全文获取类型
收费全文 | 2741篇 |
免费 | 456篇 |
国内免费 | 1165篇 |
学科分类
地球科学 | 4362篇 |
出版年
2024年 | 12篇 |
2023年 | 47篇 |
2022年 | 100篇 |
2021年 | 121篇 |
2020年 | 119篇 |
2019年 | 146篇 |
2018年 | 109篇 |
2017年 | 147篇 |
2016年 | 157篇 |
2015年 | 138篇 |
2014年 | 201篇 |
2013年 | 182篇 |
2012年 | 166篇 |
2011年 | 200篇 |
2010年 | 171篇 |
2009年 | 191篇 |
2008年 | 171篇 |
2007年 | 178篇 |
2006年 | 171篇 |
2005年 | 148篇 |
2004年 | 176篇 |
2003年 | 132篇 |
2002年 | 134篇 |
2001年 | 133篇 |
2000年 | 107篇 |
1999年 | 111篇 |
1998年 | 114篇 |
1997年 | 114篇 |
1996年 | 100篇 |
1995年 | 45篇 |
1994年 | 81篇 |
1993年 | 48篇 |
1992年 | 47篇 |
1991年 | 40篇 |
1990年 | 19篇 |
1989年 | 23篇 |
1988年 | 23篇 |
1987年 | 8篇 |
1986年 | 14篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1981年 | 3篇 |
1978年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有4362条查询结果,搜索用时 218 毫秒
21.
The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr−1, a transitional velocity between the very slow (≤20 mm yr−1) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr−1) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam,
magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is
partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order
discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating
complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first
and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace
of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to
lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the
ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to
the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief
of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments
at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit
along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the
segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively
low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and
is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the
ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken
together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining
segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this
contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather,
there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge
segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent
with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused
along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate
that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking
morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as
well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the
morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies
the segment. 相似文献
22.
W. R. Roest J. J. Dañobeitia J. Verhoef B. J. Collette 《Marine Geophysical Researches》1992,14(1):1-24
The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.Deceased 11 November 1991 相似文献
23.
本文阐述了莱州湾东部滨海砂金的物源、地貌、水动力、第四系、新构造运动和海平面变化等成矿条件以及砂金的富集特点,对该区砂金的成矿远景进行了区划和评价,初步认为该区具有良好的砂金成矿远景。 相似文献
24.
Submarine groundwater discharge (SGD) into a coastal lagoon off Perth, Western Australia, contains nitrate and silicate in concentrations two orders of magnitude higher than those of the receiving waters. This discharge delivers enough nitrate to replace that dissolved in the lagoon water mass about every eight days and enough silicate to replace the lagoon silicate in about 48 days. The delivery rate of nitrate nitrogen by SGD is equal to about 48% of that required for observed growth rates of lagoon macrophytes. Surface salinity is lower close to the shore as a result of SGD. During calm conditions a salinity front was observed in the lagoon, with a nearshore pool of nutrient-enriched water floating above the more saline ocean water. 相似文献
25.
根据库车前陆盆地水文地质条件,分析了地下水的铀成矿作用,并通过热力学计算等方法,对水中铀的存在形式及铀的溶解、迁移和沉淀富集过程作了较为详细的论述,阐明了盆地水化学及铀水文地球化学环境,并提出了成矿有利地段和有利层位。 相似文献
26.
27.
辽宁东五家子金矿的矿脉由蚀变岩和石英脉透镜体构成,可采矿体均为硫化物石英脉型.工业矿脉中,石英为烟灰色,发育他形、半自形的细粒黄铁矿等硫化物,有较大规模的矿体.矿化脉中,石英为乳白色,发育粗粒、自形的浅色黄铁矿,无可采矿体.在工业矿脉、次要矿脉、矿化脉的蚀变岩和石英脉样品中,Au,Hg与其他元素的相关性有明显区别.用石英脉样品的As-Ni-Ba图解和蚀变岩样品的Au-Ag-Ba图解预测,矿区外围的西沟1号脉和西沟3号脉属于工业矿脉. 相似文献
28.
29.
斜坡非饱和带低渗透岩石结构体风化前锋扩展过程 总被引:2,自引:0,他引:2
非饱和带低渗透岩石结构体风化前锋扩展是斜坡灾害孕育过程中的关键环节之一。到目前为止,化学风化领域的研究成果主要集中在高水-岩比的饱和环境下单矿物及矿物成分单一的碳酸盐岩的风化动力学方面,非饱和环境下的岩石风化研究还主要限于风化产物的特性描述方面。斜坡非饱和带低渗透岩石结构体风化前锋扩展是一种高度复杂的THMC过程。由于在溶解对象、溶解液及溶解过程的水动力-水化学环境等方面存在的显著差异,既有的地质材料风化动力学成果为探索这一复杂过程提供了理论储备,但还不能阐明其核心机理。斜坡非饱和带低渗透岩石结构体风化前锋的扩展过程研究不仅可以加深人们对斜坡灾害孕育的理解,为灾害预报-预警及其控制提供理论依据,同时可以促进岩石-岩体力学及浅表生水文地球化学等相关学科的交叉与发展。 相似文献
30.