MENACE OF 'EXTINCT* VOLCANOES (Continued) Research fundsthe weapon most lacking to volcanologists will enable us to understand this mys¬ terious phenomenon infinitely better than hitherto. I would like also to mention the two other new aids to observation and forecasting. One resembles the tiltmeter but provides more easily interpretable data than tilt variations which, especially with volcanoes of the circum-Pacific type, are often mis¬ leading. <strong>The</strong> method consists of measuring the diameter of a crater by means of a tellurometer. Robert W. Decker, whose idea it was, measured the dia¬ meter of Kilauea at fairly close inter¬ vals and discovered that it was increas¬ ing continuously and quite noticeably right up to the time of eruption. Before instruments were available to measure distances of up to several tens of kilometres very accurately and very rapidly, such operations were much too slow and expensive to be of practical value in volcanology. It is now quite possible that the Decker method will produce results greatly superior to those obtained by clino¬ meters. <strong>The</strong> second method, used for longrange forecasting (several months to, perhaps, several years), is based on a hypothesis deduced by Mr. C. Blot, head of the geophysics section at the ORSTOM Centre, Noumea (New Cal¬ edonia), from the relationship which appears to exist between certain deep (550 to 650 kilometres below the sur¬ face) and intermediary (150 to 250 kilo¬ metres) seismic effects, and certain eruptions in the New Hebrides archi¬ pelago. seismic effects, intermediary effects and volcanic eruptions. <strong>The</strong> relation¬ ship between deep effects and erup¬ tions cannot be a direct one, since explosions and lava flows do not ori¬ ginate at depths of 400 to 700 kilo¬ metres. "However, a certain alteration in tensions or an abrupt change of phase at these depths could set off a thermoenergy phenomenon which in zones where the critical physical conditions obtain, particularly under the volcanic arcs could cause other rapid changes producing intermediary seismic effects at depths between 250 and 60 kilo¬ metres, depending on the regional tectonics. <strong>The</strong>se would be the zones in the upper mantle where the molten pockets occur and where magma forms and where, according to the views of, for example, Dr. Shimozuru and Dr. Gorshkov, volcanic eruptions originate. "By observing seismic activity in a given area following the start of deep seismic effects and by the detection and localization of intermediary effects beneath volcanic areas, it will be possible to keep a closer watch on one sector or one volcano several months before a possible eruption." <strong>The</strong> relationships discovered in the New Hebrides and, with increasing frequency, throughout the Pacific show that there may be a constant interval between the beginnings of phenomena at different levels right up to the sur¬ face, if the depth, distances and inten¬ sities of these phenomena and other, still somewhat indeterminate tectonic, physical and chemical factors can be taken into account. 12 results S INCE submitting the first of his observations at the General Assembly of the International Union of Geodesy and Geophysics at Berkeley in August 1963, Blot has been applying the relationships which he has discovered -to attempted forecasts of the volcanic eruptions in the region of the New Hebrides. In the last three years, the volcanoes Gaua, Ambrym and Lopévi all resumed strong activity on dates forecast months in advance. In collaboration with Mr. J. Grover, chief of geological survey, Solomon Islands, it has been possible to extend these studies and forecasts to the vol¬ canoes of the Santa Cruz and Solo¬ mon Islands (Tinakula and various underwater volcanoes). At the last Pacific Scientific Con¬ gress in Tokyo, September 1966, Blot and Grover presented a paper setting out the results of these forecasts of volcanic eruptions in the south-west Pacific which ended a follows: "It seems more and more likely that a relationship exists between deep <strong>The</strong>se intervals appear to be, on average, from 10 to 14 months between the 650 and 200 kilometre levels (along the line of the 60° inclination of the deep structures of the Pacific arcs) and from 4 to 8 months between the intermediary seismic effects 200 kilo¬ metres beneath the volcanoes and the actual eruptions. If this theory proves true, it would be of the utmost value for the fore¬ casting of possible cataclysms. So far it has not been possible to verify it thoroughly outside the New Hebri¬ des area and, even there, it is a little early yet to draw any final conclusions regarding the reality of these rela¬ tionships. Though some questions remain to be answered about the mechanical, physical and chemical processes which determine the upward propagation of endogenic energy at a speed of some hundreds of kilometres a year from the depths up to the neck of a vol¬ cano, this link, if it really exists, will certainly be one of the basic criteria in volcanological forecasting in the future. Photo USIS KILAUEA, THE HELPFUL
Kilauea, on the Pacific island of Hawaii, gives due warning of its eruptions. It is one of the few volcanoes whose behaviour has so far facilitated early forecasts of impending activity. Its eruption in December 1959 was foreseen six months earlier thanks to recorded data which volcanologists used to predict with unprecedented accuracy the place and time of the eruption, thus enabling the local population to be evacuated in time. Photo shows lake of molten lava that fills the Kilauea crater (10 sq. km.: 4 sq. miles in area).
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