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71.
A. Costa F. Dell’Erba M. A. Di Vito R. Isaia G. Macedonio G. Orsi T. Pfeiffer 《Bulletin of Volcanology》2009,71(3):259-273
Tephra fallout associated with renewal of volcanism at the Campi Flegrei caldera is a serious threat to the Neapolitan area.
In order to assess the hazards related with tephra loading, we have considered three different eruption scenarios representative
of past activity: a high-magnitude event similar to the 4.1 ka Agnano-Monte Spina eruption, a medium-magnitude event, similar
to the ∼3.8 ka Astroni 6 eruption, and a low-magnitude event similar to the Averno 2 eruption. The fallout deposits were reconstructed
using the HAZMAP computational model, which is based on a semi-analytical solution of the two-dimensional advection–diffusion–sedimentation
equation for volcanic tephra. The input parameters into the model, such as total erupted mass, eruption column height, and
bulk grain-size and components distribution, were obtained by best-fitting field data. We carried out tens of thousands simulations
using a statistical set of wind profiles, obtained from NOAA re-analysis. Probability maps, relative to the considered scenarios,
were constructed for several tephra loads, such as 200, 300 and 400 kg/m2. These provide a hazard assessment for roof collapses due to tephra loading that can be used for risk mitigation plans in
the area. 相似文献
72.
Eleazar Padrón Pedro A. Hernández Theofilos Toulkeridis Nemesio M. Pérez Rayco Marrero Gladys Melián Giorgio Virgili Kenji Notsu 《Journal of Volcanology and Geothermal Research》2008
We report herein the first results of two soil CO2 efflux surveys carried out at Cuicocha lake-filled and Pululahua caldera volcanic systems, Ecuador. A total of 172 and 217 soil CO2 efflux measurements were taken at the surface environment of Pululahua and Cuicocha calderas respectively, by means of the “accumulation chamber” method during the summer of 2006 to constrain the total CO2 output from the studied area. Soil CO2 efflux values ranged from non-detectable up to 48.5 and 141.7 g m− 2 d− 1 for Cuicocha and Pululahua calderas respectively. In addition, probability graphs were used to distinguish the existence of different geochemical populations. Sequential Gaussian Simulation was used to construct an average map for 100 simulations and to compute the total CO2 emission at each studied area: 106 and 270 t d− 1 (metric tons per day) for Cuicocha (13.3 km2) and Pululahua (27.6 km2) volcanoes respectively. 相似文献
73.
Laura Valentini Bruno Capaccioni Piermaria Luigi Rossi Roberto Scandone Damiano Sarocchi 《Bulletin of Volcanology》2008,70(9):1087-1101
In order to provide new information about the source area and depositional mechanisms of the Upper Member of the Neapolitan
Yellow Tuff (NYT), a prominent pyroclastic deposit of the Campi Flegrei Volcanic District (southern Italy), statistics on
directional fabric, by means of computer-assisted image analysis on 32 rock samples, were compiled. Seventeen samples were
collected along vertical direction on two selected exposures and fifteen were taken from outcrops widely distributed all around
the Campi Flegrei Volcanic District. Fabric measurements within the investigated successions reveal a vertically homogeneous
direction of the mean particle iso-orientation, with considerable variability in the strength of particle iso-orientation
even at cm-scale. The existence of particle iso-orientation can be related to continuous sedimentation from a concentrated
bedload region beneath suspension currents, producing massive or inversely graded beds by traction carpet sedimentation. The
considerable vertical variability in the strength of iso-orientation is the result of very unstable flow regimes, up to the
extreme condition of discrete depositional events, with a variable combination of traction carpet and/or direct suspension
sedimentation. The vertical homogeneity in the mean orientation values, found in the investigated sections, may derive from
the sequential deposition of laminae to thin beds, whose relatively flat upper surfaces were unable to significantly deflect
the depositional system of the following currents. According to the observed homogeneous mean particle orientation values
along the investigated vertical profiles, samples collected through areal distribution are considered representative of the
local paleo-flow directions of the whole deposit. The mean directions of the samples collected areally show two different
coherent patterns which point to the existence of two different source areas. The first, which includes all samples from the
northern outcrops, appears to converge in a narrow area about 2 km NE of the town of Pozzuoli, largely in coincidence with
the inferred area on the basis of the pumice fall distribution. The second, which includes samples from Capo Miseno and Posillipo
areas, points to the central part of the Pozzuoli Bay, about 4 km offshore the town of Pozzuoli. 相似文献
74.
Multiple caldera collapses inferred from the shallow electrical resistivity signature of the Las Cañadas caldera, Tenerife, Canary Islands 总被引:1,自引:0,他引:1
Nicolas Coppo Pierre-Andr Schnegg Wiebke Heise Pierik Falco Roberto Costa 《Journal of Volcanology and Geothermal Research》2008,170(3-4):153-166
The Las Cañadas caldera of Tenerife (LCC) is a well exposed caldera depression filled with pyroclastic deposits and lava flows from the active Teide–Pico Viejo complex (TPVC). The caldera's origin is controversial as both the formation by huge lateral flank collapse(s) and multiple vertical collapses have been proposed. Although vertical collapses may have facilitated lateral slope failures and thus jointly contribute to the exposed morphology, their joint contribution has not been clearly demonstrated. Using results from 185 audiomagnetotelluric (AMT) soundings carried out between 2004 and 2006 inside the LCC, our study provides consistent geophysical constraints in favour of multiple vertical caldera collapse. One-dimensional modelling reveals a conductive layer at shallow depth (30–1000 m), presumably resulting from hydrothermal alteration and weathering, underlying the infilling resistive top layer. We present the resistivity distribution of both layers (resistivity images), the topography of the conductive layer across the LCC, as well as a cross-section in order to highlight the caldera's evolution, including the distribution of earlier volcanic edifices. The AMT phase anisotropy reveals the structural and radial characteristics of the LCC. 相似文献
75.
Following the emblematic flank collapse of Mount St Helens in 1981, numerous models of flank sliding have been proposed. These models have allowed to largely improve the understanding of mechanisms involved in such landslides, which represent a tremendous risk for populations living around volcanoes. In this article, a new mode of landslide formation, related to buried calderas, is described. The model emphasizes the paramount importance of the hidden ring fault that, even when the caldera is buried, still remains a plane of weakness in the core of the edifice. Under certain conditions, this plane of weakness becomes activated as the upper part of a pre-existing critical slip surface and is used in the emplacement of huge landslides which travel downslope at a very high velocity. A natural example is taken from Piton de la Fournaise Volcano (La Réunion Island, Indian Ocean). It reveals that the primary cause triggering caldera rim collapse is partial unbuttressing of the flank of the volcano. In the natural example, this occurs through regressive erosion that excavates deep canyon in the direction of the buried caldera but other mechanisms may exist. On account of the large volumes of material involved in caldera rim collapse as well as their long runout distances, such a volcanic hazard should be taken into account on every volcano where buried calderas are suspected. 相似文献
76.
Previous research indicates that Yakushima Island, southwestern Japan, may have been struck by a huge tsunami before or soon after the arrival of the Koya pyroclastic flow during the 7.3 ka caldera‐forming Kikai eruption, but this has not yet been confirmed. This paper describes sedimentological and chronostratigraphic evidence showing that Unit TG, one of three gravel beds exposed on the Koseda coast of northeast Yakushima Island and investigated here, is a tsunami deposit. Unit TG is a poorly sorted, 30 cm thick gravel bed overlying a wave‐cut bench and underlying a Koya pyroclastic flow deposit. Sparse wood fragments in Unit TG were dated at 7 416–7 167 cal year BP. The constituent gravel clasts of Unit TG are similar in composition to those of modern beach and river deposits along the Koseda coast. Unit TG also contains pumice clasts whose chemistry is identical to that of pumice derived from the 7.3 ka eruption at Kikai caldera. The long‐axis orientations and composition of gravel clasts in Unit TG suggest that they were transported by a landward‐travelling high‐particle‐concentration flow, which suggests that Unit TG was deposited by a tsunami run‐up flow during the 7.3 ka Kikai caldera eruption, just before the arrival of the major Koya pyroclastic flow at the Koseda coast. Whether the 7.3 ka tsunami was caused by a volcanic eruption or an earthquake remains unclear, but Unit TG demonstrates that a tsunami arrived immediately before emplacement of a Koya pyroclastic flow. 相似文献
77.
Abstract: Geology of the largest Roseki (pyrophyllite–clay)–mineralized, Mitsuishi mining area was re–examined, and a composite caldera model (Wake caldera) was proposed. Evidence for the caldera formation is (1) Existence of huge volume of rhyolitic tuffs in the mining area, (2) A basin structure in rhyolitic tuffs surrounded by the Permo-Triassic basement rocks, (3) Arc-like distribution of the Roseki deposits along the eastern edge of the proposed caldera, (4) Tectonic disturbance and intrusive bodies along the caldera wall, and (5) Presence of meso– and mega-breccias at the bottom of the caldera wall. The proposed caldera, sizing NNW-SSE 23 km by ENE-WSW 15 km, may have younger, Inner caldera (15 by 15 km), defined by Lower Member of the late Cretaceous rhyolitic tuffs, thus composite in character. The Roseki deposits were formed after the Inner caldera collapse by hydrothermal fluids ascended through the caldera wall, then spread into the permeable horizon. This caldera proposal bears a significant change in the regional exploration strategy for the Roseki deposits. 相似文献
78.
79.
Mount Bambouto is a polygenic stratovolcano of the Cameroon Volcanic Line,built between 21 Ma and 4.5 Ma.It is situated approximately 200 km NE of Mount Cameroon,between 09°55′and 10°15′longitude east and,05°25′and 05°50′latitude north.The volcano covers an area of 500 km~2 and culminates at 2740 m at Meletan dome and bears a collapsed caldera at the summit (13×8 km).Mount Bambouto is characterized by several natural hazards of different origins: meteorological,such as landslides and rock falls;anthropog... 相似文献
80.
山东五莲七宝山火山—次火山杂岩的演化及形成条件 总被引:1,自引:0,他引:1
山东五莲七宝山地区发育二期火山—次火山杂岩,本文通过岩石学和地球化学的系统研究,论证了火山岩、次火山岩之间存在密切的成因联系,指出二期火山—次火山杂岩分属二个岩石系列,为二个岩浆房分异演化的产物,同时还对火山—次火山杂岩形成的物理化学条件进行了定量估算。 相似文献