Ian Devereux MSc, PhD, FNZIC, AMAusIMM (1940-2020) - geoscientist and founder of Rocklabs

Obituar

Non-explosive, dome-forming eruptions at Mt. Taranaki, New Zealand

Volcanic domes may be emplaced rapidly and with few hazardous consequences, even at the summit of large stratovolcanoes. In this study the most recent activity of Mt. Taranaki in New Zealand is shown to have been a passive effusion of a c. 5.9millionm3 lava dome with minor associated explosions and little syn-eruptive hazard. This event, the Sisters eruption, appears to have been unrecorded by local indigenous populations but likely occurred between A.D. 1785 and 1820. The magma erupted is chemically distinct from the preceding A.D. 1755 Tahurangi eruption. Based on breakdown of hornblende crystal rims, the Sisters magma was probably only four days outside the hornblende stability field before cooling, and the magma ascended its last four km along a conduit at rates of 0.012ms-1. Based on dome surface morphology, a relatively low-viscosity magma is inferred. The dome remained in a metastable state for up to 70years following the eruption; eventually generating a large, cool (<350°C) collapse of at least 2millionm3 of rock, forming a highly hazardous mass flow that travelled over 5km. Factors contributing to the post-eruption dome instability include: its emplacement onto a steep flank of unconsolidated breccia or talus, possible oversteepening, fracturing due to spreading and subsidence, rapid cooling during heavy rain, and a hydrothermally altered inner core. The likely trigger for the collapse was a heavy rainstorm or an earthquake. Volcanic domes in the summit regions of volcanoes must be considered metastable and a potential source for hazardous, non-eruption related mass flows for many decades following their emplacement.16 page(s

Quaternary tephra marker beds and their potential for palaeoenvironmental reconstruction on Chatham Island, east of New Zealand, southwest Pacific Ocean

Tephras provide one of the most reliable methods of time control and synchronisation within Quaternary sequences. We report on the identification of two widespread rhyolitic tephras - the Kawakawa and Rangitawa tephras - preserved in extensive peat deposits on Chatham Island 900 km east of New Zealand. The tephras, both products of supereruptions from the Taupo Volcanic Zone, occur as pale, fine-ash dominated layers typically 10-150 mm thick. Mineralogically they are dominated by rhyolitic glass, together with subordinate amounts of quartz, feldspar, hypersthene, hornblende, Fe-Ti oxides and zircon. Phlogopite/biotite was identified additionally in Rangitawa Tephra. Ages for each tephra were obtained via mineralogical and major element glass composition-based correlation with well-dated equivalent deposits on mainland New Zealand, and we also obtained a new zircon fission-track age for Rangitawa Tephra (350 ± 50 ka) on Chatham Island. Both tephras were erupted at critical times for palaeoenvironmental reconstructions in the New Zealand region: the Kawakawa at ca. 27 cal. ka, near the beginning of the extended LGM early in marine isotope stage (MIS) 2; and the Rangitawa at ca. 350 ka near the end of MIS 10. The time constraints provided by the tephras demonstrate that Chatham Island peats contain long-distance pollen derived from mainland New Zealand, which provides a reliable proxy for identifying glacial-interglacial climate conditions, in this case during the MIS 11-10 and MIS 2-1 cycles. The two tephras thus provide important chronostratigraphic tie-points that facilitate correlation and synchronisation not only across the Quaternary deposits of the Chatham Islands group but also with climatically significant terrestrial and marine records in the wider New Zealand region

Colin George Vucetich (1918–2007)—pioneering New Zealand tephrochronologist

Many Quaternarists, tephrochronologists, and soil scientists mourned the passing in New Zealand of Colin Vucetich—gentle mentor, pedologist, and pioneering tephrochronologist—on 25 April (Anzac Day), 2007. Colin was in his 89th year. As well as forming a 25-year partnership with W.A. “Alan” Pullar, with whom he published three classic papers on tephrostratigraphy based on field work undertaken by the pair largely in their own time, Colin inspired and mentored numerous postgraduates in his later career as an academic at Victoria University of Wellington. There he taught pedology, soil stratigraphy, and tephrochronology until his retirement as Reader (Associate Professor) in 1982. In retirement he was an honorary lecturer and supervisor at Massey University (Palmerston North) until 1991 (Fig. 1, Fig. 2 and Fig. 3)

The age and origin of the Pacific islands: a geological overview

The Pacific Ocean evolved from the Panthalassic Ocean that was first formed ca 750 Ma with the rifting apart of Rodinia. By 160 Ma, the first ocean floor ascribed to the current Pacific plate was produced to the west of a spreading centre in the central Pacific, ultimately growing to become the largest oceanic plate on the Earth. The current Nazca, Cocos and Juan de Fuca (Gorda) plates were initially one plate, produced to the east of the original spreading centre before becoming split into three. The islands of the Pacific have originated as: linear chains of volcanic islands on the above plates either by mantle plume or propagating fracture origin, atolls, uplifted coralline reefs, fragments of continental crust, obducted portions of adjoining lithospheric plates and islands resulting from subduction along convergent plate margins. Out of the 11 linear volcanic chains identified, each is briefly described and its history summarized. The geology of 10 exemplar archipelagos (Japan, Izu-Bonin, Palau, Solomons, Fiji, New Caledonia, New Zealand, Society, Galápagos and Hawaii) is then discussed in detail