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241.
Density of peridotite melts at high pressure 总被引:2,自引:0,他引:2
Densities of ultramafic melts were determined up to 22 GPa by relative buoyancy experiments. Olivine and diamond were used as buoyancy markers. We confirmed that the density crossover of PHN 1611 melt and its equilibrium olivine (Fo94) occurs at around 13.5 GPa and 2030 °C and that olivine floats from deeper regions in the magma ocean of the primordial terrestrial mantle. The comparison of the compression curves of basic and ultrabasic melts implies that the basic melt is more compressible. This can be explained by the difference in the amount of compressible linkage of SiOn and AlOn polyhedra. The interstitial melt trapped by the density crossover can be the cause of the impedance anomaly of the seismic wave in the deep upper mantle. 相似文献
242.
Re-Os同位素由于其较高的半衰期特点,近年来国外广泛就于前寒武纪含Cu-Ni-PGE镁铁-超镁铁岩成岩年,结合其他地球化学数据,对壳幔岩浆源及作用进行解释研究。特别是由于Os 是6个PGE的其中之一,而多用于岩浆Cu-Ni-PGE矿床贵金属来源的判别。 相似文献
243.
Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung,Bali, Indonesia 总被引:3,自引:0,他引:3
The 1963 eruption of Gunung Agung produced 0.95 km3 dense rock equivalent (DRE) of olivine±hornblende-bearing, weakly phyric, basaltic andesite tephra and lava. Evidence for
magma mixing in the eruptive products includes whole-rock compatible and incompatible trace element trends, reverse and complex
compositional zoning of mineral phases, disequilibrium mineral assemblages, sieve-textured plagioclase phenocrysts, and augite
rims on reversely zoned orthopyroxene. Basalt magma mixed with pre-existing andesite magma shortly before eruption to yield
basaltic andesite with a temperature of 1040–1100 °C at an assumed pressure of 2 kb, f O2>NNO, and an average melt volatile content (H2O±CO2) of 4.3 wt.%. Magma-mixing end members may have provided some of the S and Cl emitted in the eruption. Glass inclusions in
phenocrysts contain an average of 650 ppm S and 3130 ppm Cl as compared with 70 ppm and 2220 ppm, respectively, in the matrix
glass. Maximum S and Cl contents of glass inclusions approach 1800 and 5000 ppm, respectively. Application of the petrologic
method to products of the 1963 eruption for estimating volatile release yields of 2.5×1012 g (Mt) of SO2 and 3.4 Mt of Cl released from the 0.65 km3 of juvenile tephra which contributed to stratospheric injection of H2SO4 aerosols on 17 March and 16 May, when eruption column heights exceeded 20 km above sea level. An independent estimate of
SO2 release from atmospheric aerosol loading (11–12 Mt) suggests that approximately 7 Mt of SO2 was injected into the stratosphere. The difference between the two estimates can be most readily accounted for by the partitioning
of S, as well as some Cl, from the magma into a water-rich vapor phase which was released upon eruption. For other recent
high-S-release eruptions of more evolved and oxidized magmas (El Chichón, Pinatubo), the petrologic method gives values two
orders of magnitude less than independent estimates of SO2 emissions. Results from this study of the Agung 1963 magma and its volatile emissions, and from related studies on eruptions
of more mafic magmas, suggest that SO2 emissions from eruptions of higher-S-solubility magma may be more reliably estimated by the petrologic method than may those
from more-evolved magma eruptions.
Received: 29 June 1994 / Accepted: 25 April 1996 相似文献
244.
Fragmentation of magma during Plinian volcanic eruptions 总被引:2,自引:0,他引:2
James E. Gardner Richard M. E. Thomas Claude Jaupart Steve Tait 《Bulletin of Volcanology》1996,58(2-3):144-162
The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V
G
/V
L
) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V
G
/V
L
in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between
pumices at a given level in a deposit. These variations in V
G
/V
L
do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges
in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point
of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary
evolution. On the timescale of Plinian eruptions, pumices with viscosities <109 Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity.
Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <105 Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates
that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically
on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices
become connected at different times throughout the time span between fragmentation and quenching. After accounting for these
effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is
a relatively constant minimum value of V
G
/V
L
of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >105 Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic
is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism
for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently
strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the
chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive
activity at volcanoes.
Received: 19 April 1995 / Accepted: 3 April 1996 相似文献
245.
浙江拔茅破火山岩浆作用:开放体系多机制复合演化 总被引:1,自引:3,他引:1
拔茅火山杂岩的成分变化范围广泛,包括玄武质,玄武粗安质,粗面安山质,粗面英安质,流纹英安质到流纹质和高硅流纹质岩石都有,它们不是由单一母岩浆演化而来,也不是由单一岩浆房喷发形成的,而是来自多种类型的岩浆房,并经历过复杂的演化过程,为开放体系多机制复合演化,其中早期双峰式基性端元是由上地幔部分熔融形成的,而酸性端无则是地壳物质边熔融边喷发(分离熔融)的结果,作为火山杂岩主体的粗面英安质岩石,其岩浆是在高位主岩浆房内由玄武质岩浆与流纹英安质岩浆混合形成的,发生混合的流纹英安质与玄武质岩浆的比例为57:43,而粗面安山质岩浆则是溶部岩浆房内由玄武质岩浆分离结晶形成的,晚期侵出-侵入的流纹英安质岩穹和高硅流纹斑岩株则分别代表高位次级岩浆房的成分及其硅质帽。 相似文献
246.
利用地震波探测、重力、地磁、地质等资料,论证了唐山地震前在震中区及其附近地区地壳内存在着的岩浆活动。认为唐山地震的发生可能是由于岩浆沿深大断裂向地壳内侵入,从而导致断裂重新破裂、扩展的结果。推断岩浆活动可能是内陆地震的一种成因。最后从岩浆活动的角度出发对一些地震现象作了讨论。 相似文献
247.
福州复式岩体的组成及其演化 总被引:7,自引:1,他引:7
本文用岩石学、岩石化学和稀土元素地球化学诸方面资料,论证了福州复式岩体具有由M型→I型→s型→A型的演化规律。晚阶段的魁岐岩体和笔架山岩体是在岩浆分异演化晚期与幔源岩浆混合作用的产物,而非典型的或单一的幔源岩浆所形成的A型花岗岩。 相似文献
248.
笔者在前人资料的基础上,通过部分野外调查工作,应用侵入体一单元一序列三级体制,将望湘岩基南部侵入体重新厘定,归并为8个单元,组合成两个序列。认为这两个序列是相隔4 Ma的同源岩浆两次脉动式上侵的结果,8个单元是两次岩浆上侵后分别结晶分异的产物。根据野外的接触关系,确认汉家山单元(B岩型)的定位时间晚于坝头岭单元和朱家冲单元(C岩型)。并依据各单元的矿物成分、化学成分、微量元素含量的变化,重新厘定了岩浆的演化顺序。 相似文献
249.
Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively,
at temperatures close to feldspar liquidus (750–700 °C), were carried out to determine the modality of water exsolution and
vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform
space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (≈0.5 vol.%)
of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size
distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius,
area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions
was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5–1000 μm
were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs
of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano
Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the
general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic
coordinates. The slope of this regression is approximately 2.8±0.1. This dependence is in striking contrast with distributions
theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution
requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous
nucleation mechanisms.
Received: 1 October 1998 / Accepted: 25 June 1999 相似文献
250.