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31.
Harold Clénet Georges Ceuleneer Patrick Pinet Bénédicte Abily Yves Daydou Esther Harris Isma Amri Céline Dantas 《Lithos》2010,114(3-4):265-281
Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 × 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al2O3], low [TiO2]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO2 content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic–troctolitic–gabbroic cumulates making up the crustal section above the Maqsad diapir.We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize. 相似文献
32.
We report on three sequences of high-resolution white-light and magnetogram observations obtained in the summer of 1989. The duration of sub-arcsecond seeing was three to four hours on each day. Study of the white-light and magnetogram data yields the following results:
- For all but one of the sunspots we have observed, both dark fibrils and bright grains in the inner part of the penumbra of sunspots move toward the umbra with a speed of about 0.5 km s-1. In the outer part of the penumbra, movement is away from the umbra. The one exception is a newly formed spot, which has inflow only in its penumbra.
- Granular flows converge toward almost every pore, even before its formation. Pores are observed to form by the concentration of magnetic flux already existing in the photosphere. The pores (or small sunspots), in turn, then move and concentrate to form bigger sunspot.
- We followed an emerging flux region (EFR) from 29 to 31 July, 1989 that was composed of a large number of bipoles with magnetic polarities mixed over a large area in the first day of its birth. As time went on, polarities sorted out: the leading polarity elements moved in one direction; the following, the opposite. During the process a large number of cancellations occurred, with some sub-flares and surges observed simultaneously. After about 24 hours, the positive and negative fluxes were essentially separated.
- We find two kinds of photospheric dark alignments in the region of new flux emergence: (a) alignments connecting two poles of opposite magnetic polarity form the tops of rising flux tubes; (b) alignments corresponding to the magnetic flux of one polarity, which we call elongated pores.
33.
By examining the absolute magnitude (H) distributions (hereafter HD) of the cold and hot populations in the Kuiper belt and of the Trojans of Jupiter, we find evidence that the Trojans have been captured from the outer part of the primordial trans-neptunian planetesimal disk. We develop a sketch model of the HDs in the inner and outer parts of the disk that is consistent with the observed distributions and with the dynamical evolution scenario known as the ‘Nice model’. This leads us to predict that the HD of the hot population should have the same slope of the HD of the cold population for 6.5<H<9, both as steep as the slope of the Trojans' HD. Current data partially support this prediction, but future observations are needed to clarify this issue. Because the HD of the Trojans rolls over at H∼9 to a collisional equilibrium slope that should have been acquired when the Trojans were still embedded in the primordial trans-neptunian disk, our model implies that the same roll-over should characterize the HDs of the Kuiper belt populations, in agreement with the results of Bernstein et al. [Bernstein, G.M., and 5 colleagues, 2004. Astron. J. 128, 1364-1390] and Fuentes and Holman [Fuentes, C.I., Holman, M.J., 2008. Astron. J. 136, 83-97]. Finally, we show that the constraint on the total mass of the primordial trans-neptunian disk imposed by the Nice model implies that it is unlikely that the cold population formed beyond 35 AU. 相似文献
34.
Harold C. Helgeson William M. Murphy Per Aagaard 《Geochimica et cosmochimica acta》1984,48(12):2405-2432
Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ? 10.6 ? (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10?9 to 10?8 cm2 and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10?13 to ~10?7 moles cm?2 sec?1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole?1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10?16 to ~10?11 moles cm?2 sec?1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole?1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction. 相似文献
35.
Occurrence of the endangered shortnose sturgeon,Acipenser brevirostrum,in the Delaware River estuary
All available capture records of the endangered shortnose sturgeon,Acipenser brevirostrum, for the Delaware River Estuary from the early nineteeth century to the present were compiled. During 1817 through 1913 some 1,949 captures were reported, most as a bycatch of the shad gill net fishery. No documented captures during 1913 to 1954 were reported in the literature. Thirty-seven shortnose sturgeon were reported captured from 1954 through 1979, mostly incidental to fishery and ecological studies. Most specimens were taken in the upper tidal freshwater portion of the estuary (rkm 200–214). Seasonal-spatial distribution appeared similar to that observed for northern shortnose sturgeon populations. Taxonomic data obtained from seven specimens generally agreed with those from other drainages. 相似文献
36.
Mercury,arsenic, antimony,and selenium contents of sediment from the Kuskokwim River,Bethel, Alaska,USA 总被引:1,自引:0,他引:1
The Kuskokwim River at Bethel, Alaska, drains a major mercury-antimony metallogenic province in its upper reaches and tributaries. Bethel (population 4000) is situated on the Kuskokwim floodplain and also draws its water supply from wells located in river-deposited sediment. A boring through overbank and floodplain sediment has provided material to establish a baseline datum for sediment-hosted heavy metals. Mercury (total), arsenic, antimony, and selenium contents were determined; aluminum was also determined and used as normalizing factor. The contents of the heavy metals were relatively constant with depth and do not reflect any potential enrichment from upstream contaminant sources. 相似文献
37.
Abstract— We use Mars Orbiter Laser Altimeter (MOLA) topographic data and Thermal Emission Imaging System (THEMIS) visible (VIS) images to study the cavity and the ejecta blanket of a very fresh Martian impact crater ?29 km in diameter, with the provisional International Astronomical Union (IAU) name Tooting crater. This crater is very young, as demonstrated by the large depth/diameter ratio (0.065), impact melt preserved on the walls and floor, an extensive secondary crater field, and only 13 superposed impact craters (all 54 to 234 meters in diameter) on the ?8120 km2 ejecta blanket. Because the pre‐impact terrain was essentially flat, we can measure the volume of the crater cavity and ejecta deposits. Tooting crater has a rim height that has >500 m variation around the rim crest and a very large central peak (1052 m high and >9 km wide). Crater cavity volume (i.e., volume below the pre‐impact terrain) is ?380 km3 the volume of materials above the pre‐impact terrain is ?425 km3. The ejecta thickness is often very thin (<20 m) throughout much of the ejecta blanket. There is a pronounced asymmetry in the ejecta blanket, suggestive of an oblique impact, which has resulted in up to ?100 m of additional ejecta thickness being deposited down‐range compared to the up‐range value at the same radial distance from the rim crest. Distal ramparts are 60 to 125 m high, comparable to the heights of ramparts measured at other multi‐layered ejecta craters. Tooting crater serves as a fresh end‐member for the large impact craters on Mars formed in volcanic materials, and as such may be useful for comparison to fresh craters in other target materials. 相似文献
38.
39.
Devin L. SCHRADER Dante S. LAURETTA Harold C. CONNOLLY jr. Yulia S. GOREVA Dolores H. HILL Ken J. DOMANIK Eve L. BERGER Hexiong YANG Robert T. DOWNS 《Meteoritics & planetary science》2010,45(5):743-758
Abstract– Sacramento Wash 005 (SaW) 005, Meteorite Hills 00428 (MET) 00428, and Mount Howe 88403 (HOW) 88403 are S‐rich Fe,Ni‐rich metal meteorites with fine metal structures and homogeneous troilite. We compare them with the H‐metal meteorite, Lewis Cliff 88432. Phase diagram analyses suggest that SaW 005, MET 00428, and HOW 88403 were liquids at temperatures above 1350 °C. Tridymite in HOW 88403 constrains formation to a high‐temperature and low‐pressure environment. The morphology of their metal‐troilite structures may suggest that MET 00428 cooled the slowest, SaW 005 cooled faster, and HOW 88403 cooled the quickest. SaW 005 and MET 00428 contain H‐chondrite like silicates, and SaW 005 contains a chondrule‐bearing inclusion that is texturally and compositionally similar to H4 chondrites. The compositional and morphological similarities of SaW 005 and MET 00428 suggest that they are likely the result of impact processing on the H‐chondrite parent body. SaW 005 and MET 00428 are the first recognized iron‐ and sulfide‐rich meteorites, which formed by impact on the H‐chondrite parent body, which are distinct from the IIE‐iron meteorite group. The morphological and chemical differences of HOW 88403 suggest that it is not from the H‐chondrite body, although it likely formed during an impact on a chondritic parent body. 相似文献
40.
Douglas S. Hall Gregory W. Henry Dietmar Böhme Peter A. Brooks Sandy Chang Ales Dolzan George L. Fortier Robert E. Fried Russell M. Genet Bruce S. Grim James Hannon Darrel B. Hoff Kevin Krisciunas Howard J. Landis Howard P. Louth Larry P. Lovell Paul Nielsen Bobby E. Powell Harry D. Powell Don Pray Thomas R. Renner Charles W. Rogers Stephen Shervais Douglas M. Slauson Samuel Slote Harold J. Stelzer Arthur J. Stokes Jack C. Troeger Louis C. Turner Norman F. Wasson Kenneth W. Zeigler 《Journal of Astrophysics and Astronomy》1991,12(4):281-287
We present photoelectric photometry of λ And never before published, obtained between February 1982 and December 1990 at 29
different observatories. Then we combine it with all other photometry available to us (previously published, contained in
the I.A.U. Commission 27 Archives, and obtained with the Vanderbilt 16-inch automatic telescope but not yet published), to
yield a 14.8-year data base. Analysis reveals a long-term cycle in mean brightness, with a full range of 0m.15 and a period of 11.4 ± 0.4 years. Because most of our new photometry was concentrated in the 1983-84 observing season,
we analyze that one well-defined light curve with a two-spot model. Spot A keeps a 0m.04 amplitude throughout four rotation cycles whereas the amplitude of spot B diminishes from 0m.09 down almost to 0m.03. The spot rotation periods were 55d.9 ± 0d.6 and 520d.8 ± 1d.0, respectively. 相似文献