effusive history of the grande decouverte volcanic complex southern basse terre guadeloupe french west indies from new k ar cassignol gillot ages abstract the grande decouverte volcanic complex gdvc active since at least 0 2 ma is the most recent volcanic complex of the basse terre island guadeloupe lesser antilles arc a detailed geochronological study using the k ar cassignol gillot technique has been undertaken in order to reconstruct the history of effusive activity of this long lived volcanic system twenty new ages permit to suggest that the gdvc experienced at least six main effusive stages from 200 ka to present time to the north of the gdvc the gds grande decouverte soufriere volcano has been active since at least 200 ka and to the south the trmf trois rivieres madeleine field started to be emplaced 100 ka morphological investigations suggest that the whole trmf volcanism was emitted from vents distinct from the gds most probably a large e w fissure network linked to the marie galante rift the mean age of 62 5 ka obtained for the e w madeleine le palmiste alignment suggests that a fissure opening event occurred at that time however whole rock major and trace element signatures are similar for both systems suggesting that a common complex magma plumbing system has fed the overall gdvc we report very young ages for lava flows from the trmf which implies that

volcanic evolution of central basse terre island revisited on the basis of new geochronology and geomorphology data twenty six new and seven previous k ar ages obtained on groundmass separates for samples from the axial chain massif guadeloupe f w i associated with geomorphological investigations allow us to propose a new model of the volcanic evolution of the central part of basse terre island the axial chain is composed of four edifices moustique mateliane capesterre and icaque mounts showing coeval activity from 681 12 to 509 10 ka which contradicts a previous hypothesis that flank collapse affected them successively our geomorphological reconstruction shows that the axial chain can be considered as a single large volcano named the southern axial chain volcano sca rather than a succession of several smaller volcanoes it raises questions regarding the formation of a large depression within the sca volcano prior to the construction of the sans toucher volcano between 451 13 and 412 8 ka given presently available evidence a slump affecting the western part of the sca volcano is the most probable scenario to reconcile the complete age dataset and the present day morphology of central basse terre finally our study shows that the sca volcano had a post activity volume of 90 km3 implying a construction rate of 0 5 km3 kyr this value strongly constrains interpretations of magma generation processes throughout the lesser antilles arc

back arc extension in the scotia sea the nature of back arc extension in the east scotia sea is re examined with the use of an enlarged geophysical data set well developed oceanic magnetic lineations confirm that the present spreading episode started about 8 ma ago that spreading is asymmetric and that the total rate increased from 50 to 70 m m a about 1 5 ma ago most of the currently active south sandwich volcanic island arc lies upon ocean floor only 6 8 m a old and generated at the current spreading ridge subsequent extension has not modified the curvature of the arc east west magnetic lineations of miocene age in the central scotia sea and contemporaneous low k arc tholeiites dredged from the eastern south scotia ridge discovery bank indicate a regime of coupled subduction and back arc extension preceding that occurring now a speculative model involving a series of collisions of parts of this earlier discovery trench with ridge crest sections of the south american antarctic plate boundary explains the transformation of this earlier regime into the present self contained sandwich plate regime the considerable small scale variability observed in the back arc region may be seen as an inevitable consequence of the action of the ridge trench collision mechanism the entire scotia sea could have formed by a similar kind of back arc extensional modification of the south american antarctic plate boundary

the eruptive history of morne jacob volcano martinique island french west indies geochronology geomorphology and geochemistry of the earliest volcanism in the recent lesser antilles arc martinique is the lesser antilles island where the most complete volcanic history of the arc can be found from the oligocene to the present time in this study we focused on the construction of morne jacob shield volcano in martinique which is the largest volcano of the lesser antilles we have dated twenty representative samples from the morne jacob by k ar based on the cassignol gillot technique used geochemical data and obtained morphological constraints that have helped us to reconstruct better the volcanic history of this shield volcano our results and the lack of reliable ages on other plio pleistocene islands show that the morne jacob is the oldest volcano of the aerial recent arc that has been dated so far it has a longer history than previously inferred with different stages ranging between 5 2 and 1 5 ma a large basaltic to andesitic shield volcano was first built between 5 2 and 4 0 ma j1 with tholeiitic lavas characterized by an increase of sio2 content through time after 800 kyr of repose calc alkaline andesites erupted over the first shield from 3 2 to 2 2 ma j2a the accumulation of lavas over a hyaloclastitic basement provoked spreading and northeast creeping of the northern flank of the volcano the mass movement induced regressive erosion particularly at the centre of the edifice then between 2 1 and 1 5 ma j2b calc alkaline lavas which show a decrease of the sio2 content through time erupted at the central vent and from peripheral fissures over the previous shield and down to the caribbean coast eruptive volumes were reconstructed permitting estimates of minimum output rates for the volcano s history minimum volume erupted during the first stage is 112 20 km3 and 33 25 km3 during the whole second stage yielding rates of about 0 107 km3 kyr and 0 019 km3 kyr respectively considering the entire history of morne jacob shield volcano between 5 14 0 07 and 1 54 0 03 ma we obtain a total volume of 145 32 km3 above the sea level and a time averaged construction rate of 0 040 0 008 km3 kyr with comparison between the reconstructed paleotopographies and the dem of present topography we have calculated an eroded volume of 18 km3 over the outcropping 210 km2 that have occurred during the last 1 5 myr finally our set of twenty k ar ages obtained on groundmass separates allow us to revise the eruptive chronology of morne jacob volcano and to date the earliest subaerial volcanic activity so far identified for the recent lesser antilles arc

eruptive chronology of tungurahua volcano ecuador revisited based on new k ar ages and geomorphological reconstructions abstract this study focuses on the evolution through time of tungurahua volcano ecuador and provides new information regarding its history eighteen new k ar ages constrain its construction and the activity of its three successive edifices we show that the volcano is much younger than expected indeed the older edifice activity only began around 293 10 ka and ended at 79 3 ka after 50 ka of quiescence the second edifice started growing at 29 2 ka after a major sector collapse and itself collapsed at 3 ka since then the third edifice filled the amphitheatre and is still active together with numerical reconstructions of the morphology of the three edifices flanks before erosion these new ages allow us to quantify the magmatic productivity rates during their construction from 0 6 0 3 and 0 9 0 2 km3 ka for the two older edifices to 2 5 1 0 km3 ka for the youngest as well as an erosion rate of 0 2 0 1 km3 ka occurring since the end of tungurahua i construction major and trace element contents of lavas from the three edifices display rather similar trends combined with our new ages the magmatic signature through time does not seem to have been significantly affected either by the sector collapses experienced by the volcano or by changes of the deep magmatic source finally our results show that the k ar dating method by the unspiked cassignol gillot technique performed on groundmass can be successfully applied to lava flows older than the holocene while the uncertainties related to younger units can prevent an accurate age determination particularly this method can be applied to quaternary volcanoes from the ecuadorian arc with many of them remaining without knowledge of the timing of their past activity

volcanic history and magmatic evolution of seguam island aleutian island arc alaska new 40 ar 39 ar dating coupled with detailed field mapping stratigraphy and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of seguam island in the aleutian island arc alaska sixty new 40 ar 39 ar ages from lavas domes and pyroclastic deposits were obtained using furnace incremental heating techniques on replicate samples of whole rock and ground mass separates and they constrain the duration of pleistocene to holocene subaerial volcanism to 318 k y the 40 ar 39 ar plateau ages indicate that over 85 of the complex 68 km 3 of material was erupted almost continuously between 318 ka and 9 ka at ca 9 ka a stratocone on the eastern half of the island partially collapsed producing a 4 km wide crater rhyolitic dome forming eruptions followed from vents in the newly created crater and were likely contemporaneous with 8 0 km 3 of basaltic and basaltic andesitic effusions from pyre peak and a 1 4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island geochemical changes over the last 318 k y are subtle most notably the earliest eruptions from 318 to 142 ka produced no andesite and basalt from this period has larger ranges in zr rb and la yb than younger basalts small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal liquid fractionation process that varied only slightly in successive eruptive phases over 318 k y we identified minor geochemical changes in magma composition during each of the three main stages of volcanism but overall the monotonic variations in major and trace element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed system fractional crystallization of basalt using the 40 ar 39 ar geochronology and estimates of individual flow volumes we calculated a time averaged eruptive rate at seguam that is similar to growth rates of other well dated arc volcanoes in the cascades and chilean southern volcanic zone but less than that of mount katmai and mount mageik which are located on the alaska peninsula the eruptive flux at seguam has been highly variable fluctuating more than an order of magnitude from 0 07 km 3 k y during the early history of bimodal volcanism to 1 18 km 3 k y over the past 9 k y

the interaction between volcanoes and strike slip transtensional and transpressional fault zones analogue models and natural examples regional strike slip faulting can control magma movements deform volcanoes and may destabilise their flanks the aim of this study is to address this problem by comparing two natural examples basse terre island volcanoes lesser antilles and maderas volcano nicaragua with analogue experiments the field and remote sensing analyses of their structures reveal that guadeloupe volcanoes which developed in a1 45 striking sinistral transtensional fault zone are dominantly fractured in a 090 e120 direction which is parallel to the maximum principal horizontal stress and to the elongation direction of the summit graben of analogue models this graben is bordered by the sigmoid i fault or y shear structure and has facilitated the formation of the beaugendre and vieux habitants valleys by faulting erosion or collapse this structure has also influenced the injection of dykes and the transport of hydrothermal fluids the comparison of maderas volcano with the analogue models confirms that the volcano has developed parallel to a 135

magnetic structure of basse terre volcanic island guadeloupe lesser antilles inferred from 3d inversion of aeromagnetic data abstract we interpret aeromagnetic data to constrain the magnetic structure of the island of basse terre guadeloupe lesser antilles aeromagnetic data are inverted in the spatial domain with a bayesian formulation to retrieve the 3d distribution of rock magnetization intensity and polarity the inversion is regularized using a correlation length and standard deviation for magnetization chosen to be consistent with results from paleomagnetic measurements on lava flow samples from basse terre the resulting 3d model of magnetization is consistent at the surface with observed polarities and at depth with a 2d model obtained from a parker and huestis 1974 inversion in the fourier domain the inferred magnetic structure is compared with the available geological information deduced from published geological geomorphological and geochronological studies in the southern part of the island very low magnetization is observed around the soufriere lava dome last activity of the grande decouverte carmichael soufriere composite volcano in relation with a high level of hydrothermal alteration high magnetizations in the south east might reflect the presence of massive lava flows and lava domes from the madeleine vents and monts caraibes medium magnetizations in the south west coincide with the location of debris avalanche deposits associated with the collapse of the former carmichael volcano and might reflect less massive lava structure at depth using the volume of normal polarity in the south part of basse terre recovered in our 3d model of rock magnetization we estimate an average construction rate of 9 4 10 4 km3 yr during the brunhes chron which provides new insights on the volcanic activity of la soufriere volcano

surface fitting in geomorphology examples for regular shaped volcanic landforms abstract in nature several types of landforms have simple shapes as they evolve they tend to take on an ideal simple geometric form such as a cone an ellipsoid or a paraboloid volcanic landforms are possibly the best examples of this ideal geometry since they develop as regular surface features due to the point like circular or fissure like linear manifestation of volcanic activity in this paper we present a geomorphometric method of fitting the ideal surface onto the real surface of regular shaped volcanoes through a number of case studies mt mayon mt somma mt semeru and mt cameroon volcanoes with circular as well as elliptical symmetry are addressed for the best surface fit we use the minimization library minuit which is made freely available by the cern european organization for nuclear research this library enables us to handle all the available surface data every point of the digital elevation model in a one step half automated way regardless of the size of the dataset and to consider simultaneously all the relevant parameters of the selected problem such as the position of the center of the edifice apex height and cone slope thanks to the highly performing adopted procedure fitting the geometric surface along with calculating the related error demonstrates the twofold advantage of the method firstly we can determine quantitatively to what extent a given volcanic landform is regular i e how much it follows an expected regular shape deviations from the ideal shape due to degradation e g sector collapse and normal erosion can be used in erosion rate calculations secondly if we have a degraded volcanic landform whose geometry is not clear this method of surface fitting reconstructs the original shape with the maximum precision obviously in addition to volcanic landforms this method is also capable of constraining the shapes of other regular surface features such as aeolian glacial or periglacial landforms

construction and destruction rates of volcanoes within tropical environment examples from the basse terre island guadeloupe lesser antilles abstract in order to better constrain the construction and the erosion rate affecting the volcanic island of basse terre island guadeloupe f w i an enlarged k ar age dataset has been combined with reconstruction of the paleo topography two different methods of interpolation of the present topography have been cross checked to better support the erosion rates obtained and their associated uncertainties the present study focusses on the monts caraibes volcanoes and on the main geomorphic feature of the piton de bouillante volcano the beaugendre valley the monts caraibes volcanoes were constructed in 83 kyr at a rate of 0 12 0 04 km 3 kyr during the last 450 kyr they have experienced an erosion rate of 610 550 t km 2 yr in the piton de bouillante volcano eleven new k ar ages have been obtained constraining the duration of its volcanic activity between 880 14 and 712 12 ka and involving a construction rate of 0 70 0 20 km 3 kyr for this volcano an erosion rate of 1220 700 t km 2 yr has been obtained for the last 700 kyr our study also shows based on the contemporaneity of the ages in the entire beaugendre valley added to the mean erosion rate of 1350 550 t km 2 yr that the flank collapse hypothesis cannot explain the formation of this valley finally the similarity of the erosion rates computed for different locations of the basse terre island shows that the time integrated erosion appears independent to the trade wind effect and suggests that the barrier effect due to the relief is not present here

morpho structural evolution of a volcanic island developed inside an active oceanic rift s miguel island terceira rift azores the evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass wasting processes more specifically volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation the island of s miguel azores has developed during the last 1 myr inside the active terceira rift a major tectonic structure materializing the present boundary between the eurasian and nubian lithospheric plates in this work we depict the evolution of the island based on high resolution dem data stratigraphic and structural analyses high precision k ar dating on separated mineral phases and offshore data bathymetry and seismic profiles the new results indicate that 1 the oldest volcanic complex nordeste composing the easternmost part of the island was dominantly active between ca 850 and 750 ka and was subsequently affected by a major south directed flank collapse 2 between at least 500 ka and 250 ka the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main e w collapse scar 3 since 250 kyr the western part of this succession furnas area was affected by multiple vertical collapses associated plinian eruptions produced large pyroclastic deposits here dated at ca 60 ka and less than 25 ka 4 during the same period the eastern part of the landslide scar was enlarged by retrogressive erosion producing the large povoacao valley which was gradually filled by sediments and young volcanic products 5 the fogo volcano in the middle of s miguel is here dated between ca 270 and 17 ka and was affected by at least one southwards flank collapse 6 the sete cidades volcano in the western end of the island is here dated between ca 91 and 13 ka and experienced mutliple caldera collapses a landslide to the north is also suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from alignments of basaltic strombolian cones the predominance of the n150 and n75 trends in the island suggest that the tectonics of the terceira rift controlled the location and the distribution of the volcanism and to some extent the various destruction events

dem based reconstruction of southern basse terre volcanoes guadeloupe archipelago fwi contribution to the lesser antilles arc construction rates and magma production abstract we combine radiometric ages and geomorphologic investigations to quantify the relief creation of well preserved volcanic surfaces applied to the volcanoes of southern basse terre guadeloupe lesser antilles arc the last 650 ka volcanic evolution of this island has been modeled using ten main stages constrained by k ar ages previously obtained on lava flows and domes from the volcanic massifs of the axial chain 1000 435 ka and from the grande decouverte volcanic complex 250 ka present based on the construction of a 250 000 point database inferred from the analysis of the guadeloupe digital elevation model 3d reconstructions of the successive volcanic stage landforms were calculated and the correlated geochronological maps drawn using arcgis software volumes and rates of construction were computed for each time span separating these ten stages the average construction rates calculated here are 2 4 0 3 10 4 km 3 yr for the last million years 0 9 0 3 10 4 km 3 yr for the last 100 ka and 0 9 0 2 10 4 km 3 yr for the last 15 ka although basse terre volcanism is characterized by a marked dominance of effusive products our estimates should be considered as minimum values because the material that went into the sea during explosive events was not taken into account however we note that a relatively high construction rate of 4 5 1 3 10 4 km 3 yr has been obtained for the icaques volcano which was emplaced during the 630 600 ka time interval within the depression formed by the first large scale flank collapse having affected southern basse terre the sudden release of the lithostatic load induced by this mass wasting event could explain this value which is significantly higher than the average value of 0 8 0 1 10 4 km 3 yr obtained for the last 650 ka finally the comparison with other analogous volcanic massifs from islands and continental arcs points to a relatively low magmatic production for southern basse terre which could be tentatively related to the relatively slow subduction rate of the atlantic plate

geomorphic evolution of the piton des neiges volcano reunion island indian ocean competition between volcanic construction and erosion since 1 4 ma abstract reunion island indian ocean is a volcanic complex whose eruptive history was dominated by the activity of two main edifices piton des neiges pn and piton de la fournaise pf volcanoes the tropical climate induces erosion processes that permanently compete with volcanic constructional processes exposed to the trade winds and associated heavy rainfalls the northeastern part of the island exhibits the most complex morphological evolution geomorphological analysis performed on a 50 m dem and associated to new k ar ages has clarified the overall history of pn volcano each massif is assigned to one of the main building stages of the edifice in addition the arrangement of these different massifs reveals that the eruptive phases have led to successive relief inversions and successive excavations of large central depressions in the proximal area as a result the younger massifs are always located in more proximal parts of the volcano the youngest being close to the edifice center in distal areas early lava flows were channeled into valleys incised along the massif boundaries leading to a more complex geochronological organization quantitative study of the dissection of pn volcano allows us to propose a minimum eroded volume of 101 44 and 105 41 km3 for the mafate and cilaos cirques depressions respectively during the last 180 kyr and a minimum average long term erosion rate of 1 2 0 4 km3 ka this leads us to estimate the removed volume during the whole history of pn volcano 1000 km3 as equivalent to the volume of the deposits identified on the submarine flanks of piton des neiges volcano therefore as regressive erosion appears to be the prevailing geomorphic process during the whole pn history it questions the presence of major flank collapses younger than 1 4 ma on this volcano erosion processes have largely been neglected in recent models but our study emphasizes them as a key component of landscape development and a major process in the morphological evolution of reunion island that has to be fully integrated in future studies

a global database of composite volcano morphometry we present a global database on the subaerial morphometry of composite volcanoes data was extracted from the 90 m resolution shuttle radar topography mission srtm digital elevation model dem the 759 volcanoes included in the database are the composite i e polygenetic volcanoes listed in the smithsonian institution global volcanism program gvp database that are covered by the srtm dem have a constructional topography and a basal width larger than 2 km the extent of each volcano edifice was defined using the netvolc algorithm which computes outlines by minimizing a cost function based on breaks in slope around the edifices morphometric parameters were then calculated using the morvolc algorithm the parameters characterize and quantify volcano size basal width summit width height and volume profile shape height basal width and summit width basal width ratios plan shape ellipticity and irregularity indexes and slopes in addition 104 well defined and relatively large summit craters calderas were manually delineated and specific parameters were computed most parameters show large variation without clear separations indicating a continuum of volcano morphologies large overlap between the main gvp morphologic types highlights the need for a more rigorous quantitative classification of volcano morphology the database will be maintained and updated through a website under construction

petrophysical properties of volcanic rocks and impacts of hydrothermal alteration in the guadeloupe archipelago west indies abstract the results of the geotref project presented here aim to develop high enthalpy geothermal energy in fractured reservoirs they focus on active volcanic islands especially the basse terre island guadeloupe archipelago as part of this project we measured several petrophysical properties to better understand how fluids flow in the hydrothermal system provide a set of parameters to modelers and build datasets for future interpretation of well logs the thermo physical rock properties of bulk and grain density porosity permeability compressional wave velocity thermal conductivity and diffusivity specific heat capacity of outcropping volcanic rocks from the guadeloupe archipelago lesser antilles were measured and a dataset was built according to field observations samples were grouped into three lithological classes and then separated by their degree of alteration according to macroscopic aspects and optical microscopy results geochemical data were obtained and magnetic minerals identified by combining anisotropy of magnetic susceptibility and variation of magnetic susceptibility with temperature the lithotypes investigated were lava flows debris flows and pyroclastics gathering ash pumice and scoria deposits for lavas and debris flows three degrees of alteration were defined fresh slightly to moderately altered and highly hydrothermalized only fresh pyroclastics were studied because the altered state was not outcropping typically samples are composed of andesite and belong to tholeiitic and calc alkaline series lava porosity consists of high density cooling joints and vesicles of different shapes whereas pores in debris flows and pyroclastics are located both between and within particles median porosities and permeabilities are clearly distinct for lavas 4 and 6 10 18 m2 debris flows 33 and 4 10 14 m2 and pyroclastics 34 and 8 10 13 m2 the porosity difference implies a thermal conductivity twice to three times higher for lavas 1 68 w m 1 k 1 specific heat capacity of all lithothypes is similar 0 75 0 97 kj kg 1 k 1 advancing hydrothermal alteration removes magnetic minerals and transforms initial rocks the original plagioclases and pyroxenes in neoformed chlorite white micas quartz epidote and clay minerals are replaced this modifies the shape of the pore network furthermore hydrothermal alteration results in a reduction of pore throat diameter it tends to decrease petrophysical differences between lavas and debris flows increasing the porosity and permeability of effusive rocks while the opposite effects are observed for debris flows

radiometric dating of three large volume flank collapses in the lesser antilles arc abstract it is now recognised that flank collapses are a recurrent process in the evolution of the lesser antilles arc volcanoes large magnitude debris avalanche deposits have been identified off the coast of dominica martinique and st lucia with associated volumes up to 20 km 3 deplus c le friant a boudon g komorowski j c villemant b harford c segoufin j cheminee j l 2001 submarine evidence for large scale debris avalanches in the lesser antilles arc earth planet sci lett 192 145 157 we present new radiometric dating of three major events using the k ar cassignol gillot technique in the qualibou depression of st lucia a collapse has been constrained by dome emplacement prior to 95 2 ka in dominica where repetitive flank collapse events have occurred le friant a boudon g komorowski j c deplus c 2002 l ile de la dominique a l origine des avalanches de debris les plus volumineuses de l arc des petites antilles c r geoscience 334 235 243 the plat pays event probably occurred after 96 2 ka inside the depression caused by this event scotts head which is interpreted as a proximal megabloc from the subsequent soufriere avalanche event has been dated at 14 1 ka providing an older bound for this event on martinique three different domes within the carbets structure dated at 337 5 ka constrain the age of this high magnitude event finally these results obtained from three of the most voluminous flank collapses provide constraints to estimate the recurrence of these events which represent one of the major hazards associated with volcanoes of the lesser antilles arc

structural style of buried ediacaran basins in saudi arabia on seismic reflection data abstract several isolated ediacaran basins subcrop phanerozoic strata in saudi arabia the structural styles of these subcropping basins are reviewed here based on interpretation of 2d and 3d seismic reflection data and comparison with arabian shield outcrops the basins are distinguished by layered seismic reflectors that are interpreted as sedimentary and volcanic strata due to limited well penetrations these packages cannot be directly dated but a relative chronology is established anchored by constraints from the arabian shield and regional studies most of the basins are bound by extensional faults and the basin fills show growth architectures and angular unconformities the bounding faults trend broadly north south and are interpreted to be late to post orogenic reactivations of oceanic terrane margins based on their structural characteristics in comparison with sub basins exposed on the shield the basin fills are correlated with the ediacaran jibalah group an equivalent of the omani nafun group these sub basins experienced inversion and exhumation prior to ongoing volcanism and unconformable deposition of late ediacaran to earliest cambrian kurayshah group sediments the kurayshah group is disconformably overlain by the widespread tayma group siliciclastics this disconformity correlates to the angudan unconformity in oman the kurayshah group includes a late ediacaran stratovolcano over 40 km in diameter in the eastern basins evaporitic structural styles indicate the presence of hormuz ara evaporites with local igneous intrusions that correlate at least in part to the kurayshah group the tayma group infills significant accommodation space variations and was affected by ongoing but declining differential subsidence and reactivation of the underlying basin bounding faults

gis methods applied to the degradation of monogenetic volcanic fields a case study of the holocene volcanism of gran canaria canary islands spain abstract modeling of volcanic morphometry provides reliable measurements of parameters that assist in the determination of volcanic landform degradation variations of the original morphology enable the understanding of patterns affecting erosion and their development facilitating the assessment of associated hazards a total of 24 volcanic holocene eruptions were identified in the island of gran canaria canary islands spain 87 of these eruptions occurred in a wet environment while the rest happened in a dry environment 45 of holocene eruptions are located along short barrancos s type less than 10 km in length 20 along large barrancos l type 10 17 km in length and 35 along extra large barrancos xl type more than 17 km in length the erosional history of holocene volcanic edifices is in the first stage of degradation with a geomorphic signature characterized by a fresh young cone with a sharp profile and a pristine lava flow after intensive field work a careful palaeo geomorphological reconstruction of the 24 holocene eruptions of gran canaria was conducted in order to obtain the digital terrain models dtms of the pre and post eruption terrains from the difference between these dtms the degradation volume and the incision rate were obtained the denudation of volcanic cones and lava flows is relatively independent both their geographical location and the climatic environment however local factors such as pre eruption topography and ravine type have the greatest influence on the erosion of holocene volcanic materials in gran canaria although age is a key factor to help understand the morphological evolution of monogenetic volcanic fields the gran canaria holocene volcanism presented in this paper demonstrates that local and regional factors may determine the lack of correlation between morphometric parameters and age consequently the degree of transformation of the volcanic edifices evolves in many cases independently of their age

revision of platte river alluvial facies model through observations of extant channels and barforms and subsurface alluvial valley fills abstract the platte river of nebraska central usa has been used by sedimentologists as a type example of a sandy braided river system for some forty years despite this designation however the details of the river s subsurface stratigraphy have never been evaluated and so the extant facies model for the platte river remains unvalidated this paper analyzes the holocene stratigraphy of the central and lower platte river it utilizes newly acquired data from wireline logs sediment cores aerial photography and ground penetrating radar gpr traverses to characterize subsurface geology in addition to surface sediment and geomorphic mapping sampling and analysis of serial aerial imagery cores and gpr profiles show that the lithosome of the active platte is 6 8 m in thickness and consists predominantly of medium sand although macroform features are common on the surface of the modern active central and lower platte river in nebraska the shallow subsurface stratigraphy is dominated by mesoform cross bedding and planar stratification overlying channel scour fills this differs markedly from the vertical succession predicted by the platte river facies model of miall 1977 data from inactive portions of the central and lower platte river however show macroform cross bedding indicative of the downstream and cross stream migration of linguoid and transverse bars which would be predicted by the published model the typical architecture of the holocene channel belt sand body in the areas studied thus comprises a lower unit dominated by macroform scale cross bedding overlain by an upper unit dominated by mesoform scale cross bedding and scour fills furthermore time series analysis of aerial photography of the river in the vicinity of grand island nebraska shows that 1 the river has become dominated by aggressively vegetated subaerially exposed channel bed domains termed vegetated channel islands that superficially resemble bars and 2 that this stretch is no longer dominated by downstream accreting macroform bars which were abundant in the 1960s and 1970s contraction of the active channel in this region has accompanied these changes somewhat independently of discharge variation new vertical sequence facies models are presented to account for these differences our data show the futility of seeking to characterize a fluvial system by means of a single vertical sequence model and furthermore indicate that the holocene facies architecture of the sandy braided platte river varies both in space and in time the dominance of mesoform scale structure in the shallow subsurface of the central and lower platte river likely reflects the ongoing process of gradual abandonment of the system it further highlights the need for caution in interpreting subsurface structure from surface geomorphology in fluvial systems

volcano tectonic and geochemical evolution of an oceanic intra plate volcano tahiti nui french polynesia abstract geological mapping accurate k ar dating and geochemical analyses of lavas allow a detailed reconstruction of the geological history of tahiti nui island french polynesia the exposed volcanic activity is first characterized by the construction of a main shield from 1 4 ma to 870 ka with a maximum aerial eruptive rate around 2 km3 kyr lavas from this early building stage are alkaline slightly silica undersaturated with 87sr 86sr and 143nd 144nd compositions rather constant and close to the enriched mantle ii type vent locations were first concentrated along a main e w rift zone which was responsible for the lateral collapse of the northern and southern flanks of the main shield around 0 87 ma ago the subsequent activity was first restricted to the northern depression corresponding to an eruptive rate of about 5 km3 kyr in the period 850 760 ka significant variations in la sm 87sr 86sr and 143nd 144nd occur in lavas erupted immediately after the main northern landslide indicating a sudden increase in the extent of partial melting likely caused by the decompression subsequent to collapse however the later activity declined and the lavas exhibit a gradual change toward strongly silica undersaturated basanites likely indicating a decreasing extent of partial melting of the upper mantle the evolution of radiogenic isotope ratios over time indicates a change in the source toward more depleted compositions until around 500 ka post erosion volcanic activity following an apparent hiatus of 240 kyr exhibits similar major and trace element and isotope compositions the volcano structural and geochemical evolution of tahiti nui and the overall alkaline character of the lavas from other eruptive complexes of the society alignment suggest a relative weakness low temperature and low eruptive rate of the society plume compared to the hawaiian hot spot

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

a global geochemical mass budget applied to the congo basin rivers erosion rates and continental crust composition rivers carry the products of continental denudation either in a dissolved form chemical erosion or in a solid form physical erosion we focus in this paper on the relationship between physical erosion and chemical erosion we establish the mass budget of the congo basin rivers using chemical complementarities between river suspended sediments sandy bedload and dissolved load of the congo basin rivers reported in a previous paper dupre et al 1995 a series of equations are presented assuming that the physical and chemical erosion processes are in a steady state during one year the total mass of river borne material dissolved and particulate transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time we show that the local continental crust on each drainage basin can be estimated and solve our steady state weathering model using an inversion procedure the very good agreement between modelled and measured values of the river suspended sediment concentrations validates the steady state hypothesis in this wet tropical area consequently in this area the sediment yield provide a good estimate of the rates of mechanical denudation this result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load erosion rates for the silicate upper crust and thus independent of the lithological variability silicates evaporites and carbonates of the drainage basins are calculated mechanical erosion rates and chemical erosion rates for the congo basin at brazzaville are 8 t km2 y and 5 t km2 y the corresponding consumption of atmospheric co2 by weathering process is estimated to 51 103 mol km2 an these weathering and consumption rates are low in spite of the severity of the weathering conditions of the high soil temperature and of the intensity of precipitations these conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering finally by estimating the local continental crust chemical composition before the onset of erosion processes especially for the most soluble elements we can test the model of taylor and mclennan

growth and erosion rates of the east carpathians volcanoes constrained by numerical models tectonic and climatic implications abstract the east carpathian volcanic range experienced an along arc late miocene to quaternary migration of eruptive activity during its 11 ma long activity here a novel and complex methodology is presented that yields new geochronological and geomorphological constraints on the evolution of the 20 volcanic edifices new unspiked k ar ages either constrain their lifespan 6 79 0 10 6 47 0 09 ma for seaca t tarca 5 47 0 08 4 61 0 07 ma for v rghis or date the youngest volcanic activity central c limani in parallel numerical reconstructions of volcanic paleo topographies were performed to quantify their shape at the end of their construction stage the inferred initial volcano size shows a wide range 3 to 592 km3 making up the four main successive volcanic segments 910 880 279 and 165 km3 for c limani gurghiu north harghita and south harghita segment respectively totalizing 2200 km3 and an average growth rate of 200 km3 ma at range scale at the volcano scale with only consideration to their respective time spans i e avoiding repose periods growth rate is characterized by two major trends a moderate growth rate 137 km3 ma for the older volcanoes 11 3 6 ma followed by a lower growth rate 28 km3 ma obtained for the plio quaternary volcanoes comparing reconstructed and current topographies yielded a total eroded volume of 524 125 km3 defining averaged denudation of 22 and a 20 m ma erosion rate erosion rates for major climatic periods were computed which highlight the contrasting climatic contexts since 11 ma the highest erosion rate 38 m ma occurred during transitional moderate subtropical continental climate period 9 5 8 2 ma an intermediate erosion rate 14 m ma characterized a moderate continental climate period 8 2 6 8 ma when conditions became less humid the lowest erosion rate 7 m ma reflects the prevailing continental but occasionally semi arid climate 6 8 5 8 ma the highest erosion rate 28 m ma was obtained for plio quaternary times during the interglacial glacial cycles such a quantitative morphometric and geochronological approach demonstrates its efficiency to study volcanic dynamism including both constructional and erosional processes through time

segmentation of the lesser antilles subduction zone abstract precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the benioff zone beneath the lesser antilles island arc there are two distinct segments to the zone one to the north of martinique which trends nnw and one to the south of st lucia which trends nne the northern zone dips at 60 50 the southern zone has a dip of 50 45 in the north but is vertical in the south pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the benioff zone mid miocene swarms of dykes in martinique and st lucia have nw and ne azimuthal modes respectively we interpret this to mean that this kink in the subduction zone existed as long ago as the miocene the kink may represent the deformation of a single subducting plate or two separate north and south american plates subducting beneath the caribbean plate with a triple junction in the region of martinique st lucia

morphometry and evolution of arc volcanoes volcanoes change shape as they grow through eruption intrusion erosion and deforma tion to study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs central america and the southern central andes using shuttle radar topography mission srtm digital elevation models we compute and defi ne parameters for plan ellipticity irregularity and profi le height width summit basal width slope shape as well as size height width volume we classify volcanoes as cones sub cones and massifs and recognize several evolutionary trends many cones grow to a critical height 1200 m and volume 10 km 3 after which most widen into sub cones or massifs but some grow into large cones large cones undergo sector collapse and or gravitational spreading without sig nifi cant morphometry change other smaller cones evolve by vent migration to elliptical sub cones and massifs before reaching the critical height the evolutionary trends can be related to magma fl ux edifi ce strength structure and tectonics in particular trends may be controlled by two balancing factors magma pressure versus lithostatic pressure and conduit resistance versus edifi ce resistance morphometric analysis allows for the long term state of individual or volcano groups to be assessed morphological trends can be integrated with geological geophysical and geochemical data to better defi ne volcano evolution models

systematic morphometric characterization of volcanic edifices using digital elevation models abstract quantitative characterization of the size and shape of volcanic edifices is an essential step towards the understanding of factors controlling volcano growth and morphology the recent advent of digital elevation models dems with worldwide coverage offers the opportunity to systematically document the morphometry of all types of volcanoes using quantitative well formalized methodologies we present a methodology for the morphometric characterization of volcanic edifices after reviewing previous studies on volcano morphometry and the various existing dem sources we describe an integrated procedure that uses a dem and its derived products slope curvature to extract a coherent set of morphometric parameters for a given volcanic edifice edifice boundaries are manually defined by searching for breaks in slope around the base the parameters describe the overall size basal and summit region area and widths height volume planar shape ellipticity and irregularity index of elevation contours profile shape height width ratios and slope of the edifice similar parameters for relatively large depending on dem spatial resolution summit craters calderas are also computed slope values and ellipticity and irregularity indexes are extracted for successive height intervals providing detailed information of volcano shape as a function of height the number of secondary peaks is also estimated the method is tested on thirteen composite volcanoes in nicaragua using three dem datasets 90 m srtm 30 m aster g dem and an 80 m topographic map derived dem and the resulting parameters are evaluated in terms of boundary delineation and dem source finally the parameters obtained for the nicaraguan volcanoes are discussed as an illustrative example of the type of data and information that can be extracted systematically for volcanoes worldwide

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

reconstructing eroded paleovolcanoes on gran canaria canary islands using advanced geomorphometry abstract original volcanic edifices of two successive stages of gran canaria are reconstructed using a geomorphometric analysis of existent or restored paleosurfaces in the reconstruction surface fitting was applied preferably to planezes i e triangular facets of original volcano flanks and quasi planar surfaces qps those occurring on planezes or scattered slightly eroded portions derived from original cone surfaces with the help of red relief image map rrim analysis out of the long lasting mid miocene to holocene subaerial evolution of the island the late miocene fataga volcano and the subsequent pliocene roque nublo volcanoes were the largest and highest the eruptive center of fataga a composite edifice 12 2 8 8 ma that may have grown up excentrically with respect to the previous tejeda caldera is well defined by both two planezes named veneguera mogan and fataga tirajana and qps remnants its calculated original volume 1000 km3 is close to the largest stratovolcanoes on earth however its 3300 m elevation obtained by exponential fit may have been significantly lower due to the complex architecture of the summit region e g a caldera responsible for ignimbrite eruptions roque nublo a 3 7 2 9 ma stratovolcanic cone which was superimposed upon the fataga rocks 3 km west of the fataga center has left no considerable paleosurfaces behind due to heavy postvolcanic erosion yet its remnant formations preserved in a radial pattern unambiguously define its center moreover surface fitting of the outcropping rocks can be corrected taking the erosion rate for the past 3 ma into account such a corrected surface fit points to a regular shaped 3000 m high cone with a 25 km radius and ca 940 km3 original volume also comparable with the dimensions of the largest terrestrial stratovolcanoes

the polygenetic kahouanne seamounts in the northern lesser antilles island arc evidence for large scale volcanic island subsidence abstract remotely operated vehicle rov explorations of the kahouanne seamounts located 25 km se of montserrat in the northern lesser antilles island arc have discovered the occurrence of honeycomb scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level mbsl these features combined with the flat topped morphology of the seamounts the occurrence of shallow water carbonates rhodoliths benthic foraminifera and the presence of oxidized highly vesicular volcanic fragments suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth the subsidence is likely to have been in response to a combination of 1 graben development southeast of the island of montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the lesser antilles volcanic arc and 2 regional subsidence inferred from studies of nearby carbonate platforms 40ar 39ar dating of basalt andesite lavas from the central seamount indicates active volcanism around 5 4 ma shallow water carbonates recovered from an apparent wave cut terrace at 600 mbsl in the same area were deposited approximately 3 ma based on sr isotope stratigraphy indicating significant subsidence of the complex since that time the total subsidence is in line with displacements in the kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of guadeloupe and martinique implying that intra arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc

eruptive history and geochronology of the mount baker volcanic field washington mount baker a steaming ice mantled andesitic stratovolcano is the most conspicuous component of a multivent quaternary volcanic field active almost continuously since 1 3 ma more than 70 packages of lava flows and 110 dikes have been mapped 500 samples chemically analyzed and 80 k ar and 40ar 39ar ages determined principal components are 1 the ignimbrite filled kulshan caldera 1 15 ma and its precaldera and postcaldera rhyodacite lavas and dikes 1 29 0 99 ma 2 60 intracaldera hydrothermally altered andesite dacite dikes and pods remnants of a substantial early postcaldera volcanic center 1 1 0 6 ma 3 unaltered intracaldera andesite lavas and dikes including those capping ptarmigan and lasiocarpa ridges and table mountain 0 5 0 2 ma 4 the long lived chowder ridge focus 1 29 0 1 ma an andesite to rhyodacite eruptive complex now glacially reduced to 50 dikes and remnants of 10 lava flows 5 black buttes stratocone basaltic to dacitic and several contemporaneous peripheral volcanoes 0 5 0 2 ma and 6 mount baker stratocone and contemporaneous peripheral volcanoes 0 1 ma to holocene glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field although more than half the material erupted has been eroded liberal and conservative volume estimates for 77 increments of known age yield cumulative curves of volume erupted vs time that indicate eruption rates in the range 0 17 0 43 km3 k y for major episodes and long term background rates of 0 02 0 07 km3 k y andesite and rhyodacite each make up nearly half of the 161 56 km3 of products erupted whereas basalt and dacite represent only a few cubic kilometers each representing 1 3 the total during the past 4 m y the principal magmatic focus has migrated stepwise 25 km southwestward from the edge of the chilliwack batholith to present day mount baker

volcanic geomorphology an overview abstract the review examines the role of geomorphology in analyzing the volcanoes on earth five objectives are stressed first classifications of volcanic landforms should be improved to take care of the complexity in volcanic landform generation as magmatic systems style of eruption and the erupted material all influence the morphology second geomorphology should contribute to the science of volcanology through its capability in reconstructing growth stages in complex volcanoes and also in analyzing the structural factors which contribute to the catastrophic collapse of volcanoes third geomorphology can contribute to physical volcanology by assessing the effects of topography on transport erosion and deposition of volcanogenic flows and identifying the sources and climatic tectonic conditions which govern the emplacement of volcaniclastic deposits fourth volcanic geomorphology a identifies sedimentary facies associations b constructs facies models for dynamic volcano delivery systems and c analyzes the characteristics of sediment gravity flows in order to determine relevant parameters for modelling their behaviour fifth process oriented geomorphology is critical in developing accurate methods for measuring rates of geomorphic processes that shape ephemeral volcanic constructs and for evaluating and comparing geomorphic impacts on disturbed catchments and the related hydrologic response before during and after eruptions this should help to refine parameters for the exponential decay model finally volcanic geomorphology is essential for risk assessment through geomorphic hazard zonation and composite risk zonation such treatments are necessary in order to face the enhanced challenge posed by the combination of natural hazards and the increasing number of people who are at risk around volcanoes

a new 3d fault model of the bouillante geothermal province combining onshore and offshore structural knowledge french west indies abstract the bouillante area hosts geothermal resources located in a complex structural area guadeloupe island french west indies on one hand faults observed on the field mainly elongate along the e w direction on the other hand offshore structures interpreted from marine seismic lines shows a larger range of directions a coherent 3d interpretation is proposed through a fault model combining onshore and offshore structural knowledge in a zone crossing the island coastline the fault network constructed reveals a hierarchy in the family of structures and highlights the prevalence of the nnw sse direction associated with secondary ne sw trending structures and the e w direction on a geographical point of view the modelled faults are gathered in 3 clusters data available to build the 3d fault model are sometimes sparse especially inland because of intense vegetation cover consequently not only the results and impacts of the 3d fault model are discussed but also its limitations as well as its possible evolution

k ar dating extended into the last millennium application to the youngest effusive episode of the teide volcano spain less than one thousand years dating of the youngest volcanic episode of the teide volcano canary islands spain has been performed using the k ar cassignol technique analyses of about 20 g of pure alkali feldspar yielded a weighted mean age of 800 300 years stratigraphic historic and archeomagnetic dating validate this age and rule out the hypothesis of the presence of significant excess argon contamination for this flow our result shows that the last effusive activity of the teide volcano took place shortly before european settlement in tenerife island this study confirms that the cassignol technique can be applied with success for historic dating and demonstrates that it can now even be extended to the last millennium

geomorphological study of long term erosion on a tropical volcanic ocean island tahiti nui french polynesia abstract a quantitative geomorphological study has been made on 27 river basins in tahiti nui volcanic island french polynesia to reconstruct the erosional evolution of a young oceanic island subjected to heavy tropical rainfall tahiti nui is composed of a main shield volcano cut by two huge landslides on each side of a main e w rift zone the northern landslide depression was rapidly buried by the construction of a second shield the late activity of which overflowed the crest and then filled the southern landslide depression the island is now volcanically inactive and is deeply dissected by erosion the present geometries of the river basins are first compared using dimensionless parameters derived from a digital elevation model the original volcanic surfaces are then reconstructed to estimate the volumes removed by erosion and determine the average rates of long term erosion the basins developed on the flanks of the main shield are wider shallower and gentler than the basins incising the post landslide second shield indicating a higher degree of evolution rainfall concentration on the windward eastern side of the island also contributed to increase the vertical lowering of the volcanic relief and the enlargement of the valleys the magnitude of erosion however is neither directly linked with the age of the units incised nor with the differential amounts of rainfall erosion rates determined over the last 1 myr range between 10 3 km 3 kyr 1 and 0 25 km 3 kyr 1 the highest values occur in the basins incising the main e w rift zone and or the lateral rims of the northern and southern landslide depressions long term dissection has thus been enhanced along the geological discontinuities of the eruptive system deep erosion was first constrained along the axis of the main e w rift zone where numerous dykes compartmentalize the volcanic structure into large unstable blocks dykes most probably acted as mechanical discontinuities along which shallow gravitational landslides recurrently occurred such mass wasting episodes produced significant amounts of debris partly preserved as highly indurated sedimentary breccias of various ages exposed at various locations subsequent dissection of tahiti nui was enhanced to the north and to the south leading to the rapid evolution of the papenoo and taharuu drainage systems over the last 500 kyr long term dissection on tahiti nui has been responsible for the removal of at least 350 km 3 of volcanic material from the surface and for the partial exhumation of a shallow intrusive complex partly composed of coarse grained plutonic rocks gabbros and syenites in the central part of the eruptive system structurally controlled erosion is thus a key component of landscape evolution on such high relief oceanic tropical islands

volcanic history reconstruction in northern ecuador insights for eruptive and erosion rates on the whole ecuadorian arc in the northern andes the ecuadorian arc presents a large number of quaternary volcanoes spread over a rather restricted area the origin of this volcanic clustering is not well understood and only a few chronological data older than the holocene are available in northern ecuador to document the arc development stages in this study we present new k ar ages obtained on lava flow and pumice samples for cushnirumi mojanda fuya fuya imbabura cubilche and cusin volcanoes located about 40 km north of quito the ecuador s capital city our results show that the volcanic activity in the northern part of the ecuadorian arc started at least at 1 ma and that construction of volcanoes mainly occurred during the last 500 ka together with the radiometric data numerical reconstructions of the paleomorphology of the volcanoes are used to estimate the volume of emitted magmas and the amount of eroded material in order to quantify their eruptive and erosion rates emission rates of ecuadorian volcanoes range between 0 2 and 3 6 2 1 km3 kyr highest rates are obtained for volcanoes constructed over time periods shorter than 100 kyr by sporadic eruptive pulses whereas lowest rates are calculated over longer periods that include quiescence phases erosion rates range between 0 02 0 01 and 0 14 0 09 km3 kyr and highlight that volcanic edifices whose activity ended recently are rapidly dismantled by physicochemical processes finally the spatial distribution of quaternary volcanoes as well as the spatio temporal evolution of lava geochemistry may reflect the progressive influence of the carnegie ridge at depth

geomorphological evolution of montserrat west indies importance of flank collapse and erosional processes analysis of topography and new swath bathymetry as well as geophysical data provides information about aerial and submarine morphological features and mass transfer processes on montserrat the island has a characteristic shallow c 4000 years ago or the boxing day event during the current eruption 26 december 1997 montserrat9s older volcanic centres the centre hills and silver hills illustrate the third stage of evolution extinction and erosion magma production long term erosion and total sedimentation rates on montserrat have been estimated as 0 17 km 3 ka 1 0 0125 km 3 ka 1 and 0 11 km 3 ka 1 i e 1 1 cm ka 1 respectively

catastrophic flank collapses and slumping in pico island during the last 130 kyr pico faial ridge azores triple junction abstract the pico island constitutes the easternmost sub aerial domain of a steep wnw ese volcanic ridge which has developed within the nubia eurasia diffuse plate boundary azores triple junction the island comprises three volcanic systems from older to younger the topo volcano the fissural system and the pico stratovolcano from a high resolution digital elevation model 10 m and new bathymetric stratigraphic structural and high precision k ar data we reconstruct the main successive stages of growth and partial destruction of the island over the last 200 kyr we especially concentrate on the central sector of the island which has recorded gradual movements through slumping and catastrophic flank collapses since ca 130 kyr the remmants of the topo volcano are partly exposed on pico s se flank and are here dated between 186 5 and 115 4 ka topo was significantly destroyed by n and s directed large scale flank collapses between ca 125 and 70 ka on pico s n flank collapse seems to have removed all the unstable material but in the s the collapse structure is composite including a major flank collapse and a remnant slump complex that is still active a first episode of deformation occurred between ca 125 and 115 ka along the master fault of the slump between ca 115 and 69 ka most of the unstable material was removed by a major flank collapse leaving behind a still considerable volume of unstable material that comprises the active slump this first collapse was catastrophic and generated a large debris deposit recognized on the high resolution bathymetry with a minimum run out of ca 17 km the scar was partially filled by volcanic products erupted from volcanic cones developed within the slump depression and possibly also from the early wnw ese fissural system subsequent deformation in the slump area affected in part the filling units leading to the individualization of secondary curved faults younger volcanic products have gradually masked the mass wasting scars unlike the well known hilina slump hawaii pico s slump evolution might be controlled by an active regional tectonics

construction and destruction of mont pelee volcano volumes and rates constrained from a geomorphological model of evolution this study presents long term volumes and construction rates for the mont conil mont pelee volcano and rate estimates at which volcanic activity creates relief an algorithm shapevolc is used to numerically model topographic surfaces volcano morphology is analyzed using current digital elevation model in combination with mapped geology to produce 10 paleotopographies at the end of four constructional stages and three destructional events volumes of each constructional stage were estimated at about 35 2 km3 26 2 km3 8 3 km3 and 2 5 km3 for a total cumulative erupted volume of 72 2 km3 we estimate that mont pelee accounted for about 10 of the lesser antilles arc production in the last 100 kyr the volcano has been built at an average rate of 0 13 km3 kyr during the last 550 kyr during that time construction rates varied by a factor of 15 from 0 04 km3 kyr in early stages up to 0 52 km3 kyr after the second flank collapse volumes displaced by each flank collapse were estimated at 14 7 km3 8 8 km3 and 3 5 km3 thus about 37 of the total constructed volume integrated over the volcano s lifetime the rate at which flank collapses removed material off the island is 0 15 km3 kyr in contrast long term erosion rates outside collapsed areas are estimated at about 0 05 0 7 km3 kyr or 11 km3 of material removed this latter rate is not negligible which strengthens the importance of taking into account recurrent small erosional events on the geomorphological evolution of a volcanic island in a tropical context

eruptive history of the late quaternary ciomadul csomad volcano east carpathians part ii magma output rates this study which builds on high precision unspiked cassignol gillot k ar age determinations presents an advanced dem based volumetrical analysis to infer long term magma output rates for the late quaternary ciomadul csomad dacitic lava dome complex east carpathians romania the volcanic field of ciomadul developed on the erosional surface of lower cretaceous flysch and 2 ma old andesites and experienced an extended eruptive history from 850 to 30 ka predominantly effusive activity took place during the first stage 850 to 440 ka producing volumetrically minor isolated peripheral domes subsequently after a 250 ky repose interval a voluminous central dome cluster developed in the second stage 200 to 30 ka during the youngest phase of evolution 60 to 30 ka highly explosive eruptions also occurred resulting in the formation of two craters mohos and st ana the calculated 8 00 0 55 km3 total volume of the lava domes which includes the related volcaniclastic 1 57 km3 as well as erosionally removed 0 18 km3 material is in line with dimensions of other medium sized dacitic lava domes worldwide this volume was extruded at an average long term magma output rate of 9 76 km3 my 0 0098 km3 ky however most of the domes 7 53 0 51 km3 were formed in the 200 to 30 ka period implying a significantly increased magma output rate of 37 40 km3 my 0 0374 km3 ky more than 30 times higher than in the first stage within these long term trends individual lava domes of ciomadul e g those with volumes between 0 02 and 0 40 km3 would have been emplaced at much higher rates over a period of years to tens of years the active periods lasting up to hundreds of years would have been followed by repose periods 30 times longer the most recent eruption of ciomadul has been dated here at 27 7 1 4 ka this age which is in agreement with radiocarbon dates for the onset of lake sediment accumulation in st ana crater dates fragmented lava blocks which are possibly related to a disrupted dome this suggests that during the last typically explosive phase of ciomadul lava dome extrusion was still ongoing in a global context the analysis of the volumetric dynamism of ciomadul s activity gives insights into the temporal variations in magma output at lava domes short term day or week scale eruption rates smooth out in long term millenia scale output rates which are tens of times lower

comment on construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores by sibrant et al 2014 and proposal of a new model for graciosa s geological evolution abstract volcanoes rising above sea level within extensional oceanic plate boundaries provide accessible locations with which to study the effects of plate tectonic and volcanic processes of such areas however relying solely on subaerial observations can lead to biased interpretations reconciling the information provided by multibeam echo sounders on the submarine parts of volcanic islands with geology and geomorphology observable above sea level can potentially provide more robust interpretations in this comment of the study of sibrant et al 2014 which is based almost solely on subaerial observations we show how the published multibeam sonar data around graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high resolution bathymetry the data reveal no large scale debris avalanche deposits or characteristic flank collapse scars where sibrant et al 2014 propose these landslides to have occurred instead the data show volcanic constructional areas some of which have simply been eroded by wave abrasion the interpretation of collapse structures appears to have originated partly from a misreading of the volcano stratigraphy and tectonic structures overall wave erosion coupled with subaerial erosion and tectonic activity can more easily explain the onshore observations of sibrant et al 2014 providing a less catastrophic explanation for the evolution of graciosa island

quantifying chemical weathering rates along a precipitation gradient on basse terre island french guadeloupe new insight from u series isotopes in weathering rinds abstract inside soil and saprolite rock fragments can form weathering clasts alteration rinds surrounding an unweathered core and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates recently uranium series u series disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds to further investigate whether the u series isotope technique can document differences in long term weathering rates as a function of precipitation we conducted a new weathering rind study on tropical volcanic basse terre island in the lesser antilles archipelago in this study for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient electron microprobe emp point measurements bulk major element contents and u series isotope compositions were determined in two weathering clasts from the deshaies watershed with mean annual precipitation map 1800 mm and temperature mat 23 c on these clasts five core rind transects were measured for locations with different curvature high medium and low of the rind core boundary results reveal that during rind formation the fraction of elemental loss decreases in the order ca na k mg si al zr ti fe such observations are consistent with the sequence of reactions after the initiation of weathering specifically glass matrix and primary minerals plagioclase pyroxene weather to produce fe oxyhydroxides gibbsite and minor kaolinite uranium shows addition profiles in the rind due to the infiltration of u containing soil pore water into the rind as dissolved u phases u is then incorporated into the rind as fe al oxides precipitate such processes lead to significant u series isotope disequilibria in the rinds this is the first time that multiple weathering clasts from the same watershed were analyzed for u series isotope disequlibrian and show consistent results the u series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a u series mass balance model the weathering advance rates generally decreased with decreasing curvature 0 17 0 10 mm kyr for high curvature 0 12 0 05 mm kyr for medium curvature and 0 11 0 04 0 08 0 03 0 06 0 03 mm kyr for low curvature locations the observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models i e that the curvature of the rind core boundary controls the porosity creation and weathering advance rates at the clast scale at the watershed scale the new weathering advance rates derived on the low curvature transects for the relatively dry deshaies watershed average rate of 0 08 mm kyr map 1800 mm and mat 23 c are 60 slower than the rind formation rates previously determined in the much wetter bras david watershed 0 18 mm kyr low curvature transect map 3400 mm and mat 23 c also on basse terre island thus a doubling of map roughly correlates with a doubling of weathering advance rate the new rind study highlights the effect of precipitation on weathering rates over a time scale of 100 kyr weathering rinds are thus a suitable system for investigating long term chemical weathering across environmental gradients complementing short term riverine solute fluxes

fast geochemical changes and rapid lava accumulation at stromboli island italy inferred from k ar dating and paleomagnetic variations recorded at 60 and 40 ka abstract forty three lava flows from the vallone di rina stromboli aeolian islands italy have been investigated for paleomagnetic geochemical and k ar analyses k ar dating shows that two fast building stages occurred at 62 1 and 40 2 ka the comparison with archeomagnetic variations for the last 2 kyr shows that the time interval covered by the vallone di rina section is at the most a few thousand years most probably on the order of a few hundred years such high accumulation rate can be related to the stromboli volcanic history which is marked by five to seven successive collapses the rapid effusion rate of these paleo stromboli pst ii lavas suggests that they have been erupted shortly after the paleo stromboli i collapse in response to pressure release due to instantaneous deloading their age of 62 1 ka is thus interpreted here as the age of the paleo stromboli i collapse event similarly the age of 40 2 ka obtained for lavas from paleo stromboli iii can be related with another collapse event as previously recognized major and trace elements of these lava flows show that they are compatible with magma sources of metasomatized mantle origin and various crustal levels of melting and mixing they display recurrent jumps from calc alkaline to high k calc alkaline and shoshonitic series within a very short time span which suggest that several magmatic systems were operating simultaneously finally we have shown that the combined approach of absolute dating provided by the k ar cassignol technique together with high resolution paleomagnetic data compared to historical variations can constrain for high extrusion rates the relative lava emplacements to within a few hundred years

volcano flank instability in the lesser antilles arc diversity of scale processes and temporal recurrence 1 the 1997 boxing day collapse a remarkable feature of the ongoing eruption of soufriere hills on montserrat has prompted new interest in the study of volcano stability in the lesser antilles building on a few cases documented in the literature we have now identified at least 47 flank collapse events on volcanoes of the caribbean arc where this type of behavior is characteristic and repetitive about 15 events occurred on active volcanoes within the last 12 000 years in the northern part of the arc flank collapses are repetitive do not exceed 1 km3 in volume occur in all directions and are promoted by intense hydrothermal alteration and well developed fracturing of the summit part of the edifices in contrast infrequent but large sector collapses with volumes up to tens of km3 are typical of the southern volcanoes they are always directed to the west as a result of the high overall slopes of the islands toward the deep back arc grenada basin because caribbean islands are small a large part of the resulting debris avalanches have flowed into the sea thus contributing voluminous and sudden inputs of volcaniclastic sediments to the grenada basin deposits from such submarine flows have been identified during the recent aguadomar and caraval oceanographic cruises and traced to their source structures on land edifice collapses have a major influence on subsequent volcanic activity but also are of high concern because of their tsunamigenic potential

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates

erosion rates and erosion patterns of neogene to quaternary stratovolcanoes in the western cordillera of the central andes an srtm dem based analysis abstract erosion patterns and rates of 33 stratovolcanoes in the arid to hyperarid central andean volcanic zone 14 s to 27 s have been constrained by morphometric modelling all selected volcanoes belong to the short lived symmetrical circular andesitic stratocone type with ages spanning 14 ma to recent starting from the initial youthful volcano morphology of this type represented in our study by parinacota volcano and comparing reconstructed volumes of progressively eroded volcanoes such a time span allows us to infer long term erosion rates typical erosion rates of