INTERESTING SURVEY BULLETIN

Evening Star, Issue 22720, 6 August 1937, Page 12

INTERESTING SURVEY BULLETIN

TAGEOLOCY OF THE ROTORUA* TAUPO SUBDIVISION ■ A welcome addition to our knowledge of New Zealand of more than general interest has appeared in the form, of the New Zealand Geological Survey’s latest Bulletin No. 37 on ‘ The Geology of the Rotorua-Taupo Subdivision.’ The field investigations in this district were carried out under the direction of Dr L. I. Grange.,now Director of "the New "Zealand Soil Surrey. ' ' The first chapter, one of General information on the region, Includes its, communications, industries, and vegetation and a short account of the climate with: valuable tables on temperature, rainfall, etc., has been added by Dr E. Kidson. Next follows a detailed account of the surface features of the country, with particular • reference to the plateaus and lakes which occur in considerable numbers. This part of the report is illustrated by two excellent block diagrams that giro a clear idea of the relief. The Rotorua-Taupo area of thermal activity occupies a huge down-faulted valley or graben directed north-north-east with marginal plateaux, Patetere on the west and Kaingaroa on the east, from I,Booft to 2,000 ft in elevation. At the southern and lowest portion of the graben is Lake Taupo (1',240ft above sea level, 238 square miles in area, and 534 ft in maximum depth), hounded by .faults’on its west and north shores, although on the east its bottom, in general, silopes gradually up to the adjoining highlands. In prehistoric times, the lake formed the vent from which violent explosive eruptions cast Volcanic ash over thousands of square miles. The lakes clustered in the RotoruaTarawera district are of somewhat diverse origins. Lake Rotorua occupies a down-warped basin; Rotomahana was gteatly deepened and enlarged by the 1886 eruption; Tikitapu (Blue Lake) occupies an early explosion crater; Rotokakahi (Green Lake) is .located in a down-faulted valley blocked at its north end by a lava flow; and Tarawera' ana Okataina appear to’ have been formed under similar conditions. The section of general geology deals with the various volcanic - ash showers and with the flows of rhyoite lava which make up the _ greater portion of the thermal district. Of particular interest are the finely crystalline, glassy rocks called ignimbrites formed by devastating eruptions of very hot gascharred sand such as was emitted from Mont Pelee in Martiuque and Mount Katmai in Alaska; A description is also, given of the eruptions during historic times such as Tarawera (1886) and-Frying Pan Flat (1917), the accompanying photographs giving a vivid idea of the phenomena. The hot% springs, fumaroles, and geysers are - dealt with in detail, and slropg similarity with those of Yellowstone National Park demonstrated. The extensive research carried out' has enabled these springs to be classified, and it is to be hoped that this work will be extended by the establishment of a vnlcanological observatory similar to those in Hawaii and Java. The final chapter on economic geology indicates the potentialities of this district as a source of diatomite and pozzolanas for cement. The utilisation of volcanic heat as in Italy is also recommended. The publication is well stocked with photographs exhibiting the different phenomena of this wonderful region, and wiJl prove of great interest, not only to New Zealanders, but also to overseas readers. A fine colour-plate of the Pink Terraces before their destruction in the Tarawera eruption adds greatly to the illustrative value of the memoir. In the accompanying portfolio are 12 coloured geological maps (scale one inch to mile) of the different survey districts, plans of the hot springs at Rotorua and Whakarewarewa. and a map of the many showers of volcanic ash that form the soils of the area.