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991.
The unequilibrated eucrite Yamato 82202 (Y82202) contains a network of relatively thick (up to 1 mm in width) glass veins. The host of the meteorite represents a monomict breccia composed of volcanic rock that crystallized > 4.3 Ga ago as a lava flow on the surface of 4 Vesta. The veins formed 3.90 ± 0.04 Ga ago, probably as a result of frictional melting associated with impact, under conditions of low effective oxygen fugacity and higher sulfur fugacity. The glass contains disordered structural elements characteristic of pyroxene and feldspar, reminiscent of the eucritic target lithology. The unequilibrated pyroxenes of the volcanic host and the pristine character of the glass indicate that this meteoritic material did not experience significant thermal metamorphism after initial crystallization of the lava. Hence, it was not affected by regional metamorphism caused by burial to a significant depth or by long-term contact metamorphism associated with a thick lava flow, large intrusion, or hot layer of impact ejecta. The meteorite resided at a shallow depth (though not at the surface) on 4 Vesta or on one of the vestoids until it was ejected and traveled to Earth, probably with other HED materials that have 36Ar exposure ages of ∼13 Ma. These data suggest that the surface of the asteroid 4 Vesta has significant, though probably small, proportions of glass, as well as unequilibrated volcanic rock.  相似文献   
992.
The focus of this paper is to relate fundamental statistical properties of landforms and drainage networks to models that have been developed in statistical physics. Relevant properties and models are reviewed and a general overview is presented. Landforms and drainage networks are clearly complex, but well-defined scaling laws are found. Coastlines, topography contours, and lakes are classic self-similar fractals. The height of topography along a linear track is well approximated as a Brownian walk, a self-affine fractal. This type of behavior has also been found in surface physics, for example the surface roughness of a fracture. An applicable model is the Langevin equation, the heat equation with a stochastic white-noise driver. This model also reproduces the statistics of sediment deposition. Drainage networks were one of the original examples of self-similar fractal trees. An important advance in quantifying the structure of drainage networks is the application of the Tokunaga fractal side-branching statistics. A classic problem in statistical physics is the diffusion-limited aggregation. The resulting tree like structures have been shown to also satisfy the Tokunaga statistics. A modified version of the diffusion-limited aggregation model reproduces the statistics of drainage networks. It is concluded that the models developed in statistical physics have direct applicability to the fundamental problems in geomorphology.  相似文献   
993.
Liquid Immiscibility and the Evolution of Basaltic Magma   总被引:9,自引:0,他引:9  
This experimental study examines relationships between alternativeevolution paths of basaltic liquids (the so-called Bowen andFenner trends), and silicate liquid immiscibility. Syntheticanalogues of natural immiscible systems exhibited in volcanicglasses and melt inclusions were used as starting mixtures.Conventional quench experiments in 1 atm gas mixing furnacesproved unable to reproduce unmixing of ferrobasaltic melts,yielding instead either turbid, opalescent glasses, or crystallizationof tridymite and pyroxenes. In contrast, experiments involvingin situ high-temperature centrifugation at 1000g (g = 9·8m/s2) did yield macroscopic unmixing and phase separation. Centrifugationfor 3–4 h was insufficient to complete phase segregation,and resulted in sub-micron immiscible emulsions in quenchedglasses. For a model liquid composition of the Middle Zone ofthe Skaergaard intrusion at super-liquidus temperatures of 1110–1120°C,centrifugation produced a thin, silicic layer (64·5 wt%SiO2 and 7·4 wt% FeO) at the top of the main Fe-richglass (46 wt% SiO2 and 21 wt% FeO). The divergent compositionsat the top and bottom were shown in a series of static runsto crystallize very similar crystal assemblages of plagioclase,pyroxene, olivine, and Fe–Ti oxides. We infer from theseresults that unmixing of complex aluminosilicate liquids maybe seriously kinetically hampered (presumably by a nucleationbarrier), and thus conventional static experiments may not correctlyreproduce it. In the light of our centrifuge experiments, immiscibilityin the Skaergaard intrusion could have started already at thetransition from the Lower to the Middle Zone. Thus, magma unmixingmight be an important factor in the development of the Fe-enrichmenttrend documented in the cumulates of the Skaergaard LayeredSeries. KEY WORDS: liquid immiscibility; Skaergaard; layered intrusions; experimental petrology  相似文献   
994.
Potential for global mapping of development via a nightsat mission   总被引:1,自引:0,他引:1  
Nightsat is a concept for a satellite system capable of global observation of the location, form and density of lighted infrastructure and development within human settlements. Nightsat’s repeat cycle should be sufficient to produce an annual cloud-free composite of surface lighting to enable detection of growth rates. Airborne and satellite imagery have been used to define the range of spatial, spectral, and detection limit options for a future Nightsat mission. Our conclusion is that Nightsat should collect data from a near-synchronous orbit in the early evening with 50–100 m spatial resolution and have detection limits of 2.5E−8 W cm−2 sr−1 μm−1 or better. Multispectral low-light imaging data would be better than panchromatic data by providing valuable information on the type or character of lighting, a potentially stronger predictor of variables such as ambient population density and economic activity.  相似文献   
995.
Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank‐full, and bed load was sampled at flows 0·7–1·7 times bank‐full. A large aperture bed load sampler (1 m by 0·45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D25 of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0·85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D70 and larger (88–155 mm), while particles in the range D30D70 (35–88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under‐represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0·75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
996.
Blooms of the toxic dinoflagellate Alexandrium fundyense commonly occur in the western Gulf of Maine but the amount of toxin observed in coastal shellfish is highly variable. In this study, a coupled physical–biological model is used to investigate the dynamics underlying the observed A. fundyense abundance and shellfish toxicity in 1993 (a high toxicity year) and 1994 (low toxicity year). The physical model simulates the spring circulation, while the biological model estimates the germination and population dynamics of A. fundyense based on laboratory and field data. The model captures the large-scale aspects of the initiation and development of A. fundyense blooms during both years, but small-scale patchiness and the dynamics of bloom termination remain problematic. In both cases, the germination of resting cysts accounts for the magnitude of A. fundyense populations early in the spring. Simulations with low net A. fundyense growth rates capture the mean observed concentration during the bloom peak, which is of similar magnitude during both years. There is little evidence that large-scale changes in biological dynamics between 1993 and 1994 were a primary driver of the differences in shellfish toxicity. Results instead suggest that the persistent southwesterly flow of the western Maine Coastal Current led to A. fundyense populations of similar alongshore extent by late May of both years. This period coincides with peak cell abundance in the region. Variations in wind forcing (downwelling favorable in 1993, upwelling favorable in 1994) and subsequent cell transport (inshore in 1993, offshore in 1994) in early June then provides a plausible explanation for the dramatic mid-June differences in shellfish toxicity throughout the western Gulf of Maine.  相似文献   
997.
Stable isotopes of faeces contain information related to the animals feeding ecology. The use of stable isotope values from subfossil faeces as a palaeoenvironmental indicator depends on how faithfully the animal records their local environment. Here we present insectivorous bat guano δ13C and δ15N values from a precipitation gradient across the southern United States and northern Mexico to compare with local vegetation and climate. We find δ13C values to be an excellent predictor of expected C4/CAM vegetation, indicating that the bats are non-selective in their diet. Moreover, we find bat guano δ13C values to be strongly correlated with summer precipitation amount and winter precipitation ratio. We also find evidence for a significant relationship with mean annual temperature. In general, we do not find δ15N values to be related to any parameters along the climatic gradient we examined. Additionally, we measured δ13C and δ15N values of bulk guano deposited annually from 1968 to 1987 in a varved guano deposit at Eagle Creek Cave, Arizona. Neither δ13C nor δ15N values were significantly related to various local meteorological variables; however, we found δ13C values of guano to be significantly related to drought and to the North American Monsoon indicating bat guano δ13C values preserve an interpretable record of large-scale atmospheric variability.  相似文献   
998.
We discuss the accuracy requirements for measuring mesoscale (roughly horizontal scales > 10 km or 5 to 10 times the planetary boundary-layer (PBL) depth) fluxes in the convective PBL, and the ability of current research aircraft to achieve this accuracy. We conclude that aircraft equipped with inertial nagivation systems capable of < 3 km hr−1 navigational accuracy are able to resolve mesoscale fluctuations in velocity, and thus variances and fluxes on the mesoscale. We then discuss measurements of velocity and scalar spectra, and cospectra of vertical velocity with horizontal velocity components and scalars, obtained from long flight legs with the National Center for Atmospheric Research Electra aircraft over the boreal forest of Canada in summer during the BOreal Ecosystem-Atmosphere Study (BOREAS), over the tropical Pacific Ocean from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and over the East China Sea during wintertime cold-air outbreaks from the Air Mass Transformation Experiment (AMTEX). Each of these studies has somewhat different forcings and boundary conditions, so we can compare their consequences on the spectra and cospectra. On average, we found no significant scalar or momentum fluxes for horizontal scales > 10 km. We also develop a simple model based on observed thermal structure to explain the phase angle between vertical velocity and the along-wind horizontal velocity as a function of height, which shows good agreement with the observed phase angle in AMTEX. The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   
999.
In a recent paper by Sergysels with the same title, he describes a problem concerning these surfaces. In this brief commentary, we resolve this problem.  相似文献   
1000.
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