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   1633篇
  免费   34篇
  国内免费   7篇
测绘学   39篇
大气科学   177篇
地球物理   290篇
地质学   466篇
海洋学   178篇
天文学   358篇
自然地理   166篇
  2021年   12篇
  2020年   10篇
  2019年   15篇
  2018年   19篇
  2017年   17篇
  2016年   39篇
  2015年   38篇
  2014年   26篇
  2013年   115篇
  2012年   50篇
  2011年   65篇
  2010年   29篇
  2009年   67篇
  2008年   76篇
  2007年   72篇
  2006年   76篇
  2005年   55篇
  2004年   60篇
  2003年   59篇
  2002年   65篇
  2001年   51篇
  2000年   35篇
  1999年   45篇
  1998年   31篇
  1997年   27篇
  1996年   22篇
  1995年   26篇
  1994年   18篇
  1993年   19篇
  1992年   17篇
  1991年   13篇
  1990年   24篇
  1989年   17篇
  1988年   10篇
  1987年   20篇
  1986年   12篇
  1985年   32篇
  1984年   31篇
  1983年   18篇
  1982年   15篇
  1981年   19篇
  1980年   13篇
  1979年   19篇
  1978年   22篇
  1977年   22篇
  1976年   13篇
  1975年   12篇
  1974年   23篇
  1973年   17篇
  1969年   10篇
排序方式: 共有1674条查询结果,搜索用时 15 毫秒
791.
Events in eastern Germany since 1989 pose major challenges for geographical interpretation. This paper examines the spatial impacts of the post-unification economic and social upheavals in the new Länder. Using unemployment rates as a key indicator of regional differentiation, spatial variability within the new Länder is examined in terms of two fundamental dimensions. First, the relationship with former patterns of economic specialization is explored, and secondly, the effects of basic geographical properties such as settlement structure, location and peripherality. Prominent regional polarities are described, notably between major agglomeration cores and rural residuals, and between ‘bridgehead’ locations and less-accessible districts. Within this context the paper then identifies some ‘green shoots’ of recovery and new growth. It concludes by emphasizing the importance of incorporating sub-regional variations into any new framework of regional policy.  相似文献   
792.
793.
Outcrops of young, sedimentary, argillaceous rocks with well developed fabric display rapid changes in their properties when subject to tropical weathering. The change in the materials is often accompanied by mass movement activity and the geomorphological consequence in terms of landforms is usually the development of badlands topography. Detailed field and laboratory studies have been undertaken on the Joe's River Formation, Barbados, and the Lichi Melange, Taiwan. Both are sedimentary mudrocks with well developed, scaly fabrics. Physical and geotechnical laboratory tests have been conducted on samples collected from type site locations to elucidate associations between material properties, earth surface processes and landform development. While the inherent physical properties show little or no difference in the transition from unweathered to highly weathered materials, by applying the critical state model, the mudrock geotechnical properties can be shown to change significantly. As weathering commences, material strength surprisingly increases. Only after a period of more extensive weathering do mechanical properties confirm increasingly incompetent materials. The initial strength increase appears to be due to weathering-induced modification of the fabric. The subsequent strength drop is a product of weathering-induced modification of both the fabric and the in situ, intact sediment. It is suggested that by applying the critical state model, a greater consideration can be gained of the geotechnical response of the sediments to weathering.  相似文献   
794.
The fields of biology and environmental health are undergoing an unprecedented transformation in the way communities are characterized and studied. One of the catalysts for this change has been the advent of high-throughput sequencing (aka next-generation sequencing). Though high-throughput sequencing is a relatively new concept, the principles behind it are fairly straightforward (PCR and pyrosequencing) and have been in the published literature since the early 1990s. Herein, I will highlight some of the more commonly encountered issues involving experimental design as well as important considerations for metagenomic sample collection (both quantity and quality) and data handling for high-throughput sequencing using the Roche/454 Life Sciences genome sequencing platform.  相似文献   
795.
W.B. Jones 《Lithos》1979,12(2):89-97
The trachyte caldera volcanoes Kilombe and Londiani have abundant syenite boulders lying on their surfaces. Kilombe also has a syenite inclusions in post-caldera flows and tuffs. Petrographic and chemical investigations show that most of the syenites are very similar to the associated lavas. An origin by almost complete crystallization of batches of trachytic liquid in magma chambers under each volcano is proposed for these. Separation of a small amount of residual melt gives rise to a few melasyenites strongly enriched in Na, Fe and lanthanides.  相似文献   
796.
The initial 87Sr/86Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO2 (56.50%) and K2O (1.29%) and are depleted in Al2O3 (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial 87Sr/86Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial 87Sr/86Sr ratios of successive flows and their concentrations of SiO2, FeO (total iron), Na2O, K2O, P2O5, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO2, Al2O3, MgO, CaO and MnO increase upward. The initial 87Sr/86Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial 87Sr/86Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different 87Sr/86Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial 87Sr/86Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their 87Sr/86Sr ratios. The chemical composition of the parent basalt (87Sr/86Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (87Sr/86Sr=0.720) is enriched in strontium (173 ppm), SiO2, FeO (total iron) and the alkalies but is depleted in Al2O3, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.Laboratory for Isotope Geology and Geochemistry, Contribution No. 33.  相似文献   
797.
Exploring social barriers to adaptation: Insights from Western Nepal   总被引:4,自引:0,他引:4  
As the challenges and opportunities posed by climate change become increasingly apparent, the need for facilitating successful adaptation and enhancing adaptive capacity within the context of sustainable development is clear. With adaptation high on the agenda, the notion of limits and barriers to adaptation has recently received much attention within both academic and policymaking spheres. While emerging literature has been quick to depict limits and barriers in terms of natural, financial, or technologic processes, there is a clear shortfall in acknowledging social barriers to adaptation. It is against such a backdrop that this paper sets out to expose and explore some of the underlying features of social barriers to adaptation, drawing on insights from two case studies in the Western Nepal. This paper exposes the significant role of cognitive, normative and institutional factors in both influencing and prescribing adaptation. It explores how restrictive social environments can limit adaptation actions and influence adaptive capacity at the local level, particularly for the marginalised and socially excluded. The findings suggest a need for greater recognition of the diversity and complexity of social barriers, strategic planning and incorporation at national and local levels, as well as an emphasis on tackling the underlying drivers of vulnerability and social exclusion.  相似文献   
798.
Technical assistance (TA) has a long and varied history as a development practice. It initially emerged as a set of ‘hard’ programs, tools, and technologies delivered to developing countries by imported First World experts, typically in the agricultural and resource sectors. Later, in response to critical and antidevelopment theories, TA morphed into its ‘soft’ version, attempting to empower marginalized people in the Global South by delivering the know-how - often collaboratively generated - sufficient to produce forms of development ‘from below’. In spite of this shift in the politics and practices of TA, it remains susceptible to neoliberal styles of development that have proceeded apace with withdrawal of state institutions in the funding and operation of social and economic development programs, and with the concomitant rise of nongovernmental organizations (NGOs).In this paper, we follow the operation of one TA program operated by an intermediary NGO in Oaxaca, Mexico. We find that the program intersects with neoliberalization in two prominent ways, relying on a form of governmentality that codifies and prescribes: (a) the social spaces of action and need, and (b) learning subjects deficient in entrepreneurial initiative and know-how. We conclude by commenting on the political economic conditions that continue to underwrite TA as a development practice in spite of a decade or more of criticism directed at it and we consider the possibilities for its subversion.  相似文献   
799.
Vertical magnetic fields for a three-dimensional numerical model, for a laboratory-analogue model, and from field stations for the Vancouver Island region of British Columbia, Canada, are compared. The numerical results are obtained using a three-dimensional finite-difference numerical technique employing a 25 × 25 × 25 mesh of grid points for a simplified mathematical model of the Vancouver Island region. The calculations are carried out for a source frequency of 0.004 Hz. The analogue model results for four traverses over the Island model and the field station values (obtained from transfer function analysis) for ten locations are those discussed previously by Nienaber et al. (1979a, b). General agreement exists between the numerical, analogue, and field station data, and comparison of results between these methods is important in three-dimensional electromagnetic induction studies of complex geomagnetic induction problems.  相似文献   
800.
The NE/SW trending Ogcheon Belt formed during the Mesozoic by deformation and metamorphism of a sedimentary sequence which includes tillite formations of late Precambrian or Lower Palaeozoic age. In this study Rb-Sr isotopic data are reported from cm-scale sampling of clasts and matrix in these tillites, which underwent metamorphism at grades ranging from the biotite zone to the kyanite zone. Sr-isotopic disequilibrium between clasts and matrix persists up to the highest grade but in four out of five rocks the matrix samples approximate to isochrons (MSWD's<12). The mechanism for this selective isotopic equilibration is considered to be related to the metamorphic reactions in the matrix, which were probably accompanied by a substantial fluid flow. The matrix isochrons for three of the rocks, together with separated white micas from associated schists, suggest that metamorphism took place in the Trias, approximately 200 Myr ago. Metamorphism was followed by slow cooling and biotite closure temperatures were not reached until 190-150 Myr. — These results suggest that where high fluid/rock ratios can be demonstrated on independent petrological grounds, the analysis of small-scale whole-rock samples may allow effective Rb/Sr dating of metamorphism.  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