Dynamics of lava flow fronts, Arenal Volcano, Costa Rica     

Andrea Borgia  Scott Linneman  Daniel Spencer  Luis Diego Morales  Jose Brenes Andre 《Journal of Volcanology and Geothermal Research》1983,19(3-4)

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.  相似文献   

Mineralogical,geochemical and microfabric evidences of gypsum crusts: a case study from Budapest     

Siegfried Siegesmund  Akos Török  Andre Hüpers  Christian Müller  Werner Klemm 《Environmental Geology》2007,52(2):385-397

High levels of SO₂ and particulate pollution enable the rapid development of gypsum-rich weathering crusts in Budapest. Two types of white crusts, thin and thick ones, and two forms of black crusts, laminar and framboidal ones, were studied in limestone buildings of the parliament and Citadella. The percentage of crust cover and damage categories were documented on selected walls. Petrographic, XRD, XRF and sulphur isotope analyses were performed under laboratory conditions to understand the mechanism of crust formation. White crusts found both on exposed and sheltered walls display a calcite-rich layer with gypsum, while black crusts are enriched with gypsum. The sulphur isotopic composition of white and black crusts overlaps, but the crusts are slightly enriched in heavy isotopes compared to rainwater. S content, Si/Al ratios and particulates in black crusts suggest that air pollution (SO₂, dust) contributes to black crust formation. The accumulation of sulphur and Zn enrichment of white crusts were also documented indicating that under high pollution levels, even these compound can accumulate on exposed facades.  相似文献   

Fine sediment transport and accumulations at the mouth of the Seine estuary (France)     

Pierre Le Hir  Andre Ficht  Ricardo Silva Jacinto  Patrick Lesueur  Jean-Paul Dupont  Robert Lafite  Isabelle Brenon  Benedicte Thouvenin  Philippe Cugier 《Estuaries and Coasts》2001,24(6):950-963

A comprehensive study of fine sediment transport in the macrotidal Seine estuary has been conducted, including observations of suspended particulate matter (SPM), surficial sediment, and bathymetric data, as well as use of a three dimensional mathematical model. Tide, river regime, wind, and wave forcings are accounted. The simulated turbidity maximum (TM) is described in terms of concentration and location according to tidal amplitude and the discharge of the Seine River. The TM is mainly generated by tidal pumping, but can be concentrated or stretched by the salinity front. The computed deposition patterns depend on the TM location and are seasonally dependent. The agreement with observations is reasonable, although resuspension by waves may be overestimated. Although wave resuspension is likely to increase the TM mass, it generally occurs simultaneously with westerly winds that induce a transverse circulation at the mouth of the estuary and then disperse the suspended material. The resulting effect is an output of material related to wind and wave events, more than to high river discharge. The mass of the computed TM remains stable over 6 months and independent of the river regime, depending mainly on the spring tide amplitude. Computed fluxes at different cross-sections of the lower estuary show the shift to the TM according to the river flow and point out the rapidity of the TM adjustment to any change of river discharge. The time for renewing the TM by riverine particles has been estimated to be one year.  相似文献   

Comment on “Positive definiteness is not enough” by M. Armstrong     

Andre G. Journel 《Mathematical Geology》1992,24(1):145-147

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Methane in an acidic bog lake: The influence of peat in the catchment on the biogeochemistry of methane   总被引：3，自引：0，他引：3  

Peter Casper  On Chim Chan  Andre L. S. Furtado  Donald D. Adams 《Aquatic Sciences - Research Across Boundaries》2003,65(1):36-46

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BEEKEEPING AND LOCAL SELF-RELIANCE IN RURAL SOUTHERN AFRICA*     

Pete M. Illgner  Etienne L. Nel  Mark P. Robertson 《Geographical review》1998,88(3):349-362

ABSTRACT. Beekeeping has the potential to supplement incomes in rural southern Africa. In light of regional economic constraints, self-reliance strategies that draw on local knowledge and skills take on a renewed importance. We consider the advantages and disadvantages of beekeeping and examine appropriate forms of development support. A short case study from Zimbabwe illustrates these issues.  相似文献   

A source of bias in rates of surface soil movement as estimated from marked particles     

Nel Caine 《地球表面变化过程与地形》1981,6(1):69-75

When estimated from measurements of introduced tracer particles, the rate of surface soil movement tends to be greater than the natural rate for equivalent particles on the same site. This consistent overestimation is greatest in the period following tracer introduction and leads to a measurement bias that may be as high as 300 per cent. The magnitude of the bias decreases with time, as the tracer is incorporated into the surface material, but remains detectable statistically for more than a year on some low-angle sites.  相似文献   

La ligne du Tonale (Alpes centrales et orientales): sens de décrochement et prolongements     

Andre Guillaume 《Tectonophysics》1978,48(1-2)

This work discusses the state of knowledge (mainly tectonic and geophysical data) about the Tonale line and other “peri-Adriatic” lines in the Central and Eastern Alps. The chain is here cut into a mosaic of independent blocs, separated by faults with basic injections in some places. The Tonale fault had a dextral movement in Oligo-Miocene times; it is connected with the Austrian “Thermenlinie”, and not to the Pusteria—Gail line. An attempt at chronology is presented.

Résumé

Ce travail fait le point des connassiances, principalement tectoniques et géophysiques, sur la linge du Tonale et les accidents “péri-adriatiques” récents des Alpes centrales et Orientales. Dans cette région, la chaine est découpée en une mosaïque de blocs indépendants, séparés par des accidents injectés ça et là de masses basiques. L'accident du Tonale, Qui a joué en décrochement dextre à l'Oligo-Miocéne, est relié à la Thermenlinie d'Autriche Et non à la linge Pusteria—Gail. Un essai de chronologie est présenté.  相似文献   

The Ebro Deep-Sea Fan system     

C. Hans Nelson  Andres Maldonado  Francis Coumes  Henri Got  Andre Manaco 《Geo-Marine Letters》1984,3(2-4):125-131

The Ebro Fan System consists of en echelon channel-levee complexes, 50×20 km in area and 200-m thick. A few strong reflectors in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee complexes prevent the development of distributary channels and midfan lobe deposits. Margin setting represents fan and/or source area  相似文献   

Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results     

E.C. Sittler Jr.  N. Andre  M. Burger  A. Coates  D. Reisenfeld  A. Persoon  H.T. Smith  R.E. Hartle  M.D. Shappirio  D.J. McComas 《Planetary and Space Science》2008,56(1):3-18

Using ion-electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Sittler et al. [2006a. Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci., 54, 1197-1210], one can estimate the ion total flux tube content, N_IONL², for protons, H⁺, and water group ions, W⁺, as a function of radial distance or dipole L shell. In Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32(14), L14S04), it was shown that protons and water group ions dominated the plasmasphere composition. Using the ion-electron fluid parameters as boundary condition for each L shell traversed by the Cassini spacecraft, we self-consistently solve for the ambipolar electric field and the ion distribution along each of those field lines. Temperature anisotropies from Voyager plasma observations are used with (T_⊥/T_∥)_W⁺∼5 and (T_⊥/T_∥)_H⁺∼2. The radio and plasma wave science (RPWS) electron density observations from previous publications are used to indirectly confirm usage of the above temperature anisotropies for water group ions and protons. In the case of electrons we assume they are isotropic due to their short scattering time scales. When the above is done, our calculation show N_IONL² for H⁺ and W⁺ peaking near Dione's L shell with values similar to that found from Voyager plasma observations. We are able to show that water molecules are the dominant source of ions within Saturn's inner magnetosphere. We estimate the ion production rate S_ION∼10²⁷ ions/s as function of dipole L using N_H⁺, N_W⁺ and the time scale for ion loss due to radial transport τ_D and ion-electron recombination τ_REC. The ion production shows localized peaks near the L shells of Tethys, Dione and Rhea, but not Enceladus. We then estimate the neutral production rate, S_W, from our ion production rate, S_ION, and the time scale for loss of neutrals by ionization, τ_ION, and charge exchange, τ_CH. The estimated source rate for water molecules shows a pronounced peak near Enceladus’ L shell L∼4, with a value S_W∼2×10²⁸ mol/s.  相似文献