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51.
BRIAN J. TODD PAGE C. VALENTINE ODDVAR LONGVA JOHN SHAW 《Boreas: An International Journal of Quaternary Research》2007,36(2):148-169
The extent and behaviour of the southeast margin of the Laurentide Ice Sheet in Atlantic Canada is of significance in the study of Late Wisconsinan ice sheet-ocean interactions. Multibeam sonar imagery of subglacial, ice-marginal and glaciomarine landforms on German Bank, Scotian Shelf, provides evidence of the pattern of glacial-dynamic events in the eastern Gulf of Maine. Northwest-southeast trending drumlins and megaflutes dominate northern German Bank. On southern German Bank, megaflutes of thin glacial deposits create a distinct northwest-southeast grain. Lobate regional moraines (>10km long) are concave to the northwest, up-ice direction and strike southwest-northeast, normal to the direction of ice flow. Ubiquitous, overlying De Geer moraines (<10 km long) also strike southwest-northeast. The mapped pattern of moraines implies that, shortly after the last maximum glaciation, the tidewater ice sheet began to retreat north from German Bank, forming De Geer moraines at the grounding line with at least one glacial re-advance during the general retreat. The results indicate that the Laurentide Ice Sheet extended onto the continental shelf. 相似文献
52.
A simulation model of alluvial stratigraphy 总被引:8,自引:0,他引:8
The quantitative model presented simulates the development of a two-dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter-related effects which influence the distribution of channel-belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel-belt deposits decrease with increasing floodplain width/channel-belt size, mean avulsion period, and channel-belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half-graben). Then channel-belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine-grained alluvium to accumulate on the inactive side. Individual channel-belt deposits thicken during aggradation, although a self-regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel-belt deposits of avulsive origin. 相似文献
53.
The endolithic lichen Lecidea auriculata is known to enhance rock surface weathering on the Little Ice Age moraines of the glacier Storbreen in Jotunheimen, central southern Norway. This study demonstrates the reduction in Schmidt hammer Rvalues that followed the rapid colonization by this lichen of pyroxene‐granulite boulders on terrain deglaciated over the last 88 years. In the absence of this lichen, the characteristic mean R‐value of boulder surfaces is 61.0 ± 0.3; where this lichen is present, R‐values are lower by at least 20 units on surfaces exposed for 30–40 years. A similar reduction in rock hardness on rock surfaces without a lichen cover requires about 10 ka. The rapid initial weakening of the rock surfaces is indicative of rates of biological weathering by endolithic lichens that may be two orders of magnitude (200–300 times) faster than rates of physico‐chemical weathering alone. If not avoided, the effects of this type of lichen are likely to negate the effectiveness of the Schmidt hammer and other methods for exposure‐age dating, including cosmogenic‐nuclide dating, in severe alpine and polar periglacial environments. The results also suggest a new method for dating rock surfaces exposed for <50 years. 相似文献
54.
Temporal Evolution of Boron Flux in the NE Japan and Izu Arcs Measured by Ion Microprobe from the Forearc Tephra Record 总被引:1,自引:0,他引:1
CLIFT PETER D.; LAYNE GRAHAM D.; NAJMAN YANI M. R.; KOPF ACHIM; SHIMIZU NOBUMICHI; HUNT JOHN 《Journal of Petrology》2003,44(7):1211-1236
The enrichment of boron relative to similarly incompatible elements,such as Be, in arc volcanic rocks has been used as a proxy forthe involvement of slab flux in petrogenesis. New ion microprobeanalyses of single glass shards in tephra layers recovered bythe Ocean Drilling Program (ODP) in the Izu and NE Japan forearcbasins now allow the temporal variation in slab flux to be chartedsince 7 and 5 Ma, respectively. B/Be ratios are typically <70in NE Japan and <130 in Izu, with no single grain exceeding200. Although moderate to high for modern arcs, these valuesare much less than those recorded in the Marianas and Tongaat 34 Ma, shortly after the start of rifting of theirback-arc basins. This observation suggests that the peak B/Bevalues seen in Tonga and the Marianas are related to the tectonicsof slab roll-back and basin opening, rather than changes inthe dynamics of the Pacific Plate. There is no temporal trendto enrichment in the high field strength elements (HFSE) orrare earth elements (REE) in either Izu or NE Japan since 7Ma, although the two elemental groups do show clear positivecorrelation. A lack of correlation between REE, HFSE and B/Besuggests that slab flux is not the only control on melting inthese arcs. 相似文献
55.
The Geochemistry and Experimental Petrology of Sodic Alkaline Basalts from Oatlands, Tasmania 总被引:2,自引:2,他引:2
Sodic basalts of Oligocene-Early Micene age occur within anEarly Tertiary graben in the Oatlands district of Tasmania.They include olivine tholeiites, alkali olivine basalts, basanites,transitional nephelinites, nepheline hawaiites, and nephelinemugearites. They have compositional characteristics in commonwith sodic alkaline basalt suites from other parts of the world.With decreasing SiO2, concentrations of CaO, alkalis, P2O5,and incompatible trace elements increase. Compositional and experimental data for the basalts are consistentwith their derivation by polybaric partial melting of a garnetlherzolite source enriched in P2O5, light rare earths, Nb, andother incompatible trace elements. Experimental data for a primitivenepheline basanite from the Oatlands district indicate thatconcentrations of H2O+CO2 need not have been more than 6?5 wt.%for the original basanite magma to have derived from an amphibole-bearinggarnet lherzolite source. In the case of more SiO2-undersaturatedolivine melilitites from the neighbouring Central Plateau, theexperimental evidence is consistent with either higher concentrationsof H2O+CO2 (approaching 14 wt.%), or higher pressures of origin(>35 kb). Petrographic and geochemical evidence suggeststhat the latter is the more probable of the alternatives. 相似文献
56.
The Significance of Multiple Saturation Points in the Context of Polybaric Near-fractional Melting 总被引:1,自引:0,他引:1
Experimental petrologists have successfully located basalticliquid compositions parental to mid-ocean ridge basalt thatare, within experimental resolution, multiply saturated withthree-phase harzburgite or four-phase lherzolite assemblageson their liquidus at some elevated pressure. Such an experimentalresult is a necessary consequence of any paradigm in which eruptedbasalts derive from single-batch primary liquids that equilibratewith a mantle residue and undergo no subsequent magma mixingbefore differentiation and eruption. Here we investigate whether,conversely, such evidence of multiple saturation is sufficientto exclude dynamic melting models wherein increments of meltare mixed after segregation from residues, during melt transportor in magma chambers. Using two independent models of crystalliquidequilibria to simulate polybaric near-fractional peridotitemelting, we find that aggregate liquids from such melting processescan display near-intersections of liquidus surfaces too closeto distinguish experimentally from exact multiple saturationpoints. Given uncertainties in glass compositions, fractionationcorrections, experimental temperature and pressure conditions,and achievement of equilibrium, these results suggest that polybaricmixtures can in fact masquerade as mantle-equilibrated single-batchprimary liquids. Multiple saturation points on the liquidussurfaces of primitive basalts do, however, preserve informationabout the average pressure of extraction of their constituentincrements of liquid. KEY WORDS: mantle melting; basaltic volcanism; experimental igneous petrology; thermodynamic modelling; inverse method 相似文献
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