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41.
Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region. 相似文献
42.
Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene
lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting
with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous
subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at
least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred
to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras
are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions
were penecontemporaneous with far-traveled lahars, probably created from newly erupted materials melting snow and glacial
ice. The most recent magmatic eruption of Mount Rainier, documented geochemically, was the 1,000 cal year BP event. Products
from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974))
are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century
eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year
BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
T. W. Sisson and J. W. Vallance contributed equally to this study. 相似文献
43.
The objective of this work was to evaluate crushed recycled glass as a medium for rapid filtration. In the first part of this work, physical and hydraulic characteristics of the glass medium were studied. In the second part, pilot scale inline filtration experiments were carried out using raw waters from three different water sources. Two physically identical filter columns were operated in parallel in all the experiments. One filter contained a silica sand medium that is widely used in Turkey, whereas the other filter contained crushed recycled glass. Experiments were repeated five times as follows: (i) Without the use of a coagulant, (ii–iii) with 5 mg/L and 10 mg/L of alum, and (iv–v) with 5 mg/L and 10 mg/L of ferric chloride. Turbidity, particle counts, and head losses were measured and compared as functions of time. The following were observed: (1) Provided that a coagulant was used, the filter containing crushed glass produced effluent turbidities and particle counts similar to those obtained with the sand filter. (2) The crushed glass medium generated both a smaller clean‐bed head loss and smaller clogging head losses than those of the sand filter. It is concluded that crushed glass shows significant promise as an alternative to silica sand in rapid filtration. 相似文献
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47.
“玻璃地球”建设的现状、问题、趋势与对策 总被引:2,自引:0,他引:2
"玻璃地球"是地质时空大数据的有效载体,是"数字地球"在地矿领域的体现。国际上"玻璃地球"建设存在的问题主要是缺乏应有的总体规划、顶层设计和协调机制,仅实现了地层框架模型的构建,缺乏多源、多主题信息的混合建模,以及复杂地质结构的快速、动态、全息、精细的多维建模,也未解决多尺度地质框架模型的集成问题。其发展趋势是从个别走向一般,由局部走向整体,从功能走向数据,从零散走向集成,从展示走向实用。由于"玻璃地球"规模大、涉及面广,建设目标与全国地质工作信息化及"智慧地球"一致,有必要从战略上进行统一规划和顶层设计,并把相关的"数字国土"、"数字矿山"、"数字油田"、"数字工程"等纳入到统一的轨道中来。 相似文献
48.
Near-infrared (NIR) absorption bands related to total water (4000 and 7050 cm−1), OH groups (4500 cm−1) and molecular H2O (5200 cm−1) were studied in two polymerised glasses, a synthetic albitic composition and a natural obsidian. The water contents of the
glasses were determined using Karl Fischer titration. Molar absorption coefficients were calculated for each of the bands
using albitic glasses containing between 0.54 and 9.16 wt.% H2O and rhyolitic glasses containing between 0.97 and 9.20 wt.% H2O. Different combinations of baseline type and intensity measure (peak height/area) for the combination bands at 4500 and
5200 cm−1 were used to investigate the effect of evaluation procedure on calculated hydrous species concentrations. Total water contents
calculated using each of the baseline/molar absorption coefficient combinations agree to within 5.8% relative for rhyolitic
and 6.5% relative for albitic glasses (maximum absolute differences of 0.08 and 0.15 wt.% H2O, respectively). In glasses with water contents >1 wt.%, calculated hydrous species concentrations vary by up to 17% relative
for OH and 11% relative for H2O (maximum absolute differences of 0.33 and 0.43 wt.% H2O, respectively). This variation in calculated species concentrations is typically greater in rhyolitic glasses than albitic.
In situ, micro-FTIR analysis at 300 and 100 K was used to investigate the effect of varying temperature on the NIR spectra of the
glasses. The linear and integral molar absorption coefficients for each of the bands were recalculated from the 100 K spectra,
and were found to vary systematically from the 300 K values. Linear molar absorption coefficients for the 4000 and 7050 cm−1 bands decrease by 16–20% and integral molar absorption coefficients by up to 30%. Depending on glass composition and baseline
type, the integral molar absorption coefficients for the absorption bands related to OH groups and molecular H2O change by up to −5.8 and +7.4%, respectively, while linear molar absorption coefficients show less variation, with a maximum
change of ∼4%. Using the new molar absorption coefficients for the combination bands to calculate species concentrations at
100 K, the maximum change in species concentration is 0.08 wt.% H2O, compared with 0.39 wt.% which would be calculated if constant values were assumed for the combination band molar absorption
coefficients. Almost all the changes in the spectra can therefore be interpreted in terms of changing molar absorption coefficient,
rather than interconversion between hydrous species.
Received: 17 December 1998 / Revised, accepted 8 July 1999 相似文献
49.
Very thick units of massive pumice and lithic clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensland, are deposits from cold, water-supported, volcaniclastic mass flows emplaced in a below-wave base submarine setting. Adjacent to syn-volcanic andesitic and rhyolitic sills and dykes, the pumice-lithic breccia shows a well-developed eutaxitic texture. The eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact. At these sites, pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening. Some lenticular pumice clasts contain small (2 mm), round, quartz-filled amygdales and spherulites. Further away from the sills and dykes, the pumice clasts have randomly oriented, delicate tube vesicle structure and are blocky or lensoid in shape. Round amygdales were generated by re-vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases involving sills, secondary welding along the contacts formed extensive, conformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit. 相似文献
50.
The Mie scattering theory shows that the presence of randomly distributed submicroscopic inclusions with narrow size distribution
and a refractive index n
i
in a matrix with different refractive index n
m
may give rise to a λ-dependent, band-like scattering (e.g., Kortüm 1969). Intensity and spectral position of this scattering
depend on a combination of several independent parameters, such as size and number of inclusions, their form and n
i
/n
m
ratio (Kortüm 1969). Recently, it was confirmed that at a certain inclusion size and n
i
/n
m
ratio the scattering bands can contribute to the UV-edge in the electronic absorption spectra of garnets, influencing their
colour (Khomenko et al. 2001). In natural minerals, however, it is impossible to differentiate between individual influence
on scattering of the above mentioned parameters because of complex and interconnected variations in number, size and type
of inclusions. Additionally, in most natural minerals variable amounts of transition metal ions are present. They may cause
UV-VIS absorption due to ligand to metal charge transfer (LMCT) in the same region where band-like scattering may occur (Khomenko
et al. 2001). At least some of these difficulties may be avoided in the case of some glass ceramics where number and size
of crystalline microinclusions can be controlled by varying ceramization conditions such that fine-grained homogeneous microstructures
are formed (e.g., James 1982; Petzoldt and Pannhorst 1991). Thus, glass ceramics of known composition, containing different
amounts of microcrystals of specified size, may serve as unique patterns for the experimental study of effects caused by submicrocrystals
on bulk properties of transparent solid materials, such as scattering, UV-VIS transparency and colour. Four types of parent
glasses and a series of glass ceramic materials produced from them by heat treatment were investigated using transmission
electron microscopy (TEM). These materials were also studied by transmission spectrometry in the range 35 000–20 000 cm−1. Different inclusions, from five to several hundred nm in size, were observed in the glass matrices depending on their compositions
and heating history. These inclusions represent two groups: early very small crystals of Ti, Zr oxides and relatively large
crystals of stuffed high-quartz type or keatite. The absorption spectra of the glass ceramics show largely varying long-wavelength
slopes of the UV absorption. UV-edge intensity correlates mostly with the size of the inclusions and changes drastically when
larger keatite-type microcrystals are growing. Small variations in the UV edges also follow the early process of Ti-phase
separation and nucleation. This may be explained by Ti depletion from the glass matrix and, thus, by reducing the measured
intensity of LMCT in the first co-ordination sphere of Ti4+ ions. The different yellowish colourations of unheated glasses studied here are caused by this effect, whereas developing
several hundred-nm-large keatite crystals leads to a strong scattering effect and a milky colour in glass ceramics.
Received: 15 February 2002 / Accepted: 11 November 2002
Acknowledgements M. Garsche and A. Kisljuck generously provided the base glass and glass ceramic samples. F. Galbert and S. Herhing-Aghte,
both at the Technical University Berlin, helped with electron microprobe analyses and refractive indices measurements, respectively.
The Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, provided financial support through grant La324/35. To all these individuals
and this institution our sincere thanks are due. 相似文献