Showing papers in "Philosophical Transactions of the Royal Society A in 1985"

The Taupo eruption, New Zealand. II. The Taupo Ignimbrite

Abstract: The ca. 30 km 3 Taupo ignimbrite was erupted as a climax to the ca. AD 186 Taupo eruption in the central North Island of New Zealand. It was erupted as a single vent-generated flow unit over a time period of ca. 400 s and was emplaced very rapidly (locally at more than 250-300 m s -1 ) and violently. The parent flow reached 8 0 + 10 km from source in all directions, crossed all but one of the mountains within its range and only stopped when it ran out of material. The ignimbrite is divisible into layers 1 and 2, and a distant facies which combines features of both layers. Layer 1 was generated as a result of strong fluidization in the flow head, caused by air ingestion, and consists of two main facies. Layer 1(P) is a pumiceous, mildly to strongly fines-depleted unit, generated by the expulsion of material from the flow front, and termed the jetted deposits. The overlying layer 1 (H) is a thinner, crystal- and lithic-rich, fines-depleted unit, generated by the sedimentation of coarse/dense constituents segregated out by strong fluidization within the flow head and termed the ground layer. Layer 2 consists of two facies with similar compositions but contrasting morphologies; during emplacement, material left behind by the flow body partially drained into depressions to form the valley-ponded ignimbrite, leaving the veneer deposit as a thin, landscape mantling layer on interfluves. The distant facies occurs in some outermost hilly areas of the ignimbrite where the flow velocity remained high but its volume had shrunk through deposition so that air ingestion fluidization affected the whole flow. The ignimbrite shows great lateral variations. Each facies, or variants therein, exhibits systematic degrees of development with varying distances from vent. Near vent, the flow consisted of a series of batches of material which by ca. 25 km had coalesced into a single wavy flow and by ca. 40 km into a single wave. Out to ca. 13 km, the flow was rather dilute and highly turbulent as it deflated from the collapsing eruption column. Beyond this distance it was fairly concentrated, being less than 100% expanded over its non-fluidized compacted state, and had acquired a fluidization-induced stable density stratification, which strongly suppressed turbulence in the flow body. Deflation from the eruption column was largely complete by ca. 13 km but influenced the flow as far as 20-25 km from vent. Grainsize and compositional parameters measured in the ignimbrite show lateral variations which equal or exceed the entire spectrum of published ignimbrite data. The flow had deflated and coalesced from the eruption column by ca. 20 km from vent. Beyond this distance most lateral variations are modelled by considering the flow to be a giant fluidized bed. As the flow moved, material was deposited from its base, and hence predictable vertical variations in the model fluidized bed are comparable with lateral variations in the ignimbrite. The agreement is excellent, and, in particular, discontinuities in the nature of the ignimbrite at 55-60 km from vent suggest that the more distal ignimbrite represents a vast segregation layer generated above the moving flow. Differences between the model and variations of some parameters reflect the influence of kinetic processes, such as shearing and local fluidization, that operated regardless of the bulk flow composition. The strong fluidization in the flow is a result of the high flow velocities (promoting air ingestion), not vice versa as is often accepted. Contrasts in the natures of layers 1 and 2 imply that the first material erupted contained significantly coarser, and a higher content of, lithics than the bulk of the flow. During emplacement, this earlier material was depleted by deposition and diluted by material introduced from the flow body. Systematic regional variations also occur in the ignimbrite: for example, it contains lower crystal: lithic ratios and higher density pumice in a northeasterly sector, and vice versa to the southwest. Ignimbrite found in mountainous areas shows changes consistent with its derivation from the upper, more mobile and pumiceous top of the flow. Fluidization processes generated structures and facies in the ignimbrite on various scales. Individual segregation bodies found at any exposure show features mimicking those of the ground layer, i.e. fines depletion and crystal- and lithic-enrichment. Fluidization-induced grading visible at individual exposures accounts for the great range of grading styles seen in the valley-ponded ignimbrite, and strong fluidization has locally generated an upper fines- and pumice- rich segregation layer (here termed layer 2c). On the largest scale, fluidization was primarily responsible for the generation of the layer 1 deposits, and for the grainsize and compositional zonation within the flow that produced the lateral variations in the ignimbrite. Ingested and heated air is inferred to have been the most important gas source for fluidization within the flow, although several other gas sources were locally dominant. It is clear that the thickness, grainsize and composition of the ignimbrite at any point are not simply related to values of these parameters in either the originally erupted material or the parent flow, and that, except for its density, the dimensions and composition of the parent flow cannot be directly inferred from the ignimbrite.

Geochemistry of diagenetic non-silicate minerals: kinetic considerations

Abstract: The early stages of burial diagenesis involve the reactions of various oxidizing agents with organic matter, which is the only reducing agent buried with the sediment. In a system in which a local equilibrium is established, thermodynamic principles indicate that, inter alia , manganese, iron and sulphate should each be consumed successively to give rise to a clearly characterized vertical zonation. However, ferric iron may not react fast enough and the relative rates of reduction of Fe III and sulphate not only control the formation of iron sulphide and associated carbonate but also may lead to extreme chemical and isotopic dis-equilibrium. This produces kinetically controlled ‘micro -environments’. On a larger scale, sulphide will diffuse upward to a zone in which its oxidation leads to a reduction of pH. The various dramatic changes in chemical environment across such an interface cause both dissolution and precipitation reactions. These explain common geological observations: the occurrence of flint nodules (and their restriction to chalk hosts) and the association of phosphate with glauconite.

A Radiometric Determination of the Stefan-Boltzmann Constant and Thermodynamic Temperatures between -40 degrees C and +100 degrees C

Abstract: The total radiant exitance of a black body at the temperature of the triple point of water, T$\_{\text{tp}}$ (273.16 K), and at a series of other temperatures in the range from about 233 K (-40 degrees C) to 373 K (100 degrees C), has been measured by using a cryogenic radiometer. From the measurements at T$\_{\text{tp}}$ a value for the Stefan-Boltzmann constant $\sigma $ has been calculated: $\sigma $ = (5.669 67 $\pm $ 0.000 76) $\times $ 10$^{-8}$ W m$^{-2}$ K$^{-4}$. This is the first radiometric determination of $\sigma $ having an uncertainty comparable with that calculated directly from fundamental physical constants. This measured value differs from the calculated one by 13 parts in 10$^{5}$, which is less than the combined standard deviations of the measured and calculated values. From the measurements of exitance at the other temperatures, values of the corresponding thermodynamic temperature T have been calculated by using Stefan's fourth-power law. Since the temperature of the radiating black body was also measured by platinum resistance thermometers calibrated on IPTS-68, values of (T-T$\_{68}$) were obtained. These range from about -(5 $\pm $ 1.6) mK at 20 degrees C to -(28 $\pm $ 2.5) mK at 100 degrees C and +(5 $\pm $ 1.5) mK at -40 degrees C. The results confirm to within a few millikelvins the departure of T$\_{68}$ from T above 0 degrees C already discovered by gas thermometry and show that similar departures, but of opposite sign, exist down to the lowest temperature measured, -40 degrees C. The uncertainties associated with these new values of T and (T-T$_{68}$) are similar to those of the best gas thermometry.

Low-Energy Scattering of Non-Abelian Magnetic Monopoles [and Discussion]

Abstract: The Bogomolny equations represent static configurations of magnetic monopoles, for some non-Abelian group such as SU(2). Geodesic motion on this configuration space represents slowly moving interacting monopoles. Geometric information on this space can then be used to investigate the scattering of monopoles. The results show surprising features, including a 90° scattering and the conversion of angular momentum into electric charge.

The Seismicity, Geomorphology and Structural Evolution of the Corinth Area of Greece

Abstract: In the eastern Gulf of Corinth, geological structure is closely reflected in the topography. In the 1981 Corinth earthquakes ground deformation accentuated existing geomorphological patterns. We explore the relationship between seismicity and landforms in space and time. The area examined extends beyond the epicentral region of the 1981 events and its evolution is traced over the last 40000 years. To understand the relation between deep and superficial structures we discuss processes that occurred in the 1981 earthquakes, and go on to propose that surface faulting appears only above linear faults at depth that move in earthquakes with magnitudes ( M s ) greater than six. Where faults at depth intersect or bend, the faulting becomes distributed and prim ary faulting does not reach the surface. Here the surface manifestation of faulting at depth is surface folding and secondary faulting. Such regions are identified in maps of the eastern Gulf of Corinth. Evidence for the evolution of the fault systems is provided by eyewitness accounts of shoreline changes following the recent earthquakes, historical and archaeological data pertaining to old sea levels, and 14 C dates for molluscs collected from fossil shorelines. The results of these studies broadly confirm an earlier view of J. A. Jackson et al . ( Earth planet. Sci. Lett . 57, 377—397 (1982)) that motion has shifted from faulting south of the city of Corinth to faulting in the Perachora peninsula. But the transfer is not complete: areas lying between the two fault systems have moved up and down in historical and archaeological times indicating that both systems are still active. The evidence for activity of the northern system points to a repeat time of 300 years for earthquakes com parable with those of 1981. The work discussed in this paper suggests that some of the features previously described as m arine terraces are fault-controlled continental surfaces. Furtherm ore, beach deposits at a num ber of different levels long regarded as successive shorelines are in fact contemporaneous. The critical ages were obtained by radiocarbon dating of shell carbonate, a technique which yields dependable results if applied to samples which have been carefully selected and pretreated. Palaeontological dating of the terraces, in contrast, is found to be misleading because the faunas are environm entally controlled and consequently diachronous.

Numerical modelling of tide-surge interaction in the Bay of Bengal

Abstract: Numerical models are described for the evaluation of the interaction between tide and surge in the Bay of Bengal. The models are used to simulate the combined tidal and surge response on 3 June 1982 along the Orissa coast of India when the landfall of a tropical cyclone led to severe inland flooding. This is one of the few events for which a reliable tide-gauge reading is available and this enables a direct comparison to be made between the model predictions and the observationally determined sea-surface elevation anomaly. The comparison, although only utilizing limited observational data, appears sufficiently good for us to assert that the principal features of the surge response are correctly reproduced. A model simulation is also made of the surge that occurred along the Andhra coast of India during the period 18—20 November 1977 when there was heavy coastal inundation. Although tide-gauge readings are not available for this event, the predicted surge response agrees well with indirect estimates of the maximum sea-surface level and eyewitness accounts of inland flooding. The principal requirement for the operational use of these models is the availability of accurate data on the surface wind field together with a reliable forecast of the track to be followed by the tropical cyclone.

Radiation hardening in magnox pressure-vessel steels

Abstract: The ferritic steels used for reactor pressure vessels undergo a marked transition from ductile to brittle fracture behaviour over a relatively narrow temperature range. For most unirradiated mild steels the ductile to brittle transition temperature (d.b.t.t.) is between — 50° and 20 °C. The process of irradiation hardening, through the formation of clusters of interstitial or vacancy defects, increases the friction stress of these steels and thereby raises the transition temperature. Given the inherent tendency of these steels to fail in a brittle manner, the raising of the transition temperature under neutron irradiation poses a problem of considerable technological importance in the nuclear industry. At the time (1962) when the first of the Central Electricity Generating Board (C.E.G.B.) Magnox nuclear stations began operation the phenomenon of brittle fracture was already comparatively well understood. A theory of the process had already been developed and applied to the problem of radiation embrittlement. However, as the results from the Magnox pressure-vessel surveillance scheme accumulated, it gradually became evident that the measured changes in yield stress in the monitoring specimens could not be accounted for simply on the basis of irradiation hardening through the formation of damage clusters. By the late 1970s, sufficient data had been gathered from the surveillance programme to enable a detailed investigation of the processes occurring in the Magnox steels to be instituted. The form of the investigation was to subsequently evolve into two phases; an initial com prehensive microstructural study of the steels, followed by the formation of an interpretative model based on the observations. In this paper we present the Magnox yield-stress monitoring measurements and then briefly describe the principal findings from our microstructural studies. The Magnox pressure-vessel steels contain between 0.05 and 0 .4 % by mass of copper and we show that under certain conditions this element may precipitate as small spherical particles within the matrix of the steels. A review of previous work on copper precipitation in ferrite is then followed by a description of our model. This assumes that the changes in yield stress generally arise from the combined effects of irradiation damage loops and copper precipitates. The formation of the latter may be enhanced by irradiation and in some steels their contribution is dominant. It is shown that the model successfully accounts for the measurements made on both plate and weld steels in all the Magnox stations. Experimental support for the model comes from our own microstructural observations and from other studies, in the U.K. and elsewhere, using techniques which allow the detection of sub-microscopic particles in steels. The model may be applied to pressure-vessel steels in other reactor systems. Indeed, it predicts that the yield-stress changes in steels with a high copper content irradiated under p.w.r. (pressurized water reactor) conditions will be dominated by the contribution from copper precipitation.

Davydov Solitons in Polypeptides

Abstract: The experimental evidence for self-trapping of amide-I (CO stretching) vibrational energy in crystalline acetanilide (a model protein) is reviewed and related to A. S. Davydov's theory of solitons as a mechanism for energy storage and transport in protein. Particular attention is paid to the construction of quantum states that contain N amide-I vibrational quanta. It is noted that the N = 2 state is almost exactly resonant with the free energy that is released upon hydrolysis of adenosine triphosphate. 30 references, 4 figures, 3 tables.

Organic-inorganic layer compounds: physical properties and chemical reactions

Abstract: In contrast with intercalation compounds, which can exist both with and without organic molecules between the planes of inorganic material, ‘molecular composite’ compounds have organic groups covalently or ionically bound to inorganic layers. In such crystals the aim is to combine magnetic or optical properties characteristic of the inorganic solid state, like magnetism and luminescence, with properties found in the organic solid state like mesomorphism or polymerization. This contribution surveys experiments from our laboratory on the structural, physical and chemical properties of one series of ‘molecular composites’, the layer perovskite halide salts (RNH 3 ) 2 MX 4 (R = organic group; M = Cr, Mn, Cd; X = Cl, Br). When R is an n -alkyl group these compounds undergo structural phase transitions triggered by variations in the hydrogen-bonding of the −NH 3 to the inorganic layer. Optical microscopy is used to follow the phase transitions and map ferroelastic domains. Compounds with M = Cr are ferromagnets with strongly temperature-dependent visible absorption spectra, whose intensity correlates with the degree of magnetic order. In Mn and Cd salts more elaborate functional groups can be inserted in the organic sidechains with the aim of making topochemical polymerizations in the solid state.

A survey of the origins and physical importance of soliton equations

Abstract: An introduction to the subject is given in an elementary way for the non-specialist, outlining why many completely integrable systems, although special, play a significant role in wave motions in applied mathematics and theoretical physics.

The geometry of the Hill equation and of the Neumann system

Abstract: Let there be given a finite-gap operator L = d$^{2}$/dx$^{2}$ + q and its Baker function $\psi $(x, p), which is analytic for p on a certain hyperelliptic curve C. It is shown that a sequence of Backlund transformations maps C to a projective space. This embedding can be interpreted as a matrix representation of the Hill equation by the Neumann system of constrained harmonic oscillators. The image curve, C$^{\prime}$, lies on a rational ruled surface; the structure of this surface is explained by use of ideas due to Burchnall & Chaundy (Proc. R. Soc. Lond. A 118, 557-583 (1928)). Baker functions and Backlund transformations are then used to define a (many-to-many) correspondence between effective divisors on the curve C and points lying on a quadric, or in the intersection of two or more quadrics. This relates the theory of the Hill equation to earlier work of Knorrer, Moser and Reid. It is then shown that the Kummer image of the Jacobian of C can be realized as a hypersurface in the space of momentum variables of the Neumann system. Further projects, such as extensions to non-hyperelliptic curves, are outlined.

The Breakdown of Superfluidity in Liquid $^{4}$He. IV. Influence of $^{3}$He Isotopic Impurities on the Nucleation of Quantized Vortex rings

Abstract: We have investigated the rate $ u $ at which negative ions nucleate charged vortex rings in a series of extremely dilute superfluid $^{3}$He/$^{4}$He solutions. Measurements of $ u $ were made at a pressure P = 23 bar (23 $\times $ 10$^{5}$ Pa) for temperatures, T, electric fields, E, and $^{3}$He/$^{4}$He isotopic ratios, x$\_{3}$, within the ranges: 0.33 < T < 0.61 K, 1.0 $\times $ 10$^{4}$ < E < 1.5 $\times $ 10$^{6}$ V m$^{-1}$, 2.1 $\times $ 10$^{-8}$ < x$\_{3}$ < 1.7 $\times $ 10$^{-7}$. A few data were also recorded at other pressures within the range 19 < P < 25 bar. For each concentration, and also for nominally pure $^{4}$He (x$\_{3}$ = 1.9 $\times $ 10$^{-10}$), $ u $ was measured for the same set of E and T. For all the chosen values of x$\_{3}$ and P, the form of $ u $(E,T) was qualitatively much the same, and considerably more complicated than for pure $^{4}$He. It was found that $ u $ became equal to the nucleation rate $ u \_{0}$ in pure $^{4}$He for large E, but that $ u \gg u \_{0}$ for smaller values of E at low T. The $^{3}$He-influenced contribution to the overall nucleation rate, $\Delta u = u - u \_{0}$, passed through a pronounced maximum at a value of E that increased with increasing T; but the magnitude of $\Delta u $ itself decreased rapidly with increasing T. Plots of $ u $ against x$\_{3}$ for fixed P, E and T show a marked upward curvature for the lower values of E and T, but become linear within experimental error above ca. 0.5 K. A model is proposed (in two variants) in which the complicated behaviour of $ u $(E,T) is accounted for in terms of changes in the average occupancy by $^{3}$He atoms of trapping states on the surface of the ion, it being proposed that the nucleation rate $ u \_{1}$, due to ions each having one trapped $^{3}$He atom, is very much greater than $ u \_{0}$ for bare ions. The nonlinearities in $ u $(x$\_{3}$) are interpreted in terms of the simultaneous trapping of two (or more) $^{3}$He atoms on a significant fraction of the ions. It is shown that the model can be fitted closely to the experimental data, thereby yielding numerical values of $ u \_{1}$, of the $^{3}$He binding energy on the ion, and of a number of other relevant quantities. From the form of $ u _{1}$(E), it is deduced that the addition of a $^{3}$He atom to a bare ion affects its propensity to create vortex rings in two ways: the critical velocity for the process is reduced by ca. 4 m s$^{-1}$, and the rate constant is increased by a factor of ca. 10$^{3}$. The implications of these results for microscopic theories of the vortex nucleation mechanism are discussed.

Reduction and prediction of sandstone reservoir potential, Jurassic, North Sea

Abstract: Porosity, permeability, mineralogical and depth data for two North Sea Jurassic sandstone sequences were analysed. Both sequences show statistically significant negative correlations between present burial depth and porosity. The influence of secondary porosity creation is subordinate to that of the general decline in porosity. For a given burial depth , sequence A is, on average, a little more porous (about 3%) than B. However, for a given porosity sequence A displays a permeability 1—3 orders of magnitude greater than B. The large permeability difference between A and B is a function of authigenic mineralogy. The only significant cement within the reservoir intervals of sequence A is quartz . Sequence B contains authigenic clays, quartz and subordinate carbonate. The abundant authigenic clay in B severely reduced permeability. In both instances, the cements are products of burial and were precipitated from pore waters expelled from shales during compaction. The expelled pore waters were both acidic and rich in solutes; a product of reactions between maturing organic matter, clays and iron oxides. The difference in authigenic mineralogy between the sequences was caused by the reaction between pore waters and sandstones with different detrital mineral compositions. Thus the present reservoir quality is a product of burial and of the reactions between evolving pore fluids and minerals in the sandstone.

Algebraic geometrical methods in Hamiltonian mechanics

Abstract: During the nineteenth century one of the main concerns in mechanics was to solve Hamiltonian systems by quadrature in terms of elliptic and hyperelliptic functions. The vein of research, abandoned for nearly a century, was entirely revived by the recent findings about the Korteweg-de Vries equation. They shed new light and perspective onto problems of finite-dimensional mechanics, which has led to effective and systematic methods for deciding about the complete integrability of Hamiltonian systems. The problems that can be captured by these methods have the common virtue that, when run with complex time, most (complex) trajectories are dense on complex algebraic tori; such a system is called algebraically completely integrable, which is stronger than the customary notion of analytic integrability. Many old and new systems enjoy this property and, in particular, a wide variety of systems that occur in the context of orbits in Kac-Moody Lie algebras. The author explains how the behaviour at the `blow-up' time of the solutions to the differential equations enables one to decide about their integrability and also how to derive precise information about the invariant surfaces of the system.

Representations of Affine Lie Algebras and Soliton Equations [Abstract Only]

Abstract: A few years ago the 'hidden symmetries’ of the soliton equations had been identified as affine Lie groups, also known as loop groups. The first extensive use of the representation theory of affine Lie algebras for the soliton equations have been developed in a series of works by mathematicians of the Kyoto school. We will review some of their results and develop them further on the basis of the representation theory. Thus an orbit of the simplest affine Lie group SL(2, C)^ in the fundamental representation V will provide the solutions of the Korteweg-de Vries equation, and similarly the solutions of the sine-Gordon equation will come from an orbit of the group (SL(2, C) x SL(2, C)) ^ in V x V*.

Charge density waves, phasing, sliding and related phenomena in NbSe 3 and other transition metal chalcogenides

Abstract: The material presented is largely drawn from a rewiev paper given at the Royal Society Symposium on Low Dimensional Solids held in May 1984 and to be published in January 1985 in The Philosophical Transactions of the Royal Society (Phil. Trans. A). The following is the abstract for that paper.

Organic superconductors: structure-property relations and new materials design

Abstract: Most known organic materials are electrical insulators having extremely low electrical conductivities of δ -10 Ω -1 cm -1 . A small number of organic materials are semiconductors having, for classification purposes, conductivities of δ ≈ 10 -10 -1 Ω -1 cm -1 . A very small, but growing, number of organic substances are metallic in nature, i.e. having conductivities that rise with decreasing tem perature (δ ≈ 1−10 10 Ω -1 cm -1 ). The latter systems comprise a class of intensely studied materials known as ‘organic metals’ of which fewer than ten can display the complete absence of electrical resistance at low temperatures, i.e. superconductivity (δ ≈ infinity). The known organic superconductors are novel, being derived from radical-cation donors and monovalent anions, X. The donors are derived from two kinds of molecules, neither of which contain any metallic elements. These are TMTSF (tetramethyltetraselenafulvalene) and BED T-TTF (bis-ethylenedithiotetrathiafulvalene, or ‘ET’ in abbreviated form). Most of the (TMTSF) 2 X and (ET) 2 X conducting materials require applied pressure to induce superconductivity that is thus far observed at very low temperatures ( T e ≈ 1–2 K). However, two materials, (TMTSF) 2 ClO 4 and (ET) 2 I 3 are ambient pressure organic superconductors ( T e = 1.2 and 1.4 K, respectively). Within each class the crystal structures have many similarities, the most important being a complex ‘infinite sheet networkߣ of short Se-Se interactions in (TMTSF) 2 X and a ‘corrugated sheet network’ of short S-S interactions in ET 2 X. In this paper we discuss structure-property relations of the (TMTSF) 2 X salts, and of the (ET) 2 X salts as far as is known. In addition, we attempt to provide insight and guidelines for the synthesis of new highly conducting anionic derivatives of TMTSF and ET. It appears that while highly conducting (TMTSF) 2 X materials can be designed before synthesis, the onset of superconductivity depends heavily on the presence of anion order in the crystal, which is a parameter not easily controlled. For the (ET) 2 X systems the structural disorder apparent at 298 and 125 K may persist to very low tem perature, making it difficult to correlate structural order with superconductivity as is the case for (TMTSF) 2 X systems.

Organic superconductors: low-dimensional conductors and anomalous superconductors

Abstract: Organic superconductors belonging to the (TMTSF)2X series are quasi-onedimensional conductors in the sense that their Fermi surface is open. The strong sensitivity of the superconducting state to pressure and non-magnetic impurities is probably a direct consequence of the low-dimensionality of the electron energy dispersion. We discuss the remarkable stabilization of semimetallic (magnetic) states observed under high magnetic fields in (TMTSF)2C104 in terms of recently proposed theoretical models. These models suggest the existence of a sequence of phase transitions between spin density wave sub-phases and the quantization of the Hall resistance within each of these sub-phases as observed experimentally.