During the emplacement and cooling of igneous bodies, fluidization of wet sediment by water vapour may occur by heating, or by pressure relief during the opening of fractures. Momentary fluidization causes sediment reconstitution and local transport. Continuous fluidization can result in substantial sediment displacement. Theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars. Studies of the emplacement of sills of andesite (Ayrshire), fluid rhyolite (Ramsey Island) and viscous rhyolite (Moelwyn Hills) recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations. The transgressive bases of the subaqueously welded ash-flow tuffs of the Capel Curig Volcanic Formation are consistent with emplacement attended by fluidization of the subjacent sediment. Evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host. Also, fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks, and can cause fractures formed during cooling to be thoroughly pervaded by sediment. Fluidization is commonly associated with the formation of peperites.
Abstract The Lorne Plateau lava succession of the north-west Grampian Highlands of Scotland is an early component of post-collisional Late Silurian to Early Devonian magmatism in the Orthotectonic Caledonides emplaced in two phases between the Late Silurian (425.0±0.7 Ma U-Pb zircon) and the Siluro-Devonian boundary at ∼419 Ma. Palaeomagnetic study comprising thermal demagnetization and component analysis embracing the time frame of the preserved succession yields a coherent mean direction of magnetization from 58 sites (D/I = 43.7/−47.4°, α 95 = 4.0°). A palaeomagnetic fold test is significantly positive with sills intruding unlithified sediment on the island of Kerrera confirming primary remanence. The ∼600 m thick succession has uniform normal polarity throughout permitting correlation with the beginning of a normal polarity chron extending from ∼426 to 403 Ma. The pole position at 2.7°N, 317.3°E ( dp / dm = 3.8/5.8°) predicts a palaeolatitude of 26°S and corresponds precisely with remanence in contemporaneous rocks from the Midland Valley of Scotland. Regional palaeofield directions are evaluated in the context of transpressional moulding of the Acadian Orogeny on the Great Glen Fault system (~416−380 Ma).
Abstract Seven lunar crater sites of granular avalanches are studied utilizing high‐resolution images (0.42–1.3 m/pixel) from the Lunar Reconnaissance Orbiter Camera; one, in Kepler crater, is examined in detail. All the sites are slopes of debris extensively aggraded by frictional freezing at their dynamic angle of repose, four in craters formed in basaltic mare and three in the anorthositic highlands. Diverse styles of mass wasting occur, and three types of dry‐debris flow deposit are recognized: (1) multiple channel‐and‐lobe type, with coarse‐grained levees and lobate terminations that impound finer debris, (2) single‐surge polylobate type, with subparallel arrays of lobes and fingers with segregated coarse‐grained margins, and (3) multiple‐ribbon type, with tracks reflecting reworked substrate, minor levees, and no coarse terminations. The latter type results from propagation of granular erosion‐deposition waves down slopes dominantly of fine regolith, and it is the first recognized natural example. Dimensions, architectures, and granular segregation styles of the two coarse‐grained deposit types are like those formed in natural and experimental avalanches on Earth, although the timescale of motion differs due to the reduced gravity. Influences of reduced gravity and fine‐grained regolith on dynamics of granular flow and deposition appear slight, but we distinguish, for the first time, extensive remobilization of coarse talus by inundation with finer debris. The (few) sites show no clear difference attributable to the contrasting mare basalt and highland megaregolith host rocks and their fragmentation. This lunar study offers a benchmarking of deposit types that can be attributed to formation without influence of liquid or gas.
Abstract The eruption on Montserrat during 1995-1999 was the most destructive in the Caribbean volcanic arc since that of Mont Pelee (Martinique) in 1902. It began on 18 July 1995 at the site of the most recent previous activity, on the flank of a c. 350-year-old lava dome within a sector-collapse scar. Phreatic explosivity occurred for 18 weeks before the onset of extrusion of an andesitic lava dome. Dome collapses produced pyroclastic flows that initially were confined by the sector-collapse scar. After 60 weeks of unsteadily accelerating dome growth and one episode of sub-Plinian explosivity, the dome eventually overtopped the confining scar. During 1997 almost two-thirds of the island was devastated following major dome collapses, two episodes of Vulcanian explosivity with fountain-collapse pyroclastic flows, and a flank failure with associated debris avalanche and explosive disruption of the lava dome. Nineteen people were killed directly by the volcanic activity and several were injured. From March 1998 until November 1999 there was a pause in magma ascent accompanied by reduced seismic activity, substantial degradation of the dome, and considerable degassing with venting of ash. The slow progress and long duration of the volcanic escalation, coupled with the small size of the island and the vulnerability of homes, key installations and infrastructure, resulted in a style of emergency management that was dominantly reactive. In order to minimize the disruption to life for those remaining on the island, following large-scale evacuations, scientists at the Montserrat Volcano Observatory had to anticipate hazards and their potential extents of impact with considerable precision. Based on frequent hazards assessments, a series of risk management zone maps was issued by administrative authorities to control access as the eruption escalated. These were used in conjunction with an alert-level system. The unpreparedness of the Montserrat authorities and the responsible UK government departments resulted in hardship, ill feeling and at times acrimony as the situation deteriorated and needs for aid mounted. Losses and stress could have been less if an existing hazards assessment had registered with appropriate authorities before the eruption.
Summary The Lower Palaeozoic Welsh Basin was founded on immature continental crust. During late Precambrian-early Cambrian times, volcanism and sedimentation were influenced by NE-SW-trending faults which defined the NW and SE margins of the basin. During the Cambrian, marine sediments infilled a graben and at the end of the Tremadoc widespread tectonism was associated with an island-arc volcanic episode. In the Ordovician this subduction-related activity was succeeded by mainly tholeiitic volcanism related to back-arc extension, with the locus of arc volcanism sited further N, in the Lake District—Leinster Zone of the Caledonides. In Wales, the Ordovician volcanic activity shifted in time and space. In S Wales volcanism persisted from the middle Arenig through the Llanvirn. In N Wales the volcanism can be broadly divided into dominantly pre-Caradoc activity in southern Snowdonia and an intra-Caradoc episode in central and northern Snowdonia. In eastern Wales, including the Welsh Borderland, and in Llŷn, both episodes are represented. In all areas faults greatly influenced both volcanism and sedimentation. Intrusive activity was dominated by high-level emplacement of sills. Granite ( s.l. ) stocks are restricted to central and northern Snowdonia and Llŷn and many were coeval with extrusive volcanism. Volcanism in the basin was essentially bimodal with voluminous eruptions of tholeiitic basalts with ocean-floor affinities, and of rhyolites. Minor volumes of andesite to rhyodacite resulted from low-pressure fractional crystallization of the tholeiitic basalts. Available evidence suggests that the rhyolites resulted mainly from crustal fusion, although in some instances evolution by crystal fractionation from intermediate magma has been proposed. Calc-alkaline assemblages are petrographically distinct, of minor occurence and, contrary to previous conclusions, are relatively insignificant in the characterization of the tectonic environment of the basin. Throughout the basin, volcanism was generally succeeded by deposition of black muds and then turbidite-dominated sequences.
Over 100 samples from the submarine Black–stones Bank Igneous Centre have been collected by scuba divers. The samples include gabbros, dolerites and metamorphosed sediments but ultramafic rocks, expected in view of the high positive gravity anomaly on the bank, have not so far been sampled. The mode of occurrence, petrography and chemical composition of the samples show many similarities between the Blackstones Bank Centre and the Tertiary igneous centres of NW. Scotland. However, K-Ar age determinations on a basaltic dyke which cuts a gabbro suggest a minimum age for the Blackstones Centre of 70 Ma, while most plutonic rocks in the British Tertiary Province have ages of c . 59 and 53 Ma.
Data from large‐scale debris‐flow experiments are combined with modeling of particle‐size segregation to explain the formation of lateral levees enriched in coarse grains. The experimental flows consisted of 10 m 3 of water‐saturated sand and gravel, which traveled ∼80 m down a steeply inclined flume before forming an elongated leveed deposit 10 m long on a nearly horizontal runout surface. We measured the surface velocity field and observed the sequence of deposition by seeding tracers onto the flow surface and tracking them in video footage. Levees formed by progressive downslope accretion approximately 3.5 m behind the flow front, which advanced steadily at ∼2 m s −1 during most of the runout. Segregation was measured by placing ∼600 coarse tracer pebbles on the bed, which, when entrained into the flow, segregated upwards at ∼6–7.5 cm s −1 . When excavated from the deposit these were distributed in a horseshoe‐shaped pattern that became increasingly elevated closer to the deposit termination. Although there was clear evidence for inverse grading during the flow, transect sampling revealed that the resulting leveed deposit was strongly graded laterally, with only weak vertical grading. We construct an empirical, three‐dimensional velocity field resembling the experimental observations, and use this with a particle‐size segregation model to predict the segregation and transport of material through the flow. We infer that coarse material segregates to the flow surface and is transported to the flow front by shear. Within the flow head, coarse material is overridden, then recirculates in spiral trajectories due to size‐segregation, before being advected to the flow edges and deposited to form coarse‐particle‐enriched levees.
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