Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber. 相似文献
Limit equilibrium analyses were applied to the 1980 Mount St. Helens and 1956 Bezymianny failures in order to examine the influence on stability of structural deformation produced by cryptodome emplacement. Weakening structures associated with the cryptodome include outward-dipping normal faults bounding a summit graben and a flat shear zone at the base of the bulged flank generated by lateral push of the magma. Together with the head of the magmatic body itself, these structures serve directly to localize failure along a critical surface with low stability deep within the interior of the edifice. This critical surface, with the safety coefficient reduced by 25-30%, is then very sensitive to stability condition variation, in particular to the pore-pressure ratio (ru) and seismicity coefficient (n). For ru=0.3, or n=0.2, the deep surface suffers catastrophic failure, removing a large volume of the edifice flank. In the case of Mount St. Helens, failure occurred within a material with angle of friction ~40°, cohesion in the range 105-106 Pa, and probably significant water pore pressure. On 18 May 1980, detachment of slide block I occurred along a newly formed rupture surface passing through the crest of the bulge. Although sliding of block I may have been helped by the basal shear zone, significant pore pressure and a triggering earthquake were required (ru=0.3 and n=0.2). Detachment of the second block was guided by the summit normal fault, the front of the cryptodome, and the basal shear zone. This occurred along a deep critical surface, which was on the verge of failure even before the 18 May 1980 earthquake. The stability of equivalent surfaces at Bezymianny Volcano appears significantly higher. Thus, although magma had already reached the surface, weaker materials, or higher pore pressure and/or seismic conditions were probably required to reach the rupture threshold. From our analysis, we find that deep-seated sector collapses formed by removing the edifice summit cannot generally result from a single slide. Cryptodome-induced deformation does, however, provide a deep potential slip surface. As previously thought, it may assist deep-seated sector collapse because it favors multiple retrogressive slides. This leads to explosive depressurization of the magmatic and hydrothermal systems, which undermines the edifice summit and produces secondary collapses and explosive blasts. 相似文献
Bivalves are commonly used to detect metal pollution in the marine environment. Commercially cultured Milky oysters (Saccostrea commercialis) were transplanted in various sites along the North Queensland coast and analyzed for two metals of potentially anthropogenic origin (Cd, Zn). To provide additional information, naturally occurring Black Lip oysters (Saccostrea echinata) were also collected at the transplantation sites. The study demonstrated that the oysters species transplanted are good bioindicators of these metal concentrations in tropical waters, sensitive to variations in the environment at concentrations which are much smaller than pollution signals commonly reported for temperate waters. Three transplant experiments were carried out from May 1999 to February 2000. Milky oysters transplanted to offshore areas (Orpheus Is., Kelso Reef) accumulated Cd in the soft parts whereas oysters sampled from cages placed in Ross Creek and the Herbert River estuaries showed a decrease in Cd concentration, which resulted from an increase in dry weight. Dry weight appeared to be an important covariant affecting Cd concentration in the oysters whereas it does not unambiguously affect Zn concentrations. For the duration of the experiments, oysters sampled from the Magnetic Is. reference site showed effectively constant Cd concentrations and total Cd contents which indicates that any seasonal cycle affecting metal concentration is weak. It was found that Cd accumulation in oysters increased as ambient dissolved Cd concentration decreased, from which it was concluded that for these oysters, the predominant source of Cd was from the particulate phase rather than the dissolved phase. 相似文献
A three-dimensional (3D) inversion program is developed to interpret gravity data using a selection of constraints. This selection includes minimum distance, flatness, smoothness and compactness constraints, which can be combined using a Lagrangian formulation. A multigrid technique is also implemented to resolve separately large and short gravity wavelengths. The subsurface in the survey area is divided into rectangular prismatic blocks and the problem is solved by calculating the model parameters, i.e. the densities of each block. Weights are given to each block depending on depth, a priori information on density and the density range allowed for the region under investigation. The present computer code is tested on modelled data for a dipping dike and multiple bodies. Results combining different constraints and a weight depending on depth are shown for the dipping dike. The advantages and behaviour of each method are compared in the 3D reconstruction. Recovery of geometry (depth, size) and density distribution of the original model is dependent on the set of constraints used. From experimentation, the best combination of constraints for multiple bodies seems to be flatness and a minimum volume for the multiple bodies. The inversion method is tested on real gravity data from the Rouyn-Noranda (Quebec) mining camp. The 3D inversion model for the first 10 km is in agreement with the known major lithological contacts at the surface; it enables the determination of the geometry of plutons and intrusive rocks at depth. 相似文献
We have developed a new method to accelerate tracer simulations to steady-state in a 3-D global ocean model, run off-line.
Using this technique, our simulations for natural 14C ran 17 times faster when compared to those made with the standard non-accelerated approach. For maximum acceleration we
wish to initialize the model with tracer fields that are as close as possible to the final equilibrium solution. Our initial
tracer fields were derived by judiciously constructing a much faster, lower-resolution (degraded), off-line model from advective
and turbulent fields predicted from the parent on-line model, an ocean general circulation model (OGCM). No on-line version
of the degraded model exists; it is based entirely on results from the parent OGCM. Degradation was made horizontally over
sets of four adjacent grid-cell squares for each vertical layer of the parent model. However, final resolution did not suffer
because as a second step, after allowing the degraded model to reach equilibrium, we used its tracer output to re-initialize
the parent model (at the original resolution). After re-initialization, the parent model must then be integrated only to a
few hundred years before reaching equilibrium. To validate our degradation-integration technique (DEGINT), we compared 14C results from runs with and without this approach. Differences are less than 10‰ throughout 98.5% of the ocean volume. Predicted
natural 14C appears reasonable over most of the ocean. In the Atlantic, modeled Δ14C indicates that as observed, the North Atlantic Deep Water (NADW) fills the deep North Atlantic, and Antartic Intermediate
Water (AAIW) infiltrates northward; conversely, simulated Antarctic Bottom Water (AABW) does not penetrate northward beyond
the equator as it should. In the Pacific, in surface eastern equatorial waters, the model produces a north–south assymetry
similar to that observed; other global ocean models do not, because their resolution is inadequate to resolve equatorial dynamics
properly, particularly the intense equatorial undercurrent. The model’s oldest water in the deep Pacific (at −239‰) is close
to that observed (−248‰), but is too deep. Surface waters in the Southern Ocean are too rich in natural 14C due to inadequacies in the OGCM’s thermohaline forcing.
Received: 18 March 1997 / Accepted: 27 July 1997 相似文献
The simulation of the mean seasonal cycle of sea surface temperature (SST) remains a challenge for coupled ocean–atmosphere
general circulation models (OAGCMs). Here we investigate how the numerical representation of clouds and convection affects
the simulation of the seasonal variations of tropical SST. For this purpose, we compare simulations performed with two versions
of the same OAGCM differing only by their convection and cloud schemes. Most of the atmospheric temperature and precipitation
differences between the two simulations reflect differences found in atmosphere-alone simulations. They affect the ocean interior
down to 1,000 m. Substantial differences are found between the two coupled simulations in the seasonal march of the Intertropical
Convergence Zone in the eastern part of the Pacific and Atlantic basins, where the equatorial upwelling develops. The results
confirm that the distribution of atmospheric convection between ocean and land during the American and African boreal summer
monsoons plays a key role in maintaining a cross equatorial flow and a strong windstress along the equator, and thereby the
equatorial upwelling. Feedbacks between convection, large-scale circulation, SST and clouds are highlighted from the differences
between the two simulations. In one case, these feedbacks maintain the ITCZ in a quite realistic position, whereas in the
other case the ITCZ is located too far south close to the equator. 相似文献
The low-frequency evolution of Indian rainfall mean-state and associated interannual-to-decadal variability is discussed for the last 6000 years from a multi-configuration ensemble of fully coupled global transient simulations. This period is marked by a shift of Indian Summer Monsoon Rainfall (ISMR) distribution towards drier conditions, including extremes, and a contraction of the rainy season. The drying is larger in simulations with higher horizontal resolution of the atmosphere and revised land surface hydrology. Vegetation–climate interactions and the way runoff is routed to ocean modulate the timing of the monsoon onset but have negligible effects on the evolution of seasonal rainfall amounts in our modeling framework in which carbon cycling is always active. This drying trend is accompanied by changes in ISMR interannual-to-decadal variability decreasing over north and south India but increasing over central India (20°–25° N). The ISMR interannual-to-decadal variability is decomposed into six physically consistent regimes using a clustering technique to further characterize its changes and associated teleconnections. From 6 to 3.8 kyr bp, the century-to-century modulations in the frequency of occurrence associated to the regimes are asynchronous between the simulations. Orbitally-driven trends can only be detected for two regimes over the whole 6–0 kyr bp period. These two regimes reflect increased influence of ENSO on both ISMR and Indian Ocean Dipole as the inter-hemispheric energy gradient weakens. Severe long-term droughts are also shown to be a combination of long-term drying and internally generated low-frequency modulations of the interannual-to-decadal variability.