Consequences of explosive eruptions on small Solar System bodies: the case of the missing basalts on the aubrite parent body     

Lionel Wilson  Klaus Keil 《Earth and Planetary Science Letters》1991,104(2-4)

Aubrite meteorites are composed of constituents which are almost certainly of igneous origin. If they were generated by the melting and fractionation of enstatite chondrite-like parental material, as seems very likely, then plagioclase-rich, basaltic complements to the aubrites should have formed. However, such materials are not known as individual meteorites, and the compositions of two plagioclase-silica clasts and one albite-silica-(diopside-anorthite) clast (probably an impact melt) in the Norton County aubrite breccia suggest that they are not the putative enstatite-plagioclase basalts. We propose a new mechanism that explains the absence of these materials, showing that the expansion of even very small amounts of volatiles present in a melt approaching the surface of a small, low-gravity body will be enough to disrupt the melt into a spray of droplets moving faster than the local escape velocity. This explosive volcanic process of melt removal requires larger melt volatile contents on larger bodies, and data on the solubility of volatiles in basaltic melts suggest that the process was limited to bodies smaller than about 100 km in radius.  相似文献   

Slope-induced changes in channel character along a gravel-bed stream: The Allt Dubhaig,Scotland     

Rob Ferguson  Phil Ashworth 《地球表面变化过程与地形》1991,16(1):65-82

Changes in channel character along a small river in the Scottish Highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0.02 to 0.00015 because of local baselevel control. This decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed. The channel pattern alters progressively rather than abruptly, in the sequence (1) near-braided, (2) meandering with active point-bar chutes, (3) meandering with active outer-bank talweg, (4) stable equiwidth sinuous. The changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical concepts than by purely empirical discriminant or trend equations. Measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload. It is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions, are typical of many streams in mountainous areas and can be understood in terms of slope-induced changes in hydraulic properties.  相似文献   

Use of ritz vectors in wave propagation and foundation response     

E. P. Bayo  E. L. Wilson 《地震工程与结构动力学》1984,12(4):499-505

The accuracy of a numerical method is demonstrated for the dynamic analysis of large complex finite element systems in which the spatial distribution of the loading is constant. The method is based on the use of a special class of Ritz vectors which were previously proposed and can be generated with minimum numerical effort. The purpose of this paper is to extend the use of these vectors to the solution of wave propagation and foundation response problems. The method is applied to one-, twoand three-dimensional problems in order to illustrate the efficiency and accuracy of the technique. Unless it is necessary to evaluate the very high-frequency behaviour of a structural system, it is shown that a small number of Ritz vectors will produce excellent results. Therefore, they can be very effective in the solution of three-dimensional soil-structure systems subjected to earthquake loading.  相似文献   

Tarawera 1886, New Zealand — A basaltic plinian fissure eruption     

George P.L. Walker  Stephen Self  Lionel Wilson 《Journal of Volcanology and Geothermal Research》1984,21(1-2)

New Zealand's biggest and most destructive volcanic eruption of historical times was that of Tarawera in 1886. The resulting scoria fall has a dispersal very similar in extent to that of the Vesuvius A.D. 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type. A new estimate of the volume (2 km³) is significantly greater than previous estimates. The basalt came mainly from a 7-km length of fissure, and emission and exit velocity were fairly uniform along at least 4 km of it, this is one of the few documented examples of a plinian eruption from a fissure vent. Primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min⁻¹. A model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1.5–3%. Violent phreatic explosions occurred in the southwestern extension of the fissure across the Rotomahana geothermal field, and it is thought that some of the water responsible for the power of the plinian eruption came from this source, though its amount was not sufficient to turn the eruption into a phreatoplinian one.  相似文献   

Oxygen isotope fractionation and disequilibrium displayed by some granulite facies rocks from the Fraser Range,Western Australia     

Allan F. Wilson  Ajoy K. Baksi 《Geochimica et cosmochimica acta》1984,48(3):423-432

The granulites of the Fraser Range are assumed to have formed in a carbon-rich fluid, and are generally devoid of hornblende, and lack obvious hydrous retrograde features. In these granulites, pyroxene, garnet, plagioclase and quartz are the minerals most likely to retain the oxygen isotope ratios fixed at an early stage of initial granulite metamorphism. Temperature estimates using these minerals commonly suggest that oxygen isotopic exchange ceased in the range 600 to 680°C. The peak metamorphic temperature was probably ～ 850°C as based on the stability fields of the coexisting minerals and some cation temperatures from coexisting pyroxenes in these rocks. Ilmenite may be slightly out of isotopic equilibrium with the other minerals. Thus, grains of quartz, feldspar, pyroxene and ilmenite have suffered considerable oxygen isotopic exchange during the retrogressive phase of the metamorphism, in spite of the fact that very little water was present in these granulites. The observed deviation from the peak metamorphic temperatures can be explained by essentially closed system solid-state diffusion (on at least a scale of centimetres) during slow cooling of the rocks from ~850 to 650°C, followed by more rapid cooling down to ～ 300°C. Such an explanation is not at variance with the radiometric data available for rocks from the area, which suggest that the latter phase could have involved uplift rates of ?0.5 mm/yr for a period of about 40 Ma. Wholerock δ¹⁸O values on non-quartzose mafic granulites, about 7.2%., fall within the range of basalts affected by seafloor weathering.  相似文献   

Simulated climate and CO2—Induced climate change over Western Europe     

C. A. Wilson  J. F. B. Mitchell 《Climatic change》1987,10(1):11-42

The use of a relatively high resolution general circulation model (the Meteorological Office 5-layer model) to determine climate changes for impact studies is evaluated. The simulation of present day climate over Western Europe is assessed by comparing not only different seasons with climatological data, but also the mean annual cycle and the frequency of extreme events. It is found that while the broad features of the simulation are satisfactory, the model produces too many cold episodes in spring, and an excessive number of wet days over northern Europe. When atmospheric CO₂ concentrations are quadrupled, and sea surface temperatures and sea ice extents changed appropriately, the number of cold episodes is reduced and precipitation is less frequent in summer and autumn over much of Europe, and throughout the year in the south. The relevance of both the model data and the statistical tests to climate impact studies is discussed.  相似文献   

Effects of weathering on site response     

Rob Davis 《地震工程与结构动力学》1995,24(2):301-309

Nearly all studies of site response under seismic excitation employ idealized site models in which uniform layers of homogeneous soils are separated by perfect interfaces. The impedance mismatch at each interface gives rise to wave reflections which strongly affect the calculated free surface response. In reality, these perfect interfaces may often not exist. Instead, a zone in which materials from both above and below are mingled may form a blurred interface between layers. This article examines the effects of such weathered zones. The zone is modelled by a linearly varying shear modulus so that no abrupt impedance change occurs. Significant differences in response of weathered and unweathered profiles are found.  相似文献   

Taupo Volcanic Zone calc-alkaline tephras on the peralkaline Mayor Island volcano, New Zealand: identification and uses as marker horizons     

C. J. N. Wilson  B. F. Houghton  B. J. Pillans  S. D. Weaver 《Journal of Volcanology and Geothermal Research》1995,69(3-4)

Mayor Island is a peralkaline rhyolitic caldera volcano characterised by numerous, sector-confined pyroclastic deposits, together with lavas forming at least five composite shields. Correlation of sequences between sectors is difficult because of the scarcity of island-wide marker beds. However, eight distal calc-alkaline fall tephras (ca. 7.3 ¹⁴C ka to 64 ka) from Okataina and Taupo volcanic centres in the nearby Taupo Volcanic Zone (TVZ) have been identified on the island. These “foreign” TVZ tephras provide marker planes to correlate activity in different sectors of Mayor Island volcano, and refine an eruptive chronology. At least seventeen pyroclastic eruptions and fourteen lava-producing events (including multiple, shield-forming events) have occurred in the past ca. 64 ka. Age controls provided by the calc-alkaline tephras confirm the extremely local dispersal characteristics of many of the Mayor Island eruptives and show that K/Ar ages as young as 25–33 ka on obsidians with 4.2–4.4% K₂O are reliable.  相似文献   

Geochemical studies in the drainage basin of the Rio Vouga,Portugal. IV. Impact of land use on the hydrogeochemistry of natural waters in the Vouzela region   总被引：1，自引：0，他引：1  

Rob N. J. Comans  Cornelis H. Van Der Weijden  Simon P. Vriend 《Environmental Geology》1987,9(2):119-128

A hydrogeochemical study of the impact of land use on the composition of natural waters in the Vouzela region in northern Portugal was carried out during the summer of 1983. Water samples were collected from spring and major streams in the area and analyzed for major constituents and some trace elements. Analysis of variance and subsequent pairwise contrast tests demonstrated that waters from agricultural areas are significantly enriched in constiluents such as Na, K, Ca, Mg, Sr, Cl, SO₄, and NO₃. High concentrations of these ions in agricultural areas are the result of both the application of fertilizers and enhanced evaporation by the intensive irrigation of agricultural lands. Constitutents such as H₄SiO₄, HCO₃, F, and probably Li, which are typically related to mineral weathering, were not affected by land use. In waters from forested areas the concentration of most constituents was about 20% higher than in waters from uncultivated areas with mainly a grass cover. This reflects the difference in the evapotranspiration of these two vegetation types. The chemistry of the Rio Zela clearly reflects differences in land use in the Rio Zela valley.  相似文献   

Vadose Zone Monitoring Concepts for Hazardous Waste Sites     

L. G. Everett  L. G. Wilson  L. G. McMillion 《Ground water》1982,20(3):312-324

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