Popular Science Monthly/Volume 64/January 1904/The Eruption of Pelee, July 9 1902
|THE ERUPTION OF PELÉE, JULY 9, 1902.|
By Professor T. A. JAGGAR, Jr.
A DESCRIPTION of the scene of devastation in Martinique was published by the writer in this magazine in August, 1902. Some of the readers of The Popular Science Monthly may be interested in the details of a great eruption, and scientific deductions from observation of the same; the present article aims to present the results of such observation. The writer returned to Martinique from Barbados in June, 1902, and had the good fortune to see an eruption of first magnitude on Mount Pelée. On the evening of the ninth of July, he was in Fort de France, and the commission sent out by the Royal Society, Drs. Tempest Anderson and John S. Flett, were living on board the sloop Minerva near Carbet; thus the scientific record kept of this eruption was unusually complete.
At 8:15 in the evening, just at twilight, there was seen from Fort de France a very high column of black dust with the characteristic cauliflower surface, boiling straight up from the direction of the volcano to an enormous height (compare Fig. 1). The blackness of this dust cloud increased, the bulbous profile gave place to a smooth balloon-shaped outline slightly flattened above, and from the moment that the interior of the cloud first became obscure, small spicular lightnings were seen dancing like a myriad of bright white sparks all over the cloud surface. At first this whole extraordinary display was perfectly silent; the writer was called out from the public library in Fort de France by an attendant at about 8:20 p. m., and there was a singularly oppressive stillness noticeable. When the phenomenon was first observed in the streets, some slight disturbance akin to panic had been audible, but the people as a whole had grown so callous to these happenings, that after a few moments very few individuals seemed to take any note of the extraordinary phenomena in the northwestern sky.
At 8:30 the cloud had the form of a balloon, swelling rapidly to the zenith, and filling with its diameter perhaps 70° of the sky north-northwest. The wonderful coruscations grew more brilliant, leaping rapidly from place to place over the surface of the cloud, but at first they had the form of points of light, rather than lines. There was no thunder accompanying these early flashes, and their frequency was incessant, the whole cloud surface fairly scintillating with dancing lights.
Gradually a sound became perceptible, which at no time developed into loud detonations. It was rather a bubbling growl, far away, different from thunder, and continuous. It lasted an hour, and varied in intensity; this noise was undoubtedly occasioned by the expansion of steam released from the crater's throat, accompanied by the tumble of a volcanic avalanche on the flanks of the mountain and at the crater. The distance of Fort de France from the crater, as the crow flies, is fourteen miles. The weather was very calm, with only a faint breeze from the direction of the volcano during the early stages of the eruption.
The writer climbed to the ridge-pole of the roof of the Hotel Ivanes, while an attendant held a candle at the open dormer window below, and the flame was not disturbed by any considerable breeze. On the high roof, the stillness of the tropical night was relieved by nothing but the simmering hum of tree-toads and crickets, and the very faint distant rumble, at that time barely audible. Here, remote from the odors of the town, there was an extremely faint odor of sulphur (sulphurous acid). At 8:50 a wave was noticed on the wharves at Fort de France, the water receding and returning through a total vertical distance of about two feet. About the same time the recently repaired cable of the telegraph company ceased to make connections; it was probably broken by submarine landslips.
During the next twenty minutes the balloon or spoon-shaped cloud spread rapidly southward until nearly the whole horizon was obscured with the exception of a narrow strip of starlit sky, the middle of which lay S. 20° E. To the north-northeast could be seen fringing showers, four or five in number, showing as vertical streaks against the horizon in that direction. These were believed at the time to represent local dust falls. At 9:10 a similar curtain was seen N. 60° W. At 9:15 some rain fell at Fort de France, but this lasted only about five minutes. At 9:18 the smoothly curved edge of the dust lowered to and merged with the horizon S. 30° E. A very remarkable flash of lightning at this time shot across the northern sky, making a complete loop overturned to the northeast with the ends downward towards the volcano. It was described by one of the bystanders as 'balloon lightning,' and had somewhat the aspect of a huge incandescent bulb, outlined by a streak of light that swept in a tremendous curve across the zenith. The bulbous effect was probably produced by the illumination of one of the cloud billows within the lightning circle. It occurred to the writer that possibly the change in the lightning forms during the two hours, from points to short lines, and from these to long serpents, was due to a change in the perspective of the cloud billows. The first view when the mushroom was expanding was 'end on,' so to speak, with reference to the individual nodes or mamelli. When the cloud spread over Fort de France these necessarily rolled out into long billows and irregular kidney-shaped forms, the lower surface of which might sometimes be seen when illumined by the lightning flashes. Discharges from node to node, with the 'end on' view, would be small points of light, like the sparks between the electrodes of an induction machine when the two are pushed together. They would also be very numerous and incessant as the individual nodes were numerous, small and possessed of intensest vertical velocities over the direct discharge from the volcano's throat. As the mushroom spread to a distance the individual dust eddies became larger and more diffuse, and the discharge between them would become less frequent and more irregular, giving rise to the worm stage. With further increase and more massive movements within the cloud, the same continually growing in volume and rising to greater height as well as spreading across the line of vision of the observer below, the lightning flashes decreased in frequency, and increased in length, intensity and irregularity, producing finally the long orange-colored serpents described below—the color being due to the dust layer which partially obscured the dazzling flashes.
Whatever the cause, the lightning clearly showed a gradual change in character; from incessant, shifting, scintillating points it changed to less frequent worm-like lines and short forking discharges, the divergence being at first downward toward the crater. One flash was noticed in a direction N. 30° W. having a distinct short S shape with three inflections, the lying horizontal. At no time were any lightning flashes seen to pass from earth to cloud or vice versa; all of these discharges were across the sky. At 9:30 the continuous grumbling from the volcano died away.
At 9:43 the middle of the dust arc southward had the bearing S. 20° E. This point corresponded to what at the start was the zenith point of the dust balloon.
At 9:45 very brilliant sheet lightning occurred showing no definite points or lines, but illumining the whole vault in dazzling flashes. The lightning seemed to be more concentrated on the periphery of the cloud. Meanwhile white cumulus clouds lifted towering thunder heads to the southeast and these marked the beginning of local thunder-storms over all the high points of the island. The high tops of these clouds lighted by the sheet lightning showed their billowy profiles white against the dark vault of dust above; some of them appeared to rise to a great height, but their summits were far below the black dome of volcanic dust, affording an excellent measure of the stupendous height of the volcanic cloud and its electrical phenomena—both entirely distinct from the rain-bearing thunder-storms.
At 9:52, looking southwest, the thin edge of the dust cloud against the starlit sky beyond appeared to have risen somewhat and to be thinner and more transparent where it had before been black and hard-edged. Its rise could be measured by the group of royal palms about the statue of the Empress Josephine, which lay in that direction. These notes were made from the middle of the open 'Savane' or common, while a negro boy held a candle for making compass observations and writing. The increase in sheet lightnings was accompanied by thunder claps from different directions. Some of these came from the east, and they were undoubtedly produced by the local showers.
At 10 o'clock it was pitch dark, with no horizon visible; white clouds rose high southwest.
At 10:08 heavy thunder was heard to the north in the direction of the high Pitons de Carbet. The lightning from the dust vault had grown more sparse and the short worms had gradually given place to elongate serpent-like flashes with increasing intervals. Following these thunder crashes from the north, there was a magnificent lightning display across the zenith. At 10:10, in the direction of the volcano north-northwest, long curved orange flashes rent the vault from east to west.
At 10:15 rumbling was renewed from the direction of the volcano and lightning was also seen. This rumbling resembled rather a slight renewal of the steam explosions than local thunder. It is worthy of note that the sound at Martinique under the dust cloud was different from that observed at great distances. At Barbados this eruption of the night of the ninth was heard distinctly as a series of heavy detonations in quick succession, resembling heavy ordnance close at hand. The distance, over one hundred miles, and the angle by which the sound traveled, probably from above the dust vault, produced a different acoustic effect from what was observed in the immediate vicinity of the volcano. Probably only the louder explosions carried so far, which would account for the discontinuous quality of the detonations. At Fort de France the effect was as though the dust billows had a muffling effect.
At 10:25 there was a cessation of active phenomena for nearly a half hour.
At 10:50 a breeze sprang up from the south.
At 10:55 there was heavy thunder in the northwest, and some serpent flashes of lightning.
At 11:15 the dust dome was obscured by rain clouds, showers fell and there was occasional distant lightning.
At 11:40 thunder was heard to the south. At this time a quadrant of the southern horizon cleared, showing itself below the straight edge of overhanging cloud. After this the night settled down to a condition of quiet cloudiness with the wind southerly, and the temperature 80° F. At midnight two sheets of white paper were placed upon an open balcony and in the morning, after six hours, these showed a film of specks of volcanic dust.
At 3:30 a.m. the moon appeared through the dust cloud with a dim reddish-yellow glow.
The next morning, July 10, at 6:30 the writer took a small steamer north, along the west coast of Martinique to Carbet, where an excellent view was obtained of the slopes of Mt. Pelée, with their newly deposited load of dust and volcanic debris. The morning dawned with high banks of cumulus cloud and a hazy sunrise; there were some light showers. When the volcano came in sight, the usual dark dust veil was seen lowering upon the ocean far to the west (Fig. 2), and the summit was veiled in billowy clouds. From a point near Morne aux Boeuf northward, the vegetation and the beach stones showed a coating of gray dust, which became more conspicuous on approaching the volcano.
The slopes of the mountain, seen at first enveloped in a rain shower, were steaming in two places along the Rivière Blanche, and a long streak of steaming deposits was seen on the high slopes above the Riviere Sêche—the Riviere des Pères was also steaming. The slopes were covered everywhere with patches of light greyish-yellow sand and gravel, hot and dry, which blew up in whirling dust clouds now and then under the puffs of easterly wind. Along the water front, at the mouths of the various rivers, there were occasional steam jets; the dust clung to the cliff surfaces of the high rock walls, outlining the rugged surface in patterns like a fresh snowfall on a cliff face in the Alps. The greatest masses of dry deposit were accumulated in drifts along the course of the Rivière Blanche. This fact is important, showing that along this gulch there accumulates in the course of a series of eruptions a very great thickness of hot dry gravel and sand, which is the cause of the violent explosions along the gorge whenever water rushes down from above.
At Carbet there had been a dust fall on the evening of July 9, averaging perhaps one centimeter in thickness. There had been a preliminary small eruption, earlier in the evening; the great eruption at 8:15 happened quite suddenly. The scintillating lightning flashes were observed at Carbet from the start; dust and small pebbles fell shortly afterwards. At Fonds St. Dénis, on the heights back of St. Pierre, angular rocks fell of sizes up to three or four inches. There was continuous rumbling from the crater noted at Carbet, and this was quite distinct from the local thunder storms which developed later. The eruption was watched from the beach at Carbet Point by Mr. Gouyer, manager of a plantation at Carbet, and he states positively that he saw no tornado or wind blast which struck St. Pierre. No one was hurt at either Morne Rouge or Carbet. It seems agreed among those living near the volcano, that these later eruptions were more 'fiery' than the earlier ones, indicating that there is more incandescent material ejected; thus they state that there was more 'fire' on June 6 than on May 20; and more slides or avalanches of incandescent material on the slopes of the mountain on July 9 than on June 6.
At about 8 o'clock on the morning of July 11, a vertical puff from the volcano rose 10,000 feet into the air, showing at first superb gray-brown cauliflower surfaces, and later, taking on smooth outlines, with a funnel-shape and a feathery fringe. There was only a single puff, and the cloud drifted away to leeward. A similar one was photographed on July 16 (Fig. 1).
It is of interest to compare with this record of observations at Fort de France, the notes of the British men of science on July 9 made from a vessel close to the volcano:
From the fissure in the volcano, clouds of pale slaty vapour rose constantly. . . . (About 7:30 p.m.)
In the rapidly-falling twilight we sat on deck intently watching the activity of the volcano, . . . when our attention was suddenly attracted to a cloud which was not exactly like any of the steam 'cauliflowers' we had hitherto seen. It was globular, with a bulging, nodular surface; at first glance not unlike an ordinary steam jet, but darker in color, being dark slate approaching black. . . . Its behaviour. . . was unique. It did not rise in the air, but rested there, poised on the lip of the fissure, for quite a while as it seemed, and retained its shape so long that we could not suppose it to be a mere steam cloud. Evidently it had been emitted with sufficient violence to raise it over the lip of the crater, but it was too heavy to soar up in the air like a mass of vapor, and it lay rolling and spouting on the slopes of the hill. The wind had no power over it, fresh protuberances spurted out from its surface, but it did not drift leeward any more than if it had been a gigantic boulder.
This cloud rolled straight down, gradually increasing in size as it came nearer. The further it traveled the faster it came. It cleared the mountains slope, increasing always in size, but still rounded, globular, with boiling, pillowy surface, pitch black, and through it little streaks of lightning scintillated. On reaching the north side of St. Pierre roadstead the black mass discharged sparkling lightnings incessantly along its contact with the water. The cloud quickly lost velocity and formed a black pall, with larger, less vigorous, more globular, bulging convolutions. It lay almost like a dead mass on the surface of the sea. The black cloud rose from the fissure about 7:40, and for twenty or thirty minutes the sloop sailed southward with a gentle breeze from the east. Then the wind fell to a dead calm. The black cloud cleared, above the fissure a faint red glare was seen, which slowly increased, and bright, glowing masses were seen describing parabolic paths through the air and then landing on the mountain slopes and rolling down the hill. These red-hot stones were projected as much as a mile from the crater.
Suddenly a great yellow or reddish glare lit up the whole cloud mass, a
This photograph was taken from the S. S. Dahome off Fort de France, looking north. On the right is the slope of the Pitons of Carbet. The steam column was over six times the apparent height of these peaks, and its base was twelve miles away. its crest subtended a vertical angle of 29° measured with a handlevel. Allowing for bulge, its height was not less than from four to six miles. The upper portion of the cloud, bent to the right (east), has passed through the trade-wind bell and is moving with the counter-current. A 'cauliflower' wall of dust could be seen moving down to the west at the base of the column, and rolling out from the mountain over the water.
prolonged angry growl burst from the mountain, not loud, but with a 'snarling character.' A red-hot avalanche rose from the cleft in the hillside and poured over the mountain slopes light down to the sea. It was dull red and in it were brighter streaks thought to be large stones, as they seemed to give off tails of yellow sparks. They bowled along apparently rebounding when they struck the surface of the ground. The dark-red avalanche had a billowy surface, and its velocity was tremendous. "The mist and steam on the mountain top did not allow us to see very clearly how the fiery avalanche arose, but we had' a perfect view of its course over the lower flanks of the hills, and its glowing, undulating surface was clearly seen." The red glow faded in a minute or two, and a round black boiling cloud rushed over the sea in front of it, filled with lightnings. This cloud was globular as seen end on, very perfectly rounded, covered with innumerable minor rounded excrescences which were filled with terrific energy. It was full of short lightnings—'A mere succession of flashing points in the great black wall of cloud.' "Many of the flashes were horizontal, others shot obliquely from one lobe to another, while along the base where the black cloud rested on the steel-gray sea, there was a line of sparkling light. constantly changing, varying in amount, but never disappearing."
A gentle puff of wind came from the southeast. The cloud lost its violence and became a gigantic wall with a velvety appearance in the moonlight, resembling a black curtain draped in folds. In size it was estimated to be two miles broad and about one mile high. The surface shimmered like silk and the colors changed to brown and gray, and even white on the edges of the folds, and these changes spread through the whole face of the cloud. The base became darker, and the paler summits soared obliquely upwards and forwards; this was believed due to sinking dust and separated steam.
The steam cloud, with a velocity of twenty miles an hour, came overhead, gray and tongue-shaped, with a blunt rounded apex, and convolutions still forming. The lightnings, reduced in number and frequency, threaded the dark mass in every direction. "A low rumbling noise was given out as the cloud worked its way across the clear, starry sky." It spread across the vault, producing darkness; when it reached the zenith, pebbles fell about the size of a chestnut, cold, and these
became a rattling hail diminishing to the size of peas. Fine gray ash followed in little moist and adherent globules, noiselessly sinking through the air and sticking to everything on which they landed. They were not warm, and there was a slight smell of sulphurous acid. After a few minutes the ash took the form of a gritty dry powder. The layer which fell on deck had a total thickness of one sixteenth of an inch.
The southerly wind rose to a fresh breeze and the naturalists took their sloop into Fort de France harbor safely. They noticed a thunderstorm to the northeast, as heretofore described.
The final comments of Doctors Anderson and Flett may be quoted in full:
The second black cloud did not differ in appearance from the first except that it was larger, had a far greater velocity and swept out at least twice as far across the sea. It was black from the first moment when we saw its boiling surface in the moonlight. Both traveled very rapidly over the lower part of the mountain, but slowed down after reaching the sea, and came to rest comparatively suddenly. The lightnings on the two clouds were similar in all respects.
No blast struck us—in fact, we were becalmed—and it seemed that when the black cloud ceased the blast was all over. Nor did the sea rage around us as some have described who were overtaken by the dust storm. When the cloud was passing overhead there was a slight rolling sea, but as the breeze freshened the boat steadied, and there was no unusual disturbance. We watched carefully for a strong indraught, such as was described by more than one observer, but the wind that rose from the southeast was very gentle, and increased gradually to a full-sail breeze. . . .In the cloud there was a dull, low rumble, but we heard no detonations, and saw no sheet of flame, so that we both agreed that there had been no sudden ignition of quantities of explosive gases. The lights in the cloud, in our opinion, were lightning and nothing else. When the slopes of the mountain were seen two days later it was evident that 'the avalanche had come down approximately along the line of the Rivière Sêche.'
Comparing the records of these observers with the observations made at Fort de France, we find the following simultaneous phenomena:
|Anderson & Flett.||Jagyar.|
|7:40 p.m.||First black cloud.||7:40 p.m.||Not watching.|
|8:00 p.m.||Glare and glowing stones.||8:00 p.m.||Not watching.|
|8:20 p.m.||Red-hot avalanche.||8:20 p.m.||High balloon-shaped column of dust.|
|8:35 p.m.||Avalanche cloud had come to rest and risen from the sea.||8:35 p.m.||Column spreading to zenith, lightnings and rumblings.|
|8:45 p.m.||Avalanche cloud reaches zenith and pebbles fall.||8:50 p.m.||Wave observed and cloud spread out to horizon.|
The dust-balloon seen from Fort de France was at least five miles high when first observed, its summit subtending an angle not less than 30°. It rose to heights so great that, after spreading out to the horizon, the dust vault was entirely above towering 'thunderhead' cumuli which developed over the mountains of the island. Its first appearance at Fort de France was coincident with the fiery avalanche seen off Carbet; the British geologists saw no upright shaft of dust, but a cloud projected horizontally two miles broad and one mile high. They state that mist and steam obscured their view of the mountain, and it does not appear that they saw the great vertically projected column of dust at any time. This was because they were too close to the volcano to see it, and the up-jet was obscured by the down-jet.
This leads to certain theoretical considerations. Flett and Anderson consider these eruptions unique with respect to the hot sand avalanche and the black cloud, speaking of the 'Peléan type' of eruption. I do not believe that these features are exceptional, nor that they were wanting in the Vesuvian eruption of '79, nor in Tarawera, Krakatoa or Bandai-san.
Anderson and Flett believe the motive power which drives the lower black cloud forward horizontally to be the falling weight of the mass. Discussing the possibility of the out-blast being caused by the resistance offered by falling materials to the ascent of subsequent discharges—an explanation offered by the writer in 1902—they write:
That no great amount of dust was seen at Carbet projected vertically, prior to the first black cloud, may be accounted for in the same way as at the time of the incandescent avalanche—it was dusk, and the mountain's summit was veiled in clouds. I have no question but that there was a preliminary up-puff, not seen by observers close to the crater. And because of the peculiar acoustic conditions mentioned, no sound would be heard, either, though at a distance this puff might have been noticed as a distinct detonation. Without disputing the possibility of dust giving weight to a vertically charged steamcloud, it is difficult to understand how mere gravitational movement could produce destructive velocities on such gentle slopes—the slopes of Pelée and Soufrière average only 15° to 17° (Fig. 3). The horizontal discharges appeared to move with greater and greater velocity until they struck the sea, then they slowed down and came to rest almost suddenly. The motion was compared to that of a toboggan on a snow-slide.
The appearance of slow movement at the crater's lip I believe to have been only apparent, probably because the mass was end on and there was no means of judging forward motion. The discharge from a cannon would 'roll, squirt and seethe' without showing any forward motion, if we were looking into the cannon's mouth. There can be no question, however, that there was an avalanche of solid materials which accompanied the second black cloud seen by Anderson and Flett, and I am tempted to ask, may there not have been a similar non-incandescent avalanche with the first cloud, representing the relatively cool material which cleared the crater's throat?
If this were true we should have with each puff a sequence of events somewhat as follows:
1. Straight puff upwards.
2. Avalanche of heavy materials downwards.
3. Horizontal dust-cloud outwards.
The sharply differentiated horizontal and vertical blasts had been noted in all the eruptions of these volcanoes. They may be seen in the photograph of the eruption of July 16, where one cloud lowers over the water, the great column is upright, and its crest is bent to the east by the high counter trade-wind. (Fig. 1.)
The three stages above mentioned, in an explosive discharge from a fissure upward, might have been expected on purely a priori grounds. Following such reasoning further, we find: (1) A rending explosion raises some very coarse and heavy material, (2) this material falls soonest and nearest the crater, (3) such falling material would have very high velocity, but would come very quickly to rest on cone slopes of 17°. Its fall would certainly deflect upward discharges of lesser intensity, and its rush down the mountain with such deflected vomitings would have the aspect of an avalanche, sending forward an avalanche blast capable of short-lived but very fierce destruction. A deflected discharge from the crater would be propelled by the same force as the up-blast, and thus become both an avalanche and a down-blast with the energy of the volcano behind it. It seems probable that such a deflected mass of incandescent gravel and bowlders formed the avalanche seen by Anderson and Flett. On this hypothesis, both gravity and deep-seated explosion contribute to the tornado violence of the black cloud, and the displacement of the air before the avalanche resembles that frequently seen in high mountain districts, where snowslips project an air blast capable of leveling forests. Such snow-slides have a tendency to hug the slope, to move very rapidly above and to come to rest quickly below, and such movement was noticed in the eruption which destroyed St. Pierre and in the eruption of Bandaisan. In the latter also, the discharge changed from upward to horizontal, the change being gradual as the upper air became overcharged with solid matter. The other eruptions of Pelée and Soufrière showed the same general characters, and that, of July 9 may be taken as an example of an eruption of first magnitude. Its direction was more to the westward than on May 8, hence the escape of Carbet and St. Pierre. The materials ejected in the fiery avalanche were believed by Anderson and Flett to be the products of exploded molten lava, which welled over the lip of the crater and was blown to shreds by expansion of gases. This view I can not accept, for I have as yet seen no glass fibers or sherds, nor any evidence of new molten lavas in any way connected with these eruptions. The angular bits of andesite and coarser rocks found on the mountain after eruption are pieces of country-rock. In some cases 'bombs' have been formed from old andesites with half-molten surfaces. All the evidence so far goes to show that these volcanoes are great steam crushers, comminuting and distributing ancient volcanic materials.
In conclusion, I wish to record my admiration of the excellent work done by the Royal Society's commission, recalling with pleasure association with Dr. Anderson and Dr. Flett in Barbados and Martinique. Their memoir is by far the most thorough and scientific work that has been published as yet on the West Indian eruptions.
- 'Report on the eruptions of the Soufrière, in St. Vincent, in 1902, and on A Visit to Montague Pelée, in Martinique,' Part I., by Tempest Anderson and John S. Flett. Phil. Trans, of the Roy Soc. of Land., Series A, Vol. 200, pp. 353-553, 1903. (See page 492 et seq.)
- 'Preliminary Report,' by same authors, Proc. Roy. Soc, Vol. 70, August 11, 1902, p. 443.
- The Popular Science Monthly, August, 1902, p. 366.