Bubble coalescence in basalt flows: comparison of a numerical model with natural examples

Gas accumulation in magma may be aided by coalescence of bubbles because large coalesced bubbles rise faster than small bubbles. The observed size distribution of gas bubbles (vesicles) in lava flows supports the concept of post-eruptive coalescence. A numerical model predicts the effects of rise and coalescence consistent with observed features. The model uses given values for flow thickness, viscosity, volume percentage of gas bubbles, and an initial size distribution of bubbles together with a gravitational collection kernel to numerically integrate the stochastic collection equation and thereby compute a new size spectrum of bubbles after each time increment of conductive cooling of the flow. Bubbles rise and coalesce within a fluid interior sandwiched between fronts of solidification that advance inward with time from top and bottom. Bubbles that are overtaken by the solidification fronts cease to migrate. The model predicts the formation of upper and lower vesicle-rich zones separated by a vesicle-poor interior. The upper zone is broader, more vesicular, and has larger bubbles than the lower zone. Basaltic lava flows in northern California exhibit the predicted zonation of vesicularity and size distribution of vesicles as determined by an impregnation technique. In particular, the size distribution at the tops and bottoms of flows is essentially the same as the initial distribution, reflecting the rapid initial solidification at the bases and tops of the flows. Many large vesicles are present in the upper vesicular zones, consistent with expected formation as a result of bubble coalescence during solidification of the lava flows. Both the rocks and model show a bimodal or trimodal size distribution for the upper vesicular zone. This polymodality is explained by preferential coalescence of larger bubbles with subequal sizes. Vesicularity and vesicle size distribution are sensitive to atmospheric pressure because bubbles expand as they decompress during rise through the flow. The ratio of vesicularity in the upper to that in the lower part of a flow therefore depends not only on bubble rise and coalescence, but also on flow thickness and atmospheric pressure. Application of simple theory to the natural basalts suggests solidification of the basalts at 1.0±0.2 atm, consistent with the present atmospheric pressure. Paleobathymetry and paleoaltimetry are possible in view of the sensitivity of vesicle size distributions to atmospheric pressure. Thus, vesicular lava flows can be used to crudely estimate ancient elevations and/or sea level air pressure.     

Some selected problems concerning the tectonics of granitoids of the Strzegom-Sobótka massif (SW Poland)

Directional structures in the SE part of the Strzegom-Sobótka massif were studied with mesostructural and petrotectonic methods. These structures are most pronounced in quarries at Strzeblów and Chwaków. They were studied there byCloos 1922a, b and taken by him as reference structures for the Q, S and L joints system. New results from this study defined more precisely the types of structures and directions of related stresses. Statistical measurements of joints and veins performed in some quarries scattered over the massif area led to the conclusion that the pluton was subjected to a complex field of tectonic stresses.

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Zusammenfassung Die Richtung der tektonischen Strukturen im Südostteil des Strzegom-Sobótka-Massives lassen sich in den Steinbrüchen von Strzeblów und Chwaków nach gefügekundlichen und petrographischen Methoden studieren. Hier hat auchHans Cloos 1922 seine Untersuchungen zum System der Q, S und L Klüfte durchgeführt. Neue Ergebnisse präzisieren diese Strukturtypen und ihre Beziehung zum tektonischen Stre\. Statistische Aufnahmen der Klüfte und Gänge über das gesamte Massiv führen zu dem Schlu\, da\ der Pluton während und nach seiner Intrusion komplexen Stre\feldern unterworfen war.

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Résumé Dans la partie sud du massif granitique de Strzegom-Sobótka les structures directionnelles ont été étudiées par les méthodes mésostructurale et pétrographique. Ces structures sont le mieux exprimées dans les carrières de Strzeblów et de Chwalkow, où d'ailleurs H.Cloos (1922 a, b) les a prises comme références pour ses systèmes de joints O, S et L. Les résultats de la présente étude permettent d'en définir avec plus de précision les divers types ainsi que les directions des contraintes qui en furent responsables. Le relevé systématique des joints et des veines, étendu à un certain nombre de carrières à travers tout le massif permet de conclure que le pluton a été le siège de champs de contraintes complexes pendant et après son intrusion.

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Evidence for A± 500my old thermal episode in southern South-West Africa

Six analyses of undeformed shales from the Late Precambrian Bogenfels Formation yielded a Rb/Sr whole-rock isochron which indicates that the rocks have been subjected to a significant thermal event some 500 my ago. It is thus established for the first time that the Damaran tectono-thermal episode of central South-West Africa (450-55 my) also affected the coastal areas farther south and may have extended as far as the Cape of Good Hope.     

The Influence of NaBr/NaCl Ratio on the Br--Catalysed Production of Halogenated Radicals

The activation of Br^- and Cl^- to atomic Br and Cl in sea-spray aerosol was investigated in smog-chamber experiments. In the presence of O₃, hydrocarbons and NaCl aerosol alone no activation was observed. By adding Br^- to the aerosol, the chain reaction: Br + O₃ BrO, BrO + HO₂ HOBr, HOBr HOBr(aq), HOBr(aq) + H⁺ + Br^- Br₂ (6), HOBr(aq) + H⁺ + Cl^- BrCl (7) was verified. The step from reaction (6) to (7) is accompanied by a decrease of the Br^-/Cl^- ratio from 1/600 to less than 1/2000. In the absence of sulphate, the chain is initiated by the reaction of OH(aq) with Br^-. The pH value decreases to less than 2 during the first minutes of the experiment and later on to almost 1 (in the absence of NO_x or SO₂). This is caused by the formation of oxalic acid from alkanes and toluene. In stopped flow experiments, the reduction of Br₂ by oxalic acid was observed to occur through a two-step mechanism: HC₂O₄ ^- + Br₂ Br^- + BrC₂O₄H (k₂₂, k_-22), BrC₂O₄H Br^- + H⁺ + 2 CO₂ (23) with the following rate constants and ratios of rate constants, k ± 2: k₂₂k_-23 / k_-22 = (2.9 ± 0.3) · 10^-4 s^-1, k_-22 / k_-23 = 7000 ± ₃₀₀₀ ¹³⁰⁰⁰ M^-1, k₂₂ = 2 ±_-1 ⁴ M^-1 s^-1, and k_-23 > 0.1 s^-1, k_-22 > 600 M^-1 s^-1. Oxalic acid may be responsible for the inhibition of the chain reaction observed at the end of the experiments.     

一个秃冻胀丘的热及水文动力学--关于其形成的暗示

Mud boils, classified as non-sorted circles, are a common patterned ground phenomenon of permafrost areas. They typically consist of a bare circle center of cryoturbated soil and are surrounded by vegetation on more stable soil. The objectives are to examine differences in seasonal hydrologic and thermal dynamic across this gradient of cryoturbation, i.e. the region below the organic border and the mud center. We installed instruments in a mud boil at a site close to Ny-@lesund, Spitsbergen, in September 1998. The bare soil circle center ranges about 1 m in diameter and is surrounded by a vegetated border consisting of a mixture of low vascular plants, mosses and lichens. Fine soil (>95% clay and silt) in the lower part of the profile is overlain by coarser grained material (silt and sand). The grain size distribution also suggests an upwelling of fine material in the center of the circle. Temperature and moisture sensors were installed over a vertical 1 m×1 m profile and hourly data recording started in September 1998. Surface irregularities, as well as variations of grain size and moisture, create a non-uniform thermal and hydrologic dynamic. We qualitatively analyze this dynamic with respect to the mud boils physical stability.     

Dust models confronted with observations

We critically discuss the two major models for interstellar dust grains developed by Greenberg and Mathis and their co-workers. So far, knowledge about the dust has been obtained mainly from extinction and polarization measurements in the UV, visual and near IR. Analysis of dust emission at longer wavelengths generally requires an evaluation of the radiative transfer. Such a procedure is necessary to assess optical grain properties at long wavelengths and also to interpret the emission from the different kinds of IR objects. The 10 m silicate resonance and the submm dust absorptivity are presented as two examples. We then briefly summarize the implications which follow from the existence of extremely small dust grains.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

S-component model computations applied to mean mm-wave measurements

Selected mm-wave observations with arc-min resolution of active regions in the central part of the solar disk obtained by the Metsähovi Radio Research Station during the years 1978–1984 are discussed from the perspective of recently developed mean S-component models.It is concluded that the mm-wave emission has a considerable (but at present unresolved) spatial fine structure. Compatibility with optical and EUV observations is only achieved by the assumption of embedding of bright plage elements in a quasi-quiet background according to filling factors of less than about 5% in the mm-wave range. Basing on model calculations, predictions are made concerning the mm-wave brightness of the sunspot and plage components. Averaging over the whole S-component sources provides a good correspondence between observation and calculation. The necessary elimination of well identified burst sources (15% of cases) and of a group of further unspecified small-scale sources (30%) from the above material indicates a greater contribution of hot-temperature radiation to the mean emission from solar active regions at mm-wavelengths than had formerly been expected.     

Revealing Uncertainties in Land Change Modeling Using Probabilities

Land change models are frequently used to analyze current land change processes and possible future developments. However, the outcome of such models is accompanied by uncertainties that have to be taken into account in order to address their reliability for science and decision‐making. While a range of approaches exist that quantify the disagreement of land change maps, the quantification of uncertainty remains a major challenge. The aim of this article is therefore to reveal uncertainties in land change modeling by developing two measures: quantity uncertainty and allocation uncertainty. We choose a Bayesian Belief Network modeling approach for deforestation in Brazil to develop and apply the two measures to the resulting probability surface. Quantity uncertainty describes the uncertainty about the correct number of cells in a land change map assigned to different land change categories and allocation uncertainty expresses the uncertainty about the correct spatial placement of a cell in the land change map. Thus, uncertainty can be quantified even in those cases where no reference data exist. Informing about uncertainty in probabilistic outcomes may be an important asset when land change projections are being used in science and decision‐making and moreover, they may also be further evaluated for other spatial applications.