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211.
Daniela Mele Roberto Sulpizio Pierfrancesco Dellino Luigi La Volpe 《Bulletin of Volcanology》2011,73(3):257-278
New volcanological studies allow reconstruction of the eruption dynamics of the Pomici di Mercato eruption (ca 8,900 cal.
yr B.P.) of Somma-Vesuvius. Three main Eruptive Phases are distinguished based on two distinct erosion surfaces that interrupt
stratigraphic continuity of the deposits, indicating that time breaks occurred during the eruption. Absence of reworked volcaniclastic
deposits on top of the erosion surfaces suggests that quiescent periods between eruptive phases were short perhaps lasting
only days to weeks. Each of the Eruptive Phases was characterised by deposition of alternating fall and pyroclastic density
current (PDC) deposits. The fallout deposits blanketed a wide area toward the east, while the more restricted PDC deposits
inundated the volcano slopes. Eruptive dynamics were driven by brittle magmatic fragmentation of a phonolitic magma, which,
because of its mechanical fragility, produced a significant amount of fine ash. External water did not significantly contribute
either to fragmentation dynamics or to mechanical energy release during the eruption. Column heights were between 18 and 22 km,
corresponding to mass discharge rates between 1.4 and 6 × 107 kg s−1. The estimated on land volume of fall deposits ranges from a minimum of 2.3 km3 to a maximum of 7.4 km3. Calculation of physical parameters of the dilute pyroclastic density currents indicates speeds of a few tens of m s−1 and densities of a few kg m−3 (average of the lowermost 10 m of the currents), resulting in dynamic pressures lower than 3 kPa. These data suggest that
the potential impact of pyroclastic density currents of the Pomici di Mercato eruption was smaller than those of other Plinian
and sub-Plinian eruptions of Somma-Vesuvius, especially those of 1631 AD and 472 AD (4–14 kPa), which represent reference
values for the Vesuvian emergency plan. The pulsating and long-lasting behaviour of the Pomici di Mercato eruption is unique
in the history of large explosive eruptions of Somma-Vesuvius. We suggest an eruptive scheme in which discrete magma batches
rose from the magma chamber through a network of fractures. The injection and rise of the different magma batches was controlled
by the interplay between magma chamber overpressure and local stress. The intermittent discharge of magma during a large explosive
eruption is unusual for Somma-Vesuvius, as well as for other volcanoes worldwide, and yields new insights for improving our
knowledge of the dynamics of explosive eruptions. 相似文献
212.
Pietro Carveni Giuliana Mele Santo Benfatto Sebastiano Imposa Maria Salleo Puntillo 《Bulletin of Volcanology》2011,73(9):1155-1162
The row of pyroclastic cones named Mts. Sartorius, outcropping on the NE flank of Etna, formed in 1865 during a lateral eruption
that lasted about 6 months. The event was eye witnessed and described by numerous scientists and reporters. In this work,
we use their observations to reconstruct the eruption chronology and scenario, and carry out a detailed geomorphologic survey
to identify the eruptive features and pyroclastic deposits. The 1865 eruption began on 29 January along a segment of the main
system of fractures oriented ENE–WSW, radial to the central conduit. After 30 January, a secondary system of fractures trending
NNW–SSE was simultaneously active. The six larger Mts. Sartorius cones developed since 3 February along the lower extension
of the radial system. They are markedly asymmetric due to the persistent winds blowing at the time and to the pre-existing
topography formed on underlying deposits, previously unreported, that we have recognized. Now, about 150 years after the eruption,
most of the eruptive vents and fractures are no longer observable in the field, being mostly hidden by products of subsequent
phases of the eruption and by younger epiclastic deposits. 相似文献
213.
A spherical approximation makes the basis for a majority of formulas in physical geodesy. However, the present-day accuracy
in determining the disturbing potential requires an ellipsoidal approximation. The paper deals with constructing Green’s function
for an ellipsoidal Earth by an ellipsoidal harmonic expansion and using it for determining the disturbing potential. From
the result obtained the part that corresponds to the spherical approximation has been extracted. Green’s function is known
to depend just on the geometry of the surface where boundary values are given. Thus, it can be calculated irrespective of
the gravity data completeness. No changes of gravity data have an effect on Green’s function and they can be easily taken
into account if the function has already been constructed. Such a method, therefore, can be useful in determining the disturbing
potential of an ellipsoidal Earth. 相似文献
214.
The rock mass structure determines the possible unstable blocks that can induce rock fall phenomena. The stability analyses
must therefore be based on an accurate geo-structural survey. In this work, the stability conditions of several steep slopes
along a motorway in the Far East have been evaluated through key block analysis based on traditional surveys and on laser
scanner acquisitions. Discontinuity orientations and positions on the rock face are derived from the point cloud in order
to perform the reconstruction of the rock mass and to identify blocks in the slope. Results obtained from both the traditional
and the new method is in good agreement. Stability analyses have been performed for evaluating the kinematic feasibility of
different failure mechanisms. The rock block shapes and volumes are computed by performing 2D and 3D analyses whereas the
failure mechanisms are examined using the key block method. Parametrical analyses have been carried on to evaluate the influence
of slope angle variation. DEM models have also been set up. The relative hazard is determined by statistically evaluating
the kinematical feasibility of different failure mechanisms. Hazard mapping has been utilized to identify the best methodology
for risk mitigation. 相似文献
215.
Odours of different sources can indicate to hermit crabs the availability of empty shells, crucial resources for the life cycle of almost all of them. Here, we compared Clibanarius erythropus and Pagurus bernhardus for the intensity of investigative behaviour exhibited towards an empty, well-fitting shell in the presence of (1) plain seawater as control and seawater conditioned by (2) dead and live snails, (3) dead and live conspecifics, (4) live predators, and (5) food. During 10 min of observation, we recorded latency (the time until the first contact with the shell), and the number and duration of shell investigation bouts. The two species behaved similarly when exposed to the odours of food, live snails, and predators, while a more intense shell investigation was induced by dead snail odour in C. erythropus and by dead or live conspecific odour in P. bernhardus. Further studies should investigate the influence of phylogeny and ecology on this interspecific difference. 相似文献
216.
Grzegorz Racki Christian Koeberl Tõnu Viik Elena A. Jagt‐Yazykova John W. M. Jagt 《Meteoritics & planetary science》2014,49(10):1851-1874
High‐velocity impact as a common phenomenon in planetary evolution was ignored until well into the twentieth century, mostly because of inadequate understanding of cratering processes. An eight‐page note, published in Russian by the young Ernst Julius Öpik, a great Estonian astronomer, was among the key selenological papers, but due to the language barrier, it was barely known and mostly incorrectly cited. This particular paper is here intended to serve as an explanatory supplement to an English translation of Öpik's article, but also to document an early stage in our understanding of cratering. First, we outline the historical–biographical background of this benchmark paper, and second, a comprehensive discussion of its merits is presented, from past and present perspectives alike. In his theoretical research, Öpik analyzed the explosive formation of craters numerically, albeit in a very simple way. For the first time, he approximated relationships among minimal meteorite size, impact energy, and crater diameter; this scaling focused solely on the gravitational energy of excavating the crater (a “useful” working approach). This initial physical model, with a rational mechanical basis, was developed in a series of papers up to 1961. Öpik should certainly be viewed as the founder of the numerical simulation approach in planetary sciences. In addition, the present note also briefly describes Nikolai A. Morozov as a remarkable man, a forgotten Russian scientist and, surprisingly, the true initiator of Öpik's explosive impact theory. In fact, already between 1909 and 1911, Morozov probably was the first to consider conclusively that explosion craters would be circular, bowl‐shaped depressions even when formed under different impact angles. 相似文献
217.
Solar Physics - The ionization and excitation equilibrium of C and O for the electron κ-distribution in the solar corona is studied. The change of the distribution shape changes the... 相似文献
218.
b
The island of Lipari is formed by Pleistocene volcanites emplaced during four main periods of volcanic activity. A study has been made of their magnetic properties, primarily with the aim of determining changes in the palaeomagnetic directions. Titanomagnetite is always the primary ferromagnetic mineral and its magnetic characteristics are common to the various lithotypes. However, the concentration of titanomagnetite and the degree of magnetic anisotropy vary systematically and correlate with the types of magma (basalt-andesite in the first and second volcanic periods; rhyolite in the third and fourth periods). All palaeomagnetic directions are of normal polarity (Brunhes epoch). Their mean overall palaeomagnetic pole (86N, 238E; dp = 5, dm = 6) is statistically indistinguishable from the geographic pole.
The variations in declination and inclination with age, however, are marked by some evidence of a discontinuity between 150 ± 10 and 127 ± 8 ka. This age can be correlated with the beginning of the Blake event of reverse polarity. The discontinuity might therefore correspond to magnetic excursions which occurred immediately before. 相似文献
The island of Lipari is formed by Pleistocene volcanites emplaced during four main periods of volcanic activity. A study has been made of their magnetic properties, primarily with the aim of determining changes in the palaeomagnetic directions. Titanomagnetite is always the primary ferromagnetic mineral and its magnetic characteristics are common to the various lithotypes. However, the concentration of titanomagnetite and the degree of magnetic anisotropy vary systematically and correlate with the types of magma (basalt-andesite in the first and second volcanic periods; rhyolite in the third and fourth periods). All palaeomagnetic directions are of normal polarity (Brunhes epoch). Their mean overall palaeomagnetic pole (86N, 238E; dp = 5, dm = 6) is statistically indistinguishable from the geographic pole.
The variations in declination and inclination with age, however, are marked by some evidence of a discontinuity between 150 ± 10 and 127 ± 8 ka. This age can be correlated with the beginning of the Blake event of reverse polarity. The discontinuity might therefore correspond to magnetic excursions which occurred immediately before. 相似文献
219.
Transfer functions are an efficient tool for the quantitative reconstruction of past climate from low to mid‐elevation pollen sites. However, the application of existing methods to high‐altitude pollen assemblages frequently leads to unrealistic results. In the aim of understanding the causes of these biases, the standard ‘best modern analogue’ method has been applied to two high‐altitude pollen sequences to provide quantitative climate estimates for the Lateglacial and Holocene periods. Both pollen sequences (Laghi dell'Orgials, 2130 m, SW aspect and Lago delle Fate, 2240 m, E aspect) are located in the subalpine belt, on opposing sides of the St. Anna di Vinadio Valley (Italian Maritime Alps). Different results were obtained from the two sequences. The largest differences occurred in palaeotemperature reconstruction, with notable differences in both the values and trends at each site. These biases may be attributed to: (1) a lack of high elevation ‘best modern analogues’ in the database of modern samples; (2) the problem of pollen taxa that have multiple climatic significance; (3) problems related to the complexity of mountainous ecosystems, such as the phenomenon of uphill transport of tree pollen by wind. Possible improvements to the reconstruction process are discussed. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
220.