A poincaré model for the earth's fluid core

Abstract

The superconducting-gravimeter data of Melchior and Ducarme (1986) has been interpreted as internal motion in the Earth's core by Aldridge and Lumb (1987) using a Poincaré model. Several low-order modes with periods of 13–16 hours have been tentatively identified in the core which is taken to be an incompressible, homogeneous fluid within a rigid, rotating container. The identification is based on asymptotic values of the frequencies which change slowly with time while the modes decay with an e-folding time of about 280 days. The slow change in frequency with time implies a small temporal variation in the rotation rate of the core. This mean flow is a nonlinear effect often observed in laboratory experiments designed to excite Poincaré modes. Interaction among modes during free ringdown is also observed in those experiments and apparently in the data of Melchior et al. (1988) as well. Laboratory work thus provides the link to extend the Poincaré model to include viscous and nonlinear effects in order to interpret the gravimetric observations as core modes.     

Petrological influence on differential weathering and inselberg development in the Kora area of central Kenya

In the Kora area of central Kenya domed inselbergs are well developed on outcrops of granitoid migmatite, while positive relief features are rare on the surrounding gneiss. Block-strewn, vegetated hills occur on restricted areas of granoblastite, gabbro, and metagabbro. Schmidt Hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness, since all the rock types have similar ‘R’ values. The results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss. Samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter-inselberg areas. The variations in potassium content probably reflect differences in protolith composition, chemical fractionation during partial melting, and the effects of metasomatism. These findings support earlier suggestions that, other things being equal, potassium-rich granitoid rocks weather more slowly than less potassic rocks.     

A model for the emplacement of the eastern compartment of the bushveld complex

The stratigraphy and geological position of the eastern compartment of the Bushveld Complex are described. A mechanical model for the initiation and growth of the eastern compartment of the Bushveld intrusion has been developed using thin elastic plate theory, assuming linked conical magma chambers. It is shown that the contribution to the pressure at the base of a cell by the restitutional force exerted by the roof of Rooiberg felsites is 10⁴ times as great as that of the layers of host in the cone. Both are minimal compared to the lithostatic pressure exerted by the magma pile. Roof deformation is therefore seen to be a more important process than sagging of the floor during intrusion—a feature which probably occurred during cooling, solidification and isostatic readjustment of the area.A stratigraphie model is proposed in which the intrusion of basic rocks into the Transvaal sequence is discussed in the light of continuous basin subsidence. Early submarine sedimentation in an irregularly-floored basin some 620 km in diameter situated on the Archaean craton gave rise to a 7.7 km thick sedimentary pile, to which was added some 7 km of subaerial basalts and felsites. Depression of the floor of the basin into the regime of maximum horizontal compression induced favourable conditions for the intrusion of a total of 2.5 km of diabase sills which further assisted the subsidence. The 9 km thick Bushveld Complex was intruded into the basal sections at points along a 010° trend in a regime characterised by shear failure. Early magma influxes gave rise to a laminated marginal zone forming a shallow cone, with associated sill activity, whilst continued later influxes filled the conical cell, transgressed the floor and uparched the roof. Partial melting in the regions beneath the Complex, exacerbated by continued crustal depression, gave rise to the late Bushveld granites.     

Excitation mechanism of atmospheric pressure waves from the 1980 Mount St Helens eruption

Summary. Atmospheric pressure waves from the Mount St Helens eruption 1980 May 18 have been clearly recorded by a sensitive microbarograph at Berkeley, California. The record shows three types of waves with different group velocities. The pressure waves can be interpreted in terms of direct waves A1, antipodean travelling waves A2 and circumnavigating waves A3, all of which are composed of several acoustic-gravity modes propagated in the lower atmosphere. Synthetic barograms appropriate to the Berkeley station have been calculated on the basis of the dynamic response of the lower atmospheric structure, together with various assumptions of source properties. Comparisons between synthetic and observed barograms provide estimates for ranges of the time history of upward particle velocity at the source, source dimensions and the velocity of the source spreading over the blast zone, as well as for the average dissipation effects over the circumferential path. The results suggest that two major compression pulses on the A1 record correlate with the arrival of pressure waves from the first (lateral) blast and second (vertical) blast, although the inferred interblast time interval is not consistent with that estimated from seismic observations.     

A procedure for identifying significant environmental aspects in sea ports

A new methodology has been designed to identify and rank the significant environmental aspects in sea ports. The main objective of the Strategic Overview of Significant Environmental Aspects (SOSEA) is to help port managers to identify significant environmental aspects and to reinforce the awareness about them in order to prioritise work in environmental management. Developed in close collaboration with port environmental managers and tested in a set of ports, it is a user-friendly tool that can be applied in approximately half a working day. It is based on ISO 14001 vocabulary and requirements and it can be considered as the base for the implementation of any Environmental Management System for port communities.     

Geomorphic response to an active transpressive regime: a case study along the Chaman strike‐slip fault,western Pakistan

The Chaman left‐lateral strike‐slip fault bounds the rigid Indian plate boundary at the western end of the Himalayan‐Tibetan orogen and is marked by contrasting topographic relief. Deformed landforms along the fault provide an excellent record for understanding this actively evolving intra‐continental strike‐slip fault. The geomorphic response of an active transpessional stretch of the Chaman fault was studied using digital elevation model (DEM) data integrated with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Visible and Near Infrared/Short Wave Infrared (VNIR/SWIR) and images from GeoEye‐1. Geologic and geomorphic mapping helped in reconstructing the Late Quaternary landscape history of this transpessional strand of the Chaman strike‐slip fault and the associated Spinatizha thrust fault in western Pakistan. Topographic analysis of a part of the transpression (the thrust bounded Roghani ridge) revealed northward growth of the Spinatizha fault with the presence of three water gaps and two corresponding wind gaps. Geomorphic indices including stream length‐gradient index, mountain front sinuosity, valley floor width to valley height ratios, and entrenchment of recent alluvial fan deposits were used to define the lateral growth and direction of propagation of the Spinatizha fault. Left‐lateral displacement along Chaman fault and uplift along the Spinatizha fault was defined using topographic analysis of the Roghani ridge and geomorphic mapping of an impressive alluvial fan, the Bostankaul fan. The landforms and structures record slip partitioning along the Indian plate boundary, and account for the convergence resulting from the difference in the Chaman fault azimuth and orientation of the velocity vector of the Indian plate. Copyright © 2012 John Wiley & Sons, Ltd.     

Niveo-aeolian deposits and denivation forms,with special reference to the great Kobuk Sand Dunes,Northwestern Alaska

The geomorphology, lithology and chronostratigraphy of extensive, late Pleistocene inland and river dune sands, aeolian sand sheets (‘cover sands’) and loess deposits of periglacial origin in northwestern Europe are well known. However, the idea that some of these aeolian sediments result from niveo-aeolian processes is still an open question, as no diagnostic sedimentary features have yet been reported. Moreover, actual niveo-aeolian sediments and related denivation forms, reported from various cold-climate regions, are not suitable analogues. Recent observations in active dune fields in northwestern Alaska indicate that interstratification of wind-driven snow and sand preferentially occurs on slip faces of transverse, barchanoid or parabolic dune ridges. Annual denivation forms develop: e.g. snow ramparts, sinkholes, snow hummocks, snow meltwater fans and tensional cracks. The surface consists of a cracked wet sand layer with a dimpled surface and spongy structure. Although the preservation potential of these features is low in this specific case, similar features may be observed in ancient sediments elsewhere and provide useful palaeoclimatic indicators. The niveo-aeolian concept should therefore not specifically be related to late Pleistocene cover sand deposition in northwestern Europe, as previously assumed.     

On the role of a monsoon factor in the variability of currents in the Indian Ocean

On the basis of long series of contemporary instrumental measurements, the dynamics of waters in the western boundary current of the Indian Ocean is studied. For the first time, the background (quasi-stationary) circulation is estimated, and summer and winter monsoon components of the velocity field are detected. It is shown that the features of water dynamics of the Somali Current pattern are caused by the intensification of circulation during the summer phase, when the background and monsoon velocity fields with the same sign of vorticity are superimposed on each other. During the winter phase, on the contrary, superposition of background and monsoon circulations, with the opposite signs, is associated with a weakening of the resultant velocity of the currents.     

Variability of monsoon circulation under different scenarios of carbon-cycle variations in the model atmosphere

The results of numerical experiments with the model of the Institute of Numerical Mathematics, RAN, aimed to reveal variability of the atmospheric global circulation under changing content of carbon dioxide, are analyzed. Variability of monsoon circulation is considered for different scenarios of carbon dioxide content in the model atmosphere. The monsoon circulation indices calculated from the zonal-wind speeds simulated in the upper and lower troposphere and model precipitation rates are studied.