storm induced response of a nearshore bar system abstract a nearshore bar system was surveyed periodically through a storm and the following recovery period the data showed a very rapid response of morphology to changing wave conditions and allowed various models on bar formation to be tested under low energy conditions prior to the storm a small bar was surveyed 13 m offshore both the high reflectivity of the beach and the cross shore distance to the bar are consistent with a model of sediment convergence at the node or antinode of a standing wave of incident period such a small scale bar may be a common feature on beaches with steep foreshores and more gentle offshore slopes with the increase in wave height during the storm the bar became better developed and migrated offshore at rates up to 2 2 m h 1 the bar maintained its form in that the ratio of trough depth to crest depth h t h c remained roughly constant the bar was in no way related to processes which would cause the convergence of sediment in the breaker zone through most of the storm the bar crest distance offshore was typically only 10 of the surf zone width analysis of the bar distance offshore in terms of a standing wave motion showed that the causative wave period must have been much longer than that of incident waves probably on the order of a minute surf zone wave data showed significant energy in the infragravity band at these periods although no definite link has been made after the height of the storm the bar had a crescentic morphology the development of this morphology occurred very rapidly with parts of the bar migrating onshore at rates up to 1 2 m h 1 in contrast to the storm during the recovery period h t h c varied by nearly a factor of three analysis of the offshore and longshore length scales showed the bar to be similar to one which would be generated by a standing mode 1 edge wave of period on the order of one minute

infragravity wave dynamics in a barred coastal region a numerical study this paper presents a comprehensive numerical study into the infragravity wave dynamics at a field site characterized by a gently sloping barred beach the nonhydrostatic wave flow model swash was used to simulate the local wavefield for a range of wave conditions including mild and storm conditions the extensive spatial coverage of the model allowed us to analyze the infragravity wave dynamics at spatial scales not often covered before overall the model predicted a wavefield that was representative of the natural conditions supporting the model application to analyze the wave dynamics the infragravity wave field was typically dominated by leaky waves except near the outer bar where bar trapped edge waves were observed relative contributions of bar trapped waves peaked during mild conditions when they explained up to 50 of the infragravity variance near the outer bar the infragravity wave growth was partly explained by nonlinear energy transfers from short waves this growth was strongest for mild conditions and decreased for more energetic conditions when short waves were breaking at the outer bar further shoreward infragravity waves lost most of their energy due to a combination of nonlinear transfers bottom friction and infragravity wave breaking nonlinear transfers were only effective near the inner bar whereas near the shoreline where losses were strongest the dissipation was caused by the combined effect of bottom friction and breaking this study demonstrated the model s potential to study wave dynamics at field scales not easily covered by in situ observations

field observations of swash zone infragravity motions and beach cusp evolution 1 fluid flows consistent with low mode edge waves were evident in video observations of swash motions during a field experiment in which beach cusps developed on an initially smooth beach as beach cusps grew energy lying along low mode dispersion curves increased the most energetic edge wave propagation direction changed from upcoast to downcoast as the orientation of the cusp horns rotated these observations suggest a coupling between morphodynamics and hydrodynamics and are evidence that beach cusp evolution might control low mode edge wave dynamics

linear modeling of infragravity waves during delilah 1 the generation of infragravity waves by directionally spread shortwaves incident on an alongshore uniform beach is investigated two mechanisms responsible for the generation of infragravity waves are considered the release of the bound infragravity waves associated with changes in the spatial variation of the incident shortwave energy and the forcing of trapped waves by obliquely incident directionally spread shortwaves the infragravity wave response is examined using linear shallow water equations taking into account the presence of bottom friction setup of the mean water level rollers and longshore current summing all infragravity contributions due to difference interactions between pairs of shortwave spectral components with the same frequency difference results in infragravity energy density spectra calculated spectra are compared with measured spectra obtained during the delilah field experiment for 11 consecutive days including calm mild and storm conditions in the comparison attention is focused on the frequency distribution of the infragravity energy density throughout the surf zone and the transformation of the root mean square infragravity wave height the measured infragravity response in the frequency range from 0 01 to 0 06 hz is reproduced by the computations with relatively broad spectra offshore and a clear nodal structure closer to shore typically 80 of the infragravity wave height variability is explained by the model of which 30 or less is due to bound infragravity waves differences occur mainly during storm events in which the infragravity energy is underpredicted in the outer surf zone and overpredicted in the inner surf zone

the role of external environmental factors in changing eruption styles of monogenetic volcanoes in a mio pleistocene continental volcanic field in western hungary abstract the occurrence shape structure and eruption style of monogenetic volcanoes such as maars tuff rings tuff cones and scoria cones are generally governed by several internal composition of the magma magmatic flux ascent rate viscosity volatile contents and external conditions regional and local tectonics topography and the presence of surfacial ground and meteoric water these controlling factors are together responsible for the eruption style distribution pattern volcanic facies architecture and morphology of the monogenetic volcanic landforms the late miocene to pleistocene bakony balaton highland volcanic field bbhvf in western hungary is a typical small sized at least 6 volcanic cycles have been identified by cluster analysis time gap between the cycles were vary from 1 66 up to 0 06 ma while the average eruption recurrence rate was 0 1078 ma event the time volume diagram of the volcanism of bbhvf have shown time predictive behavior combined with low magma flux total preserved volume 2 867 km3 and output rates 0 53 km3 ma for the entire volcanic field and 0 90 km3 ma for the last 5 cycles suggesting that volcanism was largely tectonically controlled and not magmatically controlled furthermore the topographic differences between the northern elevated and southern basin like parts of the volcanic field are also important in local differences in dominant fragmentation style because the elevated part of the field was prone to host large more magmatically evolved volcanoes than on the lower water saturated unconsolidated sediments which favored to the magma water interaction driven phreatomagmatic fragmentation the third controlling parameter which seems to play an important role in controlling the eruptive style of monogenetic volcanism at bbhvf was the paleoclimate fluctuation especially during the time interval of 3 0 to 2 5 ma thus mainly the long term environmental changes e g aridification have been response the shifting fragmentation style from phreatomagmatic to more magmatic ones

shallow plumbing systems for small volume basaltic volcanoes eruptive dynamics in basaltic volcanoes are controlled in part by the conduit geometry however uncertainties in conduit shape and dike to conduit transition geometry have limited our predictive capability for hazards assessments we characterize the subvolcanic geometry of small volume basaltic volcanoes magmatic volatile driven eruptions 0 1 to 0 5 km3 based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes conduits and vent areas 250 m depth study areas include east grants ridge new mexico usa basalt ridge east basalt ridge paiute ridge and southeast crater flat nevada usa basaltic feeder dikes 250 to 100 m deep have typical widths of 4 12 m with smooth host rock contacts rhyolite tuff at depths less than 100 m heterogeneities in the host rock form preferential pathways for small dike splays and sills resulting in a 30 m effective width at 50 m depth the development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock the overall zone of effect at depths less than 50 m is 110 m wide 220 m elongated along the feeder dike based on comparisons with theoretical conduit flow models the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1 2 m at depths greater than 500 m the flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure balanced flow conditions sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface 50 m depth but at depths greater than 100 m the difference is a factor of 2 to 4 this difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events while theoretical model results define dimensions necessary for a single steady eruption phase the complex details of magma host rock interactions observed at the study areas contact welding brecciation bifurcating dikes and sills and stoping represent the mechanisms by which the lithostatic pressure balanced geometry is attained the similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure balanced modeling approach consistent with the conclusions of wilson and head j geophys res 86 2971 3001 1981 for this type of volcano

computer simulation models of scoria cone degradation abstract long term erosional modifications of the relatively simple morphology of scoria cinder cones are ideally suited for study by field and computer simulation methods a series of temporally distinct cones in the san francisco and springerville volcanic fields of arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi arid climate progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters including scoria cone height cone height width ratio hco wco crater depth width ratio and slope angle for example holocene latest pleistocene cones in the san francisco field have a mean hco wco value of 0 178 0 041 a mean maximum slope angle of 29 7 4 2 and a mean average slope angle of 26 4 7 3 whereas the group of pliocene cones have values of 0 077 0 024 20 5 5 8 and 8 7 2 7 respectively comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields in order to better understand the degradational modifications of these volcanic landforms we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone the simulation algorithm can apply either a linear diffusion equation model or a model with a nonlinear transport law using a finite difference formulation the simulation operates upon a three dimensional scoria cone input as a matrix of elevation values utilizing both field and model results the correlation between changing hco wco value cone age and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient df for both volcanic fields for the san francisco volcanic field df had a calculated value of 21 4 m2 kyr for the linear model and 5 3 m kyr for the nonlinear model while for the springerville volcanic field df had a calculated value of 24 4 m2 kyr for the linear model and 6 3 m kyr for the nonlinear model

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

sedimentology eruptive mechanism and facies architecture of basaltic scoria cones from the auckland volcanic field new zealand abstract scoria cones are a common type of basaltic to andesitic small volume volcanoes e g 10 1 10 5 km 3 that results from gas bubble driven explosive eruptive styles although they are small in volume they can produce complex eruptions involving multiple eruptive styles eight scoria cones from the quaternary auckland volcanic field in new zealand were selected to define the eruptive style variability from their volcanic facies architecture the reconstruction of their eruptive and pyroclastic transport mechanisms was established on the basis of study of their volcanic sedimentology stratigraphy and measurement of their pyroclast density porosity scanning electron microscopy 2d particle morphology analysis and visible and near visible infrared spectroscopy collection of these data allowed defining three end member types of scoria cones inferred to be constructed from lava fountaining transitional fountaining and strombolian type and explosive strombolian type using the physical and field based characteristics of scoriaceous samples a simple generalised facies model of basaltic scoria cones for the avf is developed that can be extended to other scoria cones elsewhere the typical avf scoria cone has an initial phreatomagmatic phases that might reduce the volume of magma available for subsequent scoria cone forming eruptions this inferred to have the main reason to have decreased cone volumes recognised from auckland in comparison to other volcanic fields evolved dominantly in dry eruptive condition e g no external water influence it suggests that such subtle eruptive style variations through a scoria cone evolution need to be integrated into the hazard assessment of a potentially active volcanic field such as that in auckland

stability of lava lakes a physical model of a generic lava lake system is developed we derive the requisite conditions for the existence of an equilibrium lava lake in which magmastatic pressure at the base of the conduit balances the pressure in the underlying magmatic reservoir the stability of this lava lake system is tested by investigating the response of the system to perturbation we develop a graphical method based on the system s pressure depth profile to predict the subsequent behaviour of the system despite the simplicity of the modelled system we find a broad behavioural spectrum initially the rise of bubbles through the magma is ignored in this case both stable long lived lava lakes and unstable lakes that are prone to sudden draining are predicted the stability of the system is shown to be controlled by lake conduit geometry the solubility and gas expansion laws and the magma s volatile content we show that an unstable lake must collapse to a new stable equilibrium subsequent recharge of the system by for example conduit overturn would promote a return to the original equilibrium giving rise to cyclic behaviour such a mechanism is consistent with lava lake behaviour during the 1983 1984 pu u o o eruption of kilauea when the rise of bubbles through the magma is considered our model predicts that stable lakes must drain over time we therefore deduce that persistently degassing stable lava lakes such as those observed at mt erebus antarctica and mauna ulu kilauea hawaii must have an effective conduit convection mechanism or an exogenous supply of bubbles from depth

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

sediment transport near the break point associated with cross shore gradients in vertical eddy diffusivity sediment clouds entrained and mixed vertically into the water column at the breakpoint were observed to advect offshore with the return current following wave breaking at a natural beach an advection diffusion model is applied to this phenomenon providing for cross shore gradients in vertical eddy diffusivity within and beyond the surf zone which result in differential entrainment and settlement near the break point results indicate that the horizontal gradient in turbulent vertical sediment mixing can provide a significant mechanism for bar generation even if the surface gravity and infra gravity waves remain uncoupled quantitative estimates of the rate of bar accretion are obtained after calibrating the sediment transport model using field measurements of currents and suspended sediment loads in the absence of other processes accretion rates of order 20 mm hour associated with the horizontal gradient in vertical diffusivity are predicted on the offshore bar

the temporal evolution of three magmatic systems in the west eifel volcanic field germany abstract the quaternary west eifel volcanic field is the surface expression of a mantle plume that is thought to contain 1 melt the volcanoes range in age from 0 7 ma to 10 000 yr bp and the average recurrence rate of volcanism is around 3 5 10 4 eruptions per year giving a mean time between eruptions of 2875 years assuming that the field is monogenetic the presence of mafic and ultramafic xenoliths is taken as evidence for rapid single stage emplacement of magmas at velocities around 0 5 km h however the presence of clinopyroxene that preserves evidence of polybaric crystallisation belies this simple model this study uses variations in the fo content of olivine in mantle derived xenoliths and in olivine xenocrysts enclosed in cumulate xenoliths to calculate the time scale of evolution of three volcanic centres in the eifel field core rim variations in olivine are interpreted to result from fe mg interdiffusion between it and a surrounding melt using experimental determinations of the interdiffusion coefficient and approximations of magma temperature contact times have been calculated using planar and radial diffusion models the three volcanic centres studied were meerfelder maar gemundener maar and baarley the cumulate xenoliths at meerfelder maar formed at a depth of 13 km and record a magma emplacement event at between 1 and 3 years prior to eruption two suites of mantle derived olivine clinopyroxenites record a further two events at 1 8 days and 3 h prior to eruption the final event is interpreted to record the eruption of this volcano and the 3 h contact time records the time required for transport of peridotite xenoliths from the mantle to surface giving an average emplacement velocity of around 15 km h the cumulate xenoliths at gemundener maar crystallised at depths of between 4 and 22 km there are four distinct events recorded by the zoned olivine grains in the clinopyroxene rich cumulates from this locality the maximum residence time is similar to that at meerfelder maar however in this case the final magma resided in the chamber for between 23 days and 2 7 months prior to eruption allowing enough time for any entrained peridotite xenoliths to be filtered out the cumulate xenoliths from the baarley locality record a much longer history than those at the other localities in this case initially peridotitic olivine has been completely re equilibrated over 175 years prior to eruption in common with the other two localities there is evidence of multiple intrusive events at depths up to 7 km prior to final eruption the duration of the final eruption event and the interpreted depth to source of the xenoliths indicate an emplacement velocity of around 3 km h the data derived from modelling of diffusion profiles in olivine from cumulate xenoliths indicate that the magmatic systems feeding these three volcanoes were dynamic and very short lived in addition these data can be used to place constraints on the rate of cumulate formation in subvolcanic magma chambers on the basis of the data presented here it would seem that an appreciable thickness of cumulates can form over relatively short time spans days to a few years

edge wave trapping and amplification on barred beaches theoretical predictions show that edge waves may be trapped on bars providing a possible mechanism for bar growth simple solutions to the linear shallow water equations reveal that bar trapping is controlled by the depth of the water over the bar hbar and the geometry of the inshore trough relative to the bar for specific geometries amplification over the bar approaches an order of magnitude or more as the phase speed approaches where amplification is defined as the surface displacement over the bar relative to the surface displacement at the shoreline the amplification increases with the cross shore length scales of the profiles the amplification is somewhat frequency dependent generating stronger currents at the location of the bar in specific frequency ranges previous work showed that particular edge wave modes have the correct length scales to generate beach features such as bars and beach cusps but field observations notably shoreline run up data frequently show no evidence of edge wave energy being concentrated at the appropriate frequencies however for bar trapped modes edge wave energy is concentrated at the location of the bar therefore this edge wave energy will not necessarily be evident in shoreline measurements strong correlation exists between the energy distribution of the longshore component of the orbital velocity observed in data collected at superduck october 11 1986 and the distribution expected for a white edge wave spectrum at the shoreline this suggests that bar trapped edge waves exist in particular at incident wave frequencies

evidence for a role of the downgoing slab in earthquake slip partitioning at oblique subduction zones a new model incorporating shear deformation within a subducting slab is proposed to explain slip partitioning for oblique plate motion at subduction zones on the basis of investigation of 450 interplate earthquakes at 24 subduction zone segments we find that the degree of slip partitioning is largely correlated with the calculated slab pull force such correlation suggests that other than the upper plate deformation the slab pull force plays an important role in controlling oblique subduction our model proposes that the force balance between the slab pull force the interplate coupling resistance and the viscous mantle drag the latter two are passive forces to the former one produces a lateral shear within the slab which causes the slab to deform and change its motion direction gradually toward trench normal as it subducts the amount of direction change which would be observed as the slip partitioning during earthquakes therefore is closely related to the major plate driving force at the subduction zone which is the slab pull force in our model

morphological and textural diversity of the steens basalt lava flows southeastern oregon usa implications for emplacement style and nature of eruptive episodes this study focuses on middle miocene tholeiitic flood basalt lava flows from the oregon plateau northwestern usa steens basalt and is the first to comprehensively document and evaluate their morphology field observations of flows from several sections within and proximal to the main exposures at steens mountain have been supplemented with textural and geochemical data and are used to offer preliminary insights into their emplacement compound pahoehoe flows of variable thickness appear to be common throughout the study area laterally and vertically these tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as hawaii and the snake river plain classic a a flows with brecciated upper and basal crusts are not abundant but by no means uncommon flows with characters different from typical pahoehoe and a a are also common such flows display a range in morphology flows with preserved upper crusts but brecciated basal crusts as well as those displaying well developed flow top breccias and preserved basal crusts rubbly pahoehoe are observed the steens basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces the abundant compound pahoehoe flows often rich in plagioclase phenocrysts were likely emplaced during slow but sustained eruptive episodes their constituent lobes show clear evidence for endogenous growth the relatively aphyric flows with brecciated surfaces including a a hint at higher strain rates and or higher viscosity probably caused by higher effusion rates a couple of sections are characterized by compositionally similar but morphologically different flows that were possibly part of the same eruption while differences in pre eruptive topography could explain this it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system it is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems and deserve closer scrutiny

volcanism sedimentation interaction in the campo de calatrava volcanic field spain a magnetostratigraphic and geochronological study this work focuses on the influence of cenozoic volcanism of the campo de calatrava volcanic field on the sedimentation of two small continental basins in spain argamasilla and calzada moral basins the volcanism in this area was mainly monogenetic according to the small volume volcanic edifices of scoria cones that were generated and the occurrence of tuff rings and maars a sedimentological analysis of the volcaniclastic deposits led to the identification of facies close to the vents low density dilute pyroclastic surges secondary volcanic deposits and typical maar deposits whole rock k ar dating together with palaeomagnetic constraints yielded an age of 3 11 3 22 ma for the onset of maar formation the deposition finished in the late gauss early matuyana using both techniques and previous paleontological data allowed it to be inferred that the maar formation and the re sedimentation stage that occurred in argamasilla and calzada moral basins were roughly coeval the occurrence of syn eruption volcaniclastic deposits with small thicknesses that were separated by longer inter eruption periods where fluvial and lacustrine sedimentation was prevalent together with the presence of small volume volcanic edifices indicated that there were short periods of volcanic activity in this area the volcanic activity was strongly controlled by previous basement faults that favoured magma feeding and the faults also controlled the location of volcanoes themselves the occurrence of the volcanoes in the continental basins led to the creation of shallow lakes that were related to the maar formation and the modification of sedimentological intra basinal features specifically valley slope and sediment load

features of lava lake filling and draining and their implications for eruption dynamics lava lakes experience filling circulation and often drainage depending upon the style of activity and location of the vent features formed by these processes have proved difficult to document due to dangerous conditions during the eruption inaccessibility and destruction of features during lake drainage k lauea iki lava lake k lauea hawai i preserves many such features because lava ponded in a pre existing crater adjacent to the vent and eventually filled to the level of and interacted with the vent and lava fountains during repeated episodes a cyclic pattern of lake filling to above vent level followed by draining back to vent level preserved features associated with both filling and draining field investigations permit us to describe the characteristic features associated with lava lakes on length scales ranging from centimeters to hundreds of meters in a fashion analogous to descriptions of lava flows multiple vertical rinds of lava coating the lake walls formed during filling as the lake deepened and lava solidified against vertical faces drainage of the lake resulted in uneven formation of roughly horizontal lava shelves on the lakeward edge of the vertical rinds the shelves correlate with stable staggered lake stands shelves either formed as broken relict slabs of lake crust that solidified in contact with the wall or by accumulation accretion and widening at the lake surface in a dynamic lateral flow regime thin upper lava shelves reflect an initially dynamic environment in which rapid lake lowering was replaced by slower and more staggered drainage with the formation of thicker more laterally continuous shelves at all lava lakes experiencing stages of filling and draining these processes may occur and result in the formation of similar sets of features

syn eruptive morphometric variability of monogenetic scoria cones according to wood s model morphometric parameters such as slope angle can provide valuable information about the age of conical volcanic edifices such as scoria cones assuming that their initial slopes range from 30 to 33 measured manually on topographic maps and assuming that their inner architectures are homogenous this study examines the morphometric variability of nine young a few thousand years old small volume scoria cones from tenerife canary islands using high resolution digital elevation models in order to assess their slope angle variability because of the young age and minimal development of gullies on the flanks their morphometric variability can be interpreted as the result of syn eruptive processes including 1 pre eruptive surface inclination 2 vent migration and lava outflow with associated crater breaching and 3 diversity of pyroclastic rocks accumulated in the flanks of these volcanic edifices results show that slope angles for flank sectors differ by up to 12 among the studied volcanoes which formed over the same period of time this range greatly exceeds the 2 3 indicated by wood the greater than expected original slope range suggests that use of morphometric data in terms of morphometry based relative dating and detection of erosional processes and settings must be done with great care or detailed knowledge about absolute ages and eruption history especially in field scale morphometric investigation

the polycyclic lausche volcano lausitz volcanic field and its message concerning landscape evolution in the lausitz mountains northern bohemian massif central europe abstract the tertiary lausitz volcanic field covers a broad area encompassing parts of eastern saxony germany lower silesia poland and north bohemia czech republic volcanism was predominantly controlled by the volcano tectonic evolution of the oh e rift and culminated in the lower oligocene this paper deals with the highest volcano of this area the lausche hill 792 6 m a s l situated in the lausitz mountains we offer a reconstruction of the volcanic edifice and its eruptive history its complex genesis is reflected by six different eruption styles and an associated petrographic variety furthermore the lausche volcano provides valuable information concerning the morphological evolution of its broader environs the remnant of an alluvial fan marking a middle paleocene lower eocene 62 50 ma palaeo surface is preserved at the base of the volcano the deposition of this fan can be attributed to a period of erosion of its nearby source area the lausitz block that has undergone intermittent uplift at the lausitz overthrust since the upper cretaceous the lausche hill is one of at least six volcanoes in the lausitz mountains which show an eminent low level of erosion despite their oligocene age and position on elevated terrain these volcanoes are exposed in their superficial level which clearly contradicts their former interpretation as subvolcanoes among further indications this implies that the final morphotectonic uplift of the lausitz mountains started in the upper lower pleistocene 1 3 ma due to revived subsidence of the nearby zittau basin it is likely that this neotectonic activity culminated between the elsterian and saalian glaciation 320 ka the formation of the low mountain range was substantially controlled by the intersection of the lausitz overthrust and the oh e rift

erosion rates on different timescales derived from cosmogenic 10 be and river loads implications for landscape evolution in the rhenish massif germany we determined erosion rates on timescales of 101 104 years for two catchments in the northeastern rhenish massif in order to unravel the quaternary landscape evolution in a variscan mountain range typical of central europe spatially averaged erosion rates derived from in situ produced 10be concentrations in stream sediment of the aabach and mohne watersheds range from 47 6 to 65 14 mm ka and integrate over the last 9 13 ka these erosion rates are similar to local rates of river incision and rock uplift in the quaternary and to average denudation rates since the mesozoic derived from fission track data this suggests that rock uplift is balanced by denudation i e the landscape is in a steady state short term erosion rates were derived from suspended and dissolved river loads subsequent to 1 correcting for atmospheric and anthropogenic inputs 2 establishing calibration curves that relate the amount of suspended load to discharge and 3 estimating the amount of bedload the resulting solid mass fluxes suspended and bedload agree with those derived from the sediment volume trapped in three reservoirs however resulting geogenic short term erosion rates range from 9 to 25 mm ka and are only about one third of the rates derived from 10be model simulations in combination with published sediment yield data suggest that this discrepancy is caused by at least three factors 1 phases with higher precipitation and or lower evapotranspiration 2 rare flood events not captured in the short term records and 3 prolonged periods of climatic deterioration with increased erosion and sediment transport on hillslopes

wave variance partitioning in the trough of a barred beach the wave induced velocity field in the nearshore is composed of contributions from incident wind waves f 0 05 hz surface infragravity waves f 2 g where fis the frequency 2 f c is the radial alongshore wavenumber 2re l l being the alongshore wavelength is the beach slope and g is the acceleration due to gravity using an alongshore array of current meters located in the trough of a nearshore bar mean depth 1 5 m we investigate the bulk statistical behaviors of these wave bands over a wide range of incident wave conditions the behavior of each contributing wave type is parameterized in terms of commonly measured or easily predicted variables describing the beach profile wind waves and current field over the 10 day period the mean contributions to the total variance of the incident infragravity and shear wave bands were 71 5 14 3 and 13 6 for the alongshore component of flow mean rms oscillations of 44 20 and 19 cm s 1 respectively and 81 9 10 9 and 6 6 for the cross shore component mean rms oscillations of 92 32 and 25 cm s 1 respectively however the values varied considerably the contribution to the alongshore cross shore component of flow ranged from 44 888 4 58 5 95 8 for the incident band to 6 2 26 6 2 5 32 4 for the infragravity band and 3 433 1 0 6 14 3 for the shear wave band incident wave oscillations were limited by depth dependent saturation over the adjacent bar crest and varied only with the fide the infragravity wave rms oscillations on this barred beach are best parameterized by the offshore wave height consistent with previous studies on planar beaches comparison with data from four other beaches of widely differing geometries shows the shoreline infragravity amplitude to be a near constant ratio of the offshore wave height the magnitude of the ratio is found to be dependent on the iribarren number 0 h lo 1 2 shear waves are as previous observation and theory suggest oltman shay et al 1989 bowen and holman 1989 significantly correlated with a prediction of the seaward facing shear of the longshore current

longshore current and wave height modulation at tidal frequency inside the surf zone data were acquired continuously during the 19 day delilah nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies it is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations and since radiation stress is a function of the wave height longshore currents are forced at the tidal frequency inside the surf zone the measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions indicating that the tide is a dominant mechanism associated with longshore current variability simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by thornton and guza 1986 are used to explain observations the measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model verifying the hypothesis

volcanology geochemistry and age of the lausitz volcanic field the lausitz lusatia volcanic field is part of the central european volcanic province and its magmas represent an alkaline trend from olivine nephelinites and basanites to trachytes and phonolites typical for intraplate settings neighbouring volcanic fields are the ceske st edoho i mountains to the south west and the fore sudetic basin in lower silesia to the east more than 1000 volcanic structures associated with approximately 500 vents have been located within this volcanic field residuals of scoria cones lava lakes lava flows and maar diatreme in filling occur in situ near the level of the original syn volcanic terrain in more deeply eroded structures volcanic relicts outcrop as plugs or feeders evolved rocks occur as monogenetic domes or intrusions in diatremes while their volcaniclastic equivalents are rare twenty three localities were dated using the 40ar 39ar method the ages range from 35 to 27 ma with a focus around 32 29 ma indicating late eocene and mainly oligocene volcanism for the lvf differentiated rocks appear to be slightly younger than less differentiated no geographical age clusters are apparent

bar generating cross shore flow mechanisms on a beach random waves normally incident on a dissipative beach induce a variety of cross shore flows such as asymmetric oscillatory flow wave grouping induced long wave flow breaking induced turbulent flow and momentum decay induced undertow these flows are identified analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two dimensional case of bar generation on a beach

a quaternary monogenetic volcanic field in the xalapa region eastern trans mexican volcanic belt geology distribution and morphology of the volcanic vents abstract the most abundant volcanic manifestations along the east west trending trans mexican volcanic belt tmvb are the scoria cones these have been grouped by other authors in extended monogenetic volcanic fields such as michoacan guanajuato chichinautzin apan and los tuxtlas here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the cofre de perote volcano cp around the city of xalapa in the state of veracruz mexico within an area of about 2400 km2 the xalapa monogenetic volcanic field xmvf contains over 50 late quaternary volcanoes most of them are scoria cones but small shield volcanoes and tuff rings also occur the lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of cp between 3000 and 500 m a s l the representative rocks of the different volcanic centers include olivine basalt basaltic andesite with phenocrysts of plagioclase pyroxene and minor olivine and andesite with phenocrysts of plagioclase and pyroxene sio2 and al2o3 contents of the rocks vary between 45 and 62 wt and 15 to 18 wt respectively most of the basaltic rocks have mgo contents between 4 2 and 9 wt ni and cr concentrations between 23 and 180 and 10 to 380 ppm respectively with a typical calc alkaline behavior trace elements suggest two types of magmas the most abundant are characterized by an enrichment of lile and lree with negative anomalies of nb and ti which denote a calc alkaline affinity others are lile depleted and show high concentrations of mgo cr and ni which is typical of primary calc alkaline magmas the mean scoria cone morphological values are cone height hco 90 8 m cone diameter wco 686 38 m crater diameter wcr 208 49 m and 0 12 km3 for the cone volume we dated twelve different scoria cones using the 40ar 39ar method for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter hco wco and maximum slope angle additionally vegetation type and cone and lava flow morphologies were also considered in order to determine rough relative ages in general the ages fall into three groups those older than 2 0 ma those between 0 25 0 40 ma and those less than approximately 0 1 ma

root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar diatreme volcanoes abstract the understanding of processes within the root zone of maar diatreme volcanoes is important for the interpretation of the geology volcanology and even hazard assessment of these volcanoes in the phreatomagmatic model of pipe formation the irregularly shaped root zone is the site of the phreatomagmatic explosions and thus functions as the engine for pipe formation in this model the root zone grows over a period of time in a series of many single thermohydraulic i e phreatomagmatic explosions the explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels the ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole like fashion into the root zone many phreatomagmatic eruptions consequently result in the formation of a cone shaped diatreme thus with ongoing eruptions the cone shaped diatreme has to grow systematically both in depth and diameter during its growth processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone at the surface the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter thermohydraulic explosions which fragment both magma and the surrounding country rocks mostly occur within the bottom part of the root zone violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme in larger pipes via expansion of the mixture of highly pressurized water vapor juvenile gas phases and explosively produced tephra the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt diatreme clearing events in large pipes are difficult or impossible to maintain since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes the combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types it is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar diatreme and related volcanoes

late cenozoic partitioning of oblique plate convergence in the zagros fold and thrust belt iran the nw trending zagros fold and thrust belt is affected by two major dextral faults 1 the nw trending main recent fault that accommodates partitioning of oblique convergence at the rear of the western zagros and 2 the north trending kazerun fault located in the central zagros combined structural and fault kinematics studies and spot images analysis have shown a pliocene kinematic change accompanied by a fault pattern reorganization which has led to a modification in the accommodation of oblique convergence since the late pliocene the distributed transpressional deformation operating at the rear of the belt has become partitioned along the newly formed main recent fault this fault cuts through early pliocene nappes and transpressional structures by right laterally reactivating high angle thrusts the southeastern termination of the main recent fault connects to the northern termination of kazerun fault that consists of three fault zones that end in bent orogen parallel splay thrust faults the kazerun fault together with a series of north to nnw trending inherited basement strike slip faults define an orogen scale fan shaped fault pattern pointing toward the main recent fault kazerun fault junction this structural pattern allows slip from along the main recent fault to become distributed by transfer to the longitudinal thrust faults and folds of the zagros belt with the fan shaped fault pattern acting as a horse tail termination of the main recent fault

surf zone longshore currents and random waves field data and models abstract analytic and numerical models for longshore currents generated by obliquely incident random waves am compared with field observations five days of observations were selected during which the waves were narrow banded in both frequency and direction in keeping with model assumptions the extensive measurements included radiation stress and wave directional spectra in 9 m depth and a closely spaced array of current and pressure sensors on a line perpendicular to shore the longshore current models are based on balancing the gradient of the radiation stress with the alongshore bed shear and reynold s stresses assuming stationary wave conditions and straight and parallel bottom contours the spatial variation of wave height required to determine the gradient of the radiation stress is modeled using linear random wave theory given hrms in 9 m depth the model predicts hrms at shoreward locations with an average error of less than 9 using a nonlinear bottom shear stress formulation and the meas

longshore current on a barred beach field measurements and calculation measurements of the longshore current on a barred beach made during the 1990 duck experiment on low frequency and incident band longshore and across shore hydrodynamics delilah field data collection project conducted at duck north carolina revealed an unexpected and persistent broad peak in the current velocity in the trough between the nearshore bar and the shore this paper introduces longshore current and associated wave measurements from delilah together with a numerical model capable of describing the field observations an existing numerical model of the mean current is modified to include a general transport equation for the mean turbulent kinetic energy created by wave breaking and reynolds stress components needed to calculate the longshore current and mean water level are reexpressed to include the turbulent momentum transport in comparison to predictions from the original model the modified model produces much improved agreement with the measured current velocity on the barred profiles in the field measurements and with the velocity profile and mean water level generated on a uniformly sloping laboratory beach by monochromatic waves three forms of a bottom friction coefficient are examined and the open channel flow manning friction coefficient is selected because of best agreement and consistency values of the manning friction coefficient required to calibrate the model agree with values normally assigned to the related bottom roughness in open channel flow

bar trapped edge waves and longshore currents edge waves react to the presence of a mean longshore current as though the current was a change in bottom topography in this way the cross shore shape of the edge wave can be amplified over the current relative to the shoreline in the same way as the edge wave shape can be amplified over bars on a beach with both longshore currents and bars the degree of edge wave amplification or trapping depends on the strength and location of the longshore current shear relative to the steepness of the bottom slope and also on the direction of edge wave propagation relative to the longshore current weak longshore current shear over well developed bars does not significantly alter edge wave trapping over bars simply enhancing or reducing the existing bar trapping effect conversely strong current shears over gentle topography can govern the existence and location of edge wave trapping the extreme example being edge wave trapping on plane beaches where the amplification is entirely due to the existence of mean longshore currents in reality the possibilities are restricted since the shape of the longshore current profile is primarily determined by the pattern of incident wave breaking which itself depends on the beach profile the importance of realistic current shear relative to realistic bottom slope in determining edge wave shape is demonstrated using frequency wavenumber spectra taken during the duck experiment on low frequency and incident band longshore and across shore hydrodynamics delilah at duck north carolina these frequency wavenumber spectra show a general dominance of edge waves traveling with the longshore current in cases where the current shear is strong relative to the bottom slope the location of edge wave trapping changes as the longshore current profile changes with the tide when the bar was well defined the longshore current shear was rarely strong enough to influence the existence or location of edge wave trapping but could have a enhancing effect while past work i e schonfeldt 1995 has suggested that edge waves trapped and amplified on longshore currents may cause bars to form on plane beaches and bars to move on barred beaches these observations suggest that under most conditions edge wave trapping on longshore currents is a subtle effect modulated by sea level changes and is unlikely to produce significant morphological change unless the current is very strong

modification of edge waves by barred beach topography a numerical model to calculate edge wave profiles and wavelengths for arbitrary bottom profiles and given frequency was developed utilizing the model wave equation of berkhoff 1972 following smith and sprinks 1975 surface profiles and longshore wave numbers were found for fixed wave periods using an eigenvalue approach the model was verified by calculating profiles and dispersion for a plane beach of mild slope and comparing results with the analytic solution of eckart 1951 and ursell 1952 the model was then applied to the case of a beach with multiple offshore bars with bar positions determined by the antinodal positions of a standing incident wave of a set frequency calculation of surface profiles for a range of frequencies using a fixed beach indicated that antinodes of the edgewave profiles tend to be trapped by the bars suggesting the possibility that bars affect the long wave climate in such a way as to lead to self maintenance over extended periods no effect of the bars on the dispersion relationship was found and it was concluded that for the particular beach profile used the edge waves respond to the mean beach slope over the offshore length of the profile

reconstruction of the landeskrone scoria cone in the lusatian volcanic field eastern germany long term degradation of volcanic edifices and implications for landscape evolution abstract remnants of numerous monogenetic volcanoes are preserved in the central european volcanic province cevp the landeskrone hill a monogenetic scoria cone in the lusatian volcanic field is reconstructed here this was done using measurements of the dip of columnar jointing of lava lake basalts and the detailed mapping of the volcaniclastic rocks the reconstruction implies a large scoria cone and a lava lake filling the crater with a thickness of more than 110 m volcanic activity was characterized by strombolian eruptions possibly after an opening phreatomagmatic phase a scoria cone was developed in an initial maar diatreme volcano a late phreatomagmatic phase could explain the unusual modified crater of the scoria cone the conduit was emptied and so the crater was widened in this late eruption stage an increased magma flux probably induced a change in the eruptive style and finally the crater was filled in a single event by a lava lake this effusive phase completed the multi stage volcano development the 34 ma old volcano is an excellent example for the persistence of relicts of monogenetic volcanoes over a long period of time there are some other remnants of monogenetic scoria cones that survived degradation processes in the area with respect to the present surface the reconstruction of the paleosurface implies low uplift and erosion rates of about 3 mm ka since the oligocene these denudation values support and expand on the previously published fission track data prior to the upper cretaceous and glacial sedimentological data on neotectonic movements since the middle pleistocene the erosion rate estimated by physical volcanological data implies stagnation of tectonic uplift from the upper paleogene to the middle neogene and a reactivation of tectonic movement for the lusatian massif in the middle pleistocene thus the reconstructed edifice provides a powerful tool for the study of landscape evolution by clearly defining the characteristics of the paleosurfaces at certain times

surface morphology of columnar joints and its significance to mechanics and direction of joint growth columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes new observations show that each band contains a single plumose structure and thus represents an individual crack or joint segment formed during a discrete growth event analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies columnar joints form by nucleation and growth of new cracks on the edges of older cracks each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face where thermal stress is concentrated inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle ductile transition of lava outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava cracks often diverge slightly from the planes of previous cracks probably because of spatial and temporal changes in directions of local principal stresses mechanical interaction causes a diverging crack to overlap curve toward and usually intersect the previous crack behind its edge leaving a blind tip that points in the overall growth direction of the columnar joints this and other directional criteria are applied to determine joint growth patterns in several lava flows of the western united states in two tiered and multi tiered flows downward growing columnar joints usually meet upward growing joints well below the middle of the flows which indicates very rapid cooling of upper portions relative to lower portions this supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows whereas conduction is probably the dominant cooling mechanism at the bases

slip partitioning and inclined dextral transpression along the zagros thrust system iran abstract flexural duplex structures and shear zones are well developed in the heneshk area in southwestern iran the zagros thrust system in this area consists of eight sheets of nw striking ne dipping dextral strike slip duplexes that are linked with imbricate fans and oblique slip thrusts the zagros thrust system is a portion of the internal zone of the zagros orogenic belt this internal zone is characterized by penetrative plastic deformation and metamorphism based on the kinematic vorticity number w k 0 73 0 02 calculated from quartz porphyroclasts of the goshti shear zones along the zagros thrust system the estimated angle between the maximum instantaneous strain axis isa and the transpressional zone boundary is 33 the estimated angle of the plate convergence or the flow apophysis of the displacement field is 25 the presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence the mean estimated finite deformation w m value indicates relative contributions of 47 pure shear and 53 simple shear for the deformation which involved both strike slip and oblique slip displacements in this inclined transpression about 40 strike slip partitioning was required to accommodate the finite strain and re orientation of instantaneous strain

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

v pull apart microstructures a new shear sense indicator abstract granitoids deformed in a low grade shear zone show conspicuous development of asymmetric pull apart microstructures in feldspars characterized by v shaped gaps between the separated fragments v pull aparts they occur beside active shear planes and result from rigid body rotation of one fragment relative to the other the shear sense in the adjacent flow surface can be easily determined through the asymmetry normally shown by v pull aparts where the fracture wall at a lower angle to the shear plane indicates which fragment has rotated synthetically to the adjacent shearing another diagnostic feature is the foliation deflection that occurs adjacent to the rotated fragment the v shaped gaps are infilled by recrystallized quartz mica aggregates showing quartz c axis patterns indicative of strong participation of crystal plastic processes in these domains evidence of crack seal by precipitation c axes parallel to the direction of separation have also been found the rigid body rotation indicated by these microstructures worked as a mechanism of removal of protruding porphyroclast edges enabling the smoothing of the shear planes rigid body rotation should operate in circumstances where other mechanisms such as dissolution or crystal plasticity did not succeed in eliminating undulations along the shear planes

numerical modeling of infragravity wave response during delilah 1 the nearshore circulation model shorecirc is applied to simulate the infragravity wave conditions at two intervals during the 1990 delilah campaign at duck n c the model has been extended to solve the energy equation for random short waves on the timescale of the wave groups which acts as a wave driver for the long wave model to simulate the wave conditions an algorithm has been derived which synthesizes a directional offshore wavefield from measured data including the bound directional low frequency components the model data comparison shows that the mean short wave transformation and mean longshore current are predicted well the structure of the infragravity spectrum as measured in the nearshore array is correctly represented albeit that the energy levels are underpredicted in most locations the 2 d frequency longshore wavenumber spectra show that the infragravity wave energy consists mostly of edge waves and that the computations qualitatively resemble the measurements it is shown that for 1 day bar trapped edge waves are present the model is also used to show that the influence of the longshore variability is small for this particular beach and that the directional spreading in the offshore wave field is critical in predicting the nearshore infragravity wave conditions correctly edge waves which propagate against the current are generated by the directionally broad short wave forcing and to a lesser extent by back scattering the nonlinear terms are relatively unimportant for the prediction of the infragravity wave climate the cross shore gradient of the onshore directed radiation stress is the most important of the forcing terms

strombolian and effusive activity as precursors to phreatomagmatism eruptive sequence at maars of the pinacate volcanic field sonora mexico the pinacate volcanic field sonora mexico contains 400 cinder cones and eight maars it is noteworthy that most of the maar forming phreatomagmatic eruptions were immediately preceded by effusive and strombolian activity rather than occurring when magma first approached the surface the strombolian activity may have facilitated access of groundwater to the conduits in this arid region the field evidence suggests that phreatomagmatism is inhibited unless the magma flux is low relative to the rate of water supply and unless the top of the magma column has subsided probably below the water table the latter condition is difficult to prove in the absence of direct observation and alternative hypotheses involving disturbance of the conduit system are considered the spatial distribution of maars in the pinacate and the lithology of their associated tuffaceous ejecta both may reflect the course of an ancient river channel whose permeable gravels were pierced by the magmatic conduits

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent