全文获取类型
收费全文 | 66604篇 |
免费 | 1040篇 |
国内免费 | 601篇 |
专业分类
测绘学 | 1573篇 |
大气科学 | 4325篇 |
地球物理 | 12982篇 |
地质学 | 23973篇 |
海洋学 | 6105篇 |
天文学 | 15937篇 |
综合类 | 150篇 |
自然地理 | 3200篇 |
出版年
2022年 | 458篇 |
2021年 | 741篇 |
2020年 | 846篇 |
2019年 | 871篇 |
2018年 | 1875篇 |
2017年 | 1741篇 |
2016年 | 2053篇 |
2015年 | 1078篇 |
2014年 | 1949篇 |
2013年 | 3508篇 |
2012年 | 2130篇 |
2011年 | 2814篇 |
2010年 | 2505篇 |
2009年 | 3307篇 |
2008年 | 2784篇 |
2007年 | 2841篇 |
2006年 | 2646篇 |
2005年 | 1835篇 |
2004年 | 1808篇 |
2003年 | 1700篇 |
2002年 | 1663篇 |
2001年 | 1505篇 |
2000年 | 1420篇 |
1999年 | 1192篇 |
1998年 | 1230篇 |
1997年 | 1172篇 |
1996年 | 1019篇 |
1995年 | 987篇 |
1994年 | 938篇 |
1993年 | 782篇 |
1992年 | 752篇 |
1991年 | 749篇 |
1990年 | 845篇 |
1989年 | 676篇 |
1988年 | 654篇 |
1987年 | 813篇 |
1986年 | 666篇 |
1985年 | 884篇 |
1984年 | 918篇 |
1983年 | 875篇 |
1982年 | 814篇 |
1981年 | 766篇 |
1980年 | 702篇 |
1979年 | 641篇 |
1978年 | 655篇 |
1977年 | 564篇 |
1976年 | 565篇 |
1975年 | 565篇 |
1974年 | 536篇 |
1973年 | 564篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
981.
Petrogenesis and paleotectonic history of the Wild Bight Group,an Ordovician rifted island arc in central Newfoundland 总被引:1,自引:0,他引:1
H. Scott Swinden G. A. Jenner B. J. Fryer J. Hertogen J. C. Roddick 《Contributions to Mineralogy and Petrology》1990,105(2):219-241
The Wild Bight Group (WBG) is a sequence of early and middle Ordovician volcanic, subvolcanic and epiclastic rocks, part of the Dunnage Tectonostratigraphic Zone of the Newfoundland Appalachians. A detailed geochemical and Nd-isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks, interpret their palcotectonic environments and constrain their petrogenetic history. The lower and central stratigraphic levels of the WBG contain mafic volcanic rocks with island-arc geochemical signatures, including LREE-enriched are tholeiites with
Nd(t)
=-0.1 to +2.2 (type A-I), LREE-depleted arc tholeiites with
Nd(t)
=+5.6 to +7.1 (type A-II) and an unusual suite of strongly incompatible-element depleted tholeiites in which
Nd(t)
ranges from-0.9 to +4.6 and is negatively correlated with147Sm/144Nd (type A-III). High-silica, low-K rhyolites occur locally in the central part of the stratigraphy, associated with mafic rocks of arc affinity, and have
Nd(t)
=+4.7 to +5.4. The upper stratigraphic levels of the WBG dominantly contain rocks with non-arc geochemical signatures, including alkalic basalts with
Nd(t)
=+4.6 to +5.5 (type N-I), strongly LREE- and incompatible element-enriched tholeiites that are transitional between alkalic and non-alkalic rocks with
Nd(t)
=+4.4 to +7.0 (type N-II) and rocks with flat to slightly LREE-enriched patterns and
Nd(t)
=+5.1 to +7.4 (type N-III). Rocks with non-arc and arc signatures are locally interbedded near the stratigraphic type of the WBG. Nd-isotopic data in the type A-I and A-II rocks are generally compatible with mixing/partial melting models involving depleted mantle, variably contaminated by a subducted crustally-derived sediment. The petrogenesis of type A-III rocks must involve source mixing and multi-stage partial melting, but the details are not clear. The geochemistry and Nd isotope data for types N-I, N-II and N-III rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts. Comparison of the WBG with modern analogues suggests a 3-stage developmental model: stage 1) island-arc volcanism (eruption of type mafic volcancs); stage 2) arc-rifting (continued eruption of type A-I, A-I, eruption of types A-II and A-III mafic volcanics and high-silica, low-K rhyolites); and stage 3) back-arc basin volcanism (continued minor eruption of type A-I basalts, eruption of types N-I, N-II, N-III basalts). Stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle, while stage 3 volcanism utilized depleted-mantle sources not affected by the subducting slab. This model provides a basis for interpreting coeval sequences in central Newfoundland and a comparative framework for some early Paleozoic oceanic volcanic sequences elsewhere in the Appalachian orogen. 相似文献
982.
Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon 总被引:3,自引:1,他引:2
GRIBBLE ROBERT F.; BARNES CALVIN G.; DONATO MARY M.; HOOVER JAMES D.; KISTLER RONALD W. 《Journal of Petrology》1990,31(4):883-923
The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial 87Sr/86Sr = 0{dot}7044, 18O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (LaN/LuN = 7) and then by a suiteof K2O- and P2O5 rocks of quartz monzodioritic affinity (LaN/LuN= 1321; LaN/SmN = 2{dot}43{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(LaN/LuN= 1319; LaN/SmN = 4{dot}36 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately Ptotal = 2{dot}3kb,PH2O between 1{dot}5 and 2{dot}2 kb, and fO2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO2 togetherwith increasing 18O and decreasing initial 87Sr/86Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr1 and high (18O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in 18O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P2O5-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P2O5-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation. 相似文献
983.
A progress report on cosmic fireballs is presented. The main new results are: (a) the phenomenon should be almost universal, and most explosive -ray sources should show the characteristic fireball spectrum; (b) even if the radiation density is insufficient, pair production in electron-proton or electron-electron scattering might start the fireball; (c) some computed fireball spectra are shown. They all have in common a 1/E low-energy behaviour, a 100 keV flattening, and a 0.5 MeV cut-off.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts, held at Toulouse, France, 26–29 November, 1979. 相似文献
984.
Accurate numerical continuation of families of plane symmetric direct periodic orbits around the large primary in the Sun-Jupiter case of the restricted problem of three bodies allows the determination of the vertical branching points where families of three-dimensional symmetric periodic orbits bifurcate from the planar ones. Three families of plane periodic orbits, and the initial segments of ten bifurcating families of three-dimensional ones are determined. The stability of these families is examined and examples of their orbits are illustrated. 相似文献
985.
New spectrophotometry from 1.5 to 2.5 μm is reported for the Uranian satellites Titania, Oberon, and Umbriel. A spectrum of the rings of Uranus from 2.0 to 2.4 μm is also reported. No evidence is found for frost covering the surface of the ring material, consistent with the low albedo of the rings (PK = 0.03) previously reported by Nicholson and Jones (1980). The surfaces of the satellites are found to be covered by dirty water frost. Assuming albedos of the frost and gray components covering the Uranian satellites to be the same as the light and dark faces of Iapetus, radii are derived that are roughly twice those inferred from the assumption of a visual albedo of 0.5. 相似文献
986.
A further development of the Kostyuk-Pikelner's model is presented. The response of the chromosphere heated by non-thermal electrons of the power-law energy spectrum has been studied on the basis of the numerical solution of the one-dimensional time-dependent equations of gravitational gas dynamics. The ionization and energy loss for the emissions in the Lyman and Balmer lines have been determined separately for the optically thin and thick L-line layers. Due to the initial heating, a higher-pressure region is formed. From this region, disturbances propagate upwards (a shock wave with a velocity of more than 1000 km s-1) and downwards. A temperature jump propagates downwards, and a shock is formed in front of the thermal wave. During a period of several seconds after the beginning of this process, the temperature jump intensifies the downward shock wave and the large radiative loss gives rise to the high density jump (
2/
1 100). The numerical solution has been analyzed in detail for the case heating of the ionized and neutral plasma, and a value of this heating is close to the upper limit of the admissible values. In this case, the condensation located between the temperature jump and the shock wave front, may emit in the observed optical continuum.In their essential features, the gas dynamic processes during the flares in red dwarf atmospheres are the same as those in the solar atmosphere. However, the high atmospheric densities, smaller height scale in red dwarf atmospheres, and greater energy of this processes in stellar flares, give rise, in practice, to the regular generation of optical continuum. The photometric parameters of a source with n 015 cm-3, T 9000 K, and
z 10 km are in a good agreement with observations. 相似文献
987.
F. Kneer G. Scharmer W. Mattig A. Wyller G. Artzner P. Lemaire J. C. Vial 《Solar physics》1981,69(2):289-300
Observations with the French (L.P.S.P.) experiment on board OSO-8 of a sunspot and nearby plage region are described. The behaviour of the emission cores of the Ca II H and K and Mg II h and k resonance lines is very similar and the correspondence in intensity between the four lines persists in all observed features. In contrast, the Lyman lines show little correlation with the other lines. Their emission regions appear broader in the spectroheliograms than the underlying sunspot structure and must not necessarily possess a counterpart in lower layers. From the central intensity of L above the umbra an electron density of 4.3 × 1010 cm-3 n
e
* 2.3 × 1011 cm-3 at 20 000 K is estimated.Mitteilungen aus dem Kiepenheuer-Institut Nr. 186.Stockholm Observatorium, S-13300 Saltsjöbaden, Sweden.Laboratoire de Physique Stellaire et Planétaire, CNRS, P.O. Box 10, F-91370 Verrières-le-Buisson, France. 相似文献
988.
I. G. Mitrofanov 《Astrophysics and Space Science》1981,80(2):303-306
On grounds of Mazetset al. (1980a) conclusion that all gamma-ray bursts recorded were emitted at the distance 1 kpc from the Sun it is shown that bursts from the Galaxy and from another galaxies may be observable byEinstein Observatory (HEAO-B). The HEAO-B observing programs are considered and the numbers of possibly recorded events are estimated. It is concluded that the important information about sources of gamma-bursts may be obtained in this way. 相似文献
989.
A study of the galactic structure has been made by deriving the brightness distribution of the galactic plane at 2.2 m and 4.2 m near infrared region using infrared objects detected by ground-based sky surveys. The infrared brightness distribution shows distinct peaks at every 8–12° galactic longitude and indicates a periodic structure. The one-to-one positional correlation observed between the periodic structure in 2.2 m infrared brightness and the 2.6 mm CO emission line suggests that the near-infrared sources are strongly associated with dense clouds of molecular hydrogen distributed in the galactic plane. 相似文献
990.
R. G. Giovanelli 《Solar physics》1978,59(2):293-311
The cooling effect of emission in the spectral lines, which dominates over continuous emission in the chromosphere and becomes important first around the temperature minimum, modifies greatly the radiative relaxation timet
r in the solar atmosphere. This rises from low photospheric values to a maximum of 8 min just aboveT
min, falls in the low chromosphere to 1.5 min because of line emission, but rises again to 6 min atT 7000–8400 K in the chromosphere where hydrogen ionization increases the specific heat. 相似文献