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61.
One active and ten extinct Quaternary volcanoes are described from the Cape Hoskins area, on the north coast of New Britain. They are mostly strato volcanoes built up of lava flows, lava domes, pyroclastic flows, lahars, tephra, and derived alluvial sediments. The volcanic products range in composition from basalt to rhyolite, but basaltic andesite and andesite predominate. Much of the area is covered by tephra, several metres thick, consisting mainly of rhyolitic pumice. The active volcano, Pago, is built up of several glacier-like lava flows, the last of which was formed during an eruption in 1914–18. Pago lies within a well-preserved caldera forming the central part of a broad low-angle cone, named Witori, which consists largely of welded and unwelded pyroclastic flow deposits. C-14 dates obtained on charcoal indicate that the caldera eruption occurred about 2500 years B. P. Another caldera of similar age lies south of Witori. Of the other eight volcanoes described four are relatively well-preserved steep-sided cones formed mainly of lava flows, one is a remnant of a low-angle cone with a caldera, and three are deeply eroded cones which have none of their constructional surfaces preserved. 相似文献
62.
Potential seasonal variation in the U-series radioisotopic signatures of lake and input waters was investigated in Hawes Water, north-west England, and the sedimentary record was examined to establish whether any observed seasonality is preserved in the sediment U and Th radioisotopic activity ratios. U-series disequilibrium in the waters is indicative of both surface and shallow sub-surface origins for the dissolved U, and that this supply is influenced by changes in the prevailing hydrological regime. However, the lake is buffered from this seasonality in that the 234U/238U activity ratio remains constant during the year, and the U content undergoes only a marginal decrease during summer. The change in the mechanism of lake sedimentation from allochthonous organic input during winter to autochthonous carbonate formation during summer has a more significant influence on the U-series radioisotopic characteristics of sedimentary record in that the proportion of organic and carbonate material both exhibit a strong control on the234U/238U activity ratio and U content. In addition, significant proportions of sediment U and Th are associated with ferromanganese oxyhydroxides. Given this seasonality, it is of some significance that the 234U/238U activity ratios of the bottom sediments preserve a reasonable record of lake U-series disequilibrium and are, therefore, potential indicators of long-term changes in catchment hydrology. 相似文献
63.
Molecular orbital calculations with HF/3-21G∗∗, HF/6-311+G∗∗, and MP2/6-311+G∗∗ basis sets (HF = Hartree-Fock approximation; MP2 = 2nd-order Møller-Plesset perturbation theory) have been performed on molecular clusters in the system acetate-aluminum-water. The results model the structures, energetics, and vibrational spectra of Al3+ and Al3+-acetate complexes in the aqueous phase. An octahedral to tetrahedral coordination change is predicted in the species Al3+ (OH)m− · n (H2O) [where m + n = 6) as m increases from two to three. Calculated reaction energetics for aqueous Al3+-acetate complexation compare favorably with experimental enthalpies. In addition, the possible existence of more than one configuration for each Al` -acetate species was investigated. Theoretical vibrational spectra of the Al3+-acetate complexes provide predictions for the identification of Al3+-acetate species in aqueous solutions. 相似文献
64.
John a. Herring Daniel a. Jaffe Harald J. Beine Sasha Madronich Donald R. Blake 《Journal of Atmospheric Chemistry》1997,27(2):155-178
A seven-year record of surface ozone measurements from Denali NationalPark, Alaska shows a persistent spring maximum. These data, combined withmeasurements of NOx, hydrocarbons, O3, and PANfrom a continental site in Alaska during the spring of 1995 are used as thebasis for a sensitivity study to explore tropospheric photochemistry in thisregion. Because of the relatively high concentrations of NOx(mean of 116, median of 91 pptv), the net tendency was for photochemicalozone production. The range of net O3 production for averageconditions measured at this site during spring is between 0.96–3.9ppbv/day depending on the assumptions used; in any case, this productionmust contribute to the observed springtime maximum in O3.Model calculations showed that of the anthropogenic ozone precursors, onlyNOx had a strong effect on the rate of ozone production; themeasured concentrations of anthropogenic hydrocarbons did not significantlyaffect the ozone budget. Naturally produced biogenic hydrocarbons, such asisoprene, may also have a significant effect on ozone production, even atconcentrations of a few 10's of pptv. An observed temperature-isoprenerelationship from a boreal site in Canada indicates that isoprene may bepresent during the Alaskan spring. Measurements of isoprene taken duringthe spring of 1996 suggest that reactive biogenic hydrocarbon emissionsbegin before the emergence of leaves on deciduous trees and that theconcentrations were sufficient to accelerate ozone production. 相似文献
65.
The production of mixed magmas (streaky pumice) during flow in a volcanic conduit has been modelled in the laboratory by studying the flow of two miscible fluids of differing viscosity passing concentrically through a vertical pipe. In the experiments reported in this paper, the outermost fluid is the more viscous, as would be the case when two magmas are simultaneously tapped from a zoned chamber in which silicic magma overlies mafic magma. At a Reynolds number (Re) which is much less than that required for turbulence in isoviscous pipe flow, the interface between two liquids of different viscosity can become unstable. Growth of the instability and mixing proceed when Re, based on the properties of the inner, less viscous fluid (Re
i), is greater than approximately 3 if between 10% and 90% of the flowing fluid is composed of the more viscous fluid. Outside this range of flow rate ratios, higher Re
i and viscosity ratios are required to ensure mixing. When the viscosity ratio U10 the unstable flow takes the form of an asymmetric, sinusoidal wave and at higher viscosity ratios axisymmetric, bead-like waves are the dominant instability. Entrainment across the boundaries of these wavy interfaces results in the production of streaky mixtures of the two liquids. The degree of mixing increases with Re
1, U and distance downstream. Application of experimental results to magmatic situations shows that mixing will be possible in eruptions which tap layers of different viscosity from a stratified chamber. If a volcanic feeder is allowed to become lined by silicic magma before a mafic magma layer is drawn up from the chamber then a mixed pumice (or lava) sequence will ensue. Alternatively, if draw-up occurs when the feeder is still propagating away from the chamber, the slower flowing silicic magma may be overtaken by the faster flowing mafic magma. The advancing conduit will then have mafic or hybrid chilled margins enclosing a silicic interior, i.e. the usual arrangement in composite dykes and sills. Simultaneous tapping of silicic and underlying mafic magmas from a chamber can thus lead to magma mixing and to the emplacement of either mixed pumice sequences or composite intrusions, depending on the history of magma withdrawal and the dynamics of flow in the conduit. 相似文献
66.
The conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid-dynamical experiments. The results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number
where
1 is the viscosity of the fluid, U is the velocity, g is the acceleration due to gravity, is the density difference between the two fluids, and R is the radius of the tube. The parameter I represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls, and the buoyancy forces which tend to keep the interface horizontal. The experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube. They show that overturning of the initial density stratification and mixing occur when I>order 10-1; the two fluids remain stratified when I 10-3. Transitional states are observed when 10-3<I<10-1. These results are nearly independent of Reynolds number
and viscosity ratio
in the range of
and Re
1<300. Applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch. This mechanism can explain some occurrences of small-volume mixed lava flows. 相似文献
67.
Three models for the dynamics of seismic airgun‐generated bubbles and their associated far‐field signals are developed and compared with geophysical data. The first model of an airgun‐generated bubble uses a spherical approximation, the second is an approximate Lagrangian model which allows for small deformations from a spherical shape, whilst the final model is an axisymmetric boundary‐integral method which permits the bubble to evolve into highly non‐spherical geometries. The boundary‐integral method also allows both geometric interference and strong dynamic interactions in multi‐bubble studies. When comparing the spherical model to experimental data there are three apparent, significant differences: the magnitude of the primary pressure peak, which is greater in the model; the subsequent decay of the pressure peaks and motion – the experimental data demonstrating greater decay and a slower rise rate; and the frequency of oscillation, which is slower in the experimental data. It is believed that the first discrepancy is due to the initial stages of expansion where the compressed air is forced to sparge through the airgun ports. The other differences indicate that there is some other energy‐loss mechanism which is not accounted for in the spherical bubble model. Non‐spherical bubble behaviour is investigated through the use of two different deformable many‐bubble codes and their predictions are compared with the spherical model and experimental data. The Lagrangian model predicts the formation of a buoyancy‐driven liquid jet on the first collapse of a typical airgun bubble; however, the model breaks down when the bubble becomes significantly deformed, due to a low‐order spherical‐harmonic approximation for the potential. The axisymmetric boundary‐integral code models the jet shape accurately and it is found that these bubbles evolve to toroidal geometries when the jet impacts on the opposite surface of the bubble. This highly non‐spherical behaviour is readily observed on high‐speed films of airgun bubbles, and is one key source of energy loss; it damps the pulsations of the bubble and slows its rise speed. Inter‐bubble interactions are investigated using the two deformable bubble models, and the predictions are compared to field data. It was found that as the bubbles approach each other, their periods of oscillation increase in accordance with observations, and jets are formed in the direction of motion upon collapse. 相似文献
68.
Ruth Elaine Blake 《Geochimica et cosmochimica acta》2003,67(13):2315-2316
69.
70.
We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H2O, OH∗, C2H6, CH3OH, H2CO, CH4, HCN, C2H2, NH3, NH2, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H2O and CH4, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H2O and CH3OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C2H6 and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere. 相似文献