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Aggregation of airborne particles is an important way in which the atmosphere is cleansed of fine dust particles, such as following explosive eruptions and meteorite impacts. We identify successive stages in the growth history of particle aggregates based upon well-preserved ash aggregate--bearing pyroclastic layers on Tenerife. The layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper, widespread coignimbrite ash-fall layer. The upper part of each ignimbrite contains whole and fragmented concentric-laminated accretionary lapilli, whereas the overlying coigulmbrite ash.fall layer lacks accretionary lapilli and is composed of framework-supported smaller and nonlaminated ash pellets, sometimes slightly deformed or partly disaggregated. The pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer. These field relations are repeated numerous times in several different successions, and they indicate that ash pellets, not accretionary lapilli, form within the coignimbrite ash plumes. Some pellets fell directly to the ground, producing coignimbrite ash-fall layers, but others settled into pyroclastic density currents, where they accreted successive concentric laminations of fine ash as they circulated through the variously turbulent levels of the stratified current, and heat of the lower part of the current dried and partly lithified them into brittle accretionary lapilli. The fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli. Numerous ignimbrite veneers on Tenerife have the form of ash layers, a few centimeters thick, that drape topography and locally contain matrix-supported accretionary lapilli. Most volcanoes lack laterally continuous field exposure, and such accretionary lapilli-bearing layers might be mistaken for ash-fall deposits. We highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits, for example, during hazard assessments. doi: 10.1130/B26449.1

Deformation and alteration zones along kimberlite dikes hold clues as to how point-source volcanic vents can localize along sheet-like intrusions. Brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the Swartruggens Kimberlite Dike Swarm, South Africa. Deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments. Breccia zones indicate dilation and hydraulic fracturing, and some zones along the dikes were affected by chemical corrosion, forming fresh cores surrounded by onion-skin concentric shells of altered rock. The alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture, or by hydrothermal fluids. Consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels. Highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface. A kimberlite pipe forms after magma breaks through to the surface: the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country-rock material, as observed in layered volcanic breccias in kimberlite pipes at the Venetia Mine, South Africa. This model may have wider implications for the localization of conduits along dikes in other volcanic systems. Dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled. Chemical corrosion can further weaken these zones, which may then become the locus for initial phreatic and phreatomagmatic explosions, creating shallow vents that can then channel magma to the surface during eruption. Keywords: dike, kimberlite pipe. deformation, volcanic vent, spheroidal clasts.

On the large scale of deep-water construction, marine pipelines are extremely flexible. Construction procedures can exploit that flexibility to connect pipelines and risers to floaters, manifolds, wellheads, buoys, and platforms. The paper describes a three-dimensional physical model technique. It helps the engineer to think imaginatively and rapidly to explore different options, with the objective of minimizing construction risk and creating procedures that can be accomplished by the equipment available. The relevant governing equations are derived, and from them come the conditions required for the model to obey the correct mechanical similarity conditions. The model is exact, and can be used to derive forces and stresses; it is much more than just a picture. The paper describes a series of applications to two- and three-dimensional pipeline construction problems, most recently an application to the current Thunder Horse project.[DOI: 10.1115/1.2426982]

Exceptional longitudinal exposure through an ignimbrite sheet on the southern flanks of Las Canadas volcano, Tenerife, Canary Islands, reveals a giant regressive bed form, >520 m long. It comprises sourceward-dipping diffuse lenses, ~50 m long, of massive and diffuse-bedded lapilli tuff. Its architectural relationships indicate that at any one moment in time, deposition was restricted to an ~50-m-long longitudinal depositional zone, bounded both upslope and downslope by extensive bypassing zones. The regressive architecture shows that the zone of deposition gradually migrated sourceward as the current passed. Nonpersistent scours indicate that the current was locally and momentarily erosive, although at many locations there is little evidence for substantial erosion or deposition, and most of the pyroclastic load seems to have entirely bypassed subaerial slopes to enter the Atlantic Ocean, ~20 km from the source. The ignimbrite is diachronous, and any single vertical section through it represents only a short phase of the current's protracted depositional history. Similarly protracted and complex flow histories elsewhere may be easily overlooked at typical field exposures of ignimbrites, turbidites, and lahar deposits, leading to significantly underestimated flow durations and volumes. Keywords: pyroclastic density current, ignimbrite, bed form, bypassing, hazard.

J Neurol Neurosurg Psychiatry 2004;75:608-611. doi: 10.1136/jnnp.2003.020552 Background: Patients with neurologically unexplained symptoms (NUS) often have a previous history of other medically unexplained symptoms. A past history of such symptoms [...]

A much-revised Quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the Bandas del Sur, in southern Tenerife. New [40.sup]Ar/[39.sup]Ar data obtained for the Arico, Granadilla, Fasnia, Poris, La Caleta and Abrigo formations are presented, allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments. We also provide a minimum age of 287 [+ or -] 7 ka for a major sector collapse event at the Guimar valley. The Bandas del Sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics, including widespread basal Plinian layers, predominantly phonolite composition, ignimbrites with similar extensive geographic distributions, thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns, lateral gradations into localized massive facies within palaeo-wadis, and widespread lithic breccia layers that probably record caldera-forming eruptions. Each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean. The succession indicates that Las Canadas volcano underwent a series of major explosive eruptions, each starting with a Plinian phase followed by emplacement of ignimbrites and thin ash layers, some of co-ignimbrite origin. Several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption. Because passage of each pyroclastic density current was characterized by phases of non-deposition and erosion, the entire course of each eruption is incompletely recorded at any one location, accounting for some previously perceived differences between the units. Because each current passed into the ocean, estimating eruption volumes is virtually impossible. Nevertheless, the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the Bandas del Sur succession and probably formed a complex, nested collapse structure. Detailed field relationships show that extensive ignimbrite sheets (e.g. the Arico, Poris and La Caleta formations) relate to previously unrecognized caldera collapse events. We envisage that the evolution of the nested Las Canadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite-related caldera collapse events, and large sector collapse events, interspersed with edifice-building phases. Keywords: ignimbrite, Tenerife, explosive eruptions, phreatomagmatism, Quaternary, volcanism.