首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   494篇
  免费   25篇
  国内免费   6篇
测绘学   1篇
大气科学   49篇
地球物理   140篇
地质学   174篇
海洋学   24篇
天文学   100篇
综合类   1篇
自然地理   36篇
  2021年   6篇
  2020年   7篇
  2019年   8篇
  2018年   11篇
  2017年   11篇
  2016年   15篇
  2015年   11篇
  2014年   14篇
  2013年   23篇
  2012年   16篇
  2011年   19篇
  2010年   16篇
  2009年   21篇
  2008年   17篇
  2007年   15篇
  2006年   24篇
  2005年   18篇
  2004年   19篇
  2003年   18篇
  2002年   14篇
  2001年   14篇
  2000年   13篇
  1999年   9篇
  1998年   5篇
  1997年   4篇
  1996年   10篇
  1995年   10篇
  1994年   3篇
  1993年   5篇
  1992年   5篇
  1991年   7篇
  1990年   7篇
  1989年   3篇
  1988年   5篇
  1987年   7篇
  1985年   7篇
  1984年   10篇
  1983年   9篇
  1982年   4篇
  1981年   3篇
  1980年   6篇
  1979年   9篇
  1978年   7篇
  1977年   5篇
  1976年   6篇
  1975年   8篇
  1974年   6篇
  1973年   12篇
  1972年   3篇
  1934年   3篇
排序方式: 共有525条查询结果,搜索用时 15 毫秒
121.
122.
123.
Using Voyager results, we have made crude estimates of the rate at which Io loses volatiles by a variety of processes to the surrounding magnetosphere for both the current SO2-dominated atmosphere as well as hypothetical paleoatmospheres in which other gases, such as N2, may have been the dominant constituent. Loss rates are strongly influenced by the surface pressure on the night side, the relationship between the exobase and the Jovian magnetospheric boundary, the exospheric temperature, and the peak altitudes reached by volcanic plumes. Several mechanisms make significant contributions to the prodigious rate at which Io is currently losing volatiles. These include: interaction of the magnetospheric plasma with volcanic plume particles and the background atmosphere; sputtering of ices on the surface, if the nightside atmospheric pressure is low enough; and Jeans' escape of O, a dissociation product of SO2 gas. For paleoatmospheres, only the first two of these mechanisms would have been effective. However, they are capable of eliminating large amounts of N2 and other volatiles from Io over the satellite's lifetime. Io could have also lost large amounts of water over its lifetime due to the extensive recycling of water between its upper and lower crust, with the partial dissociation of water vapor in silicate magma chambers initiating this loss process. Significant amounts of water may also have been lost as a result of the interaction of the magnetospheric plasma with water ice particles in volcanic plumes. Once an SO2-dominated atmosphere becomes established, much water may have also been lost through the sputtering of surface water ice.  相似文献   
124.
Summary The longitudinal proper motion of sunspot groups has been shown to be a function of their size and relative extension in longitude. Two theories are proposed to explain these effects, which theories assume that the wind circulation of the spots and the wind field surrounding the spots are intimately connected, and dynamically interact. The retardation in the motion of large spot groups is hypothesized to be due to the effect of the latitudinal variation of the Coriolis parameter (on a rotating spherical body) and an assumed vortical circulation around the spots. The more rapid longitudinal motion of extended spot groups is suggested to be the result of the preferential occurrence and development of such groups in regions of above-average horizontal wind and wind shear. If these theories are essentially correct, the real solar rotation rate—i.e., the longitude- and time-averaged fluid motion field—should be derived from the motions of small spot groups, and is 1% to 11/2% higher than the usually accepted rate whichNewton andNunn derived.  相似文献   
125.
126.
Defining and interpreting shoreline change   总被引:1,自引:0,他引:1  
Successfully managing the coastal zone requires careful consideration of all the components of shoreline motion. Shoreline movement is a complex phenomenon that is the result of both natural processes and man-made effects. Some of these processes occur over millennia, while others are recent and may be cyclic. Understanding changes to the shore requires both a complete understanding of the underlying processes and an ability to accurately measure the changes. Many uncertainties exist, and predictions must be approached with an understanding of potential errors. Agencies using predictions must exercise caution and be prudent in the application of predictions for management decisions.  相似文献   
127.
The Austurhorn intrusive complex in southeastern Iceland represents an exhumed Tertiary central volcano. The geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate Austurhorn was a volcanic center analogous to Eyjafjallajökull and Torfajökull in Iceland's eastern neovolcanic zone (EVZ). Early transitional tholeiitic basalt magmatism at Austurhorn formed a shallow crustal chamber 5 km in diameter. Apparent rhythmic modal layering of, and intrusive contacts within, the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized. Felsic activity postdated near-solidification of the gabbro; numerous granitic magmas intruded along gabbro margins and within the adjacent crust. Field relations indicate that infrequent felsic replenishment prevented convective mixing of the Austurhorn chamber during this time, although commingled mafic and felsic magmas are observed in an extensive net veined complex. Late stage mafic dikes intrude the entire complex, suggesting that magmatic heat was abundantly available throughout the evolution of the Austurhorn system. Plagioclase and clinopyroxene compositions in mafic through felsic rocks, including gabbros, support a model of progressive differentiation. Field relations constrain the felsic magmas to originate at P1 kbar, presumably by fractional crystallization. The structure and geochemistry of the Austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern EVZ. The proposed rift segment was parallel to the western and eastern neovolcanic zones, and probably resulted from a reorganization of plate boundaries 7 Ma (Saemundsson 1979; Helgason 1985; Jancin et al. 1985) triggered by activity of the Iceland mantle plume.  相似文献   
128.
129.
Metapelites from the inverted Barrovian sequence in the Sikkim Himalaya (northeast India) are shown to be largely continuous with respect to their bulk rock compositions, microstructures and pressure–temperature–time–deformation (PTtD) histories. However, the upper garnet–lower staurolite zone demarcates a region of microstructurally anomalous post-kinematic garnet populations contained within strongly segregated matrices. The different microstructures within samples from this region cannot be attributed to differences in their thermobarometric histories or bulk compositions, but are instead interpreted to represent an otherwise unexposed level of the Daling Group that is now exposed along a post-metamorphic thrust splay. These heterogeneous samples contain several discrete garnet populations that progressively crystallized with increasing PT. Garnet populations that experienced the most protracted growth now form complex polycrystals that exhibit crystallographically controlled and morphologically irregular interfaces adjacent to micaceous and quartzofeldspathic domains respectively. Electron backscatter diffraction indicates that these polycrystalline garnet structures contain numerous coalesced porphyroblasts that are structurally uncorrelated across their grain boundaries. However, a crystallographically preferred orientation at the polycrystal scale is interpreted to derive from epitaxial crystallization of early-formed garnet porphyroblasts on precursor mica. Later-nucleated porphyroblasts within polycrystals preferentially concentrated towards quartzofeldspathic domains, with the overall nucleation distribution likely controlled by a complex interplay between chemical heterogeneities, strain partitioning and epitaxial crystallization. The subsequent growth of these polycrystals was equally spatially heterogeneous; it was moderated by differences in the efficiency of grain boundary transfer between quartzofeldspathic and micaceous domains that precluded thin section-scale chemical equilibration. In contrast to samples from Sikkim containing more typical porphyroblastic populations in continuous and disseminated matrices, heterogeneous availability of garnet-forming components within this strongly layered matrix is shown to have resulted in grain-scale variations in growth rates and the spatial juxtapositioning of interface-controlled microstructures and locally equilibrated chemical compositions with those that were transport controlled.  相似文献   
130.
Coastal areas in East Africa are experiencing rapid economic, resource management, demographic and technological shifts. In response diverse Community-based Natural Resource Management (CBNRM) applications have been embraced to provide mutual conservation and use benefits. These initiatives have met with mixed success in practice. Reflecting on the limitations of past research using common pool resources theory theory to study CBNRM we use insights from actor oriented theory combined with satellite image analysis to describe and discuss the forces dynamically influencing institutional and mangrove forest cover change at Kisakasaka, Zanzibar focussing on the formal CBNRM project period between 1996 and 2001, but also considering the period before and after this. We examine the shifting social relations that affected the performance and viability of the formal CBNRM arrangements. An integrated approach was taken to the presentation and discussion of results where it was possible to enrich and expand explanations of socio-environmental change, which was driven by a lack of government support, the undermining effects of party political divisions, and the lack of institutional adaptive capacity. We conclude that this was a useful approach to explain CBNRM intervention events at Kisakasaka.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号