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231.
The rate of sandstone weathering in the semi-arid climate of the Hunter Valley, New South Wales, Australia has been estimated
from observations of gravestone weathering in the area. The gravestone data points to two distinct stages in the weathering
process. The first stage covering the first century of exposure is characterised by a relatively low recession rate of 0.5 mm/100 years.
This is followed by a second stage in which the rate of weathering increases sharply to ca 2.5 mm/100 years. The non-linear
nature of the weathering trends over time suggests that during the first century of exposure, structural changes took place
within the sandstone material, which lay the foundation for accelerated weathering after further exposure. Laboratory trials
were also conducted to identify the effectiveness of different weathering processes in the decay of sandstone in this region.
Of the four processes examined, only the freeze–thaw cycle produced a significant degree of mass loss and is therefore most
likely a strong contributor to the weathering of sandstone in this region. 相似文献
232.
The digitising system installed at the Chateau Observatory, in the North Island of New Zealand, to monitor Ruapehu and Ngauruhoe volcanoes is used to digitise the seismic records from three seismometers, and to calculate the power spectra of volcanic tremor in real-time, with the smoothed power spectra being recorded every 5 minutes on magnetic disk. The 5-minute averages are later processed to remove the effects of earthquakes and interference before hourly averages are calculated. This system has a better dynamic range than visual chart records, and analysis of the data requires much less labour. 相似文献
234.
Harry H. Posey J.Richard Kyle Timothy J. Jackson Stephen D. Hurst Peter E. Price 《Applied Geochemistry》1987,2(5-6)
Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported 87Sr/86Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of 87Sr/86Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of 87Sr/86Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have 87Sr/86Sr and δ34S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have 87Sr/86Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous 87Sr/86Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic 87Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic 87Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous 87Sr/86Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin. 相似文献
235.
236.
237.
R.W. Hurst D. Bridgwater K.D. Collerson G.W. Wetherill 《Earth and Planetary Science Letters》1975,27(3):393-403
Field studies in the vicinity of Saglek Bay, Labrador, demonstrated that it was possible to subdivide the Archaean gneiss complex into distinct lithologic units and erect a geologic chronology similar to that recognized in Godthaabsfjord, West Greenland. The Uivak gneisses are the oldest quartzo-feldspathic suite in the area and are distinguished from a younger gneissic suite in the field, the undifferentiated gneisses, by the presence of porphyritic basic dykes (Saglek dykes) within the Uivak gneisses. The Uivak gneisses range in composition from tonalites to granodiorites, with the two chemically distinct suites recognized: a grey granodioritic suite and an iron-rich plutonic igneous suite which locally intrudes or grades into a grey gneiss which strongly resembles the grey Uivak gneiss. Rb-Sr isotopic studies indicate an age of 3622 ± 72 m.y. (2σ) and initial Sr isotopic composition of 0.7014 ± 0.0008 (2σ) for the Uivak gneiss suite, i.e. grey gneiss plus iron-rich suite (λRb = 1.39 × 10?11 yr?1). The grey Uivak gneiss suite, treated independently, defines a Rb-Sr isochron with an age of 3610 ± 144 m.y. (2σ) and initial Sr isotopic composition of 0.7015 ± 0.0014 (2σ) which is indistinguishable from the age and initial ratio of the total Uivak gneiss suite, grey gneisses plus iron-rich suite. The undifferentiated gneisses define a Rb-Sr isochron with an age of 3121 ± 160 m.y. (2σ), and initial Sr isotopic composition of 0.7064 ± 0.0012 (2σ). The isotopic data support field observations suggesting the undifferentiated gneisses were derived by local remobilization of the grey Uivak gneisses. The Uivak gneisses resemble the Amitsoq gneisses of Godthaabsfjord both chemically and isotopically. The interpretation of the initial Sr isotopic composition of the Uivak gneisses is interpreted as the time of regional homogenization rather than the initial ratio of the plutonic igneous parents of the Uivak gneisses as suggested for the Amitsoq gneisses. Although the undifferentiated gneisses are contemporaneous with the Nuk gneisses of West Greenland, they do not form a well-defined calc-alkaline suite and may not be associated with major crustal thickening in the Labrador Archaean. 相似文献
238.
An oil spill into Long Cove, Searsport, Maine, began on 16 March and lasted until at least 30 June 1971. It resulted in immediate and continuing soft clam mortalities which, based on before and after biological surveys, had by August 1974 exceeded 85% of the estimated 50 million market-size clams occupying the area. 相似文献
239.
Michael E. Eliot Hurst 《Geoforum》1981,12(1):71-84
The car (automobile) appears to many to be an essential ingredient of capitalist society. This paper seeks to assess the role of the car in capitalist dominated social formations by relating the production, consumption and utilization of the car to the underlying structural relationships of the capitalist mode of production as a whole. 相似文献
240.
Tony D. Peterson 《Contributions to Mineralogy and Petrology》1989,102(3):336-346
Two fractionation trends in sodic alkaline ultramafic liquids have been predicted from experiments in subsystems of the join Di-Ak-Ne-Lc-Qz. The products of these trends are equated with contrasting suites of peralkaline nephelinites from two nephelinite-carbonatite volcanos of the south Gregory Rift, Shombole (southern Kenya) and Oldoinyo L'engai (northern Tanzania). In both trends, peralkalinity is interpreted to result from fractional crystallization of aluminous clinopyroxene. The Shombole trend has olivine nephelinite as its parental magma, and the differentiation products are mildly peralkaline [(Na+K)/Al1.15] nephelinites. It is the most common lineage observed in nephelinite-carbonatite centres. The Oldoinyo L'engai trend has melilitite or olivine-melilite nephelinite as its parental magma, and produces extremely peralkaline [(Na+K)/Al=1.4–2.3] wollastonite- and combeite- (Na2 Ca2Si3O9) bearing nephelinites. The presence of a reaction relation between wollastonite and liquid to produce combeite, indicated by corroded wollastonite phenocrysts armoured by combeite in some nephelinites from Oldoinyo L'engai, is confirmed by melting experiments. Combeite nephelinites from Oldoinyo L'engai were erupted simultaneously with natrocarbonatite ash, and are very similar in composition to silicate liquids that have been shown by experiment to be immiscible with natrocarbonatite. Because the L'engai trend is rarely expressed at extrusive centres (combeite has been recorded at only three localities), and combeite nephelinites are highly evolved magmas, it is unlikely that natrocarbonatite is primary to other carbonatite types. It is proposed that carbonatite liquid is exsolved at crustal pressures from a wide range of nephelinitic liquids: Mg-rich carbonatite from primitive, olivine-bearing alkaline ultramafic liquids, Ca-rich carbonatite from olivine-free nephelinites of low peralkalinity, and natrocarbonatite from strongly peralkaline combeite nephelinites. 相似文献