Crystal size distributions and other quantitative textural measurements in lavas and tuff from Egmont volcano (Mt. Taranaki), New Zealand   总被引：2，自引：0，他引：2  

Michael D. Higgins 《Bulletin of Volcanology》1996,58(2-3):194-204

 The size, shape and orientation of plagioclase crystals have been quantified in a tuff and series of andesite lavas from the active Egmont volcano (Mt. Taranaki), New Zealand. Linear crystal size distributions (CSDs) show that if the magma had several components, then only one provided the crystals. The slope of the CSD indicates that the earliest lavas measured had a residence time of ∼50 years in the magma chamber for a growth rate of 10^–11 cm/s. Subsequent lavas had slightly longer residence times (50–75 years), but the following series returned to 50-year residence times. The youngest magmas, from both Egmont summit and the parasitic Fantham's Peak, have the shortest residence times of ∼30 years. Variations in residence time may reflect changes in the magma chamber shape or depth, or the temperature of the surrounding rocks. Crystal shapes and zonation suggest that crystallization occurred in a bottle-shape magma chamber, and not in a narrow conduit. If future eruptions use the same magma chamber as the most recent eruptions, then a delay of approximately 30 years can be expected between refilling and eruption. Received: 25 October 1995 / Accepted: 19 April 1996  相似文献   

Using GIS and statistics to study influences of geology on probability features of surface soil geochemistry in Northern Ireland     

Crawford Jordan  Chaosheng Zhang  Alex Higgins 《Journal of Geochemical Exploration》2007

The probability features of non-normality and non-lognormality are widely observed in geochemistry due to the influences of multiple factors that are difficult to quantify and model. In Northern Ireland, the pseudo-total concentrations of 14 elements (Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb and Zn) from 6138 topsoils were measured, and GIS mapping showed that the spatial distribution of these data were in line with the spatial distribution of geology in the area. Investigations into the influences of geology on the concentration data and their probability features were carried out using GIS and statistics in this study. The whole raw data sets for each element were positively skewed and none of them followed either normal or lognormal distributions. Logarithmic transformation was found to have “over-transformed” most of the data sets, changing their skewness from positive to negative values. When soil samples were classified by rock type using a GIS overlay function, obvious differences were observed in the chemical concentrations of soils derived from different rock types. Soils in basalt areas displayed the highest concentrations for most elements under study (Ca, Co, Cr, Cu, Fe, Mg, Mn, Na, Ni, P and Zn) but the lowest concentrations for K, while the highest levels for Cd and Pb occurred in the shale areas. Classifying soils by rock type produced more normally distributed data sets, especially for the igneous rock areas. To restrain the influence of soil type and land cover, samples from both gleys and pastures were extracted via a GIS and it was found the data sets then showed generally greater tendencies towards normality. However, many of the data sets would still not pass a test for normality unless the sample size was small (e.g. of the order of a couple of hundreds). Geology, soil type, land cover and sample size all played important roles in determining soil chemical concentrations and their probability features. However, the influences from other factors were still evident. Attempts made in this study show that it remains a challenging task in geochemistry to separate all the factors and to model their influence at the regional scale.  相似文献   

Alignment Trends of Velocity Gradients and Subgrid-Scale Fluxes in the Turbulent Atmospheric Boundary Layer     

Chad W. Higgins  Marc B. Parlange  C. Meneveau 《Boundary-Layer Meteorology》2003,109(1):59-83

Field experimental data in the atmospheric surface layer are analyzed using toolsfrom statistical geometry. The data consist of velocity measurements from sonicanemometer arrays. In the context of large eddy simulations (LES), these arrayspermit the spatial filtering needed to separate large from small scales. Time seriesof various quantities relevant to LES are evaluated from the data. Results show thatthe preferred filtered fluid deformation is axisymmetric extension and the preferredsubgrid stress state is axisymmetric contraction. The filtered fluctuating vorticityshows preferred alignments with the mean vorticity, with the streamwise direction,and with the intermediate strain-rate eigenvector. The alignment between eigenvectorsof the subgrid-scale stress and filtered strain rate is used to test eddy viscosity andmixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model.  相似文献   

Ecosystem Responses to Abrupt Climate Change: Teleconnections, Scale and the Hydrological Cycle     

Paul A. T. Higgins  Michael Vellinga 《Climatic change》2004,64(1-2):127-142

The occurrence of past and future abrupt climate change, such as could occur under thermohaline circulation (THC) weakening, is increasingly evident in the paleoclimate record and model experiments. We examine potential responses of ecosystem structure and function to abrupt climate change using temperature and precipitation patterns generated by HadCM3 in response to forced THC weakening. The large changes in potential ecosystem structure and function that occur are not focused in the North Atlantic region where temperature sensitivity to THC is highest but occur throughout the world in response to climate system teleconnections. Thus, THC weakening, which is often viewed as a European problem, has globally distributed ecosystem implications. Although temperature changes associated with THC weakening affect the extent of several high latitude biomes, the distribution of ecosystem change results primarily from changes in the hydrological cycle. Currently there remains large uncertainty in climate model projections of the hydrological cycle. Therefore, the predictions of the magnitude andlocation of ecosystem perturbations will also be characterized by large uncertainty, making impact assessment, and thus adaptation, more difficult. Finally, these results illustrate the importance of scale and disaggregation in assessing ecosystem responses. Small globally aggregated ecosystem responses to THC weakening, approximately five percent for NPP and biomass, mask large local and regional changes.  相似文献   

Blast Design Using Measurement While Drilling Parameters   总被引：1，自引：0，他引：1  

J. B. Segui  M. Higgins 《Fragblast: International Journal for Blasting and Fragmentation》2002,6(3):287-299

Measurement while drilling (MWD) techniques can provide a useful tool to aid drill and blast engineers in open cut mining. By avoiding time consuming tasks such as scan-lines and rock sample collection for laboratory tests, MWD techniques can not only save time but also improve the reliability of the blast design by providing the drill and blast engineer with the information specially tailored for use. While most mines use a standard blast pattern and charge per blasthole, based on a single rock factor for the entire bench or blast region, information derived from the MWD parameters can improve the blast design by providing more accurate rock properties for each individual blasthole. From this, decisions can be made on the most appropriate type and amount of explosive charge to place in a per blasthole or to optimise the inter-hole timing detonation time of different decks and blastholes. Where real-time calculations are feasible, the system could extend the present blast design even be used to determine the placement of subsequent holes towards a more appropriate blasthole pattern design like asymmetrical blasting.  相似文献   

Sapphirine I     

John B. Higgins  Paul H. Ribbe  Richard K. Herd 《Contributions to Mineralogy and Petrology》1979,68(4):349-356

Microprobe analyses of 26 natural sapphirines from 17 localities indicate that the predominant chemical substitutions in this mineral occur along the solid solution join^VI(Mg,Fe)²⁺+^IVSi⁴⁺=^VI(Al, Fe)³⁺+^IVAl³⁺. Chromium and manganese are minor substituents. Evidence for the substitution SiAl+1/2Mg+1/2 vacancy is absent within the limits of analytical error.A partitioning scheme based on electrostatic charge balance considerations has been devised permitting calculation of Fe²⁺ and Fe³⁺ from total iron content. Results are in good agreement with previous Mössbauer studies which indicate Fe³⁺ is sometimes in octahedral and/or tetrahedral coordination.Distribution coefficients for Fe²⁺-Mg exchange equilibria between sapphirine-spinel and sapphirine-orthopyroxene are similar for most mineral pairs and suggest that most of the assemblages equilibrated at about the same temperature or that the exchange reactions are insensitive to temperature.Compositions of synthetic sapphirines as a function of temperature and pressure are qualitatively predictable from crystal chemical considerations. Changes in sapphirine composition along the MgSi= AlAl solid solution join toward more aluminous compositions stabilize the sapphirine structure at high temperatures and low pressures. The limited extent of MgSi=AlAl solid solution observed in natural sapphirines appears to be related to the requirements of geometrical fit among octahedra and tetrahedra in the almost idealized cubic closest-packed anion framework.  相似文献   

Competitive Productivism and Australia's Emerging ‘Alternative’ Agri-food Networks: producing for farmers' markets in Victoria and beyond     

Peter Andrée  Vaughan Higgins  Chris Cocklin 《The Australian geographer》2010,41(3):307-322

This paper advances the theoretical argument for moving beyond the conventional/alternative divide in the analysis of emerging ‘alternative’ agri-food networks (AAFNs). In order to understand how ‘place’, ‘nature’ and ‘quality’-based food networks emerge and develop, we argue that careful attention needs to be paid to the way in which specific political-economic environments shape the options available to farmers and consumers. Australia's ‘competitive productivism’ is the outcome of an export-oriented economy and a neoliberal political orientation, and this environment affects the development of AAFNs in various ways. Most notably, a case study of farmers that sell at farmers' markets in the State of Victoria shows that the competitive-productivist policies pursued by Australian governments entice some farmers who participate in AAFNs to develop their ideas into higher output businesses tailored to compete in emerging export markets. Other farmers deliberately choose to keep marketing through alternative channels despite competitive-productivist pressures to expand. These findings demonstrate the centrality to the experience of AAFN participants of negotiating productivist pressures, adding nuances to the story of the complex relations between AAFNs and conventional supply chains.  相似文献   

X-ray digital-imaging petrography: Technique development for lunar mare soils     

S. J. Higgins  L. A. Taylor  J. G. Chambers  A. Patchen  D. S. McKay 《Meteoritics & planetary science》1996,31(3):356-361

Abstract Remote sensing studies are the primary means of solar system exploration. In particular, spectral reflectance measurements involve determinations of the nature and compositions of other worlds through analysis of absorption features characteristic of the surface chemistry and mineralogy. These studies are particularly applicable to “airless” solar system bodies (e.g., the Moon), because atmospheres, such as on Earth, tend to interfere with the reflectance spectrum. The precision of the spectral measurements is greatly increased by calibration with actual lunar soils. In the past, these calibrations were done using particle-counting data collected for the study of soil formation processes, soil classification, and provenance determination. These particle counting data, while valuable in those areas of study, neither identify the true volume percentages of soil particles, nor give the true modal values for the various phases (i.e., minerals and glasses) which make up the soil grains. These data are paramount for accurate spectral reflectance calibrations. Therefore, in this paper, a new technique is presented that involves x-ray digital-imaging of lunar soils using an energy dispersive spectrometer (EDS) on an electron microprobe. In contrast to particle counting with an optical microscope, the digital-imaging method allows precise volume percentages of soil grains to be determined, including absolute modal abundances of the various phases locked within the particles as well as their chemistry. In order to validate this method for characterization of lunar soils, the technique was applied to four Apollo 17 soils that were previously described by Heiken and McKay (1974) via particle counts with an optical microscope, and similar results were obtained. In addition to verifying the x-ray digital-imaging technique, the obtained data were applied in order to better understand the lunar-soil formational process, specifically the variation of particle types with maturity.  相似文献   

Large Wind Farms and the Scalar Flux over an Heterogeneously Rough Land Surface   总被引：2，自引：2，他引：0  

Marc Calaf  Chad Higgins  Marc B. Parlange 《Boundary-Layer Meteorology》2014,153(3):471-495

The influence of surface heterogeneities extends vertically within the atmospheric surface layer to the so-called blending height, causing changes in the fluxes of momentum and scalars. Inside this region the turbulence structure cannot be treated as horizontally homogeneous; it is highly dependent on the local surface roughness, the buoyancy and the horizontal scale of heterogeneity. The present study analyzes the change in scalar flux induced by the presence of a large wind farm installed across a heterogeneously rough surface. The change in the internal atmospheric boundary-layer structure due to the large wind farm is decomposed and the change in the overall surface scalar flux is assessed. The equilibrium length scale characteristic of surface roughness transitions is found to be determined by the relative position of the smooth-to-rough transition and the wind turbines. It is shown that the change induced by large wind farms on the scalar flux is of the same order of magnitude as the adjustment they naturally undergo due to surface patchiness.  相似文献   

Prediction of piezometric surfaces and drain spacing for horizontal drain design     

Diana I. Cook  Paul M. Santi  Jerry D. Higgins 《Landslides》2012,9(4):547-556

Horizontal drains, used independently or as part of a more complex remediation scheme, are frequently installed to mitigate the effects of increased groundwater in slope stabilization projects. Due to a general trial and error approach to their design, the need for improved design practices has been recognized. The procedures established by Crenshaw and Santi in 2004 made some advances in this direction, but did not account for slopes with drains that were not horizontal or for sloping low-permeability layers underneath the slide mass. Furthermore, the method outlined by Crenshaw and Santi is time-consuming and requires some trial and error calculations to achieve convergence. Therefore, the method has been modified to account for nonhorizontal elements, and a horizontal drain spreadsheet has been developed to streamline the design for projects where horizontal drains will be installed. The horizontal drain spreadsheet may be used to: (1) predict a conservative piezometric profile in a drained slope for use in slope stability analyses, (2) predict piezometric heads in any single piezometer in a drainage field, and (3) predict drain spacing for design purposes. This document explains the revisions to Crenshaw and Santi’s procedures and provides instructions for applying the method. The instructions may be used for hand calculations, but are specifically intended for use with the horizontal drain spreadsheet. The spreadsheet may be used for slopes composed of silty or clayey sands, silts, and silty or sandy clays.  相似文献