全文获取类型
收费全文 | 45393篇 |
免费 | 901篇 |
国内免费 | 334篇 |
专业分类
测绘学 | 1096篇 |
大气科学 | 3215篇 |
地球物理 | 9670篇 |
地质学 | 15550篇 |
海洋学 | 4050篇 |
天文学 | 10225篇 |
综合类 | 99篇 |
自然地理 | 2723篇 |
出版年
2021年 | 493篇 |
2020年 | 484篇 |
2019年 | 515篇 |
2018年 | 1130篇 |
2017年 | 1019篇 |
2016年 | 1348篇 |
2015年 | 806篇 |
2014年 | 1235篇 |
2013年 | 2362篇 |
2012年 | 1350篇 |
2011年 | 1839篇 |
2010年 | 1663篇 |
2009年 | 2164篇 |
2008年 | 1774篇 |
2007年 | 1851篇 |
2006年 | 1712篇 |
2005年 | 1397篇 |
2004年 | 1362篇 |
2003年 | 1312篇 |
2002年 | 1263篇 |
2001年 | 1125篇 |
2000年 | 1025篇 |
1999年 | 852篇 |
1998年 | 867篇 |
1997年 | 886篇 |
1996年 | 701篇 |
1995年 | 685篇 |
1994年 | 653篇 |
1993年 | 600篇 |
1992年 | 564篇 |
1991年 | 527篇 |
1990年 | 546篇 |
1989年 | 529篇 |
1988年 | 496篇 |
1987年 | 577篇 |
1986年 | 519篇 |
1985年 | 625篇 |
1984年 | 678篇 |
1983年 | 616篇 |
1982年 | 554篇 |
1981年 | 603篇 |
1980年 | 494篇 |
1979年 | 486篇 |
1978年 | 455篇 |
1977年 | 459篇 |
1976年 | 405篇 |
1975年 | 404篇 |
1974年 | 399篇 |
1973年 | 413篇 |
1971年 | 262篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
D. V. Bisikalo A. A. Boyarchuk P. V. Kaigorodov O. A. Kuznetsov T. Matsuda 《Astronomy Reports》2004,48(7):588-596
We suggest a new mechanism for the superoutbursts in SU UMa binaries, in which the increase in the accretion rate resulting in a superoutburst is associated with the formation of a spiral “precessional” wave in the inner parts of the disk, where gas-dynamical perturbations are negligible. The existence of such waves was suggested by us previously. The results of three-dimensional gas-dynamical simulations have shown that a considerable increase in the accretion rate (by up to an order of magnitude) is associated with the formation of the precessional wave. The features of the precessional spiral wave can explain both the energy release in the superoutburst and all its observational manifestations. One distinguishing feature of superoutbursts in SU UMa-type stars is the formation of a “superhump” in the light curve. Our model reproduces well both the formation of a superhump and its observational features, including its period, which is up to 3–7% longer than the orbital period, and the detectability of the superhump independent of the orbital inclination of the binary. 相似文献
962.
A possible mechanism for the formation of near-polar magnetic spots on some stars with convective envelopes is proposed. The mechanism is based on the idea that the maximum of the dynamo waves that are excited in thin convective shells by the dynamo mechanism is shifted appreciably from the maximum of the magnetic-field sources in the direction of motion of the dynamo wave. If there is no region of super-rotation near the equator for some reason (as a consequence of disruption due to tidal interaction with a companion in a binary system, for example) and the wave of stellar activity propagates toward the poles rather than toward the equator, this maximum will be in the near-polar regions. 相似文献
963.
The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition 总被引:6,自引:0,他引:6
Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·44km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an onion-skin structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km3, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure 相似文献
964.
NEKVASIL H.; DONDOLINI A.; HORN J.; FILIBERTO J.; LONG H.; LINDSLEY D. H. 《Journal of Petrology》2004,45(4):693-721
Experimental simulation of incremental crystal fractionationof a hy-normative hawaiite indicates that the spectrum of compositionsfrom mildly alkalic hawaiite to sodic rhyolite found in silica-saturatedalkalic suites of the ocean islands and continental hotspotsand rifts can be produced by fractionation at 9·3 kbarwith bulk water contents > 相似文献
965.
Sensitive high-resolution ion microprobe UPb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000300 Ma zircon. Older, 1000570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized: 相似文献
966.
KERR ANDREW C.; TARNEY JOHN; KEMPTON PAMELA D.; PRINGLE MALCOLM; NIVIA ALVARO 《Journal of Petrology》2004,45(9):1877-1906
The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceousearly Tertiary, along with pillowbasalts of the CaribbeanColombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean 40Ar39Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in SrNdPbHf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 2030% crystallization.The residual magmas from this fractionation contained 相似文献
967.
Simanovich I. M. Golovin D. I. Bujakaite M. I. Vinogradov V. I. Sakharov B. A. Matsapulin V. U. Sokolova A. L. Pokrovskaya E. V. 《Lithology and Mineral Resources》2004,39(6):555-565
The influence of geodynamic factors on the postsedimentary lithogenesis of terrigenous sequences during the replacement of the passive continental-margin regime to the active regime is considered with the Toarcian–Aalenian complexes of southern Dagestan as an example. It has been established that clayey rocks were lithified under conditions of subsidence lithogenesis, whereas the orogenic catagenesis of sandy rocks was related to stress initiated by the initial (amagmatic) subduction phase. The Kübler index has been used to reveal the postsedimentary zonality that is consistent with the intensity of folding and cleavage. The metagenesis zone fits the cleavage zone. The Rb–Sr and K–Ar datings of clayey rocks from the metagenesis zone yielded 180–190 Ma that can be interpreted as the timing of the maximal manifestation of postsedimentary lithogenesis. 相似文献
968.
Oil- and gas-bearing basins of the World Ocean spreading to the continental shelf and foothill are considered. Large hydrocarbon resources, including oil pools have been discovered in the deep-water basins. The basins are confined to passive continental margins and characterized by the common mechanism of formation. Oil and gas (hereafter, petroleum) generation and accumulation are dictated by the optimum specifics of source and reservoir rocks accumulated under favorable conditions of rifts and deep-sea fans. Halokinesis played an important role in the formation of traps and migration of hydrocarbons. The global experience shows that the northern, eastern, and southern shelves of the Russian seas, as well as their continental slopes and foothills, have a big petroleum potential. 相似文献
969.
Permian deep‐water mudstones in the Tanqua Basin, South Africa, have been studied using geochemical and spectral gamma ray techniques. The mudstones occur as thick sequences between sand‐rich submarine fans, but also occur as thinner mud‐rich units within each fan. The interfan mudstones are interpreted to have accumulated during transgression and the consequent period of relatively high sea‐level, while the submarine fans and their intrafan mudstones were deposited during regression and relatively low sea‐level. Geochemical analyses revealed systematic differences between interfan and intrafan mudstones because the two types of mudstones have slightly different source lithologies. Differences between the two types of mudstone suggest that changes in relative sea‐level played a role in controlling exposure of sediment source areas. There are geochemical signals that display systematic stratigraphic trends within both interfan and intrafan mudstones. These are best explained by gradual denudation, exposure and weathering of different lithologies within a single sediment source area. Both interfan and intrafan mudstones have uniform geochemical signals along the flow direction except for the relative amount of uranium. It is most likely that the basinward increase in uranium in the mudstones is the result of reduced clastic dilution of uranium‐bearing pelagic fallout. 相似文献
970.
Inferring the mass fraction of floc-deposited mud: application to fine-grained turbidites 总被引:1,自引:0,他引:1
K. J. Curran P. S. Hill T. M. Schell T. G. Milligan† D. J. W. Piper‡ 《Sedimentology》2004,51(5):927-944
Fine sediment deposition in the ocean is complicated by the cohesive nature of muds and their tendency to flocculate. The result is disaggregated inorganic grain size (DIGS) distributions of bottom sediment that are influenced by single‐grain and floc deposition. This study outlines a parametric model that characterizes bottom sediment DIGS distributions. Modelled parameters are then used to infer depositional conditions that account for the regional variation in the grain sizes deposited by turbidity currents on the Laurentian Fan–Sohm Abyssal Plain, offshore south‐eastern Canada. Results indicate that, on the channellized Laurentian Fan, the mass fraction of floc‐deposited mud increases only slightly downslope. The small evolution in this fraction arises because sediment concentration and turbulent energy are associated in turbidity currents. On the Sohm Abyssal Plain, however, the mass fraction of floc‐deposited mud decreases, probably as a result of lower sediment concentration at this source‐distal site. Estimates of the mass fraction of mud deposited as flocs suggest that floc deposition is the dominant mode by which sediment is lost from suspension, although single‐grain deposition contributes more to the depositional flux in proximal areas where high energy breaks flocs and in distal areas where low sediment concentration limits floc formation. It is concluded that, throughout the dispersal system, changes in the fraction of flocculated mud deposited from turbidity currents reflect changes in sediment concentration and energy downslope. 相似文献