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781.
Victor A. Ramos 《Geological Journal》2010,45(1):2-25
The analyses of the main parameters controlling the present Chile‐type and Marianas‐type tectonic settings developed along the eastern Pacific region show four different tectonic regimes: (1) a nearly neutral regime in the Oregon subduction zone; (2) major extensional regimes as the Nicaragua subduction zone developed in continental crust; (3) a Marianas setting in the Sandwich subduction zone with ocean floored back‐arc basin with a unique west‐dipping subduction zone and (4) the classic and dominant Chile‐type under compression. The magmatic, structural and sedimentary behaviours of these four settings are discussed to understand the past tectonic regimes in the Mesozoic Andes based on their present geological and tectonic characteristics. The evaluation of the different parameters that governed the past and present tectonic regimes indicates that absolute motion of the upper plate relative to the hotspot frame and the consequent trench roll‐back velocity are the first order parameters that control the deformation. Locally, the influences of the trench fill, linked to the dominant climate in the forearc, and the age of the subducted oceanic crust, have secondary roles. Ridge collisions of seismic and seismic oceanic ridges as well as fracture zone collisions have also a local outcome, and may produce an increase in coupling that reinforces compressional deformation. Local strain variations in the past and present Andes are not related with changes in the relative convergence rate, which is less important than the absolute motion relative to the Pacific hotspot frame, or changes in the thermal state of the upper plate. Changes in the slab dip, mainly those linked to steepening subduction zones, produce significant variations in the thermal state, that are important to generate extreme deformation in the foreland. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
782.
Victor W. Truesdale 《Aquatic Geochemistry》2010,16(1):101-126
Recently, the increase in dissolved concentration in the batch dissolution of various salts or sucrose has been successfully
modelled with three equations, one a cubic in time. However, from three separate earlier investigations with ocean sediments
and phytoplankton frustules, there is residual suspicion that biogenic silica does not follow this behaviour. This paper shows
that the Shrinking Object Model applies to the dissolution of sieved silica gel particles, as well as to a sample of unsieved,
freeze-dried frustules of Odentella sp. Silica gel, being readily available in quantities that can be sieved, is a useful surrogate for biogenic silica in allowing
problems of experimental design to be overcome. The dissolutions covered three possible analytic integrations that arise from
the model: an exponential for approach to saturation with excess solid silica; the approach to near saturation with either
a slight excess or deficiency of silica; dissolution at high under-saturation. Good agreement was found between experimental
results and mathematical modelling. The paper provides template calculations by which future raw results can be parameterized.
Nevertheless, the reasons for non-linear kinetics reported in earlier work have not been identified, and so controversy over
non-linear dissolution kinetics is enhanced. Stirring regime was found to be important with silica gel dissolution, and so
biogenic silica dissolution is therefore likely to be ‘transport limited’ at low stirring rate. Accordingly, all archived
and future data should be scrutinized for stirring effects before being applied to the oceanic environment. A rigorous test
for determining whether a substance’s dissolution deviates from the model is recommended as a preliminary to any future dissolutions,
whether in batch or with the chemo-stat. A fixed amount of frustule sample is added to a series of buffered mixtures containing
increasing background silicic acid concentrations. Absence of any problem is marked by a linear plot between the increase
in silicic concentration accruing over a fixed reaction period and that of the background silicic acid. A novel mathematical
proof is provided to justify the test’s use. The reasons for the earlier deviations from expected behaviour of, for example,
oceanic sediments, are discussed. Lastly, the paper provides a novel approach to the dissolution of a population of particles
of mixed sizes which will probably find ready future application in oceanography. 相似文献
783.
Michael V. Kurgansky Aldo Montecinos Victor Villagran Stephen M. Metzger 《Boundary-Layer Meteorology》2011,138(2):285-298
We report on field observations in January 2009 (austral summer) of atmospheric dust devils in the northern part of the Atacama
Desert in South America (≈20◦S). An extremely high level of dust-devil activity over the study site has been observed, dependent on local meteorological
conditions. We found a high correlation between the dust-devil frequency of occurrence and the Obukhov length scale, L, calculated from meteorological gradient measurements, with a clear tendency for this frequency to increase with decreasing
−L. The upper threshold values of −L ≈ 20–30 m, and the 2-m mean wind speed, V
2 ≈ 8m s−1, for dust-devil occurrence have been found, but the minimal V
2 threshold was not observed. Parallel routine meteorological measurements enabled us to calculate the main constituents of
the surface energy balance, to obtain direct estimates of the surface albedo (α ≈ 0.21 at the solar noon) and to summarize the local conditions. 相似文献
784.
J. Victor Owen 《Geoarchaeology》2001,16(7):785-802
The major element compositions of 15 ceramic sherds from the Bonnin and Morris factory site were determined by electron microprobe. Thirteen samples are phosphatic; the others consist of (a) “soapstone” (magnesian/plombian) and (b) true porcelain, and are interpreted as exotic artifacts, as is one compositionally distinct (relatively SiO2‐poor, P2O5+CaO‐rich) phosphatic sample. Although long considered to be virtually indistinguishable from Bow porcelain (London: ca. 1747–1776), the phosphatic Philadelphia wares have a relatively low mean CaO/P2O5 ratio (3.3 versus 3.8; molecular proportions) and high alumina content (6.6 versus 5.4 wt % Al2O3). Furthermore, unlike Bow, the Bonnin and Morris samples contain calcic plagioclase (bytownite), and in some instances, an orthoclase‐rich ternary feldspar. The preservation of calcic plagioclase indicates that Philadelphia porcelain was fired at (rather than above) the thermal minimum in the An‐SiO2‐C3P system, although the presence of Na (and other fluxes) in these wares precludes the exact determination of the maximum firing temperature from this phase diagram. These wares are also distinctive insofar as the phosphate and melt phases can contain small amounts of lead; they have bulk lead contents of approximately 0.1–1.2 wt % PbO. This component has not been detected in the body of Bow or other contemporary British phosphatic porcelains. Their principal similarity lies in the fact that both wares contain sulfate. In addition, the glazes on Bonnin and Morris porcelain (e.g., PbO ∼ 35–50 wt %; SnO2 ∼ 1–2%) compositionally resemble those used at Bow. If feldspar is formed at all, then Al‐poor phosphatic porcelain (or those with low CaO/P2O5 ratios) will have comparatively low modal calcic plagioclase contents, thereby allowing the rapid depletion of this mineral via resorption by the melt phase during vitrification. Such appears to have been the case for analyzed Bow porcelain, which is therefore interpreted to have been overfired (sensu lato) relative to its Philadelphia counterpart. Conceivably, calcic plagioclase could be preserved in low‐Al wares that were fired only briefly at vitrification temperatures. Given the role of firing history in governing the mineralogy of porcelain, compositional criteria are more reliable for distinguishing these wares. © 2001 John Wiley & Sons, Inc. 相似文献