534 Professor J. W. Gregory — The Ambrym Eruptions, 



Admiral Purey-Cust has described the upper part of the Krong 

 Point lava stream as hollow, since he heard the flow of a swift stream 

 of water along the centre (1896, p. 17). He also describes (ibid., 

 p. 13) the lava stream in the gully west of Fo-luk as concave ; the 

 upper surface has apparently sagged owing to the central lava having 

 flowed away and left the roof unsupported. 



The characteristic form of the lava-flows on Ambrym is therefore 

 that of lava pipes, in which the length may be 300 times the breadth, 

 rather than lava streams of normal proportions. 



(c) The Fissure Eruptions. 



The tubular structure of these lava-flows is the more remarkable 

 as most of them appear to have been the result of fissure eruptions. 

 According to the Rev. M. Frater the lava from the western vents 

 discharged from a series of fissures, through which the lava quietly 

 welled forth without violent explosions. Thus he says that near 

 Meltungan "streams of lava literally gushed out of the fissures 

 without intermittent explosions of steam ". According to the usual 

 conception of fissure eruptions they normally give rise to sheets of 

 lava ; but the Ambrym lava must have been so liquid that it 

 discharged through the lowest outlet from the fissure like water from 

 a reservoir ; this phenomenon is well illustrated by the narrow 

 stream which overflowed from the great lava sheet south of Ht. Maruni 

 and reached the sea at Port Vato. If the fissure had been opened 

 on the top of the level plateau it might have formed a widespread 

 lava sheet ; but as each fissure was discharged through a lateral 

 notch into some narrow gully it gave rise to a narrow lava pipe. 



That the western eruptions of Ambrym in 1894 also were from 

 fissures is indicated by the descriptions of Admiral Purey-Cust. 

 "He states (1896, No. 2, p. 600) that at Single Palm Hill "There 

 was no regular crater, and it was evident that the lava had burst up 

 from the ground in all directions, and had been either violently 

 impelled up the hillsides by hydrostatic pressure, or else the hillsides 

 had been themselves split open". 



Another striking feature of the eruption was the contrast between 

 the quiet discharge from the fissure near Fo-luk and the explosion 

 from the western end of the same fissure at Lon-wol-wol. According 

 to Mr. Frater the lava that escaped quietly from the fissures was as 

 saturated with steam as that which was thrown explosively from the 

 central craters. This difference was probably due to the varying 

 amount of water in the ground traversed by the lava. The fissure 

 near Fo-luk ran along the central ridge of the island a little to the 

 south of the crest, which there rises in places to 1,210 and 1,310 feet 

 above sea-level. This ridge being well drained on both sides was 

 relatively dry. The only steam available at the eruption there was 

 brought up by the rising lava. But the fissure at Lon-wol-wol was 

 close to the coast and only slightly above sea-level. The ground 

 there was probably saturated with water, and the ascent of the lava 

 along this fissure produced superheated steam, which suddenly found 

 relief in the explosion that formed the volcanic trough. 



