全文获取类型
收费全文 | 1649篇 |
免费 | 298篇 |
国内免费 | 557篇 |
专业分类
测绘学 | 144篇 |
大气科学 | 303篇 |
地球物理 | 277篇 |
地质学 | 1218篇 |
海洋学 | 259篇 |
天文学 | 9篇 |
综合类 | 82篇 |
自然地理 | 212篇 |
出版年
2024年 | 12篇 |
2023年 | 31篇 |
2022年 | 80篇 |
2021年 | 72篇 |
2020年 | 96篇 |
2019年 | 113篇 |
2018年 | 79篇 |
2017年 | 118篇 |
2016年 | 93篇 |
2015年 | 89篇 |
2014年 | 104篇 |
2013年 | 112篇 |
2012年 | 122篇 |
2011年 | 93篇 |
2010年 | 82篇 |
2009年 | 121篇 |
2008年 | 126篇 |
2007年 | 106篇 |
2006年 | 111篇 |
2005年 | 82篇 |
2004年 | 84篇 |
2003年 | 73篇 |
2002年 | 72篇 |
2001年 | 57篇 |
2000年 | 49篇 |
1999年 | 37篇 |
1998年 | 48篇 |
1997年 | 44篇 |
1996年 | 32篇 |
1995年 | 37篇 |
1994年 | 27篇 |
1993年 | 29篇 |
1992年 | 12篇 |
1991年 | 17篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有2504条查询结果,搜索用时 156 毫秒
41.
Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed. 相似文献
42.
Abstract The initial volcanic phase of Cretaceous island arc strata in central Puerto Rico, at the eastern end of the extinct Greater Antilles Arc, comprises a 6‐km thick pile of lava and volcanic breccia (Río Majada Group). Preserved within the sequence is a conspicuous shift in absolute abundances of the more incompatible elements, including Th, Nb, and the light rare earth elements (LREE: La, Ce, Pr and Nd). The compositional shift is marked by a decrease in La/Sm from averages of 2.11 in the lowest third of the pile (Formation A) to 1.48 at the top (Formation C), and by a distinctive flattening of LREE segments of chondrite‐normalized REE patterns. i87Sr/86Sr and ?Nd average about 0.7035 and 8.2, respectively, in early Formation A basalts. These ranges normally overlap samples from later Formations B and C. Isotope compositions of the latter group are more variable, however, and several samples are considerably more radiogenic than Formation A basalts, such that i87Sr/86Sr averages almost 0.7042 while ?Nd‐values decrease to 7.5 in Formation B and C basalts. Theoretical models of non‐modal melting processes in both amphibole peridotite and spinel lherzolite sources provide insight into the origin of depleted Th, Nb, and LREE abundances in Puerto Rican basalts. Low Nb concentrations less than normal mid‐oceanic ridge basalts in Formation A basalts indicate the wedge was slightly depleted by low‐volume decompression fusion due to induced convection in the back‐arc region prior to entry of the source into the arc melting zone. However, depleted patterns in Formation C basalts cannot be generated by relatively greater degrees of decompression fusion in the back‐arc, because addition of the La‐enriched slab‐derived component to more depleted source material invariably produces elevated rather than decreased La/Sm. Refluxing of Formation A harzburgitic residua is similarly precluded. In contrast, the observed patterns are readily reproduced by multistage melting models involving hybridized sources containing normal Formation A lherzolite source material blended with recycled, unrefluxed harzburgite residua. Successful models require hybrid sources containing large volumes of recycled harzburgite (up to 50%) during generation of Formation C basalts. Slightly elevated radiometric Sr and Nd isotopes in a few flows from Formation C are attributed to partial refluxing of the hybrid sources within the wedge. 相似文献
43.
Shallow seismicity and available source mechanisms in the Andaman–westSunda arc and Andaman sea region suggest distinct variation in stressdistribution pattern both along and across the arc in the overriding plate.Seismotectonic regionalisation indicates that the region could be dividedinto eight broad seismogenic sources of relatively homogeneousdeformation. Crustal deformation rates have been determined for each oneof these sources based on the summation of moment tensors. The analysisshowed that the entire fore arc region is dominated by compressive stresseswith compression in a mean direction of N23°, and the rates ofseismic deformation velocities in this belt decrease northward from 5.2± 0.65 mm/yr near Nias island off Sumatra and 1.12 ±0.13 mm/yr near Great Nicobar islands to as much as 0.4 ±0.04 mm/yr north of 8°N along Andaman–Nicobar islandsregion. The deformation velocities indicate, extension of 0.83 ±0.05 mm/yr along N343° and compression of 0.19 ±0.01 mm/yr along N73° in the Andaman back arc spreadingregion, extension of 0.18 ± 0.01 mm/yr along N125° andcompression of 0.16 ± 0.01 mm/yr along N35° in NicobarDeep and west Andaman fault zone, compression of 0.84 ±0.12 mm/yr N341° and extension of 0.77 ± 0.11 mm/yralong N72° within the transverse tectonic zone in the Andamantrench, N-S compression of 3.19 ± 0.29 mm/yr and an E-Wextension of 1.24 ± 0.11 mm/yr in the Semangko fault zone ofnorth Sumatra. The vertical deformation suggests crustal thinning in theAndaman sea and crustal thickening in the fore arc and Semangko faultzones. The apparent stresses calculated for all major events range between0.1–10 bars and the values increase with increasing seismic moment.However, the apparent stress estimates neither indicate any significantvariation with faulting type nor display any variation across the arc, incontrast to the general observation that the fore arc thrust events showhigher stress levels in the shallow subduction zones. It is inferred that theoblique plate convergence, partial subduction of 90°E Ridge innorth below the Andaman trench and the active back arc spreading are themain contributing factors for the observed stress field within the overridingplate in this region. 相似文献
44.
45.
46.
Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from 1900 14C yr BP to the present. Radiocarbon dating of pyroclastic deposits indicates an almost continuous activity with periods of high eruption rates alternating with shorter time spans of distinctly reduced eruptive frequency since the first appearance of high-K volcanic rocks. Geochemical data of 28 well-dated, prehistoric pyroclastic flows of the Merapi high-K series indicate systematic cyclic variations. These medium-term compositional variations result from a complex interplay of several magmatic processes, which ultimately control the periodicity and frequency of eruptions at Merapi. Low eruption rates and the absence of new influxes of primitive magma from depth allow the generation of basaltic andesite magma (56–57 wt% SiO2) in a small-volume magma reservoir through fractional crystallisation from parental mafic magma (52–53 wt% SiO2) in periods of low eruptive frequency. Magmas of intermediate composition erupted during these stages provide evidence for periodic withdrawal of magma from a steadily fractionating magma chamber. Subsequent periods are characterised by high eruption rates that coincide with shifts of whole-rock compositions from basaltic andesite to basalt. This compositional variation is interpreted to originate from influxes of primitive magma into a continuously active magma chamber, triggering the eruption of evolved magma after periods of low eruptive frequency. Batches of primitive magma eventually mix with residual magma in the magmatic reservoir to decrease whole-rock SiO2 contents. Supply of primitive magma at Merapi appears to be sufficiently frequent that andesites or more differentiated rock types were not generated during the past 2000 years of activity. Cyclic variations also occurred during the recent eruptive period since AD 1883. The most recent eruptive episode of Merapi is characterised by essentially uniform magma compositions that may imply the existence of a continuously active magma reservoir, maintained in a quasi-steady state by magma recharge. The whole-rock compositions at the upper limit of the total SiO2 range of the Merapi suite could also indicate the beginning of another period of high eruption rates and shifts towards more mafic compositions. 相似文献
47.
Abstract A controversial stratigraphic section, the Taneichi Formation, is exposed along the Pacific Coast of northeastern Honshu, the main island of the Japanese Archipelago. Although most sediments of the formation have long been dated as late Cretaceous, the northern section of it has been assigned to (i) the Upper Cretaceous; (ii) the Paleogene; or (iii) the Neogene. In the present report, we present the data of palynological and sedimentological studies, showing that the northern section should be assigned to the Neogene. A more important point in the present study is that we invoke some basic principles of fluvial sedimentology to resolve this stratigraphic subject. The lignite layers full of Paleogene–Miocene dinoflagellate cysts and pollen assemblages drape over the boulder‐sized (>40 cm in diameter) clasts in the northern section. However, the layers totally consist of aggregates of small lignite chips, indicating that the lignites are allochthonous materials. The mega‐clasts with derived microfossils in the lignites are thought to have been deposited as Neogene fluvial (flood) sediments in the newly formed Japanese Archipelago. Prior to the Miocene, the northern Honshu was part of the Eurasian Plate, thus the boulder‐sized clasts cannot be envisaged as long river flood deposits along the continental Paleogene Pacific Coast. Instead, the mega‐clasts with the draping lignites were probably derived from nearby Miocene highlands in the newly born island arc. 相似文献
48.
49.
50.
目前 ,我国气象部门相当普遍地拥有气象卫星观测资料 ,但是 ,这些资料如何在城市气象服务以及为城市建设规划等方面的决策研究中发挥作用 ,急待进一步研究并开展工作。本文试图通过合理的反演处理 ,给出卫星观测对地面温度的测量结果 ,由此可比较细致地分析了解地面热状况及其与土地利用类型 ,地面覆盖性质等的关系。本文利用有限的 NOAA- AVHRR气象卫星遥感资料 ,采用修正的 Ulivieri分裂窗方法反演出同期的地表温度场 ,通过对北京地区的卫星观测资料进行算例分析 ,揭示出一个 2 0 0 km× 2 0 0 km范围内的地面温度分布特征。反演结果可以明显地揭示出城市热岛现象的变化规律 ,并且能较为清楚地反映出相对的高温区和低温区 相似文献