全文获取类型
收费全文 | 63325篇 |
免费 | 1000篇 |
国内免费 | 556篇 |
专业分类
测绘学 | 1334篇 |
大气科学 | 4127篇 |
地球物理 | 12139篇 |
地质学 | 23264篇 |
海洋学 | 5927篇 |
天文学 | 14486篇 |
综合类 | 152篇 |
自然地理 | 3452篇 |
出版年
2022年 | 462篇 |
2021年 | 758篇 |
2020年 | 877篇 |
2019年 | 974篇 |
2018年 | 1825篇 |
2017年 | 1783篇 |
2016年 | 1979篇 |
2015年 | 1055篇 |
2014年 | 1875篇 |
2013年 | 3275篇 |
2012年 | 2109篇 |
2011年 | 2778篇 |
2010年 | 2485篇 |
2009年 | 3051篇 |
2008年 | 2713篇 |
2007年 | 2760篇 |
2006年 | 2600篇 |
2005年 | 1718篇 |
2004年 | 1677篇 |
2003年 | 1688篇 |
2002年 | 1582篇 |
2001年 | 1419篇 |
2000年 | 1319篇 |
1999年 | 1136篇 |
1998年 | 1139篇 |
1997年 | 1100篇 |
1996年 | 949篇 |
1995年 | 896篇 |
1994年 | 835篇 |
1993年 | 713篇 |
1992年 | 675篇 |
1991年 | 676篇 |
1990年 | 735篇 |
1989年 | 603篇 |
1988年 | 586篇 |
1987年 | 655篇 |
1986年 | 614篇 |
1985年 | 788篇 |
1984年 | 800篇 |
1983年 | 779篇 |
1982年 | 744篇 |
1981年 | 657篇 |
1980年 | 658篇 |
1979年 | 614篇 |
1978年 | 585篇 |
1977年 | 554篇 |
1976年 | 506篇 |
1975年 | 514篇 |
1974年 | 490篇 |
1973年 | 524篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
112.
Electrical anisotropy of young oceanic crust at mid-ocean ridges is detectable by observation of the rate and geometry of the diffusion of electromagnetic fields. The anisotropy in electrical properties arises from the presence of conductive seawater in an interconnected network of mostly ridge-parallel cracks. In this paper, we first justify the choice of a triaxial model to represent young oceanic crust, with three distinct electrical conductivities in the vertical, strike and spreading directions. We then present an algorithm to calculate the transient electromagnetic responses generated by an electric dipole source over such a triaxially anisotropic seafloor. We show that if the transient passages are measured with three distinct electric dipole-dipole configurations, it is possible to discern all three unknown conductivities independently of each other. 相似文献
113.
In our study, 48 U.S. departments that grant geography PhDs are compared along four measures of academic productivity. Using 1980–1994 as the base period, research productivity of faculty was assessed by counting book titles, as well as authorships in 77 journals. Teaching productivity was determined by placement of PhDs in graduate programs of geography in North America, and by a weighted index of teaching outcomes. Data were summed for departments, but are reported primarily as productivity per person (per FTE count). The results point to a wide variety of departmental profiles, from those departments that appear to emphasize teaching outcomes to those that are more productive in book publishing and journal authorship, with several exhibiting a balance between teaching and research. 相似文献
114.
N. V. Sobolev F. V. Kaminsky W. L. Griffin E. S. Yefimova T. T. Win C. G. Ryan A. I. Botkunov 《Lithos》1997,39(3-4):135-157
The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle. 相似文献
115.
Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil 总被引:1,自引:0,他引:1
S. A. Gibson R. N. Thompson R. K. Weska A. P. Dickin O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314
High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser 40Ar/39Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (87Sr/86Sr?=?0.704 and 143Nd/144Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C. 相似文献
116.
N. N. Ambraseys 《Journal of Seismology》1997,1(3):289-299
In this paper we assess the size and effects of the earthquakes of 12 May 1866, and 24 January 1916 in Anatolia (Turkey). We show that these events had a magnitude Ms 7.2 and that the former was associated with a 45-km long surface fault break along the north-east part of the East Anatolian Fault Zone. These two earthquakes are chosen among others in order to demonstrate how easy it is to miss out large earthquakes of the historical, even of the early instrumental period, and to draw the incompleteness of many existing catalogues to the attention of those who use them for the estimation of slip rates and the assessment of seismic hazard. Of the two earthquakes studied here, the former was only vaguely known and the latter is not included in Gutenberg and Richter's catalogue. 相似文献
117.
The evolution of views and practical decisions regarding the problem of collecting payment for the use of surface waters with regard to their quality in the former USSR and present-day Ukraine is discussed. A simple and feasible method is proposed for differentitation of regional (basin) rates of payment for the use of surface water in Ukraine with allowance made for its quality assessed based on a system of environmental classifications. 相似文献
118.
119.
The paper presents the first quantitative results of a laboratory study of the velocity field in a two-arm spiral-wave pattern generated in a steady-state fashion by a hydrodynamical instability in a differentially rotating, thin layer of liquid. The liquid layer has a free surface, and the rotational profile includes an interval where the velocity drops abruptly, as in the gaseous disks of spiral galaxies. The properties of anticyclonic vortices observed between the arms of this pattern at the corotation radius are considered. 相似文献
120.