全文获取类型
收费全文 | 997篇 |
免费 | 40篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 84篇 |
地球物理 | 218篇 |
地质学 | 340篇 |
海洋学 | 75篇 |
天文学 | 159篇 |
综合类 | 13篇 |
自然地理 | 113篇 |
出版年
2022年 | 6篇 |
2021年 | 13篇 |
2020年 | 14篇 |
2019年 | 22篇 |
2018年 | 35篇 |
2017年 | 20篇 |
2016年 | 33篇 |
2015年 | 23篇 |
2014年 | 37篇 |
2013年 | 74篇 |
2012年 | 37篇 |
2011年 | 44篇 |
2010年 | 41篇 |
2009年 | 63篇 |
2008年 | 66篇 |
2007年 | 50篇 |
2006年 | 38篇 |
2005年 | 35篇 |
2004年 | 32篇 |
2003年 | 39篇 |
2002年 | 31篇 |
2001年 | 27篇 |
2000年 | 13篇 |
1999年 | 17篇 |
1998年 | 14篇 |
1997年 | 14篇 |
1996年 | 16篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 4篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 12篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1972年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有1042条查询结果,搜索用时 31 毫秒
61.
Jeremy J. Kent Alan D. Brandon Katherine H. Joy Anne H. Peslier Thomas J. Lapen Anthony J. Irving Daniel M. Coleff 《Meteoritics & planetary science》2017,52(9):1916-1940
Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock. 相似文献
62.
Community involvement in fisheries management: Experiences in the Gulf of Thailand countries 总被引:1,自引:0,他引:1
This paper examines the involvement of coastal communities in fisheries management among the countries of the Gulf of Thailand—Malaysia, Vietnam, Cambodia and Thailand. Initiatives to decentralize management to local governing bodies, to utilize traditional management methods and to engage in community agreements to protect local resources are explored. An examination of recent experiences indicates some movement toward more local involvement in management. However, the study also leads to several suggestions for the future: in Vietnam and Cambodia, there is a need for significant legislation to control fisheries operations and greater clarity of the role of communities in management; in Malaysia, there is an overall need for more support to local fisheries management; and in Thailand, the need is for greater support of local-level enforcement and monitoring activities. 相似文献
63.
64.
Michael Anthony Velbel 《Chemical Geology》1989,78(3-4):245-253
Recent studies of alkali-feldspar hydrolysis kinetics have shown that, in the near-neutral pH range, weathering rates in natural systems are up to three orders of magnitude slower than laboratory rates. It has been hypothesized that decelerated rates may result from lower thermodynamic affinities for the hydrolysis reaction in natural systems than in laboratory systems. However, the chemical affinities for the feldspar hydrolysis reaction in two well-constrained natural systems are significantly higher than the threshold value at which affinity would exert detectable influences on the reaction rates. Thus, the hypothesis is rejected, and closer proximity to thermodynamic equilibrium in natural weathering systems does not account for the observed discrepancy between natural and laboratory rates of feldspar hydrolysis. Differences in feldspar weathering rates between natural and laboratory systems are most likely due to a combination of experimental preparation artifacts, loss of reactive surface to the formation of etch pits in naturally weathered feldspars, and inhomogeneous access of reactive fluids to those surfaces. 相似文献
65.
Pyroclastic flow emplacement is strongly influenced by eruption column height. A surface along which kinetic energy is zero theoretically connects the loci of eruption column collapse with all coeval ignimbrite termini. This surface is reconstructed as a two-dimensional energy line for the 1912 Katmai pyroclastic flow in the Valley of Ten Thousand Smokes from mapped flow termini and the runup of the ignimbrite onto obstructions and through passes. Extrapolation of the energy line to the vicinity of the source vent at Novarupta suggests the eruption column which generated the ignimbrite eruption was approximately 425 m high. The 1912 pyroclastic flow travelled about 25 km downvalley. Empirical velocity data calculated from runup elevations and surveyed centrifugal superelevations indicate initial velocities near Novarupta were greater than 79–88 m s–1. The flow progressively decelerated and was travelling only 2–8 m s–1 when it crossed a moraine 16 km downvalley. The constant slope of the energy line away from Novarupta suggests the flow was systematically slowed by internal and basal friction. Using a simple physical model to calculate flow velocities and a constant kinetic friction coefficient (Heim coefficient) of 0.04 derived from the reconstructed energy line, the flow is estimated to have decelerated at an average rate of –0.16 m s–2 and to have taken approximately 9.5 minutes to travel 25 km down the Valley of Ten Thousand Smokes. The shear strength of the flowing ignimbrite at the moraine was approximately 0.5 kPa, and its Bingham viscosity when it crossed the moraine was 3.5 × 103
P. If the flow was Newtonian, its viscosity was 4.2 × 103
P. Reynolds and Froude numbers at the moraine were only 41–62 and 0.84–1.04, respectively, indicating laminar, subcritical flow. 相似文献
66.
67.
68.
Three Australian brown coals have been separated into humin and humic acid fractions and studied by high resolution solid state 13C NMR spectroscopy and Fourier transform IR spectroscopy. The aromatic rings of the humic acids are highly substituted showing that extensive cross linking must have occurred during formation from wood lignin and tannin. However, the humins contain more aliphatic carbon and hydrogen than the corresponding humic acids. This shows that little cross linking has occurred with other components of the brown coal such as resins, waxes cutin and algal detritus, and cross linking has not rendered the aromatics alkali insoluble. The kinetics of extraction are complex and not simple first order. This is reflected in the chemical composition of the humic acid which is extraction temperature dependent. We also observed that there is a conversion of aromatic carbon to aliphatic carbon and gas during extraction, probably by alkaline oxidation, resulting in ring opening. A range of suitable model compounds have been studied to confirm this finding. Such a mechanism may account for the modification of lignin in oxidising environments such as those occurring in the initial stages of coalification (lignite or brown coal formation) and in soils. 相似文献
69.
Six plastic/rubber materials commonly encountered in marine debris and beach litter were studied under Biscayne Bay (Florida, USA) exposure conditions to determine the effect of fouling on buoyancy. Studies under restricted floating, and restricted submerged exposure conditions suggest that most plastic samples undergo fouling to an extent to cause the sample to be negatively buoyant in sea water. Rapid defouling of the submerged fouled samples was observed. The findings suggest that free-floating plastics at sea may, under certain conditions, undergo fouling-induced sinking followed by resurfacing as floating debris. 相似文献
70.
A total of 269 tropical storms and hurricanes originated in the North Atlantic basin from 1960–1989. Of these, 76 made landfall on the continental United states. This study divides the 76 tropical storms into their month of formation. Seasonal shifts in the principal areas of tropical cyclone formation over the Atlantic basin have been recognized for many decades. The results of the study suggest that the early and late season tropical cyclones develop in areas which are first affected by the position of the sun, resulting in an increase in water temperatures. These cyclones normally make landfall along the Gulf Coast and usually are of low intensity. Formation areas shift eastward in mid-summer with a slight increase in intensity. By late August and early September, the formation areas have extended to the Cape Verde Islands. These storms tend to strike the east coast of the US and are normally more intense. By the end of the hurricane season, the primary formation area has shifted back to the Gulf of Mexico, with low intensity storms affecting the Gulf Coast. 相似文献