Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 68

VII. Distribution and Effects of the Volcanic Ash

VII. Distribution and Effects of the Volcanic Ash.

The distribution of the ash from the Tarawera eruption is some what complicated, owing to the number and varying size of the vents page 61 and the action of currents of wind. In the case of a single vent, when wind is absent, the distribution of the ejected material is simple enough. The ejecta naturally fall to the greatest thickness near the vent; those which fall into the crater are again ejected, whilst, as the distance from it increases, the accumulation of matter becomes less. In this way a cone of fragmental materials is built up around a crater. The column of uprushing steam from the crater carries with it more or less of the finer materials, and, as it ceases to rise, it spreads out above equally in all directions; the solid matter begins to fall, and covers the ground for a larger or smaller circle round the cone with volcanic ash. Any wind blowing at the time will sweep the clouds of steam as they spread outwards to one side of the cone, and the ash will then be distributed over a fan-shaped area. In the present instance, however, the ejecta came from a scries of vents arranged along a line nine miles long, and the intensity of the explosion at the different vents was by no means equal : the ash from the different pints of eruption was carried to various heights in the air. Again, the distribution of the ash was influenced by the winds which blew during the eruption. Up to about 4 a.m. on the 10th June the winds blowing in the district were light and of a more or less local character, observers at different localities giving different statements. At Wairoa and Rotorua the wind seems to have been from the south. But about 4 a.m. the wind changed to the south-west and blew fiercely, driving the ash-laden cloud towards the Bay of Plenty. The rise of this wind was not caused by the eruption, but was generally felt over a wide area.

The influence of these circumstances on the distribution of the volcanic ash will be best gathered from the map prepared for this report, showing the depth of the deposit over the surrounding country (see Map I.). This map is founded on over a thousand measurements of the thickness of the deposit, made during extensive travels over the affected area. For the thickness of the deposit east of Whakatane and Galatea I am, however, indebted to gentlemen residing in the district; the other measurements were made by myself. The map, of course, represents the average thickness of the deposit, and it has not been possible (on account of the scale) to take notice of small local variations.*

Area covered with Ash.—The total area of land over which the ash fell is not less than 6,120 square miles. The lines on the map show the limits of the areas where the deposit reached or exceeded

* The chief local variation was near Te Puke, where there was a slight thickening at the deposit.

page 62 the thickness of 1in., 2in., 3in., 6in., 12in., and 3ft. respectively The following table shows the areas between the lines of equal thickness of deposit :—
Area in Square Miles.
Covered with ash over 3ft. deep 82
Ditto, between 1ft. and 3ft. deep 198
Ditto, between 1ft. and 6in. deep 263
Ditto, between 6in. and 3in. deep 245
Ditto, between 3in. and 2in. deep 354
Ditto, between 2in. and lin. deep 2,698
Ditto, lin. and less 2,280
Total area of land covered with ash 6,120

The large area of country covered with fine ash to a depth of less that 2in. testifies to the force of the wind and the fine state of division of much of the ash.

The limit of the deposit near the sea-coast is near Tairua, on the north side. The southern limit is at Anaura, according to Arch deacon Williams; but Mr. Stubbs, of the Southern Cross Petroled Works, near Waiapu, in a letter for which I am indebted to Professor Hutton, states that a little fell in Tolago Bay township, ten miles south of Anaura. The direct distance between Tairua and Tolago Bay is 165 miles.

The limit of the ash on the land can be ascertained with sufficient accuracy; but a good deal of the finer ash was blown out to see and we have no means of estimating how much, though its thickness on some of the outlying islands affords some slight indication Captain Fairchild found the deposit a quarter of an inch thick on Mayor Island, and an eighth of an inch on the Aldermen Islands. On Whale Island the ash was nearly 2¼ in. in thickness. Ash fell into the sea, as was observed by vessels crossing the Bay of Plenty, on the morning of the 10th June. Ash fell on the s.s. "Southern Cross whilst off the East Cape, which is 122 miles distant from Tarawera and on the s.s. "Wellington" near Mayor Island. When the s.s. "Hinemoa" crossed the Bay of Plenty four days after the eruption the air was still charged with fine floating dust.

Knowing the area of country covered with ash and its depth, it is not difficult to calculate approximately the volume and weigh ash which fell. The chief difficulty arises from the very unequal depth of the deposit near the fissure. It will be sufficient to state here that the estimated bulk of ash, &c., amounts in round numbers to 1,660,000,000 cubic yards. To this must be added that which feel page 63 into the sea, probably not less than 300,000,000 cubic yards. The total bulk of the ash, &c., would therefore amount to 1,900,000,000 cubic yards, or nearly two-fifths of a cubic mile.

On looking at the map it will be noticed how closely the lines of equal deposit approach each other to the south-west of the fissure. This, of course, is due to the strong south-west wind, which drove the ash-laden clouds in the opposite direction. It will further be noticed that in the opposite direction—more exactly, to N.E. by N., i.e. in the direction of Whakatane—the curved lines of equal deposit are prolonged into deep bays corresponding to the maximum deposit of ash along this line.

The curves also show that to the north-west of the fissure the ash fell more abundantly than on the other side. An examination of the ash from places in this direction throws some light on the point, At the Wairoa the ash is of a light-grey colour, but, when treated as described above, shows a proportion of black scoriæ varying from 7 to 22 per cent, by weight. The ash fell in more or less definite layers: some of these were richer in scoria; than others. The proportion of sand in the ash from the different layers varies between 37 and per cent., and the fine impalpable dust amounts to 41 to 49 per cent. Both sand and dust contain a small percentage of augite-andesite, but this does not exceed 25 per cent. It is clear, therefore, that the Wairoa deposit consists most largely of rhyolite, and, though a part of this may have been derived from the mountain, yet there can be little doubt that most of it came from the lower ground. At the west end of Rotoiti the proportion of augite-andesite in the ash is about per cent., and at Tauranga it is nearly 50 per cent. When we compare these ashes with the deposit from places more to the east we at once notice a striking difference. Even at the east end of Rotoiti the proportion of augite-andesite is much greater—about 85 per cent.—whilst passing along the coast from Tauranga to Whakatane and Opotiki we note a gradual rise in the proportion of the black andesite. At Whakatane there must be 80 per cent, of the andesite; and still farther to the east, at Opotiki and Cape Runaway, the production reaches or exceeds 90 per cent.*

To the south-east side of the fissure the ash thins out at first more rapidly than in the opposite direction, and as soon as we get a few miles away from the Rotomahana Crater the andesite forms a much larger proportion of the ash than is the case at the correspond-

* The estimates are based on a comparison of the washed ashes with various artificial mixtures of rhyolite and andesite, in which the proportions of the two rocks are known.

page 64 ing distance to the north-west. So too all along the road from Galatea to Whakatane the andesite forms three-fourths or more of the ash. The ash here is generally very coarse for the distance from the mountain, consisting mainly of lapilli or very fine scoriæ below, with a thin layer of finer sand and dust above.

These facts in the distribution of the ash are to be explained as follows : During the earlier part of the eruption a gentle wind, having a general south-east direction, was blowing in the neighbourhood of Rotomahana and Tarawera. The ash was therefore carried to the north-east, the cloud from Rotomahana and Rotomakariri passing in the direction of the Wairoa and Rotorua, and that from Tarawera keeping in a parallel direction, but more to the east. There was, of course, at the same time a large amount of mingling of the clouds. Then, before any considerable amount of ash had fallen, the strong south-west wind began to blow, and carried the cloud, which now had a very wide extent, in the direction of the Bay of Plenty, the clouds of steam struck by the cold south-west currents of air were partly condensed, and the rain carried down a quantity of the ash, including much of the finest dust, and so, from the Wairoa to Rotoiti, a good deal of the ash fell either as mud or in a moist, flaky condition, which wanted very little rain to convert it into mud. At the Wairoa it is stated that there was no difficulty in walking about on the surface of the ash next morning, hut after had been trampled on for a day or two the surface along the road was converted into mud. In a hole dug at the Wairoa for the purpose of examining the ash the depth was 2ft. 10in., and one of the deeper layers was in a very wet state. The mud cannot be supposed to have come direct from Rotomahana, for no doubt the escape of the superheated steam and the force of the explosions would dispel the water of the lake in vapour. Wherever I had an opportunity of examining sections of the ash near Rotomahana Crater (a month after the eruption), whether in slips or in holes dug for the purpose the ash, though wet on the surface by the rain, was found to be dry and warm for many feet below.

The larger proportion of the rhyolite in the ash to the north-west of the deposit thus receives a simple explanation. It must remembered, however, that the ash there still contains a considerable quantity of the augite-andesite from the mountain.

Effects of the Ash on Vegetation.—Forests of large trees grew on the sides of the Tarawera Mountain, and these of course were wholly wrecked by the eruption. Those growing on the site of the Tarawera Chasm disappeared altogether, whilst at a distance many of the trees page 65 were prostrated by the falling débris, or the electric discharges attending the eruption. One totara tree in particular presents a remarkable sight: its trunk was 8ft. in diameter, and the top of the tree has been twisted off at some distance from the ground, and the stump with long projecting splinters stands up above the ash. At a distance from the fissure the trees are denuded of all foliage and mailer branches, and their stumps, bearing a few battered lengths of the larger branches, project from the ash.

Still farther away the forests suffered a great deal, as for instance the well-known Tikitapu Bush. A remarkable difference is shown, however, in different places according to the condition in which the ash fell. In the Tikitapu Bush and the forests on Mount Edgecombe the deposit does not differ much in its thickness, though it differs widely in its physical characters. In the former place it fell in a moist condition, almost as mud, so as to adhere to the foliage of the trees. The bush was apparently completely destroyed, the foliage and smaller branches were stripped from the trees, and the large timber over several acres was prostrated by the strong wind which, owing from the south-west up Rotokakahi, was deflected by the hills at its end up the valley to Tikitapu, and down the valley to the Wairoa. In both places the trees were thrown down, but their heads point in opposite directions. On Mount Edgecombe the ash fell chiefly as fine scoriæ; and on the sides of the crater, where I found 14in of ash, all the large trees seemed quite uninjured: large rimus were none the worse for the shower, though in the Tikitapu bush they were killed. During the following summer most of smaller soft-wooded trees in the Tikitapu Bush revived, putting out numerous fresh shoots; the larger hard-wooded trees, however, like the rimu and rata, 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 tree-ferns and bracken, were scorched quickly turned brown.

Some plants appear to withstand the effects of a volcanic eruption much better than others, but these are not always the most useful kinds. The docks near the Ngae were up through 4½in. of volcanic ash within five weeks of the eruption, although it was mid-winter. During the following summer it was noticed that the bracken-fern was the plant which appeared to be able to penetrate the greatest thickness of ash When this was 2ft. in thickness the fern was able to force its way up to the surface, though it appeared rather later in the season than it would otherwise have done. We have here an excellent example of the survival of the fittest, and it is, I think, à page 66 matter worthy of consideration, whether the prevalence of the fern over extensive areas of the North Island, where showers of ashes have been of frequent occurrence in past time, is not partly connected with its power of surviving such adverse conditions. No doubt fern grows abundantly elsewhere, but the power it has of coming up through great thicknesses of volcanic ash must have been of considerable use to it in New Zealand in former times.

Effect of Ash on Agriculture.—With the exception of a few patches at the Wairoa and at the settlements on Lake Tarawera 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 character, being formed from the decay of rhyolitic rocks, and often consisted largely of pumice. The rhyolites 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, but it is the latter which must be regarded in estimating the character of the soil which the rock is likely to form, for although 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 rhyolites is in excess of any requirements, Part of the silica, too, is free in the form of quartz-crystals, which 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 rock decays 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 the atmosphere and vegetation it may be expected to yield a distinctly richer soil than the pumiceous one existing before.

Between the mouths of the Tarawera and Whakatane Rivers is a large area of low-lying land, the Te Teko Swamp. This has received a coating of ash 3in. to 6in. in thickness, and much more of the loose ash will doubtless be washed down into it from the surrounding hills. Not only is the level of the swamp raised, but the soil, rich in vegetable matter, will be distinctly improved by the addition of the ash.

In the agricultural settlements along the Bay of Plenty the deposit of ash nowhere exceeded 3in., and as a rule was less, No page 67 doubt even 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 the deposit was felt within a few months, for the old grass soon came up through the top-dressing of ash, and when I visited these settlements in the following summer an abundance of grass was to be seen everywhere, and the farmers reported that there had never been so abundant a supply as during that summer. The maize crops at Whakatane and Opotiki were also unusually heavy,

The ash, as shown by an analysis of Tauranga ash by Professor Brown, contained an appreciable amount of soluble matter (.3 per cent.), consisting of sulphates and chlorides of lime, soda and potash (the last substance in very small quantity), together with a little free hydrochloric acid. These substances were 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.

Action of Rain upon the Ash.—The action of the rain upon the deposit of ash is a matter of much interest and importance in a country like the North Island of New Zealand, where great showers of ashes have frequently occurred in the prehistoric period. Many of the river-plains are formed of the débris of volcanic rock, much of which must have been brought by running water from ground covered by volcanic showers. The valleys have been filled up to enormous depths with pumice sand and gravel, out of which the rivers have subsequently carved their channels, leaving the sides marked by terraces, which indicate the successive heights at which the rivers have flowed. The formation of these terraces must have been connected with movements of the land, but in the prevalence of volcanic showers we have the source of the vast quantities of finely-divided material. Observation therefore of the mode of treatment of Be present shower by rain and running water will throw much light on the past history of the North Island. Already evidence is afforded of a rapidity of denudation and deposit which is far in excess of that to be ordinarily observed.

At the time of the eruption fears were expressed that where the ash fell in a state approaching to mud it would, under the influence of rain, be converted into thin mud, which would slide down the Slopes of the hills and fill up the valleys, and so prove a source of much danger. Such fears proved groundless. The ash or mud seldom slipped except in places where the slopes were precipitous. On slopes of 30° the ash showed little or no tendency to slip.

page 68

What really happened was as follows: To the west of the fissure the ash contained a large proportion of fine dust-like material, which rendered it somewhat impervious to water, and everywhere at a distance the last portion of the ash to fall was naturally the finest and formed a thin layer on the surface, through which the rain did not readily penetrate. This fine material generally formed a cake on the surface with the first showers, and subsequent rain streamed off the surface rapidly. This running water wore for itself channels in the ash, and every time rain fell these were deepened. The result was the formation of great numbers of deep and narrow watercourses. These are set closely side by side everywhere except along the tops of the spurs and ridges. It is almost a matter of impossibility to travel along the sides of the hills near Rotomahana, owing to the great number of these watercourses, varying in size, according to the distance below the top of the hill, from mere gutters to narrow gullies 40ft. deep and a few feet in width. Often enough, though they may be deep, they are sufficiently narrow to jump across; but the interval before the next parallel gully is not wide enough to permit one to alight at a safe distance from the margin, which crumbles when any unusual weight is brought to bear upon it. These watercourses are, of course, arranged in systems on the sides of the hills around every minor drainage-basin. The watercourses begin as very numerous small gutters, which stretch downhill, and, as they descend, join together to form larger and deeper gutters, These, again, join to form small watercourses, and so on, all the very numerous watercourses uniting in the central channel at the bottom of the valley. The appearance of these channels in the ash is seen in Pl. p. 46. The photograph was taken three months after the eruption, and the channels have since been greatly enlarged. Where the proportion of the finest dust in the ash is large it has set to form a material like a soft inferior concrete; and here the water courses are usually very narrow. Where the deposit contained less of the fine dust and more of the sand, stones, and scoriæ the watercourses are wider and less deep.

The watercourses mentioned above are not the deepest which have been formed on the deposit. The stream which occupies the valley of the Haumi and flows into Rotomahana Crater to the south-west has excavated for itself a gully 200ft. in depth. In various parts near Rotomahana are to be found dry gullies 40ft. to 50ft. or more in depth, and 30ft. to 100ft. in width, all excavated within eight months.

On the outskirts of the deposit, for instance near Rotorua, the page 69 rapid streaming of the rain-water from the surface of the ash caused the rivers to rise in flood, and a number of bridges were washed away, whilst the water flowing in such increased volume cut down deeply into the old beds of the watercourses, so that considerable gullies were formed or enlarged. Near the Ngae a gully 20ft. deep, 120ft. long, and 40ft. wide was formed in the course of a single rain-storm. Between Lake Tarawera and Rotoiti the sides of the hills were washed out and numerous landslips were caused. So, too, near the Wairoa Road, large gullies were washed out on the hill-sides, whilst the road was rendered impassable by the numerous watercourses which cut across it. It is to be noticed, further, that this denudation is not confined to the removal of the deposit of ash, but that the underlying rocks were cut into and carried away.

The excavation of the watercourses and the washing-away of the Bill-sides implies, of course, the removal of a large quantity of material, to be deposited elsewhere. Much of that washed off the hills accumulates in the valleys, but a good deal of it has been carried down by the streams into the numerous lakes of the district. In Rotorua, Tikitapu, and Tarawera, as well as other lakes, beautiful examples of fan-shaped alluvial deltas have been thrown out into the waters. The waves on the lake tend to destroy these fans, so that the result is the accumulation of a strip of land along the margin of the shore, or the filling-up of the bays.

Much of the ash is also carried by the larger rivers to the sea. The Tarawera River, which before the eruption bad perfectly clear water, has ever since flowed with milk-white stream. Throughout the summer of 1886—87 the sea near its mouth in the Bay of Plenty was quite turbid and milky for the distance of a mile or two from the shore.

The rate of geological change, both with respect to denudation and consequent deposit, is therefore greatly in excess of that which is commonly recognised. The present shower of ashes is by no means the most considerable with which the country has been visited, and the present high rate of geological change is not a phenomenon of isolated occurrence, but one of frequent repetition.