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201.
Glonass Laser Ranging Accuracy With Satellite Signature Effect 总被引:1,自引:0,他引:1
GLONASS satellites have been tracked by the worldwide laser ranging networkas well as by the GLONASS-borne microwave-based technique. Owing to thelarge size of their corner cube reflector arrays, the amount of ranging data is enough to determine their orbits from laser ranging data alone. We found, however, that the large size of the array affected the accuracy of measurement to an extent that is dependent on the characteristics of the ranging systems. An azimuthal variation of the reflector array response was also detected in observations from single-photon laser ranging. Orbital analysis reveals that the effect makes the measured range on average 22 mm shorter than expected in the absence of the large array, which explains more than half of the offset of 39 mm previously discovered between microwave and laser orbits. 相似文献
202.
J. M. Eiler J. W. Valley C. M. Graham L. P. Baumgartner 《Contributions to Mineralogy and Petrology》1995,118(4):365-378
Retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals
in slowly cooled igneous and metamorphic rocks. Mineral zonation consistent with interdiffusion of stable isotopes has never
been directly observed, however, leaving doubt as to the mechanism responsible for the bulk-mineral isotopic compositions
commonly measured. Ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral
analyses and diffusion modeling to document the relationship between mineral zonation and the mechanism of retrogression inferred
from bulk mineral data. Two samples of magnetitebearing, quartzo-feldspathic Lyon Mountain gneiss from the Adirondack mountains,
N.Y. were studied in detail. Conventional stable isotope analysis of both samples indicates that isotope thermometers are
discordant and were reset by as much as 200°C from the estimated peak temperature of 750°C. The relative order of apparent
temperatures recorded by various thermometers differs between the two samples, however, with T
qtz-fsp≫T
mt-qtz and T
mt-fsp in one sample and T
qtz-fsp<T
mt-qtz and T
mt-fsp in the other. Diffusion modeling using the Fast Grain Boundary model shows that the former pattern of apparent temperatures
is consistent with closed system interdiffusion during cooling, whereas the latter is not. The modeling predicts that 0.5
mm diameter magnetite grains common to this rock type will contain isotopic zonation of 1‰ (rims lower in δ18O than cores), and that the cores of smaller (0.1 mm) grains will be similarly lower than to the cores of large (0.5 mm) grains.
Ion microprobe analysis reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim
zonation in multiple grains (Δcore-rim=1.1±0.4‰) and predicted grain-size vs core composition variations, consistent with
diffusion-controlled resetting of bulk mineral fractionations. In contrast, the second sample shows irregular inter-and intra-granular
variations over an 8‰ range, consistent with open system alteration. These results provide direct documentation of the importance
of interdiffusion in affecting stable isotope distributions in slowly cooled rocks. The correlations of bulk-mineral resetting
with zonation show that bulk mineral data, when interpreted with detailed modeling, can be used to determinate what processes
controlling retrogression. 相似文献
203.
K-Ar and Rb-Sr ages from glauconites are about ten to twenty per cent lower than the age of sedimentation. Previous studies have indicated that these low ages are not attributable to normal diffusion loss of Ar from glauconite crystallites.The possibility of argon loss from ‘open’ potassium sites, such as on crystal surfaces and from expanded layers, was investigated by acid dissolution techniques. These studies show that potassium is removed from glauconites with low expandabilities at three different rates. The highest dissolution rate corresponds to cation exchange and comprises five to ten per cent of the total potassium. About five per cent of the total potassium is removed at a much slower rate than that of cation exchange, but at an order of magnitude faster than the bulk of the potassium.Activation energies calculated from rate constants determined at 50° and 80°C, for one sample gave values of 19 kcal/mole for the lowest dissolution rate and 14 kcal/mole for the intermediate rate.It appears that low radiometric ages from glauconites can be largely explained by the presence of potassium in sites where argon is readily lost, although such factors as late epigenetic gain of potassium by glauconite may also contribute to their low radiometric ages. A method is described for making quantitative corrections for such daughter product loss in radiometric age determinations. 相似文献
204.
A.L. Graham A.J. Easton R. Hutchison D.Y. Jérome 《Geochimica et cosmochimica acta》1976,40(5):529-535
The Bovedy meteorite fell on 25 April 1969 in Northern Ireland; the main mass of 4·94 kg was found at Bovedy (54°57′N, 06°37′W). It is an L3 chondrite with abundant chondrules clearly visible in hand specimen. Bulk chemical analyses are presented, the total Fe content being 22·5%. The olivines are homogeneous (Fa24) but the pyroxenes are not equilibrated (Fs8–28). Brown glass is common within chondrules but a clear glass of composition An85 is present interstitially in a few orthopyroxene-rich (Fs17–28) chondrules. A bleb, 2 mm across, of clear glass, again of composition An85 was found in one stone of the meteorite and in the glass five REE (rare earth elements), (La, Sm, Eu, Yb, Lu) were determined. The low REE abundances coupled with a large positive Eu anomaly are characteristic of plagioclase, but the finer details of the pattern suggest that this glass has a closer affinity to the lunar anorthosites than to plagioclases from lunar mare basalts or eucritic meteorites. There is also evidence that the magnitude of the Eu anomaly for plagioclases and anorthosites from extra-terrestrial sources is inversely related to trivalent REE content. The existence of anorthositic material and, as a consequence, a differentiated planetary body prior to the formation of the Bovedy meteorite is suggested. 相似文献
205.
Temperature and pressure estimates for Earth's upper mantle generally are based on indirect information derived from phase equilibria studies and the measurement of temperature and pressure dependent physical and chemical properties for relevant mantle materials. This paper describes an alternative approach, based on solid-inclusion piezothermometry, which utilizes the thermoelastic properties of direct mantle derived mineral samples. In particular, this study provides the theoretical development, based on the Murnaghan equation of state for solids, for a simple method of calculating isomeke lines for host and inclusion minerals of cubic symmetry which may be extrapolated accurately to upper mantle pressure and temperature conditions. The method is demonstrated for the particular case of garnet inclusions in diamond, for which adequate laboratory thermoelastic data are available. A specific application is made in the evaluation of the depth of formation of the D1 garnet-diamond inclusion system described by Harris et al. (1970). The pressure and temperature conditions of inclusion formation lie along the calculated isomeke line within the range constrained by recent graphite-diamond phase equilibria data. However, because the isomeke line for the garnet-diamond system and the graphite-diamond phase transition are very similar in slope, a further constraint is required. Assuming, therefore, that temperature in the upper mantle is bounded by the “Oceanic” and “Shield” geotherms of Clark and Ringwood (1964), the present results indicate that the D1 garnet-diamond system formed within the depth range 138 to 155 km (about 45 to 53 kbar pressure). This result, which relates to the genesis of kimberlite xenoliths, is generally consistent with the results of other studies which utilize phase equilibria data. 相似文献
206.
Little attention has been paid to the role of sidewall processes in gully development. Simple estimates of linear incision/sidewall erosion ratios at two localities in New South Wales, Australia suggest that sidewall erosion is responsible for more than half the gully volume. Studies at three localities where overland flow and throughflow are limited to only one sidewall indicate that these processes are responsible for 10-30 per cent of gully volume. These observations have important implications for gully management. 相似文献
207.
Non-cylindrical, flexural slip folding is described from the well-known coastal section between Ardwell Bay and Kennedy's Pass, near Girvan, southwest Scotland. Bedding plane slip is recognised by the ubiquitous slickenside striations on bedding surfaces, and these linear elements define the ac kinematic plane of fold formation. This intersects the axial surfaces of folds (ab) in the common direction a which is the movement direction during fold propagation. Statistical treatment of the orientation of structural and kinematic elements yields important conclusions concerning fold formation: the Ardwell folds are markedly non-cylindrical and this is a primary feature amplified during the Ardwell Fold Phase. 相似文献
208.
209.
The Late Proterozoic ophiolite of Sol Hamed,NE Sudan 总被引:1,自引:0,他引:1
W.R. Fitches R.H. Graham I.M. Hussein A.C. Ries R.M. Shackleton R.C. Price 《Precambrian Research》1983,19(4):385-411
The Sol Hamed complex, a sequence from ultramafics, through gabbros, thin sheeted dykes, to pillow lavas, is an ophiolite. It was obducted, tilted nearly to vertical, eroded and covered unconformably by the Nafirdeib Series. This is a volcaniclastic series, dated at 712 ± 58 Ma, the lower part of which includes conglomerates with ophiolitic clasts, olistostromes, one with large oolitic limestone slabs, felsitic and dacitic tuffs, turbidites and black shales, with andesites above. Some of the structures in the ophiolite are attributed to sub-oceanic deformation. Deformation after the deposition of the Nafirdeib Series produced folds and cleavage trending about NE-SW and, more locally, a cleavage trending NW in a shear zone where the ophiolite was thrust northeastwards over the Nafirdeib Series. The ophiolite and the Nafirdeib Series were intruded by the batholithic granite dated elsewhere at ca. 669 Ma. The ophiolite is thought to occur on a suture with the Nafirdeib Series representing part of an island arc sequence. 相似文献
210.
Edgar GJ Kerrison L Shepherd SA Veronica Toral-Granda M 《Marine pollution bulletin》2003,47(7-8):276-283
Densities of fishes, invertebrates and plants at rocky intertidal and shallow subtidal sites were censused 1-2 days prior to the Jessica oil spill and compared with information obtained for the same sites one month after the spill, both for sites impacted by oil and unaffected reference sites. While the availability of pre-spill data made this analysis one of the most powerful to date for testing impacts of oil on shoreline environments, no clear changes attributable to oiling could be identified. Discharged oil appeared to cause very little impact in the intertidal and shallow subtidal zones, with such impacts lying within the range of natural spatial and temporal variation at sites investigated. Factors considered to minimize impact in Galápagos included paucity of fully sheltered shores in spill path, moderate wave action, warm temperature, high levels of sunlight, and mixing of bunker oil with diesel. 相似文献