全文获取类型
收费全文 | 573篇 |
免费 | 16篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 56篇 |
大气科学 | 28篇 |
地球物理 | 134篇 |
地质学 | 172篇 |
海洋学 | 32篇 |
天文学 | 142篇 |
综合类 | 4篇 |
自然地理 | 30篇 |
出版年
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 3篇 |
2018年 | 19篇 |
2017年 | 23篇 |
2016年 | 17篇 |
2015年 | 12篇 |
2014年 | 22篇 |
2013年 | 36篇 |
2012年 | 21篇 |
2011年 | 37篇 |
2010年 | 25篇 |
2009年 | 36篇 |
2008年 | 20篇 |
2007年 | 24篇 |
2006年 | 33篇 |
2005年 | 19篇 |
2004年 | 19篇 |
2003年 | 12篇 |
2002年 | 17篇 |
2001年 | 15篇 |
2000年 | 15篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1974年 | 3篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1970年 | 5篇 |
排序方式: 共有598条查询结果,搜索用时 15 毫秒
111.
Chiara Lesti Massimiliano Porreca Guido Giordano Massimo Mattei Raymond A. F. Cas Heather M. N. Wright Chris B. Folkes Josè Viramonte 《Bulletin of Volcanology》2011,73(10):1535-1565
Estimates of pyroclastic flow emplacement temperatures in the Cerro Galán ignimbrite and Toconquis Group ignimbrites were
determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to
the Cerro Galán volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature
estimates for the 2.08-Ma Cerro Galán ignimbrite are retrieved from 40 sites in 14 localities (176 measured clasts), distributed
at different distances from the caldera and different stratigraphic heights. Additionally, temperature estimates were obtained
from 27 sample sites (125 measured clasts) from seven ignimbrite units forming the older Toconquis Group (5.60–4.51 Ma), mainly
outcropping along a type section at Rio Las Pitas, Vega Real Grande. The paleomagnetic data obtained by progressive thermal
demagnetization show that the clasts of the Cerro Galán ignimbrite have one single magnetic component, oriented close to the
expected geomagnetic field at the time of emplacement. Results show therefore that most of the clasts acquired a new magnetization
oriented parallel to the magnetic field at the moment of the ignimbrite deposition, suggesting that the clasts were heated
up to or above the highest blocking temperature (T
b) of the magnetic minerals (T
b = 580°C for magnetite; T
b = 600–630°C for hematite). We obtained similar emplacement temperature estimations for six out of the seven volcanic units
belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we found two distinct magnetic components.
The estimation of emplacement temperatures in this latter case is constrained at 580–610°C, which are lower than the other
ignimbrites. These estimations are also in agreement with the lowest pre-eruptive magma temperatures calculated for the same
unit (i.e., 790°C; hornblende–plagioclase thermometer; Folkes et al. 2011b). We conclude that the Cerro Galán ignimbrite and Toconquis Group ignimbrites were emplaced at temperatures equal to or higher
than 620°C, except for Lower Merihuaca unit emplaced at lower temperatures. The homogeneity of high temperatures from proximal
to distal facies in the Cerro Galán ignimbrite provides constraints for the emplacement model, marked by a relatively low
eruption column, low levels of turbulence, air entrainment, surface–water interaction, and a high level of topographic confinement,
all ensuring minimal heat loss. 相似文献
112.
The distribution of diamonds within individual kimberlite pipes is poorly documented in the public domain due to the proprietary
nature of the data. The study of the diamond distribution within two pipes, Fox and Koala, from the EKATI Diamond Mine, NWT,
Canada, in conjunction with detailed facies models has shown several distinct relationships of deposit type and grade distribution.
In both pipes, the lithological facies represent grade units which can be distinguished from each other in terms of relative
size and abundance of diamonds. A positive relationship between olivine grain size and abundance with diamond size and abundance
is observed, indicating that sorting of fragmental kimberlites influences diamond distribution. Though surface geological
processes do not control the diamond potential of the erupting magma, they can be responsible for concentrating diamonds into
economically significant proportions. A good understanding of the eruption, transport and depositional processes responsible
for the individual lithological units and the diamond distribution within them is important for successful resource estimation.
This may lead to recognition of areas suitable for selective mining, making a marginal deposit economic. 相似文献
113.
The Jurassic Muskox and Jericho kimberlites (Northern Slave Province, Nunavut, Canada) contain a variety of facies exhibiting
different geometries, contact relationships, internal organisation, country rock abundance and olivine shapes, although many
have similar matrix/groundmass mineralogies and textures. Five facies are examined that either have characteristics consistent
with coherent rocks in general (i.e. intrusive and extrusive non-fragmental rocks) or are mineralogically and texturally similar
to kimberlite described as coherent (or apparent coherent). Three facies are interpreted as coherent on the basis of: (1)
geological setting, (2) apparent-porphyritic texture, (3) sharp contacts with fragmental kimberlite, (4) relative abundance
of elongate and unbroken olivine crystals and (5) paucity of country rock xenoliths, while the remaining two facies are interpreted
as fragmental on the basis of: (1) the gradational contacts with demonstrably fragmental kimberlite, (2) relative abundance
and range of sizes of country rock lithic clasts and (3) numerous broken olivine crystals. Comparisons are made with coherent
and apparent-coherent kimberlite from the literature. Our three coherent facies are similar to literature reported coherent
kimberlite dykes hosted in country rock (CKd) in terms of internal organisation, low abundance of country rock xenoliths, and apparent-porphyritic texture. Conversely,
our two fragmental facies share attributes with previously described pipe-filling coherent and apparent-coherent kimberlite
(CKpf) in terms of geometry, internal organisation and abundance of country rock xenoliths. We conclude that CKd and most CKpf, although similar in matrix/groundmass mineralogy and texture, can be distinguished on the basis of internal organisation,
country rock lithic clast abundance, texture (e.g. apparent-porphyritic texture) and possibly olivine crystal shapes and suggest
that fragmental kimberlite is more common than reported. 相似文献
114.
A first-order seismotectonic model was created for South Africa. This was done using four logical steps: geoscientific data
collection, characterisation, assimilation and zonation. Through the definition of subunits of concentrations of earthquake
foci and large neotectonic and structural domains, seismotectonic structures, systems and domains were created. Relatively
larger controls of seismicity exist between the Great Escarpment and the coast. In the south, this region is characterised
by large aeromagnetic anomalies and large EW trending faults. In the west, it is characterised by the NW–SE trending Wegener
stress anomaly, radial-trending dykes and earthquake clusters. In the east, it is characterised by a large neotectonic domain
where several large historical earthquakes occurred. In the centre of South Africa, several clusters of earthquake activity
are found, often related to mining activity. Further north, seismicity is related to both mining activity and neotectonic
deformation. This work contributes to the development of a seismotectonic model for South Africa by (1) bringing together,
digitally, several data sets in a common GIS platform (geology, geophysics, stress, seismicity, neotectonics, topography,
crustal and mantle structure and anisotropy), (2) understanding the significance of data sets for seismotectonic zonation
and limitations thereof and (3) obtaining a reasonable regional model for use in seismic hazard assessments. 相似文献
115.
Jyotiranjan S. Ray Kanchan Pande Rajneesh Bhutani Anil D. Shukla Vinai K. Rai Alok Kumar Neeraj Awasthi R. S. Smitha Dipak K. Panda 《Contributions to Mineralogy and Petrology》2013,166(6):1613-1632
The Newania carbonatite complex of India is one of the few dolomite-dominated carbonatites of the world. Intruding into Archean basement gneisses, the rocks of the complex have undergone limited diversification and are not associated with any alkaline silicate rock. Although the magmatic nature of the complex was generally accepted, its age of emplacement had remained equivocal because of the disturbed nature of radioisotope systems. Many questions about the nature of its mantle source and mode of origin had remained unanswered because of lack of geochemical and isotopic data. Here, we present results of our effort to date the complex using 147Sm–143Nd, 207Pb–206Pb and 40Ar–39Ar dating techniques. We also present mineral chemistry, major and trace element geochemistry and Sr–Nd isotopic ratio data for these carbonatites. Our age data reveal that the complex was emplaced at ~1,473 Ma and parts of it were affected by a thermal event at ~904 Ma. The older 207Pb–206Pb ages reported here (~2.4 Ga) and by one earlier study (~2.3 Ga; Schleicher et al. Chem Geol 140:261–273, 1997) are deemed to be a result of heterogeneous incorporation of crustal Pb during the post-emplacement thermal event. The thermal event had little effect on many magmatic signatures of these rocks, such as its dolomite–magnesite–ankerite–Cr-rich magnetite–magnesio-arfvedsonite–pyrochlore assemblage, mantle like δ13C and δ18O and typical carbonatitic trace element patterns. Newania carbonatites show fractional crystallization trend from high-Mg to high-Fe through high-Ca compositions. The least fractionated dolomite carbonatites of the complex possess very high Mg# (≥80) and have similar major element oxide contents as that of primary carbonatite melts experimentally produced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calcio-carbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbonatite melt and that it was derived from a carbonate bearing mantle. The Sr–Nd isotopic data suggest that the primary magma originated from a metasomatized lithospheric mantle. Trace element modelling confirms such an inference and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone. 相似文献
116.
117.
Though several alternatives/modifications to the Piper diagram are available, the Piper diagram is still the most common way of representing chemical analyses. The present note shows how to reproduce the Piper trilinear diagram in rectangular coordinates retaining all the essential features of the trilinear diagram. In this form, it can be generated using commonly used graphics packages without requiring specialized software. A sample data set of six samples from Gurur watershed, Chhattisgarh, India, was used to illustrate the method. The patterns of points produced in the trilinear Piper diagram and the reproduced Piper diagram in rectangular coordinates are essentially identical. 相似文献
118.
Lamellidens marginalis (Molluska Bivalvia Eulamellibranchiata) is an important member of the freshwater ecosystem of India, which is sustained by filtering phytoplankton, bacteria and other particulate organic matter from the available water. Hemocytes, the circulating blood cells of bivalves, function as immunological effector cells under exposure to toxins and/or parasites. The hemocytes of Lamellidens marginalis have been identified morphologically and enumerated as being qualified for use as a biomarker to analyze the threat of freshwater contamination by sublethal concentrations of sodium arsenite, a natural pollutant of freshwater ecosystems. Diverse subpopulations of hemocytes were identified as blast‐like cells, granulocytes, agranulocytes, hyalinocytes and asterocytes. Supression in the total count of hemocytes and blast‐like cells was recorded under the exposure of all the concentrations of sodium arsenite tested. Sodium arsenite elevated the relative density of granulocytes, hyalinocytes, and asterocytes. Partial restoration of the total count of the hemocytes was recorded after the post‐treated animals were maintained in arsenic‐free water for periods of 15 and 30 days. The data predict a shift in the immunological parameters of this bivalve in arsenic contaminated environments. The present study is aimed at quantifying the arsenic induced stress in Lamellidens marginalis and establishing the hemocyte density as a biomarker of aquatic pollution in selected geographical regions of India. 相似文献
119.
Summary Average SST anomalies of OCT-DEC months for Nino-3 region are predicted using the following parameters – (i) April rain over
Himachal Pradesh, (ii) Darwin pressure change (January–April), (iii) Southern Oscillation Index (Tahiti–Darwin) and (iv) SST
anomalies of Nino-3 region in the month of May. Principal component analysis is used to orthogonalise the predictors before
using them in the regression equation. The first two principal components, which explain nearly 73% of the variance, are used
to fit a regression line. The period 1951–1985 is used as the calibration period for the model and the period 1986–1997 as
the verification period for the forecast.
The Brier score with respect to a reference forecast (persistence) for the independent period is found to be 0.82 which is
indicative of good forecast skill.
Received April 1, 1999 Revised January 17, 2000 相似文献
120.
Ray von Wandruszka 《Geochemical transactions》2000,1(1):10-6
Humic acids are amphiphilic species whose behavior in aqueous solution suggests that they form pseudomicelles–aggregates akin to the micelles familiar from synthetic surfactant chemistry. It is thought that humic pseudomicelles can be formed by both intramolecular coiling and intermolecular association, depending on the molecular weight, structural characteristics, and polydispersity of the humic acid in question. The process does not feature a critical micelle concentration. Experimental evidence indicates that metal ions enhance the detergent character of dissolved humic acid by facilitating the coiling and folding of the polymer chains. A recently conceived alternative model suggests that humic acids consist of relatively small subunits that associate through weak molecular interactions. This view appears to run counter to certain experimental observations, but deserves careful attention. The strong association between metal ions and solid humic acid makes it possible to use an inexpensive commercial grade for the decontamination of polluted water. A continuous elution process through a column packed with humic acid allows for the removal of both heavy metals and organic xenobiotics from aqueous solution. 相似文献