Look out, aurorae about!     

Dr  Jim Wild 《Astronomy& Geophysics》2006,47(1):1.11-1.13

Jim Wild recounts the rationale and success of AuroraWatch, a service that alerts you when you might be able to see aurorae.  相似文献   

Mesoscale analyses of West African summer climate: focus on wave disturbances     

Leonard M. Druyan  Matthew Fulakeza  Patrick Lonergan 《Climate Dynamics》2006,27(5):459-481

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Lu–Hf geochronology and trace element distribution in garnet: Implications for uplift and exhumation of ultra-high pressure granulites in the Sudetes, SW Poland     

Robert Anczkiewicz  Jacek Szczepa&#x;ski  Stanis&#x;aw Mazur  Craig Storey  Quentin Crowley  Igor M. Villa  Matthew F. Thirlwall  Teresa E. Jeffries 《Lithos》2007,95(3-4):363-380

Combining Lu–Hf garnet geochronology with in situ trace element analyses in garnet allowed us to gain new insight into the metamorphic evolution of UHP–UHT rocks in the Stary Gierałtów region, in the Polish Sudetes. Prograde garnet growth recorded by Rayleigh-type heavy REE (HREE) zoning in the felsic granulites indicates that the obtained 386.6 ± 4.9 Ma Lu–Hf age represents the time of garnet crystallization on a prograde UHP metamorphic path. The surrounding rocks were metamorphosed at the same time as indicated by 381.2 ± 6.7 Ma Sm–Nd garnet age obtained for the mid-crustal metapelites. The second metamorphic episode, which affected most of the lower crust in the Orlica–Śnieżnik Massif (OSM) occurred at ca. 340 Ma as determined by U–Pb zircon and Sm–Nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event, which took place on a retrograde path.

Trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and HREE in garnets from mafic and felsic protoliths over the course of the metamorphic evolution. This had strong impact on the isotopic dating results and led to “decoupling” of the Sm–Nd and Lu–Hf clocks, which recorded timing of the two different metamorphic episodes separated by as much as 40 Ma. Moreover, the preservation of the HREE growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 °C implies that the Lu–Hf system under relatively dry conditions does not undergo significant diffusional re-equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth. Under hydrous conditions, at least some resetting will take place, as documented by the partially relaxed HREE zonation profile in the amphibolitised mafic granulite, which yielded a 10 Ma younger age. The HREE distribution study appeared to be a particularly valuable and essential tool, which allowed us to distinguish garnet growth from post-growth complexities and hence, provide improved age interpretation. Medium REE, on the other hand, did not show any obvious correlation with the isotopic signature of garnet.

Two distinct metamorphic episodes recorded in the Stary Gierałtów region show that buoyancy-driven uplift of UHP rocks can be arrested at the base of a continental crust if not supported by any additional force. In our case study, the UHP rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime. The multistage, discontinuous uplift revealed by the UHP rocks of the OSM provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast-track exhumation histories recognized in UHP terranes elsewhere.  相似文献   

The creative reconstruction of the Internet: Google and the privatization of cyberspace and DigiPlace   总被引：2，自引：0，他引：2  

Matthew A. Zook  Mark Graham   《Geoforum》2007,38(6):1322-1343

The Internet has often been portrayed as the ultimate leveler of information where existing hierarchies of power and privilege are undermined by meritocracy. Some websites and functions are, however, more equal than others. In particular, search engines such as Google have been a key means to construct meaning out of disorder. This ordering (or enclosing of the Internet commons), however, comes at a cost as a location within the top 10 Google search results, marks the boundary (albeit a fluid one) between the core and the periphery of the Internet. The recent incorporation of spatial elements into the Google indexing raises fresh and geographically relevant concerns. This article focuses on the construction, access and use of Google derived rankings to deploy geo-referenced information in the physical environment and the way this melding of code and place affects how people interact with place. Using the theoretical concept of DigiPlace this article analyzes how Google Maps and Google Earth are structured and shape what appears (and what does not) in cyberspace and DigiPlace. Of particular concern are the implications of a private corporation controlling this new space.  相似文献   

SSBUV and NOAA-11 SBUV/2 Solar Variability Measurements     

Matthew T. DeLand  Richard P. Cebula  Ernest Hilsenrath 《Studia Geophysica et Geodaetica》1998,42(2):83-91

The Shuttle SBUV (SSBUV) and NOAA-11 SBUV/2 instruments measured solar spectral UV irradiance during the maximum and declining phase of solar cycle 22. The SSBUV data accurately represent the absolute solar UV irradiance between 200–405 nm, and also show the long-term variations during eight flights between October 1989 and January 1996. These data have been used to correct long-term sensitivity changes in the NOAA-11 SBUV/2 data, which provide a near-daily record of solar UV variations over the 170–400 nm region between December 1988 and October 1994. The NOAA-11 data demonstrate the evolution of short-term solar UV activity during solar cycle 22.  相似文献   

Petrology of the 1979 eruption of Soufriere volcano,St. Vincent,Lesser Antilles     

Alastair M. Graham  Matthew F. Thirlwall 《Contributions to Mineralogy and Petrology》1981,76(3):336-342

Major element, trace element, and Sr isotope data are used to study the temporal variation in the chemistry of the ejecta from the 1979 eruption of Soufriere volcano, St. Vincent, and to compare the compositions of the 1979 and 1971/2 magmas. Both the 1971/2 and 1979 products were basaltic andesites almost identical in petrography. A small temporal variation in chemistry is apparent in the 1979 samples but these cannot be related to the 1971/2 lava by fractional crystallisation of phenocryst phases, and the two eruptions may therefore have sampled different batches of magma. ⁸⁷Sr/⁸⁶Sr ratios of the two magmas were identical within analytical error.Microprobe analyses of phenocryst phases and glasses from the 1979 ejecta are presented. Clinopyroxene phenocryst cores with very high Mg/Fe ratios indicate that the basaltic andesites are products of fractionation of magnesian parents. Such magmas are represented by lavas on St. Vincent similar to the microphyric alkali picrites found to the south in Grenada. A common origin for the basaltic andesites of both islands by fractional crystallisation of picritic magmas is suggested. Dacitic glass is abundant in the groundmass of scoria blocks from the eruption. It does not represent the liquid originally in equilibrium with the phenocryst phases, but rather this liquid modified by quench crystallisation. Published interpretations suggesting that dacitic glass compositions in tephra from eruptions of the Soufriere are evidence of mixed-magma eruptions are therefore rejected.  相似文献   

Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced Be in river-borne quartz     

Erik Thorson Brown  Robert F. Stallard  Matthew C. Larsen  Didier L. Bourls  Grant M. Raisbeck  Franoise Yiou 《Earth and Planetary Science Letters》1998,160(3-4):723-728

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic ¹⁰Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using ¹⁰Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.  相似文献   

Some unintended features of elastic finite-difference models     

A.J. Wild  S.C. Singh 《Geophysical Prospecting》1998,46(1):79-101

While finite-difference methods have been used extensively for many years to model wave propagation in elastic media, some of the more subtle effects observable in such models are very inadequately documented in the geophysical literature, especially in regard to their practical numerical consequences. In addition to the intended travelling waves, and the undesirable exponential instability revealed by the von Neumann test, typical second-order-time finite-difference equations also support drifting linear solutions, as can be verified, both theoretically and by numerical experiment. The necessity of these solutions, and their relationship to the incompleteness of the set of travelling-wave eigenfunctions of the finite-difference operator, can be exposed by a matrix-based analysis, and exact expressions for them can be obtained by using standard algebraic techniques. A further peculiarity of the finite-difference formulation is numerical anisotropy, which emerges in a grid of more than one spatial dimension, even when the modelled medium is intended to be isotropic. This anisotropy can be explained and quantified in terms of the exact eigenfunction solutions to the finite-difference equation, which, it is found, can be obtained in a simple, closed form, for a typical modern 3D staggered scheme.  相似文献   

Solute Changes During Aquifer Storage Recovery Testing in a Limestone/Clastic Aquifer   总被引：1，自引：0，他引：1  

June E. Mirecki  Bruce G. Campbell  Kevin J. Conlon  Matthew D. Petkewich 《Ground water》1998,36(3):394-403

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Exposed cross-sections through the continental crust: implications for crustal structure,petrology, and evolution     

David M. Fountain  Matthew H. Salisbury 《Earth and Planetary Science Letters》1981

The continental crust is exposed in cross-section at numerous sites on the earth's surface. These exposures, which appear to have formed by obduction along great faults during continental collision, may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive Bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle. From an examination of five complexes which meet these criteria, it is concluded that the most prominent layering in the crust is not compositional but metamorphic. The lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks, respectively. Post-metamorphic granitic intrusions are common at intermediate to shallow levels. Position of discontinuities in refraction velocity, where present, commonly correspond to changes in composition or metamorphic grade with depth. The continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by COCORP. Field observations, coupled with geochemical data, indicate a complex evolution of the lower crust which can include anatexis, multiple deformation, polymetamorphism and reworking of older crustal material. The complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments.  相似文献