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981.
Interactions between two identical monochromatic wave trains with a relative separation angle of 24? were experimentally investigated in a well-designed ‘X' configuration.Wave trains with different amplitudes and frequencies were generated.The results demonstrated that the interaction was strongly dependent on both wave amplitude and frequency.For nonbreaking and lower-frequency cases,the wave trains can approximately reestablish their initial state following the interaction.However,for larger waves,the interaction was enhanced,distorting the surfaces significantly-the wave trains were no longer two-dimensional after the encounter.During the interaction process,there was an obvious increase in wave height,reaching a maximum amplification in the middle of the interaction region that was approximately 1.55 times the initial height.Furthermore,the images captured by high-speed cameras illustrated that two wave trains entered the interaction region at the same time and then merged during the interaction process,resulting in an increase in wave amplitude.The combined wave crest was initially composed of two straight segments with a relative angle of 24? and gradually morphed into a single segment as is evident in the plan view.The wave then broke in the downstream,still within the interaction region,exhibiting a crescent pattern along the crest. 相似文献
982.
埃达克岩:关于其成因的一些不同观点 总被引:114,自引:57,他引:114
埃达克岩的概念是十多年前提出来的,指由俯冲的年轻洋壳熔融形成的火成岩。自从最初在现代岛弧近十几个地方报道埃达克岩以来,新近又在几个地方发现有埃达克岩(如日本西南部,外墨西哥火山岩带,等等)。但是,过去十 多年的研究也表明,埃达克岩可以由俯冲期间的其它过程产生(例如,沿俯冲板片的撕裂边,留在上地幔中的板片残余等)。另外,埃达克岩似乎与一些岩石呈共生组合,这些岩石包括高镁安山岩、富Nb的弦玄武岩(NEAB),还可能有玻安岩(几个研究者已在玻安岩中发现有埃达克岩的组分)。高镇安山岩不是来自埃达克与地幔的相互作用(Adak-type),就是来自此相互作用期间地幔的熔融(Piip-type);富Nb的弦玄武岩,据认为是来自一种被埃达我岩广泛交代的地幔的部分熔融。作为一个新的岩套,埃达克岩交代火山岩系列已被建议用来解释各种岩石组合。此外,大量的富Pb弧玄武岩也已被发现包含有超镁铁质的地幔包体,而这些包体有高亏损地幔与埃达克反应的明显证据。关于主要与下地壳熔而不是冲板片有关的埃达克岩的起源已提出几种假说,一个模型认为,下地壳熔融出现在玄武质岩浆底侵下地壳时。但是,有许多理由似乎可以排除这种模式。另一种模型认为,在大陆地壳很厚的区域,下地壳可能变成榴辉岩,从而拆离并下沉到地幔中(拆沉)。这个拆沉过程将导致下地壳下中拆沉的下地壳的上部与相对热的地幔接触,进而可引起下地壳熔融和埃达克岩的形成。这使我们认为,在中国东部发现的与俯冲作用无关的白垩纪埃达克可能是下地壳熔融与拆沉作用的产物。我们 还要强调,如果下地壳熔融与拆沉作用真能形成埃达克岩,那么埃达克岩这一术语不应该仅仅局限于与板片熔融有关的过程,而应包括那些与下地壳熔融有关的过程。太古宙的大陆地壳主要由奥长花岗岩、英云闪长岩和英安岩(TTD)组成。这种大陆地壳是来自板片熔融还是下地壳熔融仍是有争议的。然而,我们认为,太古宙期间地幔的较高温度会导致较多的洋中脊的形成,从而产生比今天“更多”的年轻洋壳的俯冲。据此,我们认为,太古宙TTD大陆地宙主要由板片熔融形成。我们也注意到,太古宙是广泛金矿化的时期。有些研究者还发现,金和铜的矿化与埃达克质交代火山岩系列有关。因此,该火山岩系列可能会寻找金属矿床的一个重要标志。 相似文献
983.
984.
Based on airborne observations during the Baltex Bridge Cloud (BBC) campaign in September 2001, the impact of two layer cloud systems, gas absorption and surface albedo on cloud radiative smoothing is investigated. Multispectral nadir radiance measurements have been conducted which cover the visible and near infrared wavelength range. The observed radiances are transformed into Fourier space where ranges of scale-invariance are identified. Associated slopes and scale breaks are determined and used to characterize the impacts on cloud radiative smoothing. The results reveal that an increase of gas absorption reduces the small scale slope and the scale break due to a decreasing likelihood of horizontal photon transport. Another impact is that the increasing gas absorption reduces the cloud surface interaction, which is indicated by an increase of the large scale slope. An increasing surface albedo results in large scale cloud radiative smoothing and is associated with a decrease of the large scale slope. This effect depends on the cloud height and the cloud morphology. Two layer cloud systems exhibit a similar behaviour in Fourier space as large surface albedos beneath a single cloud deck. It is argued that the impact of two layer cloud systems on large scale cloud radiative smoothing may not be typical for two layer clouds. 相似文献
985.
The influence of surface heterogeneities extends vertically within the atmospheric surface layer to the so-called blending height, causing changes in the fluxes of momentum and scalars. Inside this region the turbulence structure cannot be treated as horizontally homogeneous; it is highly dependent on the local surface roughness, the buoyancy and the horizontal scale of heterogeneity. The present study analyzes the change in scalar flux induced by the presence of a large wind farm installed across a heterogeneously rough surface. The change in the internal atmospheric boundary-layer structure due to the large wind farm is decomposed and the change in the overall surface scalar flux is assessed. The equilibrium length scale characteristic of surface roughness transitions is found to be determined by the relative position of the smooth-to-rough transition and the wind turbines. It is shown that the change induced by large wind farms on the scalar flux is of the same order of magnitude as the adjustment they naturally undergo due to surface patchiness. 相似文献
986.
Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for 15N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu* anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ13C vs. δ15N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in 15N and 13C from a pristine source having δ13C ? −36‰ and δ15N ? −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH4+(rock) and N2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large 13C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ15N value (+11‰), while C (up to 120 ppm and δ13C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The 15N-13C-depleted pristine source has δ 15N values from −7‰ to −4‰ and 40Ar/36Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and 40Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated. 相似文献
987.
Marc Constantin 《Geostandards and Geoanalytical Research》2009,33(1):115-132
New concentrations for Au, Ir and Ag obtained by instrumental neutron activation analysis are presented for seventy geochemical reference materials. Results in agreement with literature values for Au and Ir down to concentrations of a few ng g−1 were obtained. For Au and Ir concentrations above 10 ng g−1 , the repeatability of replicate analyses of reference materials was mostly better than 10%. For concentrations between 1 and 10 ng g−1 the RSD for Ir was 10–30%, whereas for Au it was higher and more variable (20–50%). In addition, concentrations for Cd and Hg are presented for some of the same reference materials. The high RSD at relatively high concentrations seen in gold for some RMs (e.g., WMG-1, WMS-1) did not exist for Ir and suggests homogeneity for this platinum-group element at the sub-sample size used in this study. For the following eight RMs, mostly ultramafic rocks (CHR-Pt+, OREAS-13P, OREAS-14P, PCC-1, UMT-1, WMG-1, WMS-1, WPR-1), Ir measurements agreed within ± 10% of mostly certified or recommended concentrations, which ranged from 2 ng g−1 to 6 μg g−1 . For the reference material UB-N, iridium concentration compared favourably to published results obtained by isotope dilution ICP-MS methods and a previously unrecognised heterogeneity is inferred for Au, Hg and Sb, but not for the other measured elements. 相似文献
988.
Naturally occurring asbestos in eastern Australia: a review of geological occurrence,disturbance and mesothelioma risk 总被引:1,自引:0,他引:1
Marc Hendrickx 《Environmental Geology》2009,57(4):909-926
Potential asbestos-bearing rocks account for about 0.2% of the land area of eastern Australia. The main mode of occurrence
is as narrow cross fibre and slip fibre veins of chrysotile asbestos in serpentinised ophiolite complexes along the boundaries
of major tectonic domains. Smaller deposits of chrysotile and amphibole asbestos occur in metamorphosed mafic and ultramafic
rocks associated with the Macquarie Volcanic Arc in New South Wales. Amphibole asbestos is also known from Proterozoic and
Palaeozoic amphibolite and from Devonian basalt. Natural asbestos-bearing materials in eastern Australia have been disturbed
by mining, road construction, agriculture and forestry, urban development and through natural weathering processes. Persons
most at risk of potential exposure to asbestos from natural sources include: farmers who work or live in areas where asbestos-bearing
materials may be routinely disturbed by agricultural activities; construction workers involved in large-scale earthwork projects
in areas underlain by asbestos-bearing rocks; and quarry workers who unwittingly disturb asbestos-bearing materials. Government
authorities and private enterprise need to take geological factors into account to reduce the likelihood of unplanned disturbance
of natural asbestos-bearing materials.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
989.
Rare earth element mobility in the Roffna Gneiss,Switzerland 总被引:1,自引:0,他引:1
Robert D. Vocke Jr. Gilbert N. Hanson Marc Grünenfelder 《Contributions to Mineralogy and Petrology》1987,95(2):145-154
The Roffna Gneiss, a deformed Hercynian granite porphyry within the Penninic nappes of eastern Switzerland, underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies. This resulted in the selective mobilization of major and trace elements over distances of 10's to 100's of meters and the resetting of the Rb — Sr whole rock isotopic systems some 100 my ago. The component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric. The mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline, albite and biotite and the formation of phengite and quartz. The fluids introduced Mg and H2O, promoting the development of phengite, and removed the Na being released by the breakdown of albite. The fluids were most probably derived from the surrounding Triassic carbonates and quartzites. These relatively high fO2 and carbonate rich fluids also introduced rare earth elements (REE) into the gneiss. The gneiss was progressively enriched in Eu up to 60%, Y up to 40%, and Yb up to 100%. These enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite-rich sample. While the REE were mobile, uranium and thorium were essentially immobile. The formation of xenotime was suggested to explain the observed heavy REE enrichment when large differences in the REE contents were found for replicate analyses using HF and then lithium metaborate for dissolution. These differences arose because xenotime, like monazite, can be difficult (if not impossible) to dissolve in hydrofluoric acid. Due to the possibility of incomplete sample dissolution, we now recommend fusion with lithium metaborate for all REE, Lu — Hf or Sm — Nd studies. 相似文献
990.
Abdelilah Dekayir Marc Amouric Juan Olives Claude Parron Abdelilah Nadiri Abdelkader Chergui M.Abdeljalil El Hajraoui 《Comptes Rendus Geoscience》2004,336(12):1061-1070
In Volubilis, Roman mosaics are very beautiful and reveal, from the bottom to the surface, three layers: (i) ‘hedgehog’ layer, (ii) coarse grain mortar layer (rudus + nucleus) and (iii) tesselatum. Mineralogical analysis of coarse grain mortar sampled in Flavius Germanus mosaic shows that it consisted of quartz and calcite, with some feldspar and probably mica and dolomite. Fine-grained mortar in tesselatum is made from a mixture of calcite and quartz only. Limestone tesserae (white, pink and brown) show petrographic facies that change from micritic to oolithic limestone. Conversely, black and brick red tesserae are respectively made of marble, red sandstone and from fire clay. Other colours as yellow, blue, green and grey are obtained from artificial glass with different chemical compositions. To cite this article: A. Dekayir et al., C. R. Geoscience 336 (2004). 相似文献