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191.
Soil carbon data were collected from published sources for 50 measurement sites spanning the globe's major climate and vegetation types. For each site, climate, vegetation, and land-use variables were determined and entered into a multiple curvilinear regression program to predict soil carbon. The best model incorporates an estimate of site disturbance, annual actual evapotranspiration, and annual soil moisture deficit, and yields an R = 0-803. The curvilinear regression equation was coupled with a large climatic database and computer cartography programs to produce first-generation maps of estimated soil carbon. These maps correctly portray soil carbon as high in boreal and cool temperate zones and low in deserts and tropical zones. Computer planimetry of maps of soil carbon for an ‘undisturbed’ world and for a ‘disturbed’ world resulted in estimates of 1457 × 109 mtC and 504 × 109 mtC respectively. These estimates compare favourably with recent estimates using other approaches. Clearly, the disturbance factor is critical to future refinements in estimates, suggesting the need for detailed studies of the relationship between land-use history and the creation and destruction of this important carbon pool. 相似文献
192.
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194.
James E. Gardner 《Bulletin of Volcanology》2009,71(8):835-844
Most, if not all, magmas contain gas bubbles at depth before they erupt. Those bubbles play a crucial role in eruption dynamics,
by allowing magma to degas, which causes the magma to accelerate as it ascends towards the surface. There must be a limit
to that acceleration, however, because gas bubbles cannot grow infinitely fast. To explore that limit, a series of experiments
was undertaken to determine the maximum rate at which bubbly high-silica rhyolite can decompress. Rhyolite melt that was hydrated
at 150 MPa with ~5.3 wt.% dissolved water and contained 7 to 18 vol.% bubbles can degas in equilibrium at 875°C when decompressed
at rates up to 1.2 MPa s−1 from 150 to 78 MPa, and up to 1.8 MPa s−1 when decompressed further to 42 MPa. In contrast, that same rhyolite cannot degas in equilibrium at 750°C if decompressed
faster than 0.015–0.025 MPa s−1. When combined with other published experiments, the maximum rate of decompression for equilibrium degassing is found to
increase by a factor of ten for every 50–75°C increase in temperature. When compared to predictions from conduit flow models
that assume equilibrium degassing, it is found that such models greatly over-estimate the rate at which relatively cold rhyolite
can decompress, whereas that assumption is largely correct for hot rhyolite, and thus for most other magmas, all of which
are less viscous than rhyolite. In addition, most bubbles that were 20–30 μm in size at high pressure were lost from the population
at low pressure. That absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre-eruptive
bubbles, even if small bubbles were originally present at depth. 相似文献
195.
1980年圣海伦斯火山爆发对之后30年火山学研究的启示 总被引:1,自引:0,他引:1
1980年5月18日,圣海伦斯火山发生了一次大规模喷发,这一事件令科学家和大众惊叹不已。巨大的山体滑坡、气浪以及随之而来的柱状喷发(高达25km,持续时间长达9hr)等影像记录震惊全世界,同时也激起了人们对此次火山喷发事件研究的兴趣(图1)。 相似文献
196.
Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma 总被引:2,自引:2,他引:0
Sub-Plinian to Plinian eruptions of basic magma present a challenge to modeling volcanic behavior because many models rely
on magma becoming viscous enough during ascent to behave brittlely and cause fragmentation. Such models are unable, however,
to strain low viscosity magma fast enough for it to behave brittlely. That assumes that such magmas actually have low viscosities,
but the rare Plinian eruptions of basic magma may in fact result from them being anomalously viscous. Here, we examine two
such eruptions, the 122 B.C. eruption of hawaiitic basalt from Mt. Etna and the late Pleistocene eruption of basaltic andesite
from Masaya Caldera, to test whether they were anomalously viscous. We carried out hydrothermal experiments on both magmas
and analyzed glass inclusions in plagioclase phenocrysts from each to determine their most likely pre-eruptive temperatures
and water contents. We find that the hawaiite was last stored at 1,000–1,020°C, whereas the basaltic andesite was last stored
at 1,010–1,060°C, and that both were water saturated with ∼3.0 wt.% water dissolved in them. Such water contents are not high
enough to trigger Plinian explosive behavior, as much more hydrous basic magmas erupt less violently. In addition, despite
being relatively cool, the viscosities of both magmas would range from ∼102.2–2.5 Pa s before erupting to ∼104 Pa s when essentially degassed, all of which are too fluid to cause brittle disruption. Without invoking special external
forces to explain all such eruptions, one of the more plausible explanations is that when the bubble content reaches some
critical value the fragile foam-like magma disrupts. The rarity of Plinian eruptions of basic magma may be because such magmas
must ascend fast enough to retain their bubbles. 相似文献
197.
Soil cores from river marginal wetlands from the Torridge and Severn catchments in the UK were collected to study rates of soil denitrification at different sites and at two stations (levee and backplain depression) at the river margin. Half the cores were sterilized prior to flooding to destroy the denitrifying bacteria. After flooding and equilibration, monitoring the concentration of amended nitrate in the supernatant of the sterile cores over a period of 7 days provided a simple procedure for the estimation of the diffusion coefficient of the nitrate ion in the flooded soils. An expression was developed that permitted this diffusion coefficient to be extracted from the slope of a plot of supernatant concentration versus (time)1/2. The values obtained, at 15 °C, varied from 2·4 to 6·8 × 10?10m2s?1. Sterile cores are usually treated as controls in denitrification experiments; this work develops a procedure whereby they may yield useful soil process information. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
198.
Lis D. C. Mehringer D. M. Benford D. Gardner M. Phillips T. G. Bockelée-Morvan D. Biver N. Colom P. Crovisier J. Despois D. Rauer H. 《Earth, Moon, and Planets》1997,78(1-3):13-20
We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H13CN, and H3O+ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first
detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational
transitions. The derived abundance with respect to H2O is (4-13) × 10-4. HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 ± 0.2) × 10-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) × 10-4. H3O is detected for the first time from the ground. The H13CN (3-2)transition is also detected and the derived HCN/H13CN abundance ratio is 90 ± 15, consistent with the terrestrial12C/13C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO+, CO+, and CN(the last two are first detections in a comet at radio wavelengths).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
199.
Predicting ground water nitrate concentration from land use 总被引:10,自引:0,他引:10
Ground water nitrate concentrations on Nantucket Island, Massachusetts, were analyzed to assess the effects of land use on ground water quality. Exploratory data analysis was applied to historic ground water nitrate concentrations to determine spatial and temporal trends. Maximum likelihood Tobit and logistic regression analyses of explanatory variables that characterize land use within a 1000-foot radius of each well were used to develop predictive equations for nitrate concentration at 69 wells. The results demonstrate that historic nitrate concentrations downgradient from agricultural land are significantly higher than nitrate concentrations elsewhere. Tobit regression results demonstrate that the number of septic tanks and the percentages of forest, undeveloped, and high-density residential land within a 1000-foot radius of a well are reliable predictors of nitrate concentration in ground water. Similarly, logistic regression revealed that the percentages of forest, undeveloped, and low-density residential land are good indicators of ground water nitrate concentration > 2 mg/L. The methodology and results outlined here provide a useful tool for land managers in communities with shallow water tables overlain with highly permeable materials to evaluate potential effects of development on ground water quality. 相似文献
200.
This study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions: degassing and permeability in magmas. We studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature. From the data collected, we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium. Equilibrium bubble growth occurs when the decompression rate is slower than 0.1 MPa s–1, while higher rates cause porosity to deviate rapidly from equilibrium, defining the first disequilibrium regime of degassing. If the deviation is large enough, a critical threshold of super-saturation is reached and bubble growth accelerates, defining the second disequilibrium regime. We studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions. Under these open conditions, we observed that bubbles start to coalesce at ~43 vol% porosity, regardless of decompression rate. Coalescence profoundly affects bubble texture and size distributions, and induces the melt to become permeable. We determined coalescence to occur on a time scale (~180 s) independent of decompression rate. We parameterized and incorporated our experimental results into a 1D conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust. Compared to previous models that assume equilibrium degassing of the melt during ascent, the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by ~25%, the acceleration at high porosities (>50 vol%) by a factor 5, and the associated decompression rate by an order of magnitude. The integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed, and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape.Editorial responsibility: D. Dingwell 相似文献