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51.
The Catalina Schist and Rand Schist are two high P/T terranes in southern California. The Catalina Schist is correlated with the Franciscan Complex and occurs in the continental borderland. It consists of a blueschist-facies melange tectonically overlain by a greenschist unit, which, in turn, is overthrust by an amphibolite unit. The greenschist unit itself is inversely zoned from epidote-amphibolite fades at the top through greenschist facies in the center to transitional blueschist-greenschist facies at the base. The Rand Schist is part of the eugeoclinal Pelona-Orocopia Schist terrane, which lies interior to the present continental margin, structurally beneath Precambrian to Mesozoic sialic basement. The Rand Schist is inversely zoned from epidote-amphibolite facies to transitional blueschist-greenschist facies, similar to the greenschist unit of the Catalina Schist.Two trends in amphibole composition, one from actinolite to hornblende in greenschists and epidote amphibolites (calcic series) and the other from actinolite through winchite to crossite in glaucophanic greenschists (sodic-calcic series), are present in both the Rand Schist and the greenschist unit of the Catalina Schist. The transition from actinolite to hornblende in the calcic series is defined by increases in tschermakite, edenite, and glaucophane substitution. Amphiboles of the sodic-calcic series differ mainly in the degree of glaucophanic substitution. The similarity of amphibole trends in the two terranes indicates that they were metamorphosed at approximately the same pressures and temperatures, and is evidence that the Rand Schist originated in a subduction zone, despite its present intracontinental setting.Most glaucophanic greenschists in the Rand and Catalina Schists contain both a sodic and a calcic member of the sodic-calcic series. Textural relations indicate that calcic members generally developed after the sodic ones. This implies that sodic amphibole formerly may have been present in many of the structurally higher greenschists and epidote amphibolites. Preservation of the inverted zonations, as well as microstructural evidence for the syntectonic development of calcic and sodic-calcic amphiboles, suggest that glaucophanic greenschists, greenschists, and epidote amphibolites all formed during underthrusting (subduction). This contrasts with many orogenic belts, where replacement of blueschists by greenschists to amphibolites is attributed to thermal reequilibration during erosional unroofing. 相似文献
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Reginald G. Golledge Matthew Rice R. Daniel Jacobson 《The Professional geographer》2005,57(3):339-349
The growth of the Internet and the digital revolution have meant increased reliance on electronic representations of information. Geospatial information has been readily adapted to the world of cyberspace, and most Web pages incorporate graphics, images, or maps to represent spatial and spatialized data. But flat computer screens do not facilitate a map or graph experience by those who are visually impaired. The traditional method for compensating for nonvisual access to maps and graphics has been to construct hard‐copy tactile maps. In this article, we examine an electronic accommodation for nonvisual users—the haptic map. Using new and off‐the‐shelf hardware—force feedback and vibrotactile mice—we explore how touch can be combined with virtual representations of shapes and patterns to enable nonvisual access to onscreen map or graphic material. 相似文献
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Population counts are a key anchor for much of the official statistical system and the benchmark for many commercial and research surveys and analysis. Statistical offices around the world face a wide range of challenges in counting their population, most particularly in the years between censuses, as population flows become much freer and family structures continue to evolve. This paper considers these issues, reviewing how population counts have evolved over time in the UK and other countries. The paper also looks to the future, considering ways in which population count methodology might develop. 相似文献
54.
We have obtained numerically integrated orbits for Saturn's coorbital satellites, Janus and Epimetheus, together with Saturn's F-ring shepherding satellites, Prometheus and Pandora. The orbits are fit to astrometric observations acquired with the Hubble Space Telescope and from Earth-based observatories and to imaging data acquired from the Voyager spacecraft. The observations cover the 38 year period from the 1966 Saturn ring plane crossing to the spring of 2004. In the process of determining the orbits we have found masses for all four satellites. The densities derived from the masses for Janus, Epimetheus, Prometheus, and Pandora in units of g cm−3 are , , , and , respectively. 相似文献
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To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ44Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ44Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ44Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water (Δcal–w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ44Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination. 相似文献
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Observations using the very large array (VLA) radio interferometer during the past five years have enabled the discovery of a new type of plasmasphere disturbance, the magnetic eastward-directed wave. Previous work indicated these disturbances were likely frozen to the geomagnetic field as determined from their azimuth distributions. This work provides a method to explain more accurately the azimuth distribution, thereby allowing the calculation of the disturbances location in the plasmasphere independently of the measured velocity. The measurable velocity due to corotation is calculated and subtracted from the measured trace velocity. This difference, or deviation from corotation, is attributed to electrodynamic convection; the measurement of plasmaspheric convection may lead to the eventual monitoring of mid-latitude electric fields. Disturbances are seen convecting predominantly westward, with the fastest having angular velocities greater than the anticorotating VLA line of sight. The direction of convection and conditions of observations indicate that the disturbances are likely the same phenomenon seen by the Los Alamos satellite beacon array. 相似文献