全文获取类型
收费全文 | 89739篇 |
免费 | 1772篇 |
国内免费 | 932篇 |
专业分类
测绘学 | 2323篇 |
大气科学 | 6355篇 |
地球物理 | 18240篇 |
地质学 | 31877篇 |
海洋学 | 7852篇 |
天文学 | 20353篇 |
综合类 | 256篇 |
自然地理 | 5187篇 |
出版年
2022年 | 467篇 |
2021年 | 853篇 |
2020年 | 985篇 |
2019年 | 1054篇 |
2018年 | 2338篇 |
2017年 | 2168篇 |
2016年 | 2795篇 |
2015年 | 1655篇 |
2014年 | 2671篇 |
2013年 | 4723篇 |
2012年 | 2839篇 |
2011年 | 3872篇 |
2010年 | 3318篇 |
2009年 | 4388篇 |
2008年 | 4007篇 |
2007年 | 3804篇 |
2006年 | 3633篇 |
2005年 | 2919篇 |
2004年 | 2830篇 |
2003年 | 2635篇 |
2002年 | 2426篇 |
2001年 | 2230篇 |
2000年 | 2152篇 |
1999年 | 1773篇 |
1998年 | 1856篇 |
1997年 | 1758篇 |
1996年 | 1398篇 |
1995年 | 1436篇 |
1994年 | 1266篇 |
1993年 | 1107篇 |
1992年 | 1093篇 |
1991年 | 962篇 |
1990年 | 1108篇 |
1989年 | 931篇 |
1988年 | 870篇 |
1987年 | 1036篇 |
1986年 | 871篇 |
1985年 | 1124篇 |
1984年 | 1196篇 |
1983年 | 1140篇 |
1982年 | 1099篇 |
1981年 | 911篇 |
1980年 | 906篇 |
1979年 | 805篇 |
1978年 | 782篇 |
1977年 | 732篇 |
1976年 | 692篇 |
1975年 | 675篇 |
1974年 | 654篇 |
1973年 | 626篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
381.
In the Shackleton Range of East Antarctica, garnet-bearing ultramafic rocks occur as lenses in supracrustal high-grade gneisses. In the presence of olivine, garnet is an unmistakable indicator of eclogite facies metamorphic conditions. The eclogite facies assemblages are only present in ultramafic rocks, particularly in pyroxenites, whereas other lithologies – including metabasites – lack such assemblages. We conclude that under high-temperature conditions, pyroxenites preserve high-pressure assemblages better than isofacial metabasites, provided the pressure is high enough to stabilize garnet–olivine assemblages (i.e. ≥18–20 kbar). The Shackleton Range ultramafic rocks experienced a clockwise P–T path and peak conditions of 800–850 °C and 23–25 kbar. These conditions correspond to ∼70 km depth of burial and a metamorphic gradient of 11–12 °C km−1 that is typical of a convergent plate-margin setting. The age of metamorphism is defined by two garnet–whole-rock Sm–Nd isochrons that give ages of 525 ± 5 and 520 ± 14 Ma corresponding to the time of the Pan-African orogeny. These results are evidence of a Pan-African suture zone within the northern Shackleton Range. This suture marks the site of a palaeo-subduction zone that likely continues to the Herbert Mountains, where ophiolitic rocks of Neoproterozoic age testify to an ocean basin that was closed during Pan-African collision. The garnet-bearing ultramafic rocks in the Shackleton Range are the first known example of eclogite facies metamorphism in Antarctica that is related to the collision of East and West Gondwana and the first example of Pan-African eclogite facies ultramafic rocks worldwide. Eclogites in the Lanterman Range of the Transantarctic Mountains formed during subduction of the palaeo-Pacific beneath the East Antarctic craton. 相似文献
382.
M.J. Rees 《Astrophysics and Space Science》2003,285(2):375-388
Fred Hoyle famously drew attention to the significance of apparent coincidences in the energy levels of the carbon and oxygen
nucleus. This paper addresses the possible implications of other coincidences in cosmology.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
383.
384.
S. TACHIBANA H. NAGAHARA S. MOSTEFAOUI N. T. KITA 《Meteoritics & planetary science》2003,38(6):939-962
Abstract— We have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial 26Al/27Al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1). The initial 26Al/27Al ratios of these chondrules were obtained by Kita et al. (2000) and Mostefaoui et al. (2002), corresponding to relative ages from 0.7 ± 0.2 to 2.4 ?0.4/+0.7 Myr after calcium‐aluminum‐rich inclusions (CAIs), by assuming a homogeneous distribution of 26Al in the early solar system. The measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and, thus, the chondrules in this study are regarded as representatives of ferromagnesian chondrules. The relative ages of the chondrules appear to correlate with bulk abundances of Si and the volatile elements (Na, K, Mn, and Cr), but there seems to exist no correlation of relative ages neither with Fe nor with refractory elements. Younger chondrules tend to be richer in Si and volatile elements. Our result supports the result of Mostefaoui et al. (2002) who suggested that pyroxene‐rich chondrules are younger than olivine‐rich ones. The correlation provides an important constraint on chondrule formation in the early solar system. It is explained by chondrule formation in an open system, where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation. 相似文献
385.
386.
Natalia Orlova Vladimir Korchagin Nobuhiro Kikuchi Shoken. M. Miyama Aleksei Moiseev 《Astrophysics and Space Science》2003,284(2):739-742
We have tested the applicability of the global modal approach in the density wave theory of spiral structure for a sample
of spiral galaxies with measured axisymmetric background properties. We report here the results of the simulations for four
galaxies: NGC 488, NGC 628, NGC 1566, and NGC 3938. Using the observed radial distributions for the stellar velocity dispersions
and the rotation velocities we have constructed the equilibrium models for the galactic disks in each galaxy and implemented
two kinds of stability analyses - the linear global analysis and 2D-nonlinear simulations. In general, the global modal approach
is able to reproduce the observed properties of the spiral arms in the galactic disks. The growth of spirals in the galactic
disks can be physically understood in terms of amplification by over-reflection at the corotation resonance. Our results support
the global modal approach as a theoretical explanation of spiral structure in galaxies.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
387.
388.
Abstract— In this study, we have performed pulse‐heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al2O3) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse‐heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry. 相似文献
389.
390.