收费全文 | 2051篇 |
免费 | 136篇 |
国内免费 | 10篇 |
测绘学 | 41篇 |
大气科学 | 174篇 |
地球物理 | 599篇 |
地质学 | 753篇 |
海洋学 | 244篇 |
天文学 | 246篇 |
综合类 | 14篇 |
自然地理 | 126篇 |
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 22篇 |
2021年 | 52篇 |
2020年 | 59篇 |
2019年 | 40篇 |
2018年 | 105篇 |
2017年 | 82篇 |
2016年 | 165篇 |
2015年 | 111篇 |
2014年 | 117篇 |
2013年 | 139篇 |
2012年 | 98篇 |
2011年 | 133篇 |
2010年 | 128篇 |
2009年 | 125篇 |
2008年 | 120篇 |
2007年 | 75篇 |
2006年 | 72篇 |
2005年 | 57篇 |
2004年 | 55篇 |
2003年 | 59篇 |
2002年 | 45篇 |
2001年 | 30篇 |
2000年 | 31篇 |
1999年 | 27篇 |
1998年 | 29篇 |
1997年 | 20篇 |
1996年 | 16篇 |
1995年 | 20篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 15篇 |
1990年 | 17篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1984年 | 6篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1978年 | 3篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1957年 | 3篇 |
1956年 | 3篇 |
1950年 | 2篇 |
The volcanic rocks of the Colíder and Roosevelt formations are extensively exposed in the south-central portion of the Amazonian Craton where effusive and pyroclastic rocks have been mapped. Both units, topped by chemical sediments and oceanic facies as rhyolite and andesite lavas, rhyodacite, and porphyritic dacite, with frequent intercalations of pyroclastic and epiclastic deposits. Whole-rock geochemistry for 55 samples of rhyolitic to andesitic composition suggests the involvement of fertile mantle-derived components with E-MORB to OIB compositions. The analyzed rocks display calc-alkaline to shoshonitic affinity consistent with generation related to an active continental margin. The whole-rock Sm-Nd isotope data from selected felsic volcanic rocks of the Colíder and Roosevelt formations yield negative initial εNd values between –3 and –9, indicating the predominantly crustal nature of the parental magmas with early Archean to late Paleoproterozoic (ca. 2.5–2.0 Ga) depleted mantle model ages.
相似文献In recent decades, landslide disasters in the Himalayas, as in other mountain regions, are widely reported to have increased. While some studies have suggested a link to increasing heavy rainfall under a warmer climate, others pointed to anthropogenic influences on slope stability, and increasing exposure of people and assets located in harm’s way. A lack of sufficiently high-resolution regional landslide inventories, both spatially and temporally, has prevented any robust consensus so far. Focusing on Far-Western Nepal, we draw on remote sensing techniques to create a regional inventory of 26,350 single landslide events, of which 8778 date to the period 1992–2018. These events serve as a basis for the analyses of landslide frequency relationships and trends in relation to precipitation and temperature datasets. Results show a strong correlation between the annual number of shallow landslides and the accumulated monsoon precipitation (r = 0.74). Furthermore, warm and dry monsoons followed by especially rainy monsoons produce the highest incidence of shallow landslides (r = 0.77). However, we find strong spatial variability in the strength of these relationships, which is linked to recent demographic development in the region. This highlights the role of anthropogenic drivers, and in particular road cutting and land-use change, in amplifying the seasonal monsoon influence on slope stability. In parallel, the absence of any long-term trends in landslide activity, despite widely reported increase in landslide disasters, points strongly to increasing exposure of people and infrastructure as the main driver of landslide disasters in this region of Nepal. By contrast, no climate change signal is evident from the data.
相似文献This paper presents a fundamental study on the effect of the relative humidity on the rockfill crushing strength. This aspect plays an important role in the mechanical behaviour of rockfill, and it is known that certain characteristics of the granular materials, such as compressibility and shear strength, depend on the confining stress, which is a function of the particles crushing. An increased interest has been observed regarding the effect of the relative humidity in the mechanical behaviour of rockfill. Unfortunately, limited research has been conducted until now regarding the study of individual particle crushing. Therefore, this paper thoroughly investigated particle crushing, by performing single-particle crushing tests on rockfill particles divided into four size ranges, under different relative humidity conditions. The experimental results reveal a considerable influence of the relative humidity in the studied rockfill particles, whose strength of the particles with the greatest dimensions in saturated conditions was reduced by half. Consistent macro-mechanical evidence demonstrates that particle’s size and relative humidity conditions depict the most important factors that influence particle crushing strength.
相似文献