;o8 



NA TURE 



[January 29, 1903 



CHARACTERISTICS OF RECENT VOLCANIC 

 ERUPTIONS} 



THERE is a remarkable similarity between the islands of 

 St. Vincent and Martinique. Both are roughly oval in 

 form, with the long axis almost north and south. The north- 

 west portion of each is occupied by a volcano, the Soufriere and 

 Mont Pelee, which have many points in common. Both 

 volcanoes show a single or practically single vent, and a 

 remarkable absence of parasitic cones and a scaicity of dykes. 

 In both a transverse valley exists to the south of the volcanoes, 

 and the main discharge ot ejecta during the recent eruptions, 

 which have often been nearly synchronous, has been into this 

 depression, and especially into its westerly portion. In both 

 islands, the recent eruptions have been characterised by 

 paroxysmal discharges of incandescent ashes, with comparatively 

 few larger fragments and a complete absence of lava. 



There are, however, a few points of difference. The eruptions 

 of St. Vincent have been altogether on a much larger scale than 

 those in Martinique. The area devastated was considerably 

 larger, the amount of ashes ejected probably ten times as great, 

 and if the loss of life was not so large, this is accounted lor by 

 the absence of a populous city at the foot of the mountain. 

 While both volcanoes show practically a single vent, this is 

 much more marked by the case of St. Vincent, where, excepting 

 the new crater, which is practically part of the old or main one, 

 there is not a single parasitic cone. We saw no fumaroles, no 

 hot springs, or any trace of radial cracks and fissures. 



On Mont Pelee, it is true, the main activity is confined to a 

 restricted area about the summit of the mountain, and the top 

 of the great fissure which extends or extended from this down in 

 the direction of the Riviere Blanche ; and there are no parasitic 

 cones comparable, for instance, to those which are so numerous 

 on Etna ; but there are many fumaroles, which Prof. Lacroix 

 and his colleagues speak of as emitting gases hot enough to melt 

 lead and even copper wire. A telegraph cable has been three 

 times broken at about the same place, and the broken ends on 

 one occasion, at any rate, showed marks of fusion. There are 

 also several hot springs. Judging from these and other indica- 

 tions, it is most probable that radial cracks entered deeply 

 through the substance of the mountain, and penetrated even the 

 submarine portion of its cone. 



The local distribution of erupted material in Martinique is 

 accounted for by the great fissure at the top of the valley of the 

 Riviere Blanche, which communicated with the main pipe of 

 the volcano, and out of which the eruptions took place. This 

 fissure, which was mentioned as existing in the eruption of 1S51 

 pointed almost directly towards St. Pierre, and as the eruptec 

 material flowed out almost like a fluid, it was directed straight 

 down on the doomed city. The lowest portion of the lip of the 

 crater of the Soufriere was much broader and more even, so the 

 incandescent avalanche which descended from it was spread 

 much more widely. 



The latest accounts from Prof. Lacroix indicate that the 

 recent small eruption of Mont Pelee has filled up the highest 

 parts of the fissure and formed a cone, the foot of which covers 

 up the former crater ring. In any further eruption, therefore, 

 the avalanche of incandescent sand will not be confined to the 

 district of the Riviere Blanche, but may descend on any side of 

 the mountain. 



The accompanying photograph of Mont Pelee in eruption was 

 obtained from a ten-ton sloop in a sea way and is therefore not 

 quite sharp. Attention was directed to the eruption by a 

 peculiar black cloud which appeared over the volcano and then 

 rolled down the side of the mountain to the sea. The cloud 

 was formed of surging, rolling, expanding masses, in shape 

 much like those of the previous cauliflowers, but quite black, and 

 full of lightning-flashes and scintillations, while small flashes 

 constantly struck from its lower surface on to the sea. The 

 upper slopes of the mountain cleared somewhat, and some big 

 red-hot stones were thrown out ; then the triangular crack 

 became red, and out of it poured a surging mass of incandescent 

 material, reminding us of nothing so much as a big snow- 

 avalanche in the Alps, but at a vastly different temperature. It 

 was perfectly well defined, did not at all tend to rise like the 

 previous cauliflowers, but flowed rapidly down the valley in the 

 side of the mountain which had clearly been the track of pre- 

 vious eruptions, until in certainly less than two minutes it reached 



1 From a discourse delivered by Dr. Tempest Anderson at the Royal 

 Institution on January 23. 



NO. 1735, VOL. 67] 



the sea, and was there lost to view behind the remains of the 

 first black cloud, with which it appeared to coalesce. There 

 and on the slopes of the mountain were doubtless deposited the 

 greater part of the incandescent ash, while the steam and gases, 

 wiih a certain portion of still entangled stones and ash, came 

 forward in our direction as a black cloud, but with much greater 

 rapidity than before. The cloud got nearer and nearer ; it was 

 well defined, black and opaque, formed of surging masses of the 

 cauliflower type, each lobe rolling forward, but not all with 

 one uniform rotation ; bright scintillations appeared, some in the 

 cloud itself and some like little flashes of light vertically between 

 the cloud and the sea on which it rested. This was clearly the 

 phenomena described by the survivors in the St. Vincent erup- 

 tion as " fire on the sea," occurring in the black cloud which 

 overwhelmed the windward side of that island. We examined 

 them carefully, and are quite clear that they were electric dis- 

 charges. The scintillations in the body of the cloud became 

 less numerous and more defined, and gradually took the form of 

 vivid flashes of forked lightning darling from one part of the 

 cloud to another. When the cloud had got within perhaps hah 

 a mile or a mile of us — for it is difficult to estimate distances 

 at sea and in a bad light — we could see small material falling 

 out of it in sheets and festoons into the sea, while the onward 

 motion seemed to be chiefly confined to the upper part, which 

 then came over our heads and spread out in advance and around 

 us, but left a layer of clear air in our immediate neighbourhood. 

 It was ablaze all the time with electric discharges. 



— Photograph of an eruption of Mont Pelee. 



As soon as it got overhead, stones began to fall on deck, some 

 as big as a walnut, and we were relieved to find that they had 

 parted with their heat and were quite cold. Then came small 

 ashes and some little rain. The cloud was also noticed at Fort 

 de France. It was described as like those in the previous erup- 

 tions, but was the only one in which electric scintillations had 

 been noticed. Two unbiased observers, who had seen it and 

 that of May, declared this was the larger of the two. 



As to the mechanism of the hot blast and the source of the 

 power which propelled it, both Dr. Flett and I are convinced 

 of the inadequacy of previous explanations, such as electricity, 

 vortices, or explosions in passages pointing laterally and down- 

 wards, or explosions confined and directed down by the weight 

 of the air above. Such passages into the mountain, which, to 

 be effective, would require to be closed above, do not exist in 

 the case of the Soufriere, and we are not aware that they have 

 been observed in Mont Pelee ; and as to the weight of the air, 

 this did not prevent the explosions in the pipe of the Soufriere 

 from projecting sand and ashes right through the whole thick- 

 ness of the trade-winds until they were caught by the anti-trade 

 current above and carried to Barbados. Moreover, the black 

 cloud, as we saw it emerge from Mont Pelee, seemed to balance 

 itself at the top of the mountain, start slowly to descend and 

 gather speed in its course, and the second incandescent dis- 



