全文获取类型
收费全文 | 84650篇 |
免费 | 857篇 |
国内免费 | 1327篇 |
专业分类
测绘学 | 2695篇 |
大气科学 | 5630篇 |
地球物理 | 16465篇 |
地质学 | 33116篇 |
海洋学 | 6430篇 |
天文学 | 16215篇 |
综合类 | 2269篇 |
自然地理 | 4014篇 |
出版年
2022年 | 424篇 |
2021年 | 719篇 |
2020年 | 724篇 |
2019年 | 793篇 |
2018年 | 6266篇 |
2017年 | 5490篇 |
2016年 | 4287篇 |
2015年 | 1155篇 |
2014年 | 1780篇 |
2013年 | 3174篇 |
2012年 | 2904篇 |
2011年 | 5104篇 |
2010年 | 4204篇 |
2009年 | 5226篇 |
2008年 | 4366篇 |
2007年 | 4953篇 |
2006年 | 2525篇 |
2005年 | 1892篇 |
2004年 | 2034篇 |
2003年 | 1970篇 |
2002年 | 1797篇 |
2001年 | 1448篇 |
2000年 | 1325篇 |
1999年 | 1100篇 |
1998年 | 1125篇 |
1997年 | 1063篇 |
1996年 | 897篇 |
1995年 | 905篇 |
1994年 | 816篇 |
1993年 | 689篇 |
1992年 | 671篇 |
1991年 | 660篇 |
1990年 | 740篇 |
1989年 | 649篇 |
1988年 | 591篇 |
1987年 | 739篇 |
1986年 | 626篇 |
1985年 | 783篇 |
1984年 | 857篇 |
1983年 | 826篇 |
1982年 | 738篇 |
1981年 | 761篇 |
1980年 | 650篇 |
1979年 | 631篇 |
1978年 | 613篇 |
1977年 | 547篇 |
1976年 | 539篇 |
1975年 | 526篇 |
1974年 | 503篇 |
1973年 | 566篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
461.
M. G. LAIRD 《Sedimentology》1968,10(2):111-120
Curved or planar discordant surfaces occurring within a limited stratigraphic range immediately beneath shallow marine deposits represent penecontemporaneous shear surfaces along which slumps have moved. Rotated packets of strata frequently retained within the curved discordant surfaces show both internal and external evidence of having moved laterally. Movement is considered to have been triggered off by a sudden shock or shocks which may have been either of sedimentary or tectonic origin. The slumping occurred at the change in gradient between slope and shallow marine shelf deposits. 相似文献
462.
A symposium on the genesis of till entitled Till/Sweden-76 was held in Stockholm 16–18 August 1976. This issue of Boreas contains most of the papers presented at the symposium. 相似文献
463.
464.
Gordon G. Goles 《Lithos》1975,8(1):47-58
Five Ne-normative basalts are unusually rich in Cr, to a lesser degree in Co, and have very high La/Lu ratios and high contents of several excluded trace elements resembling kimberlites in these and other aspects. They seem to represent mantle-derived magmas, only slightly modified by shallow level crystal/liquid fractionation. Cr was apparently excluded from the crystalline phases in equilibrium with these magmas in the upper mantle.The Chyulu rocks differ in several respects from other Gregory Rift basalts, but closely resemble those from Jan Mayen Island near the Mid-Atlantic ridge. The generation of these unusual basalts is therefore not related simply to mantle composition or tectonic setting, but may reflect unusually high-T (1450°C), low-P (15–29 kbars) environments in the zones of last effective equilibration of these magmas with mantle rocks. 相似文献
465.
Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric
deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ15N, ) and sulfur (δ34S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric
Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington.
The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and
all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S.
emissions data from 1950 to the mid-1970s. 相似文献
466.
Lamprophyric lavas in the Colima graben,SW Mexico 总被引:2,自引:3,他引:2
The Colima graben, located in SW Mexico, is one of three grabens which intersect about 50 km SSW of Guadalajara, forming a triple junction. The 90 km long, 20–60 km wide Colima graben represents a N-S rift of the E-W trending Mexican Volcanic Belt. Since the Early Pliocene, the Colima graben has served as a locus for the eruption of alkaline lavas, the most recent of which are basanites and minettes erupted from Late Pleistocene cinder cones (Luhr and Carmichael 1981). In this paper, we report on older alkaline lavas which crop out in the graben's walls. These rocks include phlogopite- and hornblende-bearing lamprophyres, a phlogopite-kalsilite-ankaratrite, and high-K andesites. These lavas crop out throughout the Colima graben area, and are intimately associated with calc-alkaline lavas in the field. Compared to these, the alkaline rocks are strikingly enriched in the incompatible elements, particularly Ba, Sr, P, and the LREE. Unlike the younger Late Pleistocene alkaline cinder cone lavas, most of the graben wall lamprophyres and the high-K andesites represent magmas that appear to have undergone significant evolution since their generation, including fractionation, crustal contamination, and possible magma mixing. Least-squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres. The occurrence of these alkaline lavas in an active calc-alkaline volcanic arc is unusual, and we suggest that they are a manifestation of the rifting processes which produced the Colima graben. 相似文献
467.
Significance of Europium anomalies in silicate melts and crystal-melt equilibria: a re-evaluation 总被引:1,自引:0,他引:1
Although europium speciation in silicate melts partly reflects prevailing oxygen fugacities, melt composition and structure play the major role in determining Eu2+/ Eu3+ ratios and europium partitioning into mineral phases. Experimental evidence by different investigators on the magnitude of the compositional effect on Eu2+/Eu3+ provides consistent results only if account is taken of the oxygen buffer system employed in the experiments. The medium-dependent reduction of europium can be understood in terms of the preferential stabilization of Eu2+ by a strong aluminosilicate complex in the melt phase, and to a much lesser degree by metasilicate complexes. The stability of these complexes increases as the field strength of the associated cation species decreases. Hence Eu2+-aluminosilicate complexes are preferentially stabilized relative to trivalent lanthanides in melts of appropriate composition and their presence minimizes the enthalpy of the melt. The influence of these complexes is particularly pronounced in melts with a high feldspathic component and a strongly polymerized structure. Their petrogenetic influence is best documented through the behaviour of europium relative to the other lanthanides during anhydrous anatexis in high-grade metamorphic terrains and in the anomalous europium partition coefficients of phenocryst phases which formed during the crystallization of highly silicic magmas. 相似文献
468.
NILANJAN DASGUPTA TARITWAN PAL JOYDEEP SEN TAMOGHNO GHOSH 《Journal of Earth System Science》2011,120(4):617-626
The study involves the characterization of pegmatoidal granite, southeast of Beawar, Ajmer district, Rajasthan. Earlier researchers
had described this granite as part of the BGC, basement to the Bhim Group of the Delhi Super Group rocks. However, the present
study indicates that it is younger than the rocks of Bhim Group of South Delhi Fold Belt, into which it is intrusive. The
intrusion is structurally controlled and the outcrop pattern is phacolithic. The granite had intruded post-D2 deformation of the Delhi orogeny along the axial planes of D2 folds. The intrusion has also resulted in the formation of a contact aureole about the calc gneisses. 相似文献
469.
The article presents chemical analyses of the pseudoleuciteand groundmass of a pseudoleucite tinguaite, chemical analysesof the principal minerals, chemical estimates of the mode ofthe groundmass, and both chemical and micrometric modes of thepseudoleucite. The principal minerals are hedenbergitic acmite,a nepheline containing 2.3 per cent Fe2O3 and 7.9 per cent K2O,and K-feldspar containing a little BaO and very little Na2O.The micrometric mode of the pseudoleucite, in satisfactory agreementwith the chemical mode, is nepheline 29.8 per cent, K-feldspar66.2 per cent, acmite 3.0 per cent; the chemical mode of thegroundmass is nepheline 26.8 per cent, K-feldspar 46.6 per cent,and acmite 26.7 per cent. A new analysis of a nepheline fromthe Bancroft area, made to test the analytical procedure, isalso included. 相似文献
470.