phreatomagmatic to strombolian eruptive activity of basaltic cinder cones montana los erales tenerife canary islands abstract phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide such fuel coolant reactions change the eruptive dynamics thus generating particles that reflect the degree of explosivity different eruptive phases may thus be identified from deposits allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future a detailed study of the deposits from montana los erales a 70 m high quaternary cinder cone belonging to a rift related chain of vents in the bandas del sur region in southeast tenerife was undertaken field observations on excavated sections and sem analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase through a transitional stage to one that was entirely strombolian to investigate the causes and the nature of these changes in eruptive style products from each major unit were analysed for their morphology using hand specimen observations secondary electron microscopy backscatter electron microscopy and reflected light microscopy to examine fragment size variation fragment morphologies vesicularity and the level of secondary hydrous alteration e g palagonitisation and zeolitisation study results demonstrate that the initial phase of activity was largely driven by magma water interaction where magma may have interacted with a lens of fresh ground or surface water with proceeding eruptive activity the water became exhausted giving rise to an entirely strombolian eruptive style examples of phreatomagmatic activity that occur on typical rift related basaltic vent alignments are not infrequent in the canary islands these vent systems usually erupt in strombolian fashion producing scoria and lava flows that do not generally extend far beyond the vent area however aligned feeders may intersect different strata structural features and different hydrological situations thus giving rise to activity that is less predictable in setting intensity and duration the occurrence of phreatomagmatism in an otherwise low explosivity basaltic eruptive environment increases the need for awareness of the geo hydrological situation in volcanically active areas studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment especially in densely populated areas like the canary islands

the mount hay block central australia another puzzle piece for paleo mesoproterozoic tectonic history abstract the 100 km 2 mount hay block arunta region central australia is a 15 km thick crustal cross section comprised of granulite facies tectonites containing a tectonothermal record from ca 1803 to ca 1552 ma as demonstrated by the combination of prior work and new monazite geochronology and thermobarometry igneous and sedimentary protoliths formed ca 1803 1798 ma during 1810 1790 ma stafford tanami time between ca 1790 and 1740 ma nearly continuous magmatism in the aileron province and possibly ages of recrystallized zircon and monazite in the mount hay block record subduction along the southern margin of the proterozoic australia mawson continent geologic mapping and zircon and monazite geochronology demonstrate that the mount hay block was penetratively deformed between ca 1720 ma and 1700 ma during 1735 1690 ma strangways time this includes the 8 km thick mount hay sheath fold which records ne sw subhorizontal shear once paleozoic tilting of the mount hay block is removed while still in the deeper crust 798 33 c 7 6 0 7 kbar fabrics along the northern edge of the mount hay block were transposed by the cross cutting 7 km thick capricorn ridge shear zone at ca 1551 7 5 5 ma during chewings time once restored the capricorn ridge shear zone records ne sw extension a jump in strain rate while at high temperatures 725 c initiated severe localization accommodated initially by pseudotachylite formation and cataclasis then mylonitization during fe rich fluid flux shearing may have continued to pressure temperature conditions as low as

seismic reflection profiling in the proterozoic arunta block central australia processing for testing models of tectonic evolution abstract the region extending from the northern amadeus basin to the northern arunta province is characterised by a large change in bouguer gravity 1400 s 2 abrupt changes in teleseismic travel time residuals and large thrust structures in which rocks from the lower crust are now exposed at the surface deep seismic reflection data other geophysical data and geological mapping within this region have contributed to the development of a crustal model that would result from thick skinned tectonic processes in which deformation occurred along moderately dipping thrusts or shear zones that extended from the surface down to at least the crust mantle boundary two important features of this model have emerged with the help of unconventional stacking techniques the first is a series of reflections from the redbank deformed zone that can be traced from surface outcrop to depths of more than 30 km the second is evidence of a fault extending through the crust and displacing the moho

towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

eruption processes and deposit characteristics at the monogenetic mt gambier volcanic complex se australia implications for alternating magmatic and phreatomagmatic activity the 5 ka mt gambier volcanic complex in the newer volcanics province australia is an extremely complex monogenetic volcanic system that preserves at least 14 eruption points aligned along a fissure system the complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner the facies are 1 coherent to vesicular fragmental alkali basalt effusive hawaiian spatter and lava flows 2 massive scoriaceous fine lapilli with coarse ash strombolian fallout 3 bedded scoriaceous fine lapilli tuff violent strombolian fallout 4 thin medium bedded undulating very fine lapilli in coarse ash dry phreatomagmatic surge modified fallout 5 palagonite altered cross bedded medium lapilli to fine ash wet phreatomagmatic base surges and 6 massive palagonite altered very poorly sorted tuff breccia and lapilli tuff phreato vulcanian pyroclastic flows since most deposits are lithified to quantify the grain size distributions gsds image analysis was performed the facies are distinct based on their gsds and the fine ash to coarse fine ash ratios these provide insights into the fragmentation intensities and water magma interaction efficiencies for each facies the eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles except for a magmatic horizon of effusive activity occurring at both ends of the volcanic complex simultaneously the eruption foci are located along nw se trending lineaments indicating that the complex was fed by multiple dykes following the subsurface structures related to the tartwaup fault system possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates viscosity crystallinity degassing and magma discharge rate as well as hydrological parameters

understanding the storage conditions and fluctuating eruption style of a young monogenetic volcano blue lake crater 3 ka high cascades oregon abstract blue lake crater 13 m s this study illustrates the pre eruptive and eruptive complexities of monogenetic volcanoes and highlights the minimal warning that may precede future eruptions

evolution of the canadas edifice and its implications for the origin of the canadas caldera tenerife canary islands the volcano stratigraphic and geochronologic data presented in this work show that the tenerife central zone has been occupied during the last 3 ma by shield or central composite volcanoes which reached more than 3000 m in height the last volcanic system the presently active teide pico viejo complex began to form approximately 150 ka ago the first canadas edifice ce volcanic activity took place between about 3 5 ma and 2 7 ma the ce i is formed mainly by basalts trachybasalts and trachytes the remains of this phase outcrop in the canadas wall cw sectors of la angostura 3 5 3 0 ma and 3 0 2 7 ma boca de tauce 3 0 ma and in the bottom of some external radial ravines 3 5 ma the position of its main emission center was located in the central part of the cc the volcano could have reached 3000 m in height this edifice underwent a partial destruction by failure and flank collapse forming debris avalanches during the 2 6 2 3 ma period the debris avalanche deposits can be seen in the most distal zones in the n flank of the ce i tigaiga breccia a new volcanic phase whose deposits overlie the remains of ce i and the former debris avalanche deposits constituted a new volcanic edifice the ce ii the dyke directions analysis and the morphological reconstruction suggest that the ce ii center was situated somewhat westward of the ce i reaching some 3200 m in height the ce ii formations are well exposed on the cw especially at the el cedro 2 3 2 00 ma sector they are also frequent in the s flank of the edifice 2 25 1 89 ma in tejina 2 5 1 87 ma as well as in the tigaiga massif to the n 2 23 ma during the last periods of activity of ce ii important explosive eruptions took place forming ignimbrites pyroclastic flows and fall deposits of trachytic composition their ages vary between 1 5 and 1 6 ma adeje ignimbrites to the w in the cw the upper ucanca phonolitic unit 1 4 ma could be the last main episode of the ce ii afterwards the can adas iii phase began it is well represented in the cw sectors of tigaiga 1 1 ma 0 27 ma las pilas 1 03 ma 0 78 ma diego hernandez 0 54 ma 0 17 ma and guajara 1 1 ma 0 7 ma the materials of this edifice are also found in the se flank these materials are trachybasaltic lava flows and abundant phonolitic lava and pyroclastic flows 0 6 ma 0 5 ma associated with abundant plinian falls the ce iii was essentially built between 0 9 and 0 2 ma a period when the volcanic activity was also intense in the dorsal edifice situated in the easterly wing of tenerife the so called valleys of la orotava and gu imar transversals to the ridge axis also formed during this period in the central part of tenerife the ce iii completed its evolution with an explosive deposit resting on the top of the ce for which ages from 0 173 to 0 13 ma have been obtained the cc age must be younger due to the fact that the present caldera scarp cuts these deposits on the controversial origin of the cc central vertical collapse vs repeated flank failure and lateral collapse of mature volcanic edifices the data discussed in this paper favor the second hypothesis clearly several debris avalanche type events exist in the history of the volcano but most of the deposits are now under the sea the caldera wall should represent the proximal scarps of the large slides whose intermediate scarps are covered by the more recent teide pico viejo volcanoes

preliminary deep reflection studies in the arunta block central australia summary in 1985 near vertical incidence reflection profiling was carried out across the arunta block in central australia this region consists of exposed proterozoic metasediments granites and granulites there is usually a limited sedimentary coverage generated by deep weathering the seismic sections for the deep crust are markedly different from those previously recorded in eastern australia where there is extensive sedimentary cover one of the striking features is the presence of energy with frequencies as high as 100 hz at two way times of 5 6 s reflections are found throughout the crust and there is no zone that can be characterised as non reflective the strongest reflectors commonly lie in the intervals around 4 6 s and 8 11 s and display significant dip individual shot records show fairly rapid variations in amplitude and waveform within a reflection band and the correlation between records from adjacent shots can also be somewhat limited such features are not well suited to the application of standard processing techniques designed for subhorizontal structures and call into question the utility of conventional stacking the character of the reflections changes markedly with varying frequency which suggests that they arise by interference phenomena probably associated with laterally varying lamellar structures

complex interaction between strombolian and phreatomagmatic eruptions in the quaternary monogenetic volcanism of the catalan volcanic zone ne of spain the catalan volcanic zone cvz at the ne of the iberian peninsula is one of the quaternary alkaline volcanic provinces of the european rifts system the cvz has been active during the last 12 ma despite the fact that this volcanism is significant in extension and volume and that eruptions have also occurred in holocene times it is mostly unknown compared to the contemporaneous alkaline volcanism in other parts of western and central europe volcanism younger than 0 5 ma is mostly concentrated in an area of about 100 km2 located between the main cities of olot and girona this basaltic volcanic field comprises more than 50 monogenetic cones including scoria cones lava flows tuff rings and maars magmatic eruptions range from hawaiian to violent strombolian phreatomagmatism is also common and has contributed to the construction of more than a half of these volcanic edifices frequently associated with the strombolian activity but also independently giving rise to a large variety of eruptive sequences we describe the main characteristics of this volcanism and analyse in particular the successions of deposits that form some of these volcanoes and discuss the potential causes of such a wide diversity of eruptive sequences we find that the main cause of such complex eruptive behaviour resides in the stratigraphic structural and hydrogeological characteristics of the substrate above which the volcanoes were emplaced rather than on the compositional characteristics of the erupting magma as they do not show significant variations among the different volcanoes studied

structure and evolution of the intracratonic basins of central australia summary the central australian geology is dominated by a series of parallel sedimentary intracratonic basins whose history spans time from the late proterozoic to the carboniferous the basins are separated by regions of exposed proterozoic crust that have a history of significant uplift the formation of the basins cannot be explained by conventional thermal or stretching mechanisms nor does the foreland basin model provide an adequate explanation for their occurrence the geophysical observations particularly gravity point to a structure whereby the crust is considerably thicker under the basins than it is under the regions where basement rock is exposed this suggests that the crust as a whole is involved in the process that led to the basin formation a mechanical model is developed which is based on the supposition that the lithosphere has been in a state of horizontal compression for long time intervals the lithosphere is considered as a viscoelastic plate that has been subjected to a small normal load distributed in an irregular manner under compression initial deflections due to this load are magnified according to the plate s physical properties the magnitude of the compressive force and the wavelength spectrum of the initial perturbations because of the viscosity the deflections grow with time at a rate that is determined by the balance of forces acting on the plate and by the lithosphere s rheology both the elastic and viscous deformations are much enhanced if erosion of the uplifted areas occurs with the sediments deposited into the downwarps these deformations take place at stresses that are much less than the buckling stresses as bending stresses increase with time failure by thrust faulting is predicted to occur within the basins with the exposed basement being thrust over the basins the model has been applied to the central australian region starting at about 900myr ago the first basins to develop are the officer and amadeus basins with uplift occurring in the intervening musgrave block initial rates of uplift and basin subsidence are small but they can increase rapidly with time if the compressive force is main

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

experiments on rift zone evolution in unstable volcanic edifices large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses flank deformation is often caused by slip along a decollement within or underneath the volcanic edifice we studied how such a stressed volcanic flank may bias the rift zone development the influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence here our analog experiments provide new insights we injected colored water into gelatin cones and found systematic orientations of hydro fractures dikes propagating through the cones at the base of the cone diverse friction conditions were simulated by variation of the basal creep conditions we modeled radial dike swarms collinear rift zones and three armed rift systems it is illustrated that a single outward creeping flank is sufficient to modify the entire rift architecture of a volcano the experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano s architecture

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

teleseismic travel time anomalies and deep crustral structure in central australia summary teleseismic travel times have been recorded along two lines of instruments traversing the amadeus basin and southern arunta block of central australia for the purpose of examining the deep crustal and upper mantle structure underlying these features pronounced differential travel times approaching 1 5 s and with major azimuthal variation have been recorded most of the useful observations originated from events to the north japan marianas and to the east fiji tonga at epicentral distances of 40 65d a few observations of southerly azimuth events macquarie ridge and south sandwich islands were also recorded the inferred structure beneath the basin and block exhibits major lateral variations the essential aspects of which are 1 a zone of relatively high velocity material dipping steeply northwards such that the continuation of its southern edge to the surface coincides approximately with a mylonitic shear zone known as the redbank deformed zone 2 a steeply dipping zone of relatively low velocity material immediately to the south of this shear zone and partly underlying the high velocity zone if the travel times are interpreted in terms of an undulating moho variations in moho depth of 20 km occur over horizontal distances of less than 50 km with relatively shallow moho depths occurring immediately to the north of the redbank deformed zone such an interpretation is consistent with the gravity anomalies observed across the geological structures the inferred structure is out of hydrostatic equilibrium and departures from local isostasy approach 150 mpa these stress differences must have persisted during the 300 ma following the alice springs orogeny that led to the present crustal structure evidence in the form of the surface topography and seismicity suggests that some stress relaxation has occurred and still is occurring but not at the shear zone itself at the time of the orogeny the structure may have been close to isostatic equilibrium but as erosion took place complete local rebound did not occur either because horizontal compressive forces still operated or because the strength of the lithosphere increased with time

tectonic entrapment and its role in the evolution of continental lithosphere an example from the precambrian of western canada new geophysical data acquired over the buried crystalline basement of western canada provide constraints on the history of tectonic assembly of the western canadian shield in the interval 1 75 1 85 ga specifically these data provide new perspectives on the evolution of an archean continental fragment heame province that was trapped in a tectonic vise between coeval orogenic belts that dipped beneath the hearne province the trans hudson orogen developed along the southeastern margin of the hearne province as a series of ocean floor oceanic arc and arc marginal basins were telescoped and thrust obliquely beneath the hearne along the northwest edge of the hearne collapse and subduction of a narrow marginal basin now marked by the subsurface extension of the snowbird tectonic zone led to formation of magmatic arc and collision of older proterozoic terranes the hearne province itself is characterized by regional granulite grade metamorphism and evidence of extensive and pervasive partial melting of the crust the internal character of the hearne province seen on crustal seismic reflection profiles is that of a crustal scale structural fan with reflection fabrics that verge toward the bounding orogens the deformation of the hearne is predominantly of paleoproterozoic age and constitutes a thorough reworking of this formerly archean crustal domain over a distance of more than 600 km across strike entrapment and thermal weakening of the hearne resulted from mechanical coupling of inferred buoyant subduction collision zones and removal of or modification of archean lithospheric mantle that may have originally formed the keel to the hearne long period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day hearne province which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination convective removal of thickened lithosphere in the proterozoic tectonic entrapment as illustrated by the proterozoic structural and thermal evolution of the hearne province crust and subcontinental mantle may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents

reflection diffraction seismic pattern at crustal suture zones seismic reflection profiling across strong dip more than 25 suture zones between different crustal terranes or provinces reveals a distinct seismic reflectivity pattern of dipping reflections and associated diffractions suture zones in areas dominated by thin skinned tectonics do not show this pattern profiles from southern germany deutsches kontinentales reflexionsseismisches programm great britain british institutions reflection profiling syndicate and north america consortium for continental reflection profiling and u s geological survey crossing major sutures are examined with respect to dipping reflections in the upper crust and accompanying diffractions in the lower crust the strong resemblance of seismic patterns suggests a similar origin for these structures

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

phreatomagmatic eruptive and depositional processes during the 1949 eruption on la palma canary islands abstract in 1949 a 5 week long magmatic and phreatomagmatic eruption took place along the active volcanic ridge of la palma canary islands two vents duraznero and hoyo negro produced significant pyroclastic deposits the eruption began from duraznero vent which produced a series of deposits with an upward decrease in accidental fragments and increase in fluidal ash and spatter together inferred to indicate decreasing phreatomagmatic interaction hoyo negro erupted over a 2 week period producing a variety of pyroclastic density currents and ballistic blocks and bombs hoyo negro erupted within and modified an older crater having high walls on the northern to southeastern edges southwestern to western margins of the crater lay 50 to 100 m lower strongly contrasting deposits in the different sectors n se vs sw w were formed as a result of interaction between topography weak eruptive columns and stratified pyroclastic density currents tephra ring deposits are thicker and coarser grained than upper rim deposits formed along the higher edges of the crater and beyond the crater margin valley confined deposits are thicker than more thinly bedded mantling deposits on higher topography these differences indicate that the impact zone for the bulk of the collapsing tephra laden column lay within the crater and that the high crater walls inhibited escape of pyroclastic density currents to the north and east the impact zone lay outside the low sw w rims however thus allowing stratified pyroclastic density currents to move freely away from the crater in those directions depositing thin sections

eruptive and structural history of teide volcano and rift zones of tenerife canary islands the teide and pico viejo stratocones and the northwest and northeast rifts are products of the latest eruptive phase of the island of tenerife initiated with the lateral collapse of its northern flank that formed the las canadas caldera and the icod la guancha valley ca 200 ka the eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events a set of 54 new 14 c and k ar ages provides precise age control of the recent eruptive history of tenerife particularly teide volcano the third highest volcanic feature on earth 3718 m above sea level 7 km high and unique in terms of its intraplate setting the development of the teide pico viejo volcanoes may be related to the activity of the northwest and northeast rifts volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth nested within the collapse embayment of an increasingly higher central volcano with progressively differentiated magmas the coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area where the differentiated stratocones developed current volcanic hazard in tenerife is considered to be moderate because eruptive frequency is low explosivity is modest and the eruptive activity of the teide stratocone seems to have declined over the past 30 k y with only one eruption in this period 1150 yr b p

the 2011 2012 submarine eruption off el hierro canary islands new lessons in oceanic island growth and volcanic crisis management forty years after the eruption of the teneguia volcano on la palma 1971 the last volcanic event in the canary islands a submarine eruption took place in 2011 off shore el hierro the smallest an

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

el estribo volcanic complex evolution from a shield volcano to a cinder cone patzcuaro lake michoacan mexico abstract el estribo volcanic complex evc is located in the northern part of the michoacan guanajuato volcanic field within the trans mexican volcanic belt tmvb el estribo is located at the southern edge of the e w patzcuaro fault that belongs to the patzcuaro jaracuaro graben a western extension of the e w morelia acambay fault system stratigraphy geochronology chemistry and mineral assemblages suggest that the volcanic complex was constructed in two periods separated by a 100 ka volcanic hiatus a emission of lava flows that constructed a shield volcano between 126 ka and b mixed phreatomagmatic to strombolian activity that formed a cinder cone 28 ka the magmas that fed these monogenetic volcanoes were able to use the same feeding system the cinder cone itself was constructed by strombolian fallouts and remobilized scoria beds followed by an erosion period and by a mixed phreatomagmatic to magmatic phase strombolian fallouts ending with lava flows soft sedimentary deformation of beds and impact sags cross bedding as well as pitting and hydrothermal cracks found in particles support the phreatomagmatic phase the erupted magmas through time ejected basaltic andesitic lava flows 56 21 58 88 sio 2 that built the shield volcano and then basaltic andesitic scoria 57 65 59 05 sio 2 that constructed the cinder cone although they used the same feeding system the geochemical data and the mineral chemistry of the magmas indicate that the shield volcano and the cinder cone were fed by different magma batches erupted thousands of years apart therefore the location of el estribo volcanic complex along an e w fault that has generated two sector collapses of the shield volcano to the north may be directly linked to this complex redistribution of the magmatic paths to the surface our findings show that magmatic feeding systems within monogenetic volcanic fields could be long lived questioning the classic view of the monogenetic nature of their volcanoes and yielding information about the potential volcanic risk of these settings usually considered risk free

determination of crustal structure in central australia by inversion of traveltime residuals summary teleseismic traveltime anomalies for events recorded on portable seismograph arrays across the northern and southern margins of the amadeus basin in central australia are inverted to obtain a picture of the velocity structure at a scale on the order of 5 10 km a modifled srnr algorithm is used to iteratively redistribute traveltime anomalies along incoming ray paths subject to regularization constraints to obtain a direct estimate of the lithospheric velocity freld beneath the arrays model structure is assumed fo be 2 d based on the strong east ri est strike of surface geology and gravity structure in the area spurious structures commonly generated in snr inversions are suppressed using a filter based on the density ofrays a weighting towards near surface structure is also applied to test the robustness of the inference of deep structures the most prominent feature of the solutions for all three lines is a sharp interface between slow and fast regions dipping away from the basin and beneath the adjacent basement blocks the fastest region on the upper side of the interface corresponds to a belt of high grade metamorphic rocks where they crop out at the surface the interface between slow and fast regions extends to at least 50 km depth in all cases dipping at about 50 60 on the northern lines and 60 80 on the southern 1ine on the northern lines the interface can be correlated with the redbank thrust identified in deep seismic reflection data but the velocity interface seems to dip more steeply secondary features on the profiles include a possible change in dip of the interface on the southern line and a more complex structure on one of the northern lines suggesting that one or more steeper faults cut through the hanging wall of the thrust there predicted gravity profiles derived from the velocity sections using empirical velocitydensity relations resemble observed gravity surprisingly well in their style and approximate magnitude providing support for the general features of the models the results are consistent with a thick skinned thrusting model for the deformation of the central segment of the basin margins and indicate that the style of deformation in the late proterozoic cambrian compressional event at the southern edge of the amadeus basin was similar to that in the later alice springs orogeny in the north

pleistocene forest on tenerife s southern coast a case study of montana negra montana negra is a 121 m cinder cone in the bandas del sur region of southern tenerife formed in the middle pleistocene it comprises alternating phonolitic pumice deposits and scoria layers the latter are extremely fossiliferous with good taphonomical fidelity 40 ar 39 ar age determination provides new dates of 302 7 6 ka and 299 9 11 4 ka for the lower and upper aldea blanca pumice fall deposits respectively this chronological constraint allows comparison of the palaeo habitat with the global climate at the time of pyroclastic activity abundant terrestrial gastropod species and rare disarticulated coleoptera fragments are to be found the occurrence of the endemic semi slug genus plutonia family vitrinidae is significant in indicating a woodland habitat in the region during the middle pleistocene we suggest that this may have been forest possibly dominated by laurel which is in stark contrast to the present day semi desert copyright c 2011 john wiley sons ltd

teleseismic travel time anomalies and crustal structure in central australia abstract teleseismic p wave travel time anomalies of more than 0 5s have been recorded in central australia between sites separated by less than 100 km late arrivals occur at sites that are located within sedimentary basins and early arrivals occur at sites located on the exposed and eroded basement early arrivals correspond to relative gravity highs and late arrivals correspond to gravity lows the differential travel times cannot be attributed to near surface crustal structure alone the most straightforward interpretation is in terms of a moho whose depth varies by about 10 km from a mean value and which is deepest below the basins this interpretation is consistent with the intracratonic basin evolution model proposed by lambeck

identifying magma water interaction from the surface features of ash particles the deposits from explosive volcanic eruptions those eruptions that release mechanical energy over a short time span1 are characterized by an abundance of volcanic ash2 3 this ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre existing crust host rocks interactions between rising magma and the hydrosphere oceans lakes and ground water play an important role in explosive volcanism4 5 because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy although the relative proportion of magma to host rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits2 3 4 5 here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments we find that surface features of ash grains can be used to identify the dynamic contact of magma with liquid water the abundance of such ash grains can then be related to the water magma mass ratios during their interaction

thermal and structural evolution of the intracratonic arltunga nappe complex central australia the thermal and structural evolution of a paleozoic thrust system in central australia the arltunga nappe complex has been examined through field mapping petrographic microstructural and chemical analyses k ar and 40ar 39ar thermochronology and multi diffusion domain time temperature modeling of k feldspar 40ar 39ar data the arltunga nappe complex contains three distinct tectonic units bounded by regional faults and shear zones that have undergone contrasting deformational and metamorphic histories the cooling history of the three tectonic units has been constrained by k ar and 40ar 39ar isotopic age data from k feldspars micas and amphiboles results are consistent with a piggyback history of crustal imbrication spanning over 100 m y with the hottest rocks uplifted and cooled first and with imbrication acting progressively toward the foreland implications for the intracratonic paleozoic tectonics of central australia are 1 uplift within the structurally highest tectonic unit was initiated before 400 ma 2 continued piggyback imbrication footwall failure and uplift and exhumation of the crust resulted in duplexing in the lowest structural unit at approximately 325 300 ma 3 the contrast in thermal history between the tectonic units indicates that once widely separated crustal segments were juxtaposed during thrusting above a hinterland dipping megathrust resulting in 85 km of crustal shortening

rothenberg scoria cone east eifel a complex strombolian and phreatomagmatic volcano rothenberg scoria cone eifel formed by an alternation of three strombolian and three phreatomagmatic eruptive phases eruptions took place from up to six vents on a 600 m long fissure building an early tuff ring and then two coalescing scoria cones strombolian volcanism dominated volumetrically as the supply of external water was severely limited magma water interaction only occurred during the opening stages of eruption at any vent when discharge rates were low and the fragmentation surface was below the water table the phreatomagmatic deposits consist of relatively well sorted fall beds and only minor surge deposits they contain juvenile clasts with a wide range of vesicularity and grain size implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions the transition from phreatomagmatic to strombolian eruption at any vent was rapid and irreversible and strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as by then the fragmentation surfaces were high in the growing cones and water was denied access to the magma the strombolian deposits are relatively homogenous consisting of alternating coarser and finer grained well sorted fall beds erupted during periods of open vent eruption and partial blockage of the vent respectively the intervals of strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent collapse of the growing cones and repeated recycling of clasts through the vent clear evidence of the instability of the rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones the close of strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters

cerro xalapaxco an unusual tuff cone with multiple explosion craters in central mexico puebla abstract the xalapaxco tuff cone is located on the northeast flank of la malinche stratovolcano in central mexico an unusually large number 10 of explosion craters concentrated on the central and on the uphill side of the cone expose alternating beds of stratified surge deposits and massive fall deposits the morphology of the cone and the characteristics of its deposits point to the involvement of significant quantities of groundwater during its eruption the phreatomagmatic eruptions which led to the cone s formation pierced an alluvial fan whose source is a glacially carved canyon near the summit of la malinche volcano the large canyon was cut during repeated glacial episodes the last of which ended ca 8500 years ago the present alluvial fan mostly consists of reworked glacio fluviatile andesite dacite material from la malinche rising magma encountered substantial amounts of groundwater within the limestone basement and in overlying intercalated pyroclastic and glacio fluviatile deposits of the alluvial fan short lived phreatomagmatic eruptions produced surge and airfall deposits xenoliths found in the cone beds are composed of dacite and andesite clasts limestone chert and rare ignimbrite fragments no juvenile material could be unequivocally identified but is represented most probably by porphyritic dacite similar in texture and composition to la malinche lavas the multiple craters were formed as a response to changes in water and magma supply during the short lived eruption hence the locations where ideal magma water ratios existed to fuel phreatomagmatic explosions shifted in time and space analysis of diameter depth ratios of the craters indicates that the activity shifted from the center of the cone to its periphery in the west due to the configuration of the hydrographic environment more groundwater flowing from la malinche was available from the fan on the uphill side than below the cone at later stages of the eruption the apparently anomalous position of the tuff cone on the slopes of a stratovolcano in a presently dry environment can be explained by more humid climatic conditions prevailing at the time of eruption

deep seismic profiling in central australia abstract deep seismic profiling undertaken in central australia comprised expanding spread reflection profiling long range refraction work and a small scale three dimensional refraction survey as well as the more usual near vertical incidence reflection profiling scismic reflection sections within the arunta block in central australia show abundant northerly dipping events that are interpreted as reflections from dipping faults many of which are evident in surface geological mapping the redbank deformed zone a major thrust feature has been imaged to depths of at least 30 km and defines a marked change in reflection character south of the redbank zone steep northerly dipping reflections are absent sub horizontal basement reflections are prevalent below the southern arunta province and amadeus basin seismic refraction profiles indicate that the crust is more than 50 km thick below both the boundary between the southern and central arunta provinces and the northern part of the amadeus basin expanding reflection spreads and a three dimensional refraction survey have added fine details to the results in regions of special geological interest and include the resolution of complicated velocity variations in the sedimentary rocks of the northern part of the amadeus basin and the measurement of anisotropy in the granulites of the central arunta province adjacent to the redbank zone

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

precambrian moho offset and tectonic stability of the east european platform from the urseis deep seismic profile deep seismic reflection data from the 1995 urals seismic experiment and integrated studies urseis transect of the southern urals document a distinct 5 km vertical offset of the moho beneath the uralian foreland here named the makarovo fault zone this offset does not disrupt the overlying riphean sedimentary section nor does it correlate with any major surface lineaments or known phanerozoic subduction features it is thus interpreted to be an archean or early proterozoic fault zone possibly a terrane boundary that disrupts an even older moho most important is the implication that this structure and the moho it offsets were not significantly modified by two subsequent cycles of continental collision that affected this region the baikalian 700 570 ma and the late paleozoic uralian 350 230 ma orogenic phases similarly the pronounced reflectivity of the moho beneath the uralian foreland must also be a precambrian signature not a young feature as would be implied by certain recent thermo rheological and reflectivity models that associate a reflective lower crust and moho with areas that underwent pronounced synorogenic or postorogenic collapse the makarovo fault zone provides yet another demonstration that relicts of ancient tectonic processes can survive for long periods of time in the lower crust and that the moho can be a key marker horizon in delineating the tectonic evolution of the lithosphere

deep reflection seismography in studying the earth s crust of the ukrainian shield abstract the results of studies of the ukrainian shield ush basic structures sura sinclinorium korsun novomirgorod pluton krivoy rog structure novoukrainka massif central and southern ush by the seismic reflected cdp method are considered and analysed the structures given show high effectivity of the seismic reflected cdp method in studying ush earth s crust structure the wave field is dominated by single reflections and their series associated with tectonic disturbances and contacts of rocks of different composition some of these results when compared with similar investigations in the region of post riphean consolidation show that there is much in common between the crystalline complexes of pre and post riphean consolidation in the peculiarities of spatial reflection distribution on the cdp sections similarity of reflectivity which we marked for the first time give us favourable perspectives for creating similar seismic models of the consolidated crust irrespective of the last folding age

implications for the early shield stage evolution of tenerife from k ar ages and magnetic stratigraphy abstract the combined use of field geology radioisotopic dating and magnetic stratigraphy applied to the old shield volcanoes of tenerife provides a reliable time framework for the early shield stage evolution of the island the greater part of this new set of ages obtained from sequences of lava flows is in agreement with the astronomical polarity time scale this approach illustrates that previous k ar data collected without a comprehensive stratigraphy should be viewed with caution and in some cases discarded altogether the shield volcanoes of tenerife encompass a relatively small number of magnetozones an observation consistent with the relatively short periods of growth shown by the new ages 1 2 my the island was constructed by the aggregation of three successive shields the roque del conde central shield between about 11 9 and 8 9 ma and the teno 6 2 5 6 ma and anaga 4 9 3 9 ma volcanoes this new oldest subaerial age of tenerife fits with the others obtained in the canaries in a clear west to east monotonous age progression one of the main restrictions for hotspot related island chains

stratigraphy of the bandas del sur formation an extracaldera record of quaternary phonolitic explosive eruptions from the las canadas edifice tenerife canary islands explosive volcanism has dominated the large phonolitic shield volcano of tenerife the las canadas edifice for the last 1 5 m y pyroclastic deposits of the bandas del sur formation are exposed along the southern flanks and record the last two of at least three long term cycles of caldera forming explosive eruptions each cycle began with flank fissure eruptions of alkali basalt lava followed by minor eruptions of basanite to phonotephrite lavas minor phonotephritic to phonolitic lava effusions also occurred on the flanks of the edifice during the latter stages of the second explosive cycle non welded plinian fall deposits and ignimbrites are the dominant explosive products preserved on the southern flanks of these a significant volume has been dispersed offshore many pyroclastic units of the second explosive cycle exhibit compositional zonation banded pumice occurs in most units of the third youngest explosive cycle and ignimbrites typically contain mixed phenocryst assemblages indicating the role of magma mixing mingling prior to eruption at least four major eruptions of the third cycle began with phreatomagmatic activity producing lithic poor accretionary lapilli bearing fallout and or surge deposits the repeated brief phase of phreatomagmatism at the onset of these eruptions is interpreted as reflecting an exhaustive water supply probably a small caldera lake that was periodically established during the third cycle accidental syenite becomes an increasingly important lithic clast type in ignimbrites up sequence and is interpreted as recording the progressive development of a plutonic complex beneath the summit caldera successive eruptions during each explosive cycle increased in volume with the largest eruption occurring at the end of the cycle more than ten major explosive eruptions vented moderately large volumes 1 ges 10 km 3 of phonolitic magma during the last two cycles culminating each explosive cycle was the emplacement of relatively large volume 5 10 km 3 ignimbrites with coarse vent derived lithic breccias interpreted to record a major phase of caldera collapse in the extracaldera record explosive cycles are separated by 0 2 m y periods of non explosive activity repose periods were characterized by erosion remobilization of pyroclastic deposits by discharge events and pedogenesis the current period of non explosive activity is characterized by the construction of the teide pico viejo stratovolcanic complex within the summit caldera this suggests that eruptive hiatuses in the extracaldera record may reflect effusive activity and stratovolcano or shield building phases within the summit caldera alternating effusive and explosive cycles have thus been important in the volcanic evolution of the las canadas edifice

hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on mars hydrovolcanism is a common natural phenomenon on earth and should be common on mars too since its surface shows widespread evidence for volcanism and near surface water we investigate fields of pitted cones in the nephentes amenthes region at the southernmargin of the ancient impact basin utopia which were previously interpreted as mud volcanoes the cone fields contain pitted and breached cones with associated outgoing flow like landforms based on stratigraphic relations we determined ahesperian or youngermodel age we test the hypothesis of a hydro volcanic origin based on a detailed morphological and morphometrical analysis and an analysis of the regional context an igneous volcanic origin of these cones as hydrovolcanic edifices produced by phreatomagmatic eruptions is plausible several lines of evidence suggest the existence of subsurface water ice the pitted cones display well developed wide central craters with floor elevations below the preeruptive surface their morphometry and the overall appearance are analogous to terrestrial tuff cones and tuff rings mounds that are also observed in the same region resemble terrestrial lava domes the hydrovolcanic interaction between ascending magma and subsurface water and or water ice may explain the formation of the pitted cones although other scenarios such as mud volcanism cannot be ruled out together with the mounds the cones might represent effusive and explosive edifices of amonogenetic volcanic field composed of lava domes tuff rings tuff cones and possibly maars

3d isochronal modelling of reflections from the deep crust application to reflection profiling in central australia abstract reflection profiling of the whole crust across ancient terranes encounters a range of interpretational problems and ambiguities especially with regard to the geometry of faults such problems can be addressed by seismic modelling with full inclusion of diffractions and a suitable modelling procedure for such three dimensional problems is the isochronal technique described by cao and kennett 1989 such modelling has been used to investigate the validity of some of the interpretations of a deep crustal refection profile in central australia involving planar faults of moderate dip penetrating most of the crust and considerable moho topography even with a relatively short 4 km field spread it would be possible to detect the reflected energy from faults with dips of about 40 the character of the reflections also suggests considerable variability in piysical properties within the fault zones the largest fault the redbank zone has significant displacement of the moho but most of crustal faults appear to sole out into the crust mantle interface this gives rise to an undulating mcho for which seismic modelling corresponds closely to the observed data

rift zone reorganization through flank instability in ocean island volcanoes an example from tenerife canary islands the relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented our field data aerial image analysis and 40ar 39ar chronology from anaga basaltic shield volcano on tenerife canary islands support a rift zone flank instability relationship a single rift zone dominated the early stage of the anaga edifice 6 4 5 ma destabilization of the northern sector led to partial seaward collapse at about 4 5 ma resulting in a giant landslide the remnant highly fractured northern flank is part of the destabilized sector a curved rift zone developed within and around this unstable sector between 4 5 and 3 5 ma induced by the dilatation of the curved rift a further rift arm developed to the south generating a three armed rift system this evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift and facilitates dike intrusions on the opposite side our study demonstrates a feedback relationship between flank instability and intrusive development a scenario probably common in ocean island volcanoes we therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability

subduction platform subsidence and foreland thrust loading the late tertiary development of taranaki basin new zealand borehole seismic and gravity data are used to investigate deformation of continental lithosphere at a miocene collisional zone deformation is manifested in the three following principal forms a long wavelength 500 km platform subsidence ascribed to mantle convection flexural deformation on a scale of 100 200 km due to crustal thrusting at the eastern boundary of the taranaki basin and a ductile thickening evident on the deep seismic section of taranaki basin that occurs on a scale of 10 km evidence for flexural deformation principally comes from the deep seismic section that shows a 150 km wavelength bending of the moho down toward the major zone of thrusting within the taranaki fault zone paleowater depths however provide evidence for an initial early miocene regional subsidence that is too long in wavelength to be explained by flexure induced from thrust sheet loading instead we propose that this broad platform subsidence was driven by loading from a deep source probably subduction induced flow in the mantle by 22 19 ma 1 2 km of water existed over most of the area now occupied by south taranaki basin by 19 17 ma the water depth in the zone east of the taranaki basin the taranaki fault zone had been replaced by rock due to submarine thrusting and crustal thickening this build up of submarine topography in the taranaki fault zone constitutes part of the load 25 8 mpa that created and maintains south taranaki basin gravity data place further constraints on loading at the thrust front and point to an additional intracrustal loading equivalent to 15 7 mpa over a 50 km wide zone this intracrustal load is explained as being due to thick skinned thrusting bringing denser lower crustal rocks nearer to the surface in the thrust zone the complete load on the taranaki foreland is therefore in three parts the submarine topographic load the intracrustal load and the loading of infilling sediments

phreatomagmatism and its relevance abstract phreatomagmatic explosive volcanism results from the interaction of magma with external water groundwater or surface water close to or at the earth s surface magma of any composition acid to ultrabasic may be involved in this type of explosive activity at present the relevance of phreatomagmatism is realized increasingly because it takes place in many diverse environments and affects not only small monogenetic volcanoes but also large volcanoes at some volcanoes as for example at lake taupo new zealand a volume of several tens of km 3 of magma may have been mixed with several km 3 of external water phreatomagmatism has thus become an important aspect in volcanic hazard assessments other fields involving the interaction of water with a melt are the study of nuclear and other industrial safety hazards of magma mixing of diamond carrying volcanism and of hydrothermal ore deposits as well as geothermal and groundwater reservoirs experiments on the dynamics of magma water interactions provide increasing insight into the principles and hazards of phreatomagmatic explosions this type of research will become a new field in experimental petrology

evaluation of morphometry based dating of monogenetic volcanoes a case study from bandas del sur tenerife canary islands morphometry based dating provides a first order estimate of the temporal evolution of monogenetic volcanic edifices located within an intraplate monogenetic volcanic field or on the flanks of a polygenetic volcano two widely used morphometric parameters namely cone height width ratio hmax wco and slope angle were applied to extract chronological information and evaluate their accuracy for morphometry based ordering based on these quantitative parameters extracted from contour based digital elevation models dems two event orders for the bandas del sur in tenerife canary islands were constructed and compared with the existing k ar paleomagnetic and stratigraphic data the results obtained suggest that the commonly used hmax wco ratio is not reliable leading to inappropriate temporal order estimates while the slope angle gives slightly better results the overall performance of such descriptive parameters was however generally poor i e there is no strong correlation between morphometry and age the geomorphic morphometric mismatches could be the result of 1 the diversity of syn eruptive processes i e diverse initial morphologies causing geomorphic morphometric variability 2 contrasting edifice specific degradation that depends partly upon the inner facies architecture of the volcanic edifices 3 various external environmental controls e g tephra mantling from pyroclastic density currents unrelated to the edifice evaluated and 4 differences in the scale resolution of input data the observed degradation trend and changes in morphometric parameters over time do not support a simple degradation model for monogenetic scoria cones volcanoes

moho offsets beneath the western ghat and the contact of archean crusts of dharwar craton india abstract we present the moho depth variation along a 600 km long profile from the west to the east coast of south india covering the passive continental margin and the western ghat escarpment created during india madagascar separation at 85 ma archean western and eastern dharwar craton and proterozoic basin the image is generated through three different approaches h vp vs stacking common conversion point ccp migration and inversion of teleseismic receiver functions at 38 locations the moho depth along the profile varies smoothly between 34 and 41 km except beneath the western ghat and at the contact of east and west dharwar craton where it is offset by up to 8 km the study suggests i the possible differential uplift of the western ghat as a consequence of india madagascar separation and the prominent role of deep crustal structure in the location of the escarpment compared to the surface process and ii presence of long lived steeply dipping fault separating the two distinct archean crustal blocks indicative of mechanically strong continental lithosphere beneath the dharwar craton

reconstruction of the 1706 montana negra eruption emergency procedures for garachico and el tanque tenerife canary islands in may 1706 after several earthquakes an eruptive fissure opened in the hills of the garachico district forming the montana negra volcano in 15 h the village of el tanque and part of the town of garachico including the harbour were buried under lava flows a week later another flow buried the village of san juan del reparo and much of the remaining town of garachico today over 6700 people live in the immediate vicinity of this former eruption although most of the population is concentrated in the main villages dwellings are located throughout the district and any future eruption will inevitably destroy property the reconstruction of the 1706 eruption shows that in any future event only a short time will be available to evacuate the population but it may be possible to try to prevent the effects of the flows

the intraplate character of supercontinent tectonics abstract for several decades geoscientists have recognised intraplate tectonic activity far from plate margins both from modern and ancient examples this apparent disconnect with the drivers of plate tectonics does not necessarily imply unconnected processes but rather an uncertainty in understanding exactly how these systems operate are the driving forces derived locally or do they propagate from plate margins how do these forces interact with a complex tectonic inheritance to generate the observed tectonism furthermore what novel approaches have been applied to understand these processes here we review the general literature and the contents of this special issue to develop some partial answers to these questions key observations include the critical importance of local lithospheric heterogeneities as a control on the mode of orogenesis and also the role of locally derived forces from mantle upwelling or from depositing thick piles of magmatic or sedimentary rocks these processes happen within the overarching tectonic setting provided by far field plate margins which show intimate links in both time and observed processes with many intraplate regions these insights would not be possible without a growing arsenal of geological geophysical and numerical methods that can be applied to intraplate regions and this is reflected in the varied approaches within the issue we envisage that this special issue will provide a stimulus for further progress in understanding intraplate tectonics

temperature patterns over the past eight centuries in northern fennoscandia inferred from sedimentary diatoms establishing natural climate variability becomes particularly important in remote polar regions especially when considering questions regarding higher than average warming we present a high resolution record of temperature variability for the past 800 yr based on sedimentary diatoms from a treeline lake in finnish lapland the bsizer multiscale smoothing technique is applied to the data to identify significant features in the record at different temporal levels the overall reconstruction shows relatively large multi centennial temperature variability with a total range of about 0 6 0 8 c at millennial scales the temperatures exhibit a statistically significant long term cooling trend prior to industrialization t 0 03 c century at the centennial timescale three warm time intervals were identified around ad 1200 1300 terminal phase of the medieval warm period mwp 1380 1550 and from 1920 until the present pronounced coolness occurred between ad 1600 and 1920 indicative of the little ice age lia at the decadal level certain shorter term climate excursions were revealed the warmest 10 30 yr nonoverlapping periods occurred in ad 1220 1250 1470 1500 and 1970 2000 respectively the classic events of mwp and lia are evident in our record as is also the 20th century warming

volcanic geomorphological classification of the cinder cones of tenerife canary islands spain this paper proposes a method to establish a morphological classification of tenerife s cinder cones on the basis of a dual analysis of qualitative existence geometry and disposition of craters and quantitative morphometric parameters major and minor diameters and cone elongation major and minor diameters and crater elongation the result obtained is a morphological classification of the cinder cones of tenerife which can be sub divided into four types ring shaped cones horseshoe shaped volcanoes multiple volcanoes and volcanoes without crater in tenerife there is a clear dominance of horseshoe shaped volcanoes 69 0 over ring shaped cones 13 1 volcanoes without craters 11 4 and multiple volcanoes 6 4 the classification presented in this paper is characterized by its simplicity which makes it possible to include all morphological types of volcanoes found in tenerife this fact also renders our classification a useful tool to apply in other both insular and continental volcanic areas to eventually analyze and systematize the study of eruptive edifices with similar traits

development of spheroidal composite bombs by welding of juvenile spinning and isotropic droplets inside a mafic eruption column abstract it is generally assumed that spheroidal volcanic bombs are formed during cooling of molten clots pulled up into spheres by the surface tension of the magma also volcanic bombs are regarded as non fragmental structures i e made of coherent material we describe non coherent spheroidal bombs of mafic composition from the tuff cinder cone of the cabezo segura volcano calatrava volcanic province spain that exhibit features consistent with welding of essentially non vesicular juvenile spherical droplets it is assumed that the eruption of a low viscosity magma containing suspended phenocrysts mantle clasts and cognate lithic clasts resulted in the efficient separation of volatiles and melt and the explosive fragmentation of the magma into different types of pyroclasts the finest magma fragments would yield melt blobs achneliths or lava spray ca 1 5 m small droplets largely made up also of melt and isolated solid clasts phenocryst microphenocryst and lithic the largest lava fragments incorporated phenocrysts microphenocrysts and lithic clasts all these types of clasts would be erupted in a rapid gas thrust that prevented rapid collision among particles massive aggregation and premature collapse by defluidization the rotation of the largest magma droplets during ascent in a fluidized system produced spinning droplets similar to pelletal lapilli the collision and aggregation of the fluidal clasts might start as soon as the pyroclasts gas mixture was thrust out and progressively decelerated a model is proposed for the formation of spheroidal composite bombs assuming fluidal clasts sintering either by coalescence or agglutination and welding of constituent pyroclasts inside the eruption column prior to bomb accumulation on the field

a volcanotectonic survey of ascraeus mons mars 1 ascraeus mons is one of the largest volcanoes on mars it is replete with well preserved features that can be used to understand its volcanotectonic evolution previous studies of this volcano focused on specific features and were limited by the quality and coverage of contemporary data our objective is to review and enhance the existing developmental model for ascraeus by considering all endogenic surface features on the volcano we surveyed the volcano s caldera complex flank terraces pit structures sinuous rilles arcuate grabens and small vents we report the spatial and temporal distributions of these features appraise their proposed formation mechanisms in light of our mapping results and propose a detailed geological history for ascraeus mons an initial shield building phase was followed by the formation of a summit caldera complex and small parasitic cones while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks an eruptive hiatus followed ending with the construction of expansive rift aprons to the northeast and southwest against later extensive flank resurfacing in the late amazonian continued flexure formed arcuate grabens concentric to the edifice localized eruption and surface flow of a fluid agent lava and or water from within the volcano then produced a population of rilles on the lower flanks finally in a change of flank tectonic regime from compression to extension pit crater chains and troughs developed on the main shield and rift aprons eventually coalescing to form large embayments at the northeast and southwest base of the volcano

characteristics of a kilometer scale high strain zone in the lower continental crust mt hay block central australia abstract a granulite facies high strain zone was studied on capricorn ridge part of the mt hay block granulites in the arunta inlier central australia deformation occurred in these primarily mafic composition granulites at lower crustal conditions 8 kb 800 c at 1740 1690 ma pervasive and consistently oriented foliation striking ssw and dipping steeply and lineations pitching steeply eastward occur within the shear zone the high strain zone is recognized by the presence of sl tectonites parallelism of grain shape fabrics and lithologic boundaries transposition of lithologic boundaries into parallelism with the foliation and the local occurrence of sheath like folds field based fabric mapping uses compositional foliation intensity as a proxy for finite strain to document strain variations within the capricorn ridge high strain zone there is no observed microstructural grain size reduction that corresponds to these strain variations fabric intensity increases from the centers of the major lithologic units towards the contacts 100 m scale strain localization within capricorn ridge occurs adjacent to major lithologic contacts indicating competency contrast during lower crustal deformation

growth structure instability and collapse of canarian volcanoes and comparisons with hawaiian volcanoes recent onshore and offshore investigations in the canarian archipelago especially in the western islands of la palma and el hierro have greatly improved the understanding of the genesis and evolution of these islands and allow interesting comparisons with other hotspot induced oceanic island volcanoes especially the hawaiian archipelago genesis induced by hotspot activity and consequently shield and post erosional stages of growth allow the definition of similar main stratigraphic units the canarian and hawaiian volcanoes show common constructional and structural features such as rift zones progressive volcano instability and multiple gravitational collapses however the canaries present some important geological differences from the prototypical hotspot volcanoes of the hawaiian islands particularly the absence of significant subsidence the canarian volcanoes remain emergent until completely mass wasted by gravitational collapses and erosion volcanic formations over 20 million years are observable in outcrop in the canaries including the seamount stages of growth in several of the islands

structural evolution of the harts range area and its implication for the development of the arunta block central australia abstract a division of the high grade metamorphics of the arunta block in the harts range area into two main groups on the basis of lithology structure metamorphism and igneous activity is suggested as a result of detailed mapping the lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss minor amphibolite and mafic granulite while the structurally overlying group is regarded as a cover sequence irindina supracrustal assemblage and comprises predominantly metasediments pelitic and semi pelitic gneiss calc silicate and quartzite and amphibolite anorthosite and ultramafic rock of the harts range meta igneous complex the cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre existing basement the two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover the basement s early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early precambrian terrains and developed spatially and or temporally separated from the cover cover deformation closely resembles continental margin cordillera type overthrusting and thus thin skinned tectonics associated with large scale crustal shortening during the later harts range remobilization the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation recumbent contraction fault mylonites repeatedly developed along the dislocation which was also isoclinally folded intruded by a major megacrystic granitoid bruna gneiss sheet and repeatedly rejuvenated the history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity

intracontinental orogeny enhanced by far field extension and local weak crust the accommodation of intraplate stresses in preexisting weak regions of plate interiors is here investigated using thin viscous sheet numerical models the intraplate stresses are governed by multicomponent and multidirectional stresses originating at plate boundaries the modeled scenarios mimic plate boundary conditions during the intraplate alice springs orogeny aso central australia and include 1 a northwest southeast zone of weak lithosphere within strong continental blocks to the north and southand 2 a principal south directed stress condition at the northern boundary that causes minor clockwise rotation of the northern block alternative tectonic environments are investigated in additional models that include 1 secondary compressional or extensional stresses acting at the eastern boundary representing the temporally variable stress conditions during the tasmanides orogeny and 2 an eastern wedge shaped zone of rheologically weak lithosphere mirroring rift fill of the irindina subbasin our results highlight that a low angle between major crustal features e g orogenic elongation and preexisting regional structures and the principal transmitted stresses is highly relevant in the concentration of elevated levels of differential stress and subsequent localization of deformation in plate interiors secondary stresses orthogonal to the principal acting stresses may introduce effects that explain the episodic orogenic activity in the case of the aso the combination of secondary extensional stresses at the eastern boundary of australia and weak lithosphere of the preexisting irindina subbasin strongly influences the observed spatial strain intensity localization and kinematics of deformation during the aso

historic hydrovolcanism at deception island antarctica implications for eruption hazards deception island antarctica is the southernmost island of the south shetland archipelago in the south atlantic volcanic activity since the eighteenth century along with the latest volcanic unrest episodes in the twentieth and twenty first centuries demonstrates that the volcanic system is still active and that future eruptions are likely despite its remote location the south shetland islands are an important touristic destination during the austral summer in addition they host several research stations and three summer field camps deception island is characterised by a quaternary caldera system with a post caldera succession and is considered to be part of an active dispersed monogenetic volcanic field historical post caldera volcanism on deception island involves monogenetic small volume vei 2 3 eruptions such forming cones and various types of hydrovolcanic edifices the scientific stations on the island were destroyed or severely damaged during the eruptions in 1967 1969 and 1970 mainly due to explosive activity triggered by the interaction of rising or erupting magma with surface water shallow groundwater and ice we conducted a detailed revision field petrology and geochemistry of the historical hydrovolcanic post caldera eruptions of deception island with the aim to understand the dynamics of magma water interaction as well as characterise the most likely eruptive scenarios from future eruptions we specifically focused on the crimson hill estimated age between 1825 and 1829 and kroner lake estimated age between 1829 and 1912 eruptions and 1967 1969 and 1970 events by describing the eruption mechanisms related to the island s hydrovolcanic activity data suggest that the main hazards posed by volcanism on the island are due to fallout ballistic blocks and bombs and subordinate dilute pdcs in addition deception island can be divided into five areas of expected activity due to magma water interaction providing additional data for correct hazard assessment on the island

stratigraphy sedimentology and eruptive mechanisms in the tuff cone of el golfo lanzarote canary islands the tuff cone of el golfo on the western coast of lanzarote canary islands is a typical hydrovolcanic edifice along with other edifices of the same age it was constructed along a fracture oriented nee sww that coincides with the main structural trend of recent volcanism in this part of the island we conducted a detailed stratigraphic study of the succession of deposits present in this tuff cone and here interpret them in light of the depositional processes and eruptive dynamics that we were able to infer the eruptive sequence is represented by a succession of pyroclastic deposits most of which were emplaced by flow plus a number of air fall deposits and ballistic blocks and bombs we distinguished five different eruptive depositional stages on the basis of differences in inferred current flow regimes and fragmentation efficiencies represented by the resulting deposits the different stages may be related to variations in the explosive energy eight lithofacies were identified based on sedimentary discontinuities grain size components variations in primary laminations and bedforms the volcanic edifice was constructed very rapidly around the vent and this is inferred to have controlled the amount of water that was able to enter the eruption conduit the sedimentological characteristics of the deposits and the nature and distribution of palagonitic alteration suggest that most of the pyroclastic succession in el golfo was deposited in a subaerial environment this type of hydrovolcanic explosive activity is common in the coastal zones of lanzarote and the other canary islands and is one of the main potential hazards that could threaten the human population of this archipelago detailed studies of these hydrovolcanic eruptions such as the one we present here can help volcanologists understand the hazards that this type of eruption can generate and provide essential information for undertaking risk assessment in similar volcanic environments

geotectonic remobilisation of the lower crustal segment of southern peninsular india abstract deep crustal exposures on various continental platforms have recently attracted much attention with regard to their evolution and relevance to present day tectonics the granulite terrain of the southern part of peninsular india exhibits unique characteristics the small dimensions bounded by two rifted margins high relief as a result of recent uplift and anomalous gravity magnetic and electromagnetic signatures the geophysical anomalies over this region have been cohered to produce a tentative geodynamic model in which the break up of gondwana 150 my bp subsequent rifting of the western margin of india 70 my bp and the collision with asia and post collision stresses 50 my bp play a role in the remobilisation of the crust mantle regime of south india the geophysical signatures over this deep crustal cross section differ markedly from those of its counterparts in other parts of the world and thus may reflect the consequences of recent remobilisation rather than features frozen in since proterozoic time in this sense deep crustal imaging of southern peninsular india may differ from that in other continental platforms

geomorphometric variability of monogenetic volcanic cones evidence from mauna kea lanzarote and experimental cones abstract volcanic cones are the most common volcanic constructs on earth their shape can be quantified using two morphometric ratios the crater cone base ratio wcr wco and the cone height width ratio hco wco the average values for these ratios obtained over entire cone fields have been explained by the repose angle of loose granular material i e scoria controlling cone slopes the observed variability in these ratios between individual cones has been attributed to the effect of erosional processes or contrasting eruptive conditions on cone morphometry using a gis based approach high spatial resolution digital elevation models and airphotos two new geomorphometry datasets for cone fields at mauna kea hawaii usa and lanzarote canary islands spain are extracted and analyzed here the key observation in these datasets is the great variability in morphometric ratios even for simple shape and well preserved cones simple analog experiments are presented to analyze factors influencing the morphometric ratios the formation of a crater is simulated within an analog cone i e a sand pile by opening a drainage conduit at the cone base results from experiments show that variability in the morphometric ratios can be attributed to variations in the width height and horizontal offset of the drainage point relative to the cone symmetry axis to the dip of the underlying slope or to the influence of a small proportion of fine cohesive material gis analysis and analog experiments together with specific examples of cones documented in the field suggest that the morphometric ratios for well preserved volcanic cones are controlled by a combination of 1 the intrinsic cone material properties 2 time dependent eruption conditions 3 the local setting and 4 the method used to estimate the cone height implications for interpreting cone morphometry solely as either an age or as an eruption condition indicator are highlighted

formation of composite pyroclasts by welding inside a lithic rich mafic eruption column los marteles caldera canary islands abstract the tuff ring of the los marteles phreatomagmatic maar gran canaria canary islands includes a particular unit that comprises abundant composite spheroidal clasts however these clasts differ from cored clasts elsewhere that consist of erupted lithic pyroclasts with an adhering rim of coherent chilled magma the composite clasts here studied range from lapilli to bombs and are cored or concentrically banded among the welded components several types of primary clasts are discerned that include i juvenile clasts resultant of the magma inertial fragmentation and ii cognate or accessory lithic clasts juvenile clasts include glass spheres nano and microachneliths and crystals lithic clasts include lava fragments scoriae and olivine xenocrysts primary clasts are interpreted to have been produced by the explosion of a partially crystallized mafic sio 2 50 55 magma that erupted into a maar filled up with scoriae plus lava fragments the explosion would have originated as a dense and turbulent gas thrust into which the repeated collisions of melt droplets with solid clasts generated droplet coated crystals lithic clasts and single glass spheres we interpret the cored and concentrically banded pyroclasts to be made of coalesced and or agglutinated particles coated with a melt rim to the rest of the components these composite clasts developed spheroidal shapes during transport within a violent strombolian eruption column due to spin effect and finally landed in solid state

stratigraphy structure and geochronology of the las canadas caldera tenerife canary islands after a long period of subaerial fissure fed extrusions of basaltic magmas 12 to 3 ma volcanic activity was then concentrated in the central part of tenerife phonolitic magma chambers formed and a central volcanic complex was constructed the las canadas edifice the formation of a large depression the las canadas caldera truncated the top of the edifice the active twin strato cones teide pico viejo are sited in this depression the history of the las canadas caldera and edifice are established from stratigraphy geochronology k ar dates and volcanological studies two different groups are recognized separated by a major unconformity the lower group is dated at 2 to 3 ma and includes the products of several volcanic centres which together represent several cycles the upper group ranges from 1 56 to 0 17 ma and includes three different formations representing three long term 100 to 300 ka volcanic cycles the periods of dormancy between each formation were of 120 to 250 ka duration the las canadas caldera is a multicyclic caldera which formed over the period 1 18 0 17 ma each cycle of activity represented by a formation culminated in caldera collapse which affected different sectors of the las canadas edifice geological observation and geochronology support an origin by collapse into a magma chamber the minimum volume of pyroclastic ejecta is substantially greater than the present caldera depression volume 45 km3 but approaches the inferred volume of the original caldera depression 140 km3 after the formation of the caldera sector collapses could also occur at the northern flank of the volcano causing the disappearance of the northern side of the caldera wall

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

folded basement cored tectonic wedges along the northern edge of the amadeus basin central australia evaluation of orogenic shortening abstract intracontinental deformation at the northern margin of the amadeus basin central australia is characterised by the presence of basement cored downward facing antiformal synclines these structures were formed during the devonian carboniferous alice springs orogeny and in the past were explained as erosional relics of fold nappes indicating large horizontal crustal shortening within the australian continent this notion is in stark contrast with both the uncomplicated homoclinal structure that contains the nappes and with seismic data suggestive of a simple crustal structure formed during only modest intraplate shortening balanced and sequentially restored sections across the razorback nappe and its vicinity show that such downward facing synformal anticlines can be produced by emplacement of a basement cored tectonic wedge along shortcut thrusts in the footwall of a crustal scale redbank shear zone which initially formed during the mesoproterozoic the forelandward propagating wedge proper is formed by basement and the lower two successions of the amadeus basin higher sedimentary successions accommodate shortening by hinterlandward backthrusting above a detachment horizon which is hosted in carbonates and evaporites of the bitter springs formation during the formation of the homocline which characterises much of the northern margin of the amadeus basin the tectonic wedge was rotated through 90 and now forms a downward facing antiformal syncline the orogenic shortening indicated by this succession of structural events is about 19 km which is compatible with seismic data the proposed model explains the existence of downward facing synformal anticlines as folded basement cored tectonic wedges although they resemble parts of fold nappes these folded tectonic wedges do not necessarily imply large amounts of crustal shortening

stress induced brittle fragmentation of magmatic melts theory and experiments 1 the release of kinetic energy during explosive volcanic eruptions is a key parameter for hazard assessment and civil defense the explosive production of volcanic ash by intensive fragmentation of magma and host rocks represents a substantial part of this energy for cases of explosive eruption where predominantly host rock was fragmented phreatomagmatic eruptions to form the major part of volcanic ash rock mechanical parameters could be measured and fragmentation energies assigned in cases where most of the produced ash is of juvenile origin magmatic eruptions a general method for the determination of fragmentation energy is still lacking in this article we introduce a thermodynamic approach that relates grain size data of the produced ash deposits to shear rates acting during the deformation of magma with the use of a standardized fragmentation experiment the physical parameters needed to determine the specific fragmentation energy and deformation history were measured the experiment was calibrated and tested with two case histories of the campi flegrei volcanic field southern italy both eruptions are classified as most probable worst case scenarios during the next period of activity to be expected within the next 10 100 years using the experimentally determined specific fragmentation energies the total mass of produced ash of each eruption and assuming an energy dissipation as observed in the experiments the total kinetic energy release of the worst case campi flegrei eruptive events to come were calculated with 25 and 40 kt tnt equivalent