Art. XLII.—Note on the Andesites of White Island.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 46, 1913, Page 298

Art. XLII.—Note on the Andesites of White Island. By R. Speight M.Sc., F.G.S. [Read before the Philosophical Institute of Canterbury, 3rd December, 1913.] When Mr. W. R. B. Oliver visited White Island in December, 1912, in order to examine the ecological conditions of its vegetation, he collected specimens of rocks from various points, and kindly handed them over to the Canterbury Museum. The collection includes specimens of sulphur and crystals of gypsum; but more interesting from the geological standpoint are the volcanic rocks. The only published description of igneous material from this locality is that by W. A. MacLeod,” in a paper entitled “Note on a Hypersthene Andesite from White Island.”‡ Trans. N.Z. Inst., vol. 31, 1899, p. 488. In this he describes a solitary specimen, giving its macroscopic and microscopic characters, and also a chemical analysis of the rock. The collection made by Oliver appears to be fairly complete, and includes variations from MacLeod's rock. I have therefore furnished a few additional notes on the character of the lavas of this interesting island. It has been pointed out by various authors that the island lies on a broken line of volcanic vents stretching from Ruapehu in a north-easterly direction across the Pacific, and including Mounts Tauhara, Edgecumbe, the Kermadec Islands, and various centres of activity near the Tonga Group. Marshall would terminate the line with this group,§ Report Trans. Aust. Assoc, Adv. Sci., 1909, p. 446. but if this line be continued it will pass through the Samoan and Sandwich Island volcanic lines at the point of their present maximum intensity. This may be only a coincidence, but, if so, it is a somewhat striking one. It points rather to the intersection of a line of crustal weakness running north-east with two others running north-west by west (see Dana, “Characteristics of

Volcanoes,” p. 361). There does not appear to be any general resemblance in the character of the flows from widely separated points of this north-east line, even from those points which are generally accepted as belonging to it. There is a considerable difference between the augitehypersthene andesites and the augite-hornblende-hypersthene andesites of the Ruapehu group and the augite-olivine andesites and olivine-basalts of the Kermadecs and Tofua in the Tonga group; so that the absence of close resemblance of the Savaiian and Hawaiian lavas can hardly be urged against this hypothesis, even granting that a general similarity of type could be expected from closely connected vents on the same line of weakness—a condition which is very open to question. But there is certainly a very close agreement between the White Island volcanics and those of Ruapehu and Tongariro, as the following notes will show. In the hand-specimen all the samples are of dark-grey colour, and vary from compact to acoriaceous. Small phenocrysts of feldspar and of the pyroxenes are visible to the eye. The specific gravity varies from 2·60 in the more scoriaceous varieties to 2·66 in the more compact. This corresponds with MacLeod's specific-gravity determination of 2·65. Two specimens are from sea-level on the south coast. The first (No. 379 of the collection), when examined under the microscope, shows ordinary andesitic characters. Phenocrysts of feldspar, hypersthene, augite, and very occasional olivine are distributed in a groundmass of the pilotaxitic type. The feldspar is a medium labradorite as determined by the statistical method of Michel Lévy, zonal structure being extremely common, and inclusions or signs of alteration rare; hypersthene crystals are extremely numerous, exhibiting straight extinction and moderately low polarization colours; augite occurs in well-defined crystals, frequently showing twinning, and with high extinction-angles. The olivine appears as small crystals, clear, with brilliant polarization colours, and occasionally, where it was possible to obtain it, yielding the characteristic interference figure of a mineral with an axial angle nearly equal to 90°. Magnetite occurs freely as grains. The F.M. minerals are distributed throughout the slide, but occasionally occur in nests composed of only one mineral, or of two, or sometimes of all three minerals together. The groundmass consists of a felted mass of feldspar laths and microlites of either augite or hypersthene; some of these exhibit a greenish shade, and suggest the former mineral. Another specimen from the south coast, at sea-level, is of the glassy type of groundmass with a few feldspar microlites, and with a much smaller proportion of hypersthene and augite. The feldspars, which are larger in size than the average for the rocks, contain numerous inclusions of the glassy groundmass. This shows numerous well-defined perlitic cracks, and is rendered almost opaque with a dust of some dark-coloured mineral, probably magnetite. Very similar to this one is a specimen from the base of the cliff on the south side of the crater. Another sample, from the landslip at head of the crater, is like the first rock described (No. 379); but occurrences intermediate between the pure glassy type and the pilotaxitic type come from-flows at sea-level in Crater Bay and from the north coast. A flow from sea-level on the west coast of the island shows a divergence from the usual characters of these andesites. It contains, in addition to the usual plagioclase, a certain amount of sanidine, and the groundmass is composed of untwinned feldspars of rectangular shape and augite grains

without any glass, thus showing a distinct connection with the trachytes. The ferro-magnesian elements consist of the usual hypersthene and augite, some of the latter in fairly large crystals and beautifully twinned. It will thus be seen that there is a very close resemblance in the rocks of White Island to those of the great volcanoes lying in the middle of the North Island, and to the rocks, recently described by Bartrum,* Seventh Annual Report, N.Z. Geol. Survey, Appendix C, 1913, p. 141. occurring south of Tauranga, in the Te Puke district, although these last evidently belong to a much earlier period.