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141.
Mineralogy and Petrology - The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest...  相似文献   
142.
The climatology and interannual variability of winter phytoplankton was analyzed at the Long Term Ecological Research Station MareChiara (LTER-MC, Gulf of Naples, Mediterranean Sea) using data collected from 1985 to 2006. Background winter chlorophyll values (0.2–0.5 μg chl a dm−3) were associated with the dominance of flagellates, dinoflagellates, and coccolithophores. Winter biomass increases (<5.47 μg chl a dm−3) were often recorded until 2000, generally in association with low-salinity surface waters (37.3–37.9). These blooms were most often caused by colonial diatoms such as Chaetoceros spp., Thalassiosira spp., and Leptocylindrus danicus. In recent years, we observed more modest and sporadic winter biomass increases, mainly caused by small flagellates and small non-colonial diatoms. The resulting negative chl a trend over the time series was associated with positive surface salinity and negative nutrient trends. Physical and meteorological conditions apparently exert a strict control on winter blooms, hence significant changes in winter productivity can be foreseen under different climatic scenarios.  相似文献   
143.
Global and Local Multiscale Analysis of Magnetic Susceptibility Data   总被引:3,自引:0,他引:3  
Geophysical well-logs often show a complex behavior which seems to suggest a multifractal nature. Multifractals are highly intermittent signals, with distinct active bursts and passive regions which cannot be satisfactorily characterized in terms of just second-order statistics. They need a higher-order statistical analysis. In contrast with monofractals which have a homogeneous scaling, multifractals may include singularities of many types. Here we describe how a multiscale analysis can be used to describe the magnetic susceptibility data scaling properties for a deep well (KTB, Germany), down to about 9000 m. A multiscale analysis describes the local and global singular behavior of measures or distributions in a statistical fashion. The global analysis allows the estimation of the global repartition of the various Holder exponents. As such, it leads to the definition of a spectrum, D(), called the singularity spectrum. The local analysis is related to the possibility of estimating the Lipschitz regularity locally, i.e., at each point of the support of a multifractal signal. The application of both approaches to the KTB magnetic susceptibility data shows a meaningful correlation between the sequence of Holder exponents vs. depth and the lithological units. The Holder exponents reach the highest values for gneiss units, intermediate ones for amphibolite units and the lowest values for variegated units. Faults are found to correspond to changes for H also when they are of intra-lithological type.  相似文献   
144.
This paper describes numerical models of advection/diffusion between enclaves and host magmas, applied with the aim of estimating time-scales during which enclaves can be homogenised. In particular, advection was simulated using a numerical system consisting of regular and chaotic regions. Results indicate that the homogenisation time of enclaves in chaotic regions is several orders of magnitude faster than in regular regions. For instance, an enclave with a diameter of 100 cm may be homogenised in the chaotic region in ∼ 380 years, assuming an advection velocity of 10 cm/year, whereas in the regular region it would require 6.5×105 years for complete homogenisation. This implies that, in the same magmatic system, large differences in the degree of homogenisation may co-exist, generating magmatic masses with large spatial and temporal inhomogeneities. The results of this study may have significant petrological and volcanological implications. From a petrological point of view, mafic enclaves dispersed in felsic host rocks are regarded as portions of mafic magma which, trapped inside regular regions, survived the hybridisation process. Instead, host rocks are regarded as regions where efficient mixing dynamics generated hybrid magmas. The fact that a single magmatic mass may display large compositional differences at the same time undermines the assumption of most geochemical models, which assume the temporal and spatial homogeneity of the magma body. From the volcanological perspective, the presence of magmatic enclaves in volcanic rocks allows us to estimate the mixing times of magmas by analysing chemical diffusion patterns between host rocks and enclaves. Editorial responsibility: D. Dingwell  相似文献   
145.
A safe, easy and rapid method to calculate lava effusion rates using hand-held thermal image data was developed during June 2003 at Stromboli Volcano (Italy). We used a Forward Looking Infrared Radiometer (FLIR) to obtain images of the active lava flow field on a daily basis between May 31 and June 16, 2003. During this time the flow field geometry and size (where flows typically a few hundred meters long were emplaced on a steep slope) meant that near-vertical images of the whole flow field could be captured in a single image obtained from a helicopter hovering, at an altitude of 750 m and ∼1 km off shore. We used these images to adapt a thermally based effusion rate method, previously applied to low and high spatial resolution satellite data, to allow automated extraction of effusion rates from the hand-held thermal infrared imagery. A comparison between a thermally-derived (0.23–0.87 m3 s−1) and dimensionally-derived effusion rate (0.56 m3 s−1) showed that the thermally-derived range was centered on the expected value. Over the measurement period, the mean effusion rate was 0.38±0.25 m3 s−1, which is similar to that obtained during the 1985–86 effusive eruption and the time-averaged supply rate calculated for normal (non-effusive) Strombolian activity. A short effusive pulse, reaching a peak of ∼1.2 m3 s−1, was recorded on June 3, 2003. One explanation of such a peak would be an increase in driving pressure due to an increase in the height of the magma contained in the central column. We estimate that this pulse would require the magma column to attain a height of ∼190 m above the effusive vent, which is approximately the elevation difference between the vent and the floor of the NE crater. Our approach gives an easy-to-apply method that has the potential to provide effusion rate time series with a high temporal resolution.Editorial responsibility: M. Carroll  相似文献   
146.
Summary A plethora of evidence indicates that magma mixing processes can take place at any evolutionary stage of magmatic systems and that they are extremely common in both plutonic and volcanic environments. Furthermore, recent studies have shown that the magma mixing process is governed by chaotic dynamics whose evolution in space and time generates complex compositional patterns. The fact that magma mixing processes can produce igneous bodies exhibiting a large compositional complexity brings up the key question about the potential pitfalls that may be associated with the sampling of these systems for petrological studies. In particular, since commonly only exiguous portions of the whole magmatic system are available as outcrops for sampling, it is important to address the point whether the sampling may be considered representative of the complexity of the magmatic system. Here, we attempt to address this crucial point by performing numerical simulations of magma mixing processes in 3D, and by evaluating the best conditions for sampling by considering different landscape morphologies and percentages of vegetation cover. It is shown that the goodness of sampling is strongly dependant on the roughness of the landscape, with highly irregular morphologies being the best candidates to give the most complete information on the whole magma body. Vegetation cover, on the contrary, does not appear to significantly influence the representativeness of sampling.  相似文献   
147.
Forward Looking Infrared Radiometer (FLIR) cameras offer a unique view of explosive volcanism by providing an image of calibrated temperatures. In this study, 344 eruptive events at Stromboli volcano, Italy, were imaged in 2001–2004 with a FLIR camera operating at up to 30 Hz. The FLIR was effective at revealing both ash plumes and coarse ballistic scoria, and a wide range of eruption styles was recorded. Eruptions at Stromboli can generally be classified into two groups: Type 1 eruptions, which are dominated by coarse ballistic particles, and Type 2 eruptions, which consist of an optically-thick, ash-rich plume, with (Type 2a) or without (Type 2b) large numbers of ballistic particles. Furthermore, Type 2a plumes exhibited gas thrust velocities (>15 m s−1) while Type 2b plumes were limited to buoyant velocities (<15 m s−1) above the crater rim. A given vent would normally maintain a particular gross eruption style (Type 1 vs. 2) for days to weeks, indicating stability of the uppermost conduit on these timescales. Velocities at the crater rim had a range of 3–101 m s−1, with an overall mean value of 24 m s−1. Mean crater rim velocities by eruption style were: Type 1 = 34 m s−1, Type 2a = 31 m s−1, Type 2b = 7 m s−1. Eruption durations had a range of 6–41 s, with a mean of 15 s, similar among eruption styles. The ash in Type 2 eruptions originates from either backfilled material (crater wall slumping or ejecta rollback) or rheological changes in the uppermost magma column. Type 2a and 2b behaviors are shown to be a function of the overpressure of the bursting slug. In general, our imaging data support a broadening of the current paradigm for strombolian behavior, incorporating an uppermost conduit that can be more variable than is commonly considered.  相似文献   
148.
A geological-geophysical expedition (Ev-K2–CNR 1988) visited the area from West Kun Lun to Karakorum (K2–Gasherbrum). Seven tectonic units including sedimentary, magmatic and metamorphic rocks were distinguished in this area; the northernmost are suggested to belong to the Kun Lun and Qiangtang Microplates. The sedimentary sequence of Shaksgam is proved to extend from the Permian to the Jurassic, with Carboniferous and Cretaceous ages more doubtful. This sequence shows intermediate affinities between the Karakorum and the Qiangtang. The two southernmost units belong to the Karakorum Microplate. The Karakorum Fault Zone comprises a complex pattern of faults and thrusts, with brittle deformation and uplifting of granitoid bodies.  相似文献   
149.
We present a method for calibrating every instrument working with magneto-optical filter (MOF) technology, in order to take into account the effect in the observations due to the aging of the vapour cells. The method allows the evaluation of some characteristics of the transmission profile by modelling the blue and/or the red line intensity images acquired with the MOF system. In particular, it reveals the presence of unwanted spurious transmissions and prevents cross-talk between the intensity and velocity fluctuations. The method also gives information about the position of the “working point” of the instrument, i.e. the average wavelength position of its transmission profile, which is necessary for estimating the right solar atmospheric layer observed by the MOF. This analysis is powerful and fast: applied before every observing campaign, it enables setting Q the most suitable value for the cell temperature, which is one of the parameters that controls the MOF transmission profile.  相似文献   
150.
In the seismic microzonation the evaluation of the local amplification issue may be solved using different methodological approaches. Taking the hint from the studies made within VEL project framework, sponsored by Tuscany Region on Fivizzano's area we are herewith comparing the results of geotechnical and geophysical integrated multidisciplinary studies with the existing data available from previous studies, mainly using a macroseismic approach, on the same area. This qualitative method does not seem to be able to make any forecasting on the scale whereas generally microzoning is performed. On the contrary, the knowledge of the geological-technical model of the investigated area, obtained by punctual multidisciplinary integrated studies, allows a direct collection of the data to insert in input step of numerical modelling, aimed to local effects evaluation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
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