PREHISTORIC ERUPTION.

Shannon News, 11 April 1928, Page 3

PREHISTORIC ERUPTION.

NAPIER VOLCANIC DEPOSITS.

•MUD GLOBULES IN BEDS

New Zealand’s geological history is crammed with evidences of numerous volcanic eruptions. Probably one °* the motet extensive and violent resulted in the deposit of the vast pumice beds on the east coast of the North Island. This eruption occurred during the La-ter-Pleistocene period, or about three of four million years ago. Fresh information regarding this violent volcanic disturbance is given by Dr. J. Allen Berry, of Napier, in a study of the deposits as .in the Napier hills. According to Dr. Berry, it greatly exceeded in intensity even the eruption of Vesuvius, which overwhelmed Pompeii and Herculaneum, in 79 A.D. He has discovered in the* Napier deposits, small globular bodies composed of pumice, the formation of which, on the theory he propounds, was similar to that of hailstones. This ingenious theory is called by its originator “the hailstone theory. ” o VAST BEDS. OF VOLCANIC ASH. Napier is built largely on what was once an island, now known as Scinde Island. It and the surrounding country are composed largely of limestones which were raised above sea level, and by denudation assumed very much their present contour. Lying uncomformably upon these limestones, showing that they are of subaerial deposition, are beds of pumiceous clays and of pure voleanic ash. Within these layers of volcanic ash are millions of globular bodies varying in size from a pin’s head to a very large pea. A great volcanic eruption took place possibly, somewhere in the vicinity of the National Park, and vast clouds of volcanic ash were taken over Napier by the prevailing westerly .winds. How intense this eruption was is shown by the fact that even at a distance of between 80 and 90 miles from the probable volcano, there is a deposit of volcanic ash up to about/:sft in depth. During the course of tho eruption there must baVe been a darkness which literally could be felt, and all forms of animal and vegetable life, which then existed, were overcome. The discovery of m'oa bones and footprints suggests this. FORMATION OP MUD-BALLS.

■ On the/‘hailstone theory,’’ it is assumed there was a gredt amount of water vapour. Under the influence of surface tension and electrical attraction, .these particles would coalesce to form a spherical mass of mud. f In the formation of hailstones it is well known that the laminated structure they present is due to ascending currents of air causing the hailstone/o be surrounded by alternate layers of ice and snow, due to its being carried up into layers of the atmosphere having varying degrees of temperature. In the same manner, these masses of mud were carried up'into a layer of pumice dust, where there was not much water vapour, resulting in a dense lamination. As the result of increased weight, or a diminution iii the velocity of the ascending currents, and these ascending currents are quite common, they fell back into the layer of pumice dust containing a large quantity of water vapour and a less dense lamination resulted. This process of ascent and descent repeated several times would result in the formation of what Dr. Berry has called chalazoidites, from Greek words meaning “stone resembling hail.”

Dr. Berry lias shown from staining and X-ray methods that these lamina;tions are characteristic of these bodies; in fact they are very similar to the layers of an onion. Although these pumice bodies, or chalazoid>w?> must in New Zealand have a widespread distribution, no geologist has previously, directed attention to them. It is also remarkable that although they are probably a concomitant of every great volcanic eruption in any part of the world where dust with large quantities of water vapour is being ejected, the references to their pccurrrence in the literature are remarkably few. SIMILAR PHENOMENA.. The earliest description of them appears to be that of Dr. Edward Otis Hovey, who encountered them after the eruptions on Martinque in 1902. They have also been recorded in the great explosion of Kilauea, Hawaii, in 1879, by Dr. Jaggar, in a bulletin of the Hawaiian Observatory, A full description was given by Pratt of their occurrence in the Philippines. They have been found in recent eruptions of Vesuvius and are we. 1 -known in the volcanoes of Japan, particularly Oshima, in the Pacific Ocean, near Tokio. Dr. Berry has recently discovered that ehalazoidites fell during the eruption of Vesuvius which overwhelmed Pompeii in 79 A.D., and that they were similar to specimens which have been found in Napier and Taupo. With a complete knowledge of the distribution of these ehalazoidites in New Zealand it will be possible to locate the exact site of the volcano or volcanoes responsible, and throw much fascinating light on the past volcanic history of our country. Mr J. A. Bartrum, professor of geology at the Auckland University, says the only specimens comparable to them which he has seen, though they are of different origin, are the deposits of larger concretions which have been noted in the c'lt near the diversion tunnel at the Arapuni dam by Mr J. 11. Gardner. The Arapuni concretions are about the size of marbles, and form in little clusters which occur in definite planes of the silt, They are probably formed during floods in sluggish waters by the rolling along of initial sandgrains with accretion of smaller particles. Professor Bartrum considers Dr. Berry’s theory to be highly reasonable, although the Napier ehalazoidites may possibly have been formed by gentle rain falling through pumice dust clouds during the violent eruption: which undoubtedly took pla«e.