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561.
562.
The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km2. Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km3 D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism. 相似文献
563.
In the southern part of Rhodes, Greece, rhyolitic subaqueous pyroclastic deposits are interbedded with Tertiary, deep water, marine sediments. The lowermost and best exposed of these deposits — the Dali Ash — is described here. The deposit has been previously described as a deep water welded subaqueous ignimbrite. This paper shows that there is no evidence of welding, and texture previously reported were misidentified. The Dali Ash consists of a lower massive unit (5 m thick), overlain by a sequence of ash-turbidites (2.5 m thick). The lower unit was deposited by a high concentration turbidity current and the ash-turbidites by dilute turbidity currents. Foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water/particulate systems. A palaeomagnetic study also suggests they were deposited cold. The Dali Ash can be interpreted as the lateral equivalent of a subaerial pumiceous pyroclastic flow deposit (ignimbrite). The ash-turbidites then may be redeposited slumps off the submarine slope of the lower massive unit, or, may represent later, smaller pyroclastic flows in the eruption. Other alternatives for the origin of the Dali Ash are fully discussed to show the problems in interpreting submarine volcanigenic sediments. It is possible that the deposits are not even a primary eruptive product and are remobilized pyroclastic debris, slumped, for example, off the sides of a shallow marine rhyolitic tuff ring. 相似文献
564.
Daniel G. Wright 《大气与海洋》2013,51(3):225-241
Abstract In this note some aspects of the dynamics involved in the process of baroclinic instability are discussed using simple physical arguments. In particular, the connection between the requirement for unstable perturbations to release potential energy from the mean state and the conservation of potential vorticity is examined. It is shown how the conservation of potential vorticity results in perturbation phase propagation relative to the mean flow, which is a necessity for potential energy to be released from the mean state. Eady's (1949) problem is discussed as an illustrative example. 相似文献
565.
The DOPE (Doppler Pulsation Experiment) HF Doppler sounder located near Tromsø, Norway (geographic: 69.6°N 19.2°E; L = 6.3) is deployed to observe signatures, in the high-latitude ionosphere, of magnetospheric ULF waves. A type of wave has been identified which exhibits no simultaneous ground magnetic signature. They can be subdivided into two classes which occur in the dawn and dusk local time sectors respectively. They generally have frequencies greater than the resonance fundamentals of local field lines. It is suggested that these may be the signatures of high-m ULF waves where the ground magnetic signature has been strongly attenuated as a result of the scale size of the waves. The dawn population demonstrate similarities to a type of magnetospheric wave known as giant (Pg) pulsations which tend to be resonant at higher harmonics on magnetic field lines. In contrast, the waves occurring in the dusk sector are believed to be related to the storm-time Pc5s previously reported in VHF radar data. Dst measurements support these observations by indicating that the dawn and dusk classes of waves occur respectively during geomagnetically quiet and more active intervals. 相似文献
566.
567.
Petrographic analysis of ooids from the Upper Triassic (Mercia Mudstone Group) of southwest England provides an opportunity to assess in detail the origins, transport pathways and diagenesis of an ancient oolite. The Clevedon Oolite is dolomitized and contains a variety of dissolved ooids (oomoulds) and associated grains. The oomoulds occur in well‐sorted, planar and cross‐stratified grainstones, packstones, sandstones and conglomerates associated with shoreface, intershoal, foreshore, beachrock and littoral strandplain deposits. The ooids grew in suspension in the shoreface zone and developed a radial aragonite microstructure. The ooids grew to 0.80 mm in diameter, after which they fractured or ceased growing. Broken grains deposited on or near mobile shoals were rapidly recoated, while other grains, deposited in less agitated, intershoal and lower foreshore areas, were micritized or microbially bound into grapestone aggregates. Locally peloids, intraclasts, quartz grains and micritized grains from intershoal areas supplied nuclei for ooids on nearby shoals. Grains deposited in foreshore areas were rapidly cemented into beachrock and reworked into conglomerates. Soon after deposition, the ooids were subjected to widespread aragonite dissolution followed by dolomitization. The lack of pre‐dolomitization calcite, together with the abundance of early (pre‐compaction) dolospar cements and fabric‐selective dolomitization of micritic fabrics, suggest aragonite dissolution by dolomitizing fluids. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献