Airline networks and the international diffusion of severe acute respiratory syndrome (SARS)   总被引：2，自引：0，他引：2  

JOHN T BOWEN JR    CHRISTIAN LAROE† 《The Geographical journal》2006,172(2):130-144

In fewer than four months in 2003, severe acute respiratory syndrome (SARS) spread from China to 25 countries and Taiwan, becoming the first new, easily transmissible infectious disease of the twenty-first century. The role of air transport in the diffusion of the disease became obvious early in the crisis; to assess that role more carefully, this study relates the spatial-temporal pattern of the SARS outbreak to a measure of airline network accessibility. Specifically, the accessibility from those countries that were infected by SARS, beginning with China, to other countries was measured using airline schedules. The country-pair accessibility measure, along with other country-level factors relevant to the disease, were tested as determinants of the speed with which SARS arrived in infected countries as well as its failure to arrive in most countries. The analyses indicate that airline network accessibility was an especially influential variable but also that the importance of this variable diminished in the latter weeks of the outbreak. The latter finding is partly attributable to public health measures, particularly health screening in airports. The timing and geography of those measures are reviewed using data from media reports and interim World Health Organization (WHO) documents during the outbreak. The uneven effort to curtail the international diffusion of SARS suggests further planning is needed to develop a concerted response to contain future epidemics.  相似文献   

Inconsistent patterns of historical seismicity and earthquake-triggered landsliding in southeastern Sicily: an alarm bell?     

Pier Giorgio Nicoletti   《Geomorphology》2005,65(3-4):257-278

In southeastern Sicily, the distribution pattern of earthquake-triggered landslides is inconsistent with historically known seismic activity. In particular, historical seismicity has been stronger in the eastern sector, while earthquake-triggered landsliding is much more developed in the western sector, and this contrasts with expectations. This paper describes the patterns of seismicity and landsliding, and tackles the problem of whether greater landsliding in the western sector may be due (a) to seismicity of the eastern sector or (b) to a high-magnitude, low-recurrence seismicity generated in the western sector itself. It is statistically shown that lithostructural characteristics of rock masses and geometry of hillslopes cannot justify, either singly or jointly, greater landsliding in the western sector if triggering earthquakes are generated in the eastern sector. As a consequence, it is possible that landsliding of the western sector is due to earthquakes generated locally, but having higher magnitude and longer recurrence intervals than historically observed.  相似文献   

Volcanological implications of crystal-chemical variations in clinopyroxenes from the Aeolian Arc, Southern Tyrrhenian Sea (Italy)   总被引：1，自引：0，他引：1  

Sabrina Nazzareni  Gianmario Molin  Angelo Peccerillo  Pier Zanazzi 《Bulletin of Volcanology》2001,63(1):73-82

Crystal chemistry and structural data for clinopyroxene from the Aeolian islands (Southern Tyrrhenian Sea, Italy) were determined with the aim of obtaining geobarometric information and exploring implications for the structure of volcanic plumbing systems. Cell and M1 site volumes for clinopyroxenes, which are known to decrease with increasing pressure of crystallization, revealed variable values, both within some single islands and along the entire arc, indicating polybaric conditions of crystallization. The lowest cell and M1 volumes were found at Filicudi, plotting close to values of clinopyroxenes from high-pressure ultramafic xenoliths entrained in alkali basalts. Indications of high-pressure crystallization were also found at Salina and, to a lesser extent, at Alicudi, all situated in the western sector of the Aeolian Arc. The central and eastern islands of Lipari, Vulcano, Panarea and Stromboli generally show higher values of cell parameters, suggesting crystallization in shallow magma chambers. These islands are characterized by the occurrence of large calderas, which are apparently lacking at Salina and Filicudi. Time-related variations were observed for cell and M1 volumes of clinopyroxene for some islands. At Salina, the early-erupted products display low values of cell parameters with respect to later activity, thus indicating a decrease in crystallization pressure with time. A similar, although less striking, pattern is observed at Alicudi and Lipari. An overall increase in cell parameters with time was observed at the scale of the entire arc. The observed variations in clinopyroxene structural parameters highlight the significance of pyroxene crystal chemistry for petrogenetic and volcanological interpretation. Correlation with time and the structural characteristics of volcanoes suggest significant regional and temporal modifications in the plumbing systems of Aeolian volcanoes. Clinopyroxenes from Filicudi and the older Salina crystallized at high pressure in deep magma chambers, in the lower crust or at the mantle-crust boundary. The lower crystallization pressure in the younger Salina is interpreted as evidence of upward migration of magma chambers with time. Similar evolution can be envisaged for Alicudi. Instead, the entire evolutionary history of the central and eastern islands was dominated by low-pressure crystallization, with formation of calderas and generation of abundant acid products that are scarce or absent in the western islands. Evolution of the plumbing system of single volcanoes and of the Aeolian arc in general is probably related to modification of stress regimes and/or thinning of the arc basement, due to the effect of uprising mantle material above the Ionian subduction zone.  相似文献   

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton   总被引：6，自引：1，他引：6  

DOWNES  HILARY; MACDONALD  RAY; UPTON  BRIAN G. J.; COX  KEITH G.; BODINIER  JEAN-LOUIS; MASON  PAUL R. D.; JAMES  DODIE; HILL  PETER G.; HEARN  B. CARTER  JR 《Journal of Petrology》2004,45(8):1631-1662

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette  相似文献   

The greenhouse marathon: A proposal for a global strategy     

Pier Vellinga  Rob Swart 《Climatic change》1991,18(1):vii-xii

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The La Breña — El Jagüey Maar Complex,Durango, México: I. Geological evolution     

Jose J Aranda-Gómez  James F Luhr  G Pier 《Bulletin of Volcanology》1992,54(5):393-404

The La Breña — El Jagüey Maar Complex, of probable Holocene age, is one of the youngest eruptive centers in the Durango Volcanic Field (DVF), a Quaternary lava plain that covers 2100 km² and includes about 100 cinder and lava cones. The volcanic complex consists of two intersecting maars — La Breña and El Jagüey — at least two pre-maar scoria cones and associated lavas, and a series of nested post-maar lava and scoria cones that erupted within La Breña Maar and flooded its floor with lava to form one or more lava lakes. We believe that El Jagüey Maar formed first, but pyroclastic deposits associated with its formation are exposed at only a few places in the lower maar walls. A perennial lake in the bottom of El Jagüey marks the top of an aquifer about 60 m below the lava plain. Interaction of the rising basanitic magmas with this aquifer was probably responsible for the hydromagmatic eruptions at the maar complex. In the southeastern quadrant of La Breña and in most parts of El Jagüey, the upper maar walls expose a thick pyroclastic sequence of tuffs, tuff breccias, and breccias that is dominated by thinly layered sandwave and plane-parallel surge beds and contains minor interlayered scoria-fall horizons. We conclude that these deposits in the upper walls of both maars erupted during the formation of La Breña, based on: (1) thickness variations in a prominent scoria-fall marker bed interlayered with the surge deposits; (2) inferred transport directions for ballistic clasts, channels, and dune-like bedforms; and (3) lateral facies changes in the surge deposits. Some of the surge clouds from La Breña apparently travelled down the inner southwestern wall of El Jagüey, fanned out across its floor, and climbed up the opposite walls before emerging onto the surrounding lava plain. These clouds deposited steep, inward-dipping surge deposits along the lower walls of El Jagüey. Following this hydromagmatic phase, which was responsible for the formation of the maars, a series of strombolian eruptions took place from vents within La Breña. At many places along the maar rims these eruptions completely buried the surge beds under a thick sequence of post-maar scoriae and ashes. The outer flanks of the maar complex and the surrounding lava plain are also blanketed by post-maar ashes. The final phase of activity involved effusive eruptions of post-maar lavas from vents on the floor of La Breña. The evolutionary sequence from hydromagmatic eruptions during formation of the maars, through strombolian eruptions of the post-maar scoriae and ashes, and finally to the post-maar lavas appears to reflect the declining influence of magma-groundwater interactions with time. Basanitic magmas from all eruptive stages carried spinel-lherzolite and feldspathic-granulite xenoliths to the surface. The La Breña — El Jagüey Maar Complex contains the only known hydromagmatic vents in the DVF and the largest spinel-lherzolite xenoliths, which range up to 30 cm diameter. These two observations indicate an unusually rapid ascent rate for these basanitic magmas compared to those from other DVF vents.  相似文献   

Erosion rather than plant dieback as the mechanism of marsh loss in an estuarine marsh     

J. A. Nyman  M. Carloss  R. D. Delaune  W. H. Patrick JR. 《地球表面变化过程与地形》1994,19(1):69-84

Coastal marsh loss in Louisiana is attributed to plane dieback caused by processes that stress vegetation, and a common landscape pattern is broken marsh that expands at the expense of surrounding unbroken marsh. We tested the hypothesis that vegetation is more stressed in broken marsh than in adjacent unbroken marsh, as indicated by vegetation aboveground biomass, species diversity and soil Eh, on transects that extended from broken marsh to unbroken marsh at Marsh Island, Louisiana. Soil Eh, vegetation above-ground biomass and species diversity did not differ between broken marsh and unbroken marsh, and above-ground biomass was similar to that reported from other marshes. Thus, we rejected the hypothesis that marsh loss is related to vegetation stress. Two factors were related to vegetation vigour: soil drainage and soil bulk density. Surprisingly, significant soil drainage occurred in broken marsh but not in unbroken marsh. Above-ground biomass of the dominant plant, Spartina patens (Aiton) Muhl., was lowest where soil bulk density was less than 0-08 gcm⁻³, which illustrated the importance of mineral matter accumulation in submerging coastal marshes. The mechanism of marsh loss appeared to be erosion below the living root zone, as indicated by the vertical and often undercut marsh-water interface, and by the separation of sod clasts. This is different from more rapid marsh loss associated with plant stress which we observed in other Louisiana marshes only 135 km away, indicating that marsh loss mechanisms can vary spatially even within a relatively small region.  相似文献   

Flow properties of turbidity currents in Bute Inlet, British Columbia   总被引：1，自引：0，他引：1  

JIANJUN ZENG  DONALD R. LOWE  DAVID B. PRIOR  WILLIAM J. WISEMAN  JR  BRIAN D. BORNHOLD 《Sedimentology》1991,38(6):975-996

Bute Inlet, a fiord along the southwestern coast of British Columbia, Canada, includes a sea-floor sedimentation system 70 km in length which resembles those developed on some large submarine fans. Turbidity currents originate at the head of the flord on the submerged delta fronts of the Homathko and Southgate rivers. They move downslope for about 30 km within a single large incised channel, spill onto a depositional area termed the channel lobe complex, and finally spread out over a low-relief distal splay area that passes 55 km downslope into a flat basin floor. During the present study, turbidity currents in Bute Inlet were studied using sea-floor morphology, bottom sediment distribution, and in-situ instrument packages. The mean velocities of the most recent flows, estimated from surface sediment grain size, has varied between 100–120 cm s^–1 in the incised channel, 20–50 cms^–1 in the channel lobe complex, and < 5 cm s^–1 on the basin floor. Velocities based on channel morphology are poorly constrained but are in the range of 160-425 cm s^–1 in the upper part of the incised channel and 66 cm s^–1 in the lower channel. Calculated flow densities range from 1.049 to 1.028g cm^–3. Turbidity flows monitored in 1986 using submerged instrument packages exceeded 32 m in thickness in the upper part of the incised channel, where the maximum measured velocity was 330 cm s^–1. At the head of the channel lobe complex the maximum velocity had declined to 75 cm s^–1. The density of the monitored flows is estimated at 1.025-1.03g cm^–3. The cored sediments and channel morphology yield estimates of mean flow velocities that are generally greater than those measured by the in-situ instrument packages and estimated from modern surface sediments. The former suggest past flow velocities up to 500 cm s^–1 in the incised channel, about 20 cm s^–1 in spillover deposits along the lower part of the incised channel, and 100-140 cm s^–1 in the distal splay. The contrast between the velocities of modern and past flows suggests that past flows may have been considerably larger and more energetic than those presently occurring in Bute Inlet. The size properties of sediments in the monitored turbidity flows suggest a strong vertical size gradient in the suspended load during transport. The surface and cored sediments fine downslope from the channel lobe complex to distal splay area. Distinctive sedimentary sequences are recognized in cores from the spillover lobes, channel lobe complex, distal splay, and basin floor depositional areas. Many individual turbidites grade downslope from massive T_a divisions in the channel lobe complex and probably in the incised channel to T_a divisions overlain by slurried divisions on the distal splay and largely slurried beds on the basin floor. These facies suggest that individual currents commonly evolve from largely cohesionless suspensions in the incised channel and channel lobe complex to dilute cohesive slurries downslope on the distal splay and basin floor. Many flows in Bute Inlet fail to develop a traction state of sedimentation and the resulting turbidites lack well-developed T_b. T_c, and T_d divisions.  相似文献   

Mineralogy, Petrology, and Magmatic Conditions from the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado   总被引：2，自引：4，他引：2  

WHITNEY  JAMES A.; STORMER  JOHN C.  JR. 《Journal of Petrology》1985,26(3):726-762

The Fish Canyon Tuff is one of the largest currently recognizedash-flow tuffs (> 3000 km³). It is a crystal-rich quartzlatite containing about 40 per cent phenocrysts of plagioclase,sanidine, biotite, hornblende, quartz, magnetite, sphene, andilmenite. Pyrrhotite occurs as inclusions in magnetite, sphene,and hornblende. The consistency of mineralogy and whole rockchemistry confirms that the Fish Canyon tuff is remarkably homogeneous.Most chemical variations can be accounted for by phenocryst-matrixfractionation, probably due to glass winnowing during eruptionand emplacement. The composition of the parent magma, correctedfor such winnowing, is very similar to that of calc-alkalinebatholiths such as the Boulder and the Sierra Nevada batholiths. Fe-Ti oxide geothermometers indicate temperatures of 800 ? 30?C for most of the outflow tuff. No evidence for a regular thermalgradient in the magma chamber could be detected. Two feldsparand Fe-Ti oxide equilibria indicate that the magma developedat depths of 25 to 30 km (about 9 kb pressure), and was eruptedwithout time for phenocryst re-equilibration. The reconstructedcomposition of the liquid in equilibrium with the phenocrystsalso suggests a deep source for this ash flow. A late, upperpackage of flow units have mineralogical characteristics whichmay reflect partial re-equilibration in a shallower environment. Oxygen fugacities are moderately high (log fO₂ = — 11.5?0.3) but are similar to those obtained from other continentalcalc-alkaline ash-flow tuffs. The water fugacity is limitedby calculations using biotite equilibria and experimental workrelating to the stability of the phenocryst assemblage. Bestestimates are that water fugacity was 2000 ? 1000 bars. Theactivities of sulphurous gases are estimated at fSO₂ = 2 to4 bars, fso₂ = 150 to 200 bars, fH₂S = 70 to 80 bars. The Fish Canyon Tuff therefore came from a deep, homogeneous,granitic magma body of batholithic proportions. Calculationsof its probable viscosity, density, and size indicate that thesystem should convect with any reasonable thermal gradient.Convective mixing may account for the homogeneity of the parentmagma body.  相似文献   

Lower Temperature Terminations of the Three-phase Region Plagioclase-Alkali Feldspar-Liquid     

STEWART  D. B.; ROSEBOOM  E. H.  JR. 《Journal of Petrology》1962,3(3):280-315

Geological and experimental evidence indicate that the three-phasefield, plagioclase-alkalifeldspar-liquid, may terminate in severaldifferent ways. The possible terminations have been developedfrom Schreinemakers' rules governing the disappearance of three-phasefields. In igncous rocks, these different terminations may arisefrom variations in the relative amounts of additional componentsin magmas, or from changes of total pressure, or from structuralchanges that effect the extent of solid solution in feldspar. The three-phase region originates from the intersection of thesolidus and the feldspar solvus. The available evidence regardingthis intersection is reviewed, as is the evidence for the existenceand form of the boundary curve on the feldspar liquidus. Thedata are used to project a series of isobaric polythermal andisobaric isothermal diagrams for each possible termination.Subsequent discussions relate the theoretical arguments to thenatural evidence, suggest the more probable geological environmentsof some of the terminations, and indicate characteristic featuresof each termination. It may be possible to identify the type of termination involvedin the crystallization of some rocks. The necessary data arethe amounts and compositions of both kinds of coexisting feldsparsand of the feldspar components of the coexisting liquid at oneor more stages of the crystallization process. Volcanic rocksare most suitable for such studies. The terminations explain many of the compositional relationshipspossible between feldspar crystals and silicate melt under geologicaland experimental conditions. These compositional relationshipsare fundamental to understanding the crystallization of igneousrocks and the formation of melts by partial fusion. Other geologicalapplications include a simple explanation for some resorbedfeldspars, the separation, correlation, and comparison of porphyriticrock units, interpretation of compositional changes of successivezones of zoned feldspars, and mantling of one feldspar by another.With all types of termination, extensive fractionation yieldsa liquid rich in Or and Ab that may crystallize to alkali feldsparor feldspars.  相似文献