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121.
The 79 ad Plinian eruption of Vesuvius produced first a white pumice fallout from a high steady eruptive column, and then a grey pumice
fallout originating from an oscillatory eruptive column with several partial column collapse events after which there was
a total column collapse. This first total collapse was followed by renewed Plinian activity and produced the last grey pumice
(GP) fallout deposit of the eruption. Textural characteristics (vesicularity and microcrystallinity) of a complete sequence
of the pumice fallout deposits are presented along with the major element compositions and residual volatile contents (H2O, Cl) to constrain the degassing processes and the eruptive dynamics. Large variations in residual volatile contents exist
between the different eruptive units. Textural features also strongly differ between white and grey pumices, but also within
the grey pumices. The degassing processes were thus highly heterogeneous. We propose a new model of the 79 ad eruption in which pre-eruptive conditions (H2O saturation, magma temperature and viscosity) are the critical controls on the diversity of the syn-eruptive degassing processes
and hence the eruptive dynamics. Cl contents measured in melt inclusions show that only the white pumice and the upper part
of the grey pumice magma were H2O saturated prior to eruption. The white pumice eruptive units represent a typical closed-system degassing evolution, whereas
the first grey pumice one, stored under similar pre-eruptive saturation conditions, follows a particular open-system degassing
evolution. We suggest that the oscillatory regime that dominated the grey pumice eruptive phase is linked to pre-eruptive
water undersaturation of most of the grey magma, and the associated time delays necessary for H2O exsolution. We also suggest that the high residual H2O content of the last grey pumice, deposited after the renewal of Plinian activity following the first total column collapse
event, is due to syn-eruptive saturation of GP magma and reduced H2O exsolution efficiency resulting from speciation of dissolved H2O in the melt. 相似文献
122.
The Transfiguration Cu–Pb–Zn–Ag deposit, enclosed within reduced grey sandstone, is associated with continental red beds of
the Lower Silurian Robitaille Formation in the Quebec Appalachians, Canada. The Robitaille Formation rests unconformably on
foliated Cambro-Ordovician rocks. The unconformity is locally cut by barite veins. The basal unit of the Robitaille Formation
comprises green wacke and pebble conglomerate, which locally contain calcite nodules. The latter have microstructures characteristic
of alpha-type calcretes, such as “floating” fabrics, calcite-filled fractures (crystallaria) and circumgranular cracks. Massive,
grey sandstone overlies the basal green wacke and pebble conglomerate unit, which is overlain, in turn, by red, fine-grained
sandstone. Mineralisation occurred underneath the red sandstone unit, chiefly in the grey sandstone unit, as disseminated
and veinlet sulphides. Chalcopyrite, the most abundant Cu sulphide, replaced early pyrite. Calcrete, disseminated carbonate
and vein carbonate have stable isotope ratios varying from −7.5‰ to −1.1‰ δ13C and from 14.7‰ to 21.3‰ δ18O. The negative δ13C values indicate the oxidation of organic matter in a continental environment. Sulphur isotope ratios for pyrite, chalcopyrite
and galena vary from −19‰ to 25‰ δ34S, as measured on mineral concentrates by a conventional SO2 technique. Laser-assisted microanalyses (by fluorination) of S isotopes in pyrite show an analogous range in δ34S values, from −21‰ to 25‰. Negative and positive δ34S values are compatible with bacterial sulphate reduction (BSR) in systems open and closed with respect to sulphate. We interpret
similarly high δ34S values for sulphide concentrates (25.1‰) and for vein barite (26.2‰) to result from rapid and complete thermochemical reduction
of pore-water sulphate. Two early to late diagenetic stages of mineralisation best explain the origin of the Transfiguration
deposit. The first stage was characterised by the ponding of groundwater over the Taconian unconformity, recorded by calcrete
and early pyrite formation via BSR in grey sandstone. Early pyrite contains up to 2 wt.% Pb, which is consistent with Pb fixation
by sulphate-reducing bacteria. The second stage (II) is defined by the replacement of early pyrite by chalcopyrite, as well
as by sulphide precipitation via either BSR or thermochemical sulphate reduction (TSR) in grey sandstone. This event resulted
from the synsedimentary fault-controlled percolation and mixing of (1) an oxidising, sulphate-bearing cupriferous fluid migrating
per descensum from the red-bed sequence and (2) a hydrocarbon-bearing fluid migrating per ascensum from the Cambro-Ordovician basement. Mixing between the two fluids led to sulphate reduction, causing Cu sulphide precipitation.
The positive correlation between Cu and Fe3+/Fe2+ bulk rock values suggests that Fe acted as a redox agent during sulphate reduction. Stage II diagenetic fluid migration is
tentatively attributed to the Late Silurian Salinic extensional event. 相似文献
123.
Pierre Gioan Josephine Rasendrasoa Jacques-Marie Bardintzeff Georges Rasamimanana 《Journal of African Earth Sciences》1996,22(4):597-608
New petrological and geochemical data are presented on basic igneous rocks from the south of the Morondova basin located in the southwestern part of Madagascar. The structural setting is well defined and, with the aid of a schematic map, the spatial relationships between the magmatic centres and major faults can be deduced from seismic data. On the other hand, new petrographic and geochemical data enable the different types of basaltic magmatism manifested in this area to be distinguished. It appears that tholeiitic magmatism evolved towards transitional magmatism from the south to the north of the Morondava basin. This geochemical evolution of basaltic magmatism can be linked to the initiation of rifting between Madagascar and Africa along the Mozambique channel and the length of the Davie ridge associated with the opening of the Indian ocean. These observations, together with structural data and recently obtained geochronological data, contribute to a better understanding of the mechanism and evolution of Madagascan rifting. 相似文献
124.
Benoît Villemant Georges Boudon J.-C. Komorowski 《Earth and Planetary Science Letters》1996,140(1-4):259-267
Volcanic rocks from subduction zones are widely believed to originate by partial melting of mantle lherzolite modified by the addition of a fluid or melt extracted from the down-going slab. U-series disequilibrium in such magmas is commonly attributed to this particular melting process. A detailed study of U-series isotopes in the 650 y. B.P. eruptive sequence of Mt. Pelée (Martinique) shows that plinian products are in radioactive equilibrium, whereas dome-forming products of the same eruption are characterized by 238U-230Th disequilibrium. The same features apply to other plinian and dome-forming products of this volcano and systematically correspond to different eruptive styles. We attribute these characteristics to variable superficial interaction of magmas with the hydrothermal system during the final stages of eruption rather than to deep magma genesis processes. This conclusion might be generally applicable to arc magmas. 相似文献
125.
126.
Résumé On rencontre souvent des produits de fonctions de Legendre en géodésie mathématique, en mécanique céleste et quantique, et en géophysique. Nous donnons les expressions générales des coefficients de la décomposition d'un produit quelconque sur la base des fonctions de Legendre ellesmêmes. Quelques applications immédiates sont envisagées. 相似文献
127.
128.
129.
The precambrian postorogenic pluton of the Fort-Trinquet area (Northern Mauretania) is composed by a series of granitic rocks in which amphiboles are the characteristic mafic minerals. Twenty six amphiboles have been separated and chemically analysed; the optical properties and the unit-cell data are also given.The crystallographic and chemical differences between these minerals reflect the variations in bulk composition of the host-rocks. In the plutonic suite, two igneous trends have been recognized: 1) a granitic trend (quartz monzonite-adamellite-granite) and 2) a syenitic trend (syenite-quartz bearing syenite-alkali granite). The amphiboles of the first trend belong to the tremolite-hastingsite series; they become richer in hastingsite mole with increasing proportion of quartz and decreasing Mg/(Mg+Fe) ratio in the rocks. The granitoids of the second trend are characterized by the occurrence of two amphiboles: a primary prismatic green-coloured actinolitic hornblende generally surrounded by a dark blue rim of riebeckite composition; the riebeckite may also form some acicular crystals associated to needles of stilpnomelane. These coexisting amphiboles would result from autometasomatic reactions which affected the rocks of the syenitic trend and which gave rise to the late alkali pegmatites where the constituent is a low arfvedsonitic riebeckite. The major substitutions involved in that amphibole transformation are Nax R3+Cax R2+ and NaxSiCaxAlIV.Comparison with experimental data allows to estimate the physical conditions during the emplacement and the tardimagmatic evolution of this granitoid series. 相似文献
130.
Using definite examples the authors show that it is possible to find, in a given granite, relations between uranium and the other elements, but these relations cannot be generalized. They cannot depend on the petrographical and mineralogical properties rather than on the chemical composition. One finds with the essential minerals some apparent relations but they are valid only for a given rock. The nature of a granite depends, mainly, on its formation and consequently on the original material from which it derived. The present data on radioactivity of granites cannot be generalized. 相似文献