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121.
A new genus and species of a microtine rodent, Proneofiber guildayi, is named on the basis of rooted teeth with cement from the warm Gilliland local fauna, Seymour Formation of Pleistocene age, Knox County, Texas. Proneofiber guildayi is considered to be ancestral to Neofiber, the round-tailed water rat, now living in the extreme southeastern United States. The presence of Pleistocene faunas prior to the first continental glaciation and the existence of three Pearlette-like volcanic ashes in the Plains Region suggest a revision of the age assignment of Pleistocene deposits from the nonglaciated region. 相似文献
122.
The Late Pleistocene to Holocene eruptive history of Pico de Orizaba can be divided into 11 eurptive episodes. Each eruptive episode lasted several hundred years, the longest recorded being about 1000 years (the Xilomich episode). Intervals of dormancy range from millenia during the late Pleistocene to about 500 years, the shortest interval recorded in the Holocene. This difference could reflect either changes in the volcano's activity or that the older stratigraphic record is less complete than the younger. Eruptive mechanisms during the late Pleistocene were characterized by dome extrusions, lava flows and ash-and-scoria-flow generating eruptive columns. However, in Holocene time plinian activity became increasingly important. The increase in dacitic plinian eruptions over time is related to increased volumes of dacitic magma beneath Pico de Orizaba. We suggest that the magma reservoir under Pico de Orizaba is stratified. The last eruptive episode, which lasted from about 690 years bp until ad 1687, was initiated by a dacitic plinian eruption and was followed by effusive lava-forming eruptions. For the last 5,000 years the activity of the volcano has been gradually evolving towards such a trend, underlining the increasing importance of dacitic magma and stratification of the magma reservoir. Independent observations of Pico de Orizaba's glacier early this century indicate that some increase in volcanic activity occurred between 1906 and 1947, and that it was probably fumarolic. 相似文献
123.
A geochemical reconnaissance heavy mineral survey has been carried out in the area of the Gatineau River Valley, over a part of the Grenville Province of Québec. The majority of samples show gold concentrations below the detection limit of 5 ppb, and anomalies are erratic. To circumvent these problems and to present an accurate image of the gold-bearing potential of the area, a regression analysis has been applied to the samples whose gold concentrations were above the detection limit. The regression of gold on chalcophile pathfinder elements (As, Se and Pb) explains 11% of the gold variance.Bedrock lithologies, overburden cover and physico-chemical characteristics are important factors influencing the chalcophile element dispersion pattern. Principal component analysis was used to characterize the principal inter-element relationships and to identify the factors influencing the dispersion of chalcophile elements. A filtering procedure was used to remove the background effects influencing the pathfinder elements, and to enhance the influence of mineralization. This procedure uses standard formulas for partial reconstruction of the data matrix from results of principal component analysis. The regression model constructed with these corrected data explains 20% of gold variance. It constitutes an improvement over the model which uses non-filtered data.The equation inferred from regression was applied to the whole data set and provided an estimation of gold at each sampling point. Exploration targets are defined from highest gold concentrations estimated by regression. The regression model aided in the definition of some well defined anomalies associated with geological structures favourable to gold mineralization. 相似文献
124.
To study the amount of heat generated by radioactive decay in the continental crust, the usual practice in the literature is to fit to the heat flow and radioactivity data a relationship of the form: Q = Qr + D · A where Q and A are the observed heat flow and radiogenic heat production. Qr is the “reduced” heat flow and D is a depth scale. This procedure implicitly assumes that uranium, thorium and potassium have identical distributions in the crust. We suggest that significant information may be lost as the three radioelements may in fact be affected by processes operating over different depths.Data published for four heat flow provinces throughout the world are used to estimate the distributions of uranium, thorium and potassium in the continental crust. These distributions are characterized by a depth scales defined as follows: Di =∫0h Ci(z)Ci(0)dz where h is the thickness of the layer containing the bulk of radioactivity and Ci(z) the concentration of element i at depth z. Three depth scales are computed from a least-squares fit to the following relationship: Q = Qr + DU · AU + DT · AT + DK · AT where Q is the observed heat flow and Qr some constant (a reduced heat flow). Ai is the heat generation rate due to the radioactive decay of element i, and Di is the corresponding depth scale.The analysis suggests that the three distributions are different and that they have the same basic features in the four provinces considered. The depth scale for potassium is large in granitic areas, that for thorium is small and that for uranium lies between the other two.We propose a simple model according to which each radioelement essentially provides a record for one process. Potassium gives a depth scale for the primary differentiation of the crust. Thorium gives the depth scale of magmatic or metamorphic fluid circulation. Finally, the uranium distribution reflects the late effects of alteration due to meteoric water. We show that the heat flow and radioactivity data are compatible with this model.Our analysis and numerical results are supported by data from deep boreholes and by geochemical evidence, such as detailed investigations of plutonic series and studies of U-Th-Pb systematics. 相似文献
125.
Bernard Dalmayrac Gérard Laubacher René Marocco Claude Martinez Pierre Tomasi 《International Journal of Earth Sciences》1980,69(1):1-21
Résumé Une chaîne hercynienne, remaniant des noyaux précambriens, forme le substratum de la chaîne des Andes centrales depuis le Nord du Pérou jusqu'en Argentine moyenne. L'évolution préorogénique de cette chaîne s'amorce, dès la fin du Précambrien, par une fracturation sur la bordure ouest de l'actuelle plaque sud-américaine. Au Paléozoïque inférieur s'individualise un vaste sillon coudé, sur croûte amincie, où s'accumulent 8000 à 15 000 m de sédiments marins essentiellement detritiques. Ce sillon intracratonique est bordé, à l'Est, par le Bouclier brésilien et, à l'Ouest, par une aire continentale: le massif précambrien d'Arequipa que prolongeait, vers l'ouest, le »Paléocontinent sud-est pacifique«. Deux phases tectoniques: la phase majeure éohercynienne (350-330 Ma) puis la phase tardihercynienne (265-260 Ma), sont responsables de la formation de la chaîne hercynienne laquelle résulte du serrage, du sillon paléozoïque entre les deux blocs précambriens rigides.C'est une chaîne intracontinentale apparemment simple, d'allure symétrique, à structures généralement verticales. Le métamorphisme est épizonal sauf dans certaines zones très localisées où la montée de granites syntectoniques s'accompagne d'un métamorphisme méso-à catazonal intermediaire de basse pression.En conclusion et dans une optique de tectonique globale, il est proposé une hypothèse sur l'évolution paléogéodynamique de la chaîne hercynienne sud-américaine des Andes centrales, en mettant l'accent sur les mécanismes particuliers de sa formation; mécanismes où la compression se traduit par du plissement associé à des décrochements.
A Hercynian Foldbelt, reworking elements of Precambrian, constitues the basement of the Central Andes that extend from the northern Peru to the central Argentina. The preorogenic evolution of the belt initiate after the end of the Upper Precambrian with continental fracturation on the western boundary of the actual South American Plate. During the Paleozoïc a great elbowed trough, lying on thinned crust, was filled by 8000 to 15000 m of essentially detritic marine sediments. That intracratonic trough was bounded to the East by the Brasilian Shield and also to the West by an other continental area that acted during the Lower Paleozoïc as an important sediment source area: the Precambrian Massif of Arequipa that extend toward the West with the South-East Pasific Paleocontinent. The Hercynian Foldbelt resulted from the compression of the paleozoïc basin between that rigid precambrian blocs. Two main compression stages are distinguished: an Eochercynian Phase (350-330 My) that is the most important and a Tardi hercynian Phase (265-260 My). That Hercynian intracontinental Foldbelt of the Central Andes is obviously simple with symetrical aspect and subvertical structures. The metamorphism is epizonal except in very limited areas where syntectonic granits are associated with meso-and catazonal metamorphism of low-pressure type.To conclude a paleogeodynamic evolution model of the south american hercynian foldbelt is proposed in terms of global tectonic, with emphasis on the mecanism of its formation, mechanism where folding is associated with strike-slip faulting.
Zusammenfassung Ein variskischer Faltengürtel, der auch einige präkambrische Elemente umfaßt, bildet die Basis der Zentralanden, die sich von Nordperu bis Zentralargentinien erstrecken. Die präorogene Entwicklung dieses Gürtels beginnt am Ende des oberen Präkambriums mit einem Kontinentalbruch an der westlichen Grenze der heutigen südamerikanischen Platte. Während des Paläzoikums wurde eine große Senke, die auf einer dünnen Kruste lag, mit 8000–15 000 m ditritischen, marinen Sedimenten gefüllt. Die intrakratonische Senke war auf der östlichen Seite durch den brasilianischen Schild begrenzt und auf der westlichen Seite durch eine Kontinentalfläche, die während des unteren Paläozoikums als ein wichtiges Liefergebiet für Sedimente diente. Dieser westliche Kontinent setzte sich aus dem präkambrischen Massiv von Arequipa und dem südostpazifischen Kontinent zusammen. Die Entwicklung des variskischen Faltengürtels kann auf die Kompression des paläozoischen Beckens zwischen den regiden präkambrischen Blöcken zurückgeführt werden. Hier unterscheidet man zwei Hauptkompressionsphasen: eine wichtigere frühvarskische Phase (350–330 mJ) und eine spätvariskische Phase (265–260 mJ). Es ist aber deutlich, daß der variskische interkontinentale Faltengürtel der Zentralanden einfach symmetrisch aufgebaut ist und von subvertikalen Strukturen bestimmt wird. Die Metamorphose ist epizonal mit Ausnahme von sehr begrenzten Gebieten, wo die syntektonischen Granite mit einer mezobis catazonalen Druckmetamorphosen verbunden sind.In der vorliegenden Arbeit wird ein geodynamisches Entwicklungsmodell für den südamerikanischen variskischen Faltengürtel vorgeschlagen, das im Sinne einer Globaltektonik die Faltung mit Blocktektonik erklärt.
, . - . , 8000–15000 . , , . , - . . : (350-330 ) (265–260 ). , , , . — , — . - , .相似文献
126.
Annie Michard-Vitrac Joël Lancelot Claude J. Allègre Stephen Moorbath 《Earth and Planetary Science Letters》1977,36(3):449-451
Using the single zircon technique, two areas which give some of the oldest ages on earth have been investigated, namely the Minnesota River Valley and West Greenland.The results on single zircons from the Minnesota River Valley and from the Amîtsoq gneisses (West Greenland) do not differ significantly from the results of Farhat (1975) and Baadsgaard (1973) at about 3.3 and 3.65 b.y., respectively.The U-Pb analyses of single zircons from acid boulders from the conglomerate unit at Isua (West Greenland) yield the oldest age so far reported for a terrestrial rock, namely 3.824?0.009+0.012 b.y. 相似文献
127.
We present an aeromagnetic survey of the Gulf of Valencia and the Balearic Islands (western Mediterranean). A total field anomaly map and a map of the anomalies reduced to the pole have been obtained. From these maps, it is apparent that there are two regions of opposing magnetic style: the Balearic archipelago which is magnetically very smooth, and the north Balearic basin (or Gulf of Valencia) where anomalies are in places very intense. From a comparison of these two domains, we conclude that the Valencian basin was created during an extensional tectonic phase. 相似文献
128.
We have analyzed the nickel isotopic composition of meteoritic materials by high-precision mass spectrometry. The samples analyzed include almost all meteorite types for which large isotopic anomalies have been reported for oxygen, silver, magnesium and titanium. These samples are C1, C3, L, LL, H and E chondrites, IVB irons, Eagle Station pallasite and inclusion, matrix and “whole rock” samples of the Allende meteorite. The result is that we have not found any anomaly for nickel isotopic compositions within our accuracy of 0.7‰ for61Ni/60Ni, 0.4-0.08‰ for62Ni/60Ni and 1–1.5‰ for64Ni/60Ni. 相似文献
129.
Gérard Manhes Claude J. Allègre Bernard Dupré Bruno Hamelin 《Earth and Planetary Science Letters》1980,47(3):370-382
In order to explore mantle heterogeneity through geological time, Pb isotopic compositions have been determined for 8 differentiated-layered intrusions whose ages are between 2.7 and 0.05 b.y.. The Pb-Pb ages of these intrusions and the U-Pb characteristics of their parent sources are discussed.The Pb-Pb dating method is found to be applicable for this type of basic or ultrabasic rocks and agrees satisfactorily with other available methods. Significant differences are found between the calculated values for the parent bodies of these intrusions. This could reflect either mantle heterogeneity since Archean times, or contamination of some of the bodies by continental crust. Discrimination between these hypotheses can be proposed from the positions of the initial Sr ratios of these massives in respect to the supposed “terrestrial” evolution line. The two intrusions which plot on that closed system Sr evolution line (Muntsche Tundra, U.S.S.R., and Skaergaard, Greenland) belong also to a simple two-stage evolution model for Pb, with a low first-stage μ value of 7.8. If these two bodies are considered as pieces of a “primitive” closed-system mantle, a4.55 ± 0.01 age of the earth can be calculated from their Pb initial ratios. 相似文献
130.