Popular Science Monthly/Volume 25/July 1884/The Volcanic Eruption of Krakatau
|THE VOLCANIC ERUPTION OF KRAKATAU.|
CONSIDERING that the volcanic eruption, of which the Straits of Sunda have been for the last eight months the center, is among the most stupendous of our times, and that the attendant phenomena have given rise to many questions of the highest scientific and, we may add, geographical interest, a résumé of the facts compiled from all the latest available sources may be interesting to our readers.
The Island of Krakatau (such, and not Krakatoa, is the native name) is situated in latitude 6° 7' south, longitude 105° 26' east, in the fairway of the Sunda Straits, about equally distant from Java and Sumatra, close on twenty-six miles west-southwest from the village and lighthouse of Anjer, the call-port or signal-station, prior to the present eruption, for all vessels passing through that frequented channel. It was a small, uninhabited island about five miles in length and three in breadth, culminating in two elevations, the taller of which, known as the Peak of Krakatau, rises (or did rise) some 2,750 feet above the sea. Surrounding it on all sides are numerous volcanic cones. The Tengamoes (or Kaiser's Peak) to its northwest is situated at the head of the Semangka Bay, and the quiescent Rajabasa to its northeast in the southern promontory of Sumatra; in the east by south the Karang smolders in Bantam, and southeast rise the active cones of the Buitenzorg Mountains. Standing in the straits and very little to the north of Krakatau are the two dormant or dead cones of Sebesie and Sebooko. A line drawn from Rajabasa, passing along the western side of Krakatau, and continued thence to Prince's Island, which lies off Java Head, would mark the boundary on the eastward side of the shallow Java Sea, which rarely exceeds fifty fathoms, and on the west side of the deep Indian Ocean. On looking at the accompanying map of the locality before the eruption it will be seen that close to the east and northwest sides of Krakatau there are two small fragments of land, Lang and Verlatin Islands respectively. It is Mr. Norman Lockyer's opinion that these are two higher edges of the old rim of a subsided crater, overflowed in part by the sea through inequalities in the margin between them; that the heights on Krakatau itself, the remaining portion of the old volcano summit, are cones elevated on this old crater-floor; and that the ancient funnel is practically coextensive with the area inclosed by these three islets, though till the 20th of May last blocked up by volcanic débris.The earliest accounts of Krakatau we have been able to obtain are contained in a curious old volume, "Aenmerckelijke Reysen van Elias Hesse nae en in Oost-Indien van't jaar 1680 tot 1684" ("Remarkable Journeys of Elias Hesse to the East Indies from the Year 1680 to 1684"), published in Utrecht in 1694. The author relates that he passed on the 19th of November, 1681, "the Island of Cracatouw, which is uninhabited. It had about a year before broken out in eruption. It can be seen far at sea, when one is still many miles distant from it, on account of the continually ascending smoke of the fire; we were with our ship very close under the shore; we could perfectly well and accurately see the wholly burned trees on the top of the mountain, but not the fire itself." About the same period Johann Wilhelm Vogel, one of the Dutch East India Company's servants, who published in 1716 a very interesting account of his travels there, passed through the straits. He says: "On February 1, 1681, by God's help, in front of the Straits of Sunda, where, with great astonishment, I saw that the island of Cracketouw, which on my former journey to Sumatra appeared so very green and gay with trees, lay now altogether burned up and waste before our eyes, and spued out fire from great fire-holes. And on inquiry at the ship, Captain . . ., at what time it broke out, . . . I was told that it was in May, 1680. . . . The former year, and when he was on his voyage from Bengal, he had met with a
great storm, and about ten miles from this island he encountered an earthquake on the sea, followed by most frightful thunders and cracklings, from which he imagined that an island, or else a piece of the land, had burst up, and shortly thereafter, as they drew a little closer with the ship to the land, and were come near to the mouth of the Sunda Straits, it was evident that the Island of Cracketouw had burst out; and his conjecture was correct, for he and all the ship's company perceived the strong sulphur-atmosphere, also the sea covered with pumice, . . . which they scooped up as curiosities." Save for the observations of passing travelers, by whom the great beauty of its tree-clad slopes, the first verdant spot to meet the eye after weary weeks at sea, has been gratefully described, the volcano, after it died out, has had an uneventful and unrecorded history.
On the 20th of May last year, at half-past ten in the forenoon, the inhabitants of Batavia were astounded by hearing a dull, booming noise, whether proceeding from the air or from below was doubtful, soon followed by the forcible drumming and rattling of all the doors and windows in the place. The commotion was strongest between half-past ten and one o'clock in the day, and between seven and eight in the evening. About midday a curious circumstance was observed—that in some spots in the city no vibrations were perceived, although the surrounding buildings were experiencing them. It was at once concluded that a volcanic eruption of an alarming character had taken place, but for some time it was impossible to localize the direction of the sounds, though the west was the quarter of the compass to which most people assigned them.
A report, issued next day by the director of the observatory in Batavia, stated that, as he had no instruments for recording the intensity and direction of earthquake-shocks, he could certify only that no increase of earth magnetism accompanied the tremblings—the photographs indicating nothing abnormal; and that the quivering was absolutely vertical throughout the periods mentioned above; for a suspended magnet with an exact registering apparatus gave no indications of the slightest horizontal oscillations, but alone of vertical vibrations. This was verified by the observations of one of the philosophical-instrument makers in the town on a pendulum in his shop, where only vertical trillings were observable at a time when the windows and glass doors of the house were rattling, just as if shaken by the hand, in so violent a way that it was difficult to carry on conversation. Nowhere, however, do there seem to have been observed any shocks of a true or undulatory earthquake. From midnight of the 20th throughout the forenoon of the 21st the tremulations continued very distinct. The same morning a thin sprinkling of ashes fell, "whence, is not known," both at Telok-betong and at Semangka, situated in Sumatra at the head of the Lampong and Semangka Bays respectively. At Buitenzorg, thirty miles south of Batavia, the same phenomena were observed; while in the mountains farther to the southwest they were even more pronounced, and the Karang, a mountain situated about west from Batavia, it was thought must be the seat of disturbance. By this time the general opinion had decidedly ascribed to the west or northwest the direction whence the movements were proceeding. Krakatau itself was even named; but some of the Sumatran mountains were considered more likely to be the delinquents. Batavia being connected with that island by a telegraph line passing along the north coast of Java to Anjer, across the Straits of Sunda to Telok-betong, thence northward to Palembang on the east, and to Padang on the west coast, intelligence from all parts soon began to come in; but none of any eruption anywhere, beyond the notice of the fall of ashes mentioned above. Anjer telegraphed, "Nothing of the nature of an earthquake known or felt here." This was dated the 21st; a message in much the same terms had been received on the previous day, as well as the report of one of the Government officials to the following effect: "On Sunday morning, the 20th, I landed at Anjer, and there staid till one o'clock in the afternoon; at half-past three I reached Serang, and halted an hour. Neither I nor my coachman, either at Anjer or at Serang, or on my whole journey to Tangerang (near Batavia), felt or heard any earthquake or disturbance, or anything at all remarkable."
Anjer lies on the narrowest part of the Sunda Strait, twenty-seven miles from Krakatau, which formed a prominent object in one's seaward view from the veranda of its quiet little hotel on the sea-margin. This hotel was kept by one of Lloyd's agents, Mr. Schuit (whose family perished in the subsequent disaster), who had in his veranda a powerful telescope for reading the signals of ships for report to Batavia, and by whom consequently any occurrence in the strait could scarcely fail to be observed. Thus during the period of greatest disturbance in Batavia and Buitenzorg, when men there were referring the origin to Krakatau, eighty miles away, at Anjer, only twenty-seven miles distant from it, nothing was felt or heard. The same report was made from Merak, likewise situated on the straits, thirty-five miles from and presenting a clear outlook to the volcano. The winds prevalent in this region during the month of May are from the east, and would tend to drive any smoke and ashes toward the Indian Ocean, which might explain their not being detected from Anjer; but the direction of the wind fails to account for the entire absence in that and the surrounding villages of the phenomena which were most conspicuous in Batavia.
Not till the evening of the 21st was smoke observed to be issuing from Krakatau; on the 22d the volcanic vent there seems to have been fully established, and the vibrations and other phenomena experienced in Batavia quickly subsided. Now, in a letter to "Nature," Mr. H. O. Forbes has recorded the passage, during the 11th and 12th of July, of the ship (on board which he was returning to England) through extensive fields of pumice spread over the ocean north and south as far as the eye could reach. The vessel passed the volcano on the 9th, but till the evening of the 10th, when the steamer would be about a degree to the west (a little northerly) of her noon position, which was 102° 25' east longitude, 6° 20' south latitude, no pumice was observed. During the whole of the 1 1th the vessel was surrounded by the pumice-sheet, which about noon of the 12th, in 93° 54' east longitude, 5° 53' south latitude, suddenly terminated, shortly after it had appeared in greatest amount, while a current had been encountered after leaving the entrance to the straits, running against the ship's course at the rate of a quarter of a mile an hour. The pumice -nodules were considerably worn, but many pieces were observed as large as a child's head. Several lumps were picked up infested with barnacles, of from one to one and a half inch in length, which represented at least some four or five weeks' growth.
The specimens of pumice obtained at sea have been submitted to Professor Judd and the committee appointed by the Royal Society for the examination of the phenomena connected with the eruption. If, on analysis, they should prove different in composition from specimens obtained directly from the volcano, a different origin will have been established for them; but, should both turn out to have identically the same components, it will not necessarily prove that both have come from the same crater. The Peninsular and Oriental Company's steamer Siam, on her voyage from King George's Sound to Colombo, sailed for four hours, on August 1st, through a similar "lava" (puraice) sheet, in latitude 6° south, and 89° east longitude, the nearest land, the coast of Sumatra, being seven hundred miles off, and the current then running eastward at from fifteen to thirty miles a day. The soundings at the spot reached two thousand fathoms. Mr. Forbes, who incidentally referred to the eruption when reading his paper before the society on the 28th of January last, suggested that the sounds heard in Batavia on the 20th of May, which were altogether unperceived at spots so near Krakatau as Anjer, Merak, and Telok-betong, which would be inexplicable if they really originated there, were the result of a submarine eruption in the Indian Ocean, somewhere southwesterly from Java Head; and that the tremors were propagated thither perhaps by continuous strata connecting the locale of the outburst with Batavia, Buitenzorg, and more especially with the hills to the southwest, where the manifestations were so distinctly perceived. We know from Mr. Darwin's and Mr. Forbes's observations, that the center of volcanic disturbance does exist in that direction, in the Keeling Atoll, situated six hundred miles west by south from the mouth of the straits. Whether or not anything unusual has been experienced in these islands about the third week of May, no intelligence has yet reached this country. We know, from what occurred at Graham's Island, that pumice ejected from the sea-bottom rises to the surface, and an examination of the chart of the currents in the Indian Ocean at once shows that any flotsam in the region between west and south of Java Head in that longitude could be drifted to the locality in which it was observed in the month of July. If such a submarine outburst did take place, Mr. Forbes suggested that somehow the orifice very soon became blocked after a great in-rush of water had taken place, which, becoming transformed into steam under enormous pressure, shaped its course for the nearest old earth-scar, and found vent in Krakatau, by an offshoot probably of the funnel of the eruption of 1680. That such large lumps of pumice should be carried seven hundred miles westward into the Indian Ocean does not seem probable, and is not supported by any observations. The earlier outbursts were not of very unwonted vigor, for no pieces of any size are reported to have fallen on the neighboring coasts of Java and Sumatra; even after those of August, no ship farther off than one hundred miles speaks of the fall of any but the "finest dust and sand."
On the 23d of May, a ship encountered at Flat Cape, in Sumatra, a large amount of pumice on the sea, which increased in amount as Krakatau was neared. Of the appearance of the volcano on the 27th, we have a graphic account in the "Algemeen Dagblad" newspaper, of Batavia, by one of a party that ascended to the crater on that day. As they approached the scene, the neighboring islands had the appearance of being covered with snow. The crater was seen to be situated not on the peak, but in a hollow of the ground, which lay from southeast to northwest, sloping toward the north point, in front and to the north side of the lower summit, looking toward Verlaten Island. Both heights were seen; the southerly green, and the more northerly and much lower one quite covered with dust and ashes. The volcano was ejecting, with a great noise, masses of pumice, molten stone, and volumes of steam and smoke, part of which was being carried away westward by the monsoon wind, dropping all round and close at hand its larger pieces, while a higher rising cloud is specially recorded as driving away eastward, having evidently encountered a current in that direction in the upper air. Some of this dust-cloud was carried far to the eastward, for Mr. Forbes relates that on the morning of the 24th of May, when in the Island of Timor, twelve hundred miles distant, he observed on the veranda of his hut, situated high in the hills behind Dilly, a sprinkling of small particles of a grayish cinder, to which his attention was more particularly drawn later on that and the next day by their repeated falling with a sudden pat on the page before him. The visitors to the crater seemed to have viewed with most amazement the grandeur of the smoke-column whirling upward with a terrific roar like a gigantic whirlwind, through whose sides the ascending ejecta, vainly trying to break, were constantly sucked back and borne upward round and round in the center of its Stygian coils. The trees which once clothed this portion of the island presented only bare stems from which their crowns had disappeared, evidently not by fire, for there was no charring visible on them, but rather as if wrenched off by a whirlwind—perhaps of the crater itself.
After the 28th, curiosity in these volcanic phenomena seems to have abated, and during the next eight or nine weeks, though the eruption continued with great vigor, little is recorded of its progress; indeed, so completely did it seem to have been forgotten, that visitors to Batavia, unless they had made inquiries, might have failed to hear of its existence at all. During this period no local disturbances to attract attention or to cause the least alarm are recorded. From the logs of ships in the neighborhood of the straits, about the middle of August, numerous extracts have been published; but many of them show that they have been written either with the mind bewildered and confused by the terrifying incidents amid which the officers found themselves, or from the after-recollection of the events, of which under such conditions the important dry facts of time, place, and succession, are liable to be unconsciously misstated. Much is therefore lost which might have been known; but a few are of the utmost value.
On the 21st of August the volcano appears to have been in increased activity; for the ship Bay of Naples reports being unable to venture into the straits on account of the great fall of pumice and ashes.
The first, however, of the more disastrous effects were experienced on the evening of the 26th, commencing about four o'clock in the afternoon. They were inaugurated by violent explosions heard in Anjer, Telok-betong, and as far as Batavia, accompanied by high waves, which after first retreating rolled upon both sides of the straits, causing much damage to the villages there, and were followed by a night of unusually pitchy darkness. These horrors continued all night with increasing violence, till midnight, when they were augmented by electrical phenomena on a terrifying scale, which enveloped not only the ships in the vicinity but embraced those at a distance of even ten to a dozen miles. As the lurid gleams that played on the gigantic column of smoke and ashes were seen in Batavia, eighty miles off in a straight line, we can form some idea of the great height to which the débris, some of which fell as fine ashes in Cheribon, five hundred miles to the east, was being ejected during the night.
Between five and seven o'clock (for the hour is uncertain) in the morning of the 27th, there was a still more gigantic explosion, heard in the Andaman Islands and in India, which produced along both shores of the strait an immense tidal movement, first of recession and then of unwonted rise, occasioning that calamitous loss of life of which we have all heard.
The material thrown out rose to an elevation which we have no means of estimating, but so tremendous was it that on spreading itself out it covered the whole western end of Java and the south of Sumatra for hundreds of square miles with a pall of impenetrable darkness. During this period abnormal atmospheric and magnetic displays were observed; compass-needles rotated violently, and the barometer rose and fell many tenths of an inch in a minute. Following at no great interval, and somewhere between ten and twelve o'clock in the forenoon of the same day, either by successive rapid outbursts or by one single supreme convulsion, the subterranean powers burst their prison-walls with a detonation so terrific as to have been, as it seems, inaudible from its very immensity to human ears in its close vicinity, but which spread consternation and alarm among the dwellers within a circle whose diameter lay across nearly three thousand miles, or fifty degrees of longitude.
With sunrise on the 28th the dense curtain which had enveloped so wide an area in darkness gradually began to clear off, and the light broke on a scene of devastation of the saddest kind, but on one of comparative placidity, as if Nature lay exhausted after her frantic paroxysm. Krakatau was seen reduced to a fraction of its original size; the whole of the northern portion, with the height in front of which the volcano first broke out, and half of the peak itself, had vanished (see the accompanying map). To the northward, however, two new pieces of land, which have received the names of Steers and Calmeyer Islands, raised their tops above the surface of the sea, where the morning previous thirty to forty fathoms of water had existed. Of the two islets on each side of Krakatau, Lang Island is left practically unaltered, while Verlaten Island seems elevated somewhat, and is reported to be in eruption. But, where the volcano had been so active a few hours before, a sea fathomless with a line of a thousand feet is now to be found.
Having thus followed the succession of events, there remains little doubt that the crater on the 26th of August by its constant action had either cleared out the old funnel into its submerged portion, or that a rent by subsidence or otherwise was formed, through which a volume of water was admitted to the heated interior, resulting in explosion after explosion in increasing violence, as more material for generating steam was finding its way into the underground recesses.
The first great waves on the evening of the 26th and the early part of the 27th were probably caused by a portion of Krakatau being shot out northward for eight miles and dropped where we have now Steers Island; while the appalling detonation in that forenoon and the greater wave accompanying it resulted perhaps from that still more Titanic effort which lifted the greater portion of Krakatau—several thousand million cubic yards of material—out by its one hundred and seventy fathom root, hurled it through the air over Lang Island, and plunged it into the sea some seven miles to the northeast, where Calmeyer Island now blocks the channel which mariners have known so long as the East Passage.
The reports we have as to the tidal phenomena differ from different places. At many points it was observed that a distinct withdrawal of the water preceded the rise or great tide; while from others, as in the canal at Batavia, the opposite is given as the order of occurrence. Everything, however, depends on the moment of the observation. It will be apparent that these waves were the most natural consequents of the events, and were due certainly not to any seismic movement of the sea-bed, but, on the one hand, to the in-rush of water to fill the deep chasms out of which the ejected portions of the island came, which was naturally followed first by a withdrawal of the water, and then by a disastrous recoil over the low fore-shores of Java and Sumatra; and on the other hand to the tremendous stroke—the splash, in fact—imparted to the sea by such a gigantic block of matter, square miles in size, which must have resulted first in a great rise of water, followed by a withdrawal.
It is a remarkable circumstance that in the logs of several ships which were in the close vicinity of the volcano in the forenoon of the 27th, no mention is made of the great wave which proved so destructive, and which could scarcely, one conceives, have failed to attract attention. May the explanation not lie in the supposition that these two great waves—the in-rush and the splash waves—which would follow each other after a short interval, had neutralized each other at the spots where these vessels chanced to be at the moment? Issuing from the narrow straits into the oceans east and west, these waves started off on their journey round the globe, and, from the records of the tide-gauges which are now coming in, we have a most remarkable tale unfolded. On the afternoon of the same day that the greater of them swept away the Javan villages, the undulations were registered unmistakably in Mauritius, the Seychelles, in South Africa, and on the shores of the Pacific islands; but, as Mr. Lockyer informs us, they did not vanish there, but proceeded onward, and, crossing each other on the antipodes of Krakatau, journeyed back to the spot whence they had emanated, and this they did no fewer than four times before the equilibrium of the sea was restored so far as to be insensible to our instruments. While the tide-gauges have recorded their story, the delicate fingers of the barometrical registers of the world have also borne uninfluenced testimony of a similar kind. The blow which hurled such a mass of matter into the air, which originated a hurricane there and caused the barometers in the neighborhood of the volcano to rise and fall with unparalleled rapidity and a vessel distant three hundred miles to tremble, started an atmospheric wave also round the globe. It was first detected in the Kew registers, we believe, by General Strachey, who has now examined a large number of barographs, from which he has been able to fix the dates at which the atmospheric undulations passed various places on the earth's surface. As in the sea, so in the air, two waves, one to the east and one to the west, started from Krakatau, whose rate of progress has been found to be that of sound. One surprising circumstance, of which we have as yet observed no explanation, is how those ships which were near the volcano at the moment of the supreme explosions, of the enormity of which they seem not to have been cognizant, notwithstanding that they were heard at such immense distances, did not only not suffer from the concussion, but were not blown off the face of the water altogether. Almost coincident with the record of the abnormal atmospheric fluctuations, magnificent sunlight effects, unusually lurid skies, prolonged dawns, lengthened twilights, and green or blue or moon-like suns, began to be observed. From the dates at which these phenomena first appeared in different parts of the world—on the east coast of Africa on the second day, the Gold Coast on the third, Trinidad on the sixth day, at four thousand miles in the Pacific west of Panama on the seventh, and at Honolulu on the ninth day—it can be seen that the volcanic cloud followed a straight path.
To what height the supreme outburst propelled the smoke, dust, and the lighter portion of matter, it is impossible at present to estimate. Mr. Whymper saw Cotopaxi, in by no means one of its extraordinary expirations, eject a column over 20,000 feet in height; but many multiples of this distance will doubtless be required to measure the spire that was shot sky-ward on the forenoon of the 27th of August last. At all events it rose so high that months have been required for it to descend. Those places situated below the direct westward path of the cloud, which would be elevated at first as a narrow column, as they were carried under it by the eastward rotation of the earth, were the first to have the usual light of the sun changed into ominous displays or delightful after-glows, varying in intensity according to their time-distance away, and therefore to the amount of the obstructing dust, which would also condense moisture in the upper part of the air, and give special absorption effects, that had by the hour they were reached subsided from the atmosphere. This narrow band, gradually spreading out north and south, enabled the inhabitants of all lands to obtain a view of the gorgeous effects of broken and absorbed sunbeams, and a demonstration of the vastness of the power of imprisoned steam.
Many questions connected with the subject remain at present unexplained; but the difficulties will in great part doubtless disappear before our fuller information. A committee of the Royal Society, consisting of our highest authorities in meteorological, volcanic, and light phenomena, has, as we have said, been appointed to fully investigate the subject, and from their labors we shall by-and-by be in possession of the first really accurate and scientific examination of the effects of volcanic eruptions, which in this case bids likely to result, to meteorological science at least, in a gain whose immense importance it is impossible now to calculate. Nor is it unlikely that this "biggest terrestrial experiment" afforded us by Nature may ultimately prove to have been not the least of her beneficent gifts to humanity.—Proceedings of the Royal Geographical Society.
- "Narrative of Survey Voyages of the Adventure and Beagle," vol. iii.
- "Proceedings of the Royal Geographical Society," December, 1879.
- Cf. "Nature," February 21, 1884, pp. 381, 882.