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   321篇
  免费   14篇
  国内免费   1篇
测绘学   4篇
大气科学   37篇
地球物理   58篇
地质学   89篇
海洋学   26篇
天文学   76篇
自然地理   46篇
  2022年   1篇
  2021年   11篇
  2020年   11篇
  2019年   10篇
  2018年   11篇
  2017年   8篇
  2016年   13篇
  2015年   10篇
  2014年   9篇
  2013年   25篇
  2012年   28篇
  2011年   23篇
  2010年   14篇
  2009年   30篇
  2008年   17篇
  2007年   15篇
  2006年   14篇
  2005年   16篇
  2004年   15篇
  2003年   13篇
  2002年   9篇
  2001年   8篇
  2000年   8篇
  1999年   5篇
  1998年   3篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1994年   1篇
  1993年   2篇
  1983年   1篇
  1981年   1篇
  1972年   1篇
排序方式: 共有336条查询结果,搜索用时 234 毫秒
21.
Here we reconstruct the last advance to maximum limits and retreat of the Irish Sea Glacier (ISG), the only land-terminating ice lobe of the western British Irish Ice Sheet. A series of reverse bedrock slopes rendered proglacial lakes endemic, forming time-transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, optically stimulated luminescence (OSL) bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples. This chronology constrains what is globally an early build-up of ice during late Marine Isotope Stage 3 and Greenland Stadial (GS) 5, with ice margins reaching south Lancashire by 30 ± 1.2 ka, followed by a 120-km advance at 28.3 ± 1.4 ka reaching its 26.5 ± 1.1 ka maximum extent during GS-3. Early retreat during GS-3 reflects piracy of ice sources shared with the Irish-Sea Ice Stream (ISIS), starving the ISG. With ISG retreat, an opportunistic readvance of Welsh ice during GS-2 rode over the ISG moraines occupying the space vacated, with ice margins oscillating within a substantial glacial over-deepening. Our geomorphological chronosequence shows a glacial system forced by climate but mediated by piracy of ice sources shared with the ISIS, changing flow regimes and fronting environments.  相似文献   
22.
23.
Geographies of age: thinking relationally   总被引:1,自引:0,他引:1  
Peter Hopkins  Rachel Pain 《Area》2007,39(3):287-294
In contrast to recent treatment of other social identities, geographers' work on age still focuses disproportionately on the social-chronological margins – the very young and (to a far lesser extent) the very old – and rarely connects them directly. We outline the benefits of creating relational geographies of age, in order to build out from the recent explosion of children's geographies, and discuss three helpful concepts: intergenerationality, intersectionality and lifecourse. We suggest that participation provides one epistemological vehicle for getting beyond geographies which are mainly adults'.  相似文献   
24.
Ever wanted to give school students or the public a closer look at the stars, without worrying about the vagaries of the UK weather? Paul Roche and Rachel Dodds from the Faulkes Telescope Project explain how you can do just that – without even leaving the classroom!  相似文献   
25.
26.
27.
Apatite fission track thermochronology (AFTT) and paleomagnetic (PM) results have been used to constrain the Late Paleozoic to Cenozoic landscape evolution of the Lachlan Fold Belt (LFB) around the Northparkes copper-gold deposit in east-central New South Wales. The present-day landscape of this region of the LFB is relatively flat with little expression of the underlying rock and has previously been interpreted to indicate long-term stability of the region since the end of LFB orogenesis in the Early Carboniferous. This was presumably borne out by PM analyses from thick weathered horizons within open pits at the mine, which suggested that significant periods of weathering, and hence relative landscape stability, prevailed during the Early to middle Carboniferous and at some time during the Cenozoic. Results from AFTT analyses, however, indicate that the region must have experienced significant episodes of cooling/denudation during the mid-Permian to mid-Triassic and during the early Cenozoic, as well as episodes of heating/burial during the Late Carboniferous to mid-Permian and during the late Mesozoic. When combined, the AFTT and PM results are in fact consistent and indicate that since the late Paleozoic the landscape of the LFB around the Northparkes deposit has evolved through multiple episodes of denudation and deposition as well as periods of relative stability during which the thick weathering horizons formed. Together these results establish a complementary chronological framework that constrains the Late Palaeozoic to Cenozoic landscape evolution of the Northparkes region and highlights the importance of using dual data sets in elucidating the long-term landscape evolution of similar "stable" terranes.  相似文献   
28.
The lunar rock and mineral characterization consortium (LRMCC) has conducted coordinated mineralogy/petrography/spectroscopy analyses of a suite of pristine lunar basalts. Four basalt slabs (two low‐Ti, two high‐Ti) and paired thin sections were analyzed. Thin sections were analyzed for mineralogy/petrography, while the slabs were used to prepare particulate separates of major mineral phases and bulk samples. Mineral separates and particulate bulk samples were crushed to controlled grain sizes and their reflectance spectra measured in the NASA RELAB at Brown University. The resulting data set provides an essential foundation for spectral mixing models, offers valuable endmember constraints for space weathering analyses, and represents critical new ground truth results for lunar science and exploration efforts.  相似文献   
29.
Abstract– Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe2+ is responsible for the dominant pyroxene absorptions in the visible and near‐infrared, but substitutions of other cations such as Ca2+ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca2+, Mg2+, and Fe2+), we focus on a suite of pyroxenes synthesized with only Ca2+, Mg2+, and Fe2+ in the two octahedral sites, specifically examining the effect of Ca2+ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthopyroxenes and low‐Ca clinopyroxenes, Ca2+‐content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo30, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca2+ is added. At Wo50, when most of the M2 sites are filled by Ca2+, band positions do not change drastically, although the presence and strengths of the 1 and 2 μm bands are affected by even trace amounts of Fe2+ in the M2 site. It is thus apparent that next‐nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe2+ cations and control absorption band properties.  相似文献   
30.
This paper presents a new optimization model to help cities in seismically active developing countries decide (1) How much to spend on pre-earthquake mitigation versus waiting until after an event and paying for reconstruction or simply not rebuilding damaged buildings? (2) Which buildings to mitigate and how? and (3) Which buildings to reconstruct and how? It extends previously developed optimization models to consider the particular issues that arise in such countries. First, the model allows for the possibility that some damaged buildings will not be reconstructed immediately and keeps track of any lost building inventory. Second, buildings can be mitigated to, or when damaged, reconstructed to, any appropriate structural type and seismic design level. Finally, the model objectives include minimizing the chance of an extremely high death toll in any one earthquake and minimizing the average annual death toll across earthquakes. The model is illustrated through a case study analysis for Tehran, Iran.  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