首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   252488篇
  免费   5183篇
  国内免费   3293篇
测绘学   6757篇
大气科学   18838篇
地球物理   53241篇
地质学   87384篇
海洋学   21134篇
天文学   54859篇
综合类   979篇
自然地理   17772篇
  2021年   2164篇
  2020年   2578篇
  2019年   2839篇
  2018年   3161篇
  2017年   2852篇
  2016年   5475篇
  2015年   4176篇
  2014年   6881篇
  2013年   14172篇
  2012年   6326篇
  2011年   7510篇
  2010年   6612篇
  2009年   9236篇
  2008年   8141篇
  2007年   7486篇
  2006年   9594篇
  2005年   7602篇
  2004年   7553篇
  2003年   7050篇
  2002年   6640篇
  2001年   5934篇
  2000年   5904篇
  1999年   5183篇
  1998年   5208篇
  1997年   5008篇
  1996年   4653篇
  1995年   4400篇
  1994年   4075篇
  1993年   3820篇
  1992年   3593篇
  1991年   3565篇
  1990年   3745篇
  1989年   3499篇
  1988年   3148篇
  1987年   3708篇
  1986年   3243篇
  1985年   4100篇
  1984年   4617篇
  1983年   4294篇
  1982年   4210篇
  1981年   3833篇
  1980年   3584篇
  1979年   3429篇
  1978年   3423篇
  1977年   3219篇
  1976年   2956篇
  1975年   2901篇
  1974年   2866篇
  1973年   3058篇
  1972年   1990篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
141.
142.
143.
144.
145.
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them.  相似文献   
146.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
147.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems.  相似文献   
148.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during 1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles. The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed.  相似文献   
149.
150.
Sediment archives from a mountain lake are used as indicators of seismotectonic activity in the Grenoble area (French western Alps, 45°N). Sedimentological analysis (texture and grain-size characteristics) exhibits several layers resulting from instantaneous deposits in Lake Laffrey: six debris flow events up to 8 cm thick can be attributed to slope failure along the western flank of the basin. Dating with 210Pb and 137Cs gamma counting techniques and the reconnaissance of historical events, provide a constrained age-depth model. Over the last 250 years, five of such debris flow deposits could be related to historical earthquakes of MSK intensities greater than VI over an area of <60 km. One debris flow deposit triggered at the beginning of the last century can be related to an historical landslide possibly triggered by the artificial regulation of the lake level.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号