Journal of the Asiatic Society of Bengal/Volume 29/Account of a Visit to Barren Island in March 1858
JOURNAL
OF THE
ASIATIC SOCIETY.
No. I. 1860.
Account of a Visit to Barren Island in March 1858. By G. von Liebig, M. D.[1]
Barren Island is a volcanic island, situated in Lat. 12° 17′ N. and in Long. 90° 54′ E. Its smallest distance from the Andaman Archipelago is in a straight line only 36 miles East. The distance from the nearest point of the main land, near Tavoy, is about 270 miles W. S. W. It lies not far out of the straight course between Port Blair and Amherst, about 63 miles from the former, and 330 from the latter place. The Semiramis approached the island on the morning of the 19th March, 1858, coming from the N. E., and steamed round it by S. keeping close to the shore, until the ship was opposite the entrance of the crater (Fig. 2.) bearing about W. and by N. from the centre of the island, where she hove to, and we landed.
It is stated in former accounts, that all round the island the lead finds no bottom at 150 fathoms, only ¼ mile distance from the shore. Captain Campbell found however ground at that distance on one side of the island, its centre bearing N. E. at a depth varying from 4½ to 14 fathoms.
Nearing the island from the North and passing round to the South East of it, it looks from a distance like an oval-topped hill; but coming closer, the sides of the mountain are discovered to belong to a steep circular elevation, sending out spurs towards the sea and enclosing a central valley. The sides of the enclosing circle being lower in the direction of the spectator, the upper circumference of this valley is seen in the shape of an oval ring, formed by the crest of the surrounding ridge. In the middle of this ring, the upper part of a regular cone is visible, from the apex of which small white vapour-like clouds emanate. It is also distinguished from the surrounding darker masses by its grey colour, and some large white marks on it, like fields of snow. An entrance is not discernible.
The slopes towards the sea are generally covered with shrubby vegetation, presenting however some bare patches towards the upper edge; small trees grow about the base, where large rounded stones are washed by the sea.
Turning now to the S. and S. W. the enclosing wall is higher than the cone and the crest of the opposite ridge, and both therefore disappear from the view. On this side the vegetation down the spurs to the sea may be called rich, and consists of different forest trees of moderate height, interspersed with graceful palms; and where the descent is rocky, the rocks are frequently covered with ferns.
Passing to the westward of the centre of the island, and continuing the survey towards the northern end, one of the first turns discovers a large gap in the circular wall, extending quite down to the base of the island, through which the interior of the valley, with the cone in the middle, opens at once into full view.
The sides of this gap or fissure in the circular wall form a regular cut or short transverse valley through it, opening towards the sea into a small bay, and on the other side into the circular valley, to which it is the only way of access. Opposite this entrance, in the centre of the valley, rises the cone of grey ashes, and surrounding its base the bottom of the valley is filled with black masses of cold lava, which are continued like a congealed stream through the gap, breaking off abruptly when they arrive near the water's-edge. At its termination the steam is about 10 or 15 feet high, and its breadth seems less than farther up. It looks like a black perpendicular wall drawn across the entrance and facing the sea.
The lava consists of a black basalt mass (matrix) throughout which are disseminated innumerable semi-transparent little crystals of a variety of common felspar (orthoclase), and also many bright green granules of olivine. the lower part of its thickness is homogeneous, with a smooth fracture, but from the upper surface to a depth of several feet it is cleft in all directions, whereby the upper part is divided into rough blocks, possessing a spongy texture as well as countless sharp edges and corners.
The older lava, composing the rocks on the side of the valley and also the strata of the surrounding ridge is slightly different from this. The colour of its principal mass is a reddish grey, felspar and olivine crystals are embedded in it in the same proportions as before, and in addition small pieces of black augite of the granular kind, with conchoidal fracture. From underneath the black lava, where it terminates near the sea, issues a broad but thin sheet of hot water, mixing with the sea water between the pebbles of the beach. The Thermometer I had with me was not graduated high enough to measure its temperature, its highest mark being 104°F. (40°C.) The water where escaping from the rock must have been nearly at the boiling point, judging from the heat felt when the hands were dipped into it, or when the hot stones were touched. When bathing, we found the sea water warm for many yards from the entrance of the hot spring and to a depth of more than 8 feet. It is not impossible that a jet of hot steam or water may emerge from the rocks below the level of the sea. The hot water tasted quite fresh, and not saline as might have been expected, showing that it could not have been long in contact with the rocks.
We ascended to the base of the cone, passing along the sloping sides of the transverse valley through dry grass and brushwood or over sandy ridges, so long as the solidified stream of lava in the middle left us room to do so. At last we had ascend the rugged surface of the black lava itself, and cross the circular valley, which has about the same breadth as the transverse valley (not quite one-eighth of a mile), until we arrived at the base about half a mile from the sea. The cone rises from the lava accumulated in the circular valley, and its base is about 50 feet higher than the level of the sea, at a rough estimate. It is quite round and smooth, and the inclination of its sides is 40 degrees. No vegetation of any kind was visible along its surface. We turned to the left and went up from the north side, where the appearance of a ravine, some way up, only two or three feet deep and very narrow with some tufts of grass growing along it, promised an easier ascent for a part of the way, and where a rocky shoulder at about two-thirds of the hight would offer a place to rest. Our ascent commenced at about 2½ p.m., and was certainly the most fatiguing expedition many of us remember ever to have undertaken. The sky was almost cloudless, and the heat consequently was great. The lower third and more of the slope consisted of a powder of ashes gained. A little higher, stones loosening when the foot stepped on them and rolling down in long jumps, were dangerous to any one following.
Arrived at the rocks mentioned, their nature and the manner in which the side of the cone bulged out in their neighbourhood, showed that they marked the point from whence an effusion of lava of the same kind, as we has seen below, had taken place from the side of the cone, not reaching the mouth of the tube at the apex. The last third of the way from the rocks upwards offered a firmer footing, the ashes being cemented by sulphate of lime (gypsum) which, where it was present, formed the white patches we had already observed from a great distance when approaching the island. The ground now became very hot, not however intolerably so, until about 30 feet from the apex a few rocks again offered a convenient seat, not affected by the heat of the ground. There the Aneroid barometer and the temperature of the air were observed in the shade of an umbrella.
About half way between these rocks and the highest point cracks and fissures commenced to intersect the ground, widening higher up to the breadth of several inches, where clouds of hot watery vapour issued from them. They were filled with sulphur, often accompanied with beautifully crystallised white needles of gypsum, and a sulphurous smell also accompanied the vapour (sulphurous acid). This smell was however not very strong and did not prevent us from penetrating the clouds, when we discovered that what had appeared from below as the summit was in fact the edge of a small crater, about 90 or 100 feet wide, and 50 or 60 deep. At that depth it had a solid floor of decomposed lava and tufa and volcanic sand. Its walls were made up of rocks, in appearance like those of the older lava and were highest on the north and south sides. Towards the west the crater opened with a similar cleft, to that which had permitted us to enter the island. The vapours rose principally from the northern and southern quarters of the edge where the fissures were largest and longest, running both parallel and across the edge. The rocks where the sulphurous vapours issued from between them, were covered with reddish and white crusts, indicating the beginning of decomposition of their substance. From the top the horizon and more or less of the sea were visible in all directions, with the exception of the quarter between South and West. The inner slope of circular elevation enclosing the valley, had no spurs, but was like a plain wall, falling off with a steep descent all round towards the centre. It had a uniform brownish colour, appertaining either to the surface of larger masses of the rock itself, or being derived from the dry grass and smaller shrubs covering the slope. There were no trees or brushwood visible to correspond to the richer vegetation on the external circumference. Horizontal parallel lines, traceable throughout the circle and rising somewhat like the borders of the receding steps, indicated the thickness and strike of the different sheets of lava and tufa which, superimposed upon one another, formed the substance of the circular elevation. A very good transverse section of it had already attracted my attention, where the left side of the transverse valley debouches into the sea. Several strata of tufaceous formation, alternating with older rock like lava, could be seen there rising from the rocky beach. One of the most remarkable amongst these was a stratum of rounded stones, like large pebbles, cemented by tufa, exactly like those of the present beach, but at a considerable elevation (about 20 feet) above the high water mark, showing that the sub-marine base of the Island must have been raised since those pebbles had been washed by the sea. All these strata dipped out-wards from the centre of the island, parallel with the external slope of the encircling wall. It is interesting to observe that this slope continues under sea level on three sides of the Island at least, at the same inclination as above water, which averages about 35°. This is shown by the soundings, which exceed 150 fathoms at a distance of a quarter of a mile from the shore.
Judging from what we saw, as I have here attempted to describe it, I would conclude that the circular valley and its walls constitute the crater of a huge volcanic cone of sub-marine basis, which had been the vent for fluid masses of rock, when such eruptions took place on a larger scale than in more recent times. The smaller cone in the centre of the old crater, corresponding in its size to the diminished forces of volcanic action, is of recent origin, and represents those smaller cones of still active volcanoes which are usually distinguished as cones of eruption from the original cones, also called the cones of elevation.
We have it on record that about 60 years ago, the crater of the little cone was throwing out showers of red hot stones of several tons weight and enormous volumes of smoke (Captain Blair's account Asiatic Researches 1795), and but for the island position of the volcano preventing its more frequent observation, we should doubtless be able to fix the date of the eruption that left the stream of lava behind, which is now filling the valley and its outlet into the sea. Since that time it has entered the period of decline of volcanic activity, without however leaving us the assurance that it will not some day revive again.
From barometrical observations, I deduced the height of the cone by Gauss's formula, allowing for the time of the day and the influence of the hot ground near the summit, to be about 980 feet, from the level of the sea to the northern edge of the crater. This height is confirmed by a trigonometrical measurement of Lieutenant Heathcote, I. N., to whom I am indebted for the communication of his results. He visited the Island about four months earlier than we did, when he found the height of the cone 975 feet above the level of the sea, and the diameter of the Island 2,970 yards, 1.69 miles North and South.
The few notes I could glean respecting the recent history of the Island, are derived from the Island itself, from the records of the Asiatic Society, and from Horsburgh. We found on a rock in the transverse valley the inscription "Galathea 1846", showing that since then no alteration has taken place. The same conclusion can be extended farther back to the year 1831 or 1832, judging from an account communicated to the Asiatic Society (Asiatic Society's Journal, April 1832) by Dr. J. Adam, whose informant landed in the month of March, and reached the base of the cone. By this explicit account, the descriptions of the Island "Lyell,"[2] dated 1843, and in Humboldt's Cosmos, both apparently derived from the same source, must be rectified. The narrator states (in "Lyell") that the sea filled the circular valley round the cone.
Horsburgh states that in 1803 the volcano was observed to explode regularly every 10 minutes, projecting each time a column of black smoke, perpendicularly, to a great height, "and in the night a fire of considerable size continued to burn on the east side of the crater, which was then in view."
The oldest account on record is that of Captain Blair, already quoted, taken from his Report of the survey of the Andaman Islands. He must have visited the Island about 1790, as far as I am able to conclude from the publication in the researches and the date of his chart of the Andamans which is 1790. He approached nearly to the base of the cone, which he describes as the lowest part of the island, very little higher than the level of the sea, but he does not mention the black stream of lava. The acclivity of the cone he states to be 32° 17′, and its height 1,800 feet nearly, which, says he, is also the elevation of the other parts of the Island. On the other hand he remarks that the cone is visible in clear weather at a distance of twelve leagues, which would require a height of not more than from 900 to 1,000 feet. I think therefore that Captain Blair could have taken no accurate measurements, contenting himself with a rough estimate. If it could be proved otherwise, the Island would have subsided 820 feet since he visited it.
From the description in some of these accounts it would appear that the high vegetation which we found on the external slope of the Island, is of quite recent origin.
Mr. Adam's authority (1831) states as follows :—
"The summits to the N. E. were completely smooth and covered with ashes; those to the S. W., although partly covered with ashes, also have a good many small shrubs over them, with dry and parched grass growing on the surface."
He conjectures from this that the eruptions would take place only in the S. W. Monsoon or rainy season, at which time the S. W. wind will blow the dust and ashes on the hills in the opposite direction, or N. E.; such a conjecture is hardly admissible on the ground given, it being easier to account for the vegetation on the south-western slope by its angle of descent being much smaller than that of the north-eastern slope.
The sulphur on the top of the cone occurs in such quantity in the cracks and fissures, often lining them to the thickness of more than half an inch, that the question naturally arises, whether the sulphur could not be worked with advantage.
Although in the immediate neighbourhood of the crater, where the fissures are numerous, the ground seems to be completely penetrated with sulphur, this is not so evident in other parts, only a few feet lower, where the surface is unbroken. There are however some reasons which seem to promise that a search might be successful. In eruptive cones, like that of Barren Island, there is always a central tube, or passage, connecting the vent in the crater with the heat of volcanic action in the interior. In this tube the sulphur, generally in combination with hydrogen, rises in company with the watery vapour, and is partly deposited in the fissures and interstices of the earth near the vent, the remainder escaping through the apertures.
If in the present case we admit the sensible heat of the ground of the upper third of the cone to be principally due to the condensation of steam, a process of which we have abundant evidence in the stream of hot water rushing out from underneath the cold lava, it is not improbable that the whole of the upper part of the interior of the cone is intersected with spaces and fissures filled with steam and sulphurous vapour, these being sufficiently near the surface to permit the heat to penetrate. It is therefore not unlikely that at a moderate depth we should find sulphur saturating the volcanic sand that covers the outside of the cone.
I only speak of the outside, as we may conclude from the evidence we have in the rocks of lava in the crater and those bulging out on the side, that the structure of the cone is supported by solid rock nearly to its summit, the ashes covering it only superficially.
From what has been said above, the probability of sulphur being found near the surface disposed in such a way as to allow of its being profitably exhausted, will depend on the following conditions:
First.—That the communication of the central canal, through which the vapours rise, with its outlets, be effected not through a few large, but through many and smaller passages, distributed throughout the thickness of the upper part of the cone.
Second.—That some of these passages communicate with the loose cover of ashes and stones which envelopes the rocky support of the cone.
Although I have mentioned some facts which seem to indicate the existence of such favorable conditions, and which are moreover strengthened by an observation by Captain Campbell, who saw vapour issuing, and sulphur being deposited near a rocky shoulder about two-thirds of the height on the eastern descent of the cone, still their presence can only be ascertained satisfactorily by experimental digging.
The Solfatara at Puzuoli, near Naples, is a similar instance of the production of sulphur. It is a crater in which exhalations of watery vapour, sulphurous acid and hydrochloric acid take place, and where sulphur is also deposited. The sulphur is gained there by distilling it out the sand of the crater, to a depth of 10 metres or 32 feet—it becomes too hot lower down—and returning the sand which after 25 or 30 years is again charged with sulphur. The permanency of the volcano of Barren Island as a source of sulphur would depend on the rapidity with which the sulphur would be replaced after the sand had been once exhausted. The time required for this is not necessarily fixed to periods of 25 or 30 years. In Iceland, at a similar spot, the sulphur is renewed every two or three years.
If a preliminary experiment should make it appear advantageous to work the cone regularly, the material about the apex, after being exhausted of the sulphur that is present, could by blasting and other operations be disposed in such a way as to direct the jets of vapour in the most convenient manner through uncharged portions of ground. If the sulphur should aggregate in periods of not too long duration, it would be possible to carry on the work of filling up new ground on one side, and taking away saturated earth on the other at the same time, so that after working round the whole circumference, the earth that had been first put on would be ready to be taken away.
If the periods should prove too long to allow the work permanently to be carried on, an interval of time might be allowed to pass, before resuming operations.
Water for the labourers could always be obtained from the warm spring at the entrance of the Island.
The distilling or melting of sulphur to separate it from adherent earth is a matter of comparatively little expense or trouble. If the sulphur be abundant, it might be effected as in Sicily by using a part of it as fuel. It is not necessary to do it on the spot; it might be done at any place where bricks and fuel are cheap.
It is impossible to predict certain and lasting success to and undertaking of this kind, all depending on the quantity of sulphur present and the rapidity with which it will be replaced.
The situation of Barren Island offers every facility for a preliminary trial. The near proximity of the Andamans insures a supply of convict labour, timber, bricks, and lime. All the wood and iron work required for facilitating the transport of loads up and down the hill could be made on the Andamans.