全文获取类型
收费全文 | 25988篇 |
免费 | 364篇 |
国内免费 | 213篇 |
专业分类
测绘学 | 520篇 |
大气科学 | 1939篇 |
地球物理 | 5200篇 |
地质学 | 9056篇 |
海洋学 | 2106篇 |
天文学 | 6011篇 |
综合类 | 47篇 |
自然地理 | 1686篇 |
出版年
2021年 | 161篇 |
2020年 | 197篇 |
2019年 | 209篇 |
2018年 | 497篇 |
2017年 | 462篇 |
2016年 | 566篇 |
2015年 | 436篇 |
2014年 | 606篇 |
2013年 | 1211篇 |
2012年 | 731篇 |
2011年 | 1015篇 |
2010年 | 875篇 |
2009年 | 1219篇 |
2008年 | 1052篇 |
2007年 | 1054篇 |
2006年 | 978篇 |
2005年 | 818篇 |
2004年 | 837篇 |
2003年 | 799篇 |
2002年 | 739篇 |
2001年 | 688篇 |
2000年 | 636篇 |
1999年 | 571篇 |
1998年 | 576篇 |
1997年 | 576篇 |
1996年 | 440篇 |
1995年 | 419篇 |
1994年 | 380篇 |
1993年 | 335篇 |
1992年 | 311篇 |
1991年 | 276篇 |
1990年 | 304篇 |
1989年 | 281篇 |
1988年 | 235篇 |
1987年 | 311篇 |
1986年 | 257篇 |
1985年 | 346篇 |
1984年 | 384篇 |
1983年 | 371篇 |
1982年 | 335篇 |
1981年 | 310篇 |
1980年 | 312篇 |
1979年 | 282篇 |
1978年 | 311篇 |
1977年 | 263篇 |
1976年 | 267篇 |
1975年 | 275篇 |
1974年 | 234篇 |
1973年 | 238篇 |
1972年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Little is known of Holocene landform development in Upland Britain. This paper describes a site at Middle Langdale in the Howgill Fells of Cumbria where large, but now stabilized and inactive gullies cut through periglacial material. At the base of the gullies large debris cones have buried earlier alluvial sediments on the valley floor. On these sediments and buried by the debris cones is a well-developed organic soil from which two 14C dates have been obtained in an attempt to estimate the age range of the soil. These dates range from 2580±55 years BP for the fine particulate fraction from the base of the organic horizon to 940±95 years BP for fossil rootlets from the uppermost organic layer, immediately below the overlying debris cones. The pollen evidence suggests that the valley floor site was initially dominated by alder carr and later by a Juncus marsh with birch, alder and hazel nearby. The pollen, from the surrounding upland area suggests woodland on the valley sides, dominated by oak and elm that was later replaced by a more open environment rich in heath species and in which disturbed ground species were present. The magnetic evidence indicates a stable local environment during soil formation but shows a sudden inwash of unweathered debris at the top of the buried soil. The evidence suggests that the valley floor was geomorphologically stable throughout the period of soil formation, although there was a local change in valley floor vegetation and a reduction of woodland cover on the valley sides at sometime during the period. The evidence then points to major geomorphological changes; a wave of soil erosion, gully development and debris cone deposition, perhaps following the Scandinavian introduction of sheep farming in the tenth century A.D. 相似文献
992.
F. Machado 《Bulletin of Volcanology》1972,36(2):319-327
The trachytic volcanoes of San Miguel Island are emplaced on either side of a young basaltic area, which could correspond to some active branch of the mid-Atlantic rift. Geological and geomagnetic surveys suggest crustal drift of about 1 cm/year on either direction. From the seismic anomaly data, shallow magma chambers have been inferred for those acid volcanoes, where owing to the crustal drift the main vents appear to have been shifted relatively to the fractures feeding the chambers from the deep upper mantle supply. The primitive magma is certainly basaltic; when the surface vents are directly above the deep feeding fractures, only basaltic lavas of some primitive composition can be extruded; but, when the upper vents are laterally displaced, the magma is delayed in the chambers and will differentiate, eventually into trachytic material capable of producing huge pumice explosions. 相似文献
993.
994.
J.F. Payne 《Marine pollution bulletin》1977,8(5):112-116
Several phyla from the coastal Northwest Atlantic were investigated for mixed function oxidases. Enzyme activity was related to petroleum hydrocarbon metabolism and detection in the marine environment. 相似文献
995.
Aqueous extracts of two petroleum oils, No. 2 fuel oil and Southern Louisiana crude, were tested on two amphipods, Gammarus muccronatus and Amphithoe valida, for survival. The oils were toxic at concentrations of 0.8 ppm (fuel oil) and 2.4 ppm (S. Louisiana crude). Mortalities increased with the concentration and length of exposure. Few or no young were produced at these and higher concentrations (breeding adults were decreasing rapidly in numbers). The amphipods are more sensitive to aqueous extracts of these oils than benthic polychaetes and shrimp, for which data are available. 相似文献
996.
997.
This paper examines present and future resource utilization in the tidal section of the Georges River in the southern part of Sydney. Over the last 30 years, the physical characteristics of the river have changed as a result of urbanization, changed hydrologic regime and sand dredging. Urbanization has resulted in an increase in population from 190,000 in 1947 to 550,000. This has resulted in an increase in runoff, sediment input to the river and waste disposal. The change in hydrologic regime has resulted in a substantial increase in the magnitude and frequency of flooding which has increased erosion. Sand dredging has increased the channel capacity, enlarged the tidal prism and increased water turbidity. It has also resulted in extensive bank erosion in the upper part of the river and extensive deposition lower down as tailings are deposited. Although most of these changes have resulted in the deterioration of environmental quality, use of the river for recreation and as an urban amenity has increased. 相似文献
998.
W. F. Giggenbach 《Bulletin of Volcanology》1975,39(1):132-145
Investigations into the chemistry of volcanic gases depend on the availability of complete and accurate analyses of volcanic exhalations. The wide variety of sampling and analysis methods hitherto used, often supplying only partial analyses of low precision, made intercomparison, and thus a systematic study of volcanic gases, difficult. With the method proposed here, complete volcanic gas samples are obtained permitting the accurate determination of all major species by standard analytical methods without the need for highly specialised ancillary equipment. The samples are collected in evacuated 300 ml pyrex flasks through titanium tubes deeply inserted into the gas vent. Two types of flask are used, a single compartment flask allowing the easy determination of the major constituents and containing 50 ml 4 N NaOH, and a double compartment flask for the separate analysis of the sulfur species and containing 25 ml 0.1 N As2O3 in 1 N HClO4 in the first, and 50 ml 4 N NaOH in the second compartment. Non-absorbed gases are determined by gas chromatography, the rest by standard analytical procedures. The determination of H2O, CO2, SO2, SO2, S2, H2S, HCl, HF, H2, N2, O2, CH4, CO and NH2 is described. 相似文献
999.
For outcropping bodies an approximate direct interpretation of the associated gravity anomaly is generally obtained with the flat plate formula. Results can be significantly improved if the causative body is approximated by a bell shape instead of a flat plate. A set of parameter curves allows the conversion to depth data. The validity of the method is borne out by synthetic models and by field examples in a Nevada valley with Tertiary and in the Los Angeles Basin. The method provides structural definition more accurate than can be obtained with the flat plate formula, particularly in the case of narrow anomalies. 相似文献
1000.
More than fifty heat flow measurements in Italy are examined. The values, corrected only for local influences (when present), are related to the main geological features with the following results: foreland areas, 55±19 mW m–2, foredeep areas, 45±21 mW m–2; folded regions and intermountain depressions, 76±29 mW m–2. In volcanic areas the heat flow rises to in excess of 600 mW m–2. From a tectonic point of view, these values are consistent with the hypothesis that the Apennine chain is intersected by two arcuate structures: the first from Liguria to Latium is very probably a continental arc, that is an are which occurs within a continent, and the second from Campania to Calabria is very similar from geophysical evidence to the classic island arcs. 相似文献