Pyroclastic deposits and volcanic history of Mayor Island

The emergent summit of Mayor Island, Bay of Plenty, New Zealand, is a peralkaline rhyolite volcano constructed by: a sequence of lava flows, the Tutaretare Rhyolite Formation new; and pyroclastic deposits, the Oira Pyroclastite Formition (new). These 2 formations constitute the Mayor Island Group new. The pyroclastic deposits mantle most of the outer slopes of the island, in places exceeding 100 m in thickness, and also occur interbedded with lava flows of the main cone. The pyroclastics have been informally assigned on the basis of their compositional, welding and textural, and sedimentary structural characteristics to one or other of 15 lithotypes which may be related to particular modes of eruption and emplacement, of both airfall (phreatic, phreatomagmatic, phreatoplinian, and plinian types) and pyroclastic flow (ignimbrite, nuée ardente, and base surge types origins). A sixteenth lithotype comprises epiclastic deposits formed possibly by catastrophic overspill from an ancestral crater lake. Two new radiocarbon dates on logs from the pyroclastic deposits are recorded: (Wk105) 8000 ± 70 years B.P., and (Wk77) 6340 ± 190 years B.P. Recognition of the calcalkaline Rotoehu and possibly Rotoma Ashes on Mayor Island, together with the new radiocarbon dates, enables definition of 8 phases of major volcanic activity, each separated by relatively quiescent periods with erosion and paleosol formation. Volcanism commenced sometime prior to 42 000 years ago and has continued intermittently up to the eruption of the young dome lavas, possibly less than 1000 years ago. At present, only I Mayor Island-derived tephra has been identified on the mainland of the North Island, namely the Tuhua Tephra dated (Wk77) at source as 6340 ± 190 years B .P. However, the character and magnitude of several of the pyroclastic units on Mayor Island is such that recognition of other peralkaline tephras is anticipated in northern North Island

Provenance history of a Late Triassic-Jurassic Gondwana margin forearc basin, Murihiku Terrane, North Island, New Zealand: petrographic and geochemical constraints

The Murihiku Terrane in the North Island was a forearc basin adjacent to a volcanic arc along the eastern margin of Gondwana during the Mesozoic. The rocks that infill the basin are mainly volcaniclastic sandstones and mudstones, often turbiditic, with sparse shellbeds, rhyolitic tuffs, carbonaceous sandstones, plant beds, concretionary horizons, and rare thick granitoid-rich conglomerates. Petrographic studies of the rock fragments in the sandstones show that andesites are the dominant lithic type, but there is a wide range of other lithologies, including dacites, rhyolites, ignimbrites, granitoids, quartzofeldspathic mica schists, rare amphibolites, and reworked mudstones and sandstones. The sandstones are texturally and mineralogically immature and suggest deposition relatively close to a source of high relief, undergoing physical rather than chemical weathering in cool- to cold-temperate conditions. Geochemical analyses of 67 whole-rock volcaniclastic sandstones and siltstones indicate that they were derived from an active and dissected volcanic arc in a convergent margin setting built upon relatively thin continental crust. Modal petrographic data and whole-rock geochemistry both confirm that there were systematic variations with time in the composition of clastic material being supplied to the basin. From the Late Triassic to Middle Jurassic, there was a decrease in silicic volcanic material, plutonics, and metamorphics, and an increase in the supply of andesitic detritus. This was followed in the Late Jurassic by a broader range of volcanic detritus, varying from basaltic andesite to rhyolite, which may have been caused by progressive extension of the volcanic arc and thinning of the crust, a precursor to the breakup of Gondwana in the Early-Middle Cretaceous. Comparison with the Southland segment of the Murihiku Terrane in the South Island suggests that there were significant along-arc source variations, with relatively less silicic but greater andesitic and continental crust contributions in the North Island than in Southland. This may be analogous to the modern Taupo-Kermadec arc where there is a south-north along-arc transition from a continental to an oceanic arc

Ages of the Pliocene-Pleistocene Alexandra and Ngatutura Volcanics, western North Island, New Zealand, and some geological implications

The Alexandra and Ngatutura Volcanics are the two southernmost of the Pliocene-Quaternary volcanic fields of western and northern North Island, New Zealand, northwest of Taupo Volcanic Zone TVZ. The Ngatutura Basalts are an alkalic basaltic field comprising monogenetic volcanoes. The Alexandra Volcanics consist of three basaltic magma series: an alkalic (Okete Volcanics), calcalkalic (Karioi, Pirongia, Kakepuku, and Te Kawa Volcanics), and a minor potassic series. Twenty new K-Arages are presented for the Alexandra Volcanics and 9 new ages for the Ngatutura Basalts. Ages of the Alexandra Volcanics range from 2.74 to 1 .60 Ma, and the ages of all three magma series overlap. Ages of the Ngatutura Basalts range from 1 .83 to 1.54 Ma. Each basaltic field has a restricted time range and there is a progressive younging in age of the basaltic fields of western North Island from the Alexandra Volcanics in the south, to Ngatutura, to South Auckland, and then to the Auckland field in the north. Neither of the Alexandra nor Ngatutura Volcanics shows any younging direction of their volcanic centres or any age pattern within their fields, and there is no systematic variation in age with rock composition. Any correlation of age with degree of erosion of volcanic cones is invalid for these basaltic fields; instead, the degree of erosion may be controlled by the lithology of the cones and possibly by the extent of preservation offered by the thick cover deposits of the Kauroa, Hamilton, and younger tephra beds. Stratigraphic relations have enabled the earliest member of the Kauroa Ash Formation to be dated at 2.3 Ma. This formation represents a series of widespread rhyolitic plinian and ignimbrite eruptions probably derived from TVZ and initiated during the Late Pliocene

Ages on weathered Plio-Pleistocene tephra sequences, western North Island, New Zealand

Using the zircon fission-track method, we have obtained five ages on members of two strongly-weathered silicic, Pliocene-Pleistocene tephra sequences, the Kauroa and Hamilton Ash formations, in western North Island, New Zealand. These are the first numerical ages to be obtained directly on these deposits. Of the Kauroa Ash sequence, member K1 (basal unit) was dated at 2.24 ± 0.29 Ma, confirming a previous age of c. 2.25 Ma obtained (via tephrochronology)from K/Ar ages on associated basalt lava. Members K2 and K3 gave indistinguishable ages between 1.68 ± 0.12 and 1.43 ± 0.17 Ma. Member K12, a correlative of Oparau Tephra and probably also Ongatiti Ignimbrite, was dated at 1.28 ± 0.11 Ma, consistent with an age of 1.23 ± 0.02 Ma obtained by various methods on Ongatiti Ignimbrite. Palaeomagnetic measurements indicated that members K13 to K15 (top unit, Waiterimu Ash) are aged between c. 1.2 Ma and 0.78 Ma. Possible sources of the Kauroa Ash Formation include younger volcanic centres in the southern Coromandel Volcanic Zone or older volcanic centres in the Taupo Volcanic Zone, or both. Of the Hamilton Ash sequence, the basal member Ohinewai Ash (HI) was dated at 0.38 ± 0.04 Ma. This age matches those obtained by various methods on Rangitawa Tephra of 0.34-0.35 Ma, supporting correlation with this Whakamaru-caldera derived deposit. The origin of the other Hamilton Ash beds is unknown but various younger volcanic centres in the Taupo Volcanic Zone are possible sources. The topmost member, Tikotiko Ash (H6-H7), is estimated to be aged between c. 0.18 and 0.08 Ma. Various silicic pyroclastic deposits documented in North Island and in marine cores may be co-eval with members of the Kauroa Ash and Hamilton Ash sequences on the basis of their age

The medical student

The Medical Student was published from 1888-1921 by the students of Boston University School of Medicine

The Murchison Widefield Array: Design Overview

The Murchison Widefield Array (MWA) is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations, but is initially focused on three key science projects. These are detection and characterization of 3-dimensional brightness temperature fluctuations in the 21cm line of neutral hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10, solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources,and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broad-band active dipoles, arranged into 512 tiles comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5km in diameter, with a small number of outliers extending to 3km. All tile-tile baselines are correlated in custom FPGA-based hardware, yielding a Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point spread function (PSF) quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment,allowing full exploitation of the instrumental capabilities.Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings of the IEE