Dynamics of lava flow fronts, Arenal Volcano, Costa Rica

Arenal Volcano has effused basaltic andesite lava flows nearly continuously since September, 1968. The two different kinds of material in flows, lava and lava debris, have different rheologic properties and dynamic behavior. Flow morphology depends on the relationship between the amount and distribution of the lava and the debris, and to a lesser extent the ground morphology.Two main units characterize the flows: the channel zone and the frontal zone. The channel zone consists of two different units, the levées and the channel proper. A velocity profile in the channel shows a maximum value at the plug where the rate of shear is zero, and a velocity gradient increasing outward until, at the levées, the velocity becomes zero. Cooling produces a marked temperature gradient in the flow, leading to the formation of debris by brittle fracture when a critical value of shear rate to viscosity is reached. When the lava supply ceases, much of this debris and part of the lava is left behind after the flow nucleus drains out, forming a collapsed channel.Processes at the frontal zone include levée formation, debris formation, the change in shape of the front, and the choice of the flow path. These processes are controlled primarily by the rheological properties of the lava.Frontal zone dynamics can be understood by fixing the flow front as the point of reference. The lava flows through the channel into the front where it flows out into the levées, thereby increasing the length of the channel and permitting the front to advance. The front shows a relationship of critical height to the yield strength (τ₀) surface tension, and slope; its continued movement is activated by the pressure of the advancing lava in the channel behind. For an ideal flow (isothermal, homogeneous, and isotropic) the ratio of the section of channel proper to the section of levées is calculated and the distance the front will have moved at any time t_x can be determined once the amount of lava available to the front is known. Assuming that the velocity function of the front {G(t)} during the collapsing stage is proportional to the entrance pressure of the lava at the channel-front boundary, an exponential decrease of velocity through time is predicted, which shows good agreement with actual frontal velocity measurements taken on two flows. Local variations in slope have a secondary effect on frontal velocities.Under conditions of constant volume the frontal zone can be considered as a machine that consumes energy brought in by the lava to perform work (front advancement). While the front will use its potential energy to run the process, the velocity at which it occurs is controlled by the activation energy that enters the system as the kinetic energy of the lava flowing into the front. A relation for the energy contribution due to frontal acceleration is also derived. Finally the entrance pressure, that permits the front to deform, is calculated. Its small value confirms that the lava behaves very much like a Bingham plastic.     

Black dimensional stones: geology, technical properties and deposit characterization of the dolerites from Uruguay

Dimensional stones with a black color occupy a prominent place on the international market. Uruguayan dolerite dikes of andesitic and andesitic–basaltic composition are mined for commercial blocks of black dimensional stones. A total of 16 dikes of both compositions were studied and samples collected for geochemical and petrographical analysis. Color measurements were performed on different black dimensional stones in order to compare them with the Uruguayan dolerites. Samples of the two commercial varieties (Absolute Black and Moderate Black) were obtained for petrophysical analysis (e.g. density, porosity, uniaxial compressive strength, tensile strength, etc.). Detailed structural analyses were performed in several quarries. Geochemistry and petrography determines the intensity of the black color. The Uruguayan dolerite Absolute Black is the darkest black dimensional stone analyzed in this study. The petrophysical properties of Uruguayan dolerites make them one of the highest quality black dimensional stones. Structural analyses show that five joint sets have been recognized: two sub-vertical joints, one horizontal and two diagonal. These joint sets are one of the most important factors that control the deposits, since they control the block size distribution and the amount of waste material.     

Bacterial response to seasonal changes in labile organic matter composition on the continental shelf and bathyal sediments of the Cretan Sea

Bacterial abundance, biomass and cell size were studied in the oligotrophic sediments of the Cretan Sea (Eastern Mediterranean), in order to investigate their response to the seasonal varying organic matter (OM) inputs. Sediment samples were collected on a seasonal basis along a transect of seven stations (ranging from 40 to 1570 m depth) using a multiple-corer. Bacterial parameters were related to changes in chloroplastic pigment equivalents (CPE), the biochemical composition (proteins, lipids, carbohydrates) of the sedimentary organic matter and the OM flux measured at a fixed station over the deep basin (1570 m depth). The sediments of the Cretan Sea represent a nutrient depleted ecosystem characterised by a poor quality organic matter. All sedimentary organic compounds were found to vary seasonally, and changes were more evident on the continental shelf than in deeper sediments. Bacterial abundance and biomass in the sediments of the Cretan Sea (ranging from 1.02 to 4.59 × 10⁸ cells g⁻¹ equivalent to 8.7 and 38.7 μgC g⁻¹) were quite high and their distribution appeared to be closely related to the input of fresh organic material. Bacterial abundance and biomass were sensitive to changes in nutrient availability, which also controls the average cell size and the frequency of dividing cells. Bacterial abundance increased up to 3-fold between August '94 and February '95 in response to the increased amount of sedimentary proteins and CPE, indicating that benthic bacteria were constrained more by changes in quality rather than the quantity of the sedimentary organic material. Bacterial responses to the food inputs were clearly detectable down to 10 cm depth. The distribution of labile organic compounds in the sediments appeared to influence the vertical patterns of bacterial abundance and biomass. Cell size decreased significantly with water depth. Bacterial abundance and biomass were characterised by clear seasonal changes in response to seasonal OM pulses. The strong coupling between protein flux and bacterial biomass together with the strong bacterial dominance over the total biomass suggest that the major part of the carbon flow was channelled through the bacteria and the benthic microbial loop.     

Synoptic and mesoaerological study of radiation fog

Summary The present study was for the purpose of determining the conditions necessary to the formation of radiation fog. The temperature and relative humidity of the lowest stratum of the troposphere were sounded with a tethered balloon and radiosonde during a number of nights when the conditions were considered favourable for the formation of radiation fog: clear weather with effective net radiation, light wind, and high relative humidity at ground level. Advection fog was also studied on some occasions for comparison. The measurements were carried out in Uppsala. Meteorological phenomena of possible importance to the study were recorded concurrently with the measurements. The weather was analyzed synoptically and aerologically also on each occasion.The results of the investigation suggest that the conditions necessary to the formation of radiation fog are not only an effective net radiation, light wind, and high relative humidity at ground level, but also that the wind must be light and the relative humidity high at some distance above the ground. This permits the more or less marked surface inversion to leave ground level and to rise as a layer of rapid temperature transition. This is brought about by the radiation gradually emitting from the upper surface of the layer of mist or fog, formed in the lowest and chilled air strata, which are subsequently cooled from above until an unstable lapse rate develops. As a results, the layer of rapid temperature transition is maintained rather intact as it rises. If the radiation continues without obstruction and the wind is ligh aloft, this layer migrates upwards until it reaches a level at which the relative humidity is so low that the cooling necessary for condensation does not occur by radiation, limiting the extension of the underlying fog. If the relative humidity is high only in the undermost air strata, the surface inversion cannot leave ground level and no fog will be formed aloft. If the wind is strong aloft, the process is hindered by the increased turbulent exchange occurring between the high and low strata.

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Zusammenfassung Die vorliegende Untersuchung, die in Uppsala durchgeführt wurde, hat zum Zweck, die für die Bildung von Strahlungsnebel notwendigen Bedingungen festzulegen. Temperatur und relative Feuchtigkeit der untersten Troposphärenschicht wurden mit Fesselballon und Radiosonde während einer Anzahl Nächte ausgemessen, in denen die Bedingungen für die Bildung von Strahlungsnebel günstig erschienen, somit bei klarem Wetter mit effektiver Ausstrahlung, leichtem Wind und hoher relativer Feuchtigkeit am Boden. Zum Vergleich wurden auch einige Fälle von Mischungsnebel (Advektionsnebel) untersucht. Die Bestimmungen wurden in Uppsala durchgeführt. Gleichzeitig wurden auch weitere meteorologische Vorgänge, die für diese Untersuchung von Bedeutung sein konnten, aufgezeichnet. Zudem wurde das Wetter synoptisch und aerologisch bei jeder Gelegenheit un tersucht.Die Ergebnisse dieser Untersuchung legen die Annahme nahe, daß die für die Bildung von Strahlungsnebel not wendigen Bedingungen nicht nur in effektiver Ausstrahlung, leichtem Wind und hoher relativer Feuchtigkeit am Boden bestehen, sondern daß auch in gewisser Entfernung über dem Boden der Wind schwach und die Feuchtigkeit hoch sein muß. Dadurch wird es der mehr oder weniger ausgesprochenen Bodeninversion ermöglicht, das Bodenniveau zu verlassen und als Schicht eines schnellen Temperaturüberganges in die Höhe zu steigen. Dies wird bewirkt durch die Ausstrahlung, die allmählich von der Oberseite der Nebelschicht ausgeht, welche sich in den untersten und ausgekühlten Luftschichten bildet, die sich nach und nach von oben her abkühlen, bis sich ein instabiler Temperaturgradient entwickelt. Infolgedessen wird die Schicht des schnellen Temperaturübergangs beim Emporsteigen nahezu unverändert aufrechterhalten. Falls die Ausstrahlung ungehindert andauert und der Wind auch in der Höhe schwach ist, wandert diese Schicht aufwärts, bis sie ein Niveau erreicht, wo die relative Feuchtigkeit so niedrig ist, daß die für Kondensation notwendige Abkühlung nicht mehr durch die Ausstrahlung hervorgerufen wird, wodurch die Ausdehnung des darunter liegenden Nebels begrenzt wird. Wenn dagegen die relative Feuchtigkeit nur in den alleruntersten Luftschichten hoch ist, kann die Bodeninversion die Höhe des Erdbodens nicht verlassen und es bildet sich haher in der Höhe kein Nebel. Falls in der Höhe starker Wind weht, wird der Prozeß der Nebeldung durch die verstärkten Austauschvorgänge zwischen den hohen und den niederen Schichten verhindert.

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Résumé Le but de la présente étude faite à Uppsala est de déterminer les conditions de formation du brouillard de rayonnement. On a mesuré à cet effet la température et l'humidité relative des couches basses de la troposphère à l'aide de ballons captifs et de radiosondes pendant un certain nombre de nuits apparement favorables à la formation de brouillard, done par temps clair, vent faible et forte humidité au sol. Quelques cas de brouillard d'advection ont été aussi retenus à titre de comparaison. En même temps on a relevé d'autres processus météorologiuqes pouvant être utiles; la situation météorologique fit l'objet d'une analyse synoptique et aérologique dans chaque cas.Les résultats obtenus font penser que ce ne sont pas seulement le rayonnement, la faible agitation de l'air et la forte humidité relative au sol qui sont déterminants dans la formation du brouillard; le vent doit être faible et l'humidité élevée également à une certaine hauteur au-dessus du sol: c'est alors que l'inversion plus ou moins prononcée peut s'élever. Cela résulte du refroidissement par rayonnement de la surface du brouillard bas; la couche froide au sol augmente alors d'épaisseur jusqu'à ce qu'un gradient vertical instable apparaisse. La couche de décroissance rapide de température se maintient tout en gagnant vers le haut, et le processus ne s'arrête que lorsqu'un niveau à faible humidité est atteint; la couche de brouillard a alors acquis son épaisseur maximum. Si l'humidité relative n'est forte que dans les couches les plus basses, l'inversion ne peut par contre quitter le sol et il ne se forme pas de brouillard plus haut. S'il y a du vent fort a une certaine hauteur, le processus de formation du brouillard est rendu impossible par le brassage des couches basses et moyennes.

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With 12 Figures

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Dedicated to Dr.Anders K. Ångström on the occasion of his 70th birthday.     

The most frequent interfaces in olivine aggregates: the GBCD and its importance for grain boundary related processes

Rocks consist of crystal grains separated by grain boundaries that impact the bulk rock properties. Recent studies on metals and ceramics showed that the grain boundary plane orientation is more significant for grain boundary properties than other characteristics such as the sigma value or disorientation (in the Earth’s science community more frequently termed misorientation). We determined the grain boundary character distribution (GBCD) of synthetic and natural polycrystalline olivine, the most abundant mineral of Earth’s upper mantle. We show that grain boundaries of olivine preferentially contain low index planes, in agreement with recent findings on other oxides (e.g. MgO, TiO₂, Al₂O₃ etc.). Furthermore, we find evidence for a preferred orientation relationship of 90° disorientations about the [001] direction forming tilt and twist grain boundaries, as well as a preference for the 60° disorientation about the [100] axis. Our data indicate that the GBCD, which is an intrinsic property of any mineral aggregate, is fundamental for understanding and predicting grain boundary related processes.     

Stable Atmospheric Boundary-Layer Experiment in Spain (SABLES 98): A Report

This paper describes the Stable AtmosphericBoundary Layer Experiment in Spain (SABLES 98),which took place over the northern Spanish plateaucomprising relatively flat grassland,in September 1998. The main objectives of the campaign were to study the properties of themid-latitude stable boundary layer (SBL).Instrumentation deployed on two meteorologicalmasts (of heights 10 m and 100 m)included five sonic anemometers, 15 thermocouples,five cup anemometers and three propeller anemometers,humidity sensors and radiometers.A Sensitron mini-sodar and a tetheredballoon were also operated continuously. Atriangular array of cup anemometers wasinstalled to allow the detection ofwave events. Two nocturnal periods analysedon 14–15 and 20–21 September are used toillustrate the wide-ranging characteristics of the SBL.     

Semi-Classical Cosmological Models with Scalar, Dirac and Electromagnetic Fields

Exact solutions of the semi-classical Einstein equations for conformally invariant free quantum fields in an homogeneous and isotropic space-time, with cosmological constant and containing a classical scalar field, dust matter, an unquantised Dirac field and electromagnetic radiation are found. The initial behaviour of the semiclassical models is investigated. Some of the solutions found avoid the singularity and do not have particle horizons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of monticellite–forsterite and merwinite–forsterite symplectites in the CaO–MgO–SiO<Subscript>2</Subscript> model system: influence of temperature and water content on microstructure evolution

Homogeneous single crystals of synthetic monticellite with the composition \({\text{Ca}}_{0.88}{\text{Mg}}_{1.12}{\text{SiO}}_4\) (Mtc I) were annealed in a piston-cylinder apparatus at temperatures between 1000 and \(1200\,^{\circ }\hbox {C}\), pressures of 1.0–1.4 GPa, for run durations from 10 min to 24 h and applying bulk water contents ranging from 0.0 to 0.5 wt% of the total charge. At these conditions, Mtc I breaks down to a fine-grained, symplectic intergrowth. Thereby, two types of symplectites are produced: a first symplectite type (Sy I) is represented by an aggregate of rod-shaped forsterite immersed in a matrix of monticellite with end-member composition (Mtc II), and a second symplectite type (Sy II) takes the form of a lamellar merwinite–forsterite intergrowth. Both symplectites may form simultaneously, where the formation of Sy I is favoured by the presence of water. Sy I is metastable with respect to Sy II and is successively replaced by the latter. For both symplectite types, the characteristic spacing of the symplectite phases is independent of run duration and is only weeakly influenced by the water content, but it is strongly temperature dependent. It varies from about 400 nm at \(1000\,^{\circ }\hbox {C}\) to 1200 nm at \(1100\,^{\circ }\hbox {C}\) in Sy I, and from 300 nm at \(1000\,^{\circ }\hbox {C}\) to 700 nm at \(1200\,^{\circ }\hbox {C}\) in Sy II. A thermodynamic analysis reveals that the temperature dependence of the characteristic spacing of the symplectite phases is due to a relatively high activation energy for chemical segregation by diffusion within the reaction front as compared to the activation energy for interface reactions at the reaction front. The temperature dependence of the characteristic lamellar spacing and the temperature-time dependence of overall reaction progress have potential for applications in geo-thermometry and geo-speedometry.     

Scheduling the EChO survey with known exoplanets

The Exoplanet Characterization Observatory (EChO) is a concept of a dedicated space telescope optimized for low-resolution transit and occultation spectroscopy to study the exoplanet diversity through the composition of their atmospheres. The scope of this paper is to answer the following question: Can we schedule a nominal EChO mission, with targets known today (in mid 2013), given the science requirements, realistic performances and operational constraints? We examine this issue from the point of view of duration of the mission and the scheduling restrictions with a sample of exoplanet systems known nowadays. We choose different scheduling algorithms taking into account the science and operational constraints and we verified that it is fairly straightforward to schedule a mission scenario over the lifetime of EChO compliant with the science requirements. We identified agility as a critical constraint that reduces significantly the efficiency of the survey. We conclude that even with known targets today the EChO science objectives can be reached in the 4.5 years duration of the mission. We also show that it is possible to use gaps between exoplanet observations, to fit the required calibration observations, data downlinks and station keeping operations or even to observe more exoplanet targets to be discovered in the coming years.