RUAPEHU

Evening Post, Volume CXL, Issue 59, 7 September 1945, Page 9

RUAPEHU

A TECHNICAL REPORT

SEISMIC INVESTIGATION ADVISED

Mr. H. E. Fyfe, field geologist on the staff qf the Geological Survey, in a preliminary report on his observations at Mount Ruapehu during August, pointed- out that so far seismic activity preceding the present eruption had not been investigated. "It is rather important that this should be done to decide whether the activity derives from some deep-seated source, or merely from the 're-heating' of a residual pocket of magma left from some previous outburst. Reheating of magma, he writes, might be objected to by volcanologists, and perhaps the term was inappropriate; what he wished to imply was that a shallow pocket of magma had become activated by relief of pressure. The minor historic eruptions from Crater Lake might be cited as evidence. Investigation of the seismic history of the area over the past few years might throw considerable light on the present activity, and point to either a deep-seated or a shallow magma source. If it arose from the former, prolonged and probably increasing activity might be expected from Ruapehu; and if from the latter, the activity might be expected to wane within a comparatively short time. A MINOR ACTIVITY. Again, said Mr. Fyfe, if the magma were deep seated; there was every possibility of a major eruption from Ruapehu with attendant violently explosive outbursts; but it was his opinion, founded chiefly on comparison with the history of other vo.lcanian types of eruption, that the present activity was of a minor order and would not be accompanied by disi astrous consequences. "So little is defintely known of the actual mechanism of volcanoes that it was rather difficult to predict the future course of Ruapehu's activity," he said. "From the copious steam emitted, it is reasonable to assume that the magma is rich in volatiles, chiefly water, and if the magma is deep seated the present activity may be controlled by physical conditions in the conduit restricting and controlling the physico-chemical reactions in the upper levels of the lava column. With the removal of any construction in the vent, more violent phases of eruptive activity may ensue. On the other hand, the cooling effect of the rapidlyexpanding steam and gas, carbon dioxide, hydrogen sulphide, and sulphur dioxide, is likely to maintain a viscous or semi-solid plug in the conduit, and so control the course of the eruption. The balance between the rigidity of this plug, and the evolution of steam lower in the conduit, will determine the violence of the explosive outbursts. Steam pressure may build up sufficiently to shatter the crater walls, and in such a case it was likely that the violence of such an outburst would be directed . towards the Wangaehu side, the weakest part of the crater rim." STEAM EXPLOSION IMPROBABLE. Violent steam or phreatic explosions were, however, usually engendered by access of sub-surface water to some part of the laval column, and at the present stage of eruption that possibility was not entertained, though in the dying phases of the present eruption surface water might find its way into the top of the lava column. Mr. Fyfe inferred that such a steam explosion was improbable. Different aspects of Ruapehu's present and future activity had been discussed with the Director and Mr. A. C. Beck in the field, and the conclusion was reached that the present outbreak would not be attended with disastrous consequences. Nevertheless, it was urged that every available method, seismologic and geophysical, should be employed in an endeavour to establish the source and extent of the magma. It was well established that volcanic outbursts were preceded by tilting of the volcanic edifice, measurable by seismographs, tilt-meters, and precise levelling, and magnetic anomalies accompanied the sub-sur-face movement of magma.

A WARNING

"Investigation should proceed along these lines in an endeavour to find out what' is happening and likely to happen at Ruapehu. Further, we can no longer assume, that activity in the Tongariro-Ruapehu zone is dying or dead. It has been > merely . dormant. The structural set-up and deep-seated fault complex extending from the Bay of Plenty to Ruapehu along which there has been profound vulcanicity in times past, is ideal for repeated disastrous outbursts. Sooner or later they will eventuate."