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311.
Chris B. Folkes Shanaka L. de Silva Heather M. Wright Raymond A. F. Cas 《Bulletin of Volcanology》2011,73(10):1455-1486
By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components,
we show that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr.
The Cerro Galán system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides,
apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro
Galán ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite
contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide
and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially
hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice
is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone.
Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from
40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and
Yb between the Galán ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent
magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute
this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like
magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume
ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galán
system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts.
A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance
of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly
identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma
source in a subduction zone volcanic setting. At Cerro Galán, we propose that there is a zone between mantle magma input and
upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures.
This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation,
storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve
only to slightly modify the geochemistry of the magmas. 相似文献
312.
Charles W. Querfeld Raymond N. Smartt Veronique Bommier Egidio Landi Degl'innocenti Lewis L. House 《Solar physics》1985,96(2):277-292
The Stokes components of He i D3 emission in two quiescent prominences, using full spectral profile measurements, are analyzed to derive vector magnetic fields. Two independently developed schemes, based on the Hanle effect, are used for interpretation. They involve solutions of the statistical equilibrium equations for the He i D3 multiplet, including the effect of coherency and full level crossing, which predict the magnetic field dependence of the observed polarization. Derived magnetic field vector solutions for each pair of linear polarization Stokes profiles corresponding to an observational point in the prominence are, intrinsically, not uniquely determined, and a set of possible solutions is usually obtained. However, mutual consistency of these solutions with those independently predicted by the form of the circular polarized component, allow, in almost all cases, rejection of all solutions of a set except one symmetrical pair. Of such a pair, a unique solution can be determined with a high confidence level by reference to independent potential field information. Field vectors are found usually to be close to horizontal and normal to the prominence surface, but extreme exceptions are found. Field values range from 6 G to 60 G. The derived vectorfield configurations and their magnitudes are briefly discussed relative to these prominences and to different quiescent prominence models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation. 相似文献
313.
Natalie C. Williams Karen A. Bjorndal Margaret M. Lamont Raymond R. Carthy 《Estuaries and Coasts》2014,37(4):986-994
The foraging ecology and diet of the green turtle, Chelonia mydas, remain understudied, particularly in peripheral areas of its distribution. We assessed the diet of an aggregation of juvenile green turtles at the northern edge of its range during winter months using two approaches. Stomach content analyses provide a single time sample, and stable isotope analyses integrate diet over a several-month period. We evaluated diet consistency in prey choice over time by comparing the results of these two approaches. We examined stomach contents from 43 juvenile green turtles that died during cold stunning events in St. Joseph Bay, Florida, in 2008 and 2011. Stomach contents were evaluated for volume, dry mass, percent frequency of occurrence, and index of relative importance of individual diet items. Juvenile green turtles were omnivorous, feeding primarily on seagrasses and tunicates. Diet characterizations from stomach contents differed from those based on stable isotope analyses, indicating the turtles are not feeding consistently during winter months. Evaluation of diets during warm months is needed. 相似文献
314.
The Kangankunde Carbonatite Complex from the Cretaceous Chilwa Alkaline Province in southern Malawi contains ankeritic and siderite carbonatite that are affected by late stage remobilisation by a carbothermal or hydrothermal fluid. The coarse pegmatitic siderite carbonatite that hosts exotic minerals like monazite, synchysite, bastnasite, strontianite and apatite in vugs and cavities constitutes some of the richest rare earth deposits in the world. Besides these minerals, our studies reveal the presence of collinsite and aragonite from the siderite carbonatite. Fine drusy monazites are seen as overgrowths on thin veinlets of siderite within the rare earth mineralised zones. We present unambiguous SEM-based surface textural evidence such as presence of dissolution-corrosion features like etching along cleavage, solution channels, solution pits, sinstered scaly surface, etc. along with rare earth mineralisation that suggests the exotic minerals in the siderite carbonatite did not crystallise from carbonate magma and are a result of sub-solidus processes involving carbonatite-derived fluids. We believe that the monazite-synchysitebastnasite-strontianite-collinsite assemblages were formed by juvenile post magmatic hydrothermal alteration of pre-existing carbonatite by a complex CO2-rich and alkali chloride-carbonate-bearing fluid at ~250 to 400°C in an open system. This late ‘magmatic’ to ‘hydrothermal’ activity was responsible for considerable changes in rock texture and mineralogy leading to mobility of rare earth elements during fluid-rock interaction. These aspects need to be properly understood and addressed before using trace and rare earth element (REE) geochemistry in interpreting carbonatite genesis. 相似文献
315.
Lucille Le Corre Vishnu Reddy Nico Schmedemann Kris J. Becker David P. O’Brien Naoyuki Yamashita Patrick N. Peplowski Thomas H. Prettyman Jian-Yang Li Edward A. Cloutis Brett W. Denevi Thomas Kneissl Eric Palmer Robert W. Gaskell Andreas Nathues Michael J. Gaffey David W. Mittlefehldt William B. Garry Holger Sierks Christopher T. Russell Carol A. Raymond Maria C. De Sanctis Eleonora Ammanito 《Icarus》2013
NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt. 相似文献
316.
317.
318.
Jacob D. Hosen George H. Allen Giuseppe Amatulli Sara Breitmeyer Matthew J. Cohen Byron C. Crump YueHan Lu Jérôme P. Payet Brett A. Poulin Aron Stubbins Byungman Yoon Peter A. Raymond 《水文研究》2021,35(5):e14124
Most terrestrial allochthonous organic matter enters river networks through headwater streams during high flow events. In headwaters, allochthonous inputs are substantial and variable, but become less important in streams and rivers with larger watersheds. As allochthonous dissolved organic matter (DOM) moves downstream, the proportion of less aromatic organic matter with autochthonous characteristics increases. How environmental factors converge to control this transformation of DOM at a continental scale is less certain. We hypothesized that the amount of time water has spent travelling through surface waters of inland systems (streams, rivers, lakes, and reservoirs) is correlated to DOM composition. To test this hypothesis, we used established river network scaling relationships to predict relative river network flow-weighted travel time (FWTT) of water for 60 stream and river sites across the contiguous United States (3090 discrete samples over 10 water years). We estimated lentic contribution to travel times with upstream in-network lake and reservoir volume. DOM composition was quantified using ultraviolet and visible absorption and fluorescence spectroscopy. A combination of FWTT and lake and reservoir volume was the best overall predictor of DOM composition among models that also incorporated discharge, specific discharge, watershed area, and upstream channel length. DOM spectral slope ratio (R2 = 0.77) and Freshness Index (R2 = 0.78) increased and specific ultraviolet absorbance at 254 nm (R2 = 0.68) and Humification Index (R2 = 0.44) decreased across sites as a function of FWTT and upstream lake volume. This indicates autochthonous-like DOM becomes continually more dominant in waters with greater FWTT. We assert that river FWTT can be used as a metric of the continuum of DOM composition from headwaters to rivers. The nature of the changes to DOM composition detected suggest this continuum is driven by a combination of photo-oxidation, biological processes, hydrologically varying terrestrial subsidies, and aged groundwater inputs. 相似文献
319.
Bogdan Valcu Peter L. Smith Larry D. Gardner John C. Raymond Mari-Paz Miralles John L. Kohl 《Solar physics》2007,243(1):93-104
We have tracked the spectral responsivity of the ultraviolet channels of the UVCS (Ultraviolet Coronagraph Spectrometer) instrument
on SOHO by repeated observations of a stable hot star. We demonstrate first that the ultraviolet spectral irradiance of the
Be star ζ Tau (HD 37202) for the 100- to 125-nm wavelength range has been sufficiently constant for our purposes when measured periodically
over the course of the SOHO mission. We then use ζ Tau as a radiometric transfer standard to determine an average decrease beginning in November of 1998 of 13.0% per year in
the responsivity of the UVCS O vi channel for wavelengths near H i Ly α and for a particular UVCS unvignetted aperture used for science observations. The calibration tracking method involves
separating two ζ Tau spectral regions that are overlapped on part of the detector. The change in the responsivity of UVCS/SOHO began in late
1998 as determined by comparison of simultaneous observations of the corona carried out with UVCS/SOHO and the freshly-calibrated
UVCS instrument on the Spartan 201 satellite in early November of 1998.
B. Valcu now at BrainLAB, Inc, 3 Westbrook Corporate Ctr., Tower 3, Suite 400, Westchester, IL 60154, USA, e-mail: bogdan.valcu@brainlab.com
or Bogdan_valcu@hotmail.com 相似文献
320.
We review the atomic processes responsible for astrophysical emission line spectra, paying particular attention to the reliability
of the rates used to determine the physical conditions in the emiting gas. We discuss particular cases where often-neglected
processes play important roles. 相似文献