Evidence for cospatial optical and radio polarized emission in active galactic nuclei

We investigate the relationship between the optical and radio emission of active galactic nuclei (AGN) by analysing optical and 15+22+43 GHz Very Long Baseline Array (VLBA) polarization observations simultaneous to within a day for 11 BL Lacertae (BL Lac) objects and the blazar 3C279. We have determined and corrected for the Faraday rotation measures in the very long baseline interferometry (VLBI) cores, enabling us to compare the intrinsic (zero-wavelength) VLBI-core polarization angles and the optical polarization angles χ_opt. A clear alignment between these two angles emerges in the transition toward higher radio frequencies, and a prominent peak at 0° is visible in the distribution of |χ_opt−χ_43 GHz|. This correlation implies that the magnetic-field orientations in the regions giving rise to the optical and radio polarization are the same, and can be easily understood if the radio and optical polarization are roughly cospatial. It is difficult to rule out the possibility that they arise in different regions in a straight jet with a uniform magnetic-field structure, but this seems less likely, since the VLBI jets of AGN are often bent on parsec-scales. This may suggest that much or all of the strong optical polarization in these sources arises in the inner radio jets, possibly associated with the formation and emergence of compact new VLBI components.     

The origin of plagioclase phenocrysts in basalts from continental monogenetic volcanoes of the Kaikohe-Bay of Islands field,New Zealand: implications for magmatic assembly and ascent

Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands area, New Zealand, provide an opportunity to explore the crystallization and ascent history of small volume magmas in an intra-continental monogenetic volcano field. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is compositionally similar to groundmass plagioclase (~?An₆₅) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~?An_20–45) or more calcic (>?An₇₀), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (<?~?An₄₅), have ⁸⁷Sr/⁸⁶Sr ratios that are either mantle-like (~?0.7030) or crustal-like (~?0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics. It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent mafic recharge of this system, as indicated by the final crystal rim growths around the entrained antecrystic and xenocrystic cargo. The recharge also entrained cognate gabbros that occur as inclusions, and produced mingled groundmasses. Multi-stage magmatic ascent and interaction is indicated, and is consistent with the presence of a partial melt body in the lower crust detected by geophysical methods. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a hazards perspective, the magmatic system inferred here increases the likelihood of detecting eruption precursor phenomena such as seismicity, degassing and surface deformation.     

Investigation of failures in Irish raised bogs

This paper presents the results of field geophysical testing and laboratory testing of peat from Carn Park and Roosky raised bogs in the Irish Midlands. The motivation for the work was highlight the importance of these areas and to begin to attempt to understand the reasons for the failure of the bogs despite them having surface slopes of some 1°. It was found that the peat is typical of that of Irish raised bogs being up to 8 m thick towards the “high” dome of the bogs. The peat is characterised by low density, high water content, high organic content, low undrained shear strength and high compressibility. The peat is also relatively permeable at in situ stress. Geophysical electrical resistivity tomography and ground penetrating radar data shows a clear thinning of the peat in the area of the failures corresponding to a reduction in volume from dewatering by edge drains/peat harvesting. This finding is supported by detailed water content measurements. It was also shown that the peat base topography is relatively flat and indicates that the observed surface movement has come from within the peat rather than from the material below the peat. Potential causes of the failures include conventional slope instability, the effect of seepage forces or the release of built-up gas in the peat mass. Further measurements are required in order to study these in more detail.     

Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates

We obtained U–Th disequilibrium age data on zircons from each of the four rhyolite eruptions that built Tarawera volcano in the last 22 ka within the Okataina Volcanic Center (OVC), caldera, New Zealand. Secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near-eruption age to ~100 ka prior to eruption. Age-depth profiling of crystals reveals long periods of continuous (~34 ka) and discontinuous growth (~90 ka). Growth hiatuses of up to ~40 ka duration occur, but do not all relate to obvious resorption surfaces. Age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts. These features are best explained by periodic, complete, or partial, sub-solidus storage and/or inclusion in larger crystal phases, followed by rapid liberation prior to eruption. This is supported by high abundances of U and Th (~500 − >2,000 ppm) in some zircons consistent with periods of high crystallinity (>70%) in the magmatic system, based on crystal/melt partitioning. Contemporaneous but contrasting rim-ward trends of these elements within crystals, even in the same lava hand sample, require synchronous growth in separate melt bodies and little connectivity within the system, but also significant crystal transport and mixing prior to eruption. Many crystals record continuity of growth through the preceding ~60 ka OVC caldera-collapse and subsequent eruptions from Tarawera. This demonstrates a decoupling between eruption triggers, such as shallow crustal extension and mafic intrusion, and the crystallization state of the OVC silicic magmatic system. The data highlights the need to distinguish between the time for accumulation of eruptible magma and the long-term magma residence time based on the age of crystals with high closure temperatures, when assessing the potential for catastrophic eruptions.     

The eruptive history and chemical stratigraphy of a post-caldera,steady-state volcano: Yasur,Vanuatu

The persistent activity of Yasur volcano, a post-caldera scoria cone in the southern Vanuatu Arc, along with the uniformity exhibited by its eruptive products, indicates that it is a “steady-state” volcano. This implies that rates of magma replenishment and tapping are in equilibrium. Examination of recently exposed tephra sequences suggests that Strombolian-style activity at Yasur has persisted in its current form for the last 630–850 years. Eruption of tephra with uniform grain size and texture throughout this period indicates invariant eruption magnitude and style. Based on tephra accumulation rates, a uniform, time-averaged eruption flux of ～410–480 m³ days^?1 is estimated. Major and trace element analyses of glass shards and mineral grains from these tephra deposits show limited variation in magma composition throughout that time, consistent with a chemically buffered magma reservoir and models for steady-state volcanism. Similarly, mineral crystallisation temperature estimates are within error, suggesting the magma reservoir has retained a constant temperature through this time, while pressure estimates suggest shallow crystallisation. Eruptions appear to be driven by gas release, with small fluctuations in magma chemistry and eruptive behaviour governed by perturbations in volatile flux. This period of steady-state activity was preceded by ～600 years of higher-magnitude, lower-frequency eruptions during which less evolved compositions were erupted. Variation between these two styles of eruptive behaviour may be explained by a shift from a periodically closed to fully opened conduit, allowing more regular magma release and changes to degassing regimes. New radiocarbon ages suggest a period of irregular eruptive behaviour extending >1,400 year B.P. Overall, a transition from an irregular to a very steady magmatic system has occurred over the past ～2 kyr. Previously determined tectonic indicators for caldera resurgence in the area suggest revived magma replenishment after a hiatus following the caldera-forming Siwi eruption. This replenishment, while now supplying today’s constant activity, has not yet manifested itself in variations in composition or style/magnitude of eruptions.     

Hydraulic,physical and rheological characteristics of rain‐triggered lahars at Semeru volcano,Indonesia

Lahars (volcanic debris flows) have been responsible for 40% of all volcanic fatalities over the past century. Mount Semeru (East Java, Indonesia) is a persistently active composite volcano that threatens approximately one million people with its lahars and pyroclastic flows. Despite their regularity, the behaviour and the propagation of these rain‐triggered lahars are poorly understood. In situ samples were taken from lahars in motion at two sites in the Curah Lengkong River, on the southeast flank of Semeru, providing estimates of the particle concentration, grain size spectrum, grain density and composition. This enables us to identify flow sediment from three categories of lahars: (a) hyperconcentrated flow, (b) non‐cohesive, clast‐ and matrix‐supported debris flow, and (c) muddy flood. To understand hyperconcentrated flow sediment transport processes, it is more appropriate to sample the active flows than the post‐event lahar deposits because in situ sampling retains the full spectrum of the grain‐size distribution. Rheometrical tests on materials sampled from moving hyperconcentrated flows were carried out using a laboratory vane rheometer. Despite technical difficulties, results obtained on the <63, <180, and <400 µm fractions of the sampled sediment, suggest a purely frictional behaviour. Importantly, and contrary to previous experiments conducted with monodisperse suspensions, our results do not show any transition towards a viscous behaviour for high shear rates. These data provide important constraints for future physical and numerical modelling of lahar flows. Copyright © 2010 John Wiley & Sons, Ltd.     

Welcoming a monster to the world: Myths,oral tradition,and modern societal response to volcanic disasters

Volcanic eruptions can overwhelm all senses of observers in their violence, spectacle and sheer incredibility. When an eruption is catastrophic or unexpected, neither individuals nor communities can easily assimilate the event into their world view. Psychological studies of disaster aftermaths have shown that trauma can shake the very foundations of a person's faith and trigger a search – supernatural, religious, or scientific – for answers. For this reason, the ability to rapidly comprehend a traumatic event by “accepting” the catastrophe as part the observer's world represents an important component of community resilience to natural hazards. A relationship with the event may be constructed by adapting existing cosmological, ancestral, or scientific frameworks, as well as through creative and artistic expression. In non-literate societies, communal perceptions of an event may be transformed into stories that offer myth-like explanations. As these stories make their way into oral traditions, they often undergo major changes to allow transmission through generations and, in some cases, to serve political or religious purposes. Disaster responses in literate societies are no different, except that they are more easily recorded and therefore are less prone to change over time.     

Developing probabilistic eruption forecasts for dormant volcanoes: a case study from Mt Taranaki,New Zealand

The majority of continental arc volcanoes go through decades or centuries of inactivity, thus, communities become inured to their threat. Here we demonstrate a method to quantify hazard from sporadically active volcanoes and to develop probabilistic eruption forecasts. We compiled an eruption-event record for the last c. 9,500 years at Mt Taranaki, New Zealand through detailed radiocarbon dating of recent deposits and a sediment core from a nearby lake. This is the highest-precision record ever collected from the volcano, but it still probably underestimates the frequency of eruptions, which will only be better approximated by adding data from more sediment core sites in different tephra-dispersal directions. A mixture of Weibull distributions provided the best fit to the inter-event period data for the 123 events. Depending on which date is accepted for the last event, the mixture-of-Weibulls model probability is at least 0.37–0.48 for a new eruption from Mt Taranaki in the next 50 years. A polymodal distribution of inter-event periods indicates that a range of nested processes control eruption recurrence at this type of arc volcano. These could possibly be related by further statistical analysis to intrinsic factors such as step-wise processes of magma rise, assembly and storage.     

Consequences of artificially opening coastal lagoons on their fish assemblages

The consequences of artificially opening lagoon entrances on the fish assemblages of coastal lagoons in south-eastern Australia were investigated. Impacts of lagoon opening were assessed by comparing changes in the fish assemblages of Werri and Shellharbour Lagoons, both of which intermittently opened to the sea with that of Lake Illawarra, a third coastal lagoon which remained closed during the study period. Numbers of fish species and individuals did not differ significantly after lagoon openings at Werri Lagoon and Shellharbour Lagoon. However, higher numbers of species and individuals were found in Lake Illawarra before the other lagoons had opened. Abundances of some economically significant species (namelyMugil cephalus, Myxus elongatus andLiza argentea) significantly increased after openings, mainly due to small juveniles (<40 mm FL) recruiting to the seagrass beds. In contrast, these species were caught as larger juveniles (>70 mm FL) in Lake Illawarra. Furthermore, their abundance in Lake Illawarra declined during successive months, coinciding with declining salinity. Abundances of resident species (such asPseudomugil olorum andAfurcagobius tamarensis) remained largely unchanged after openings. Although it was clear that marine-spawning species must benefit from lagoon openings, the number and composition of species able to move into lagoons from adjacent waters may be dependent upon the frequency, duration and time of year that the entrance is connected to the sea. The present study thus provides data which may be useful for fisheries management in intermittently open lagoons. However, these results should not be cited as reason to artificially open lagoon entrances as further detailed studies of other aspects of lagoon biota, such as their invertebrate and avian faunas, are also needed to provide sufficient basis for broader management frameworks.