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981.
Bark samples isolated from brown coal deposits in Victoria, Australia, and buried wood from Rhizophora mangle have been studies by high-resolution solid-state nuclear magnetic resonance (NMR) techniques. Dipolar dephasing 13C NMR appears to be a useful method of detecting the presence of tannins in geochemical samples including barks, buried woods, peats and leaf litter. It is shown that tannins are selectively preserved in bark during coalification to the brown coal stage. 相似文献
982.
The method of Koiwa and Ishioka (Philos Mag A 47:927–938, 1983) is used, with slight modification, to evaluate the correlation factor for vacancy-mediated diffusion of impurity atoms on the sublattice of dodecahedral sites in garnet, as a function of the relevant vacancy-jump frequencies. The required values of the lattice Green’s function were obtained from multiple Monte Carlo simulations in lattices of progressively larger size, extrapolated to an infinite lattice using a model that linearizes the dependence of the functional value on lattice size. As Online Resources, codes are provided that permit evaluation of the correlation factor for any chosen set of vacancy-jump frequencies, for implementation in either Mathematica ® or Matlab ®. 相似文献
983.
984.
Alan J. Wilson David R. Cooke Benjamin J. Harper Cari L. Deyell 《Mineralium Deposita》2007,42(5):465-487
The alkalic porphyry gold–copper deposits of the Cadia district occur in the eastern Lachlan Fold Belt of New South Wales,
Australia. The district comprises four porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill, and Cadia East) and two iron–copper–gold
skarn deposits (Big Cadia and Little Cadia). Almost 1,000 tonnes of contained gold and more than four million tonnes of copper
have been discovered in these systems, making Cadia the world’s largest known alkalic porphyry district, in terms of contained
gold. Porphyry gold–copper ore at Cadia is associated with quartz monzonite intrusive complexes, and is hosted by central
stockwork and sheeted quartz–sulfide–(carbonate) vein systems. The Cadia porphyry deposits are characterized by cores of potassic
and/or calc–potassic alteration assemblages, and peripheral halos of propylitic alteration, with late-stage phyllic alteration
mostly restricted to fault zones. Hematite dusting is an important component of the propylitic alteration assemblage, and
has produced a distinctive reddening of feldspar minerals in the volcanic wall rocks around the mineralized centers. Sulfide
mineralization is strongly zoned at Ridgeway and Cadia East, with bornite-rich cores surrounded by chalcopyrite-rich halos
and peripheral zones of pyrite mineralization. The Cadia Hill and Cadia Quarry deposits have chalcopyrite-rich cores and pyrite-rich
halos, and Cadia Hill contains a high-level bornite-rich zone. Distinctive sulfur isotopic zonation patterns have been identified
at Ridgeway, Cadia Hill, and Cadia East. The deposit cores are characterized by low δ34Ssulfide values (−10 to −4‰), consistent with sulfide precipitation from an oxidized (sulfate-predominant) magmatic fluid at 450 to
400°C. Pyrite grains that occur in the propylitic alteration halos typically have δ34Ssulfide values near 0‰. There is a gradual increase in δ34Ssulfide values outwards from the deposit cores through the propylitic halos. Water–rock interaction during propylitic alteration
caused magmatic sulfate reduction and concomitant oxidation of ferrous iron-bearing minerals, resulting in enrichment of 34S in pyrite and also producing the distinctive reddened, hematite-rich alteration halos to the Cadia deposits. These results
show that sulfur isotope analyses have potential applications in the exploration of alkalic porphyry-style deposits, with
zones of depleted δ34Ssulfide values most prospective for high-grade mineralization. 相似文献
985.
986.
The 1886 Plinian eruption of Tarawera, New Zealand, is a unique basaltic fissure-fed eruption with exceptionally well preserved
fall deposits to within 200 meters of the source vents. These proximal deposits form a series of spatter/cinder half-cones
along the northeastern 8-km-long segment of the 1886 fissure. Here we examine these deposits using grain size and clast componentry
techniques. We contrast the products of the phreatomagmatic (phases I and III) and Plinian (Phase II) stages of the eruption
and examine deposit variability as a function of contrasting eruptive intensity within the climactic phase (II) of the eruption.
The opening phreatomagmatic phase I of the eruption involved gas-rich magma interacting with water and fragmenting at least
300 meters below the surface. The deposits of the climactic phase that followed have relatively uniform grain size but marked
contrasts in the relative abundance of juvenile and wall rock (lithic) clasts. Deposits linked to vents associated with the
high Plinian plume are more uniform than those characterized by a weaker cone-forming eruption style. During the third, and
closing, phase of the eruption, magma withdrawal accompanied the onset of decoupling of the exsolved gas phase, leading to
fragmentation at increasingly greater depths and significant wall rock collapse into the erupting vents. Variability in eruptive
style during phase II along the fissure appears to be a function of shallow seated controls, in particular the variable extent
of incorporation of lithic wall rock into the erupting jet, as a consequence of vent wall collapse. Widely dispersed beds
centralized around Plinian sources along the fissure have very low lithic content; cone-forming beds at other craters that
contain very high lithic contents. This incorporation led to a significant reduction of the velocity and stability of the
jet at these latter steep-walled craters, and induced episodicity in the form of vent-clearing explosions. The result is a
large reduction of the physical and thermal ability of these vents to contribute to a stable high eruptive plume. Instead
large volumes of ejecta were sedimented prematurely from shallow heights at rates an order of magnitude greater than for historical
Strombolian, Hawaiian and subPlinian eruptions. This study illustrates that sustained powerful Plinian eruptions can be accompanied
by heterogeneities and instabilities of the eruptive jet. At Tarawera, the record of complex proximal transport and deposition
processes in the eruptive jet cannot be inferred from the eruption products at distances greater than 400 m from the eruptive
fissure. We suggest that study of ultraproximal deposits, as seen at Mt Tarawera, provides the only opportunity to document
the complex, dynamic behavior of the jet region of Plinian eruptions. 相似文献
987.
988.
Red mud is a waste product in the reduction of bauxite in the production of aluminium. The exact composition of red mud depends on the source of the bauxite and its treatment during production. This variation has been responsible for conflicting reports about the damage caused by dumping red mud at sea. 相似文献
989.
990.