全文获取类型
收费全文 | 486篇 |
免费 | 12篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 38篇 |
地球物理 | 91篇 |
地质学 | 206篇 |
海洋学 | 29篇 |
天文学 | 52篇 |
综合类 | 1篇 |
自然地理 | 59篇 |
出版年
2023年 | 2篇 |
2021年 | 3篇 |
2019年 | 3篇 |
2018年 | 4篇 |
2017年 | 8篇 |
2016年 | 12篇 |
2015年 | 6篇 |
2014年 | 11篇 |
2013年 | 26篇 |
2012年 | 18篇 |
2011年 | 18篇 |
2010年 | 20篇 |
2009年 | 19篇 |
2008年 | 25篇 |
2007年 | 29篇 |
2006年 | 25篇 |
2005年 | 24篇 |
2004年 | 18篇 |
2003年 | 19篇 |
2002年 | 23篇 |
2001年 | 4篇 |
2000年 | 11篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 11篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 3篇 |
排序方式: 共有502条查询结果,搜索用时 0 毫秒
21.
Summary A geometrically simple volcano is considered, havig a spherical magma chamber of 2.5 km radius centred at 10 km depth. The Curie point isotherm is assumed to be a plane at 20 km depth, except for the spherical volume which is also non-magnetic. The stress pattern in the vicinity of the spherical chamber, due to regional stress of sufficient intensity to cause an eruptions, is used to calculate the change in magnetization which results from the piezomagnetic effect through the volume of solid rock. The consequent magnetic field anomaly at the surface is then obtaied by numerical integration of the dipole law of force over the stressed volume. For rocks of the type found on the volcanic island of St. Vincent (West Indies), this model gives a maximum local volcano-magnet c effect of about 7 gammas. 相似文献
22.
23.
Systematic variations in atmospheric heat exchange, surface residence time, and groundwater influx across montane stream networks commonly produce an increasing stream temperature trend with decreasing elevation. However, complex stream temperature profiles that differ from this common longitudinal trend also exist, suggesting that stream temperatures may be influenced by complex interactions among hydrologic and atmospheric processes. Lakes within stream networks form one potential source of temperature profile complexity due to the spatially variable contribution of lake-sourced water to stream flow. We investigated temperature profile complexity in a multi-season stream temperature dataset collected across a montane stream network containing many alpine lakes. This investigation was performed by making comparisons between multiple statistical models that used different combinations of stream and lake characteristics to represent specific hypotheses for the controls on stream temperature. The compared models included a set of models which used a topographically derived estimate of the hydrologic influence of lakes to separate and quantify the effects of stream elevation and lake source-water contributions to longitudinal stream temperature patterns. This source-water mixing model provided a parsimonious explanation for complex stream-network temperature patterns in the summer and autumn, and this approach may be further applicable to other systems where stream temperatures are influenced by multiple water sources. Simpler models that discounted lake effects were more optimal during the winter and spring, suggesting that complex patterns in stream temperature profiles may emerge and subside temporally, across seasons, in response to diversity of water temperatures from different sources. 相似文献
24.
25.
Ben K. Greenfield Geoffrey S. Siemering Joy C. Andrews Michael Rajan Stephen P. Andrews David F. Spencer 《Estuaries and Coasts》2007,30(4):627-640
Management actions to control invasive aquatic species can have significant ecosystem-scale effects. We evaluated the water
chemistry and nutrient effects of mechanical shredding to control water hyacinth (Eichhornia crassipes) in an agricultural slough and a tidal wetland on the Sacramento-San Joaquin River Delta, California. Shredding was conducted
with two types of shredder boats in fall of 2003 and another boat in spring of 2004. Shredding measurably affected water quality,
but specific effects varied as a function of shredding site and season. Significant increases were observed for total Kjeldahl
nitrogen and total phosphorus for all experiments. Dissolved oxygen effects varied by site, decreasing after shredding at
the agricultural slough but increasing at the tidal wetland. The increase in dissolved oxygen likely resulted from tidal incursions
from the adjacent river. A year-long time series of dissolved oxygen data indicated a negative relationship between hyacinth
abundance and dissolved oxygen concentrations. Hyacinth contained similar tissue concentrations of mercury to underlying sediments,
suggesting that plant harvesting could aid mercury remediation efforts. Simple mass calculations indicated that Delta-wide
shredding operations could cause between 0.1% and 9.6% increases in the overall abundance of carbon, nitrogen, and phosphorus
in the Delta water column. Results suggest that local effects of management actions to control invasive aquatic plants will
vary widely as a function of site-specific hydrology, but that estuary-wide effects would be limited. 相似文献
26.
Geoffrey Caruso Gilles Vuidel Jean Cavailhès Pierre Frankhauser Dominique Peeters Isabelle Thomas 《Journal of Geographical Systems》2011,13(1):31-48
We present a model that simulates the growth of a metropolitan area on a 2D lattice. The model is dynamic and based on microeconomics.
Households show preferences for nearby open spaces and neighbourhood density. They compete on the land market. They travel
along a road network to access the CBD. A planner ensures the connectedness and maintenance of the road network. The spatial
pattern of houses, green spaces and road network self-organises, emerging from agents individualistic decisions. We perform
several simulations and vary residential preferences. Our results show morphologies and transition phases that are similar
to Dieletric Breakdown Models (DBM). Such similarities were observed earlier by other authors, but we show here that it can be deducted from the functioning
of the land market and thus explicitly connected to urban economic theory. 相似文献
27.
Peter S. Meyer Henry J. B. Dick Geoffrey Thompson 《Contributions to Mineralogy and Petrology》1989,103(1):44-63
A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16 E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO2 contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge. 相似文献
28.
The Eocene Maoming oil shale from Guangdong Province occurs as a laterally uniform stratigraphic section, typically 20–25 m thick, from which the aliphatic hydrocarbon constituents of six representative samples were investigated using GC and C-GC-MS. The sediments evaluated included the basal lignite, a vitrinite lens from the overlying claystone, and four intervals from the massive oil shale bed. As expected, the lignite and vitrinite differ markedly from the oil shales. The lignite is dominated by bacterial hopanoids and components of higher plant origin, including C29 steroids and triterpenoids such as oleanenes. Visually, the oil shale samples show corroded and degraded phytoclasts, spores, wispy particles of fluorescent organic material attributable to dinoflagellates and, especially in the uppermost sample, colonial algal bodies. The distributions of biological markers in the oil shales show many features in common, notably a dominance of dinoflagellate-derived 4-methylsteroids, and a significant proportion of higher-plant derived n-alkanes with marked odd-over-even carbon number predominance. Overall, they exhibit several features that resemble characteristics of the Messel shale. The hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales. The aliphatic hydrocarbons of the uppermost shale are dominated by a number of C31 and C33 botryococcane homologues and other unusual branched alkanes possibly derived from green algae. All of the samples are immature. Overall, molecular and microscopic examination of the stratigraphic succession of the Maoming oil shale suggests a shallow, lacustrine environment within which peats were deposited. This lake subsequently deepened to support abundant algal populations, especially dinoflagellates, culminating in a dominance of botryococcoid algae. 相似文献
29.
The effects of low-temperature on the crystal structure of a natural epidote [Ca1.925Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6)? and β = 115.396(8)° at room conditions, Sp. Gr. P21
/m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction
data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe3+ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible
H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is
supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000–2,500 cm−1 are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)–O(4).
Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and
Fe3+ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to
protonation of O(10)–O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected
for “rigid body motions”. A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra,
especially when corrected for “non-correlated motion”. The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction
of the magnitude of the thermal displacement ellipsoids, with a variation of U
eq
(defined as one-third of the trace of the orthogonalised U
ij
tensor) by ~40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U
eq
of the octahedral and oxygen sites decrease similarly by ~35%, whereas those of the tetrahedral cations by ~22%. 相似文献
30.
Richard Christopher Chiverrell Geoff Stephen Powell Thomas Matthew Burke Alicia Medialdea Rachel Smedley Mark Bateman Chris Clark Geoffrey A. T. Duller Derek Fabel Geraint Jenkins Xianjiao Ou Helen Marie Roberts James Scourse 《第四纪科学杂志》2021,36(5):752-779
Here we reconstruct the last advance to maximum limits and retreat of the Irish Sea Glacier (ISG), the only land-terminating ice lobe of the western British Irish Ice Sheet. A series of reverse bedrock slopes rendered proglacial lakes endemic, forming time-transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, optically stimulated luminescence (OSL) bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples. This chronology constrains what is globally an early build-up of ice during late Marine Isotope Stage 3 and Greenland Stadial (GS) 5, with ice margins reaching south Lancashire by 30 ± 1.2 ka, followed by a 120-km advance at 28.3 ± 1.4 ka reaching its 26.5 ± 1.1 ka maximum extent during GS-3. Early retreat during GS-3 reflects piracy of ice sources shared with the Irish-Sea Ice Stream (ISIS), starving the ISG. With ISG retreat, an opportunistic readvance of Welsh ice during GS-2 rode over the ISG moraines occupying the space vacated, with ice margins oscillating within a substantial glacial over-deepening. Our geomorphological chronosequence shows a glacial system forced by climate but mediated by piracy of ice sources shared with the ISIS, changing flow regimes and fronting environments. 相似文献