全文获取类型
收费全文 | 1723篇 |
免费 | 73篇 |
国内免费 | 50篇 |
专业分类
测绘学 | 59篇 |
大气科学 | 131篇 |
地球物理 | 366篇 |
地质学 | 609篇 |
海洋学 | 118篇 |
天文学 | 412篇 |
综合类 | 11篇 |
自然地理 | 140篇 |
出版年
2024年 | 10篇 |
2023年 | 14篇 |
2022年 | 10篇 |
2021年 | 38篇 |
2020年 | 42篇 |
2019年 | 39篇 |
2018年 | 81篇 |
2017年 | 68篇 |
2016年 | 78篇 |
2015年 | 74篇 |
2014年 | 79篇 |
2013年 | 113篇 |
2012年 | 67篇 |
2011年 | 98篇 |
2010年 | 82篇 |
2009年 | 93篇 |
2008年 | 92篇 |
2007年 | 92篇 |
2006年 | 76篇 |
2005年 | 59篇 |
2004年 | 64篇 |
2003年 | 49篇 |
2002年 | 48篇 |
2001年 | 36篇 |
2000年 | 44篇 |
1999年 | 34篇 |
1998年 | 21篇 |
1997年 | 23篇 |
1996年 | 19篇 |
1995年 | 11篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有1846条查询结果,搜索用时 15 毫秒
71.
Alexander Zdravkov Irena Kostova Jordan Kortenski 《International Journal of Coal Geology》2007,71(4):488
Several Mio-Pliocene aged lignite seams occur as part of a non-marine transgressive sequence in the Elhovo graben in south-eastern Bulgaria. The present study is focused on 45 samples collected from three boreholes in the eastern part of the basin. Petrographic data along with ash and sulphur contents were used in order to determine the lateral and vertical variations of the coal facies and depositional environment of the Elhovo lignite.The lignite seams accumulated in a rheotrophic, low-lying mire with high pH value and are characterized by high ash yields and sulphur contents. Despite of the neutral to weakly alkaline environment the bacterial activity was limited and the tissue preservation and gelification were mainly controlled by the redox conditions.Vegetation rich in decay resistant conifers dominated in the Elhovo basin together with mesophytic angiosperm species. The absence of algal remains and sapropelic coal indicated that open water areas were not present during peat accumulation. The latter processed in an environment, characterized by low subsidence rate, in which prior to the burial the woods were subjected to severe mechanical destruction. According to our interpretation, the enhanced impregnation of the tissues bacteria and fungi played only a secondary role in the process of humification. The lignite from borehole 122 and partly from BH 145 deposited in an environment characterized by relatively low (ground)water table, whereas to the south an area dominated by a flooded forest swamp (BH 104) formed. This is suggested by the better tissue preservation and gelification of the organic matter in BH 104. The vertical variation of the maceral composition in the studied lignite is interpreted as a consequence of vegetational changes, rather than to changes in the depositional environment. The low contents of inertinite macerals indicate that despite of the low water level the environment was relatively wet and the thermal and oxidative destruction of the tissues was limited.Peat accumulation was terminated by a major flooding event and a short term establishment of a lake. In contrast to the West Maritsa basin, no seam formed in the Elhovo basin during the filling stage of the lake. 相似文献
72.
Natural Hazards - Flooding is the most costly type of natural disaster, as well as the most frequent. To provide risk-based flood insurance, providers must be able to accurately determine an... 相似文献
73.
Jürgen Konzett Daniel J. Frost Alexander Proyer Peter Ulmer 《Contributions to Mineralogy and Petrology》2008,155(2):215-228
Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO2–TiO2–Al2O3–Fe2O3–FeO–MnO–MgO–CaO–Na2O–K2O–P2O5 and SiO2–TiO2–Al2O3–MgO–CaO–Na2O to investigate the Ca-Eskola (CaEs Ca0.5□0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 ± TiO2 ± kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative
anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison,
no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these
textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites
cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes
are not necessarily restricted to ultrahigh pressure metamorphic conditions.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
74.
75.
76.
The mollusc Watsonella crosbyi was studied on the basis of material from the lower Cambrian Dahai Member of the Zhujiaqing Formation in eastern Yunnan, China and the Pestrotsvet Formation of the Aldan River region in the Siberian Platform. This fossil had been excessively described under different names in the literature, such as Heraultia varensalensis Cobbold, 1935, H. sibirica Missarzhevsky, 1974, Heraultipegma yunnanensis He and Yang, 1982, H. yannanese He and Yang, 1982, Watsonella yunnanensis, W. crosbyi Grabau, 1900, etc. Taxonomic revision shows that other species are junior synonyms of W. crosbyi. Analysis of functional morphology implies that W. crosbyi may be rather a helcionelloid than a rostroconch since its univalved conch is untorted endogastric and lacks a rostrum and a true internal pegma. Replicas of the original microstructures preserved on the surfaces of phosphatic internal moulds confirm that the shell consists of two layers. The inner layer is lamello-fibrillar, and the outer layer is spherulitic prismatic. The widespread geographical occurrences in both siliciclastic and carbonate rocks of the late Terreneuvian indicate that W. crosbyi is an important index fossil for global correlations and subdivision of the Cambrian Series 1 (Terreneuvian). The first appearance datum (FAD) of W. crosbyi is suggested to be a potential GSSP (Global Boundary Stratotype Section and Point) candidate marker for defining the base of the Cambrian Stage 2. 相似文献
77.
78.
Alexander S. Lloyd Terry Plank Philipp Ruprecht Erik H. Hauri William Rose 《Contributions to Mineralogy and Petrology》2013,165(1):129-153
We have investigated the loss of H2O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volcán de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H2O contents (loss of ~1 wt% H2O) and 10 % post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D~10?9 to 10?10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H2O concentrations in olivine-hosted MIs may reflect unrecognized H2O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent. 相似文献
79.
Three choices of control variables for meteorological variational analysis (3DVAR or 4DVAR) are associated with horizontal
wind: (1) streamfunction and velocity potential, (2) eastward and northward velocity, and (3) vorticity and divergence. This
study shows theoretical and numerical differences of these variables in practical 3DVAR data assimilation through statistical
analysis and numerical experiments. This paper demonstrates that (a) streamfunction and velocity potential could potentially
introduce analysis errors; (b) A 3DVAR using velocity or vorticity and divergence provides a natural scale dependent influence
radius in addition to the covariance; (c) for a regional analysis, streamfunction and velocity potential are retrieved from
the background velocity field with Neumann boundary condition. Improper boundary conditions could result in further analysis
errors; (d) a variational data assimilation or an inverse problem using derivatives as control variables yields smoother analyses,
for example, a 3DVAR using vorticity and divergence as controls yields smoother wind analyses than those analyses obtained
by a 3DVAR using either velocity or streamfunction/velocity potential as control variables; and (e) statistical errors of
higher order derivatives of variables are more independent, e.g., the statistical correlation between U and V is smaller than the one between streamfunction and velocity potential, and thus the variables in higher derivatives are more
appropriate for a variational system when a cross-correlation between variables is neglected for efficiency or other reasons.
In summary, eastward and northward velocity, or vorticity and divergence are preferable control variables for variational
systems and the former is more attractive because of its numerical efficiency. Numerical experiments are presented using analytic
functions and real atmospheric observations. 相似文献
80.
John Alexander Stevenson Neil Charles Mitchell Fiona Mochrie Michael Cassidy Harry Pinkerton 《Bulletin of Volcanology》2012,74(1):33-46
The Nesjahraun is a basaltic lava flow erupted from a subaerial fissure, extending NE along the Tingvellir graben from the
Hengill central volcano that produced pāhoehoe lava followed by ‘a‘ā. The Nesjahraun entered Iceland’s largest lake, Tingvallavatn,
along its southern shore during both phases of the eruption and exemplifies lava flowing into water in a lacustrine environment
in the absence of powerful wave action. This study combines airborne light detection and ranging, sidescan sonar and Chirp
seismic data with field observations to investigate the behaviour of the lava as it entered the water. Pāhoehoe sheet lava
was formed during the early stages of the eruption. Along the shoreline, stacks of thin (5–20 cm thick), vesicular, flows
rest upon and surround low (<5 m) piles of coarse, unconsolidated, variably oxidised spatter. Clefts within the lava run inland
from the lake. These are 2–5 m wide, >2 m deep, ∼50 m long, spaced ∼50 m apart and have sub-horizontal striations on the walls.
They likely represent channels or collapsed tubes along which lava was delivered into the water. A circular rootless cone,
Eldborg, formed when water infiltrated a lava tube. Offshore from the pāhoehoe lavas, the gradient of the flow surface steepens,
suggesting a change in flow regime and the development of a talus ramp. Later, the flow was focused into a channel of ‘a‘ā
lava, ∼200–350 m wide. This split into individual flow lobes 20–50 m wide along the shore. ‘A‘ā clinker is exposed on the
water’s edge, as well as glassy sand and gravel, which has been locally intruded by small (<1 m), irregularly shaped, lava
bodies. The cores of the flow lobes contain coherent, but hackly fractured lava. Mounds consisting predominantly of scoria
lapilli and the large paired half-cone of Grámelur were formed in phreatomagmatic explosions. The ‘a‘ā flow can be identified
underwater over 1 km offshore, and the sidescan data suggest that the flow lobes remained coherent flowing down a gradient
of <10°. The Nesjahraun demonstrates that, even in the absence of ocean waves, phreatomagmatic explosions are ubiquitous and
that pāhoehoe flows are much more likely to break up on entering the water than ‘a‘ā flows, which, with a higher flux and
shallow underlying surface gradient, can penetrate water and remain coherent over distances of at least 1 km. 相似文献