Neutralizing Homelessness, 2015: Tent cities and ten year plans     

Tony Sparks 《Urban geography》2017,38(3):348-356

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Single-target echo detections of jellyfish   总被引：2，自引：1，他引：2  

Brierley  Andrew S; Axelsen  Bjorn Eric; Boyer  David C; Lynam  Christopher P; Didcock  Carol A; Boyer  Helen J; Sparks  Conrad A.J; Purcell  Jennifer E; Gibbons  Mark J 《ICES Journal of Marine Science》2004,61(3):383-393

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Sedimentation of tephra by volcanic plumes. Part 2: controls on thickness and grain-size variations of tephra fall deposits     

R S J Sparks  M I Bursik  G J Ablay  R M E Thomas  S N Carey 《Bulletin of Volcanology》1992,54(8):685-695

A model for sedimentation from turbulent suspensions predicts that tephra concentration decreases exponentially with time in an ascending volcanic column and in the overlying umbrella cloud. For grain-size distributions typical of plinian eruptions application of the model predicts for thickness variations in good agreement with the exponential thinning observed in tephra fall deposits. The model also predicts a proximal region where fallout from the plume margins results in a more rapid decrease in thickness so that the deposit shows two segments on a thickness versus distance plot. Several examples of deposits with two segments are known. The distance at which the two segments intersect is a measure of eruption column height. The thickness half-distance ( equivalent to the dispersal index of Walker) is strongly correlated with column height, but is also weakly dependent on grain-size distribution of the ejecta. For a dispersal index of 500 km² (the plinian/subplinian boundary of Walker) column heights between 14 and 18 km are calculated. For ultraplinian deposits with D>50000 km² column heights of at least 45 km are implied. Model grain-size distributions of the deposits have sorting values comparable to those observed in tephra fall deposits formed from eruption columns in a weak or negligible cross-wind. Median diameter decreases exponentially with distance as is observed. Sorting () improves with distance as is observed in plinian deposits in a weak wind. However, tephra fall deposits formed in strong winds do not show improved sorting with distance and proximal deposits are typically somewhat better sorted than the model calculations. Differences are attributed to the influence of wind which disperses particles further than predicted in our model and which has an increasing influence as particle size decreases.  相似文献   

Control on the emplacement of the andesite lava dome of the Soufriere Hills volcano, Montserrat by degassing-induced crystallization     

Sparks  Murphy  Lejeune  Watts  Barclay  & Young 《地学学报》2000,12(1):14-20

Lava solidification is controlled by two mechanisms: external cooling and gas exsolution, the latter inducing crystallization due to increasing liquidus temperature. The andesite lava dome of the Soufriere Hills Volcano, Montserrat, is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement. In the magma chamber the magma has an estimated viscosity of 7 × 10⁶ Pa s. During ascent, gas exsolution caused the magma to extrude in a highly crystalline state, with only 5–15% residual melt, viscosities in the range 10¹³–10¹⁴ Pa s and mechanical strength > 1 MPa. Deformation can be heterogeneous with extrusion along shear zones. Rheological stiffening in the upper conduit also causes large overpressures, shallow seismicity, and cyclic patterns of dome extrusion. Gas-rich porphyritic andesites tend to be the least mobile kind of lava, because transition from magma into hot crystalline material was reached during ascent.  相似文献   

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展   总被引：15，自引：13，他引：2  

第五春荣  孙勇  董增产  王洪亮  陈丹玲  陈亮  张红 《岩石学报》2010,26(4):1171-1174

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。  相似文献   

Formation of nano-crystalline todorokite from biogenic Mn oxides     

Xiong Han Feng  Mengqiang Zhu  Chaoying Ni  Donald L. Sparks 《Geochimica et cosmochimica acta》2010,74(11):3232-6063

Todorokite, as one of three main Mn oxide phases present in oceanic Mn nodules and an active MnO₆ octahedral molecular sieve (OMS), has garnered much interest; however, its formation pathway in natural systems is not fully understood. Todorokite is widely considered to form from layer structured Mn oxides with hexagonal symmetry, such as vernadite (δ-MnO₂), which are generally of biogenic origin. However, this geochemical process has not been documented in the environment or demonstrated in the laboratory, except for precursor phases with triclinic symmetry. Here we report on the formation of a nanoscale, todorokite-like phase from biogenic Mn oxides produced by the freshwater bacterium Pseudomonas putida strain GB-1. At long- and short-range structural scales biogenic Mn oxides were transformed to a todorokite-like phase at atmospheric pressure through refluxing. Topotactic transformation was observed during the transformation. Furthermore, the todorokite-like phases formed via refluxing had thin layers along the c^∗ axis and a lack of c^∗ periodicity, making the basal plane undetectable with X-ray diffraction reflection. The proposed pathway of the todorokite-like phase formation is proposed as: hexagonal biogenic Mn oxide → 10-Å triclinic phyllomanganate → todorokite. These observations provide evidence supporting the possible bio-related origin of natural todorokites and provide important clues for understanding the transformation of biogenic Mn oxides to other Mn oxides in the environment. Additionally this method may be a viable biosynthesis route for porous, nano-crystalline OMS materials for use in practical applications.  相似文献   

Deep Hubble space telescope ultraviolet imaging of the M87 jet     

Juan P. Madrid  William B. Sparks  D. E. Harris  Eric S. Perlman  Duccio Macchetto  John Biretta 《Astrophysics and Space Science》2007,311(1-3):329-333

Near-ultraviolet imaging with HST offers the best possible spatial resolution currently available for optical/UV astronomical imaging. The giant elliptical galaxy M87 hosts one of the most spectacular, best studied and nearest (d=16 Mpc) galactic-scale relativistic (synchrotron emitting plasma) jets. We have extracted from the HST archive all 220 nm images of the jet of M87, taken with the STIS MAMA camera and co-added them to provide the deepest image ever at this wavelength. The combination of highest spatial resolution and long integration time, 42500 seconds, reveals a wealth of complex structure, knots, filaments and shocks. We compare this image with deep X-ray observations obtained with the Chandra X-ray telescope.  相似文献   

The fluid dynamics of a basaltic magma chamber replenished by influx of hot,dense ultrabasic magma     

Herbert E. Huppert  R. Stephen J. Sparks 《Contributions to Mineralogy and Petrology》1981,75(3):279-289

This paper describes a fluid dynamical investigation of the influx of hot, dense ultrabasic magma into a reservoir containing lighter, fractionated basaltic magma. This situation is compared with that which develops when hot salty water is introduced under cold fresh water. Theoretical and empirical models for salt/water systems are adapted to develop a model for magmatic systems. A feature of the model is that the ultrabasic melt does not immediately mix with the basalt, but spreads out over the floor of the chamber, forming an independent layer. A non-turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion. Both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock. The convection continues until the two layers have almost the same temperature. The compositions of the layers remain distinct due to the low diffusivity of mass compared to heat. The temperatures of the layers as functions of time and their cooling rate depend on their viscosities, their thermal properties, the density difference between the layers and their thicknesses. For a layer of ultrabasic melt (18% MgO) a few tens of metres thick at the base of a basaltic (10% MgO) magma chamber a few kilometres thick, the temperature of the layers will become nearly identical over a period of between a few months and a few years. During this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling. Olivines only settle after the two layers have nearly reached thermal equilibrium. At this stage residual basaltic melt segregates as the olivines sediment in the lower layer. Depending on its density, the released basalt can either mix convectively with the overlying basalt layer, or can continue as a separate layer. The model provides an explanation for large-scale cyclic layering in basic and ultrabasic intrusions. The model also suggests reasons for the restriction of erupted basaltic liquids to compositions with MgO<10% and the formation of some quench textures in layered igneous rocks.  相似文献   

The Los Chocoyos Ash, guatemala: A major stratigraphic marker in middle America and in three ocean basins     

John W. Drexler  W. I. Rose  Jr.  R. S. J. Sparks  M. T. Ledbetter 《Quaternary Research》1980,13(3):327-345

The Los Chocoyos Ash, having erupted from vents near the Lake Atitlán caldera, Guatemala, is perhaps the largest Quaternary silicic pyroclastic unit in Central America. It consists of an underlying H-tephra member and an overlying ash-flow member. One-hundred-and-five samples of ash from the Guatemalan Highlands and deep-sea cores in the equatorial Pacific and Gulf of Mexico were analyzed by neutron activation and/or electron microprobe. Glass shard chemistry, determined by microprobe, is useful for distinguishing several very widespread, distinct, deep-sea ash layers, but needs support from trace-element data when applied on land to distinguish between many individual eruptions from the same province. Data from this study support the correlation of the Worzel ‘D’ layer and the Los Chocoyos Ash proposed by Hahn et al. (1979) and Bowles et al. (1973). Chemical data from this study are used to correlate the Y-8 ash layer of the Gulf of Mexico with the Los Chocoyos Ash. The recognition of the Los Chocoyos Ash in the Gulf of Mexico and equatorial Pacific increases the known areal extent of the unit to more than 6 × 10⁶ km² and allows an age of 84,000 yr B.P. to be assigned to the formation on the basis of oxygen-isotope stratigraphy, biostratigraphy, and Pa-Th-isotope data. Trace-element data obtained from seven other ash layers in the Gulf of Mexico and the equatorial Pacific, when combined with new land-based data, should allow further correlation and dating of ash units in Central America.  相似文献   

The dimensions and dynamics of volcanic eruption columns   总被引：5，自引：1，他引：5  

R. S. J. Sparks 《Bulletin of Volcanology》1986,48(1):3-15

Eruption columns can be divided into three regimes of physical behaviour. The basal gas thrust region is characterized by large velocities and decelerations and is dominated by momentum. This region is typically a few hundred metres in height and passes upwards into a much higher convective region where buoyancy is dominant. The top of the convective region is defined by the level of neutral density (heightH _B ) where the column has a bulk density equal to the surrounding atmosphere. Above this level the column continues to ascend to a heightH _T due to its momentum. The column spreads horizontally and radially outwards between heightH _T andH _B to form an umbrella cloud. Numerical calculations are presented on the shape of eruption columns and on the relationships between the heightH _B and the mass discharge rate of magma, magma temperature and atmospheric temperature gradients. Spreading rate of the column margins increases with height principally due to the decrease in the atmospheric pressure. The relationship between column height and mass discharge rate shows good agreement with observations. The temperature inversion above the tropopause is found to only have a small influence on column height and, eruptions with large discharge rates can inject material to substantially greater heights than the inversion level. Approximate calculations on the variation of convective velocities with height are consistent with field data and indicate that columns typically ascend at velocities from a few tens to over 200 m/s. In very large columns (greater than 30 km) the calculated convective velocities approach the speed of sound in air, suggesting that compressibility effects may become important in giant columns. Radial velocities in the umbrella region where the column is forced laterally into the atmosphere can be substantial and exceed 55 m/s in the case of the May 18th Mount St. Helens eruption. Calculations on motions in this region imply that it plays a major role in the transport of coarse pyroclastic fragments.  相似文献