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31.
Sitiraju Srinivasa Rao Dinesh Kumar Sahadevan Milind R. Wadodkar M/ S. S. Nagaraju Sudipta Chattaraj William Joseph Prasanth Rajankar Titu Sengupta M. V. Venugopalan Subrata N. Das Asok Kumar Joshi Jaswanth Raj Sharma Eadara Amminedu 《Journal of the Indian Society of Remote Sensing》2016,44(2):277-285
32.
By measuring the ratio of wavelength to arclength of folds in closely associated disharmonically folded competent layers,
it is possible to determine which layer has undergone a greater layer-parallel strain and has a smaller competence. This method
may lead to a paradoxical situation. For example when foliated quartzite and mica schist layers are folded together, the mica
schist laminae show a much larger buckle shortening than the quartzite layers. On the other hand, the geometry of folds in
quartzite indicates that quartzite was more competent than the mica schist. The structure can be explained by different modes
of buckling, general buckling in quartzite layers and internal buckling in strongly anisotropic mica schists. 相似文献
33.
Dr. Sudipta Sengupta 《International Journal of Earth Sciences》1977,66(1):175-192
The Singhbhum Shear Zone separates the rocks of a highly metamorphosed northern group from an unmetamorphosed southern group. It had been recognised by earlier workers as a thrust zone in which the thrust movements were achieved by slip along a pre-existing schistosity. Deformed conglomerates crop out in discontinuous bands along the Shear Zone. Strain determinations from these conglomerates have been made to ascertain the strain pattern within the Shear Zone. None of the existing methods of determining strain from stretched pebbles can be applied to these conglomerates. An approximate value for shortening perpendicular to the schistosity can however be obtained from cross-cutting buckled veins. Detailed study of various small-scale structures clearly indicates that the principal structures of this region cannot be explained simply by a slipping on the schistosity.
Zusammenfassung Die Singhbhum-Scherzone trennt die Gesteine der stark metamorphen nördlichen Gruppe von denjenigen einer nicht metamorphen südlichen Gruppe. Nach früheren Autoren ist es eine Überschiebungszone, worin die Überschiebungsbewegungen durch das Gleiten entlang einer älteren Schieferung zustande kamen. Deformierte Trümmergesteine treten in ununterbrochenen Bändern entlang der Scherzone auf. Man hat Deformationsbestimmungen von diesen Trümmergesteinen gemacht, um das Deformationsbild in der Scherzone zu erklären. Eine ausführliche Untersuchung der verschiedenen kleinmaßstäblichen Strukturen zeigt deutlich, daß die Hauptstrukturen dieser Gegend nicht einfach durch das Gleiten auf der Schieferung erklärt werden können.
Résumé La zone de cisaillement de Singhbhum sépare les roches très métamorphiques au nord, de celles d'un groupe non-métamorphique au sud. Elle avait été reconnue lors de recherches précédentes comme étant la base de charriages suivant laquelle la poussée se serait faite par glissement le long d'une schistosité pré-existante. Des conglomérats déformés affleurent par bandes discontinues le long de cette zone. Grâce à ces conglomérats, des estimations sur la déformation ont été faites pour définir le style de déformation dans la zone de glissement. Une étude détaillée de différentes structures à petite échelle indique clairement qu'on ne peut pas expliquer les principales structures de cette région uniquement par un glissement sur la schistosité.
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34.
A Paleocene granodiorite pluton on Jamaica has been subject to extensive weathering caused by the tropical marine environment of the island. The natural remanence of 29 samples obtained from relatively fresh rock in two localities was found to consist of two components with overlapping coercivity ranges. Alternating field treatment proved ineffective for removing the secondary component without destroying the primary one. Thermal demagnetization of samples from the two localities was more effective and yielded paleomagnetic poles at 14.7°N, 11.6°W and 58.9°N, 15.9°E respectively. These pole positions are different from those available from contemporaneous North American rocks and from poles derived from Jamaican Cretaceous and Upper Miocene rocks. Mineralogical studies showed that the granodiorite has undergone an extensive maghemitization superposed on earlier class 2 deuteric oxidation and related to the weathering process. Some of the titanomaghemite has, however, been converted to titanohematite. Hence although the secondary remanence carried by the former was removable by thermal treatment at 500°C, its part carried by the latter could not be removed without simultaneously destroying the primary remanence carried by the residual titanomagnetite. The observed paleopole positions do not, therefore, represent the true Paleocene geomagnetic field, but suggest that the direction of magnetization of the pluton has been approximately equatorial and was probably acquired in a reversed geomagnetic field. This could be interpreted as having been caused by the behavior of the geomagnetic field during a polarity transition, but a more favorable interpretation appears to be a large anticlockwise tectonic rotation of the islands since the Paleocene. 相似文献
35.
Exsolution textures in orthopyroxene in aluminous granulites as indicators of UHT metamorphism: New evidence from the Eastern Ghats Belt, India 总被引:1,自引:0,他引:1
Highly aluminous orthopyroxene, coexisting with sapphirine, cordierite, sillimanite, quartz and garnet in various combinations, constitute granoblastic mosaic peak metamorphic assemblages in aluminous granulites from three localities in the Eastern Ghats Belt, India. Orthopyroxene contains four types of intergrowths: (a) involving sapphirine with or without cordierite, (b) involving spinel, but without sapphirine, (c) involving cordierite, but without sapphirine and spinel, and (d) involving garnet, without sapphirine, spinel or cordierite. On the basis of textural and compositional data, origin of the intergrowths is ascribed to breakdown of Mg-Tschermak component, locally also involving Fe- and Ti-Tschermak. An attempt is made to compute the “pre-breakdown” compositions of orthopyroxene by image analysis, which shows maximum Al2O3 content of 13.4 wt.% in the pristine orthopyroxene. Geothermometry, phase equilibria consideration and application of existing experimental data on alumina solubility in orthopyroxene coexisting with sapphirine and quartz, collectively indicate extreme thermal conditions of metamorphism (> 1000 °C) for the studied assemblages. This re-affirms the notion that Al2O3 solubility in orthopyroxene is the most powerful indicator of UHT metamorphism (Harley, S.L., 2004. Extending our understanding of ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158). The intergrowths are considered to have formed due to cooling from the thermal peak spanning a temperature range of approximately 150 °C. Appearance of diverse types of intergrowths is probably related to subtle differences in bulk composition, particularly Fe:Mg ratios. 相似文献
36.
Sudipta Lahiri 《Journal of the Indian Society of Remote Sensing》1996,24(4):265-272
Spatial characteristies of drainage geometry of Brahmaputra river and its tributaries were studied based on visual interpretation of 1RS IB imagery (on 1:250,000 scale) and topographical maps of different periods. Observed features of drainage lines alongwith possible mechanism of their formation in terms of neotectouie adjustment are discussed It is inferred that selected stretches of Brahmaputra river viz. between Dibrugarh and Jorhat and between Guwahati and Goafpara may represent channels with active boundaries that have restrieted/controlled the width of sinuous lines of discharge of the river within the valley. Local tilting of ground, lying to the south-east of Brahmaputra river, between Noa Dihing and Burhi Dihing rivers, may explain some of the observed features of these drainage lines Movement along some lineaments in recent past has occurred. This inference is based on the evidence of adjustment of several drainage lines viz, Dihing. Burhi Dihing. Dikrang. and Mora Bhareh rivers and displacement of rocks of younger age. 相似文献
37.
Ilvaite, Ca(Fe2+, Fe3+)Fe2+Si2O8(OH), a black mixed valence iron silicate shows considerable Fe2+?Fe3+ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P2 1/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe2+ and Fe3+ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400?21 K. Below 343±1K, a continuous rotation of the molar susceptibility K ∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. X a, X b and X c increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe2+ and Fe3+ spins on A sites was suggested in previous Mössbauer and neutron powder diffraction studies. In addition, 1/X a shows an antiferromagnetic maximum at 50±3 K, whereas 1/X b and 1/X c at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe2+ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe2+(A)-Fe3+(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K ∥-K⊥) suggests a singlet ground State 5A1 for the Fe2+ ions, in agreement with previous Mössbauer studies. 相似文献
38.
Ajoy Kumar Dasgupta 《Astrophysics and Space Science》1979,60(2):455-464
Charged dust grains of radiia3×10–63×10–5 cm may be driven out of the galaxy due to radiation pressure of starlight. Once clear of the main gas-dust layer, dust grains may then escape into intergalactic space. Such grains are virtually indestructible-being evaporated only during galaxy formation. The dust grains, once injected into the intergalactic medium, may acquire suprathermal energy, thus suprathermal grains in collision with magnetized cloud by the Fermi process. In order to attain relativistic energy, suprathermal grains have to move in and out (scattering) of the magnetic field of the medium. It is now well established that high energy cosmic rays are of the order 1020 eV or more. We have speculated that these high energy (>-1018 eV) cosmic ray particles are charged dust grains, of intergalactic origin. This is possible only if there exists a magnetic field in the intergalactic medium. 相似文献
39.
Abundance ratios in QSOs Q 1246-057, 0453-423, PKS 0528-250, Q 0002-422, and PKS 2126-158 have been calculated by the method of curve of growth. Relative abundance of carbon with respect to silicon has been calculated in the QSOs Q 1246-057, PKS 0528-250, Q 0002-422, and PKS 2126-158. Relative abundance of aluminium with respect to silicon has been calculated in the QSOs PKS 0528-250 and PKS 2126-158. Relative abundance of iron with respect to magnesium has been also calculated in the QSOs Q 0453-423 and Q 0002-422. The ratiosN(C)/N(Si) andN(Al)/N(Si) andN(Fe)/N(Mg) in QSOs considered as a class are 3.45±2.33, 0.063±0.054, and 1.08±0.70, respectively.The ratiosN(C)/N(Si) andN(Al)/N(Si) show a mild trend of increase with decreasing emission redshift. On the other hand, the ratioN(Fe)/N(Mg) shows a mild trend of decrease with decreasing emission redshift. This suggests the possibility of the chemical evolution of QSOs.The comparison ofN(C)/N(Si), the relative abundance of carbon-one of the four most plentiful elements — with respect to silicon in QSOs and normal galaxies suggests that QSOs might belong to an early phase in the evolutionary sequence of galaxies and that they evolve chemically into normal galaxies in course of time.Mrs. Alpana Gangopadhyaya after marriage. 相似文献
40.
In the Schirmacher Hills, most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism. The microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth. The variation in microstructure of the sheared rock is partly due to heterogeneity of the intensity of strain from domain to domain, producing protomylonites, orthomylonites and ultramylonites. However, a large part of the microstructural variation has resulted from syn- to post-tectonic recrystallization and grain growth of constituent minerals. Both quartz and feldspar have deformed by crystal plastic processes with dominant grain boundary migration. The present aspect ratio of the feldspar grains is a result of various degrees of dynamic recrystallization along the grain boundary. The ratio varies between 1.5 and 2. Presence of exsolution lamellae in perthites and formation of myrmekite at the strained grains of K-feldspar suggest diffusion assisted dislocation creep. These mylonites are characterized by the presence of weakly strained or unstrained long quartz ribbons. The development of quartz ribbons with the absence of significant strain suggests grain recovery and grain growth during high temperature mylonitization. The growth of quartz ribbons took place by coalescing neighbouring grains both along and across the ribbon length. At the ultramylonite stage the fine-grained matrix of quartz and feldspar mostly accommodates the bulk strain. 相似文献