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排序方式: 共有154条查询结果,搜索用时 156 毫秒
91.
Hornblende dehydration reactions during magma ascent at Soufrière Hills Volcano, Montserrat 总被引:1,自引:1,他引:0
V. J. E. Buckley R. S. J. Sparks B. J. Wood 《Contributions to Mineralogy and Petrology》2006,151(2):121-140
Hornblende phenocrysts in recent andesites of the Soufrière Hills Volcano display reaction rims of microcrystalline plagioclase,
pyroxene, Fe-oxides and interstitial glass, formed by decompression during magma ascent. Mass balance calculations give the
following reactions with mineral proportions in agreement with modal abundances:
These reactions require an open chemical system with exchange of selected components with surrounding melt. Volatiles, TiO2 and alkalis are expelled and SiO2 and FeOT are consumed. Matrix glasses fall into two compositional groups. Glasses in pumice are relatively rich in CaO and poor in
K2O and Na2O compared to glasses in dome samples. The former glasses formed by moderate amounts of groundmass crystallisation of plagioclase,
associated with rapid magma ascent in explosive eruptions. The later glasses evolved in response to hornblende breakdown,
groundmass crystallisation and mixing of melts from different levels during slow magma ascent and extended residence time
in the dome. Interstitial glass compositions in reaction rims reflect the compositions of the surrounding matrix glasses,
but show variable compositional differences mostly consistent with the proposed open-system reactions. 相似文献
92.
J. W. Atkinson J. L. Collett A. Marconi D. J. Axon A. Alonso-Herrero D. Batcheldor J. J. Binney A. Capetti C. M. Carollo L. Dressel H. Ford J. Gerssen M. A. Hughes D. Macchetto W. Maciejewski M. R. Merrifield C. Scarlata W. Sparks M. Stiavelli Z. Tsvetanov R. P. van der Marel 《Monthly notices of the Royal Astronomical Society》2005,359(2):504-520
93.
Diana I. Montemayor Eric L. Sparks Oscar O. Iribarne Just Cebrian 《Estuaries and Coasts》2018,41(3):765-771
Herbivory is a common process in salt marshes. However, the direct impact of marsh herbivory on nutrient cycling in this ecosystem is poorly understood. Using a 15N enrichment mesocosm study, we quantified nitrogen (N) cycling in sediment and plants of black needlerush (Juncus roemerianus) salt marshes, facilitated by litter decomposition and litter plus grasshopper feces decomposition. We found 15 times more 15N recovery in sediment with grasshopper herbivory compared to sediment with no grasshopper herbivory. In plants, even though we found three times and a half larger 15N recovery with grasshopper herbivory, we did not find significant differences. Thus, herbivory can enhance N cycling in black needlerush salt marshes sediments and elevate the role of these salt marshes as nutrient sinks. 相似文献
94.
Arvind Singh R.A. Jani R. Ramesh 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(11):1422-1431
A new data set of oxygen isotopic composition (δ18O) and salinity (S) of surface and sub-surface waters of the northern Indian Ocean, collected during the period 1987–2009, is presented. While the results are consistent with positive P?E (excess of precipitation over evaporation) over the Bay of Bengal and negative P?E over the eastern Arabian Sea, a significant spatiotemporal variability in the slope (also intercept) of the δ18O–S relation is observed in the Bay; the temporal variability is difficult to discern in the Arabian Sea. The slope and intercept are positively and negatively correlated, respectively, with the annual rainfall over India, a rough measure of river runoff. Both the slope and intercept appear to be sensitive to rainfall; the slope (intercept) is higher (lower) during years of stronger monsoon. The observed variability in the δ18O–S relation implies that caution needs to be exercised in paleosalinity estimations, especially from the Bay of Bengal, based on δ18O of marine organisms. 相似文献
95.
The distribution of clasts deposited around a volcano during an explosive eruption typically contoured by isopleth maps provides important insights into the associated plume height, wind speed and eruptive style. Nonetheless, a wide range of strategies exists to determine the largest clasts, which can lead to very different results with obvious implications for the characterization of eruptive behaviour of active volcanoes. The IAVCEI Commission on Tephra Hazard Modelling has carried out a dedicated exercise to assess the influence of various strategies on the determination of the largest clasts. Suggestions on the selection of sampling area, collection strategy, choice of clast typologies and clast characterization (i.e. axis measurement and averaging technique) are given, mostly based on a thorough investigation of two outcrops of a Plinian tephra deposit from Cotopaxi volcano (Ecuador) located at different distances from the vent. These include: (1) sampling on a flat paleotopography far from significant slopes to minimize remobilization effects; (2) sampling on specified-horizontal-area sections (with the statistically representative sampling area depending on the outcrop grain size and lithic content); (3) clast characterization based on the geometric mean of its three orthogonal axes with the approximation of the minimum ellipsoid (lithic fragments are better than pumice clasts when present); and (4) use of the method of the 50th percentile of a sample of 20 clasts as the best way to assess the largest clasts. It is also suggested that all data collected for the construction of isopleth maps be made available to the community through the use of a standardized data collection template, to assess the applicability of the new proposed strategy on a large number of deposits and to build a large dataset for the future development and refinement of dispersal models. 相似文献
96.
97.
Elizabeth P. Turtle Laszlo P. Keszthelyi Jani Radebaugh Damon P. Simonelli David A. Williams Windy L. Jaeger H.Herbert Breneman Cynthia B. Phillips the Galileo SSI Team 《Icarus》2004,169(1):3-28
We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼400 km high plume—over a relatively short period of time (∼13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io. 相似文献
98.
Summary During the 1979 eruption of the Soufrière of St Vincent, West Indies, a 14 min period of explosive activity occurred on April 22. The first three minutes of this activity were filmed. Five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km. The ascent velocities of the fronts of these plumes ranged from 8.5 to 61.7 ms-1 . The half-angle of spreading of the plume fronts ranged from 21.5 to 24°. One of the plumes was observed to 8 km height and decelerated steadily from 51.5 to 23 ms-1 . The main column fed by individual explosions was, however, observed to accelerate from 8.5 to 58 m s-1 in the same height interval.
A theoretical treatment of volcanic plume motion is presented. Measurements of dimensions, velocities and spreading rates from the film are used to estimate plume parameters such as temperature, particle content and volume discharge rate of magma from the theoretical relationships. These calculations show that the individual plumes became hotter and richer in juvenile ash with time. The acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash-rich explosions. An average volume discharge rate of 12600m3 s-1 is estimated from the analysis of the plume motions. This value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column. This agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case, as long as the effect of hot entrained ash particles is considered. 相似文献
A theoretical treatment of volcanic plume motion is presented. Measurements of dimensions, velocities and spreading rates from the film are used to estimate plume parameters such as temperature, particle content and volume discharge rate of magma from the theoretical relationships. These calculations show that the individual plumes became hotter and richer in juvenile ash with time. The acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash-rich explosions. An average volume discharge rate of 12600m
99.
T. K. Hincks W. P. Aspinall P. J. Baxter A. Searl R. S. J. Sparks G. Woo 《Bulletin of Volcanology》2006,68(3):266-284
Frequent ash fallout from long-lived eruptions (with active phases greater than 5 years) may lead to local populations experiencing
unacceptably high cumulative exposures to respirable particulate matter. Ash from Montserrat has been shown to contain significant
levels of cristobalite and other reactive agents that are associated with an increased risk of developing pneumoconiosis (including
silicosis) and other long-term health problems. There are a number of difficulties associated with estimating risks in populations
due to uncertain and wide ranging individual exposures, change in behaviour with time and the natural variation in individual
response. Present estimates of risk in workers and other population groups are simplifications based on a limited number of
exposure measurements taken on Montserrat (1996–1999), and exposure−response curves from epidemiological studies of coal workers
exposed to siliceous dust. In this paper we present a method for calculating the long-term cumulative exposure to cristobalite
from volcanic ash by Monte Carlo simulation. Code has been written to generate synthetic time series for volcanic activity,
rainfall, ash deposition and erosion to give daily ash deposit values and cristobalite fraction at a range of locations. The
daily mean personal exposure for PM10 and cristobalite is obtained by sampling from a probability distribution, with distribution parameters dependent on occupation,
ground deposit depth and daily weather conditions. Output from multiple runs is processed to calculate the exceedance probability
for cumulative exposure over a range of occupation types, locations and exposure periods. Results are interpreted in terms
of current occupational standards, and epidemiological exposure−response functions for silicosis are applied to quantify the
long-term health risk. Assuming continuing volcanic activity, median risk of silicosis (profusion 1/0 or higher) for an average
adult after 20 years continuous exposure is estimated to be approximately 0.5% in northern Montserrat to 1.6% in Cork Hill.
The occupational group with the highest exposure to ash are gardeners, with a corresponding 2% to 4% risk of silicosis. In
situations where opportunities for in-depth exposure studies are limited, computer simulations provide a good indication of
risk based on current expert knowledge. By running the code for a range of input scenarios, the cost-benefit of mitigation
measures (such as a programme of active ash clearance) can be estimated. Results also may be used to identify situations where
full exposure studies or fieldwork would be beneficial.
Editorial responsibility: J Stix 相似文献
100.
Cladistics is a systematic method of classification that groups entities on the basis of sharing similar characteristics in
the most parsimonious manner. Here cladistics is applied to the classification of volcanoes using a dataset of 59 Quaternary
volcanoes and 129 volcanic edifices of the Tohoku region, Northeast Japan. Volcano and edifice characteristics recorded in
the database include attributes of volcano size, chemical composition, dominant eruptive products, volcano morphology, dominant
landforms, volcano age and eruptive history. Without characteristics related to time the volcanic edifices divide into two
groups, with characters related to volcano size, dominant composition and edifice morphology being the most diagnostic. Analysis
including time based characteristics yields four groups with a good correlation between these groups and the two groups from
the analysis without time for 108 out of 129 volcanic edifices. Thus when characters are slightly changed the volcanoes still
form similar groupings. Analysis of the volcanoes both with and without time yields three groups based on compositional, eruptive
products and morphological characters. Spatial clusters of volcanic centres have been recognised in the Tohoku region by Tamura
et al. (Earth Planet Sci Lett 197:105–106, 2002). The groups identified by cladistic analysis are distributed unevenly between the clusters, indicating
a tendency for individual clusters to form similar kinds of volcanoes with distinctive but coherent styles of volcanism. Uneven
distribution of volcano types between clusters can be explained by variations in dominant magma compositions through time,
which are reflected in eruption products and volcanic landforms. Cladistic analysis can be a useful tool for elucidating dynamic
igneous processes that could be applied to other regions and globally. Our exploratory study indicates that cladistics has
promise as a method for classifying volcanoes and potentially elucidating dynamic and evolutionary volcanic processes. Cladistics
may also have utility in hazards assessment where spatial distributions and robust definitions of a volcano are important,
as in locating sensitive facilities such as nuclear reactors and repositories. 相似文献