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111.
Pumice samples from Fukutoku-oka-no-ba in the Izu–Bonin – Mariana (IBM) arc were analysed for 40 trace elements and Sr, Nd, and Pb isotopic compositions. These samples are shoshonites (59.4–61.8 wt% SiO2 ), characterized by high contents of K2 O (3.74–4.64 wt%), Ba (1274–1540 p.p.m.), Rb (91–105 p.p.m.), and light rare earth elements. The characteristics of alkali-element enrichment are similar to those of other parts of the Alkalic Volcano Province (AVP) in the northern Mariana and southernmost Volcano arcs. Sr (87 Sr/86 Sr = 0.7036–0.7038) and Pb isotopic compositions (206 Pb/204 Pb = 19.08–19.11, 207 Pb/204 Pb = 15.62–15.63, 208 Pb/204 Pb = 38.85–38.91) of Fukutoku-oka-no-ba pumice are relatively radiogenic, whereas Nd is unradiogenic (143 Nd/144 Nd = 0.51283–0.51286). Fukutoku-oka-no-ba is isotopically distinct from Iwo Jima and is similar to the Hiyoshi Volcanic Complex, suggesting that Fukutoku-oka-no-ba might have a magma source similar to that of the Hiyoshi volcanic complex. Plots of Pb and Nd isotopes for AVP lavas trend toward the fields of ocean island basalt (OIB) source and pelagic sediments, which are possible sources of AVP enrichments. 相似文献
112.
ROBERT W. DALRYMPLE 《Sedimentology》1984,31(3):365-382
Intertidal sandwaves in the Minas Basin and Cobequid Bay, Bay of Fundy, occur under a wide range of conditions (mean grain size 0.274-1.275 mm; velocity strength index (V1)0.46-3.34; and velocity symmetry index (V2) 0.011-0.294), and they vary from symmetrical to strongly asymmetrical in cross-section. Heights and wavelengths average 0.81 and 37.9 m respectively. They are straight to weakly sinuous and laterally continuous in plan, occasionally show crestal branching reminiscent of wave ripples, and are commonly skewed relative to the strongest currents because of differential migration rates along their length. The average migration rate is 0.11 m/tidal cycle. Megaripples occur on each sandwave crest, at least during spring tides, but the areal extent, sinuosity and size of the megaripples increases as the dominant current speed increases. The megaripples have heights averaging 24% of the sandwave height, are oriented perpendicular to the fastest dominant currents, and have life spans of several tidal cycles. They are believed to be in quasi-equilibrium with the sandwaves and play a key role in sandwave dynamics and internal structure formation: periods of lee face steepening and rapid forward migration (megaripple crest at sandwave brink) alternate with times of non-deposition or erosion and slowed or reversed migration (trough at brink). Dominant-current cross-bedding predominates in the two intergradational varieties of translation structure observed: Inclined Cross-Bedding—decimetre-scale cross-beds separated by gently inclined (9° average) erosional surfaces; and Large-Scale Foresets—cross-beds with thicknesses greater than half the sandwave height, interrupted by weakly erosional to conformable discontinuity surfaces. These are overlain by a vertical growth or repair structure, Complex Cross-Bedded Cosets, that consists of nearly equal volumes of dominant- and subordinate-current cross-beds stacked without a preferred set-boundary dip. The translation structures correspond well to forms predicted by Allen (1980a, fig. 8) but the inclined set boundaries and discontinuity surfaces (master bedding planes) are produced by megaripple troughs rather than by current reversals. Consequently, Allen's regime diagram is unable to predict structure occurrences. The repair structures suggest that ‘curvature-related mass-transport’ (Allen, 1980a, b) is important in tidal sandwave maintenance, although it is not necessarily responsible for sandwave initiation. 相似文献
113.
Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan 总被引:12,自引:1,他引:11
TAYLOR REX. N.; NESBITT ROBERT W.; VIDAL PHILLIPE; HARMON RUSSELL S.; AUVRAY BERNARD; CROUDACE IAN W. 《Journal of Petrology》1994,35(3):577-617
The Bonin archipelago represents an uplifted fore-arc terrainwhich exposes the products of Eocene supra-subduction zone magmatism.Chichijima, at the centre of the chain, represents the typelocality for the high-Mg andesitic lava termed boninite. Therange of extrusives which constitute the boninite series volcanicsare present on Chichijima, and are disposed in the sequenceboninite-andesite-dacite with increasing height in the volcano-stratigraphy.Progression to evolved compositions within the Chichijima boniniteseries is controlled by crystal fractionation from a boniniteparental magma containing 15% MgO. Olivine and clinoenstatiteare the initial liquidus phases, but extraction of enstatiticorthopyroxene, followed by clinopyroxene and plagioclase, isresponsible for the general evolution from boninite, throughandesite, to dacite. Some andesites within the overlying MikazukiyamaFormation are petrographically distinct from the main boniniteseries in containing magnetite phenocrysts and a high proportionof plagioclase. As such, these andesites have affinities withthe calc-alkaline series. Major and trace element data for 74 boninitic series rocks fromChichijima are presented. Although major element variation isdominantly controlled by high-level crystal fractionation, thelarge variations in incompatiable trace element concentrationsat high MgO compositions cannot be explained by this mechanism.Nd, Pb, and Sr isotopic data indicate the following: (1) a strongoverprint on 87Sr/86Sr by seawater alteration; (2) Pb isotopeslie above the northern hemisphere reference line (NHRL) andare thus similar to the <30-Ma are and basin lavas of theIzuBonin system, and (3) Nd(40 Ma) ranges between 2.8and 6.8 within the boninite series volcanics. Differences inrare-earth elements (REE), Zr, Ti, and 143Nd/144Nd at similardegrees of fractionation can be explained by the addition ofa component of fixed composition from the down-going oceaniccrustal slab to a variably depleted source region within theoverlying wedge. Data presented for Sm/Zr and Ti/Zr indicatethat boninite series volcanics are characterized by low valuesfor both of these ratios. In particular, boninites appear tohave uniquely low Sm/Zr ratios. These characteristics may bethe result of slab melting in the presence of residual amphibole;the resultant melt could combine with typical slab dehydrationfluids and infiltrate the overlying mantle wedge. Such a fluidmeltcomponent could mix either with shallow mantle or directly withprimitive melts from depleted mantle. Trace elements, REE, andisotope data thus point to a model for boninite genesis whichrequires tightly constrained pressuretemperature conditionsin the slab combined with melting of a variably depleted sourcein the overlying wedge. Such constraints are rarely met exceptduring the subduction of juvenile oceanic crust beneath a young,hot overriding plate. 相似文献
114.
ROBERT PARK 《Sedimentology》1976,23(3):379-393
The lamination found in recent stromatolites, an intrinsic feature of many of them, comprises a number of sediment-rich and organic-rich couplets. The significance of this lamination has figured in at least two avenues of research. In one is its possible influential role upon geochemical and diagenetic trends, in the other its possible calendar role. Algal growth follows a diurnal pattern but sedimentation, which is largely marine, is far less regular or predictable. The monitoring of modern algal mat surfaces has yielded much useful and accurate information concerning rates of mat accretion. Millimetre-scale lamination may be produced on a number of interacting processes but as yet no one has been able to determine with any great assurance whether it represents a daily, monthly, annual or some other regular periodic growth. The evidence from the Trucial Coast tends to show, rather, that burial, subsequent compaction, distortion and, in the case of sabkha-type sections, destruction owing to evaporite mineral growth, all tend to obliterate or at best mask any regular patterns of growth. In conclusion, one is forced to acknowledge that any computations concerning the calendar role of stromatolites are to be regarded with the utmost suspicion. 相似文献
115.
ROBERT MEYER 《Sedimentology》1976,23(2):235-253
In the eastern Paris Basin, on the boundary of two French ‘départements’, Marne and Meuse, the Cretaceous deposits begin with Wealden facies assigned to the Valanginian. This is overlain by marine Hauterivian and Lower Barremian. Several techniques have been used for studying these deposits: light-microscope, scanning electron microscope, grain-size analysis, chemical and mineralogical analysis. Environmental interpretations of the various facies are based on sedimentological, geochemical and pedological criteria (fossil soils are present). Clay minerals are dominantly illite and kaolinite. In the soil horizons kaolinite is dominant. A mixed-layer illite-smectite appears at the base of the transgression. Under marine influence a more stable illite dominant suite is formed. The Wealden sediments appear to have been deposited in a very mature flood-plain. Part of the sequence was subjected to weathering in situ by soil processes under a warm climate with repeated wetting and drying. Paraconformably above, the marine Hauterivian is the product of a shallow sea that tended to form lagoons. A true marine environment was established in the Lower Barremian, when mixing homogenized the sediments. 相似文献
116.
火山灰的提取及测试技术 总被引:6,自引:0,他引:6
同一次火山爆发产生的火山灰 ,能在广阔的区域内形成等时标志层 ,成为地层定年和对比的极有价值的工具。文中介绍了提取肉眼不可见火山灰的 4种方法 :烧失法、稀碱法、重液浮选法和酸化法 ,还介绍了火山玻璃的显微镜下识别技术及电子探针测试技术。 相似文献
117.
A key element in achieving Ocean Drilling Program Leg 183 scienceobjectives is determining the age of volcanism at differentlocations across the Kerguelen Plateau and Broken Ridge. Thispaper reports crystallization ages derived from 40Ar39Arincremental heating experiments, for whole rocks and feldsparsseparated from basement units recovered at Sites 1136, 1137,1138, 1139, 1140, 1141 and 1142. The subaerial environment oferuption at most sites and the generally evolved, high K contentof these lava flows contributed to precise and reproducibleage determinations. Volcanic activity at southern KerguelenPlateau Site 1136 occurred at 118119 Ma; at Elan BankSite 1137, 107108 Ma; at central Kerguelen Plateau Site1138, 100101 Ma; at Skiff Bank Site 1139, 6869Ma; at northern Kerguelen Plateau Site 1140, 3435 Ma;and at Broken Ridge Sites 1141 and 1142, 9495 Ma. Thenew ages allow calculation of melt production rates throughthe 相似文献
118.
Distributed hydrological models are becoming increasingly complex with respect to spatial phenomena, and with the widespread availability of spatial data from GIS, this trend is likely to increase. In all such models the spatial arrangement of phenomena, such as soil properties and land-use categories is fundamental, and so the arrangement should have an influence on the model output. Testing for this influence we term spatial sensitivity analysis. Here, we report on the spatial sensitivity of two widely used models, AgNPS (agricultural non-point source pollution model) and ANSWERS (areal nonpoint source watershed environment response simulation). The input spatial data were subjected to spatially random mixing to varying degrees, such that the organized landscape became disorganized. The chemical discharge from AgNPS, and the sediment and water discharge from ANSWERS, are examined. In both cases most outputs exhibited little or no sensitivity to the spatial distribution of most input data. Only infiltration-related inputs produced large variations, but these changes were not in the sense that might have been predicted. Although the analytical methods used require further refinement, there must now be some doubt as to the validity of the models, and whether they repay their computational complexity. Furthermore, it is felt that spatial sensitivity analysis should become a fundamental part of the verification of all such models. © 1997 by John Wiley & Sons, Ltd. 相似文献
119.
120.
ROBERT ANDRLE 《地球表面变化过程与地形》1996,21(10):955-962
Mandelbrot (Science, 1967, 156, 636–638) used the west coast of Britain as an example of a naturally occurring statistically self-similar fractal. Evidence from this study indicates that the west coast of Britain is not statistically self-similar over the range of scale of measurement, and that complexity reaches maxima at characteristic scales related to identifiable features on the coastline. A fractal analysis is conducted using the divider method, and although the resulting log–log plot of measured length against steplength appears linear, statistical tests for linearity strongly suggest that the coastline is not statistically self-similar. An angle measure technique (AMT) developed by the author to examine changes in line complexity with scale, shows that within the range of scale of measurement there are two peaks in complexity for the west coast of Britain, suggesting that two processes acting at different scales have influenced coastal development. The AMT is also used to identify differences in complexity between northern and southern sections of the coastline. Additionally, high r2 values associated with regressions of log L(G) against log G are shown to be insufficient evidence of statistical self-similarity, and apparently linear segments (fractal elements) often found in Richardson plots may contain systematic curvature revealed only by more rigorous tests for non-linearity. 相似文献