Tarawera 1886, New Zealand — A basaltic plinian fissure eruption

New Zealand's biggest and most destructive volcanic eruption of historical times was that of Tarawera in 1886. The resulting scoria fall has a dispersal very similar in extent to that of the Vesuvius A.D. 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type. A new estimate of the volume (2 km³) is significantly greater than previous estimates. The basalt came mainly from a 7-km length of fissure, and emission and exit velocity were fairly uniform along at least 4 km of it, this is one of the few documented examples of a plinian eruption from a fissure vent. Primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min⁻¹. A model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1.5–3%. Violent phreatic explosions occurred in the southwestern extension of the fissure across the Rotomahana geothermal field, and it is thought that some of the water responsible for the power of the plinian eruption came from this source, though its amount was not sufficient to turn the eruption into a phreatoplinian one.     

A REVIEW OF MAP REVISION BY PHOTOGRAMMETRY

Revision has become a task equally as important as new mapping. Photogrammetric methods, however, have developed around the latter rather than the former. Modern instrumentation rejects this growth process. This paper examines the efficacy of various photogrammetric approaches for revision with respect to both detection and plotting of change.     

Pollen analysis and quaternary research in Scotland

The development of pollen analysis in Scotland and its role in present-day Quaternary studies are examined. Methodological advances in field and laboratory techniques, in the treatment and interpretation of pollen analytical data and in palaeoecological investigations involving pollen analysis are considered in the context of research in Scotland. The applications of pollen analysis in specific areas of Scottish Quaternary research are then discussed. These include the reconstruction of interglacial, interstadial and Late Devensian Lateglacial environments, Flandrian plant migrations and forest history, man and landscape change in the mid- and late Flandrian, the dating of geomorphological events, and the study of land and sea-level changes.     

Finite strain patterns and their significance in Permian rocks of the Alpes Maritimes (France)

More finite strain data has been obtained from autochthonous Permian mudstones of the Alpes Maritimes, S.E. France. These new data were computed from field measurements of green spots on all available sections, deformed mudcracks and from the quantitative correlation between magnetic susceptibility anisotropy and finite strain in these rocks. Previously published finite strain data and the new results are presented on a series of structural maps and cross-sections for the Dôme de Barrot, the Tinée and Vionène region and the Roya region. As in previous studies difficulties arise in explaining the apparently variable extension parallel with the 100°, subhorizontal bedding-cleavage intersection: either this is real or there were large volume changes during the tectonic deformation. Study of quartz fibres, developed in deformed mudcracks in the Tinée valley, suggest that early in the tectonic history incremental stretching directions were parallel with the bedding—cleavage intersection, while later they were down-dip in the 100° trending cleavage. Since these Permian rocks have remained stuck to the Argentera basement they also record displacements and deformations in the basement. The early 100°, subhorizontal stretching is consistent with NW-SE dextral, strike-slip basement faulting, while later, down-dip stretching in the cleavage is consistent with contraction faults in the basement. This information and new palaeomagnetic data on the same samples are combined with recent geophysical evidence and regional tectonic studies, to provide a new precision to the tectonic history of this part of the Western Alpine External Zone.     

The distribution and production of the hard clam,Mercenaria mercenaria,in Wassaw Sound,Georgia

Estuaries and Coasts - Hard clams,Mercenaria mercenaria, occurred in four intertidal habitats in the outer, high salinity (18‰) region of Wassaw Sound, Georgia: small feeder creeks $$(\bar x...     

Melting equilibria in multicomponent systems and liquidus/solidus convergence in mantle peridotite

The melting of undepleted mantle peridotite proceeds through a temperature interval which decreases with increasing pressure. If liquidus and solidus actually meet in the range 100–150 Kb, as suggested by Herzberg (1983), peridotite must transform there directly to a melt of its own composition. Thermodynamic analysis shows that such a liquidus/solidus meeting would be very unlikely in a system as chemically complex as mantle peridotite and would require that unanticipated phase equilibrium relations suppress all incongruent melting behavior. But Takahashi and Scarfe's (1985) preliminary experiments suggest that the upper mantle itself may indeed have a special composition with respect to phase equilibrium relations between liquids and solids at very high pressure. If so, mantle peridotite composition cannot be generated as a crystal accumulate or melting residue, because these two popular theories of origin are difficult to reconcile with a supposed eutecticlike composition. If upper mantle peridotite were itself a solidified liquid composition produced either as a partial melt or, more likely, as a crystallization residue of some more primitive melt composition representative of the whole mantle, an approach of liquidus to solidus might be expected at high pressure although the phase relations of Herzberg (1983) and Herzberg and O'Hara (1985) remain implausible.     

Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids

Experimental study of natural alkalic lava compositions at low pressures (pO₂QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without obvious reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl₂)(MgSi₂)_–1 and Fe³⁺ (Al)_–1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi.     

The early Cenozoic tectonic history of the southeast Pacific

It is suggested that the hydrothermal vents of the South Pacific Rise produce a beta-governed circulation at mid-depth, and that perhaps the associated plume of excess³He (Lupton and Craig [1]) points westward because of the dynamics of this circulation rather than as a passive tracer.     

Iron metal production in silicate melts through the direct reduction of Fe(II) by Ti(III), Cr(II), and Eu(II)

The production of metallic iron in silicate melts by the chemical reactions, 2Ti³⁺_(melt) + Fe²⁺_(melt) → 2Ti⁴⁺_(melt) + Fe⁰_(crystal)2Cr²⁺_(melt) + Fe²⁺_(melt) → 2Cr³⁺_(melt) + Fe⁰_(crystal)2Eu²⁺_(melt)+ Fe²⁺_(melt) → 2Eu³⁺_(melt) + Fe⁰_(crystal) has been demonstrated under experimental conditions in a simplified basaltic liquid, Such reactions may occur in lunar basalts and other reduced systems, and, thus, may aid in the understanding of the reduced nature of lunar basalts. The reactions were studied in a glass-forming Na-Ca-Mg-Al-silicate composition at a melt temperature of 1250°C and an imposed oxygen fugacity at the C/CO buffer (1 atm total pressure). Microtitrations of individually-doped samples were used in the quantitative assessment of their redox ratios and for the calibration of visible and near-infrared spectral absorptions. These spectral absorptions were then applied to the evaluation of the mutual redox interactions in dual-doped samples.     

40Ar39Ar and KAr dating of altered glassy volcanic rocks: the Dabi Volcanics,P.N.G.

KAr and ${}^{40}\text{Ar}{}^{39}\text{Ar}$ ages have been determined for altered submarine tholeiitic and boninite (high-Mg andesite) lavas from the Dabi Volcanics, Cape Vogel Peninsula, Papua New Guinea. ${}^{40}\text{Ar}{}^{39}\text{Ar}$ whole rock total fusion and plateau ages identify a Late Paleocene age for the tholeiitic lavas (58.9 ± 1.1 Ma) and also for the boninitic lavas (58.8 ± 0.8 Ma). Apparent KAr ages for the same samples range from 27.2 ± 0.7 to 63.9 ± 4.5 Ma, and young KAr ages for glassy boninites are probably due to variable radiogenic ⁴⁰Ar (⁴⁰Ar¹) loss. These new ages effectively reconcile previously ambiguous age data for the Dabi Volcanics and indicate contemporaneous tholeiitic and boninitic volcanism occurring in southeast PNG during the Late Paleocene.Smectites, developed as alteration products after glass in oceanic lavas commonly do not retain ³⁹Ar during or subsequent to irradiation, but in some cases may contain ⁴⁰Ar1. In the absence of other factors modifying K and Ar contents, samples which have not lost ⁴⁰Ar¹ from smectite and suffer ³⁹Ar loss only, are interpreted to have been altered immediately subsequent to the crystallization of the lava; whereas samples which have lost ⁴⁰Ar¹ as well as ³⁹Ar may be the result of either recent alteration, or of continuous ⁴⁰Ar¹ loss since the time of crystallization.