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1.
Philippe Machetel 《Comptes Rendus Geoscience》2003,335(1):91-97
The numerical models of mantle convection agree to depict avalanches behaviour according to the level of endothermicity of the spinel → perovskite phase change. Their potential effects on the global thermal and dynamical states of the mantle have been computed thanks to a numerical code, which takes into account both the 400-km exothermic and the 660-km endothermic phase changes. The cycle followed by the avalanches is: local layering, destabilization of the 660-km thermal layer, travelling and spreading on the core, and reappearing of the local layering. Therefore, mantle convection is characterized by quiet periods of partial layering embedded in catastrophic events. During the avalanche, the amplitude of the surface velocity is multiplied by two, which would imply an enhanced plate tectonic and ridge activities. The global thermal effects of the avalanche are compatible with a high mantle temperature and an acceleration of Earth's rotation during the Cretaceous. They also offer a coherent explanation to locate the origin of mantle plumes both within the CMB and just below the transition zone. 相似文献
2.
The thermodynamic structure on top of a numerically simulated severe storm is examined to explain the satellite observed plume formation above thunderstorm anvils. The same mechanism also explains the formation of jumping cirrus observed by Fujita on board of a research aircraft. A three-dimensional, non-hydrostatic cloud model is used to perform numerical simulation of a supercell that occurred in Montana in 1981. Analysis of the model results shows that both the plume and the jumping cirrus phenomena are produced by the high instability and breaking of the gravity waves excited by the strong convection inside the storm. These mechanisms dramatically enhance the turbulent diffusion process and cause some moisture to detach from the storm cloud and jump into the stratosphere. The thermodynamic structure in terms of the potential temperature isotherms above the simulated thunderstorm is examined to reveal the instability and wave breaking structure. The plumes and jumping cirrus phenomena represent an irreversible transport mechanism of materials from the troposphere to the stratosphere that may have global climatic implications. 相似文献
3.
Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics? 总被引：2，自引：0，他引：2
Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts.Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to 350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to 2.7 Ga: Kenorland or Ur 2.7–2.4 Ga; Columbia 1.6–1.4 Ga; Rodinia 1100–750 Ma; and Pangea 230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean. 相似文献
4.
5.
SELFE: A semi-implicit Eulerian–Lagrangian finite-element model for cross-scale ocean circulation 总被引：1，自引：0，他引：1
Unstructured-grid models grounded on semi-implicit, finite-volume, Eulerian–Lagrangian algorithms, such as UnTRIM and ELCIRC, have enjoyed considerable success recently in simulating 3D estuarine and coastal circulation. However, opportunities for improving the accuracy of this type of models were identified during extensive simulations of a tightly coupled estuary–plume–shelf system in the Columbia River system. Efforts to improve numerical accuracy resulted in SELFE, a new finite-element model for cross-scale ocean modeling. SELFE retains key benefits, including computational efficiency of existing semi-implicit Eulerian–Lagrangian finite-volume models, but relaxes restrictions on grids, uses higher-order shape functions for elevation, and enables superior flexibility in representing the bathymetry. Better representation of the bathymetry is enabled by a novel, “localized” vertical grid that resembles unstructured grids. At a particular horizontal location, SELFE uses either S coordinates or SZ coordinates, but the equations are consistently solved in Z space. SELFE also performs well relative to volume conservation and spurious oscillations, two problems that plague some finite-element models. This paper introduces SELFE as an open-source code available for community use and enhancement. The main focus here is on describing the formulation of the model and on showing results for a range of progressively demanding benchmark tests. While leaving details to separate publications, we also briefly illustrate the superior performance of SELFE over ELCIRC in a field application to the Columbia River estuary and plume. 相似文献
6.
Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300°, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, ‘plume’ connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Ωm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%.A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397–3400] at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 102 Ωm (1450–1500 °C) within the seismic LVR increasing to an off-swell resistivity of >103 Ωm (<1300 °C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry. 相似文献
7.
Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics. 相似文献
8.
Nobuyuki Kinoshita 《Boundary-Layer Meteorology》2003,106(1):61-91
Turbulent flow data of wind velocity and temperature in the unstably stratifiedatmospheric boundary layer, derived from steel tower observations in the field and wind-tunnel experiments were used to study the relationship between the plumes and the small-scale eddies in the inertial subrange. Flow visualisation experiments in the wind tunnel were also conducted to observe the structure of the flow in the plumes, and time series data were analysed by using wavelet transforms. The results show that variances of velocity and temperature due to the small-scale eddies are large in the plumes and small outside of the plumes, and that the momentum and heat fluxes due to the small-scale eddies follow the same tendency as found in the variances. The ratios of the variances caused by the small-scale eddies in the plumes to the whole of the variances caused by the small-scale eddies in and out the plumes increase with non-dimensional height -z/L in which L is the local Obukhov length. Similar ratios of the fluxes caused by the small-scale eddies also show the same tendency. These ratios can be expressed as functions of -z/L for results based on field observationand the wind tunnel experiments. This relation hardly changes even if the wavelet function is changed. The flow visualisation experiments show that the plumes have a complicated structure in which mushroom type flows are stacked on top of each other. This characteristic structure seems to increase the energy of the small-scale eddies in the plumes. 相似文献
9.
In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe–Ni core resulted in extensive the melting of the silicate mantle and in the formation of a near-surface magma mush ocean. Progressive, continuous degassing and chemical–gravitational differentiation of the crust–mantle system accompanied this Hadean stage, and has gradually lessened during the subsequent cooling of the planet. Mantle and core overturn was vigorous in the Hadean Earth, reflecting deep-seated chemical heterogeneities and concentrations of primordial heat. Hot, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of much thermal energy, but gradually decreased in importance with the passage of time. Formation of lithospheric scum began when planetary surface temperatures fell below those of basalt and peridotite solidi. Thickening and broadening of lithospheric plates are inferred from the post-Hadean rock record. Developmental stages of mantle circulation included: (a) 4.5–4.4 Ga, early, chaotic magma ocean circulation involving an incipient or pre-plate regime; (b) 4.4–2.7 Ga, growth of small micro-oceanic and microcontinental platelets, all returned to the mantle prior to 4.0 Ga, but increasing in size and progressively suturing sialic crust-capped lithospheric amalgams at and near the surface over time; (c) 2.7–1.0 Ga, assembly of cratons surmounting larger, supercontinental plates; and (d) 1.0 Ga–present, modern, laminar-flowing asthenospheric cells capped by gigantic, Wilson-cycle lithospheric plates. Restriction of komatiitic lavas to the Archean, and of ophiolite complexes ± alkaline igneous rocks, high-pressure and ultrahigh-pressure metamorphic terranes to progressively younger Proterozoic–Phanerozoic orogenic belts supports the idea that planetary thermal relaxation promoted the increasingly negative buoyancy of cooler oceanic lithosphere. The Thickening of oceanic plates enhanced the gravitational instability and the consequent overturn of the outer Earth as cold, top-down oceanic mantle convection. The scales and dynamics of deep-seated asthenospheric circulation, and of lithospheric foundering + shallow asthenospheric return flow evidently have evolved gradually over geologic time in response to the progressive cooling of the Earth. 相似文献
10.
Y. CHEN: Department of Civil & Environmental Engineering University of Bradford Bradford West Yorkshire BD DP United Kingdom P. A. MACKINNON: School of Civil Engineering The Queen''''s University of Belfast Belfast BT AG United Kingdom G. THOMPS 《国际泥沙研究》2000,(3)
I INTRODUCTIONSediment dredging operations are effective means of maintaining the natural navigation channel ofshipping for transportation of passengers and cargoes (Goldbeck 1998, Wakeman 1998). Expansions ofexisting ports to facilitate the groWth in marine traffic often necessitate navigation channel dredgingactivities (Foxworthy et. al. 1995, Luger et. al. 1998). Dredging is also heavily used for marine mining ofthe scabed deposits for gold or minerals (Anon 1996, Gamett 1996), and … 相似文献