes within a few yards of one another, at others perhaps with page 66 a mile or so intervening—were terraces or "steps" running athwart them, of various heights, from a foot or two up to 20ft., 30ft., or 50ft. A peculiarity about them is that they do not all face the same way—i.e., the descent is not always the same as that of the valley. It constantly occurs that a valley may have a gentle regular descent of 4° or 5° inclination for half a mile or so, then a nearly perpendicular step will have to he ascended, and then the regular descent is continued to the next step, which may be another ascent, or, on the other hand, it may be a descent. All of these valleys are, of course, clue to the action of Nature's most powerful graving-tool, running water; though rarely is running water now to be seen in them. What becomes of the water which after very heavy rain does come down them? The earthquakes attendant on the recent eruption have clearly explained this, as well as the nature of the steps. Every recent earthquake-crack follows along the face of these steps, generally about half-way up, sometimes on top, sometimes at the bottom, and more rarely a few yards off. Clearly the steps are old lines of earthquake-cracks, which have formed fissures—faults, in fact—in which the downthrow varies, sometimes in one direction, sometimes in the opposite. The running water coming down the valleys after rain, on meeting these fissures, naturally disappears into them. At the foot of the steps large holes or depressions are frequently seen, sometimes extending along them for considerable distances, marking where a subsidence of the surface has taken place. In the valley leading from Tutchu (see map) to Maungaongaonga (sec Plate 18A for drawing of the mountain) the following notes of the cracks were taken: The three nearest the latter mountain have a downthrow to the north-west of amounts varying from 6ft. to 12ft.; the other one a downthrow to the south-east of about 15ft. In the valley leading from Maungakakaramea towards Pareheru the three nearest the former mountain have a downthrow to the north-west of 10ft., 30ft., and 25ft.; the other three a downthrow to the south-east of 40ft., 50ft., and 15ft. The combined evidence of these two series tends to show that there has been a subsidence of the central area, which is north-west of the great fissure, of about 25ft. to 30ft. (See section in Plate No. 18B.) The long earthquake-crack shown near the Pareheru Bush was traced over hill and dale a distance of one and a half miles, the southern part of which has a parallel direction to the great fissure, but the northern portion tends more to the west. It has a downthrow to the south-east or towards the fissure varying from nothing to 60ft. or 80ft. In mentioning the downthrow, however, it must be understood that this is not due to the recent earthquakes, but to the more ancient page break
Maungaongaonga.

Maungaongaonga.

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Plate 18B

Plate 18B

Section Along Valley Near Pareheru,

Shewing Earthquake Faults.

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Regent Subsidence in old Earthquake Crack

Regent Subsidence in old Earthquake Crack

Also furrowed appearance of the mud covered hills. Road Rotorua to Wairoa

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Plate No 19

Plate No 19

Recent Subsidence on Old Earthquake Crack

From View

(Ninety feet deep)

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Recent Subsidence Follwing Old Earthquake Crack.

Recent Subsidence Follwing Old Earthquake Crack.

Road Rotorua to Wairoa. Looking South West

From a photograph by J .C. Blythe.

page 67 ones, which were of a much more serious nature; or perhaps the constant earthquakes for ages past may each have performed its part in causing a gradual subsidence.

Although these ancient lines of subsidence, or faults, are most plainly to be seen in the valleys, there is no difficulty in tracing them over the hills: in every case a depression will be seen where the subsidence has taken place. Plate No. 19 shows what is meant. This illustrates a recent subsidence along an old crack which occurs on the coach-road from Rotorua to the Wairoa. On the right of the picture the terrace or step which runs across the valley is seen, and by following it out with the eye it will be observed to cross the ridge in the background just under the little hillock. Similar depressions on the ridges can be seen in every other case noticed. The subsidence which has here taken place has, no doubt, been caused by the opening of the old crack by recent earthquakes in the first instance; and, as the covering of mud which there hides the face of the country no longer allows the rain-water to percolate slowly through the naturally porous surface, it has gathered in considerable volume near the crack, until its weight, charged with mud, has broken through into the old subterranean channel which had previously carried off the waters of the valley. This subsidence is, however, on a much larger scale than usual: the depth of the chasm formed is about 90ft.; it can be traced for a long distance over the hill to the north.*

In every case, in the neighbourhood of the great fissure, where these ancient lines of subsidence were seen, the recent earthquakes

* The study of the mud-and ash-covered surface of the country around the seat of the eruption affords some very instructive examples of the power of running water acting on loosely-compacted materials. The views generally held as to the slowness with which meteoric agencies act in altering the form of surface are likely to be a good deal modified—at any rate, in a country of similar origin—after a visit to the district. It is quite astonishing the additional power that running water gains when charged with mud as an excavating agent. Many of the little streams draining the mud-covered hills, which were never known to flood to any appreciable extent, now come down as great torrents, carrying along an immense load of dêbris, to be spread over the lower levels, whilst the channels are cleared out and widened in a remarkable manner. In several instances watercourses were seen, which owe their origin to the eruption, that had been worn out to a depth of over 100ft., whilst on the hills the furrows were running down the sides every few feet, and often very deep. In parts the country exhibits an excellent example of the usual form of hill-shading on maps, the hachures being formed by the watercourses; and, as these, of course, run in the same perpendicular lines, the resemblance is complete. Plato No. 17 gives some idea of this appearance. The water when charged with mud and sand, is able to effect in an hour or two what under ordinary circumstances could only be accomplished by water alone in many ages. It is perhaps the action of water charged in this manner that has caused the excavation of the many gullies in this country down which no water ever runs now, and the origin of which is most difficult to arrive at. Possibly the "pluvialists" would quote this country as evidence of the truth of their theory of a former state of the earth in which a vastly greater amount of rain fell than at present; but we have only to observe what water can do when the conditions are favourable—as they are here—to account for almost any amount of denudation, and that, too, within a very short space of time.

page 68 have cracked the surface—and nowhere else. The cracks are not large—generally from a few inches to 4ft. or 5ft. wide—hut they are sufficiently numerous in some parts to make it difficult to take a horse across country.

On the map will he seen a place called Waikorua, or Earthquake Plat. Here are to be seen several of the old lines of subsidence; and, as they are in this place closer together than usual, the intervening ground can plainly be seen to have subsided bodily for a few feet. The recent cracks follow the same lines, and have a general direction roughly parallel to the great fissure.* Some of them open out to a considerable width; one was noticed which appeared to have been in part occupied by hot springs or fumaroles formerly, for the sides were coloured as in many of the active fumaroles, and the rock had been in some cases decomposed by the action of the gases. The flat itself is a somewhat remarkable feature, having much the same appearance, though on a smaller scale, that Rotomahana Crater will have after the vegetation shall have covered its steep sides. It looks, in fact, just like one of the hydro-thermal craters which has possibly been occupied by a lake at a subsequent date, though the evidence of this is not very clear. Leading from it at each end is a long, dry, winding valley, with a gradual descent in both directions. The earthquake-cracks and faults in it are subsequent to its being occupied by a lake, if ever such lake existed.

Although the neighbourhood of the great fissure is that in which the greatest number of the steps or faults are found, they exist in considerable numbers in other parts, especially towards Rotorua, or between the fissures which have been described. They have a very general parallel direction, though some run nearly at right angles, and in one place they appear to radiate from Tikitapu Lake, as before referred to. It will be found that if the direction of the cracks southwest of the Southern Crater is continued for a few miles, it will lead, first, directly through a country in which are a large number of extinct hot springs and fumaroles, and then, at a short distance further, into the existing group of hot springs at Otamakokore and Waikite. These springs are situated immediately at the foot of the steep range of hills forming the northerly continuation of the Paeroa Mountains, which there is every reason to believe forms one side of a great fault or fissure.

* Major Mair states that after a heavy earthquake in 1865 he observed these same cracks to have been opened afresh.

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Ash field and Craters, as seen from Pareheru bush.

Ash field and Craters, as seen from Pareheru bush.

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Hills covered with mud as seen from Kaiteriria, Pah.

Hills covered with mud as seen from Kaiteriria, Pah.