全文获取类型
收费全文 | 58766篇 |
免费 | 836篇 |
国内免费 | 703篇 |
专业分类
测绘学 | 1905篇 |
大气科学 | 4852篇 |
地球物理 | 11744篇 |
地质学 | 20787篇 |
海洋学 | 4551篇 |
天文学 | 13163篇 |
综合类 | 204篇 |
自然地理 | 3099篇 |
出版年
2021年 | 396篇 |
2020年 | 428篇 |
2019年 | 504篇 |
2018年 | 1243篇 |
2017年 | 1177篇 |
2016年 | 1610篇 |
2015年 | 981篇 |
2014年 | 1535篇 |
2013年 | 2917篇 |
2012年 | 1639篇 |
2011年 | 2157篇 |
2010年 | 1823篇 |
2009年 | 2426篇 |
2008年 | 2095篇 |
2007年 | 1937篇 |
2006年 | 2015篇 |
2005年 | 1696篇 |
2004年 | 1605篇 |
2003年 | 1563篇 |
2002年 | 1572篇 |
2001年 | 1397篇 |
2000年 | 1395篇 |
1999年 | 1214篇 |
1998年 | 1141篇 |
1997年 | 1184篇 |
1996年 | 1070篇 |
1995年 | 1001篇 |
1994年 | 922篇 |
1993年 | 796篇 |
1992年 | 753篇 |
1991年 | 769篇 |
1990年 | 790篇 |
1989年 | 740篇 |
1988年 | 695篇 |
1987年 | 838篇 |
1986年 | 750篇 |
1985年 | 864篇 |
1984年 | 975篇 |
1983年 | 960篇 |
1982年 | 903篇 |
1981年 | 817篇 |
1980年 | 764篇 |
1979年 | 711篇 |
1978年 | 714篇 |
1977年 | 652篇 |
1976年 | 583篇 |
1975年 | 577篇 |
1974年 | 644篇 |
1973年 | 651篇 |
1972年 | 417篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
Pre-weighed blocks of a Jurassic Limestone were exposed on the ground surface in the coastal Namib Desert for a period of 2 years. The environment is both salty and foggy. Some of the blocks suffered extensive disintegration, and laboratory analyses (including geochemistry, XRD and SEM) indicate that the weathered samples have a high halite (sodium chloride) content. Cycles of wetting and drying associated with the frequent fog events of the area cause cycles of halite crystallization. Rocks exposed at the surface absorb salts from the surrounding desert surface and then disintegrate, contributing to planation of the landscape. 相似文献
112.
This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the Pn phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the Pn phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P -wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s−1 at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed Pn amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s−1 in the fast direction to 0 s−1 in the slow direction of horizontal P -wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived. 相似文献
113.
Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil 总被引:1,自引:0,他引:1
S. A. Gibson R. N. Thompson R. K. Weska A. P. Dickin O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314
High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser 40Ar/39Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (87Sr/86Sr?=?0.704 and 143Nd/144Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C. 相似文献
114.
Sources of Water Supplying Pumpage from Regional Aquifer Systems of the United States 总被引:1,自引:0,他引:1
Richard H. Johnston 《Hydrogeology Journal》1997,5(2):54-63
115.
116.
117.
118.
The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered. 相似文献
119.
120.