[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.