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   382篇
  免费   14篇
  国内免费   1篇
测绘学   7篇
大气科学   17篇
地球物理   76篇
地质学   119篇
海洋学   36篇
天文学   113篇
综合类   1篇
自然地理   28篇
  2021年   4篇
  2020年   3篇
  2019年   5篇
  2018年   7篇
  2017年   9篇
  2016年   7篇
  2015年   4篇
  2014年   11篇
  2013年   22篇
  2012年   3篇
  2011年   13篇
  2010年   12篇
  2009年   16篇
  2008年   15篇
  2007年   13篇
  2006年   13篇
  2005年   13篇
  2004年   22篇
  2003年   21篇
  2002年   14篇
  2001年   13篇
  2000年   14篇
  1999年   11篇
  1998年   14篇
  1997年   5篇
  1996年   4篇
  1995年   3篇
  1994年   4篇
  1993年   7篇
  1992年   6篇
  1991年   9篇
  1990年   2篇
  1989年   8篇
  1988年   2篇
  1987年   9篇
  1985年   7篇
  1984年   10篇
  1983年   6篇
  1982年   3篇
  1981年   3篇
  1980年   3篇
  1979年   3篇
  1978年   6篇
  1976年   2篇
  1975年   3篇
  1974年   2篇
  1970年   1篇
  1968年   1篇
  1967年   2篇
  1966年   3篇
排序方式: 共有397条查询结果,搜索用时 15 毫秒
271.
272.
273.
274.
Rocks with ages of ca. 1 Ga occur in central and southern Mexico as inliers surrounded by ubiquitous Mesozoic and Cenozoic rocks. They appear to share a common history consisting of: (i) ca. 1300–1200 Ma arc magmatism and deposition of sediments including evaporites; (ii) ca.1160–1100 Ma intrusion of syenite, granite and anorthosite, the later part of which is synchronous with migmatization; (iii) intrusion of a ca. 1035–1010 Ma anorthosite–gabbro–charnockite–granite (AMCG) suite; (iv) a 1000–980 Ma granulite facies tectonothermal event with a stretching axis parallel to the long axis of Oaxaquia; (v) gradual exhumation at 750 and/or 545 Ma; and (vi) 517 Ma intrusion of an isolated calcalkaline granitoid pluton. The common Precambrian geological record of these outcrops suggests that they belonged to a single terrane (Oaxaquia) and formed a juvenile arc/backarc bordering a continent that underwent collision with, and overthrusting of, the Avalonian arc at 1000–980 Ma. This buried Oaxaquia to 25–30 km and was followed by further supra-subduction zone magmatism at ca. 917 Ma. These Precambrian rocks are unconformably overlain by uppermost Cambrian and Silurian platform rocks containing Gondwanan fauna and ca. 1 detrital zircons of Oaxacan provenance. The neighbouring Mixteca terrane includes lower Paleozoic, rift-passive margin sedimentary rocks that also contain 900–750 Ma detrital zircons probably derived from the Goiás arc in eastern Amazonia. The arc-backarc tectonic setting inferred for the 1300–900 Ma rocks also suggests that Oaxaquia lay on an active periphery of Amazonia until ca. 900 Ma, well after the amalgamation of Rodinia. This precludes a location for Oaxaquia off southern and western Amazonia that are inferred to have been juxtaposed against eastern Laurentia; contiguity with eastern Amazonia is also unlikely given the absence of the 900–750 Ma convergent tectonics in the Goiás arc. This leaves northern Amazonia as the most likely position, a location that requires the least relative displacement between Oaxaquia and Amazonia. The inferred 750 and 545 Ma exhumation episodes of Oaxaquia correspond to two proposed breakup stages of Rodinia.  相似文献   
275.
The recent discovery of a fossilized assemblage of juvenile Ediacaran rangeomorph macro‐organisms, fern‐like impressions from the Drook Formation of Newfoundland, suggests that the morphological diversity present within the earliest known communities of the Ediacaran biota was relatively high. The fledgling population of these Proterozoic organisms was smothered by volcanic material from a nearby eruption, preserving the standing community on the seafloor. As many as six different genera are identified, present less than three million years after the end of the Gaskiers glacial event. This hints at a hidden evolutionary history for these organisms that is yet to be discovered. Co‐occurring filamentous impressions, and other fossils interpreted to represent the remains of decayed and ‘effaced’ organisms, suggest that the bedding plane exhibits unusual ecological completeness, with the assemblage as a whole documenting an early example of secondary community succession. Discoveries such as this reveal the potential for high‐quality preservation of soft‐bodied Ediacaran organisms in siliciclastic successions, and provide valuable insights into rangeomorph development and palaeoecology.  相似文献   
276.
Examination with scanning electron microscopy (SEM) and scanning force microscopy (SFM) revealed etch pits, layers and islands on dolomite crystal faces synthesized from calcite in Ca‐Mg‐Cl solutions at 200 °C and a wide variety of natural dolomites. Layers are broad, flat structures bounded by steps less than 100 nm high and greater than 1 μm wide. Islands are rounded topographic highs <20 nm high and <200 nm wide. The nanotopography of synthetic dolomite changed from islands throughout most of the reaction to layers at 100% dolomite. Island nanotopography formed on both Ca‐rich and near‐stoichiometric dolomite. Analyses of reaction products from dolomite synthesis indicates that there are no SFM‐detectable products formed in <10 h. SEM‐detectable products formed in 15 h. X‐ray diffraction (XRD)‐detectable products formed in ≈18 h, and the reaction went to completion in ≈40 h. Based on SFM analyses, the induction period for dolomitization in these experiments accounts for ≈20% of the total reaction time necessary to dolomitize CaCO3 completely under the experimental conditions used here. Island nano‐ topography is inferred to occur at higher degrees of supersaturation than layer nanotopography for three reasons. First, island nanotopography on synthetic calcite and gypsum forms at higher supersaturations than layer nanotopography. Secondly, island nanotopography formed in solutions with higher degrees of supersaturation with respect to dolomite. Thirdly, the greater surface roughness of a crystal face composed of islands compared with layers indicates that island surfaces have higher surface energy than layer surfaces. Therefore, the stability of island surfaces requires a higher degree of supersaturation. Because islands and layers form under a wide range of conditions, their presence provides broadly applicable criteria for evaluating relative degrees of supersaturation under which ancient dolomite formed. Comparison of synthetic dolomites with natural dolomites demonstrates (1) similar nanotopography on natural and synthetic dolomites and (2) both natural planar and non‐planar dolomite may have island nanotopography.  相似文献   
277.
We present a catalogue of X-ray luminosities for 401 early-type galaxies, of which 136 are based on newly analysed ROSAT PSPC pointed observations. The remaining luminosities are taken from the literature and converted to a common energy band, spectral model and distance scale. Using this sample we fit the L X  :  L B relation for early-type galaxies and find a best-fit slope for the catalogue of ∼2.2. We demonstrate the influence of group-dominant galaxies on the fit and present evidence that the relation is not well modelled by a single power-law fit. We also derive estimates of the contribution to galaxy X-ray luminosities from discrete-sources and conclude that they provide     . We compare this result with luminosities from our catalogue. Lastly, we examine the influence of environment on galaxy X-ray luminosity and on the form of the     relation. We conclude that although environment undoubtedly affects the X-ray properties of individual galaxies, particularly those in the centres of groups and clusters, it does not change the nature of whole populations.  相似文献   
278.
Solar limb scanning at 5 wavelengths from 0.35 to 2 mm on the JCMT has revealed significant limb extension which increases rapidly with wavelength, in agreement with other measurements. This appears to be related to the increasing opacity of overlapping spicules which appear to become optically thick at about 1mm.  相似文献   
279.
We have derived the X-ray luminosities of a sample of galaxies in groups, making careful allowance for contaminating intragroup emission. The L X: L B and L X: L FIR relations of spiral galaxies in groups appear to be indistinguishable from those in other environments, however the elliptical galaxies fall into two distinct classes. The first class is central-dominant group galaxies, which are very X-ray luminous and may be the focus of group cooling flows. All other early-type galaxies in groups belong to the second class, which populates an almost constant band of L X/ L B over the range 9.8< log  L B<11.3 . The X-ray emission from these galaxies can be explained by a superposition of discrete galactic X-ray sources together with a contribution from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In the region where the optical luminosity of the non-central group galaxies overlaps with the dominant galaxies, the dominant galaxies are over an order of magnitude more luminous in X-rays.
We also compared these group galaxies with a sample of isolated early-type galaxies, and used previously published work to derive L X: L B relations as a function of environment. The non-dominant group galaxies have mean L X/ L B ratios very similar to those of isolated galaxies, and we see no significant correlation between L X/ L B and environment. We suggest that previous findings of a steep L X: L B relation for early-type galaxies result largely from the inclusion of group-dominant galaxies in samples.  相似文献   
280.
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