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991.
Claudio Marchesi José María González-Jiménez Fernando Gervilla Carlos J. Garrido William L. Griffin Suzanne Y. O’Reilly Joaquín A. Proenza Norman J. Pearson 《Contributions to Mineralogy and Petrology》2011,161(6):977-990
Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc
tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies
enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities
at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = −7.1 to +1.6, relative
to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite
which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that
was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity
of their mantle sources on the 10–100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling
of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The
distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable
Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater
Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the
most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains
of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other
hand, most PGM are crystallized by melts that tapped highly refractory mantle sources. 相似文献
992.
Vyacheslav M. Zobin Juan José Ramírez Hydyn Santiago Eliseo Alatorre Carlos Navarro 《Bulletin of Volcanology》2011,73(1):91-99
Continuous tilt changes during the 2004–2005 effusive-explosive episodes at Volcán de Colima (México) were recorded simultaneously
by two tiltmeters installed on opposite sides of the volcano at elevations of 2200 m and 3060 m above sea level. Data indicate
that the 2004 lava extrusion was preceded by an inflation accompanied by a deflation. The 2005 explosion sequences were associated
with a deflationary–inflationary tilt. The period between the 2004 extrusion and the 2005 explosions was characterized by
an inflationary tilt during a 3 month period. Two deformation sources were located. The first was situated at a depth between
300 m and 1800 m beneath the crater at the northern flank of the volcano and was responsible for volcano deformation during
the preliminary September 2004 stage, the October 2004 extrusion, and the initial stage of the transition period and the March
2005 explosion sequence. The second source was located at a depth between 1800 m and 2800 m beneath the crater at the southern
flank of the volcano and was responsible for volcano deformation during the final stage of the transition period and the May–June
2005 explosion sequence. 相似文献
993.
On the crystal structure and compressional behavior of talc: a mineral of interest in petrology and material science 总被引:1,自引:1,他引:0
G. Diego Gatta Marco Merlini Giovanni Valdrè Hanns-Peter Liermann Gwilherm Nénert André Rothkirch Volker Kahlenberg Alessandro Pavese 《Physics and Chemistry of Minerals》2013,40(2):145-156
The crystal structure of a natural triclinic talc (1Tc polytype) [with composition: (Mg2.93Fe0.06)Σ2.99(Al0.02Si3.97)Σ3.99O10(OH)2.10] has been investigated by single-crystal X-ray diffraction at 223 and 170 K and by single-crystal neutron diffraction at 20 K. Both the anisotropic X-ray refinements (i.e. at 223 and 170 K) show that the two independent tetrahedra are only slightly distorted. For the two independent Mg-octahedra, the bond distances between cation-hydroxyl groups are significantly shorter than the others. The ditrigonal rotation angle of the six-membered ring of tetrahedra is modest (α ~ 4°). The neutron structure refinement shows that the hydrogen-bonding scheme in talc consists of one donor site and three acceptors (i.e. trifurcated configuration), all the bonds having O···O ≤ 3.38 Å, H···O ~ 2.8 Å, and O–H···O ~ 111–116°. The three acceptors belong to the six-membered ring of tetrahedra juxtaposed to the octahedral sheet. The vibrational regime of the proton site appears being only slightly anisotropic. The elastic behavior of talc was investigated by means of in situ synchrotron single-crystal diffraction up to 16 GPa (at room temperature) using a diamond anvil cell. No evidence of phase transition has been observed within the pressure range investigated. P–V data fit, with an isothermal third-order Birch-Murnaghan equation of state, results as follows: V 0 = 454.7(10) Å3, K T0 = 56(3) GPa, and K′ = 5.4(7). The “Eulerian finite strain” versus “normalized stress” plot yields: Fe(0) = 56(2) GPa and K′ = 5.3(5). The compressional behavior of talc is strongly anisotropic, as reflected by the axial compressibilities (i.e. β(a):β(b):β(c) = 1.03:1:3.15) as well as by the magnitude and orientation of the unit-strain ellipsoid (with ε 1:ε 2:ε 3 = 1:1.37:3.21). A comparison between the elastic parameters of talc obtained in this study with those previously reported is carried out. 相似文献
994.
Eduardo M. Bretas José V. Lemos Paulo B. Lourenço 《Rock Mechanics and Rock Engineering》2013,46(2):327-339
A numerical model for the hydromechanical analysis of masonry dams based on the discrete element method is presented. The dam and the rock foundation are represented as block assemblies, and a coupled flow-stress analysis is performed in an integrated manner for the entire system. Complex block shapes may be obtained by assembling elementary blocks into macroblocks, allowing the application of the model to situations ranging from equivalent continuum to fully discontinuum analysis. A contact formulation was developed based on an accurate edge–edge approach, incorporating mechanical and hydraulic behavior. The main numerical aspects are described, with an emphasis in the flow analysis explicit algorithm. An application to an existing masonry dam is presented, analyzing its present condition, with excessive seepage, and the proposed rehabilitation intervention. An evaluation of sliding failure mechanisms was also performed, showing the expected improvement in the safety of the structure. 相似文献
995.
Over the last three decades, many regional studies in mountain ranges under temperate climate revealed that it is possible to discriminate debris-flow and fluvial fans from morphometric indicators measured at the scale of the catchment and the fan itself. The most commonly used indicators are the Melton index (R), a normalized index of the gravitational energy of the catchment, and the fan slope (S). A wide range of thresholds have been proposed for discriminating purpose, but these are generally based on a small population of catchments and may be highly influenced by ambiguous fans included in the data set. A database of 620 upland catchments from several mountain ranges under temperate climate was compiled from the literature to propose robust discriminant morphometric thresholds for debris-flow versus fluvial responses. Linear discriminant analysis (LDA) and logistic regression (LR) were performed using the whole data set, and a leave-one-out cross-validation was used to evaluate performances of the models. Sensitivity and specificity scores obtained for LDA and LR were 0.96 and 0.73, and 0.95 and 0.75, respectively. It is also shown that the channel slope above which debris-flow is observed decreases with the gravitational energy of the catchment. Limitations of the morphometric discrimination are discussed. 相似文献
996.
The influence of the consolidation on a strutted excavation in soft soil is analysed using a computer code based on the finite element method. A base jet-grout slab is considered in order to improve stability against bottom heave failure and minimize wall displacements. The numerical model incorporates the Biot consolidation theory (coupled formulation of the flow and equilibrium equations) with soil constitutive relations simulated by the p–q–θ critical state model. Special emphasis is given to the analysis, during and after the construction period, of the pore pressures, shear stresses, stress levels and displacements in the ground, as well as strut compression loads, wall displacements and bending moments, earth pressures on the wall faces and compression loads and bending moments on the jet-grout slab. The safety factor against bottom heave is also evaluated from the finite element analysis considering formulations of the critical state soil mechanics, and also compared to values obtained with traditional methods that use limit equilibrium approach and bearing capacity fundamentals. 相似文献
997.
RésuméL’évolution structurale et métamorphique de la zone des Montagnes Noires, qui borde vers le sud le bassin carbonifère de Châteaulin, est contrôlée par le fonctionnement d’un cisaillement trans-current dextre. Le caractère unique, mais progressif et inhomogène, de la déformation régionale d’âge carbonifère supérieur conduit à remettre en cause le concept d’une structuration hercynienne polyphasée et celui de « phase bretonne ». 相似文献
998.
The interaction of the Mid-Atlantic Ridge with the North Atlantic Mantle Plume has produced a magmatic plateau centred about Iceland. The crust of this plateau is 30 km thick on average. This abnormal thickness implies that, unlike other slow-spreading ridges, addition of magmatic material to the crust is not balanced by crustal stretching. The thermal effect of the plume also reduces the strength of the lithosphere. Both mechanisms affect the rifting process in Iceland. A structural review, including new field observations, demonstrates that the structure of the Iceland plateau differs from that of other slow-spreading oceanic ridges. Lithospheric spreading is currently accommodated in a 200 km wide deformation strip, by the development of a system of half-grabens controlled by growth faults. Similar extinct structures, with various polarities, are preserved in the lava pile of the Iceland plateau. These structures are identified as lithospheric rollover anticlines that developed in hanging walls of listric faults. We introduce a new tectonic model of accretion, whereby the development of the magmatic plateau involved activation, growth and decay of a system of growth fault/rollover systems underlain by shallow magma chambers. Deactivation of a given extensional system, after a lifetime of a few My, was at the expense of the activation of a new, laterally offset, one. Correspondingly, such systems formed successively at different places within a 200 km wide diffuse plate boundary. Unlike previous ones, this new model explains the lack of an axial valley in Iceland, the dip pattern of the lava pile, the complex geographical distribution of ages of extinct volcanic systems and the outcrops of extinct magma chambers. 相似文献
999.
Lapierre Henriette Angelier Jacques Cogné Xavier Grand Thierry Mascle Georges 《Geodinamica Acta》2013,26(4):197-206
RésuménUne analyse tectonique du couloir faillé d’Arakapas est menée à partir de la mesure de plus de 3 000 plans striés relevés dans les formations ophiolitiques du Troodos et du Kellaki. Des mesures ont également été effectuées dans les formations du Crétacé terminal — Paléogène et du Néogène, afin de préciser l’âge des différents stades de la déformation. Elles ont été traitées par les méthodes des dièdres droits et du calcul du tenseur des contraintes. Il est montré que le couloir d’Arakapas a été le siège de plusieurs déformations successives. Au Néogène règne un régime en extension qui succède à un jeu décrochant dextre contemporain des compressions d’âge Crétacé terminal à Miocène. Un jeu décrochant antérieur peut-être mis en évidence. Il serait compatible avec le fonctionnement en faille transformante océanique proposé par divers auteurs. 相似文献
1000.
AbstractThe gullying action on Jurassic and Cretaceous marls in the Baronnies is observed, warm season after cold season, thanks to a network of measuring stakes arranged in 12 gullies. The climate, characterized by summer heath, rather cold but luminous winters, and quite heavy rain, can be classified as mountainous-mediterranean. Seasonal ablation/sedimentation measures have been carried out upon 119 stakes five times within two years (768 days): two in automn, two in early spring (april), and one at end-of-spring.Ablation and sedimentation sequences are described among the stakes population by Correspondance Factor Analysis (CFA), and their covariations in space and time are explained by Canonical Correspondance Analysis (CCA) according to biophysical environment variables.Axis 1 of the CFA opposes ground level variations according to their amplitude, high or moderate, regardless of the direction of variation and of measurement date. Axis 2 presents a temporal variation, opposing, on one side, sedimentation sequences at post-winter terms and ablation sequences at postsummer terms, to their converse on the other side.The CCA indicates that talwegs in their medium and lower parts are more likely to undergo hard gullying or strong accumulation, especially on steep, or conversely flat, vegetationfree slopes of oxfordian marls; on the contrary, interfluve ridges and Gonson moderate gentle to moderate slopes on cenomanian marls, possibly with some vegetation are less dynamic (canonical axis 1). This result allows to extend former observations on rythmic, contrasted variations of gullies bottoms, well different of interfluves and gonson, progressive, regular, ones.Owing to their varied reactions to seasonal erosive agents (cryoclasty, concentrated run-off), upper parts of talwegs and lower parts of gully slopes and interfluves (canonical axis 2) originate seasonal contrasts revealed by CFA’s axis 2. However, to insure this point of view, more sampling is needed. 相似文献