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   997篇
  免费   40篇
  国内免费   5篇
测绘学   40篇
大气科学   84篇
地球物理   218篇
地质学   340篇
海洋学   75篇
天文学   159篇
综合类   13篇
自然地理   113篇
  2022年   6篇
  2021年   13篇
  2020年   14篇
  2019年   22篇
  2018年   35篇
  2017年   20篇
  2016年   33篇
  2015年   23篇
  2014年   37篇
  2013年   74篇
  2012年   37篇
  2011年   44篇
  2010年   41篇
  2009年   63篇
  2008年   66篇
  2007年   50篇
  2006年   38篇
  2005年   35篇
  2004年   32篇
  2003年   39篇
  2002年   31篇
  2001年   27篇
  2000年   13篇
  1999年   17篇
  1998年   14篇
  1997年   14篇
  1996年   16篇
  1995年   12篇
  1994年   7篇
  1993年   10篇
  1992年   4篇
  1991年   9篇
  1990年   7篇
  1989年   7篇
  1988年   12篇
  1987年   4篇
  1986年   4篇
  1985年   9篇
  1984年   10篇
  1983年   15篇
  1982年   12篇
  1981年   5篇
  1980年   11篇
  1979年   9篇
  1978年   5篇
  1976年   6篇
  1975年   6篇
  1974年   4篇
  1972年   3篇
  1969年   3篇
排序方式: 共有1042条查询结果,搜索用时 15 毫秒
961.
A national, representative survey of the U.S. public found that Americans have moderate climate change risk perceptions, strongly support a variety of national and international policies to mitigate climate change, and strongly oppose several carbon tax proposals. Drawing on the theoretical distinction between analytic and experiential decision-making, this study found that American risk perceptions and policy support are strongly influenced by experiential factors, including affect, imagery, and values, and demonstrates that public responses to climate change are influenced by both psychological and socio-cultural factors.  相似文献   
962.
Early in the crystallization of many tholeiitic basaltic magmas, plagioclase crystals cluster together into a 3-D cellular network, which forms a passive marker capable of recording the deformation that accompanies compaction of crystal mush. Although irregular in detail, the overall network is initially isotropic and only becomes anisotropic as a result of compaction. We have developed four independent methods to quantify the 3-D textural anisotropy of a basalt sample using at least three non-parallel thin sections. Three of the methods are based on the geometrical properties of digitized maps of the feldspar chain networks. One approach focuses on the angular variation of the mean intercept along parallel traverses through the network, another examines the orientation and size distribution of individual links, and the third considers the average shape of interstitial regions outlined by the plagioclase network. The fourth technique approximates the textural anisotropy by the variogram anisotropy of a scanned thin section image. We illustrate the methods using five oriented non-parallel thin sections from a sample of diabase 146 m above the base of the 300-m-thick Palisades sill of New Jersey. Compaction of crystal mush in this sill has previously been postulated on the basis of chemical evidence. The 3-D feldspar network anisotropy based on the first three approaches suggests nearly uniaxial compaction on the order of 8.6% in a direction within 3° of the intersection of the columnar joints at the sample site. A rigorous statistical test based on the statistics of elliptically contoured non-normal multivariate distributions documents that the link-vector distribution in vertical sections are statistically anisotropic at a 95% confidence level and that the overall compaction is 7.9±2.6%. The orientation and magnitude of the 3-D textural anisotropy determined by the image variogram of the non-opaque minerals is almost identical to the mean feldspar network anisotropy; 8.5% compaction in a direction 10° from the columnar joint intersections. The major silicate textural and feldspar network anisotropy axes both plunge almost directly down dip of the sill. On the other hand, the major axis of the variogram anisotropy of the opaque minerals is approximately parallel to the strike of the sill and to the major axis of the anisotropic magnetic susceptibility. The anisotropy of the silicate mineral fabric may reflect down-dip flow of a deformable melt-rich crystal mush, whereas the AMS and opaque textural anisotropy reflects the influence of gravitational stresses during the growth of magnetite in the final stages of melt crystallization. Evidently the Palisades sill was not originally horizontal but was intruded in an orientation close to its present attitude.  相似文献   
963.
Metamorphic remobilization of arsenopyrite-rich ores is a globally important process which can lead to significant concentrations of gold. In order to understand this and related processes, relations of sulphur isotopes can give a number of important clues. To resolve such relations in detail, we have successfully calibrated and applied a laser combustion system for in situ analysis of sulphur isotopic compositions of arsenopyrite. Experimental calibration of the laser fractionation factor (+0.4‰) was obtained by using compositionally and isotopically homogeneous natural samples from ore deposits at Boliden (Sweden) and Freiberg (Germany); subsequent to detailed microscopic study, the S isotope ratios of these samples were measured by conventional and laser combustion techniques.The present application to different types of arsenopyrite in the Palaeoproterozoic metamorphosed VHMS ores of the Boliden Au–Cu–As deposit, Skellefte district, northern Sweden, shows that the sulphur isotope composition of arsenopyrite is essentially unmodified during medium-grade metamorphic recrystallization and remobilization. Here, massive arsenopyrite ore is crosscut by later veins that carry a complex quartz–sulphosalt–sulphide assemblage. The latter ore type is markedly Au-rich compared to the host ore, and thus of significant economic interest. We find that both ore types exhibit very similar sulphur isotope compositions, ca. +2‰ to +3‰ (V-CDT), which is similar to most massive sulphide deposits in the Skellefte district. Thus, the crosscutting Au-rich vein ore has inherited the sulphur isotope composition from sulphur liberated by metamorphic reactions affecting the massive ore, and most likely also inherited the Au through this mechanism. The latter finding clearly has important implications both for the general discussion on, and the prospecting for, similar high-grade Au ores in this world-class mining region.  相似文献   
964.
965.
High-temporal resolution meteorological output from the Parallel Climate Model (PCM) is used to assess changes in wildland fire danger across the western United States due to climatic changes projected in the 21st century. A business-as-usual scenario incorporating changing greenhouse gas and aerosol concentrations until the year 2089 is compared to a 1975–1996 base period. Changes in relative humidity, especially dryingover much of the West, are projected to increase the number of days of high fire danger (based on the energy release component (ERC) index) at least through the year 2089 in comparison to the base period. The regions most affected are the northern Rockies, Great Basin and the Southwest –regions that have already experienced significant fire activity early this century. In these regions starting around the year 2070, when the model climate CO2 has doubled from present-day, the increase in the number ofdays that ERC (fuel model G) exceeds a value of 60 is as much as two to three weeks. The Front Range of the Rockies and the High Plains regions do not show a similar change. For regions where change is predicted, new fire and fuels management strategies and policies may be needed to address added climatic risks while also accommodating complex and changing ecosystems subject to human stresses on the region. These results, and their potential impact on fire and land management policy development, demonstrate the value of climate models for important management applications, as encouraged under the Department of Energy Accelerated Climate Prediction Initiative (ACPI), under whose auspices this work was performed.  相似文献   
966.
Montjoly is a headland‐bound embayed sandy beach in Cayenne, French Guiana, that shows long‐term plan shape equilibrium in spite of periodic changes in accretion and erosion that alternately affect either end of the beach. These changes are caused by mud banks that move alongshore from the Amazon. The mechanisms involved in changes in the plan shape of the beach in response to the passage of one of these mud banks were monitored between 1997 and 2000 from airborne video imagery and field work. The beach longshore drift to the northwest, driven by the incident easterly to northeasterly swell usually affecting this coast, became temporarily reversed as the mud bank, migrating from east to west, initially sheltered the southeastern end of the beach. The difference in exposure to waves engendered a negative wave height gradient alongshore towards the southeast, resulting in the setting up of a cell circulation and counter‐active longshore drift from the exposed northwestern sector to the southeast. Sand eroded from the exposed sector accumulated first in the southeastern, and then the central sectors of the beach. The effect of increasing beach sheltering by the mudbank moving west is highlighted on the videographs by an ‘arrested’ pattern of beach shoreline development. The videographs show hardly any changes in beach plan shape since January 1999, due to sheltering of the beach from wave attack by the mud bank. It is expected that the eroded sector will recover in the future as the mud bank passes, leading to re‐establishment of the northwesterly sand drift. This temporally phased bi‐directional drift within the confines of the bounding headlands results in a rare example of mud‐bank‐induced beach rotation, and probably explains the long‐term equilibrium plan shape of Montjoly beach. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
967.
The UV-visible absorption cross-sections of HOI have been recorded over the wavelength range 278-494 nm and at 298 K following generation of HOI in the gas phase using laser flash photolysis. The gas phase reaction of OH with I2 was used to produce HOI, and the absorption spectrum of HOI was calibrated relative to the consumption of I2. The HOI spectrum recorded exhibits 2 broad absorption maxima of = 3.99 × 10-19 cm2 and = 2.85 × 10-19 cm2, centred at 338.4 nm and 404.8 nm respectively. The spectrum is adequately described by a parameterisation consisting of two semi-logarithmic Gaussian distribution functions. The HOI spectrum is more intense than that recorded in previous work of Jenkin, but is in good agreement with more recent work by Bauer et al. The parameterised HOI absorption spectrum recorded in this work was used in a radiative model to calculate the atmospheric photolysis rate (J-value) of HOI. These results indicate that, under most sunlit conditions, HOI has a lifetime with respect to solar photolysis of the order of minutes. Experiments attempting to generate HOI by the reaction of O atoms with C2H5I led to complex absorption spectra containing a negative contribution to the absorption from the photolytic removal of an unidentified species. In addition, evidence was found for adsorption and desorption of an iodine-containing species in the reaction vessel. This behaviour is rationalised in terms of the disproportionation of HOI to I2O, and an uncalibrated spectrum tentatively attributed to I2O has been recorded.  相似文献   
968.
Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The 87Sr/86Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the 87Sr/86Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.  相似文献   
969.
Highly deformed regionally metamorphosed rocks of the Natal-Namaqua belt north of Durban preserve the features of a major Precambrian geosynclinal sequence, with possible reactivated oceanic and continental floor material. The broad scale grouping of similar lithologies into mappable geologic units is possible. Formation status is assigned to certain units, classified in the Natal group. Variation of lithologic character of these rocks within the belt is correlated with facies of orogenic deposition. Three zones of regional progressive metamorphism are recognised, and the metamorphosed rocks of the belt can be divided into a high-pressure intermediate, intermediate-pressure and intermediate-low pressure facies series types corresponding to the upper-greenschist facies, low-amphibolite facies and middle-amphibolite facies respectively. These three metamorphic terrains display considerable differences in composition and structural style, and correspond to the major tectonic units of the belt. Evidence gained from a study of the distribution of regional metamorphism in the Natal-Namaquarides north of Durban, has reinforced the favourability of applying the theory of plate tectonics to the development of the orogen in an attempt to obtain a broader understanding of the geologic history of the belt. Furthermore, the complex suite of metamorphites and associated intrusive phases constituting the belt involves the deformation and metamorphism of lithological types similar to those described in orogenic belts in other parts of the world.  相似文献   
970.
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