全文获取类型
| 收费全文 | 19701篇 |
| 免费 | 4242篇 |
| 国内免费 | 5666篇 |
专业分类
| 测绘学 | 2010篇 |
| 大气科学 | 2749篇 |
| 地球物理 | 5713篇 |
| 地质学 | 12987篇 |
| 海洋学 | 2761篇 |
| 天文学 | 282篇 |
| 综合类 | 1373篇 |
| 自然地理 | 1734篇 |
出版年
| 2024年 | 57篇 |
| 2023年 | 222篇 |
| 2022年 | 550篇 |
| 2021年 | 732篇 |
| 2020年 | 769篇 |
| 2019年 | 971篇 |
| 2018年 | 795篇 |
| 2017年 | 946篇 |
| 2016年 | 949篇 |
| 2015年 | 1080篇 |
| 2014年 | 1347篇 |
| 2013年 | 1245篇 |
| 2012年 | 1357篇 |
| 2011年 | 1403篇 |
| 2010年 | 1251篇 |
| 2009年 | 1413篇 |
| 2008年 | 1349篇 |
| 2007年 | 1514篇 |
| 2006年 | 1482篇 |
| 2005年 | 1271篇 |
| 2004年 | 1193篇 |
| 2003年 | 998篇 |
| 2002年 | 847篇 |
| 2001年 | 750篇 |
| 2000年 | 728篇 |
| 1999年 | 685篇 |
| 1998年 | 609篇 |
| 1997年 | 545篇 |
| 1996年 | 480篇 |
| 1995年 | 397篇 |
| 1994年 | 398篇 |
| 1993年 | 290篇 |
| 1992年 | 254篇 |
| 1991年 | 178篇 |
| 1990年 | 125篇 |
| 1989年 | 149篇 |
| 1988年 | 96篇 |
| 1987年 | 67篇 |
| 1986年 | 31篇 |
| 1985年 | 23篇 |
| 1984年 | 9篇 |
| 1983年 | 7篇 |
| 1982年 | 6篇 |
| 1981年 | 4篇 |
| 1980年 | 12篇 |
| 1979年 | 3篇 |
| 1978年 | 4篇 |
| 1977年 | 2篇 |
| 1972年 | 1篇 |
| 1954年 | 14篇 |
排序方式： 共有10000条查询结果，搜索用时 505 毫秒
91.
Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclasephenocrysts were dredged in the rift valley and adjacent flanksof the ultraslow-spreading Mohns and Knipovich ridges. The PUBsshow large variations in crystal morphologies and zoning. Thelarge variations suggest that single basalt samples containa mixture of plagioclase crystals that aggregated at differentlevels in the magma conduits. Resorbed crystals and repeatedreverse zones suggest that the magma reservoirs were replenishedand heated several times. Thin concentric zones with melt inclusions,and sharp reductions in the anorthite content of 3–7%,are common between the reverse zones. These zones, and skeletalcrystals with distinctly lower anorthite contents than massivecrystals, are interpreted to be the result of rapid crystalliztionduring strong undercooling. The changes between short periodsof cooling and longer periods with reheating are explained bymultiple advances of crystal-rich magma into cool regions followedby longer periods of gradual magma inflow and temperature increase.The porphyritic basalts are characterizd by more depleted andmore fractionated compositions than the aphyric basalts, withlower (La/Sm)N, K2O and Mg-numbers. This relationship, and theobservation that PUBs are sampled only close to segment centresalong these ridges, suggests that the PUBs formed by higherdegrees of melting and evolved in more long-lived magma reservoirs.We propose that the zoning patterns of plagioclase crystalsand crystal morphologies of these PUBs reflect the developmentand flow of magma through a stacked sill complex-like conduitsystem, whereas the aphyric equivalents represent later flowof magma through the conduit. The formation of voluminous higher-degreemelts may trigger the development of the magma conduits andexplain the generally depleted compositions of PUB magmas. KEY WORDS: basalt; mineral chemistry; MORB; magma mixing; magma chamber; major element 相似文献
92.
V. A. Lebedev S. N. Bubnov O. Z. Dudauri G. T. Vashakidze 《Stratigraphy and Geological Correlation》2008,16(2):204-224
Isotopic-geochronological study of the Pliocene magmatic activity in western part of the Dzhavakheti Highland (northwestern region of the Lesser Caucasus) is carried out. The results obtained imply that the Pliocene magmatic activity lasted in this part of the highland approximately 2 million years from 3.75 to 1.75–1.55 Ma. As is established, the studied volcanic rocks correspond in composition mostly to K-Na subalkaline and more abundant normal basalts. Time constraints of main phases in development of basic volcanism within the study region are figured out. We assume that individual pulses of silicic to moderately silicic volcanism presumably took place in the Dzhavakheti Highland about 3.2 and 2.5 Ma ago. 相似文献
93.
94.
95.
96.
Numerical estimation of REV and permeability tensor for fractured rock masses by composite element method 总被引：2，自引：0，他引：2
The Monte Carlo method is used to generate parent stochastic discrete fracture network, from which a series of fractured rock samples of different sizes and orientations are extracted. The fracture network combined with a regular grid forms composite element mesh of the fractured rock sample, in which each composite element is composed of sub‐elements incised by fracture segments. The composite element method (CEM) for the seepage is implemented to obtain the nodal hydraulic potential as well as the seepage flow rates through the fractured rock samples. The application of CEM enables a large quantity of stochastic tests for the fractured rock samples because the pre‐process is facilitated greatly. By changing the sizes and orientations of the samples, the analysis of the seepage characteristics is realized to evaluate the variation of the permeability components, the existence of the permeability tensor and the representative element volume. The feasibility and effectiveness are illustrated in a numerical example. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
97.
塔里木盆地西部的吐木休克断裂带是中央隆起(前身是晋宁期碰撞造山带)的次级单元巴楚断隆与北部坳陷的次级单元阿瓦提凹陷之间的分界。本文以大量的地质和地球物理证据,证明它是一条内部结构复杂且切穿地壳的深断裂。结合区域构造演化恢复了断裂发育史,指出它经历过三期重大的冲断活动,分别发生在加里东期、海西期和新近纪。新近纪的冲断与巴楚断隆的南界断裂带有相同的构造样式,可分为两幕:中新世的冲断受制于南天山前陆冲断带的向南扩展,更新世的冲断主要受制于西昆仑前陆冲断带的向北扩展。吐木休克断裂带的东西走向段和北西走向段的构造特征尚有若干差异,前者在加里东期活动较强,反映了构造的继承性;北西走向段则是加里东期出现的新生构造,在海西期进一步发展,更新世时因被卷入西昆仑的前陆冲断作用,表现出强烈而复杂的变形。 相似文献
98.
There could be some discontinuities in a soil media such as layers, earth structures, cracks, and fissures so that estimation
of stresses and deformations in these types of soil masses are somewhat different from continuous masses. The discontinuities
in a soil mass could be considered as a special link between two blocks. Transmitted swelling pressures affected by the soil
properties of the backfill especially at the interface between the backfill and retaining wall. In order to estimate the transmitted
swelling pressure distribution behind a retaining wall, using with interface element, a new finite element model and a code
(SWELPRES) have been developed. To evaluate the effect of backfill characteristics and interface element from the transmitted
lateral swelling pressures, four cases of thickness of backfills with or without interface elements are investigated. 相似文献
99.
激光粒度仪的广泛应用带来了与历史数据的对比问题,因此需要建立激光粒度仪与早期分析结果之间的关系。根据江苏海岸潮滩沉积物样品的激光粒度仪和移液管-筛析法分析,对粒度参数、粒度组分等进行了对比,对两种方法的差异进行了分析。结果表明,对于江苏潮滩沉积物粒度参数中平均粒径的激光粒度仪与移液管-筛析分析结果之间有良好的线性关系;筛分法测得的粗颗粒物质较激光法偏少,而移液管法法测得的细颗粒物质较激光法偏多;将样品分类之后再进行两种方法所获粒度参数的回归分析,相关性得以提高,说明不同粒度组成的沉积物对分析结果的对比有不同的影响。两种方法之间的换算关系不仅与研究区域有关,而且与沉积物本身的粒度组成有关。 相似文献
100.