全文获取类型
收费全文 | 39060篇 |
免费 | 641篇 |
国内免费 | 241篇 |
专业分类
测绘学 | 761篇 |
大气科学 | 2590篇 |
地球物理 | 7692篇 |
地质学 | 14143篇 |
海洋学 | 3554篇 |
天文学 | 8770篇 |
综合类 | 88篇 |
自然地理 | 2344篇 |
出版年
2022年 | 261篇 |
2021年 | 445篇 |
2020年 | 500篇 |
2019年 | 579篇 |
2018年 | 1024篇 |
2017年 | 1028篇 |
2016年 | 1088篇 |
2015年 | 590篇 |
2014年 | 1040篇 |
2013年 | 1861篇 |
2012年 | 1206篇 |
2011年 | 1646篇 |
2010年 | 1447篇 |
2009年 | 1808篇 |
2008年 | 1637篇 |
2007年 | 1698篇 |
2006年 | 1563篇 |
2005年 | 1064篇 |
2004年 | 1076篇 |
2003年 | 1118篇 |
2002年 | 1002篇 |
2001年 | 874篇 |
2000年 | 808篇 |
1999年 | 728篇 |
1998年 | 717篇 |
1997年 | 727篇 |
1996年 | 589篇 |
1995年 | 574篇 |
1994年 | 503篇 |
1993年 | 454篇 |
1992年 | 411篇 |
1991年 | 425篇 |
1990年 | 439篇 |
1989年 | 392篇 |
1988年 | 371篇 |
1987年 | 401篇 |
1986年 | 415篇 |
1985年 | 510篇 |
1984年 | 547篇 |
1983年 | 545篇 |
1982年 | 496篇 |
1981年 | 454篇 |
1980年 | 432篇 |
1979年 | 408篇 |
1978年 | 375篇 |
1977年 | 386篇 |
1976年 | 344篇 |
1975年 | 353篇 |
1974年 | 340篇 |
1973年 | 372篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
202.
203.
F. Moseley J.C. Cuttell E.W. Lange D. Stevens J.R. Warbrick 《Journal of Structural Geology》1981,3(3):237-251
The Pre-Betic is the most northerly of the Alpine zones forming the Betic Cordilleras of southern Spain. It consists of strongly folded and faulted Mesozoic and Tertiary rocks, the oldest of which are ferruginous and gypsiferous Triassic mudstones, followed by a predominantly carbonate facies of Cretaceous, Palaeogene and Miocene age. Although this sequence is interrupted by a number of minor unconformities, the major structures were formed during the middle or late Miocene. The highly incompetent Triassic rocks are the most strongly deformed, and form diapiric intrusions discordant to regional structural trends in the younger rocks. The latter are essentially of two facies: massive competent limestones which are deformed by relatively simple folds of large wavelength, and highly incompetent marl-limestone interbeds with complex disharmonic folds and crush belts. Faults include low-angle and high-angle thrusts, gravity slides and wrench faults. The regional tectonic strike is ENE to NE, but the diapiric intrusions mostly follow WNW and N directions. These intrusions have pushed the younger rocks aside, the result being polyphase structures of several trends.Less intense post-Miocene tectonics are mostly associated with continued diapirism and have resulted in the folding and tilting of the late Miocene to Quaternary elastic sediments. 相似文献
204.
205.
The present article is the second in a series of baseline water quality evaluation studies in parts of south-eastern Nigeria.
Study results indicate that the water samples from the study are generally acidic, soft and fresh in addition to low sodium
and salinity hazard. Regression equations indicate a good positive correlation between conductivity and Na, Ca, Mg and SO4. Ca-SO4 and Ca-Cl constitute the major facies in the area. Generally the waters in the area are good for most domestic and agricultural
purposes.
Received: 14 April 1997 · Accepted: 3 February 1998 相似文献
206.
The physicomathematical features of the plasma explosion region dynamics at altitudes higher than 100 km at the early stage of its development, which culminates in the transfer of a considerable portion of the initial plasma energy into the environment, are discussed in the first part of the work. The radiation gas dynamic, inertial, and shock stages of the disturbed region development are calculated. 相似文献
207.
208.
The first results of U–Pb isotopic dating (LA–ICP–MS) of detrital zircons from metasedimentary rocks of the pre-Devonian basement of the SW part of western Spitsbergen (from Upper Mezoproterozoic Gulliksenfellet quartzite) showed ages ranging from 1700 ± 25 to 2948 ± 27 Ma. 相似文献
209.
Meagan E. Ankney Clark M. Johnson Charles R. Bacon Brian L. Beard Brian R. Jicha 《Contributions to Mineralogy and Petrology》2013,166(2):563-585
Uranium-series isotope ratios determined for 35 volcanic rocks and 4 glass separates erupted from ~36 to 4.8 ka at Mt. Mazama, Crater Lake, Oregon, identify both 230Th-excess and 238U-excess components. U–Th isotope compositions cover a wide range, exceeding those previously measured for the Cascade arc. Age-corrected (230Th/232Th) and (238U/232Th) activity ratios range from 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % 230Th-excess to 8.8 % 238U-excess), respectively. The most distinctive aspect of the data set is the contrast in U–Th isotope ratios between low and high Sr (LSr, HSr) components that have been previously identified in products of the 7.7 ka caldera-forming climactic eruption and preclimactic rhyodacite lavas. The LSr component exclusively contains 238U-excess, but the HSr component, as well as more primitive lavas, are marked by 230Th-excess. 230Th-excesses such as those recorded at Mt. Mazama are commonly observed in the Cascades. Melting models suggest that high 230Th-excesses observed in the more primitive lavas evolved through mixing of a mantle melt with a partial melt of a mafic lower crustal composition that contained garnet in the residuum that was produced through dehydration melting of amphibolite that was initially garnet free. Dehydration melting in the lower crust offers a solution to the “hot-slab paradox” of the Cascades, where low volatile contents are predicted due to high slab temperatures, yet higher water contents than expected have been documented in erupted lavas. The 238U-excess observed at Mt. Mazama is rare in Cascade lavas, but occurs in more than half of the samples analyzed in this study. Traditionally, 238U-excess in arc magmas is interpreted to reflect slab fluid fluxing. Indeed, 238U-excess in arcs is common and likely masks 230Th-excess resulting from lower crustal interaction. Isotopic and trace element data, however, suggest a relatively minor role for slab fluid fluxing in the Cascades. We propose that 238U-excess reflects melting and assimilation of young, hydrothermally altered upper crust. The processes related to generating 238U-excess are likely important features at Mt. Mazama that accompanied development of a large-scale silicic magma chamber that led to the caldera-forming eruption. 相似文献
210.
J. A. Vorpahl E. G. Gibson P. B. Landecker D. L. McKenzie J. H. Underwood 《Solar physics》1975,45(1):199-216
132 soft X-ray flare events have been observed with The Aerospace Corporation/Marshall Space Flight Center S-056 X-ray telescope that was part of the ATM complement of instruments aboard Skylab. Analyses of these data are reported in this paper. The observations are summarized and a detailed discussion of the X-ray flare structures is presented. The data indicated that soft X-rays emitted by a flare come primarily from an intense well-defined core surrounded by a region of fainter, more diffuse emission. Loop structures are found to constitute a fundamental characteristic of flare cores and arcades of loops are found to play a more important role in the flare phenomena than previously thought. Size distributions of these core features are presented and a classification scheme describing the brightest flare X-ray features is proposed. The data show no correlations between the size of core features and: (1) the peak X-ray intensity, as indicated by detectors on the SOLRAD satellite; (2) the rise time of the X-ray flare event, or (3) the presence of a nonthermal X-ray component. An analysis of flare evolution indicates evidence for preliminary heating and energy release prior to the main phase of the flare. Core features are found to be remarkably stable and retain their shape throughout a flare. Most changes in the overall configuration seem to be the result of the appearance, disappearance or change in brightness of individual features, rather than the restructuring or re-orientation of these features. Brief comparisons with several theories are presented. 相似文献