Art. XLI.—The Recent Volcanic Eruptions on Ambrym Island.
[Read before the Olago Institute, 1st December, 1914.]
It is well known that geologically the New Hebrides Islands are mainly of volcanic origin, and of all of them Ambrym Island is the best known because of its volcanic activity. The island is near the middle of the group, in latitude 16° 10′ S. and longitude 168° E. It is more or less triangular in shape, with its greater dimension extending for thirty miles in a direction that is nearly east and west. It measures twenty-one miles in a north-to-south direction. Its surface is mountainous, the highest point, Mount Marum, rising to 4,380 ft., while the whole of the central portion of the island rises to more than 2,000 ft. Volcanic activity was in progress on the island when it was first sighted by Captain Cook, in 1774, and its activity has been frequently mentioned by travellers since that date.
The best description of the island is that given by Captain Purey Cust.* Whilst he was surveying the island in H.M.S. “Dart” in 1894 he witnessed an eruption of great magnitude. Captain Purey Cust describes a great central ash plain, as much as five or six miles in diameter, in the more elevated part of the island. On the edge of this, on its western side, are situated the two volcanoes Marum and Benbow, 4,380 ft. and 3,720 ft. respectively. The latter of these was active at the time of Purey Cust's visit, though the activity that took place in the crater of the volcano was explosive only, for the lava-streams which were emitted at that time escaped from orifices two to five miles to the west of the volcano. One of these streams flowed north-west and then north, finally reaching the coast at Krong Point after flowing for a distance of six miles. Another stream, which flowed from a point farther to the west, filled a small lake. The former of these streams is said to be no more than 10 yards wide in places.
There was a small eruption near the extreme east of the island in 1889. In 1910 a lava-stream issued from near the base of Mount Marum, and flowed in a N.N.W. direction, finally reaching the sea at a point about five miles distant from its point of origin.
Dr. Bowie has kindly given me the following account of the devastating eruptions of December, 1913, which were almost confined to the extreme western end of the island.
Some of the essential features of the island are shown in the appended map, which has been copied from that of Captain Purey Cust. On this map Dr. Bowie has kindly indicated the direction of the lava-flows of 1910, 1913, and 1914. A few of the surface features of the island are indicated in the accompanying sketch-map, but in order to obtain a good idea of its topography it is necessary to consult the map of Captain Purey Cust.
[Footnote] * Journ. & Proc. Roy. Geog. Soc., 1896, p. 586.
It appears tolerably evident that the central ash plain marks the site of a former large cone which was truncated by an immense explosive eruption, though Mawson refers to it merely as an old crater.
Report of Eruption in Ambrym Island, New Hebrides, 7th December, 1913, by Dr. Bowie.
On the 6th December, 1913, a little after 5 p.m. it was observed that Mount Benbow, Ambrym Island, was in active eruption to an exceptional degree. Gigantic columns of steam were seen to rise many thousands of feet in the air. After each explosion a fresh pillar could be made out, boring its way in a spiral fashion through the cloud which extended upwards from the mouth of the crater. The succeeding explosions followed each other in more and more rapid succession, as well as in added force, until within an hour the intervals between the explosions were reduced from about three minutes to under one minute, and ultimately there was apparently no interval at all.
(a.) Just before dark, dense columns of steam and smoke were observed rising somewhere in the western edge of the “Great Ash Plain,” and as darkness fell we could easily see a great river of incandescent lava rushing towards Baulap; another was running towards Port Vato. We climbed the hill immediately behind the hospital at Dip Point, and watched the Baulap stream until it reached the sea, which it did a little after 9 p.m. Lava, however, continued to flow down that valley for at least a whole day after this.
(b.) About 11 p.m. a new lava-stream was sighted rushing down the hillside about three miles and a half north-east along the coast. It reached the sea immediately to the west of Krong Point at about 2 a.m.
(c.) Soon after the last stream reached the sea it became very apparent that another lava-flow was on its way to the sea. During the whole night the sky was brilliantly lit up, but now, though there were high hills between us and the various rivers of lava, the valley in which we were was so illuminated that we could discern objects hundreds of yards away. This stream reached the sea two miles to the north-east of the hospital about 2.30 a.m., 7th December.
(d.) Almost simultaneously with stream (c) another made its appearance back in the hills at the head of Lowea Valley. This stream, one could easily observe, was coming much nearer to us, and we calculated the speed of the flow at about five miles an hour. Certainly the incline was, on the whole, pretty steep, and at one place the lava poured over a precipice about 60 ft. or 70 ft. high. The sight was magnificent as well as awe-inspiring. The lava, which was quite incandescent, came quickly on, burning up great forest-trees, tossing them all aglow in the air. As they fell again into the torrent they rebounded high in the air, emitting sparks like a thousand catherine-wheels. Soon after 3 a.m. the lava plunged with a savage hiss and a mighty roar into the sea. The sight was then superb, and never to be forgotten.
The din as the lava flowed through the forest reminded one very much of the noise of a hurricane with the boom of the ocean and the crashing of great forest-trees.
The last stream mentioned reached the sea within three-quarters of a mile of the hospital compound, but the hospital was built in a valley surrounded by hills except on the sea side. The valley behind was about a mile and a half long by about a mile broad at its widest part, and narrow-
ing to within half a mile at the sea. The hills around ranged from 500 ft. to 1,245 ft., and we considered everything inside this barrier as being quite safe.
Just about the time, or shortly before, the last stream reached the sea we saw a dense volume of black smoke rising from the hilltop on the east side, near the entrance to the valley and overlooking the hospital. When day broke we discovered the top of the hill was being blown out, but as yet there was no appearance of fire; only some continuous expulsive force was driving the hilltop out. There were no separate explosions observable. The force was continuous. Soon, however, fire appeared, and the force increased quickly, so that one could observe the hill steadily diminishing in height. The side of the hill looking west was acted upon, and decreased more quickly than the other sides, and about 10 a.m. on the 7th lava began to flow down the hillside into the valley and through part of the hospital compound.
(e.) The ground in the valley could be felt in motion—a kind of heave—not violently, but reminding one of a slight motion at sea. This motion was not continuous, but at very short intervals.
Shortly after 10 a.m. an explosive eruption blew the hospital and other buildings into the air. The force of this explosion was terrific. First a great sheet of what appeared to be electric flame flashed from the ground, and immediately following was a mighty roar and crash like myriad thunder-claps one piled on another. The heat, too, was intense. We could see for an instant only the corrugated iron spreading out like scintillating glass, and then it was gone. Explosion then followed explosion almost continuously for four days at least. Sometimes there would be a short interval, but the intervals did not last long.
(f.) We discovered at daylight that another lava-stream had flowed behind our hills towards Craig's Cove. This stream stopped on the plain about 500 yards from the beach. This stream was the biggest of all, being in some parts about three-quarters of a mile wide.
(g.) About 3 p.m. on the 7th a submarine volcano broke out about a mile from the shore off Lamb Point. Soon an island was formed, and within sixteen hours had joined the mainland. After four days the activity of this volcano ceased.
On Tuesday, the 9th December, a geyser suddenly broke out a few hundred yards inland from Craig Point. Water must have been forced to a considerable height, for we were drenched in a mud bath while over 500 yards from the shore. The geyser lasted for probably fifteen minutes or more. Immediately after the geyser ceased a roar like a rushing torrent was heard. Very soon a river extending in width from the trading-station at Craig Point to Malver tore its mad rush to the sea For half an hour or so there was a great volume of water, which gradually diminished in volume, and apparently in speed. This river ran for at least six hours.
On Sunday, the 7th December, there were six distinct craters in the vicinity of Dip Point, West Ambrym. The whole district was undoubtedly a fairly recent volcanic area. In the valley behind the hospital, as well as at Craig's Cove, outcrops of lava could be seen at various points, and there was little evidence of weathering, the lava in many places appearing rugged, as it does soon after cooling.
The rocks of Ambrym do not appear to have been described hitherto, and it has therefore been thought advisable to add to this account of the eruption by Dr. Bowie a few notes on the rock-specimens that have been
brought from this island. Some of these have been kindly forwarded by the Rev. Peter Milne, a missionary on Nguna Island, and others by Professor W. Morris Davis.
The locality of the specimens sent by Mr. Milne is not stated, but Dr. Bowie assures me that the rock is the commonest type that is found at the western end of the island. This rock is highly vesicular, and contains large rounded phenocrysts of feldspar as much as 1 cm. in diameter, but no other minerals can be distinguished in hand-specimens.
In section the rounded crystals of feldspar are found to be aggregates of bytownite, with an extincton-angle of 40° in those sections which are at right angles to the brachypinacoid. In the central portions of these aggregates there are a great many inclusions of magnetite and of glassy matter. These can be compared with the feldspar glomerules of the Mau basalt previously described by Mawson.* In the present instance, however, there is not a marginal zone of inclusions, nor are the marginal portions less basic than the central area. Smaller crystals of greenish augite and of perfectly colourless olivine also occur, and in some instances rounded grains of olivine are actually embedded in the feldspar. The groundmass consists almost equally of microlites of bytownite, short crystals but more frequently rounded grains of augite, and sometimes also of olivine. There is a considerable residue of brownish glass, densely filled with magnetite-dust.
From Dip Point Professor W. M. Davis has sent me a specimen of a compact grey rock which contains numerous feldspar phenocrysts. As in the previous case, the phenocrysts when examined with the microscope are found to be aggregates of several crystals, though they are less regularly arranged than in the preceding rock. The mineral in these aggregates is again bytownite. The groundmass consists mainly of bytownite with large grains of diopside and of colourless olivine, and small octahedrons of magnetite. There is a base of brownish glass, which contains numerous needles of ilmenite.
A specimen of the lava of 1913 at Dip Point was also sent to me by Professor W. M. Davis. The hand-specimen is iron-grey, and it is highly vesicular. There is no porphyritic feldspar, though green augite and olivine are distinct. In section there is found to be much feldspar in crystals of small size but much twinned; it is of a basic type, bytownite-labradorite. The augite is near diopside, but slightly pleochroic. The crystals of olivine are generally crowded with magnetite-dust. This appears to be an alteration of a somewhat unusual nature, as no mention of it can be found in text-books. A similar alteration occurs in the olivine of a dyke which traverses the scoria cone of Mount Eden, at Auckland, and also in basaltic rocks which occur near the margin of the pipe of the great central volcano of Tahiti, though in the last instance the change is marginal or restricted to the border of the cracks which traverse the mineral. It is still more noticeable in the peridotite that forms part of the pipe of Tahiti.
The groundmass of this Ambrym rock consists of feldspar and augite with magnetite. The augite of the groundmass is greenish, but often with a brown margin.
It will be seen that these rocks are all basalts, and it is stated by Mawson that all the recent products of volcanic activity in the New Hebrides are basaltic, though earlier eruptions were associated with the emission of andesitic rocks.
[Footnote] * “Geology of New Hebrides,” Proc. Linn. Soc. N.S.W., 1905, p. 463, pl. xxiii, fig. 4.