The charge distribution on interstellar grains—The effect of induced charge

A calculation of the equilibrium charge acquired by interstellar grains is given, which takes account of polarization charges that are induced in a grain by incident ions and electrons.Both metal and dielectric grains are considered and photoionization of the latter grains by UV radiation is taken into account where necessary. It is found that the inclusion of the polarization charges in the calculation is only important in gas clouds where the mean charge on a grain is low (<1e); that is, for HI regions and dense molecular clouds. In such clouds, the effect of the polarization charges is to increase the amount of negative charge acquired by a grain. A discussion is given concerning the validity of the classical electrostatic theory employed in the paper for small grains of radius 10^–6 cm, and some astrophysical consequences of the modification of the grain charge by polarization effects are considered.     

Interdisciplinary survey in Messenia,Southwest Peloponnese,Greece

The survey of Messenia by the University of Minnesota Messenia Expedition (UMME) was recently followed by the Pylos Regional Archaeological Project (PRAP). Despite improvements in survey methodology, problems remain, particularly with the interpretation of field data and with extrapolation, especially when this is made based on survey results alone, without excavation. In addition, the work of the archaeologists has not yet been fully integrated with that of the earth scientists. Some examples are given of the difficulties encountered in this survey, especially concerning retrodictions (predictions about the past) of ancient settlement patterns and numbers. Suggestions are made here for further survey and selective excavation, especially in eastern Messenia, where fieldwork has been far less extensive than in western Messenia. More fieldwork is needed to determine the pattern of geomorphic changes in the region, especially in relation to the varying degrees of visibility and/or preservation of the ancient sites. Meanwhile, increased recent human activity is rapidly destroying or endangering those remains that have hitherto survived, so that survey is often de facto of a rescue nature. © 2007 Wiley Periodicals, Inc.     

Large-scale mass wasting in ancient volcanic materials

Giant landslides are significant hazards associated with many active volcanic edifices. We describe a similar feature on ancient (>4 Ma) volcanic deposits subject to active tectonism. The landslide is approximately 3 km long by 1 km wide, with an estimated depth of 400 m. Side margins are straight and parallel, mimicking regional structure; narrow valleys incised down these margins provide low-strength side-release surfaces. Between these is a giant slump consisting of at least four, largely intact, discrete blocks that have moved down-dip a distance of >500 m. A series of flows with areal extents ranging from 0.01 to 0.5 km² extends from the front of the failure. The materials represent an eroded sequence of andesite flows on the flanks of a stratovolcano. These have undergone two phases of hydrothermal alteration, and are deeply weathered to low-density (1040±80 kg m⁻³) silt (59%) and clay (35%) materials with strength properties typical of weathered silts (c=26±3 kN m⁻²; φ=42±8°). The size and location of this landslide preclude detailed geotechnical investigation of the failure. The worth of numerical stability analysis as an alternative technique in assessing the nature of the failure and hence the risk it poses to nearby communities is investigated. Sensitivity analysis identified likely conditions under which initial failure may have occurred: analyses for sensitivity to strength and earthquake acceleration needed conversion to critical combinations (F=1.0) of water table and strength/acceleration to remove the overriding influence of water table fluctuations. Failure was likely initiated either by a high water table level (83-84%), or some combination of intensity VII-IX earthquake waves together with water table heights of 40-80%. A general hazard assessment indicates that the risk associated with creep and catastrophic failure of the main mass is small, whereas the risk from flow failures near the toe of the landslide may be high. Important parameters (hydrological regime, flow failure morphology, age of initiation, and rates of movement) requiring closer investigation are identified. Development of a model is crucial to assessing the hazard associated with a feature such as that described here. With limited resources, a detailed stability analysis is a powerful tool as an initial stage in hazard analysis.     

Energy input and dissipation in a temperate lake during the spring transition

ADCP and temperature chain measurements have been used to estimate the rate of energy input by wind stress to the water surface in the south basin of Windermere. The energy input from the atmosphere was found to increase markedly as the lake stratified in spring. The efficiency of energy transfer (Eff), defined as the ratio of the rate of working in near-surface waters (RW) to that above the lake surface (P ₁₀), increased from ～0.0013 in vertically homogenous conditions to ～0.0064 in the first 40 days of the stratified regime. A maximum value of Eff～0.01 was observed when, with increasing stratification, the first mode internal seiche period decreased to match the diurnal wind period of 24 h. The increase in energy input, following the onset of stratification was reflected in enhancement of the mean depth-varying kinetic energy without a corresponding increase in wind forcing. Parallel estimates of energy dissipation in the bottom boundary layer, based on determination of the structure function show that it accounts for ～15% of RW in stratified conditions. The evolution of stratification in the lake conforms to a heating stirring model which indicates that mixing accounts for ～21% of RW. Taken together, these estimates of key energetic parameters point the way to the development of full energy budgets for lakes and shallow seas.     

Mesozoic magnetic lineations in the Mozambique Basin

East-west-trending Mesozoic magnetic anomalies M2 through M22 have been identified in the northern Mozambique Basin. These anomalies are best matched by sea floor created at 50°S trending N120°E and spreading at a rate of around 1.5 cm/yr. The northward increase in age inferred from the identifications of these anomalies are compatible with observed decrease in the “reliable” heat flow values from 1.4 to 1.1 μcal/cm² s to the north in the basin. The anomalies terminate in the southern part of the Mozambique Channel against a magnetic quiet zone to the north. Both the Mozambique Basin anomalies and those recently observed off Antarctica are strong evidence in favour of a Gondwanaland reconstruction that places Dronning Maud Land against southern Mozambique, and a late Jurassic or older separation between Africa and Antarctica.     

Porphyroclast systems as kinematic indicators

Porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material. The geometry of a porphyroclast and its tails, called a ‘porhyroclast system’, can serve as a valuable indicator of the sense of vorticity. Porphyroclast systems have been divided into σ- and δ-types on the basis of the geometry of the tails. σ-Types have wedge-shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system. σ-Types are further subdivided into a σ_a-types, in which the porphyroclast is isolated in a relatively homogeneous matrix, and σ_b-types, in which the porphyroclast system is associated with a shear band foliation in the matrix. δ-Types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast. Consequently, embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends.A porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non-coaxial flow in the adjacent matrix. This behaviour has been modelled in shear box experiments. Passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear. The experiments were modified to simulate a change in recrystallization rate (R) with respect to rate of deformation (γ) by decreasing the diameter of the rigid cylinder during deformation at variable rates. The ratio R/γ appears to be one of the most important factors in determining which porphyroclast system will develop. At high R/γ values, flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge-shaped σ_a-type tails develop. At low R/γ values, relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast. In addition, most porphyroclast systems at low shear strains are of σ_a-type or lack monoclinic symmetry, whereas δ-types are only developed at high shear strain values. Complex porphyroclast systems, characterized by two generations of tails, are observed in many of the natural and model shear zones studied and may form due to fluctuating R/γ. Conditions that allow isolated σ_a- and δ-type porphyroclast systems to be used as sense of vorticity indicators are: the systems should have a monoclinic shape symmetry; matrix grain size should be small with respect to porphyroclast size; matrix fabric should be homogeneous; deformation history should be simple, and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite.     

Development of large‐scale seismites in Upper Cretaceous fluvial sandstones in a fault‐proximal setting

Large‐scale soft‐sediment deformation structures occur within fluvial sandstone bodies of the Upper Cretaceous Wahweap Formation in the Kaiparowits basin, southern Utah, USA. These structures represent an exceptional example of metre‐scale fault‐proximal, seismogenic load structures in nearly homogenous sandstones. The load structures consist of two types: large‐scale load casts and wedge‐shaped load structures. Large‐scale load casts penetrate up to 4·5 m into the underlying sandstone bed. Wedge‐shaped load structures include metre‐scale, parallel, sub‐vertical features and decimetre‐scale features along the periphery of the large‐scale load casts or other wedge‐shaped load structures. Wedge‐shaped load structures contain well‐developed, medial cataclastic shear deformation bands. All load structures contain pervasive well‐defined millimetre‐thick to centimetre‐thick internal laminae, oriented parallel to the outside form of the load structures and asymptotic to deformation bands. Both types of load structures formed because of an inverted density profile, earthquake‐triggered liquefaction and growth of irregularities (a Rayleigh–Taylor instability) on the sandstone–sandstone erosional contact. The internal laminae and deformation bands formed during deformation and clearly demonstrate polyphase deformation, recording a transition from liquefied to hydroplastic to brittle modes of deformation. Decimetre‐scale wedge‐shaped load structures on the edge of the large‐scale load casts probably formed towards the end of a seismic event after the sediment dewatered and increased the frictional contact of grains enough to impart strength to the sands. Metre‐scale wedge‐shaped load structures were created as the tips of downward foundering sediments were driven into fractures, which widened incrementally with seismic pulsation. With each widening of the fracture, gravity and a suction effect would draw additional sediment into the fracture. Superimposed laminae indicate a secondary syndeformational origin for internal laminae, probably by flow‐generated shearing and vibrofluidization mechanisms. Large‐scale and wedge‐shaped load structures, polyphase deformation and secondary laminae may characterize soft‐sediment deformation in certain fault‐proximal settings.     

Urban effects of Chennai on sea breeze induced convection and precipitation

Doppler radar derived wind speed and direction profiles showed a well developed sea breeze circulation over the Chennai, India region on 28 June, 2003. Rainfall totals in excess of 100 mm resulted from convection along the sea breeze front. Inland propagation of the sea breeze front was observed in radar reflectivity imagery. High-resolution MM5 simulations were used to investigate the influence of Chennai urban land use on sea breeze initiated convection and precipitation. A comparison of observed and simulated 10m wind speed and direction over Chennai showed that the model was able to simulate the timing and strength of the sea breeze. Urban effects are shown to increase the near surface air temperature over Chennai by 3.0K during the early morning hours. The larger surface temperature gradient along the coast due to urban effects increased onshore flow by 4.0m s⁻¹. Model sensitivity study revealed that precipitation totals were enhanced by 25mm over a large region 150 km west of Chennai due to urban effects. Deficiency in model physics related to night-time forecasts are addressed.