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971.
972.
Danielle Alloin Carme Gallart Emmanuel Fleurence Emanuela Pompei Daniel Raimann Uta Fritze- Von Alvensleben Sukyoung Yi 《Astrophysics and Space Science》2002,281(1-2):109-113
The goal of this project is to analyze the integrated-light spectrum of a region in the LMC bar, for which HST images are
already available yielding color-magnitude diagrams reaching the oldest main-sequence turnoff. Then, we can derive the star
formation history in this field through evolutionary/population synthesis of the integrated light from 330 to 990 nm by a
variety of independent methods and compare the results with the - in principle - more reliable results derived from the analysis
of the color-magnitude diagram (CMD). This confrontation is the key ‘truth-test’ needed for the understanding of the abilities
and the limitations of the evolutionary/population synthesis methods that must be applied of necessity to unresolved nearby
and far-away galaxies. We report here on the observational part of the project.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
973.
Stress models for Tharsis formation, Mars 总被引:1,自引:0,他引:1
A critical survey is presented of most stress models proposed for the formation of the tectonic structures in the Tharsis volcano-tectonic province on Mars and provides new constraints for further models. First papers, in the 1970s, attempted to relate the Tharsis formation to asthenospheric movements and lithosphere loading by magma bodies. These processes were then quantified in terms of stress trajectory and magnitude models in elastic lithosphere (e.g. Banerdt et al., J. Geophys. Res. 87(B12), 9723–9733, 1982). Stresses generated by dynamic lithosphere uplift were rapidly dismissed because of the poor agreement between the stress trajectories provided by the elastic models and the structural observations. The preferred stress models involved lithosphere loading, inducing isostatic compensation, and then lithosphere flexure. Some incomsistency with structural interpretation of Viking imagery has been found. In the early 1990s, an attempt to solve this problem resulted in a model involving the existence of a Tharsis-centred brittle crustal cap, deteched from the strong mantle by a weak crustal layer (Tanaka et al., J. Geophys. Res. 96(E1), 15617–15633, 1991). Such a configuration should produce loading stresses akin to those predicted by some combination of the two loading modes. This model has not been quantified yet, however it is expected to reconcile stress trajectories and most structural patterns. Nevertheless, some inconsistencies with observed structures are also expected to remain. Parallel to this approach focused on loading mechanisms, the idea that volcanism and tectonic structures could be related to mantle circulation began to be considered again through numerical convection experiments, whose results have however not been clearly correlated with surface observations. Structural clues to early Tharsis dynamic uplift are reported. These structures have little to do with those predicted by elastic stress modelling of dynamic lithosphere uplift. They denote the existence of unsteady stress trajectories responsible for surface deformations that cannot be readily predicted by elastic models. These structures illustrate that improving current stress models for Tharsis formation shall come from deeper consideration of rock failure criterion and load growth in the lithosphere (e.g. Schultz and Zuber, J. Geophys. Res. 99(E7), 14691–14702, 1994). Improvements should also arise from better understanding rheological layering in the lithosphere and its evolution with time, and from consideration of stress associated to magma emplacement in the crust, which may have produced many tectonic structures before loading stress resulting from magma freezing became significant (Mège and Masson, Planet. Space Sci. 44, 1499–1546, 1996a). 相似文献
974.
A detailed analysis of chemical zoning in two garnet crystals from Harpswell Neck, Maine, forms the basis of an interpretation of garnet nucleation and growth mechanisms. Garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation, giving rise to complex two‐dimensional zoning patterns depending on the orientation of the thin section cut. Contours of Ca zoning cross those of Mn, Fe and Mg, indicating a lack of equilibrium among these major garnet constituents. Zoning of Fe, Mg and Mn is interpreted to reflect equilibrium with the rock matrix, whereas Ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal. Image analysis reveals that the growth of garnet is more rapid along triple‐grain intersections than along double‐grain boundaries. Moreover, different minerals are replaced by garnet at different rates. The relative rate of replacement by garnet along double‐grain boundaries is ordered as muscovite > chlorite > plagioclase > quartz. Flux calculations reveal that replacement is limited by diffusion of Si along double‐grain boundaries to or from the local reaction site. It is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet, with Si and Al transport being rate limiting in these samples. 相似文献
975.
Warming permafrost in European mountains 总被引:3,自引:0,他引:3
Charles Harris Daniel Vonder Mühll Ketil Isaksen Wilfried Haeberli Johan Ludvig Sollid Lorenz King Per Holmlund Francesco Dramis Mauro Guglielmin David Palacios 《Global and Planetary Change》2003,39(3-4):215-225
Here we present the first systematic measurements of European mountain permafrost temperatures from a latitudinal transect of six boreholes extending from the Alps, through Scandinavia to Svalbard. Boreholes were drilled in bedrock to depths of at least 100 m between May 1998 and September 2000. Geothermal profiles provide evidence for regional-scale secular warming, since all are nonlinear, with near-surface warm-side temperature deviations from the deeper thermal gradient. Topographic effects lead to variability between Alpine sites. First approximation estimates, based on curvature within the borehole thermal profiles, indicate a maximum ground surface warming of +1 °C in Svalbard, considered to relate to thermal changes in the last 100 years. In addition, a 15-year time series of thermal data from the 58-m-deep Murtèl–Corvatsch permafrost borehole in Switzerland, drilled in creeping frozen ice-rich rock debris, shows an overall warming trend, but with high-amplitude interannual fluctuations that reflect early winter snow cover more strongly than air temperatures. Thus interpretation of the deeper borehole thermal histories must clearly take account of the potential effects of changing snow cover in addition to atmospheric temperatures. 相似文献
976.
Kazuto SAIKI Didier LAPORTE Daniel VIELZEUF Satoru NAKASHIMA Pierre BOIVIN 《Meteoritics & planetary science》2003,38(3):427-444
Abstract— Two types of pallasites can be distinguished on the basis of the grain shape of olivine (rounded or angular). It has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body. In order to characterize the kinetics of rounding of olivine grains in an Fe‐Ni matrix, we carried out a series of annealing experiments using a mixture of olivine and Fe‐Ni powder. We were able to reproduce, at a miniature scale, the range of textures in pallasites. The rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 μm, even though the experiments were performed below the solidus of the Fe‐Ni metal. For instance, grains ?14 mm in diameter became nearly spherical within 7 days at 1400°C. For the morphological analysis of olivine grains, we used two independent techniques: the “critical diameter method” and the “Gaussian diffusion‐resample method,” a new technique specially developed for our study. Both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice, not by surface diffusion along the olivine‐metal boundaries. We used our experimental data to estimate the time scales required for the development of olivine‐metal textures in natural pallasites. We determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals: ?100 years to produce rounded olivine grains 0.1 mm in radius at 1300–1400°C. 相似文献
977.
Eleanor T. DIXON Donald D. BOGARD Daniel H. GARRISON 《Meteoritics & planetary science》2003,38(3):341-355
Abstract— This study presents the first determinations of 39Ar‐40Ar ages of R chondrites for the purpose of understanding the thermal history of the R chondrite parent body. The 39Ar‐40Ar ages were determined on whole‐rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak 39Ar‐40Ar ages of the four chondrites are 4.35, ?4.47 ± 0.02, 4.30 ± 0.07 Ga, and 4.37 Ga, respectively. These ages are similar to Ar‐Ar ages of relatively unshocked ordinary chondrites (4.52–4.38 Ga) and are older than Ar‐Ar ages of most shocked ordinary chondrites («4.2 Ga). Because the meteorites with the oldest (Rumuruti, ?4.47 Ga) and the youngest (Acfer 217, ?4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end‐member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ?170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact‐induced thick regolith layer, or within a rubble pile type parent body following parent body re‐assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (?4.47 Ga) impact events and breccia formation on the R chondrite parent body. 相似文献
978.
Jennifer A. GRIER David A. KRING Timothy D. SWINDLE Andrew S. RIVKIN Barbara A. COHEN Daniel T. BRITT 《Meteoritics & planetary science》2004,39(9):1475-1493
Abstract— We have studied the petrography, reflectance spectra, and Ar‐Ar systematics of the Orivinio meteorite. Orvinio is an H chondrite not an L chondrite as sometimes reported. The material in the meteorite was involved in several impact events. One impact event produced large swaths of impact melt from H chondrite material surrounding relict clasts of chondrule‐bearing material. Not only were portions of a bulk H chondrite planestesimal melted during the impact event, but shock redistribution of metal and sulfide phases in the meteorite dramatically altered its reflectance spectra. Both the melt and relict clasts are darker than unshocked H chondrite material, bearing spectral similarities to some C‐class asteroids. Such shock metamorphism, which lowers the albedo of an object without increasing its spectral slope, may partially explain some of the variation among S‐class asteroids and some of the trends seen on asteroid 433 Eros. Noble gases record the evidence of at least two, and perhaps three, impact events in the meteorite and its predecessor rocks. The most significant evidence is for an event that occurred 600 Ma ago or less, perhaps ?325 Ma ago or less. There is also a signature of 4.2 Ga in the Ar‐Ar systematics, which could either reflect complete degassing of the rock at that time or partial degassing of even the most retentive sites in the more recent event. 相似文献
979.
Daniel H. Stuermer Robert B. Spies Paul H. Davis Douglas J. Ng Clarence J. Morris Sharon Neal 《Marine Chemistry》1982,11(5):413-426
Hydrocarbon analyses on samples of sediment, interstitial water, and dissolved and particulate fractions of the water column from the vicinity of the Isla Vista seep near Santa Barbara, California, are presented. The results demonstrate that a very heterogeneous distribution of hydrocarbon composition and concentration exists in the seep environment and that the life-style and feeding habits of the organisms in this environment will have a dramatic effect on their hydrocarbon exposure. 相似文献
980.