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   23265篇
  免费   253篇
  国内免费   131篇
测绘学   377篇
大气科学   1227篇
地球物理   4429篇
地质学   9153篇
海洋学   2255篇
天文学   5311篇
综合类   42篇
自然地理   855篇
  2022年   264篇
  2021年   431篇
  2020年   400篇
  2019年   465篇
  2018年   948篇
  2017年   870篇
  2016年   889篇
  2015年   371篇
  2014年   787篇
  2013年   1307篇
  2012年   915篇
  2011年   1125篇
  2010年   1070篇
  2009年   1243篇
  2008年   1053篇
  2007年   1231篇
  2006年   1076篇
  2005年   578篇
  2004年   542篇
  2003年   545篇
  2002年   571篇
  2001年   511篇
  2000年   414篇
  1999年   336篇
  1998年   325篇
  1997年   335篇
  1996年   258篇
  1995年   263篇
  1994年   238篇
  1993年   186篇
  1992年   210篇
  1991年   184篇
  1990年   197篇
  1989年   193篇
  1988年   160篇
  1987年   186篇
  1986年   176篇
  1985年   208篇
  1984年   208篇
  1983年   200篇
  1982年   194篇
  1981年   172篇
  1980年   163篇
  1979年   189篇
  1978年   163篇
  1977年   145篇
  1976年   135篇
  1975年   139篇
  1974年   127篇
  1973年   169篇
排序方式: 共有10000条查询结果,搜索用时 699 毫秒
911.
The paper examined the concept of development in line with the role of geospatial technology applications in governance and geospatial management in Nigeria. It also evaluates the previous efforts made by successive governments in managing governance process through geospatial technology initiatives using Lagos State as a proof-of-concept. It empirically demonstrates the role of monitoring spatial development initiatives using Landsat multi-dates satellite images 1985, 1990, 1995, 2000, 5005, 2010 and 2016 and Geographic Information System technique. The paper then identified, mapped and examined the settlement growth of Lagos using Ikorodu Division as an empirical proof-of-concept. The study reveals a geometric increase in uncoordinated human activities resulting from sporadic spatial development (builtup area) from 27.67 sq km (7.7%) in 1985–313.02 sq km (87.3%) spatial coverage in 2016. Based on the regression line, the study reveals a strong correlation between increasing settlement development and time. Low level of community involvement and lack of sense of ownership of development project initiatives in developing a local “vision” and strategy are identified as a problem. The paper re-emphasized that sustaining democratic processes as well as the fight against poverty will be won or lost mostly in the rural and less in the urban areas. The rural area is home to about 70% of the world’s poor due to lack or inadequate government services and basic amenities. The study suggest that making the future better for the yet unborn requires appreciable fundamental dataset, technologies, human resources and reliable government structures.  相似文献   
912.
Comparison of benthic foraminiferal assemblages from the core obtained within the Peterburgskoe ore field (Mid-Atlantic Ridge) and from the core taken five kilometers away from the ore field revealed evident differences in their composition, in the appearance of their shells, and also in the benthic–plankton species ratio. It was noted that the foraminiferal assemblage from the ore-bearing sediments of the Petersburg field was characterized by a higher relative content of benthic species and a large number of chemically altered and broken shells. The first occurrence of the species Osangularia umbonifera, which is able to exist in lowoxygen and CaCO3-undersaturated bottom waters at the boundary of biogenic sediments surrounding the ore field and in the ore-bearing sediments, was established. In the core section sampled beyond the ore field, the composition of foraminiferal assemblages differs insignificantly from typical oceanic ones.  相似文献   
913.
The methodical basis, development, and current state of a new method of chronostratigraphic studies, i.e., strontium isotope stratigraphy (SIS), are considered. This method makes it possible to date and correlate geographically distant sedimentary sequences without involving the biostratigraphic and isotope geochronological data. SIS is based on secular variations in 87Sr/86Sr in the paleocean, resulting from the redistribution of the roles of two global strontium flows formed in the mantle and continental reservoirs of the Earth. Isotopic homogeneity of Sr in the paleoceans and in the linked seas leads to the fact that the 87Sr/86Sr ratio in the sea basins is individual for each geological time point and is inherited in marine chemogenic sediments under deposition of dissolved Sr as an isomorphic impurity. Low-Mg calcite and also fragments of fossilized paleontological remains buried in situ are the best minerals that are capable of retaining the Sr isotopic signature of the sedimentation environment. SIS is carried out with geochemical diagnostics of secondary alteration of the studied material and selective dissolution of the samples to produce a carbonate material that adequately reflects isotopic signature of the sedimentary basin. Interregional correlations of the Proterozoic and Cenozoic sea sediments and their relation to the SIS-based stratigraphic scale are given as an example.  相似文献   
914.
Data on mineral-hosted melt, fluid, and crystalline inclusions were used to study the composition and evolution of melts that produced rocks of Changbaishan Tianchi volcano, China–North Korea, and estimate their crystallization parameters. The melts crystallized within broad ranges of temperature (1220–700°C) and pressure (3100–1000 bar), at a drastic change in the redox potential: Δ log \(f_{O_2}\) from NNO + 0.92 to +1.42 for the basalt melts, NNO –1.61 to –2.09 for the trachybasaltic andesite melts, NNO –2.63 to –1.89 for the comendite melts, and NNO –1.55 to –3.15 for the pantellerite melts. The paper reports estimates of the compositions of melts that produced the continuous rock series from trachybasalt to comendite and pantellerite. In terms of trace-element concentrations, all of the mafic melts are comparable with OIB magmas. The silicic melts are strongly enriched in trace elements and REE. The most strongly enriched melts contain concentrations of certain elements almost as high as in ores of these elements. The paper reports data on H2O concentrations in melts of different composition. It is demonstrated that the variations in the H2O concentrations were controlled by magma degassing. Data are reported on the Sr and Nd composition of the rocks. The deviations in the Sr isotopic composition are proportional to the 87Sr/86Sr ratio and could be produced in a melt with a high enough 87Sr/86Sr ratio during a geologically fairly brief time period. The evolution of melts that produced rocks of the volcano was controlled by crystallization differentiation of the parental basalt magmas at insignificant involvement of melt mixing and liquid immiscibility of silicate and sulfide melts. The alkaline salic rocks were generated in shallow-sitting (13–3.5 km) magmatic chambers in which the melts underwent profound differentiation that gave rise to pantellerites and comendites strongly enriched in trace elements (Th, Nb, Ta, Zr, and REE). Data on the composition of the magmas and parameters of their derivation are used to develop a generalized petrologic–geodynamic model for the origin of Changbaishan Tianchi volcano.  相似文献   
915.
The paper presents data on plutonic and metamorphic rocks dredged during Cruise 249 of the German R/V Sonne to the Stalemate Ridge, Northwest Pacific Ocean and the Shirshov Rise, western Bering Sea. Dredges in the northwestern sector of the Stalemate Ridge and central portion of the Shirshov Rise show that the plutonic and metamorphic rocks obtained here are amazingly similar. Our petrologic and geochemical data led us to view the rocks as members of a mafic–ultramafic assemblage typical of cumulate portions of ophiolite complexes and backarc spreading centers. The plutonic complexes of the Shirshov Rise and Stalemate Ridge show similarities not only in the petrography and mineralogy of their protoliths but also in the character of their metamorphic transformations. Plutonic rocks from both areas display mineralogical evidence of metamorphism within a broad temperature range: from the high-temperature amphibolite facies to the greenschist facies. Relations between the index mineral assemblages indicate that the metamorphic history of plutonic complexes in the Stalemate Ridge and Shirshov Rise proceeded along a retrograde path. Hornblende schists accompanying the plutonic rocks of the Stalemate Ridge and Shirshov Rise are petrographically close to foliated amphibolites in subophiolitic metamorphic aureoles. Within the framework of geodynamic interpretations of our results, it is realistic to suggest that the examined plutonic complexes were exhumed from subduction zones of various age.  相似文献   
916.
The age of the Paleozoic sedimentary successions of the Yuryung-Tumus Peninsula in the Khatanga Gulf is determined as Middle Devonian, late Eifelian, and the beginning of the early Givetian on the basis of the study of brachiopods, ammonoids, nautiloids, bactritoids, gastropods, and conodonts, and in the terminology of the standard conodont succession, these deposits are not older than the late Eifelian Tortodus kockelianus Zone, but include the terminal Eifelian Polygnathus ensensis Zone and the first Givetian Polygnathus hemiansatus Zone and are no younger than the beginning of the early Givetian Polygnathus varcus Zone. The Middle Devonian sedimentary successions of the Yuryung-Tumus Peninsula show the level of the global sedimentary Ka?ak Event. The areas of distribution of the Devonian sedimentary deposits in the Yuryung-Tumus Peninsula in the Khatanga Gulf should be considered as one of the fragments of the regional geological structures of a remote (isolated?) region of the Taymyr Fold System rather than a component of the structures of the northern Siberian Platform.  相似文献   
917.
The results of U—Th—Pb (LA-ICP-MS) geochronological studies of detrital zircons from terrigenous rocks of the Dzhida terrane of the Central Asian Fold Belt (CAFB) are presented. The data obtained allow us to distinguish the following age maxima (Ma): 578 and 634 (Vendian); 720, 823, and 919 (Late Riphean); 1922, 2090, 2225, and 2321 (Early Proterozoic). A number of zircons have Late Archean age in the interval of 2670–2980 Ma. Taking into account Late Cambrian age (504–506 Ma) of intrusive rocks that intruded the Dzhida terrane, a possible sedimentation period of sequences of this terrane is estimated to be in the interval of 580–510 Ma (from Vendian to Late Cambrian). The possible provenance areas of terrigenous sediments are proposed and the previously proposed models of geodynamic evolution of the Dzhida terrane are correlated with new geochronological data.  相似文献   
918.
Mineralogy and Petrology - The new mineral ziminaite, ideally Fe3+VO4, was found in fumarole sublimates at the Bezymyannyi volcano, Kamchatka, Russia. Ziminaite occurs as lamellar, tabular or...  相似文献   
919.
A deterministic seismic risk and monetary loss model is presented for the capital of Santorini volcanic Island, the town of Fira, on a building block scale. A local seismic source of M5.6 inferred from a recent volcano unrest in 2011–2012, detailed seismic vulnerability of 435 buildings and site conditions deduced from free-field ambient noise measurements were combined toward assessing the EMS-98 damage grade and its probability to occur. The seismic scenario yielded no damage or slight damage for 84% of the buildings, 16% of the constructions are expected to present moderate-to-heavy damage, while the economic loss amounts to 4 million euros. Although the model predicts low damage and direct economic loss, interaction with the touristic business activities might produce cascade side effects for the economy of the island and consequently Greece’s GDP, an important part of which emanates from Santorini.  相似文献   
920.
The average seismic strain rate is estimated for the seismotectonic zone of the northern/central parts of the Gulf of Suez. The principal strain rate tensor and velocity tensor were derived from a combination of earthquake focal mechanisms data and seismic moment of small-sized earthquakes covering a time span of 13 years (1992–2004). A total of 17 focal mechanism solutions have been used in the calculation of the moment tensor summation. The local magnitudes (MLs) of these events range from 2.8 to 4.7. The analysis indicates that the dominant mode of deformation in the central and northern parts of the Gulf of Suez is extension at a rate of 0.008 mm/year in N28°E direction and a small crustal thinning of 0.0034 mm/year. This low level of strain means that this zone experienced a little seismic deformation. There is also a right lateral shear motion along the ESE–WNW direction. This strain pattern is consistent with the predominant NW–SE normal faulting and ESE–WNW dextral transtensive faults in this zone. Comparing the results obtained from both stress and strain tensors, we find that the orientations of the principal axes of both tensors have the same direction with a small difference between them. Both tensors show a predominantly extensional domain. The nearly good correspondence between principal stress and strain orientations in the area suggests that the tectonic strength is relatively uniform for this crustal volume.  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