The Bandai-san Eruption
Tokio, October 12th, 1888
In my letter dated 28th of July, giving some account of the Bandai-san explosion, I mentioned that, pending the reports of Japanese experts then at work upon the problem, it would be idle to look for any full and scientific explanation of that melancholy and most appalling catastrophe.
To the mere visitor little was evident but its tremendous effects.
Chaos, death and havoc, a deluge of earth, mud, and rocks, buried lands and hamlets, wrecked forests, shattered buildings, dammed-up rivers, and a vast, steaming crater-these left an impression on eye and mind awful and vivid indeed, yet to the last degree bewildering from their very profusion and immensity.
But the bare spectacle of results, even when supplemented by such accounts as might be gathered from scared survivors among the peasantry, was, after all, only half satisfying.
Though accomplished facts were patent and plentiful enough, the how and why of many strange particulars were still veiled in more or less mystery; and one came away from the spot reluctant and perplexed-athirst for the fuller explanation of things seen but not well understood, which skilled and protracted investigation alone could give.
This explanation is at length before us, as far, at least, as the patient labours of highly-trained specialists, extended over a period of between two and three months, have resulted in eliciting the truths of the case and throwing light on its greater mysteries.
Professor Sekiya, a young seismoligist already known to the world, and Mr. Y. Kikuchi, a geologist - both of the Imperial University - have embodied, in a paper read by the former last evening before the Seismological Society of Japan, as much as we are ever likely to know of a volcanic manifestation of the first order, investigated under highly favourable circumstances, and full of interest for the student of natural science.
Sho-Bandai-san, the peak that was destroyed on the 15th of July, is, or was, one of a group of four conical mountains, known collectively as Bandai-san, forming the walls of an old elevated crater basin, and rising to a height of some 6,000 ft.above the sea.
Stratified volcanic rocks, for the most part gneiss and andesite, form the bulk of this mountain mass, and are mainly disposed in six great layers, the fruits of as many successive eruptions.
Lava, apparently of prehistoric date, is found on the slopes.
But though Japanese records often speak of fire and smoke and poisonous vapours issuing from Bandai-san, the latest known active eruption took place 1,081 years ago, and all that remained to warrant the mountain's retention in the list of live volcanoes were a few solfatara in and near the old crater, Numanotaira, which from time immemorial have given off steam.
On the morning of the 15th of July, however, this condition of tranquility was suddenly and violently disturbed.
Soon after the mild preliminary earthquake, which took place at about half-past 7, there came a second and prolonged shock of fearful intensity.
Then, while the ground in the whole region was still heaving and groaning and making the houses rock, a dense black column was shot forth from Sho-Bandai-san to a height of some 4,000ft.
During the next minute there were 15 or 20 repetitions of this phenomenon, all of them accompanied by horrible and tremendous noises.
In the last of them, the ejectamenta took a course highly inclined to the vertical.
Zigzag flashes of lightning, resulting from the electricity generated by the steam explosions, were seen to shoot forth from the ascending columns.
Then, for another half-hour, the thunder of minor explosions were heard at frequent intervals.
Meanwhile, the lighter particles of the black columns, consisting of mingled steam and dust, rose steadily upward, attaining an altitude of some 12,000 or 15,000 feet above the volcano, and spreading out into a vast cloud like an open umbrella in shape, which shrouded the earth beneath it in midnight darkness, until dispersed and wafted away by the north-westerly wind.
From this cloud descended the shower of blue-gray ash, so-called, which has been mentioned in every account of the catastrophe-in reality, volcanic dust or powder(augite-andesite), caused by the violent mechanical disintegration of ejecterd rocks, hurled swiftly through the air after having been rendered brittle and soft by the action of steam and gases.
Highly heated itself, and mingling with the condensing steam, it assumed a fine granular shape and fell on the adjacent country in a solid, scalding rain, which caused shocking injuries to many individuals and clothed the ground with a hot mantle on which it was difficult and painfil to walk.
On the map this dust-strewn region has the shape of a half-open fan, and covers 1,040 square miles of land area, attaining at the Pacific shore, 62 miles from the volcano, a breadth of some 41 miles, and spreading yet farther over the ocean.
About six inches deep at and near its origin, the layer gradually diminished in thickness, till at the coast it was a barely perceptible film.
The noises of the explosions were heard some 30 miles to windward of Bandai-san, and 62 miles to leeward.
But the earthquake which preceded and attended the outburst, though so prolonged and terrible in intensity, was, strange to say, not felt beyond a radius of 30 miles from the volcano - a fact accounted for by Messrs. Sekiya and Kikuchi on the ground that the seat of violent action was doubtless but a little way below the earth's surface, if indeed, not above the mean periphery and in the bowels of the mountain itself.
Steam - a well-known and powerful cause of seismic phenomena - was, as has been aleady indicated, the agent of the explosions.
The great volumes of steam that must be generated whenever, from any cause, subterranean waters are brought into contact with the molten interior, expand and fill up the rock-fissures.
If not deep enough down, or if lacking sufficient pressure and volume to break through the superincumbent masses, such ebullitions, though they may wrench and strain and tear the earth's crust internally, are yet hopelessly imprisoned and can only produce on the surface the phenomena of earthquakes or minor seismic vibrations.
But there are cases - happily none too common in this our day - when the pent-up vapour succeeds in bursting open its prison roof along some line of least resistance and working havoc on a prodigious scale.
Of such was the explosion which lately rent Sho-Bandai-san in twain.
Besides the lighter erupted matter, whose nature and behaviour have been sketched above, there was the solid body of the peak itself, which, tossed in gigantic masses high into the air, fell upon the slopes and glens, and rushing down them with fearful velocity until brought to rest on level or nearly level ground or by impassable obstacles, buried 27 square miles of country fathoms deep in debris, in the short space of about ten minutes from the first explosion.
One of the toughest of the many problems which beset the Japanese investigators was that of accounting for the wonderful and apparently eccentric fashion in which this mighty volume of matter had been propagated and disposed.
Persevering examinations, however, soon brought them to intelligent conclusions: and these were confirmed before the very eyes of Professor Sekiya by an occurence which, though doubtless gratifying to that ardent seismologist, was not, as he drily remarked, a particularly comfortable incident of a solitary ramble.
One day, while he was at work in the crater, a huge slice of the precipitous rear-wall that had been bared by the explosions fell of a sudden, quite near to but happily clear of him, and crashed with a tremendous uproar down the steep mountain side.
This slab was about 1,000 feet high and of considerable thickness.
He witnessed its fall and its long descent.
He saw how the great masses of earth and rock were shattered as they fell, and broken up into bits ever growing smaller as the velocity and the distance increased and as the fragments were dashed against one another and against obstacles in their way, until they finally lost cohesion, and were reduced to a pulverised, almost impalpable state, not very different from that of sand.
The behaviour of the mass now resembled the rush of a headlong torrent.
Tough boulders, able to survive the ordeal, were of course mixed with the finer matter, and great rock masses from 20 to 30 feet in diameter were floated down on the surface.
But, as a whole, the movement approximated to that of a fluid.
No words, says the Professor, can describe the "fierceness and force" of that magnificent and impetuous downpour - its mad surgings this way and that, and the bold leaps with which it would now and then bound over low hills that hindered its progress, and shoot onward down the neighbouring depression.
Similar, though on a vastly greater scale, must have been the awful avalanches which darted down from Sho-Bandai-san in two principal streams on the fatal morning of the 15th of July.
These, it is now known, dashed over hills and ridges fully 100 feet in height, and Professor Sekiya's estimate that they must have attained a velocity of nearly 50 miles an hour sufficiently accounts for the swiftness of the fate that befell the doomed peasantry in the uplands and valleys below.
A part, doubtless, of the descending matter, mingling with the waters of ponds and lakes in its course, became a kind of mud, and may have been thus assisted in its flow; while that which reached the Nagase river and swallowed up so many of the Nagasaka villagers, as described in my last letter, acquired the consistency of a paste.
But by far the greater volume was never moistened, and must have derived its fluid or semi-fluid properties, from a rapid process of pulverization, after the manner witnessed by Professor Sekiya.
As for the crater, the researches conducted by the Japanese explorers now assign to the disrupted matter dimensions far in excess of all previous estimates.
In form, the crater-bed is roughly that of a horse-shoe, opening northward, and inclined slightly down from the apex to the mouth, where it is nearly 1.5 mile wide.
Its whole area is about 650 acres.
Round the crown of the shoe is a nearly vertical wall, 1,660 feet high, in front of which everything has been blown away.
But the peak itself, which was 540 feet higher than the summit of the crown, lay within the now empty space.
Thus, the three greatest dimensions of this gigantic projectile were, respectively, about 2,200, 7,500, and 7,800 feet.
From these and other particulats it has been possible to estimate very approximately both the volume and weight of the disrupted matter.
In my former narrative I ventured on the statement that, assuming the mean depth of debris over the buried area to be 15 feet, its weight would not be less than 700 millions of tons.
I added, however, that this depth was probably far short of the truth.
It proves to be only about one-fourth of the truth.
No fewer than 1,587 millions of cubic yards, weighing 2,880 millions of tons, and spread over 27 square miles of country to an average depth of 57 feet, are the approximate figures with which to estimate the power exerted in this latest manifestation of plutonic energy.
A great fissure, doubtless correpondent with the original line of least resistance, runs through the crater from its vertex nearly to its mouth.
It is marked by a long range of steam jets, large and small, which puff and hiss forth immense volumes of white, pungent vapours.
But Bandai-san is now perfectly at rest.
Delicate tromometers fail to detect the faintest throb upon its surface.
Only that row of jets remains, to tell of the fever that rages far beneath.
The terrible coup de vent that accompanied the explosions, and wrought such havoc in the forests and villages, were, of course corresponding phenomena to those which break windows and lay low the grass and plants at the firing of ordnance forged by men.
The difference was one of degree only, though some idea of its vastness may be gathered from the fact that in this case villages many miles from the scene were literally wrecked, while in the forests near the crater hundreds of trees three or four feet in diameter were laid prostrate on the ground.
Suddenness, from first to last, characterized the whole of this remarkable phenomenon.
There had been slight shocks of earthquake on the 8th, 9th, 10th, and 13th; also a momentary spasm at about 7 o'clock on the morning of the eruption, so feeble, however, that many persons failed to notice it.
Strange rumblings, taken for distant thunder, were heard in the mountains soon after 7.
But of palpable warning there was virtually none, with, perhaps, the bare exception that animals in the neighbourhood are said to have shown signs of uneasiness and fear shortly before the outburst.
That animals are highly susceptible to minute tremors of the ground is a well-established fact; and, as the earth in the vicinity must have been more or less affected before such an explosion as that of Bandai-san, it is quite conceivable that there may have been a succession of microseisms perceptible only to the delicate senses of quadrupeds and other dumb creatures.
Well-waters are said to have diminished in some places before eruption occurred.
But neither before nor after did the large Lake Inawashiro, to the south of the volcano, give any sign of being affected by it.
And, generally, it must be owned that the Bandai-san catastrophe and the phenomena preceding it have not brought us any nearer than we were before to the power of saying when-or even where - volcanic mountains may be expected to give vent to their hidden fury.
The Times - Nov.24th
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