全文获取类型
收费全文 | 1438篇 |
免费 | 156篇 |
国内免费 | 166篇 |
专业分类
测绘学 | 155篇 |
大气科学 | 153篇 |
地球物理 | 279篇 |
地质学 | 509篇 |
海洋学 | 172篇 |
天文学 | 122篇 |
综合类 | 235篇 |
自然地理 | 135篇 |
出版年
2023年 | 3篇 |
2022年 | 27篇 |
2021年 | 30篇 |
2020年 | 33篇 |
2019年 | 25篇 |
2018年 | 13篇 |
2017年 | 26篇 |
2016年 | 54篇 |
2015年 | 40篇 |
2014年 | 279篇 |
2013年 | 188篇 |
2012年 | 167篇 |
2011年 | 146篇 |
2010年 | 144篇 |
2009年 | 132篇 |
2008年 | 95篇 |
2007年 | 43篇 |
2006年 | 42篇 |
2005年 | 31篇 |
2004年 | 34篇 |
2003年 | 19篇 |
2002年 | 22篇 |
2001年 | 28篇 |
2000年 | 23篇 |
1999年 | 19篇 |
1998年 | 29篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 11篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1980年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有1760条查询结果,搜索用时 18 毫秒
51.
伊春地区晚印支期I型花岗岩带特征及其构造背景 总被引:8,自引:0,他引:8
晚印支期花岗岩岩石类型主要为一套含角闪石黑云母二长花岗岩及含黑云母正长花岗岩,岩石中暗色闪长质岩包体发育较普遍,富含角闪石及镁质黑云母,副矿物以榍石、磁铁矿、磷灰石常见,Al2O3/(Na2O十K2O+CaO)〈1.1,显示出I型花岗岩特征。利用岩石矿物组合、岩石化学特征判别其产于碰撞后构造环境,认为造山后伸展体制是这期花岗岩形成的重要原因。岩石(^87 Sr/^86 Sr);及δEu值反映其源区为壳幔过渡区,氧同位素(δ^18 O)测定值在5.5‰-10‰之间,具壳幔混源的特点,表明岩石来源于下地壳或壳幔过渡部分,与下地壳的部分熔融或壳幔过渡区部分熔融作用有关。 相似文献
52.
Analyses of aircraft observations of the stratocumulus-topped boundary layer during the First ISCCP (International Satellite
Cloud Climatology Project) Regional Experiment (FIRE I) show the frequent presence of clear, but relatively moist, air patches
near the stratocumulus cloud-top interface. A conditional sampling of measurements in these clear air patches shows that their
thermodynamic properties do more resemble boundary-layer air characteristics than those of free troposphere air. From an aircraft
leg through cloud tops it is demonstrated that turbulent mixing across the cloud-top interface can lead to the local dissipation
of the cloud top. Analogous to the terminology used for shallow cumulus parameterizations this process can be considered as
detrainment, with which we mean that after a mixing event across the cloud-top boundaries, mixed unsaturated parcels become
part of the clear environment of the cloud. 相似文献
53.
国家遥感应用工程技术研究中心NationalEngineeringResearchCenterforGeomatics(NCG)国家遥感应用工程技术研究中心于1996年12月25日由国家科委正式批准组建(国科发计字[1996]603号文件)。中国科学... 相似文献
54.
星载微波SSM/Ⅰ遥感在中国东北华北农田的辐射特征分析 总被引:2,自引:1,他引:1
本文研究了星载微波SSM/Ⅰ1996年在中国东北华北平原农田上7个通道辐射亮度温度(TB)的遥感数据,提出用几个通道TB组合的散射指数和极化指数来分析中国平原地区农田的微波辐射特征,及其随生长季节的时间性变化。星载SSM/Ⅰ数据可以监视农作物的生长和平原地区地面湿度的变化。本文还给出了大气和农作物地表矢量辐射传输的数值模拟结果。 相似文献
55.
本文提出一个既具有五次旋转的20面体点群(Ih,5m3)也显示由5个共轭立方格子表征的三维平移周期性的几何构型──正12面体框架构型(RDFC)所对20面体相原子结构进行了诠释,同时也诠释了1981年发现的20面体相(Ⅰ相)的电子衍射图(EDPs)和高分辨电子显微象。 相似文献
56.
Feldspar phenocrysts of silicic volcanic rocks are commonly in Sr-isotopic disequilibrium with groundmass. In some cases the feldspar is more radiogenic, and in others it is less radiogenic. Several explanations have been published previously, but none of these is able to accommodate both senses of disequilibrium. We present a model by which either more- or less-radiogenic feldspar (or even both within a single eruptive unit) can originate. The model requires a magma body open to interaction with biotite- and feldspar-bearing wall rock. Magma is incrementally contaminated as wall rock melts incongruently. Biotite preferentially melts first, followed by feldspar. Such melting behavior, which is supported by both field and experimental studies, first contaminates magma with a relatively radiogenic addition, followed by a less-radiogenic addition. Feldspar phenocrysts lag behind melt (groundmass of volcanic rock) in incorporating the influx of contaminant, thus resulting in Sr-isotopic disequilibrium between the crystals and melt. The sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process. This model is testable by isotopic fingerprinting of individual feldspar crystals. For a given set of geologic boundary conditions, specific core-to-rim Sr-isotopic profiles are expectable. Moreover, phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history. Initial results of Sr-isotopic fingerprinting of sanidine phenocrysts from the Taylor Creek Rhyolite are consistent with the model. More tests of the model are desirable. 相似文献
57.
58.
59.
Victoria Bannister Peter Roeder Alexei Poustovetov 《Journal of Volcanology and Geothermal Research》1998,87(1-4)
Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe2+/(Fe2++Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe3+/(Fe3++Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe3+/(Fe3++Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H2O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H2O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H2O>1 wt.%, and the log fO2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H2O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites. 相似文献
60.