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31.
32.
D. R. Nelson A. J. Crawford M. T. McCulloch 《Contributions to Mineralogy and Petrology》1984,88(1-2):164-172
Rocks with boninitic affinities have been recognised in a number of ophiolites, including the Cambrian Heathcote and Mt Wellington Greenstone Belts of Victoria. Boninites and high-Mg andesites from the Heathcote Greenstone Belt show a restricted range of initial
Nd values of between +3.3 to +5.8. Extremely refractory boninites from the Mt Wellington Greenstone Belt have
Nd ranging from +1.3 to –9. Ti/Zr is positively correlated with Sm/Nd with the Heathcote lavas generally possessing greater depletion of Ti and enrichment of Zr relative to the middle and heavy REE with increasing LREE/HREE. These data are consistent with the generation of boninites by partial melting of refractory peridotite following invasion by LREE- and Zr-enriched, low
Nd fluids. Tholeiites overlying the boninites in both greenstone belts have flat REE patterns and
Nd+5, lower than that anticipated for lavas derived from depleted MORB source reservoirs in the Cambrian, suggesting that their source was also contaminated by a LREE-enriched, low
Nd component similar to that involved in the generation of the Howqua boninites. The added components have characteristics compatible with their derivation from subducted altered oceanic crust and/or from wet subducted sediments. The identification of boninites and other low-Ti lavas in the Victorian greenstone belts is strong evidence for island arc development in southeastern Australia during the Lower Cambrian and provides further support for a subduction-related origin for many ophiolites. 相似文献
33.
Major and trace element data for a sequence of peralkaline silicic lavas and pyroclastic flows, exposed in the caldera wall of the Paisano volcano, west Texas, document systematic fractional crystallization during magmatic evolution and an open system, magma mixing event in the upper parts of the sequence. Stratigraphically lowest flows are comendite and comenditic quartz trachyte lavas and ash flow tufts. Overlying these units is a trachyte with compositional, textural and mineralogical features indicating that it is the product of magma-mixing; similar flows occur in other parts of the volcano at the same stratigraphic level. This composite trachyte is considered to be a mixture of mugearitic or mafic trachytic magma, derived from a similar source region which yielded the earlier caldera wall flows. Trace element concentrations of the post-trachyte comenditic quartz trachyte lavas suggest they were erupted from a chamber whose magma was diluted by an influx of mugearitic or mafic trachytic magma during a magma mixing event.Rayleigh fractionation calculations show that the comendites and comenditic quartz trachytes can be derived from a parental mugearite magma by 88% to 93% fractionation of dominantly plagioclase and alkali feldspar, with lesser amounts of clinopyroxene, magnetite and apatite. Zircon was not a significant fractionating phase. The composition, mineralogy and depth of the source region(s) which generated these magmas cannot be constrained from the present data set. 相似文献
34.
Stratigraphic and chemical data from the ice core of an ‘anthropogenic palsa’ at Toolik Lake, Alaska, indicate that the mound formed as a consequence of hydrostatic pressure developed in an isolated hydrologic system within the active layer. Survey data for five palsas over a three-year period suggest that growth was essentially complete at the time of the initial survey; a net decrease of summit elevation is apparent in all five mounds, but complete degradation of the palsas would require several decades at observed rates. Because accurate field measurements of thermal and hydrologic evolution in such features are extremely difficult, simulation of the environmental conditions and events involved in palsa growth is an important supplement to field observation. Both analytic and finite-element models yield results that are in substantial agreement with inferences drawn from observational data. 相似文献
35.
36.
The coal fields of the Unites States can be divided into six major provinces. The Appalachian and Interior Provinces contain dominantly bituminous coal in strata of Pennsylvanian age. The coal seams are relatively thin and are mined both by surface and underground methods. Sulfyur content is low to moderate in the Appalachian Province, generally high in the Interior province. The Gulf Coastal Plain Province, in Texas and neighboring states, contains lignite of Eocene age. The seams are 3–25 ft (0.9–7.5 m) thick and are minded in large open pits. The Northern Great Plains Province has lignite and subbituminous coal of Cretaceous, Paleocene and Eocene age. The coal, largely very low in sulfur, occurs in beds up to 100 ft (30 m) thick and is strip-mined. The Rocky Mountain Province contains a great variety of coal deposits in numerous separate intermontane basins. Most of it is low-sulfur subbituminous to bituminous coal iof Creatceous and early Tertiary age. The seams range from a few feet to over 100 ft (30 m) thick. Strip-mining dominates but underground mines are important in Utah and Colorado. The Pacific Coast Province, which includes Alaska, contains enormous cola resources but has seen little mining. The coal is highly diverse in physical character and geologic setting. 相似文献
37.
38.
Vertical distributions of particulate silica, and of production and dissolution rates of biogenic silica, were determined on two N-S transects across the Pacific sector of the Antarctic Circumpolar Current during the austral spring of 1978. Particulate silica profiles showed elevated levels in surface water and near the bottom, with low (35–110 nmol Si · 1?1) and vertically uniform values through the intervening water column. Both the particulate silica content of the upper 200 m and the production rate of biogenic silica in the photic zone increased from north to south, reaching their highest values near the edge of the receding pack ice. A significant, but variable, fraction (18–58%) of the biogenic silica produced in the surface layer was redissolving in the upper 90–98 m. Net production of biogenic silica in the surface layer (production minus dissolution) was proceeding at a mean rate of ca. 2 mmol Si · m?2 · day?1. This is ca. 4 times greater than the most recent estimate of the mean accumulation rate of siliceous sediments beneath the ACC. We estimate, based on mass balance, that the mean dissolution rate of biogenic silica in subsurface water column in the Southern Ocean is 1.2–2.9 mmol Si · m?2 · day?1. 相似文献
39.
Carlos M. Palacios Nelson S. Guerra Patricio B. Campano 《International Journal of Earth Sciences》1983,72(2):733-738
Ti, Zr, Y and P, known to be chemically immobile elements during alteration and metamorphism, have been ploted in a set of diagrams in order to discriminative between calc-alkaline andesites from island arcs and continental margin. This method may be an appropiate procedure to help in the discrimination between ancient volcanics of both groups. 相似文献
40.
Polycyclic aromatic hydrocarbons and their molecular diagnostic ratios in urban atmospheric respirable particulate matter 总被引:2,自引:5,他引:2
María del Rosario Sienra Nelson G. Rosazza Margarita Prndez 《Atmospheric Research》2005,75(4):267-281
Atmospheric concentrations of polycyclic aromatic hydrocarbons (PAHs) in Santiago de Chile city were evaluated to study particulate PAHs profiles during cold and spring weather periods. Urban atmospheric particulate matter PM10 was collected using High Volume PM10 samplers. Fifteen samples of 24 h during austral winter and 20 samples of 24 h during spring, 2000 were collected at two sampling sites (North–East and Central areas of the city) whose characteristics were representative of the prevailing conditions. Seventeen PAHs were quantified and total PAHs concentration ranged from 1.39 to 59.98 ng m−3, with a seasonal variation (winter vs. spring ratio) from 0.5 to 12.6 ng m−3. Molecular diagnostic ratios were used to characterize and identify PAHs emission sources such as combustion and biogenic emissions. Results showed that the major sources of respirable organic aerosol PM10 in Santiago are mobile and stationary ones. 相似文献