magnetization of 0 26 5 ma seafloor at the ultraslow spreading southwest indian ridge 61 67 e 1 we investigate the magnetic signature of volcanic and nonvolcanic seafloor areas along the southwest indian ridge between 61 e and 67 e and analyze their relationship with crustal thickness variations and past to present ridge segmentation this part of the southwest indian ridge is an end member for the global ridge system in terms of low melt supply thin crust and ultraslow spreading rates it is characterized by large expanses of seafloor that show no evidence for a volcanic upper crustal layer we find that variations of intrinsic magnetization and thickness of the basaltic extrusive layer where it is present dominate the present day along axis crustal magnetization off axis the magnetization contrast is on average higher for volcanic seafloor than for smooth nonvolcanic topography indicating that the contribution of the basaltic upper crustal layer to the production of magnetic anomalies remains important however magnetic anomalies that record past magnetic polarity events are found almost everywhere in the survey area even over domains that lack a volcanic upper crustal layer arguing thus for the contribution of other sources we propose that both gabbros and serpentinized peridotites contribute to these anomalies although not systematic and weak over most parts of the survey area an induced component of magnetization is clearly present in some nonvolcanic seafloor domains serpentinized peridotites are the likely carriers of this induced magnetization component

the bouma division a and the possible duration of turbidity currents abstract experimental data arnott and hand 1989 combined with theoretical arguments suggest that the transition in a turbidite from division a to division b occurs at a deposition rate of unsupported grains of about 6 7 10 4 m s 1 in the case of fine medium quartz sand a second rate may be calculated from properties measured at the transition from division b to division c allen 1971b these rates together permit an estimate to be made of the order of magnitude of the duration of deposition from a turbidity current the calculated order of a few tens of minutes for a mio pliocene turbidite described and interpreted hydraulically by komar 1985 is in broad agreement with his independent estimate based on the sequenc of sedimentary structures

tsunami generation by a rapid entrance of pyroclastic flow into the sea during the 1883 krakatau eruption indonesia 1 the 1883 eruption of krakatau volcano in indonesia was one of the most explosive volcanic events in history it was a marine caldera forming eruption that resulted in voluminous ignimbrite deposits and huge tsunamis we have used numerical simulations to investigate three major mechanisms for tsunami generation caldera collapse phreatomagmatic explosion and pyroclastic flow and have constrained the source parameters computed tsunami characteristics for each hypothesis are compared with observations at locations along the coasts of the sunda strait where tsunami data were obtained immediately after the eruption for the pyroclastic flow hypothesis two types of two layer shallow water models dense and light type models were used under different initial conditions pyroclastic flows are erupted from a circular source following a sine function that assumes waning and waxing phases caldera collapse was performed using a simple piston like plunger model in which collapse duration was assumed to be up to 1 h the phreatomagmatic explosion hypothesis was examined using simple empirical models for underwater explosions in shallow water with explosion energy between 1016 and 1017 j the results show that when a pyroclastic flow with a volume of 5 km3 and an average discharge rate of the order of 107 m3 s enters the sea the computed tsunami heights are broadly consistent with historical records in coastal areas including a tide gauge record at batavia now jakarta we conclude that a pyroclastic flow entering the sea is the most plausible mechanism of the 1883 krakatau tsunami

may 18 1980 mount st helens deposits in south coldwater creek washington beginning at 0832 pacific daylight time pdt mount st helens washington produced a rockslide debris avalanche and a lateral explosion blast that generated a laterally directed pyroclastic surge called here a blast surge the resulting deposits are thickest in the east west trending valley of north fork toutle river immediately north of the volcano but are almost as thick in south coldwater creek a small tributary of the toutle river in the lee of an east west trending ridge johnston ridge just north of the north fork toutle river the avalanche surmounted johnston ridge 300 m relief and flowed down the valley of south coldwater creek closely following the avalanche and in part contemporaneously with it the blast surge left thin up to 0 5 m deposits on johnston ridge and thick up to 15 m deposits in south coldwater creek these deposits have a lower layer layer a1 which is friable massive poorer in fines particles 1 16 mm and more poorly sorted than the upper layer layer a2 layer a2 is less friable because it contains more silt and clay sized material beneath layer a1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface this zone designated as layer a0 is a poorly mixed layer containing schlieren of soil shattered volcanic fragments and considerable quantities of organic material layer a1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge layer a2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head layer a0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge it is concluded that a strongly fines depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high velocity particulate density current

submarine pyroclastic deposits formed at the soufriere hills volcano montserrat 1995 2003 what happens when pyroclastic flows enter the ocean the soufriere hills volcano montserrat west indies has undergone a series of dome growth and collapse events since the eruption began in 1995 over 90 of the pyroclastic material produced has been deposited into the ocean sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea the coarse components pebbles to boulders are deposited proximally from dense basal slurries to form steep sided near linear ridges that intercalate to form a submarine fan the finer ash grade components are mixed into the overlying water column to form turbidity currents that flow over distances 30 km from the source the total volume of pyroclastic material off the east coast of montserrat exceeds 280 106 m3 with 65 deposited in proximal lobes and 35 deposited as distal turbidites

an overview of lava dome evolution dome collapse and cyclicity at soufriere hills volcano montserrat 2005 2007 1 the third episode of lava dome growth at soufriere hills volcano montserrat was characterised by higher average magma discharge rates than either previous dome growth episode at this volcano and yet fewer collapses during sustained dome growth at moderate high average rates 6 m3 s we identified 2 6 week discharge pulses that each supplied c 20 mm3 magma from depth our observations are consistent with some existing models but we explain discrepancies by a combination of higher volatile contents and higher ascent rates cycles of c 11 16 days were evident in rockfall lp rockfall and shallow lp earthquake counts related to dome growth and degassing we speculate that degassing at the conduit margins together with stick slip conduit flow may drive these cycles only one major collapse 10 mm3 occurred during the third episode on may 20 2006 as a new magma pulse entered the dome and coincided with heavy rainfall

syneruptive and intereruptive lithofacies in lacustrine environments the cretaceous beolkeum member wido island korea abstract the cretaceous beolkeum member was deposited in a lacustrine environment affected by explosive volcanism and a number of syneruptive lithofacies are intercalated with lacustrine mudstones the syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features which can be grouped into four syneruptive lithofacies assemblages sla 1 to sla 4 from bottom to top primary syneruptive lithofacies are represented by welded massive lapilli tuff in sla 1 and a normally graded lapilli tuff in sla 2 in sla 1 sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water resulting in the formation of a welded texture and columnar joints in sla 2 unsteady pyroclastic density currents rapidly disintegrated mixed with ambient water and transformed into high density turbidity currents depositing thick normally graded lapilli tuff resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions when volcaniclastic sediments were continuously supplied into the lake by debris flows a volcaniclastic fan can be formed sla 1 resulting in a coarsening upward trend and progradational geometry in case of relatively small amounts of volcaniclastic sediment supply turbidity currents would be a main depositional process depositing a series of normally graded tuff on the primary syneruptive lithofacies sla 2 showing a fining upward trend in sla 3 and sla 4 there are only a few cm thick normally graded tuff reflecting minor volcaniclastic sediment supply however overlying normally graded sandstones in sla 3 showing a coarsening upward trend and a thick normally graded sandstone in sla 4 suggest favorable conditions for the generation of the sediment gravity flows probably due to an increase in volcaniclastic sediment supply after eruptions

spreading rate dependence of melt extraction at mid ocean ridges from mantle seismic refraction data a variety of observations indicate that mid ocean ridges produce less crust at spreading rates below 20 mm yr 1 refs 1 3 reflecting changes in fundamental ridge processes with decreasing spreading rate the nature of these changes however remains uncertain with end member explanations being decreasing shallow melting or incomplete melt extraction each due to the influence of a thicker thermal lid here we present results of a seismic refraction experiment designed to study mid ocean ridge processes by imaging residual mantle structure our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo spreading rate changes in mantle velocity gradient basement topography and crustal thickness all correlate with this spreading rate change these observations can be explained by variations in melt extraction at the ridge with a gabbroic phase preferentially retained in the mantle at slower spreading rates the estimated volume of retained melt balances the approximately 1 5 km difference in crustal thickness suggesting that changes in spreading rate affect melt extraction processes rather than total melting

local recharge processes in glacial and alluvial deposits of a temperate catchment summary this study demonstrates that the composition and structure of quaternary deposits and topography significantly influence rates of recharge and distribution of diffuse agricultural pollution at the hillslope scale analyses were made of vertical profiles of naturally occurring chloride and nitrate and artificially introduced bromide in unsaturated and saturated sections of borehole cores of glacial till and alluvium under different land uses in the upper eden valley uk estimates of local potential recharge were made based on chloride mass balance and nitrate peak methods persistent chloride bulges below the root zone were observed and are interpreted to result from filtration processes at lithological boundaries changes in the shape of chloride profiles downslope corroborated by nitrate profiles indicate the roles of surface or near surface runoff and runon and the existence of lateral subsurface flows at depth these findings have implications for estimation of recharge rates through unsaturated zones in quaternary deposits and the interpretation of potential hot spots of diffuse agrochemicals particularly nitrates moving through quaternary deposits into groundwater

long term trends in paleointensity the contribution of dsdp odp submarine basaltic glass collections abstract the deep sea drilling project and the ocean drilling program have been collecting fresh appearing submarine basaltic glass from the world s oceans for over three decades this glass has proved nearly ideal for estimating paleointensity variations of the earth s magnetic field we compile here data for 726 paleointensity experiments from six publications on paleointensity using dsdp odp glass we also include new data for an additional 225 specimens these were obtained through the so called izzi paleointensity experiment of tauxe l staudigel h 2004 strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite geochem geophys geosyst 5 2 q02h06 doi 10 1029 2003gc000635 whereby infield zerofield steps are alternated with the zerofield infield steps to enhance quality assessment of the resulting data the entire collection of data from 951 experiments was prepared for uploading to the magic data base http earthref org including original measurements interpretations and useful metadata excellent results were obtained throughout the depth 1400 mbsf and age 0 160 ma range sampled dsdp odp glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1 160 ma paleolatitudes were estimated for all cooling units in a self consistent manner for use in calculating virtual axial dipole moments we conclude 1 there is about a 20 difference in mean values between the sbg and the lava flow data 48 36 and 57 29 zam2 respectively the difference is caused by the fact that there are more higher values in the lava flow data than in the sbg data set rather than a difference in the minimum values lava flows cooling over a periods of days to months can account for the discrepancy 2 the positive relationship between polarity interval length and average paleofield intensity first hypothesized by cox a v 1968 lengths of geomagnetic polarity intervals j geophys res 73 3247 3260 is supported by data compiled here the brunhes data for which we have only a minimum estimate for polarity interval length are consistent with a long polarity interval suggesting that instead of racing toward reversal hulot g eymin c langlais b mandea m olsen n 2002 small scale structure of the geodynamo inferred from oersted and magsat satellite data nature 416 620 623 we could instead be in the midst of a long stable polarity interval 3 because the average value appears to be a function of polarity interval length it is probably not useful to calculate a mean value nonetheless it appears that most of the time apart from the brunhes and the longest polarity intervals the average dipole moment is substantially less than the present day value as suggested by juarez t tauxe l gee j s pick t 1998 the intensity of the earth s magnetic field over the past 160 million years nature 394 878 881

lateral moisture flow beneath a sandy hillslope without an apparent impeding layer part of a small drainage basin on the sevilleta national wildlife refuge about 25 km north of socorro nm was intensively instrumented with soil monitoring equipment to estimate natural ground water recharge soil moisture data were analysed with special attention to characterizing the influence of topography on the direction of vadose water flow paths in fine to medium aeolian sand moisture content data were obtained by the neutron scattering technique and hydraulic head data were obtained using tensiometers in addition tracer experiments were conducted on a sandy hillslope to delineate the flow paths of vadose water the results indicate that there is a strong lateral component to unsaturated flow on a hillslope even in the absence of apparent sublayers of much lower permeability darcian calculations estimate the long term steady deep flux beneath a concave location to be about 4 per cent of an assumed mean annual precipitation of 20 cm the deep soil water flux downward varied by several orders of magnitude during the 17 month period of record

hyperfiltration of sodium chloride through kaolinite membranes under relatively low heads implications for groundwater assessment abstract three relatively low head hyperfiltration experiments were conducted to investigate clay membrane behavior at lower hydraulic heads dilute cl solutions 187 and 336 mg l were forced through thin layers of kaolinite 0 67 1 62 mm under heads ranging between 1 45 and 1 89 m at the end of each experiment there was a cl concentration increase 15 29 within the cell the concentration increase is attributable to solute sieving by the kaolinite calculated final concentration increases at the membrane face ranged between 1 51 and 1 65 times the initial concentration and the calculated values of the reflection coefficient ranged between 0 35 and 0 40 these experiments show that clays are capable of significant hyperfiltration effects at lower pressure heads more typical of natural aquifers consequently hyperfiltration effects may need to be considered in a broad spectrum of shallow subsurface processes in which these effects have been regarded as inconsequential particularly in perched aquifers

resonance oscillations of the soufriere hills volcano montserrat w i magmatic system induced by forced magma flow from the reservoir into the upper plumbing dike abstract short period deformation cycles are a common phenomenon at active volcanoes and are often attributed to the instability of magma flow in the upper plumbing system caused by fluctuations in magma viscosity related to cooling degassing and crystallization here we present 20 min periodic oscillations in ground deformation based on high precision continuous borehole strain data that were associated with the 2003 massive dome collapse at the soufriere hills volcano montserrat west indies these high frequency oscillations lasted 80 min and were preceded by a 4 hour episode of rapid expansion of the shallow magma reservoir strain amplitude ratios indicate that the deformational changes were generated by pressure variations in the shallow magma reservoir and with reversed polarity the adjacent plumbing dike the unusually short period of the oscillations cannot be explained with thermally induced variations in magma properties we investigate the underlying mechanism of the oscillations via a numerical model of forced magma flow through a reservoir dike system accounting for time dependent dilation contraction of the dike due to a viscous response in the surrounding host rock our results suggest that the cyclic pressure variations are modulated by the dynamical interplay between rapid expansion of the magma chamber and the incapacity of the narrow dike to take up fast enough the magma volumes supplied by the reservoir our results allow us to place first order constraints on the viscosity of crustal host rocks and consequently its fractional melt content hence we present for the first time crustal scale in situ measurements of rheological properties of mush zones surrounding magmatic systems

geochemical estimates of paleorecharge in the pasco basin evaluation of the chloride mass balance technique the pasco basin in southeastern washington state provides a unique hydrogeologic setting for evaluating the chloride mass balance technique for estimating recharge this basin was affected by late pleistocene catastrophic floods when glacial dams in western montana and northern idaho were breached it is estimated that multiple missoula floods occurred between 13 000 and 15 000 years b p and reached a high water elevation of 350 m these floods removed accumulated chloride from the sediment profile effectively resetting the chloride mass balance clock at the beginning of the holocene the rate of chloride accumulation qcl in the sediments was determined by two methods and compared the first method measured qcl by dividing the calculated natural fallout of 36cl by a measured ratio of 36cl cl in the pore water while the second method used the total mass of chloride in the profile divided by the length of time that atmospheric chloride had accumulated since the last flood although the two methods are based on different approaches they showed close agreement in laboratory studies the sediment to water ratio for chloride extraction was sensitive to the grain size of the sediments low extraction ratios in silt loam sediments led to significant underestimation of pore water chloride concentration br cl ratios were useful for distinguishing nonatmospheric e g rock sources of chloride field studies showed little spatial variability in estimated recharge at a given site within the basin but showed significant topographic control on recharge rates in this semiarid environment an extension of the conventional chloride mass balance model was used to evaluate chloride profiles under transient time varying annual precipitation conditions this model was inverted to determine the paleorecharge history for a given soil chloride profile and the parameters of the root extraction model required to estimate paleoprecipitation

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

detachment faulting at ocean core complexes intracrustal reflections from a cretaceous inside corner are interpreted as a detachment fault that dips gently within the basement and can be followed up the flank of a domed high the fault heave is sufficient for ridge normal extension to have exhumed deep crustal and probably mantle rocks as sampled at some inside corner domed massifs the morphology of these massifs closely resembles that of the cretaceous high implying that the structures are analogous and supporting the notion that modern inside corner massifs are oceanic core complexes formed by unroofing the footwalls of similar detachment faults partially exposed as corrugated slip surfaces this is the first demonstration of the overall geometry of an oceanic core complex detachment fault

aggregation in particle rich environments a textural study of examples from volcanic eruptions meteorite impacts and fluidized bed processing aggregation is a common process occurring in many diverse particulate gas mixtures e g those derived from explosive volcanic eruptions meteorite impact events and fluid bed processing it results from the collision and sticking of particles suspended in turbulent gas air to date there is no generalized model of the underlying physical processes here we investigate aggregates from 18 natural deposits 16 volcanic deposits and two meteorite impact deposits as well as aggregates produced experimentally via fluidized bed techniques all aggregates were analyzed for their size internal structuring and constituent particle size distribution commonalities and differences between the aggregate types are then used to infer salient features of the aggregation process average core to rim ratios of internally structured aggregates accretionary lapilli is found to be similar for artificial and volcanic aggregates but up to an order of magnitude different than impact related aggregates rim structures of artificial and volcanic aggregates appear to be physically similar single sub spherical regularly shaped rims whereas impact related aggregates more often show multiple or irregularly shaped rims the particle size distributions psds of all three aggregate types are similar 200 m this proves that in all three environments aggregation occurs under broadly similar conditions despite the significant differences in source conditions particle volume fraction particle size distribution particle composition temperature residence times plume conditions e g humidity and temperature and dynamics of fallout and deposition impact generated and volcanic aggregates share many similarities and in some cases may be indistinguishable without their stratigraphic context

magnetic polarity structure of the lower oceanic crust a newly developed rock drill was used to take the first fully oriented samples from in situ gabbros exposed on atlantis bank which formed 11 m y ago on the southwest indian ridge swir these paleomagnetic data locate the seabed outcrop of a normal polarity magnetic lineation within an outcrop of lower crust that correlates with overlying deep towed and sea surface magnetic field measurements a systematic lateral offset between the seabed boundary and measured anomalies implies that the reversal boundary dips 25 to the south away from the ridge axis paleomagnetic results suggest that following magnetization acquisition the platform tilted south by 20 range 33 south to 7 north implying an original reversal boundary dip of only 5

the significance of grain size breaks in turbidites and pyroclastic density current deposits turbidite beds and pyroclastic density current deposits commonly show abrupt internal changes in grain size with stepwise upward fining grain size breaks divide a turbidite bed into a lower sandy zone a middle silty zone and an upper muddy zone zone boundaries need not correspond with bouma division boundaries but the sandy zone usually consists of a b and occasionally c divisions the silty zone is composed of one or more of the b c and d divisions and the muddy zone correlates with division e pyroclastic density current deposits likewise display a tripartite zonation with prominent grain size breaks divergence of particle laden gravity currents into a lower dense body and an upper dilute turbulent wake can explain the formation of the lower grain size break fine particles are elutriated into the wake region by ambient entrainment through the current head while coarse grains are transported in the body particle size and concentration stratification is sustained by the turbulence and velocity structure of gravity currents with a velocity maximum and turbulence minimum also dividing currents into two regions two stages of deposition result the upper grain size break may be produced by buoyant lofting with fine particles convected into the water column or atmosphere as the flow wanes

pyroclastic density currents resulting from the interaction of basaltic magma with hydrothermally altered rock an example from the 2006 summit eruptions of mount etna italy after 16 months of quiescence mount etna began to erupt again in mid july 2006 the activity was concentrated at and around the southeast crater sec one of the four craters on the summit of etna and eruptive activity continued intermittently for 5 months during this period numerous vents displayed a wide range of eruptive styles at different times virtually all explosive activities took place at vents at the summit of the sec and on its flanks eruptive episodes which lasted from 1 day to 2 weeks became shorter and more violent with time volcanic activity at these vents was often accompanied by dramatic mass wasting processes such as collapse of parts of the cone highly unusual flowage processes involving both old rocks and fresh magmatic material and magma water interaction the most dramatic events took place on 16 november when numerous rockfalls and pyroclastic density currents pdcs were generated during the opening of a large fracture on the se flank of the sec cone the largest pdcs were clearly triggered explosively and there is evidence that much of the energy was generated during the interaction of intruding magma with wet rocks on the cone s flanks the most mobile pdcs traveled up to 1 km from their source this previously unknown process on etna may not be unique on this volcano and is likely to have taken place on other volcanoes it represents a newly recognized hazard to those who visit and work in the vicinity of the summit of etna

facies architecture of a continental below wave base volcaniclastic basin the ohanapecosh formation ancestral cascades arc washington usa the 800 m thick oligocene ohanapecosh formation records voluminous sedimentation of volcanic clasts in the ancestral cascades arc washington state usa most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition now entirely devitrified and altered all beds are laterally continuous and have uniform thickness fine sandstone and mudstone beds have features typical of deposits from low density turbidity currents and suspension settling erosion surfaces cross beds and evidence of bi directional oscillatory currents i e wave ripples and swaley and hummocky cross stratification are almost entirely absent we infer that the setting was subaqueous and below wave base the abundance of angular pumice clasts crystals and dense volcanic clasts and the extreme thickness of several facies suggest they were derived from magmatic volatile driven explosive eruptions the extremely thick beds are ungraded or weakly graded and lack evidence of hot emplacement suggesting deposition from subaqueous water supported high concentration volcaniclastic density currents some of the thickest beds contain coarse rounded dense clasts at their base and are interbedded with accretionary lapilli bearing mudstone these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra basinal scoria cones that may have been partly subaerial the range in facies in the ohanapecosh formation is typical of below wave base continental lacustrine basins that form in proximity to active volcanic arcs and includes eruption fed and resedimented facies extreme instantaneous aggradation rates are related directly to explosive eruptions and sediment pathways reflect the locations of active volcanoes in contrast to conventional sedimentation processes acting in non volcanic environments

seismic structure of oceanic crust in the western north atlantic we examine the seismic structure of cretaceous aged ocean crust north of the blake spur fracture zone in the western north atlantic by using a combination of multichannel seismic and wide angle reflection refraction data although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure it displays large variations in crustal reflectivity on both ridge normal and ridge parallel profiles the upper crust is highly reflective and contains both subhorizontal or shallow dipping events and more steeply dipping reflectors subhorizontal reflectors are typically present between 1 5 and 2 km depth and are in some cases laterally continuous for distances of 15 to 20 km generally these events do not correlate with the depth of the seismic layer 2 layer 3 boundary determined from refraction data and their origin is still poorly understood the steeply dipping events are generally confined to the upper 2 5 km of the crust we interpret these reflectors as the subsurface expression of ridge parallel extensional faults commonly mapped at mid ocean ridges the lower crust in this area exhibits alternating regions of high and low reflectivity the highly reflective zones are made up of packages of linear or concave upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust some of these dipping reflectors appear to cut through the whole crustal section although most fade out in the acoustically transparent midcrust on ridge normal profiles the majority of these events dip to the east toward the paleo spreading center whereas on ridge parallel profiles the events typically dip south toward the blake spur fracture zone the base of the crust is generally not associated with a strong discrete moho reflection but is a comparatively indistinct boundary usually associated with a 1 to 2 km thick band of diffuse high reflectivity we interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8 10 km with the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge normal and ridge parallel profiles we may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults alternatively these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere

catastrophic lava dome failure at soufriere hills volcano montserrat 12 13 july 2003 the lava dome collapse of 12 13 july 2003 was the largest of the soufriere hills volcano eruption thus far 1995 2005 and the largest recorded in historical times from any volcano 210 million m3 of dome material collapsed over 18 h and formed large pyroclastic flows which reached the sea the evolution of the collapse can be interpreted with reference to the complex structure of the lava dome which comprised discrete spines and shear lobes and an apron of talus progressive slumping of talus for 10 h at the beginning of the collapse generated low volume pyroclastic flows it undermined the massive part of the lava dome and eventually prompted catastrophic failure from 02 00 to 04 40 13 july 2003 large pyroclastic flows were generated these reached their largest magnitude at 03 35 when the volume flux of material lost from the lava dome probably approached 16 million m3 over two minutes the high flux of pyroclastic flows into the sea caused a tsunami and a hydrovolcanic explosion with an associated pyroclastic surge which flowed inland a vulcanian explosion occurred during or immediately after the largest pyroclastic flows at 03 35 13 july and four further explosions occurred at progressively longer intervals during 13 15 july 2003 the dome collapse lasted approximately 18 h but 170 of the total 210 million m3 was removed in only 2 6 h during the most intense stage of the collapse

magnetization of 0 29 ma ocean crust on the mid atlantic ridge 25 30 to 27 10 n a sea surface magnetic survey over the west flank of the mid atlantic ridge from 0 to 29 ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust we find that magnetization decays rapidly within the first few million years although the filtering effect of water depth on the sea surface data and the slow spreading rate 13 km m y preclude us from resolving this decay rate a distinctly asymmetric along axis pattern of crustal magnetization is rapidly attenuated off axis suggesting that magnetization dominated by extrusive lavas on axis is reduced off axis to a background value off axis we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust indicative of induced magnetization in thin inside corner ic crust in general we find that off axis segment ends show an induced magnetization component regardless of polarity and that ic segment ends tend to have slightly more induced component compared with outside corner oc segment ends possibly due to serpentinized uppermost mantle at ic ends we find that remanent magnetization is also reduced at segment ends but there is no correlation with inside or outside corner crust even though they have very different crustal thicknesses this indicates that remanent magnetization off axis is independent of crustal thickness bulk composition and the presence or absence of extrusives remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in oc crust and a combination of crustal thinning and alteration in ic crust from all these observations we infer that the remanent magnetization of extrusive crust is strongly attenuated off axis and that magnetization of the lower crust may be the dominant source for off axis magnetic anomalies

a hydropedological approach to quantifying groundwater recharge in various glacial settings of the mid continental usa data from six monitoring stations were combined with a soil water dynamics model hydrus 1d to achieve physically based estimates of shallow water table recharge in representative hydropedological settings of the glaciated midwestern u s a calibration involved inverse modeling that yielded optimized hydraulic parameters root mean square errors for modeled versus measured soil moisture contents were generally within 3 for all soil layers at the six study sites the optimized models also accurately simulated recharge values that corresponded to observed water table fluctuations optimized parameter values were consistent with estimates from a pedotransfer function lab analyses and field experiments forward modeling indicated that shallow water table recharge in mid continent glacial settings is approximately 35 of precipitation but interannual and monthly variability is significant soil parent materials and horizon characteristics influence recharge primarily through their control on ks with clay rich till parent materials producing values as low as 16 and coarse grained outwash parent materials producing values as high as 58 of precipitation during the three year study period distinct seasonality of recharge was observed with most recharge occurring in the winter seasonal mean of all sites was 66 of precipitation and lesser but interannually stable amounts in the spring 44 summer 13 and autumn 16 this research underscores the importance of incorporating pedological information into models of soil water dynamics and groundwater recharge 2015 the authors hydrological processes published by john wiley sons ltd

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

groundwater recharge estimation using chloride deuterium and oxygen 18 profiles in the deep coastal sands of western australia the depth distributions of environmental chloride deuterium and oxygen 18 in the deep sands 20 m were monitored with a view to estimate average recharge to groundwater the investigated area is located on the southwestern coast of western australia and supports a perennial native woodland vegetation by using a steady state model based on the conservation of chloride the average areal recharge was estimated to be approximately 15 of the average precipitation 775 mm yr 1 whilst the cl concentration of groundwater was relatively uniform there was a considerable spatial variability in the depth distribution of cl in the unsaturated zone an approximate method of data analysis is developed application of this to cl data shows that possibly some 50 of the total annual recharge occurs via movement of water through preferred pathways thus bypassing the soil matrix stable isotope deuterium and oxygen 18 data show that there has been some isotopic enrichment due to evaporation of the rainfall but the extent of enrichment appears to be too small to estimate recharge rates reliably it is argued that relatively small isotopic enrichment occurs in the system studied since the direct evaporation from soil is a rather small component of the total evapotranspiration the observations on the depth distribution of isotopes qualitatively support the presence of a bimodal system for water movement through the profile as was suggested by the analysis of chloride data

dynamics and deposits generated by the kos plateau tuff eruption controls of basal particle loss on pyroclastic flow transport the explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents while these punctuated events dramatically alter the landscape and have potential climate altering impact our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations we utilize the natural boundary conditions encountered by the eruption of the kos plateau tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large 60 km3 event at the time of the eruption 161 ka the lower sea level in the mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent while flows to the south may have encountered an expanse of water steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed we use an eulerian eulerian lagrangian computational approach coupled with overwater and overland boundary conditions including topography to determine the role of bed load versus suspended load in the transport of these flows we find that a ring vent structure and eruptive fluxes greater than 2 106 m3 s are required for the spatial distribution of the kpt the maximum grain size and deposit locations of the first voluminous ignimbrite unit d can be explained by suspended flow to the south consistent with overwater transport and bed load and suspended load transport to the north consistent with overland transport however the maximum lithic size for the largest and last ignimbrite unit e requires some bed load transport in all directions we propose that the boundary conditions were significantly altered during the course of the eruption through either the in filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation on the basis of the inferred eruptive flux we estimate that the duration of the eruption climax in which most of the material was erupted likely only lasted from a few hours to a day

characterization of the in situ magnetic architecture of oceanic crust hess deep using near source vector magnetic data marine magnetic anomalies are a powerful tool for detecting geomagnetic polarity reversals lithological boundaries topographic contrasts and alteration fronts in the oceanic lithosphere our aim here is to detect lithological contacts in fast spreading lower crust and shallow mantle by characterizing magnetic anomalies and investigating their origins we conducted a high resolution near bottom vector magnetic survey of crust exposed in the hess deep tectonic window using the remotely operated vehicle rov isis during rrs james cook cruise jc21 in 2008 hess deep is located at the western tip of the propagating rift of the cocos nazca plate boundary near the east pacific rise epr 2 15 n 101 30 w rov isis collected high resolution bathymetry and near bottom magnetic data as well as seafloor samples to determine the in situ lithostratigraphy and internal structure of a section of epr lower crust and mantle exposed on the steep 20 dipping south facing slope just north of the hess deep nadir ten magnetic profiles were collected up the slope using a three axis fluxgate magnetometer mounted on rov isis we develop and extend the vertical magnetic profile vmp approach of tivey 1996 by incorporating for the first time a three dimensional vector analysis leading to what we here termed as vector vertical magnetic profiling approach we calculate the source magnetization distribution the deviation from two dimensionality and the strike of magnetic boundaries using both the total field fourier transform inversion approach and a modified differential vector magnetic analysis overall coherent long wavelength total field anomalies are present with a strong magnetization contrast between the upper and lower parts of the slope the total field anomalies indicate a coherently magnetized source at depth the upper part of the slope is weakly magnetized and magnetic structure follows the underlying slope morphology including a bench and lobe shaped steps imaged by microbathymetry the lower part of the slope is strongly magnetized with a gradual reduction in amplitude from east to west across the slope surface morphology and recent drilling results indicate that the slope has been affected by mass wasting but the observation of internally coherent magnetization distributions within the upper and lower slopes suggest that the disturbance is surficial we attribute the spatial differences in magnetization distribution to the combination of changes in in situ lithology and depth to the source these survey lines document the first magnetic profiles that capture the gabbro ultramafic and possibly dike gabbro boundaries in fast spreading lower crust

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

inland directed base surge generated by the explosive interaction of pyroclastic flows and seawater at soufriere hills volcano montserrat the largest and most intense lava dome collapse during the eruption of soufriere hills volcano montserrat 1995 2004 occurred 12 13 july 2003 the dome collapse involved around 200 x 106 m3 of material and was associated with a phenomenon previously unknown at this volcano large pyroclastic flows at the peak of the dome collapse interacted explosively with seawater at the mouth of the tar river valley and generated a hot dry base surge that flowed 4 km inland and 300 m uphill the surge was destructive to at least 25 m above the ground and it carbonized vegetation the resulting two layer deposits were as much as 0 9 m thick although the entire collapse lasted 18 h the base surge greatly increased the land area affected by the dome collapse in a few minutes at the peak of the event illustrating the complex nature of the interaction between pyroclastic flows and seawater

refining the formation and early evolution of the eastern north american margin new insights from multiscale magnetic anomaly analyses to investigate the oceanic lithosphere formation and early seafloor spreading history of the north atlantic ocean we examine multiscale magnetic anomaly data from the jurassic early cretaceous age eastern north american margin enam between 31 40 n we integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids satellite derived gravity anomaly and satellite derived and shipboard bathymetry data we evaluate these data sets to 1 refine magnetic anomaly correlations throughout the enam and assign updated ages and chron numbers to m0 m25 and eight pre m25 anomalies 2 identify five correlatable magnetic anomalies between the east coast magnetic anomaly ecma and blake spur magnetic anomaly bsma which may document the earliest atlantic seafloor spreading or syn rift magmatism 3 suggest pre existing margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the atlantic jurassic quiet zone jqz 4 suggest that if the bsma source is oceanic crust the bsma may be m series magnetic anomaly m42 168 5 ma 5 examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the bsma to m25 suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ecma to the more linear m25 and 6 observe anomalously high amplitude magnetic anomalies near the hudson fan which may be related to a short lived propagating rift segment that could have helped accommodate the crustal alignment during the early atlantic opening

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

pyroclastic flows and surges over water an example from the 1883 krakatau eruption pyroclastic deposits from the 1883 eruption of krakatau are described from areas northeast of the volcano on the islands of sebesi sebuku and lagoendi and the southeast coast of sumatra massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the krakatau islands the distal deposits exhibit a decrease in sorting coefficient median grain size and thickness with increasing distance from krakatau crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards the deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 august at approximately 10 a m energetic flows spread out away from the volcano at speeds in excess of 100 km h and traveled up to 80 km from source the flows retained temperatures high enough to burn victims on the sw coast of sumatra historical accounts from ships in the sunda straits constrain the area affected by the flows to a minimum of 4x103 km2 at the distal edge of this area the flows were relatively dilute and turbulent yet carried enough material to deposit several tens of centimeters of tephra the great mobility of the krakatau flows from the 10 a m activity may be the result of enhanced runout over the sea it is proposed that the generation of steam at the flow seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective the buoyant distal edge of these ash and steam laden clouds lifted off into the atmosphere leading to cooling condensation and mud rain

facies variation of eruption units produced by the passage of single pyroclastic surge currents hopi buttes volcanic field usa abstract base surges are a significant volcanic hazard associated with phreatomagmatism but their transport and depositional dynamics are incompletely understood stratigraphic analysis and lateral tracing of surge eruption units produced by the passage of single base surges from haskie tuff ring hopi buttes volcanic field usa extend to 1 km from the vent and show similar downcurrent changes in lithofacies within 200 m of the vent eruption units are primarily disorganized lapilli tuff that is rich in juvenile pyroclasts at distances 200 m from the vent eruption units transform into a couplet of juvenile rich lapilli that grades upward into bedform bearing ash tuff at distances 900 m upper and lower beds of the eruption unit couplet transform into planar stratified beds at distances 1100 m couplets composing different eruption units are almost entirely composed of planar beds and amalgamate to form almost identical planar bedded ash tuff the collapse of dense eruption columns following explosive magma wet sediment interaction generated turbulent surge currents as the surges traveled away from the haskie vent they expanded and partitioned into a basal juvenile rich traction carpet and overlying accidental rich ash cloud surge near the limit of their runout the surges cooled and lost their pyroclastic load leaving planar beds of accidental rich ash the deposition of multiple surge eruption units with similar lateral and vertical facies transitions and a paucity of interbedded fallout suggests an eruption characterized by recurring phreatomagmatic explosions of similar size and the generation of surges with similar flow characteristics

unprecedented pressure increase in deep magma reservoir triggered by lava dome collapse 1 the collapse of the soufriere hills volcano lava dome on montserrat in july 2003 is the largest such event worldwide in the historical record here we report on borehole dilatometer data recording a remarkable and unprecedented rapid 600s pressurisation of a magma chamber triggered by this surface collapse the chamber expansion is indicated by an expansive offset at the near dilatometer sites coupled with contraction at the far site by analyzing the strain data and using added constraints from experimental petrology and long term edifice deformation from gps geodesy we prefer a source centered at approximately 6 km depth below the crater for an oblate spheroid with overpressure increase of order 1 mpa and average radius 1 km pressurisation is attributed to growth of 1 3 of gas bubbles in supersaturated magma triggered by the dynamics of surface unloading recent simulations demonstrate that pressure recovery from bubble growth can exceed initial pressure drop by nearly an order of magnitude

quantification of nitrate storage in the vadose unsaturated zone a missing component of terrestrial n budgets national terrestrial nitrogen budgets for many developed countries have been calculated as part of the management of impacts of n on the environment but these rarely represent the subsurface explicitly using estimates of vadose zone travel time and agricultural nitrate loading we quantify for the first time the total mass of nitrate contained in the vadose zone of aquifers in england and wales this mass peaked in 2008 at 1400 kt n 800 to 1700 kt n from sensitivity analyses which is approximately 2 5 to 6 times greater than saturated zone estimates for this period and indicates that the subsurface is an important store of reactive nitrogen about 70 of the nitrate mass is estimated to be in the chalk with the remainder split between the permo triassic sandstones the jurassic oolitic limestones and minor aquifers current controls on fertiliser application mean that the vadose zone is now a nitrate source and in 2015 we estimate the net flux from the unsaturated zone to groundwater to be 72 kt n annum the mass of nitrate in the vadose zone should be included in future terrestrial nitrogen budgets at national and global scales to improve ecosystem management this article is protected by copyright all rights reserved

the crustal structure of the canary basin accretion processes at slow spreading centers multichannel seismic reflection and gravity data define the structure of mesozoic ocean crust of the canary basin formed at slow spreading rates single and multichannel seismics show a transition from smooth to rough basement topography from jurassic to cretaceous crust and a coeval change in crustal structure internal reflectivity of the rough basement area comprises upper upper middle or whole crust cutting discrete dipping reflections lower crustal reflectivity is almost absent and reflections from the crust mantle transition are short and discontinuous or absent for several kilometers in contrast crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust mantle boundary the crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust in the smooth basement area fracture zones exhibit neither a clear topographic expression nor crustal thinning we interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from jurassic to cretaceous times in addition the crust imaged across the path of the cape verde hot spot in the canary basin exhibits a widespread lower crustal reflectivity very smooth topography and apparently thick crust our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the mesozoic mid atlantic ridge

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples

anatomy of a submarine pyroclastic flow and associated turbidity current july 2003 dome collapse soufriere hills volcano montserrat west indies the 12 to 13 july 2003 andesite lava dome collapse at the soufriere hills volcano montserrat provides the first opportunity to document comprehensively both the sub aerial and submarine sequence of events for an eruption numerous pyroclastic flows entered the ocean during the collapse depositing approximately 90 of the total material into the submarine environment during peak collapse conditions as the main flow penetrated the air ocean interface phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling the bulk of the flow was submerged and rapidly mixed with sea water forming a water saturated mass flow efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth the coarsest components 60 of the total volume were deposited proximally from a dense granular flow while the finer components 40 were efficiently elutriated into the overlying part of the flow which evolved into a far reaching turbidity current

application of actualistic models to unravel primary volcanic control on sedimentation taveyanne sandstones oligocene northalpine foreland basin abstract this work is focused on the taveyanne sandstones gres de taveyanne an oligocene volcaniclastic turbidite sequence cropping out in the northern alpine molassa between se france and central switzerland with the aim to investigate the temporal relationship between volcanic activity and sediment supply detailed stratigraphic sedimentological and petrographic xrd analyses on mudstones and point counts on sandstones studies conducted on three sections col de l oulette and flaine in se france taveyanne in sw switzerland allow a discrimination of three main facies among which only one is extremely enriched in volcaniclastic detritus and characterized by features similar to those of disaggregated pyroclastic density current deposits the other two facies are characterized by variable to no volcanic detritus but supplied by crystalline and sedimentary detritus such sediment trends are similar to those of modern volcanically controlled source to sink systems this allows a reinterpretation of the taveyanne sandstones as a syn volcanic turbidite system episodically supplied by large amounts of volcanic detritus which periodically modified the drainage paths moreover the well known temporal and spatial persistence of such modifications in modern settings leads to conciliate the syn volcanic supply with the location of the volcanic centers in the internal part of the alps without invoking particular climatic and tectonic conditions controlling foreland sedimentation

melt supply variations to a magma poor ultra slow spreading ridge southwest indian ridge 61 to 69 e the southwest indian ridge swir to the east of the melville fracture zone receives anomalously low volumes of melt on average however a small number of ridge segments appear to receive more melt than this regional average we use off axis bathymetry gravity and magnetic data to show that this melt distribution pattern quite distinct from what is observed at the mid atlantic ridge mar has been a characteristic of the easternmost swir for at least the past 10 myr we also show that segments of the easternmost swir are substantially shorter lived than most segments of the mar melt distribution in our swir study area is therefore both more focused and more variable in time than at the mar we tentatively propose a mechanism by which strong and transient melt focusing events could be initiated by a localized increase in the volume of melt supplied by the melting mantle to the base of the axial lithosphere causing thermal thinning of this lithosphere and along axis melt migration these two processes may combine to effectively focus larger volumes of melt toward the center of future thick crust segments rapid melt extraction by dikes that feed large volcanic constructions on the seafloor followed by tectonic disruption of these volcanic constructions by deep reaching faults may then cool the axial lithosphere back to its original thickness and end the melt focusing events the easternmost swir is also characterized by a common departure from isostatic compensation of seafloor topography and by a pronounced asymmetry of crustal thickness and seafloor relief between the two ridge flanks at the faster spreading mar similar characteristics are found near the ends of ridge segments we propose that spreading at the ultra slow swir during periods when the melt supply is low i e most of the time for the easternmost swir is dominated by large offset asymmetric normal faulting with significant flexural uplift of the footwalls faults face either north or south and changes in fault polarity are frequent both along axis and along flow lines i e with time producing large faults and maintaining high uncompensated reliefs require the axial lithosphere to be thick a predictable characteristic for this ultra slow ridge which has an anomalously low regionally averaged melt supply

the lateral facies of ignimbrites at laacher see volcano valley fill ignimbrites of the middle laacher see tephra mlst in the proximal depositional fan south of laacher see volcano are laterally continuous with fine grained overbank facies deposits 0 5 1 5 m thick covering higher elevations and interfluves between the paleovalleys the overbank deposits consist of up to 12 ash layers each 4 10 cm thick which show internal structures typical of ash flow transport such as poor sorting reverse size grading of pumice local normal grading of coarse ash sized lithics above a fine grained basal layer cross stratification behind obstacles and erosional unconformities thickness median grain size and number of individual layers decrease systematically with distance from the vent overbank ash layers thicken at the valley slopes and form discrete valley fill ignimbrite flow units in the paleochannels with median grain size increasing from 63 m to 350 m toward the center of paleochannels however the well defined overbank facies is obscured by mutual erosion of individual flow units overlapping data fields in ternary grain size variation diagrams indicate the overbank facies to have evolved from ash flows chiefly through depletion of lapilli and coarse ash overbank facies ash layers do not represent dust layers resulting from elutriation clouds of ash flows co ignimbrite ash or surge deposits developed on higher ground due to low concentration of solids they are similar in some parameters to taupo type ignimbrite veneer deposits but are interpreted differently the thin fine grained laacher see ash layers are thought to have been deposited from diluted portions of the flow proceeding directly from the eruption column while the main pyroclastic flows were confined to the paleovalleys radiating away from the eruptive center the wide distribution of the thin ash layers is attributed to the balance of deposition from various flow parts and turbulent entraining and heating of ambient air that sustained sufficient mobility of the diluted flows to spread across hills and level ground

tephra deposits associated with a large lava dome collapse soufriere hills volcano montserrat 12 15 july 2003 abstract the 12 13 july 2003 dome collapse at soufriere hills volcano montserrat was the largest event of its kind during the eruption thus far 1995 2005 involving the removal of 210 million m 3 of the lava dome complex over 18 h less than 2 of the total volume of material involved in the dome collapse was deposited on land a pyroclastic density current deposit alongshore and inland from the tar river fan was generated from a single blast originating at the shoreline the blast was caused by the interaction of pyroclastic flows with seawater we propose that at the peak of the lava dome collapse a sharp increase in the volume flux of pyroclastic flows caused substantial displacement of seawater from the shoreline followed by inrush of seawater when the flux decreased a few minutes later the tsunami allowed penetration of seawater into the main body of the pyroclastic flow at the shoreline which led to explosive fragmentation of pyroclastic blocks tephra fall deposits accumulated at a high rate on montserrat causing extensive damage to vegetation and buildings their stratigraphy recorded fallout from high co pyroclastic flow clouds from a vulcanian explosion cloud at the peak in collapse rate caused by the fragmentation of degassed lava dome and from four vulcanian explosion clouds after the dome collapse caused by fragmentation of bubbly magma in the conduit the total tephra fall volume is estimated at 10 20 million m 3

origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife 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 coignimbrite 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

revised pacific m anomaly geomagnetic polarity timescale summary the current m anomaly geomagnetic polarity timescale gpts is mainly based on the hawaiian magnetic lineations in the pacific ocean m anomaly gpts studies to date have relied on a small number of magnetic profiles a situation that is not ideal because any one profile contains an uncertain amount of geologic noise that perturbs the magnetic field signal compiling a polarity sequence from a larger array of magnetic profiles is desirable to provide greater consistency and repeatability we present a new compilation of the m anomaly gpts constructed from polarity models derived from magnetic profiles crossing the three lineation sets hawaiian japanese and phoenix in the western pacific polarity reversal boundary locations were estimated with a combination of inverse and forward modelling of the magnetic profiles separate gpts were established for each of the three pacific lineation sets to allow examination of variability among the different lineation sets and these were also combined to give a composite timescale owing to a paucity of reliable direct dates of the m anomalies on ocean crust the composite model was time calibrated with only two ages one at each end of the sequence these two dates are 125 0 ma for the base of m0r and 155 7 ma for the base of m26r relative polarity block widths from the three lineation sets are similar indicating a consistent pacific wide spreading regime the new gpts model shows slightly different spacings of polarity blocks as compared with previous gpts with less variation in block width it appears that the greater polarity chron irregularity in older models is mostly an artifact of modelling a small number of magnetic profiles the greater averaging of polarity chron boundaries in our model gives a gpts that is statistically more robust than prior gpts models and a superior foundation for late jurassic early cretaceous geomagnetic and chronologic studies

submarine pyroclastic deposits formed during the 20th may 2006 dome collapse of the soufriere hills volcano montserrat the 20th may 2006 lava dome collapse of the soufriere hills volcano montserrat had a total non dense rock equivalent non dre collapse volume of approximately 115 106 m3 the majority of this volume was deposited into the ocean the collapse was rapid 85 of the mobilized volume being removed in just 35 min giving peak pyroclastic flow flux of 66 103 m3 s 1 channel and levee facies on the submarine flanks of the volcano and formation of a thick steep sided ridge suggest that the largest and most dense blocks were transported proximally as a high concentration granular flow of the submerged volume 30 was deposited from the base of this granular flow forming a linear high relief ridge that extends 7 km from shore the remaining 70 of the submerged volume comprises the finer grain sizes which were transported at least 40 km by turbidity currents on gradients of 2 at several localities the may 2006 distal turbidity currents ran up 200 m of topography and eroded up to 20 cm of underlying substrate multiple turbidites are preserved representing current reflection from the graben margins and deflection around topography the high energy of the may 2006 collapse resulted in longer submarine run out than the larger 210 106 m3 soufriere hills dome collapse in july 2003

slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the mid atlantic ridge south of the kane fracture zone mark area 1 the remanent magnetization of gabbroic material of the mid atlantic ridge south of the kane fracture zone mark area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment the remanence of these gabbroic samples is often complex with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales tens of centimeters to a few meters moreover several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components given the nominal age 1 ma of the crust we suggest that this pattern of normal reversed normal polarity most plausibly reflects emplacement and or cooling through three successive polarity intervals jaramillo normal polarity interval 1 07 0 99 ma a portion of the matuyama reversed polarity interval 0 99 0 78 ma and the brunhes normal polarity interval 0 78 ma to present a small number of samples with three well defined magnetization components have magnetic characteristics compatible with a remanence carried by fine grained possibly single domain magnetite laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the jaramillo matuyama 0 99 ma and matuyama brunhes 0 78 ma polarity transitions together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as 350 475 c for a minimum of 0 21 m y after emplacement we suggest that continued injection of liquid in the form of sills or small magma bodies over a broad region half width of 3 km is responsible for this slow cooling in addition inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole suggesting that little if any resolvable tilt occurred during uplift of these rocks to the seafloor

unique and remarkable dilatometer measurements of pyroclastic flow generated tsunamis pyroclastic flows entering the sea may cause tsunamis at coastal volcanoes worldwide but geophysically monitored field occurrences are rare we document the process of tsunami generation during a prolonged gigantic collapse of the soufriere hills volcano lava dome on montserrat on 12 13 july 2003 tsunamis were initiated by large volume pyroclastic flows entering the ocean we reconstruct the collapse from seismic records and report unique and remarkable borehole dilatometer observations which recorded clearly the passage of wave packets at periods of 250 500 s over several hours strain signals are consistent in period and amplitude with water loading from passing tsunamis each wave packet can be correlated with individual pyroclastic flow packages recorded by seismic data proving that multiple tsunamis were initiated by pyroclastic flows any volcano within a few kilometers of water and capable of generating hot pyroclastic flows or cold debris flows with volumes greater than 5 10 6 m 3 may generate significant and possibly damaging tsunamis during future eruptions

syneruptive features and sedimentary processes associated with pyroclastic currents entering the sea the ad 79 eruption of vesuvius bay of naples italy the ad 79 plinian eruption of vesuvius that buried pompeii and herculaneum began with pumice falls deposited towards the se of the volcano followed by pyroclastic currents directed towards the sw these currents reached herculaneum and rapidly entered the sea forming a fan the interpretation of seismic reflection profiles and gravity cores collected off herculaneum documents a submarine fan shaped pyroclastic body at 10 140 m water depth that we interpret as the submarine counterpart of the onshore pyroclastic current deposits this fan c 0 3 km 3 in volume displays a chaotic seismic facies that changes seaward to parallel reflectors and then to wavy reflectors gravity cores reveal a succession consisting of centimetre thick sand or silt sized ash couplets followed by a graded gravelly sand sized bed up to 180 cm thick containing shell fragments and beach derived pebbles overlain by centimetre thick graded and laminated sandy ash layers the depositional textures and sedimentary structures of the submarine pyroclastic fan have been interpreted as the product of the interactions between pyroclasts water waves and tsunamis induced by the ad 79 pyroclastic density currents into the bay of naples

seafloor spreading history of the western north atlantic basin derived from the keathley sequence and computer graphics abstract an analysis of sea surface magnetic profiles and an aeromagnetic contour chart west of bermuda yields a new tectonic framework for the keathley m series seafloor spreading magnetic anomaly sequence changes in spreading direction derived from the western keathley sequence occur at times corresponding to m24 m21 and m11 of which m11 is a time of continental breakup the models facilitate new identifications of the anomalies between m16 and m4 obviating the need for anomalously slow spreading between m11 and m4 previously suggested by other workers and by us and placing the location of the well defined rough smooth basement boundary at m11 the models show that spreading rates contemporaneously vary among profiles collected between flowlines as determined by a recently presented kinematic history this strongly suggests that individual ridge axis segments act independently of another in detail our reconstruction technique records in map form where the ridge crest has made significant departures from perfect symmetry and a predictable spreading rate during its evolutionary history among eight profiles collected parallel to seafloor spreading flowlines in the western north atlantic encompassing only 2700 km of normal oceanic crust we can identify more than fourteen ridge jump events there is evidence for a small 30 km ridge jump to the east of many ridge axis segments at m24 this is supported in the eastern north atlantic conjugate crust of the canary basin where m25 and m24 are poorly represented a series of four ridge jumps to the east along one paleoridge axis segment now located at 36 n 65 w is related to the passage of the mesozoic mid atlantic ridge crest over the verde hotspot at 137 m y b p the project magnet magnetic anomaly contour chart for the area between 29 n and 40 n has been used to determine stage poles of motion for processes occurring at the ridge crest related to its segmentation which have a component oblique to spreading between chrons m21 and m11 these trends may relate to the motion of the ridge crest over the mantle

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

origin of the smooth zone in early cretaceous north atlantic magnetic anomalies 1 late jurassic early cretaceous marine magnetic anomalies observed in the north atlantic exhibit an abrupt change in character in m5 m15 crust the anomalies are smoother with low amplitudes and are difficult to correlate among nearby profiles the accepted explanation for the origin of this smooth zone is diminished resolution and anomaly interference due to slow spreading rates which narrows the widths of polarity reversals in the crust and causes interference among sea surface anomalies magnetic modeling of these anomalies indicates that neither slow spreading rates alone nor slow spreading rates in combination with a decrease in geomagnetic field intensity can explain the basic character of the smooth zone combined with other geophysical evidence our study suggests that one consequence of slow spreading rates that is responsible for the magnetic smooth zone is a thinned crustal basalt layer or a non basaltic magnetic source layer resulting from low melt supply during a period of ultra slow spreading

the spatial and temporal distribution of grain size breaks in turbidites grain size breaks are surfaces where abrupt changes in grain size occur vertically within deposits grain size breaks are common features in turbidites around the world including ancient and modern systems despite their widespread occurrence grain size breaks have been regarded as exceptional and not included within idealized models of turbidity current deposition this study uses ca 100 shallow sediment cores from the moroccan turbidite system to map out five turbidite beds for distances in excess of 2000 km the vertical and spatial distributions of grain size breaks within these beds are examined five different types of grain size break are found type i in proximal areas between coarse sand and finer grained structureless sand type ii in proximal areas between inversely graded sand overlain by finer sand type iii in proximal areas between sand overlain by ripple cross laminated finer sand type iv throughout the system between clean sand and mud and type v in distal areas between mud rich debrite sand and mud this article interprets types i and v as being generated by sharp vertical concentration boundaries controlled by sediment and clay concentrations within the flows whilst types ii and iii are interpreted as products of spatial temporal fluctuations in flow capacity type iv are interpreted as the product of fluid mud layers which hinder the settling of non cohesive grains and bypasses them down slope decelerating suspensions with sufficient clay will always form cohesive layers near to bed promoting the generation of type iv grain size breaks this may explain why type iv grain size breaks are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere therefore type iv grain size breaks should be understood as the norm not the exception and regarded as a typical feature within turbidite beds

timing and emplacement dynamics of newly recognised mass flow deposits at 8 12ka offshore soufriere hills volcano montserrat how submarine stratigraphy can complement subaerial eruption histories this contribution describes two mass movement deposits total volume 0 5 km3 identified in seven marine cores located 8 to 15 km offshore southern montserrat west indies the deposits were emplaced in the last 35 ka and have not previously been recognised in either the subaerial or distal submarine records age constraints provided by radiocarbon dating show that an explosive volcanic eruption occurred at ca 8 12 ka emplacing a primary eruption related deposit that overlies a large 0 3 km3 reworked bioclastic and volcaniclastic flow deposit formed from a shelf collapse between 8 and 35 ka the origin of these deposits has been deduced through the correlation of marine sediment cores component analysis and geochemical analysis the 8 12 ka primary volcanic deposit was likely derived from a highly erosive pyroclastic flow from the soufriere hills volcano that entered the ocean and mixed with the water column forming a water supported density current previous investigations of the eruption record suggested that there was a hiatus in activity at the soufriere hills volcano between 16 and 6 ka the ca 8 12 ka eruptive episode identified here shows that this hiatus was shorter than previously hypothesised and thus highlights the importance of obtaining an accurate and complete marine record of events offshore from volcanic islands and incorporating such data into eruption history reconstructions comparisons with the submarine deposit characteristics of the 2003 dome collapse also suggests that the 8 12 ka eruptive episode was more explosive than eruptions from the current eruptive episode

entrance of hot pyroclastic flows into the sea experimental observations the entrance of hot pyroclastic flows into water has been observed in a series of experiments in which shooting granular flows of hot ignimbrite ash 403 c of bulk density near that of water run down a smooth chute and enter a water filled tank at an angle of 26 flows of relatively cool ash 250 c mixing across the water surface over some distance from shore generates steam explosions forming fountains of wet and dry ash and convectively rising fine ash plumes steam explosions increase in strength and lateral extent towards higher ash temperatures and mass fluxes the explosions generate water waves that remain driven by massive fountain fallout across some distance from shore the ash fountains feed pyroclastic surges which advance down tank over water at high speeds underwater plumes of sediment falling from the ash fountains and surges drop coarse load onto the floor whereas the finer load forms a turbidity current high temperature and poor size sorting of pyroclastic flows are key parameters that determine the processes of interaction with water and the associated hazards poor sorting allows for the formation of ash cloud surges moving over water even from cool flows high temperatures cause littoral explosions and facilitate extensive mass transport over water all flows generate tsunami waves by different mechanisms

magmato tectonic cyclicity at the ultra slow spreading southwest indian ridge evidence from variations of axial volcanic ridge morphology and abyssal hills pattern on axis deep tow side scan sonar data are used together with off axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra slow spreading southwest indian ridge differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments we interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development including periods of volcanic construction and periods of tectonic dismemberment using off axis bathymetric data we identify numerous abyssal hills with a homogeneous size for each segment these abyssal hills all display an asymmetric shape with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away we suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment i e a magmato tectonic cycle we observe that large abyssal hills are in ridge sections of thicker crust whereas smaller abyssal hills are in ridge sections of thinner crust this suggests that the magma supply controls the size of abyssal hills the abyssal hills in ridge sections of thinner crust are regularly spaced indicating that the magmato tectonic cycle is a pseudoperiodic process that lasts 0 4 m y about 4 to 6 times shorter than in ridge sections of thicker crust we suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long lived segments by contrast when magma supply strongly decreases and becomes highly discontinuous regular abyssal hills patterns are no longer observed