全文获取类型
收费全文 | 1720篇 |
免费 | 67篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 123篇 |
地球物理 | 496篇 |
地质学 | 567篇 |
海洋学 | 165篇 |
天文学 | 281篇 |
综合类 | 6篇 |
自然地理 | 127篇 |
出版年
2021年 | 19篇 |
2020年 | 23篇 |
2019年 | 24篇 |
2018年 | 42篇 |
2017年 | 32篇 |
2016年 | 56篇 |
2015年 | 36篇 |
2014年 | 42篇 |
2013年 | 83篇 |
2012年 | 45篇 |
2011年 | 62篇 |
2010年 | 78篇 |
2009年 | 75篇 |
2008年 | 66篇 |
2007年 | 83篇 |
2006年 | 61篇 |
2005年 | 55篇 |
2004年 | 51篇 |
2003年 | 52篇 |
2002年 | 49篇 |
2001年 | 26篇 |
2000年 | 33篇 |
1999年 | 28篇 |
1998年 | 27篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 31篇 |
1993年 | 14篇 |
1992年 | 31篇 |
1991年 | 29篇 |
1990年 | 38篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1971年 | 11篇 |
排序方式: 共有1805条查询结果,搜索用时 31 毫秒
961.
962.
The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium
feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous
loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing
plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles
from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely
narrow range of composition, about an average of Ms93.5Pg6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH2O slowly increased from initial values less than Ptotal + rock strength, under conditions of low permeability, the actual value of PH2O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening
of thermal gradients following the onset of fractional melting.
The isograd reaction is dependent in only a minor way upon the anorthite content of the plagioclase. Below the isograd, a
continuous reaction takes place leading to a diminution in paragonite content of muscovite stable in the presence of quartz.
It is possible that this reaction leads to the nearly ubiquitous normal zoning of the plagioclase.
Changes in the composition of biotite at the isograd are not conspicuous, and can be satisfactorily explained by the release
of Mg, Fe, and Ti impurity from the muscovite, and a continuous reaction between ilmenite, quartz, and muscovite. Garnets
are not abundant and are high in Mn, both facts probably due to the low pressure of metamorphism, The presence of garnet probably
relates to the Mn content of the rock, and seems to be independent of the Mg/Fe ratio of the biotite. The garnets are zoned
with respect to Mn and Mg, but often Mn is enriched and Mg depleted in the marginal zone. The Mg/Fe ratio of the biotite varies
twofold depending on the presence or absence of pyrrhotite. The transition: microcline → orthoclase depends upon the amount
of dissolved albite; the polymorph is orthoclase in the pelitic schists but microcline in the calc-silicate rocks which are
much lower in sodium. The plagioclases are of “low” structural type, although
is slightly greater than many other “low” plagioclases. A correlation of d(002) of muscovite and paragonite solid solution
for the range 0 to 20 % paragonite is given. An appreciable positive volume of mixing for the binary system muscovite-paragonite
is indicated. 相似文献
963.
Kirk Ellsworth Gerald Schubert Charles G. Sammis 《Geophysical Journal International》1985,83(1):199-213
Summary. We determine the variation of effective viscosity η across the lower mantle from models of the Gibb's free energy of activation G * and the adiabatic temperature profile. The variation of G * with depth is calculated using both an elastic strain energy model, in which G * is related to the seismic velocities, and a model which assumes G * is proportional to the melting temperature. The melting temperature is assumed to follow Lindemann's equation. The adiabatic temperature profile is calculated from a model for the density dependence of the Grüneisen parameter. Estimates of η depend on whether the lower mantle is a Newtonian or power law fluid. In the latter case separate estimates of η are obtained for flow with constant stress, constant strain rate, and constant strain energy dissipation rate. For G * based on the melting temperature, increases in η with depth range from a factor of about 100 for Newtonian deformation or power-law flow with constant stress to about 5 for non-Newtonian deformation with constant strain rate. For G * based on elastic defect energy, increases in η with depth range from a factor of about 1500 for Newtonian deformation or power-law flow with constant stress to about 10 for non-Newtonian deformation with constant strain rate. Among these models, only a non-Newtonian lower mantle convecting with constant strain rate or constant strain energy dissipation rate is consistent with recent estimates of mantle viscosity from post-glacial rebound and true polar wander data. 相似文献
964.
The abundance of holmium (Z=67) in the Sun remains uncertain. The photospheric abundance, based on lines of Hoii, has been reported as + 0.26±0.16 (on the usual scale where log(H) = 12.00), while the meteoritic value is + 0.51±0.02. Cowan-code calculations have been undertaken to improve the partition function for this ion by including important contributions from unobserved levels arising from the (4f
116p+4f
10(5d+6s)2) group. Based on 6994 computed energy levels, the partition function for Hoii is 67.41 for a temperature of 6000 K. This is 1.5 times larger than the value derived from the 49 published levels. The new partition function alone leads to an increase in the solar abundance of Ho to log (Ho) =+ 0.43. This is within 0.08 dex of the meteoritic abundance. Support for this result has been obtained through LTE spectrum synthesis calculations of a previously unidentified weak line at 3416.38. Attributing the feature to Hoii, the observations may be fitted with log (Ho) =+ 0.53. This calculation assumes log (gf)=0.25 and is uncertain by at least 0.1 dex. 相似文献
965.
Biological pollution in the Mediterranean Sea: invasive versus introduced macrophytes 总被引:17,自引:0,他引:17
The authors have listed 85 species of macrophytes that have probably been introduced to the Mediterranean. Among them, nine species can be considered as invasive, i.e., playing a conspicuous role in the recipient ecosystems, taking the place of keystone species and/or being economically harmful: Acrothamnion preissii, Asparagopsis armata, Lophocladia lallemandii, Womersleyella setacea (Rhodophyta), Sargassum muticum, Stypopodium schimperi (Fucophyceae), Caulerpa racemosa, Caulerpa taxifolia and Halophila stipulacea (Plantae). These data fit well the Williamson and Fitter's "tens rule", which states that, on average, 1 out of 10 introduced species becomes invasive. Though some features (e.g. life traits, geographical origin) can increase the likelihood of a successful invasion, the success of invaders is far from being predictable. Since the beginning of the 20th century, the number of introduced species to the Mediterranean has nearly doubled every 20 years. Should these kinetics continue, and according to the tens rule, it can be expected that 5-10 newly introduced macrophytes shall become invasive in the next 20 years. 相似文献
966.
Adnand Bitri Michel Ballèvre Jean-Pierre Brun Jean Chantraine Denis Gapais Paul Guennoc Charles Gumiaux Catherine Truffert 《Comptes Rendus Geoscience》2003,335(13):969-979
The structure of the Hercynian collision zone in the southeast of the Armorican Massif is illustrated by a 70-km long deep seismic profile acquired in September 2000. The profile images a previously unknown south-dipping thrust that brought the Champtoceaux Domain on top of the Central Armorican Domain during Carboniferous times. Dextral strike-slip motions along the South Armorican Shear Zone, which is downward cut by the thrust zone, are partly coeval with northward thrusting. A major discontinuity, hidden by the thrust front, is also imaged in the lower crust between the Champtoceaux area and the Central Armorican Domain. These new data lead to a structural and kinematic re-interpretation of this part of the Hercynian collision zone. To cite this article: A. Bitri et al., C. R. Geoscience 335 (2003). 相似文献
967.
Jenneke M. Visser Charles E. Sasser Robert H. Chabreck R. Greg Linscombe 《Estuaries and Coasts》2002,25(6):1184-1195
In order to document the effect of the recent drought and the resulting marine intrusion event on plant-community shifts in a Louisiana estuary, we analyzed two vegetation data sets collected in Barataria estuary in 1997 and 2000 and compared community shifts to surface salinity changes at four points along the estuarine gradient within the study area. We used the major vegetation types identified in our previous research of larger data sets and tested the use of a simple vegetation classification technique. This vegetation classification technique is based primarily on the dominant and co-dominant species, and secondarily on the number of taxa observed. To distinguish vegetation types with similar dominant species but different associated species, the vegetation classification technique used a salinity score derived from the species composition. Surface water salinity increases were reflected by a change in species composition in the mesohaline to fresh marshes. The largest species composition shift observed was the shift from oligohaline wiregrass (species rich vegetation type dominated bySpartina patens) to mesohaline wiregrass (vegetation type dominated byS. patens with few other species). Shifts in vegetation composition may have been enhanced by the presence of the major dominant species at a low abundance in other vegetation types. The vegetation classification technique used could classify over 95% of the stations. This vegetation classification technique provides a simple method to classify Louisiana's coastal vegetation based on plant species composition. 相似文献
968.
David M. Quanrud Konrad Quast Otakuye Conroy Martin M. Karpiscak Charles P. Gerba Kevin E. Lansey Wendell P. Ela Robert G. Arnold 《Ground Water Monitoring & Remediation》2004,24(2):86-93
The fate of estrogenic activity in waste water effluent was examined during surface transport and incidental recharge along the Santa Cruz River in Pima County, Arizona. Based on measurement of boron isotopes, the fractional contribution of reclaimed water in surface waters and ground water wells proximate to the river was determined for a contemporary sample set. Estrogenic activity decreased by −60% over the 25 mi length of the river below effluent discharge points in Tucson. In ground water samples obtained from monitoring wells that are proximate to the Santa Cruz River, both dissolved organic carbon ( p = 0.0003) and estrogenic activity ( p = 3 × 10−6 ) were highly correlated to fractional waste water content. Results indicate that proximate ground water quality is sensitive to incidental recharge of reclaimed water in the Santa Cruz River bed. In a few locations, little attenuation of estrogenic activity was apparent during percolation of effluent in the river channel to well withdrawal points. 相似文献
969.
970.
Building and Editing a Sealed Geological Model 总被引:1,自引:0,他引:1
Guillaume Caumon François Lepage Charles H. Sword Jean-Laurent Mallet 《Mathematical Geology》2004,36(4):405-424