全文获取类型
收费全文 | 85417篇 |
免费 | 1287篇 |
国内免费 | 1454篇 |
专业分类
测绘学 | 3034篇 |
大气科学 | 6461篇 |
地球物理 | 17100篇 |
地质学 | 32019篇 |
海洋学 | 6300篇 |
天文学 | 16233篇 |
综合类 | 2289篇 |
自然地理 | 4722篇 |
出版年
2021年 | 453篇 |
2020年 | 528篇 |
2019年 | 624篇 |
2018年 | 5872篇 |
2017年 | 5085篇 |
2016年 | 4088篇 |
2015年 | 1227篇 |
2014年 | 1682篇 |
2013年 | 3038篇 |
2012年 | 2722篇 |
2011年 | 4902篇 |
2010年 | 4057篇 |
2009年 | 5066篇 |
2008年 | 4353篇 |
2007年 | 4798篇 |
2006年 | 2493篇 |
2005年 | 2078篇 |
2004年 | 2320篇 |
2003年 | 2203篇 |
2002年 | 2007篇 |
2001年 | 1672篇 |
2000年 | 1552篇 |
1999年 | 1363篇 |
1998年 | 1360篇 |
1997年 | 1331篇 |
1996年 | 1063篇 |
1995年 | 1004篇 |
1994年 | 926篇 |
1993年 | 811篇 |
1992年 | 743篇 |
1991年 | 718篇 |
1990年 | 735篇 |
1989年 | 717篇 |
1988年 | 630篇 |
1987年 | 770篇 |
1986年 | 676篇 |
1985年 | 832篇 |
1984年 | 981篇 |
1983年 | 889篇 |
1982年 | 857篇 |
1981年 | 789篇 |
1980年 | 733篇 |
1979年 | 666篇 |
1978年 | 717篇 |
1977年 | 612篇 |
1976年 | 585篇 |
1975年 | 599篇 |
1974年 | 536篇 |
1973年 | 579篇 |
1972年 | 395篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
152.
The Dergaon (H5) chondrite: Fall,classification, petrological and chemical characteristics,cosmogenic effects,and noble gas records 总被引:1,自引:0,他引:1
P. N. SHUKLA A. D. SHUKLA V. K. RAI S. V. S. MURTY N. BHANDARI J. N. GOSWAMI A. C. MAZUMDAR P. PHUKON K. DUORAH R. E. GREENWOOD I. A. FRANCHI 《Meteoritics & planetary science》2005,40(4):627-637
Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ~10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ~340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ~106 cm?2 in samples from the largest fragment to approximately (4–9) × 105 cm?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ~20 cm for the Dergaon meteorite. 相似文献
153.
Tidal effects on temperature front in the Yellow Sea 总被引:5,自引:0,他引:5
Temperature front (TF) is one of the important features in the Yellow Sea, which forms in spring, thrives in summer, and fades
in autumn as thermocline declines. TF intensity ⋎S
T
⋎ is defined to describe the distribution of TF. Based on the MASNUM wave-tide-circulation coupled model, temperature distribution
in the Yellow Sea was simulated with and without tidal effects. Along 36°N, distribution of TF from the simulated results
are compared with the observations, and a quantitative analysis is introduced to evaluate the tidal effects on the forming
and maintaining processes of the TF. Tidal mixing and the circulation structure adapting to it are the main causes of the
TF.
Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No.
49736190). 相似文献
154.
155.
Non-Newtonian topographic relaxation on Europa 总被引:1,自引:0,他引:1
Models of topographic support on Europa by lateral shell thickness variations have previously assumed a Newtonian ice viscosity. Here I show that using a more realistic stress-dependent viscosity gives relaxation times which can be significantly different. Topography of wavelength 100 km cannot be supported by lateral shell thickness variations for ∼50 Myr, unless the shell thickness is <10 km or the ice grain size >10 mm. Shorter wavelength topography would require even thinner shells, but may be supported elastically. Global-scale variations in shell thickness, however, can be supported for geological timescales if the shell thickness is O(10 km). 相似文献
156.
157.
158.
Jose A.P. Rodriguez Sho Sasaki James M. Dohm Hideaki Miyamoto Goro Komatsu Justin C. Ferris 《Icarus》2005,175(1):36-57
The undulating, warped, and densely fractured surfaces of highland regions east of Valles Marineris (located north of the eastern Aureum Chaos, east of the Hydraotes Chaos, and south of the Hydaspis Chaos) resulted from extensional surface warping related to ground subsidence, caused when pressurized water confined in subterranean caverns was released to the surface. Water emanations formed crater lakes and resulted in channeling episodes involved in the excavation of Ares, Tiu, and Simud Valles of the eastern part of the circum-Chryse outflow channel system. Progressive surface subsidence and associated reduction of the subsurface cavernous volume, and/or episodes of magmatic-driven activity, led to increases of the hydrostatic pressure, resulting in reactivation of both catastrophic and non-catastrophic outflow activity. Ancient cratered highland and basin materials that underwent large-scale subsidence grade into densely fractured terrains. Collapse of rock materials in these regions resulted in the formation of chaotic terrains, which occur in and near the headwaters of the eastern circum-Chryse outflow channels. The deepest chaotic terrain in the Hydaspis Chaos region resulted from the collapse of pre-existing outflow channel floors. The release of volatiles and related collapse may have included water emanations not necessarily linked to catastrophic outflow. Basal warming related to dike intrusions, thermokarst activity involving wet sediments and/or dissected ice-enriched country rock, permafrost exposed to the atmosphere by extensional tectonism and channel incision, and/or the injection of water into porous floor material, may have enhanced outflow channel floor instability and subsequent collapse. In addition to the possible genetic linkage to outflow channel development dating back to at least the Late Noachian, clear disruption of impact craters with pristine ejecta blankets and rims, as well as preservation of fine tectonic fabrics, suggest that plateau subsidence and chaos formation may have continued well into the Amazonian Period. The geologic and paleohydrologic histories presented here have important implications, as new mechanisms for outflow channel formation and other fluvial activity are described, and new reactivation mechanisms are proposed for the origin of chaotic terrain as contributors to flooding. Detailed geomorphic analysis indicates that subterranean caverns may have been exposed during chaos formation, and thus chaotic terrains mark prime locations for future geologic, hydrologic, and possible astrobiologic exploration. 相似文献
159.
160.
F. J. Ormeling Sr. 《GeoJournal》1986,13(4):413-416
Conclusion The above account is just a selection of Perthes' multifarious activities. As such it is fragmentary and incomplete. Many other publications and collaborators could have been mentioned. Even in its incompleteness the report may justify the statement that the success of Justus Perthes Geographische Verlagsanstalt, was apart from the managing qualities of its governors, undoubtedly due to the long series of scholarly competent collaborators whom they succeeded in engaging. As illustrated above most of theme were among the avant garde of the profession. Apparently they were at ease with Justus Perthes where they were granted enough freedom of action to realise their ambitious projects under the vigilant eye of the management. In fact the history of the firm is a continuous story of their achievements. Their combined effort initiated modern scientific atlas-making, laid down the foundation of methodic school cartography and accelerated the growth of thematic cartography. The Justus Perthes maps and atlasses stand for intellectual honestly and have an authoritative appearance of truth and exactness. For a long time to come the name of the firm will be considered as a classical example of the promotion of geography and cartography by private enterprise. 相似文献