V Olcanic Ash Southern Style

Nature Vol. 286 28Augusf 1980 841

only incapacitate the gene but result in the The molecular analysis of globin genes is arising in the more northerly parts of the expression of a new protein detrimental to probably more advanced than any other North Island with a redeposited the whole animal. Although such mutant mammalian gene system at this time. It tephra material which might be classed as a genes might be expected to rapidly seems likely that when other gene families loess. According to Kennedy tephric loess disappear from the population, further are equally well characterized new is deposited during arid climatic periods inactivating mutations might occur that pseudogenes will be identified. Indeed, and is always found sandwiched between could then allow these genes to become extrapolating from the situation in globin, tephras of 15,000-20,000 yrs BP. Thus the fixed in the population. There is, in any one in four gene sequences may be period 15-20,000 BP is regarded as cold and case, no reason to believe that pseudogenes pseudogenes. Put in simple terms this arid. In the Rotorua basin the average rate per se have any function although, of suggests that one quarter of our genes of accumulation during that critical period course, new genes could evolve from them. might be dead. 0 was 0.3 cml yr. D. Seward (N.Z. lnst. Nuclear Sci.) reported on progress in fission track dating of tephra deposits. Tephras represent time planes within both marine and continental sediments and are most suitable for stratigraphic chronology and correlation. Glass was the first material used for fission V olcanic ash southern style track dating of tephras but most glasses from fan Smalley have undergone varying amounts of annealing and this has reduced the size and WITH the eruptions of Mt. St Helens still Kidnappers tephras may enable events on density of the spotaneous fission tracks, making the headlines in the northern land to be tied into the sea-level chrono and produced a 'young' age. The most hemisphere this was a good time to have a logy. Nine discrete rhyolitic tephras, which suitable mineral in the tephras is zircon . conference* on volcanic ash deposits (alias Kamp has named informally A to I, occur The Tertiary-Quaternary sequences in New tephras) in New Zealand. It might be noted in the 350 m thick Kidnappers Group Zealand contain sufficient lephras for a that the St Helens eruption has produced succession. tight time scale to be realised throughout. about I kmJ of ash but the last eruption of V. Neall (Massey University) offered However, because of error limits the fission the Taupo volcano in the mid North Island some comparisons between the volcanoes track ages of individual tephras must be of New Zealand produced about 24 km 3, in the Cascade Range in the Western closely analysed before tephras from one possibly all in about 15 minutes through a United States (including Mt. St. Helens) sequence can be absolutely correlated with vent 100 m wide. and Mt Egmont in New Zealand. The those from another. C. Vucetich (Victoria P. Froggatt (Victoria University, geological setting of each volcano and its University, Wellington) offered some Wellington) reviewed the tephras relationship to non-volcanic rocks Quaternary tephra correlations, from from Taupo. There have been 8 provides useful comparisons, particularly c.40,OOO yrs BP to c.300,OOO yrs BP. The major rhyolitic eruptions during the past with respect to control of pyroclastic flow chronology was based on two fission track 10,000 years. Their stratigraphy and and lahar mudfJow distribution. Fre dates for zircons by B. Kohn: Puketirau distribution is well understood and provides quency of tephra eruptions in Holocene tephra (Tirau) 0.22 ± 0.03 Myr and useful marker beds over most of the North times varies from 1 per 100-200 years at St. Omokoroa tephra (Tauranga) 0.36 ± 0.04 Island. A source vent for each eruption has Helens (based on the last 4,000 years Myr and indirect dating per c.120,OOO yr now been established. The most useful and record), to 1 per 250-350 years at Egmont, marine transgression. widespread marker bed is the Taupo 1 per 500-1,000 years at Rainier and 1 per D. Lowe (Waikato University) pumice (1,820 yrs BP) which erupted from 800 years at Shasta. The Egmont tephra demonstrated the usefulness of tephra within Lake Taupo at Horomatangi Reefs. succession displays a remarkably uniform layers preserved in peats. The recent The stratigraphy and nature of the Taupo mineralogy in marked contrast to tephras identification of more than twelve pumice have now been revised and show a within the last 500 years. Pyroclastic flows unweathered and unmixed thin discrete air variety of eruptive mechanisms. The last from Egmont have been as frequent as fall tephras in piston cores from shallow phase of the eruptions was the most those produced from St Helens and Shasta, peaty lakes near Hamilton have provided a voluminous and destructive, being a series but are much less in volume. stratigraphic record of late Pleistocene of pyroclastic flows which swept radially A. Palmer (Victoria University, (post-Kawakawa tephra) and Holocene outward over all topography for more than Wellington) considered the occurrence of tephra deposits in the Waikato region. 60 km . Reconstruction of the eruptive airfall tephra in the loess of the Wairarapa, Identification of the tephras was greatly history of this ignimbrite has important at the southern end of the North Island. aided by using X-radiography techniques implications for older eruptions such as the The 20,000 yr BP Kawakawa tephra occurs and these are being actively developed by Kawakawa tephra (20,000 yrs BP) and the as a pinkish white 5-10 cm band of coarse the Waik'ato group, with particular older welded ignimbrites. silt to fine sand-sized glass and pumice possibilities seen in the techniques of P . Kamp (Waikato University) discussed fragments and is consistently found at two xeroradiography which utilises a reusable the Pleistocene tephras in Hawkes Bay and thirds the depth of the top (Ohakean) loess photo receptor plate instead of their implications for Quaternary events. unit. The tephra influences bulk density conventional X-ray film. The tephras provide an absolute time-scale and water contents for at least 50 cm above The situation may appear hazardous but for the Kidnappers Group sedimentary the primary airfall material and similar like the farmers persistently cultivating the succession, the deposition of which was bulk density and water content variations slopes ofEtna the scientists find rich results predominantly glacio-eustatically con within the older loesses probably indicate are derived from volcanic ash . The scale of trolled and which matches the standard volcanic ash. Some further observations on New Zealand vulcanism in the past has deep-sea chronostratigraphy (core loess were provided by N. Kennedy (DSIR been colossal with the Taupo eruptions V28-238). The correlation of Pleistocene Soil Bureau, Rotorua) but he was judged by G.P.L. Walker [0 have been tephras elsewhere in New Zealand with concerned with what has come to be known perhaps among the largest ever. The as 'tephric' loess. Some problems arc enormous spread of volcanic material in New Zealand means that the datable fan Smalley is in the Soil Bureau, Department of *Tephra Workshop meeting. held at Victoria University. volcanic events in the north can offer useful Wellington, on 30 June and I July 1980. Organised by C. Scientific and Industrial Research, Lower Hutt, Vucetich, R. Howarth and P. Froggatt; proceedings to be stratigraphic indicators even down into the New Zealand. published by the Geology Deparrmem. Victoria University. glaciated south. 0