evolution of the eastern volcanic ridge of the canary islands based on new kar data the results of 64 new k ar age determinations together with 32 previously published ages show that after a period of erosion of the basal complex miocene volcanic activity started around 20 ma in fuerteventura and 15 ma in lanzarote forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs this series includes five separate volcanic edifices each one with its own eruptive history in fuerteventura several miocene eruptive cycles have been identified in the central edifice one around 20 17 ma followed by two others centred around 15 and 13 ma in the southern edifice the maximum of activity took place around 16 14 ma whereas in the northern one the main activity occurred between 14 and 12 ma in lanzarote a first cycle of activity took place in the southern edifice between 15 5 and 14 1 ma followed by another between 13 6 and 12 3 ma in the northern edifice three pulses occurred 10 2 8 3 6 6 5 3 and 3 9 3 8 ma an important temporal gap greater in fuerteventura than in lanzarote separates series i from the plio quaternary series ii iii and iv formed by multi vent basaltic emissions in fuerteventura the following eruptive cycles have been identified 5 2 9 2 4 1 8 1 7 0 8 0 4 and 0 1 ma in lanzarote the activity was fairly continuous from 2 7 ma to historic times with a maximum in the lower pleistocene eruptive rates in the series i edifices were on the average 0 1 0 01 km3 ka comparable but slightly smaller than in similar edifices in tenerife and la gomera but much lower than in gran canaria average post miocene eruptive rates were about 0 013 0 027 km3 ka in lanzarote and 0 003 0 007 km3 ka in fuerteventura all these volcanic edifices show a similar general sequence fissural eruptions erosion multi vent volcanism repeated at different periods in different parts of the eastern islands of the canaries the model of growth of the series i edifices is comparable to those in tenerife and la gomera long periods of activity sometimes greater than 6 m y with pulses separated by gaps however salic and intermediate differentiates frequent in tenerife and la gomera are very scarce in these islands the fuerteventura lanzarote ridge shows a decrease in volcanic activity with time and also a certain ssw nne polarity in the temporal development of volcanism

analysis and modelling of the aeromagnetic anomalies of gran canaria canary islands abstract in this paper we present a crustal magnetic model of the volcanic island of gran canaria based on aeromagnetic data this magnetic study has made it possible to constrain the location and geometry of 1 the mafic core of gran canaria built as the result of the crystallization of magmas which rose from the mantle during the submarine and shield basaltic stages of its evolution the most intense magnetic high displayed over the nw part of the island likely shows the main feeding system of the shield volcano 2 the residual syenitic shallow magma chamber which fed the salic volcanic activity in the center of the island linked to the formation of the tejeda caldera and to the intrusion of felsic rocks which made up a cone sheet 3 a reversely magnetized linear intrusion located in the marine area to the nw of the island which could be related to the magmatic source of the submarine volcanism between gran canaria and tenerife most of these magnetic sources show a linear pattern with trends that are in close agreement with the orientations of previously identified fractures the magmatic intrusion to the nw of the island could be related with a wsw ene active fault between gran canaria and tenerife while the main trend and location of the mafic core agree with a nw se fault suggested by geological studies this means that these magnetic sources are the result of the ascent and intrusion of magma along regional fractures therefore this study provides fresh data which demonstrate the influence of regional tectonics on the growth of gran canaria during its entire evolution finally the spectral analysis of the magnetic anomaly map suggests that it is possible that rocks located at mantle like depths from the moho to about 23 km behave as magnetic sources a fact that can be related with the magmatic underplating detected by other geophysical techniques

giant quaternary landslides in the evolution of la palma and el hierro canary islands in the past large morphological escarpments in the canaries have been generally related to explosive and or erosive processes recent onshore and offshore investigations drastically changed this interpretation by providing evidence of the importance of giant lateral collapses in the evolution of the islands especially in their earlier stages of growth giant landslide scars and deposits are readily observed both onshore and offshore in the younger western canaries and seem to be a common feature of the development of the entire archipelago at least one catastrophic collapse is apparent on la palma the cumbre nueva giant landslide which occurred about 560 ka ago this collapse removed some 200 km3 of central western la palma forming a large embayment three successive giant landslides and evidence of an aborted attempt at another are seen in the island of el hierro the combined volume of those collapses estimated at about 400 500 km3 considerably exceeds the present subaerial volume of the island about 140 km3 giant landslides and erosion during the past million years have removed more than half of the total subaerial volume of la palma and el hierro

unrest signals after 46 years of quiescence at cumbre vieja la palma canary islands abstract monogenetic eruptions are the most common volcanic activity in the world however unrest monitoring data are scarce due to the long intervening quiescence periods this study analyzes unrest signals recorded in one of the largest monogenetic fields in the canary islands cumbre vieja la palma two seismic swarms were registered in october 2017 and february 2018 with b values of 1 6 0 1 and 2 3 0 2 respectively suggesting an intense magmatic fluids contribution gas and or magma both swarms were linked to changes in gas emissions increases in hydrogen concentration and r ra c up to 7 52 0 05 were recorded before the first swarm at the sampling point closest to where seismicity was located indicating a deep gas input after the second swarm increases in r ra c and thoron soil concentration were recorded at two locations this dataset is compatible with a stalled magmatic intrusion at ca 25 km depth with an estimated volume between 5 5 10 4 km3 and 3 10 2 km3

internal structure of the western flank of the cumbre vieja volcano la palma canary islands from land magnetotelluric imaging 1 large scale mass wasting is a natural part of the evolution of volcanic islands where deformation and indications of flank instability such as large scale faulting and seismic and aseismic slip are common the cumbre vieja volcano on the island of la palma canary islands provides an ideal setting to address fundamental questions about the structure evolution and stability of island volcanoes the island of la palma is still in a shield building stage and it has been postulated that the western side of the island lies over a pre existing zone of weakness that can nucleate ruptures we undertook an audiomagnetotelluric amt survey over the proposed unstable western flank to try to image structures that may be associated with the zone of weakness the magnetotelluric method mt is a geophysical technique used to map the presence of fluids or image important structural contrasts the goals of this study were 1 to delineate the unstable flank 2 to map the structures underneath and 3 to determine the presence and geometry of fluids the results show a 1 km thick top resistive layer overlaying an area of reduced resistivity enhanced conductivity interpreted as a layer consisting of an alteration zone and also fluids our results confirm previous studies that suggested the existence of a western flank lying over collapse debris material and hyaloclastites and also they allow us to map part of the subaerial southern extent of the cumbre nueva units that lie beneath the more recent cumbre vieja rocks in addition dimensionality analysis maps the rotation of the dike emplacement off ridge along the western flank in an en echelon fashion

lyttelton volcano banks peninsula new zealand primary volcanic landforms and eruptive centre identification abstract lyttelton volcano banks peninsula new zealand has historically been viewed as a simple volcanic cone this paper uses digital terrain models dtm and primary volcanic landforms to reinterpret lyttelton volcano as having multiple eruptive centres primary volcanic landforms are features produced during active volcanism classified as constructional hypabyssal and erosional volcanic features constructional volcanic features are lava flows scoria cones and domes hypabyssal volcanic features are dykes and sills and erosional volcanic features are valley and ridge patterns and orientations lava flow trends are recognised from aerial photograph analysis and supported by field observations highlighting radiating lava trends around specific locations within lyttelton harbour scoria cones and domes occur on the outer flanks of volcanic cones and are used as such in the identification of remnant cone surfaces dyke orientations are plotted and then projected to the interior of the volcano defining 13 zones of convergence the projected arrays of these orientations highlight defined regions along the erosional crater rim each indicating a radial dyke swarm from which the projected trends of the associated dykes indicate an eruptive centre valley and ridge orientations are projected from the longest valley or ridge segment towards the inner harbour radiating erosive patterns are incepted during the growth and degradation of a volcanic cone with the resulting trends orienting to the summit zones of convergence eruptive centres are identified from lava flow orientations onlapping lava sequences scoria cones and intrusive locations the summit of a volcanic edifice can be identified from the orientations of valleys and ridges while radial dyke systems determine whether this summit was a volcanic centre or simply a local topographic high volcanic landforms are used to identify cone sectors the preserved sector associated with a particular eruptive centre cone sector limits are defined by a basal footprint and an erosional crater rim with similar arcuate features remnant cone features being exposed in the interior of the volcano lyttelton volcano comprises fifteen volcanic cones with vent locations controlled by underlying fault lineaments the growth and erosion of each volcanic cone is reflected in primary volcanic landforms with the preserved features of cones confined to cone sectors and cone artefacts

time evolution of deformation using time series of differential interferograms application to la palma island canary islands differential interferometry is a very powerful tool for detecting changes in the earth s crust where coherence conditions are good but is difficult to employ in some volcanic areas due to dense vegetation we apply two differential insar methods using the time series associated with the interferograms to perform a phase analysis on a data set for la palma island canary islands from the ers 1 and ers 2 european space agency esa satellites for the time period 1992 to 2000 both methods involve choosing a master image from the database and creating a series of interferograms with respect to this image the coherent pixel time series cpts technique chooses pixels with good average coherence aligns the unwrapped interferograms with a stable area and then performs an inversion to calculate the linear velocity to quantify the deformation the coherent target modeling ctm method calculates the temporal coherence of each pixel to identify stable targets and then determines the best velocity for each pixel by using a linear fit that maximizes the temporal coherence using these two methods we have been able to detect deformation on la palma island that has been previously undetectable by conventional insar methods there is a roughly circular region on the southern part of the island that is actively deforming at 4 to 8 mm yr this region is located near the teneguia valcano the host of the last known eruption on la palma in 1971 a thorough investigation of the possible sources for this deformation revealed that it was most likely created by a subsurface thermal source

long term 17 ma turbidite record of the timing and frequency of large flank collapses of the canary islands volcaniclastic turbidites on the madeira abyssal plain provide a record of large volume volcanic island flank collapses from the canary islands this long term record spans 17 ma and comprises 125 volcaniclastic beds determining the timing provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the canary islands especially the western islands of tenerife la palma and el hierro these turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers furthermore the last large volume failures from tenerife coincide with explosive volcanism at the end of eruptive cycles many large volume canary island landslides also occurred during periods of warmer and wetter climates associated with sea level rise and subsequent highstand however these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant

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

shear zones as a result of intraplate tectonics in oceanic crust the example of the basal complex of fuerteventura canary islands ductile shear zones affecting igneous rocks pyroxenites syenites gabbros and carbonatites belonging to the basal complex of the island of fuerteventura are described here they develop typical mylonites with their cortege of structures and crystallographic fabrics an evolution from brittle to ductile conditions has been recognized and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks carbonatites syenites along the shear zones local metamorphic changes favour the ductile behaviour via a reaction enhanced softening mechanism the shear zones are arranged in a nearly orthorhombic pattern kinematic criteria suggest an extensional tectonic scenario with a bulk irrotational nonplane deformation the long axis of the finite deformation ellipsoid is e w and horizontal the available data allow us to establish this deformation as being from the late oligocene to the early miocene this is tentatively related to the plate tectonics evolution and mantle anomalies in the northwestern corner of the african plate

changing depths of magma fractionation and stagnation during the evolution of an oceanic island volcano la palma canary islands la palma canary islands represents an oceanic island volcano with an active rift zone inferred to have formed during the last 800 ka following southward growth of the former radial symmetrical stratovolcano taburiente we carried out clinopyroxene melt thermobarometry and microthermometry of fluid inclusions to reconstruct the evolution of the magma plumbing systems over time and to understand the genetic relationship between taburiente and the presently active cumbre vieja rift zone clinopyroxene melt equilibria of phenocryst rims and glassy groundmass indicate pressures of 0 60 1 04 gpa 19 34 km depth for taburiente 0 47 1 17 gpa 16 40 km for the former cumbre nueva rift arm of taburiente and 0 50 0 78 gpa 16 26 km for bejenado volcano that formed after collapse of the cumbre nueva rift these pressures are interpreted to reflect depths of magma storage and major crystal fractionation co2 dominated fluid inclusions hosted by clinopyroxenes and olivines indicate pressures of formation or re equilibration within an overall range of 0 25 0 61 gpa 8 19 km depth respective frequency maxima are at 0 41 0 50 gpa for taburiente dunite xenoliths 0 26 0 43 gpa for cumbre nueva ankaramites and 0 26 0 32 gpa for bejenado cumulate xenoliths these pressures are interpreted to reflect levels of temporary magma stagnation during ascent our data show that the magma pathways during all volcanic phases including the presently active cumbre vieja rift klugel a hansteen t h galipp k 2005 magma storage and underplating beneath cumbre vieja volcano la palma canary islands earth and planetary science letters 236 211 226 are characterized by two distinct storage levels a system of prolonged storage within the upper mantle and a system of short term stagnation within the lower crust or near to the moho both the mantle and crustal storage systems show a migration to shallower levels from 1 0 ma to present probably as a result of changing thermomechanical properties of the mantle and crust and possibly stoping our combined barometric data and field observations suggest that the extinct taburiente cumbre nueva and the active cumbre vieja represent two distinct volcanoes with separate magma plumbing systems in this case the present rift configuration does not reflect continuous growth of the taburiente shield volcano during the last 800 ka none of the la palma volcanoes shows any indicators of a long lived shallow magma reservoir where magmas fractionate and from which rift zones emanate which is an important difference to hawaiian shield volcanoes characterized by shallow subcaldera magma chambers

volcanic evolution of the island of tenerife canary islands in the light of new k ar data new age determinations from tenerife together with those previously published 93 in all provide a fairly comprehensive picture of the volcanic evolution of the island the oldest volcanic series with ages starting in the late miocene are formed mainly by basalts with some trachytes and phonolites which appear in anaga teno and roque del conde massifs in anaga ne three volcanic cycles occurred one older than 6 5 ma a second one between 6 5 and 4 5 ma with a possible gap between 5 4 and 4 8 ma and a late cycle around 3 6 ma in teno nw after some undated units the activity took place between 6 7 and 4 5 ma with two main series separated by a possible pause between 6 2 and 5 6 ma in the zone of roque del conde s the ages are scattered between 11 6 and 3 5 ma between 3 3 and 1 9 ma the whole island underwent a period of volcanic quiescence and erosion the large canadas volcano made up of basalts trachytes and phonolites was built essentially between 1 9 and 0 2 ma to the ne of this central volcano linking it with anaga is a chain of basaltic emission centers with a peak of activity around 0 8 ma the canadas caldera had several collapse phases associated with large ignimbrite emissions there were at least an older phase more than 1 ma old on the western part of the volcano and a younger one less than 0 6 ma old in the eastern side the two large valleys of guimar and la orotava were formed by large landslides less than 0 8 ma ago and probably before 0 6 ma ago the present canadas caldera was formed by another landslide less than 0 2 ma ago this caldera was later filled by the huge teide volcano which has been active even in historic times during the same period a series of small volcanoes erupted at scattered locations throughout the island the average eruptive rate in tenerife was 0 3 km3 ka with relatively small variations for the different eruptive periods this island and la gomera represent a model of growth by discontinuous pulses of volcanic activity separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years the neighbouring gran canaria on the other hand had an initial rapid shield building phase during which more than 90 of the island was built and a series of smaller pulses at a much later period a comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation several gaps are present on each island overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the canaries

submarine landslides around the canary islands the morphology and structure of the submarine flanks of the canary islands were mapped using the gloria long range side scan sonar system bathymetric multibeam systems and sediment echosounders twelve young 2 ma giant landslides have been identified on the submarine flanks of the canary islands up to now older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the canary islands most slides were found on the flanks of the youngest and most active islands of la palma el hierro and tenerife but young giant landslides were also identified on the flanks of the older 15 20 ma but still active eastern islands large scale mass wasting is an important process during all periods of major magmatic activity the long lived volcanic constructive history of the islands of the canary archipelago is balanced by a correspondingly long history of destruction resulting in a higher landslide frequency for the canary islands compared to the hawaiian islands where giant landslides only occur late in the period of active shield growth the lower stability of the flanks of the canaries is probably due to the much steeper slopes of the islands a result of the abundance of highly evolved intrusive and extrusive rocks another reason for the enhanced slope instability is the abundance of pyroclastic deposits on canary islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas dike induced rifting is most likely the main trigger mechanism for destabilization of the flanks flank collapses are a major geological hazard for the canary islands due to the sector collapses themselves as well as triggering of tsunamis in at least one case a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse

the contribution of landslides to landscape evolution in europe abstract after a review of some analytical and conceptual models of slope evolution with the intervention of mass movements a proposal is made of a series of indicators for the quantitative description of landscape evolution in relation to mass movements the ratio between landslide mobilisation rate and downwearing rate is proposed as a quantitative measure of the significance of mass movements in landscape evolution a subdivision of european regions with respect to their susceptibility to landsliding as a process of landscape evolution is proposed the role of landsliding for the evolution of landscape in some regions is described the data presented show that landsliding may in some cases be the main contributor to landscape change

geodetic and structural research in la palma canary islands spain 1992 2007 results we review the results of the geodetic and structural studies carried out on la palma island using geodetic and geophysical data during the period 1992 2007 insar and gps observation techniques were applied to study the existence of deformation on the island and gravity observations were carried out for structural studies gravity data were inverted using a nonlinear three dimensional gravity inversion approach to obtain the geometry of the anomalous bodies constructed in a random growth process with respect to an exponentially stratified background the main structural feature is a large central body under the caldera de taburiente with high density which was interpreted as the pliocene age uplifted seamount and a relatively dense intrusive plutonic complex magma body the cumbre vieja series is characterized by elongated minima distributed according to the rift structure insar results show a clear subsidence located on the teneguia volcano where the last eruption took place in 1971 a thermal source is the most probable origin for this deformation a gps network composed of 26 stations covering the total island surface was set up vertical displacements determined comparing the gps coordinates obtained in 2007 with coordinates determined in 1994 are consistent with the insar results obtained in the southern part of the island this is not the case for the northern part from the comparison of 2006 and 2007 coordinates it is clear that more time is needed to obtain significant displacements but observed trends are also consistent with insar results all the observed significant displacements are in stations located outside of the large high density central body

k ar chronology of the volcanic eruptions in the canarian archipelago island of la gomera a geochronological study of the island of la gomera canaries has been carried out by the k ar method the 26 new ages obtained together with the 17 previous determinations show that above the main unconformity of the island separating the basal complex from the later volcanic series there is a unit of lower old basalts more than 10 m y old polymictic volcanic breccias were emplaced between 10 and 9 m y ago the upper old basalts above them were formed between 9 and 6 m y ago with a peak of activity around 7 m y after a period of erosion 6 5 m y a thick series of young basalts associated with trachytic and phonolitic domes and flows were rapidly emplaced between 4 5 4 m y ago finally local basaltic activity took place 2 8 m y ago the age of the basal complex is not well known although three ages 14 6 15 5 and 19 3 m y have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex

structure composition and age of the small islands of santa luzia branco and raso cape verde archipelago abstract the northern alignment of cape verde archipelago contains three small islands in its central sector santa luzia branco and raso on which there is very scarce geological petrological or geochronological information available the three islands along with the island of sao vicente arise on a continuous underwater relief less than 50 m deep and about 70 km long that formed from separate volcanic edifices the southernmost island raso is formed by accumulation of several tens of lava flows and some remaining strombolian cones an erosion surface covered by sediments allows the separation of two different episodes of growth the lower one older than 2 ma and the upper one between 0 9 and 1 2 ma the smaller island of branco is the remnant of a large stratovolcano about 6 km in radius 1000 m of altitude and between 5 and 6 ma in age santa luzia the northernmost and largest island of this particular group is essentially a remaining rift zone between 4 and 7 ma old at least 600 m high about 10 km long and 7 km wide the se extreme of the island is occupied by rocks of an older partially submarine stage of growth which also include gabbroic intrusions crossed by a dense network of dikes whose age may be estimated in 8 ma or more on the contrary the nw end shows the only remains of late post erosion activity that is of much less volumetric importance and it has been dated between 0 8 and 1 4 ma the three islands as a whole are formed by moderately or strongly alkaline basic and ultrabasic rocks similar to those found in the neighboring island of sao vicente ultraalkaline rocks only appear in raso and differentiate types as trachyandesites and phonolites are exclusively found in santa luzia the similar age found for the main stages of growth recorded in the mentioned four islands and even more in the other two islands santo antao and sao nicolau situated at the end of the barlovento alignment are not easily comprehensible with evolution models implying a simple spatial progression of the volcanic activity from east to west in this array of the archipelago on the other hand the temporal coincidence in periods of quiescence observed in the different islands seems to point to the existence of a cyclical activity for the ensemble

helium emission at cumbre vieja volcano la palma canary islands abstract we report herein the results of three soil helium surveys undertaken at cumbre vieja volcano la palma island canary islands helium content analyses at the surface environment of the volcano were carried out in the summers of 2002 2003 and 2004 to estimate the diffusion contribution of helium emission soil porosity was estimated indirectly from the granulometry data measured on a previous soil survey carried out in 1997 helium enrichments in the soil layer with respect to the air concentration measured on cumbre vieja indicate a strong structural control in the degassing processes of the volcano and the excess helium seems to be emitted mainly along both n s and n w rifts of the volcano 4 he flux values calculated following a pure diffusive model ranged between non detectable values and 1 0 mg m 2 d 1 in 2002 2003 and 2004 surveys total 4 he emission rate at cumbre vieja was estimated in the range of 18 38 kg d 1 isotopic composition of helium measured in soil gases along the n s volcanic rift of cumbre vieja showed small contributions of morb type helium increasing towards the southern part of the volcano a 3 he 4 he ratio of 9 95 0 12 r a was measured at a cold spring geothermal sample showing a 7 2 contribution of plume type helium 3 he emission results in the range of 0 6 0 7 mol yr support the hypothesis of contemporary degassing at cumbre vieja coming from magmas stored at different depths under la palma island

constructive and destructive episodes in the building of a young oceanic island la palma canary islands and genesis of the caldera de taburiente the results of new field observations 23 new k ar determinations and sixteen previously published determinations provide the basis for the reconstruction of the subaerial volcanic history of the island of la palma after the seamount activity represented by the materials of the basal complex an eruptive phase between 2 0 and 1 3 ma formed a large shield a period of volcanic quiescence followed until around 1 ma during which a large lateral collapse partly destroyed the former edifice between 1 05 and 0 7 ma activity was renewed in the shield and a n s ridge was built in the southern part of the island around 0 7 ma two new large lateral collapses affected the western part of both edifices and they were followed by eruptions between 0 71 and 0 65 ma which built a new edifice that partly filled the depressions thus created the caldera de taburiente constitutes the eroded remnants of the depression formed in the northern shield from 0 65 ma to present activity has been restricted to the n s ridge which has continued to grow southwards there was a general n s migration of volcanic activity with time but in the shield the trend was northwest to southeast eruptive rates seem to have been fairly constant during the different eruptive phases considered between 0 15 and 0 37 km3 ka a very similar succession of constructive and destructive episodes has been obtained for the neighboring island of hierro but in this case the activity started around 0 8 ma and eruptive rates were about 0 5 km3 ka

eruptive response of oceanic islands to giant landslides new insights from the geomorphologic evolution of the teide pico viejo volcanic complex tenerife canary large sector collapses are a major component of oceanic islands evolution here we show that voluminous events such as the icod landslide on tenerife canary islands cause dramatic changes on the magma feeding system and control the subsequent volcanic and geomorphologic evolution of the eruptive complex over a period of more than 150 kyr instantaneous unloading by the icod landslide is marked by the development of a large phonolitic explosive eruption dated at 175 3 ka and interpreted as reflecting the immediate emptying of a shallow pre existing magma chamber geochronological geomorphological and geochemical analyses carried out on the post landslide volcanic succession sampled in a 4 4 km long underground water recovery gallery provide further evidence for an enhanced extrusion of primitive lavas starting in the 10 kyr time interval following the failure rapid construction 40 kyr of a thick basaltic volcano in the landslide scar at high eruptive rates up to 8 km3 kyr 1 increased the lithostatic pressure which then favored the intermittent storage of basic magma under the edifice this resulted in more episodic construction evidenced by a significant decrease in output rates and the increasing occurrence of lavas with intermediate composition from 117 7 to 52 7 ka an apparent volcanic gap is observed between 52 7 and 18 1 ka after which highly differentiated lavas have been dominantly erupted we propose that part of the gap can be explained by the individualization of a shallow magma reservoir a few kilometers below the base of the teide volcano during recent periods vertical and lateral extrusions of trachytic and phonolitic viscous bodies from this storage area contributed to increase the slope of the main edifice up to 35 overall favoring its present day instability

ar ar ages from transitionally magnetized lavas on la palma canary islands and the geomagnetic instability timescale 1 a detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of barranco de los tilos on the island of la palma canary islands reveals a complex temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events the matuyama brunhes m b reversal is recorded in five transitionally magnetized lava flows from the north tn section the isochrons obtained from three of the lower four m b lavas are defined by 14 incremental heating experiments that together with a previous age determination yielded a weighted mean of 798 4 6 2 ka all uncertainties 2s in addition a 780 3 10 3 ka isochron was determined for the overlying transitionally magnetized flow indicating that it was erupted during a distinctly younger portion of the transition near the base of the south ts section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole vgp positions in the southwest indian ocean between latitudes 56 s and 65 s suggesting instability of the geomagnetic field beyond that of typical secular variation 40 ar 39 ar isochrons from three of these flows defined by 11 separate incremental heating experiments gave a weighted mean of 822 2 8 7 ka this anomalous field behavior recorded 24 11 kyr prior to the m b reversal may coincide with an event featured in several marine sediment records directly above two normal polarity flows 40 ar 39 ar isochrons of 751 9 18 1 ka and 675 0 15 7 ka are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude vgps spanning 23 60 n these flows are then capped by a single flow possessing normal polarity based on 12 incremental heating experiments 40 ar 39 ar isochrons of five of these nine lavas along with the uppermost flow gave a weighted mean age of 580 2 7 8 ka for this period of transitional to normal field behavior from these same transitional lavas quidelleur et al 1999 reported three unspiked k ar ages with a weighted mean of 602 24 ka and proposed a new event called the la palma excursion however the 40 ar 39 ar age presented here is three times more precise than the k ar age and is indistinguishable at the 95 confidence level from the 40 ar 39 ar age of a lava from the snake river plain idaho that originally defined the big lost event transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the big lost event recorded at la palma was indeed global in extent rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process index terms 1560 geomagnetism and paleomagnetism time variations secular and long term 1520 geomagnetism and paleomagnetism magnetostratigraphy 1035 geochemistry geochronology 1513 geomagnetism and paleomagnetism geomagnetic excursions keywords argonargon dating lavas excursion paleomagnetism reversal

volcanic evolution of sao vicente cape verde islands the praia grande landslide abstract the island of sao vicente has undergone continuous volcanic activity from pliocene to pleistocene times the earliest evidence of activity corresponds to some 9 million years ago when a submarine edifice of alkaline basaltic affinity had already started to develop the island resulted from the growth of a single major edifice the sao vicente edifice built up in several distinguishable growth stages the early main stage occurred at 6 5 to 4 5 ma giving rise to an edifice of about 10 12 km in radius and more than 2500 m in height whose centre was located south of the present city of mindelo although the edifice in its earlier phase showed typical characteristics of a shield volcano the mindelo formation it adopted later on those of an ordinary composite volcano the madeiral monte cara formation a giant landslide event the praia grande landslide destroyed the ne sector of the edifice and left a 10 12 km large depression that was successively refilled by nephelinitic lava flows and some associated carbonatites the monte verde formation 4 5 3 1 ma the volcanic activity ceased about 3 2 ma ago when the edifice started to be deeply eroded until disappearing about 90 of its total volume only in recent times 0 3 ma very scarce and local strombolian activity has been developed aside the sao vicente edifice in the eastern sector of the island

the chemically zoned 1949 eruption on la palma canary islands petrologic evolution and magma supply dynamics of a rift zone eruption the 1949 rift zone eruption along the cumbre vieja ridge on la palma involved three eruptive centers 3 km spaced apart and was chemically and mineralogically zoned duraznero crater erupted tephrite for 14 days and shut down upon the opening of llano del banco a fissure that issued first tephrite and after 3 days basanite hoyo negro crater opened 4 days later and erupted basanite tephrite and phonotephrite while llano del banco continued to issue basanite the eruption ended with duraznero erupting basanite with abundant crustal and mantle xenoliths the tephrites and basanites from duraznero and llano del banco show narrow compositional ranges and define a bimodal suite each batch ascended and evolved separately without significant intermixing as did the hoyo negro basanite which formed at lower degrees of melting the magmas fractionated clinopyroxene olivine kaersutite ti magnetite at 600 800 mpa and possibly 800 1100 mpa abundant reversely zoned phenocrysts reflect mixing with evolved melts at mantle depths probably as early as 1936 hoyo negro basanite entered the deep rift system at 200 350 mpa some shallower pockets of this basanite evolved to phonotephrite through differentiation and assimilation of wall rock a few months prior to eruption a mixing event in the mantle may have triggered the final ascent of the magmas most of the erupted tephrite and basanite ascended from mantle depths within hours to days without prolonged storage in crustal reservoirs the cumbre vieja rift zone differs from the rift zones of kilauea volcano hawaii in lacking a summit caldera or a summit reservoir feeding the rift system and in being smaller and less active with most of the rift magma solidifying between eruptions

enhancement of sub daily positioning solutions for surface deformation surveillance at el hierro volcano canary islands spain el hierro island in the canary archipelago recently experienced a submerged eruption a few kilometers off its southern shore detected 2011 october 10 on the island s south rift alignment the seismic activity suddenly increased around mid 2011 july and ground deformation was then detected on the only geodetic benchmark that is continuously observed by global navigation satellite systems techniques and provides public data access based on that information several other global navigation satellite system signal receivers were deployed on the island to provide continuous observation for data collected by these receivers a processing strategy was applied to achieve millimeter level half hourly positioning solutions position updates every 24 h are satisfactory to determine tectonic plates velocities updates near 1 s or less are required to characterize seismic waves in between minute level updates are well suited for monitoring active volcano s inflation or deflation providing an optimal time resolution of the local ground deformation in half hourly positioning solutions the heterogeneous satellites distribution in their orbital planes gives different constraints during satellite constellation revolution which can bias the solutions also several geophysical influences can bias the solutions including those related to gravitational movements these influences have mostly semi diurnal periodicities and may be considered gaussian colored noise on the position s time series daily solutions that average out these influences can be applied in active volcanoes but they can impose some limitations because they average the daily deformation and the update waiting time is not suitable when near real time surveillance is mandatory these semi diurnal biases do need to be removed or minimized to achieve millimeter level sub daily positioning solutions however and to do so a discrete kalman filter was applied to enhance the half hourly positioning solutions required during el hierro s 2011 2012 unrest and eruption throughout el hierro s volcanic activity there were correlations between ground deformation and seismic activity many times the deformation preceded the earthquakes though at other times the seismic activity was followed by the ground deformation response this correlation is the outcome of ground deformation taking place as the result of energy accommodation whereas seismic events correspond to energy release hence those observed correlations indicate that the kalman filter enhanced half hourly positioning solutions measured local ground deformation accurately they were not a mathematical trick producing a spurious precision

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

giant miocene landslides and the evolution of fuerteventura canary islands abstract fuerteventura is the oldest island in the canaries archipelago following the growth of a pre miocene seamount its shield stage began 20 6 ma ago and ended in the mid miocene there followed a very extended period of quiescence then very minor quaternary post erosional volcanism the shield stage produced volcanoes up to 3000 m above sea level which were rapidly eroded by 17 5 ma both fuerteventura and lanzarote the older eastern most islands are in a post erosional stage and have subdued topography with elevations rarely rising above 400 m the western islands la palma and el hierro together with tenerife are the youngest in the archipelago and are still in their shield stage which began at most 7 5 ma ago they have a rugged topography with peaks rising to several thousand metres volcanic activity persists in these islands and in lanzarote the shield stage construction of the volcanically active western islands demonstrates that recent eruptive activity has concentrated in volcanic centres aligned in well defined rift zones in which major dyke emplacement has taken place and that volcano flank collapse has occurred with giant landslides initiated on the rifts in fuerteventura most of the shield stage volcanic rocks have been removed however evidence has now been advanced to show that major dyke swarms were emplaced in the oligocene and miocene these belonged to a line of intrusive complexes above which by the early to mid miocene volcanic peaks had been constructed three volcanic centres have been identified and in the central one a peak as high as the present mount teide on tenerife was rapidly denuded in less than 2 ma in the west of the island this erosion has exposed a window of more than 300 km 2 of submarine volcanics and sediments belonging to the pre shield phase seamount the principal mechanism of erosion seems to have been the generation of giant landslides analogous to those seen in the younger islands these appear to have removed some 3500 km 3 of lavas and volcaniclastics stripping the western sectors of the miocene volcanoes down to the pre shield phase basal complex and transporting the mass of volcanic material into the atlantic ocean as debris flows remnants of the shield volcanoes still exist in the eastern part of the island present day topographic variation indicates that the miocene base level of erosion is still visible in a central depression which runs through the axis of the island the eastern hills are composed of the miocene shield stage volcanics which still preserve a drainage pattern initiated on these peaks and the basal complex in the west has a juvenile landscape suggesting incision on a post miocene domal uplift of the volcano core structural links between intra island volcano tectonic events and the south atlas fault zone are not thought to be likely

the primitive volcano of mount pelee its construction and partial destruction by flank collapse abstract the geology of mount pelee the active volcano of martinique lesser antilles has been actively studied during the past decade this paper reviews the current knowledge about the oldest least well known part of the volcano here called paleo pelee and the relations between paleo pelee and the mont conil volcanic complex paleo pelee consists mostly of volcanic breccias with a few lava flows it differs in this respect from neighbouring active volcanoes such as the grande decouverte soufriere in guadeloupe and the soufriere in st vincent the oldest products of which are mainly lava flows paleo pelee initially formed a regular cone which was comparable in size with the present edifice it is now largely eroded and its products form prominent radial ridges on the western flank of mount pelee these are buried under younger deposits on the eastern flank the uniformly andesitic breccias of paleo pelee are for the most part brecciated lava flows it is inferred that the activity of mount pelee was less explosive at this stage than during following periods neo pelee the paleo pelee formations overlap those of mont conil the geology of mont conil is poorly known however its lowest exposed products are submarine andesitic breccias cut by many dikes this contrasts with paleo pelee where all breccias are of subaerial origin pleistocene limestones are interbedded within the mont conil breccias between 120 and 150 m above sea level though mont conil seems to be closely linked to mount pelee on the basis of available petrological data a major uplift must have occurred sometime during or after the construction of mont conil and forms a recognizable geological boundary between the two volcanic systems the paleo pelee edifice was partially destroyed by a flank collapse event that occurred on the southwest flank during the early stage 40 000 20 000 y b p of neo pelee this event is evidenced by a large 6 2 5 km horseshoe shaped structure which is limited by well preserved morphological scarps on land and by a 300 m offshore scarp as shown by seabeam bathymetry paleo pelee breccias and lava flows are truncated by the boundary of the collapse structure lava domes occupied the upslope part of the structure at a later time these domes are intersected by the present crater etang sec which has probably been the active vent for at least 2000 years the active crater is located at the intersection between the flank failure depression and an older caldera formed at the paleo pelee stage but probably enlarged later the flank failure structure appears as a major structure of the volcano and creates a strong structural asymmetry in the edifice this fact should be considered when dealing with such various aspects as the hydrogeology of the volcano the structural control of past and future eruptions and the volcanic hazards

turbidite record of frequency and source of large volume 100 km3 canary island landslides in the last 1 5 ma implications for landslide triggers and geohazards during the last two decades numerous studies have focused on resolving the landslide histories of the canary islands issues surrounding the preservation and dating of onshore and proximal submarine landslide deposits precludes accurate determination of event ages however submarine landslides often disaggregate and generate sediment gravity flows volcaniclastic turbidites sampled from madeira abyssal plain piston cores represent a record of eight large volume failures from the western canary islands in the last 1 5 ma during this time there is a mean recurrence rate of 200 ka while the islands of el hierro and tenerife have individual landslide recurrences of 500 ka and 330 ka respectively deposits from the 15 ka el golfo landslide from el hierro and 165 ka icod landslide from tenerife are examined this study also identifies potential deposits associated with the orotava 535 ka guimar 850 ka and rogues de garcia landslides 1 2 ma from tenerife el julan 540 ka and el tinor 1 05 ma landslides from el hierro and the cumbre nueva landslide 485 ka from la palma seven of eight landslides occurred during major deglaciations or subsequent interglacial periods which represent 55 of the time however all of the studied landslides occur during or at the end of periods of protracted island volcanism which generally represent 60 of the island histories although climate may precondition failures it is suggested that volcanism presents a more viable preconditioning and trigger mechanism for canary island landslides

site amplification from coda waves validation and application to s wave site response abstract the subject of this article is to see if the response to s waves at a specific site can be determined from coda waves we used aftershock records of the 1992 landers and big bear earthquakes observed at four terrascope stations pas pfo svd and gsc in southern california the wide dynamic range of the terrascope instrument provided a broadband seismogram for s waves and at the same time coda waves over a long lapse time window we analyzed a total of 708 three component waveforms from 65 events whose magnitudes ranged from 3 3 to 5 4 the station svd is located on quaternary alluvium and the remaining stations are on rocks of cretaceous age we first evaluated the coda site amplification factor using a spectral ratio method may eda et al 1991 koyanagi et al 1992 for frequency bands centered at 0 75 1 0 1 5 2 0 3 0 4 0 and 6 0 hz for lapse time windows after 70 sec from the origin time we found that the site amplification factor for the radial component is similar to that for the transverse component this similarity was expected because codas consist of backscattered waves coming from all directions into the station resulting in a directionally averaged site response we then evaluated the s wave site amplification factor using an inversion method andrews 1986 on the direct s wave portion of the same data set we found that the coda site response agrees with the s wave site response within a factor of about 1 5 supporting the earlier findings by tsujiura 1978 and justifying the use of coda waves in determining site specific amplification for s waves this agreement between s wave site response and coda site response is understandable if we assume that coda waves are dominated by s waves as theoretically predicted by aki 1992 and zeng 1993

why fine tree roots are stronger than thicker roots the role of cellulose and lignin in relation to slope stability plant roots help to reinforce the soil increase slope stability and decrease water erosion root tensile strength plays an important role in soil reinforcement and slope stabilization the relationship between tensile strength and internal chemical composition of roots is unknown due to limited studies thus it is difficult to determine why root tensile strength tends to decrease with increasing root diameter in this study biomechanical and biochemical tests were performed on the roots of chinese pine pinus tabulaeformis to determine the relationships among tensile strength and the contents of the main chemical composition cellulose alpha cellulose and lignin in the roots with different diameters our results confirmed that the tensile strength of chinese pine roots decreased with increasing root diameter and this relationship might be a power function the chemical contents of the roots and root diameter were also related to each other with significant power regression with increasing root diameter the cellulose content and alpha cellulose content increased but the lignin content decreased in addition the lignin content exhibited a significantly positive relationship with tensile strength furthermore the ratios of lignin cellulose and lignin alpha cellulose decreased with increasing root diameter following significant power regressions and they also demonstrated a positive relationship with tensile strength taken together these results may be useful for studies on root tensile strength soil reinforcement and slope stability

landslides and the evolution of el hierro in the canary islands seismic and sonar data have been used to evaluate the extent and characteristics of giant landslides on the flanks of el hierro in the canary islands as the youngest and most southwesterly of the canary islands el hierro has experienced rapid growth and destructive events in its 1 12 million year history at least four giant landslides el golfo el julan san andres and las playas have modified 450 km3 of el hierro during the last 200 300 thousand years with each landslide event removing around 3 of the total edifice volume the extent of landsliding indicates that it is the main process of decay we characterise flank morphology around el hierro and distinguish between rugged unfailed flank failed flank and steep gullied ridge flanks affected by landsliding have downslope long profiles with distinctive b coefficients and exponential forms the el golfo landslide is the most recent 15 ka best described and clearly defined landslide in the canary islands the el julan landslide sw flank has an estimated volume of 130 km3 an age of 200 ka and is characterised by gravitational slumping on the se flank two new landslide events are reported the younger landslide las playas occurred 145 176 ka has a narrow steep sided embayment and a corresponding blocky debris avalanche deposit the older landslide san andres is recognised on the basis of a highly chaotic seismic facies offshore and reduced upper flank gradients its lack of an upper flank embayment and offshore blocky debris avalanche lead us to interpret that the landslide involved gravitational slumping possibly a series of events which reduced upper flank gradients but did not catastrophically collapse to produce a debris avalanche

the history of intrusive activity on the island of la palma canary islands abstract the island of la palma located in the northwest of the canary archipelago consists of two main geological units 1 the subaerial extrusive coberta series covers most of the island and consists of alkali basaltic lavas phonolitic plugs and pyroclastic rocks erupted between 1 61 ma and the recent 2 the basement complex is exposed in the barranco de las angustias and the caldera de taburiente and consists of the submarine intrusive and extrusive seamount series and a basal plutonic complex alkali basaltic dikes of different ages and orientations intruded in both units three groups of dikes and sills can be distinguished based on their field relationships petrography geochemistry and ages the oldest group i represent the feeder dikes to the seamount series intruded between 2 9 and 4 0 ma b p they were originally steep with a n05 w strike and perpendicular to the originally nearly flat bedding of the layered seamount series subsequent to emplacement group i dikes and the extrusives of the seamount series were rotated into their present orientation group ii sills intruded parallel to the bedding of the seamount series 45 230 are coeval with or younger than group i but older than the erosional unconformity separating the coberta and the basement complex i e 1 6 ma 50 vol of the volcanic edifice of the la palma seamount and highlight the importance of intrusive processes for island growth intrusions into central oceanic volcanoes like la palma may result in an uplift and rotation of more than 30 km 3 by up to 45

paleomagnetic determinations on lanzarote from magnetic and gravity anomalies implications for the early history of the canary islands the bouguer and aeromagnetic anomaly maps of lanzarote show a gravity high and a dipolar magnetic anomaly over the central part of the island indicating one isolated source assuming that the structure responsible for both anomalies is the same a methodology has been designed to estimate the total magnetization vector of the source which is interpreted as a large intrusive body mafic core positioned as a result of magma rising to the surface during the early stages of growth of lanzarote considering its geometry to be known from a previous three dimensional 3 d gravity model the approach proposed in this paper is based on the delineation of magnetic contacts through analysis of the horizontal gradient of the reduced to the pole anomaly map comparison between the gravity and the pseudogravity anomalies and 3 d forward magnetic modeling the total magnetization vector obtained by this method is defined by a module of 4 5 a m 1 and a direction d 20 degrees and i 30 degrees comparing the paleomagnetic pole obtained from this direction with the apparent polar wander path of africa for the last 160 myr it is concluded that the main component of the total magnetization vector is probably a primary natural remanent magnetization nrm which could have been acquired between 60 and 100 ma this result suggests that the emplacement of magmas at shallow depths linked to the beginning of volcanism in lanzarote took place during the upper cretaceous thus providing the first evidence of a timeline for the early formative stages of this volcanic island

magnetic imaging of the feeding system of oceanic volcanic islands el hierro canary islands summary el hierro is the youngest of the canary islands a volcanic archipelago in the central atlantic near the african coast the subaerial part of the island shows the characteristic shape of three convergent ridges that has been interpreted as a triple arm rift system at least four giant landslides formed wide horseshoe embayments that separate these ridges recent studies based on high resolution bathymetry however showed that the submarine rift structure is much more complex we analysed an aeromagnetic anomaly data set acquired in 1993 by the spanish national geographic institute in order to obtain a structural model of the island from a magnetic point of view a digital elevation model of the volcanic edifice was divided into a mesh of prismatic cells each of them with a top corresponding to the topographic height or bathymetric depth in the marine area and a bottom at a constant depth of 4000 m below sea level a three dimensional 3 d inversion algorithm and forward modelling along representative profiles provided us with a magnetization distribution containing valuable information about the inner structure of the island the magnetic model cast new light on the rift structure of el hierro in particular high magnetization values have been mainly interpreted as intrusive complexes on which rift zones are rooted their location confirms the hypothesis of a complex rift structure in the marine area the inverse magnetization that characterizes the ne submarine rift area implies that this part of the volcanic edifice formed during the matuyama and therefore predates the nw submarine rift zone which is normally magnetized the n s rift zone extending southwards from the island seems to be shifted to the west with respect to the bathymetric high in this area this result suggests that the original rift zone was located in the area where the highest magnetizations presently occur so that the present morphology may reflect the westward collapse of the original ridge in addition very low magnetizations characterize the areas affected by giant landslides indicating that magnetic anomalies can provide important constraints on the distribution of these catastrophic events

magma storage and underplating beneath cumbre vieja volcano la palma canary islands we have conducted a barometric study of basaltic lavas and mafic to ultramafic xenoliths from cumbre vieja volcano la palma in order to reconstruct magma storage plumbing and related intrusive island growth the samples were collected both along the volcano s rift zone and from its subaerial and submarine flanks up to 30 km off the rift axis clinopyroxene melt barometry of lavas yields a well defined pressure range of 410 770 mpa average 600 mpa and shows no systematic variation with sample locality this pressure range reflects a major fractionation level at ca 15 26 km depth within the uppermost mantle where phenocrysts and melt last equilibrated in contrast microthermometry of co2 dominated fluid inclusions in phenocrysts and xenoliths gives pressure estimates of 240 to 470 mpa average 330 mpa within the lower oceanic crust these pressures are interpreted to reflect an underplating zone at 7 14 km depth where ascending magmas become ponded prior to eruption and where passing magmas temporarily stagnate during eruption the underplating zone is inferred to extend to a radius of ca 30 km at least and its depth appears to increase slightly with distance to the rift axis our data thus show a remarkable pressure bimodality for long term and short term storage of magma that appears to be characteristic for cumbre vieja by using a simple model based on the cumulative volume of material added to the crust we have estimated uplift rates of 0 5 m ka due to underplating that occurs on the island scale on a time scale of some 10 ka these uplift rates are of similar magnitude as global changes in sea level a second more local mode of uplift and related faulting is caused by shallow intrusions into the volcanic edifice as is indicated by the highly irregular morphology of the cumbre vieja we stress that intrusion related uplift should be qualitatively considered when reconstructing the evolution of oceanic island volcanoes since most of the underplated material is inferred to consist of cumulate rocks being intimately mixed and juxtaposed with the lower oceanic crust and having similar densities it is probably impossible to detect such an underplating zone seismically

the 40ar 39ar age dating of the madeira archipelago and hotspot track eastern north atlantic the 40ar 39ar ages for 35 volcanic rocks and 14c ages for two charcoal samples from the madeira archipelago and ampere seamount eastern north atlantic are presented the volcanic evolution of madeira can be divided into a voluminous shield stage 4 6 0 7 ma and a subsequent low volume posterosional stage 70 myr old hotspot which formed porto santo island 11 1 14 3 ma seine ampere 31 ma corral patch and ormond 65 67 ma feraud et al 1982 1986 seamounts and the serra de monchique 70 72 ma mcintyre and berger 1982 complex in southern portugal age and spatial relationships result in a calculated absolute african plate motion above the hotspot of 1 2 cm yr around a rotation pole located at 43 36 n 24 33 w

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

basanite to phonolite differentiation within 1550 1750 yr u th ra isotopic evidence from the a d 1585 eruption on la palma canary islands u th ra disequilibria of basanites tephrites and phonolites from the a d 1585 eruption on la palma canary islands constrain magma differentiation times in an ocean island rift zone the insignificant difference in 230th 232th implies differentiation from basanite to phonolite in 15 k y 226ra has a half life of 1600 yr however and permits higher temporal resolution 226ra 230th disequilibria are highest in the phonolites 46 54 and basanites 44 47 and lowest in the tephrites 38 41 the higher 226ra excesses in the end member compositions model basanite phonolite differentiation within 1550 1750 yr at a rate of 0 04 fractional crystallization per year such a short time interval is in sharp contrast to the 200 k y proposed for phonolite differentiation on the neighboring island of tenerife and could reflect different volcanic systems with a mantle fed rift system on la palma versus a crustal magma reservoir on tenerife

growth of large volcanoes on venus mechanical models and implications for structural evolution the structure tectonics and evolution of large volcanoes on venus as revealed by data from the magellan mission appear to be distinct from those on earth and mars to determine the conditions and processes that account for these differences we model the evolution of stress and deformation in growing volcanic edifices on venus using the finite element method large volcanoes on venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons the surfaces of both edifice and apron consist dominantly of radially oriented flows tectonic features usually the surface expression of shallowly intruded dikes also have predominantly radial orientations features similar to hawaiian style linear rift zones or large scale flank failure common on large volcanoes on earth and mars are absent by comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features we determine that a welded condition is more likely horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness te contravening stress increments from magma chamber expansion differential thermal contraction or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored incremental volcano growth results in stress distributions that decrease with height in the edifice dike intrusion is easiest in the uppermost youngest layers consistent with the observed shallow radial dikes on venus volcanoes small values of te greatly inhibit the formation of large shields in that conical edifice topography cannot be supported and large contravening stresses are required for further growth large volcano formation is much more likely at large te where more moderate contravening stresses are sufficient for growth the near surface manifestation of mantle upwelling on venus may thus depend on te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile leading to the formation of coronae but a thick elastic lithosphere is required to support the growth of the largest volcanoes the transitional value of te between these modes of evolution likely depends on the horizontal scale of the upwelling

magmatic underplating in the canary archipelago abstract the location of the canary archipelago at the east edge of the north atlantic a few hundred kilometres from the west african coast has for years posed a serious difficulty in understanding the complex interaction between long lasting volcanic activity and one of the oldest seafloor basements 150 ma on earth many different hypotheses have been proposed in the last two decades to understand the genesis of the canary archipelago there is an increasing acceptance to explain the archipelago as the product of the slow passage of the african plate over a mantle hotspot in rival tectonic models stretching and thinning of the lithosphere would determine the islands as independent volcanic blocks we review recent geophysical and geological evidence supporting a mantle plume origin for the canary islands based on the recognition of crustal thickening by mafic intrusions produced by the interaction between the mantle plume and the old oceanic lithosphere beneath the canary archipelago

paleointensity variations across the last geomagnetic reversal at la palma canary islands spain a detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal from a succession of 69 distinct lava flows in two adjacent sections on the island of la palma the correlation between the two sections relies on the positions of the brunhes matuyama boundary and on the flow chronology obtained by a refined potassium argon k ar dating technique several pretransitional flows exhibit very shallow paleomagetic inclinations but they cannot be considered as really transitional paleointensity experiments conducted on 288 specimens provided 168 successful determinations 14 were obtained after correction for 50 lava flows that were considered to represent 45 distinct units the evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows the period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 t which decrease down to 7 t prior to the transition the 100 kyr long interval following the reversal is associated with field intensities as high as 60 t and depict a strong and rapid restoration of the dipole field the virtual dipole moments vdms have been compared with sedimentary vdms generated from the record of relative paleointensity from ocean drilling program site 851 to which have been added non dipole components similar to the present geomagnetic field a reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the k ar age determinations thus these results can support but do not fully demonstrate the existence of asymmerical saw toothed variations in field intensity

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

the morphology of the submarine flanks of volcanic ocean islands a comparative study of the canary and hawaiian hotspot islands the submarine flanks of volcanic islands are shaped by volcanic constructional processes landslides erosion sediment deposition and tectonic movements we use a newly acquired multibeam sonar dataset from the westerly canary islands el hierro la palma and tenerife to develop a comparison with the hawaiian islands which suggests differences in the processes constructing and modifying their flanks landslides affect the flanks of both island groups debris avalanches fast moving shallow landslides have left smooth chutes and blocky deposits in both cases but blocks within some hawaiian avalanche deposits are markedly larger we attribute the larger block sizes in the hawaiian islands to the fact that their avalanches were relatively unconfined whereas many canary and hawaiian avalanches with small block sizes appear to have been constrained down narrow chutes forcing interactions between blocks within the flows and encouraging disintegration furthermore the hawaiian avalanches with the largest blocks initiated near sea level whereas many of the canary avalanches initiated above sea level so hydraulic resistance of water entering cracks may be an additional factor in resisting block disintegration during flow slow moving deep seated slumps or volcanic spreading have produced submarine benches and tabular escarpments due to thrust faulting adjacent to several hawaiian rift zones but are not well developed in the canaries although volcanic morphology is partly obscured by sedimentation in the canaries we are able to interpret lava terraces around the deep flanks of el hierro which are similar to those found in the hawaiian islands however cones rather than terraces are the most common volcanic forms in the canary islands implying that flank eruptions have involved magma with significant volatile contents assuming that volatile contents dictate whether cones or terraces are formed these differences may ultimately originate from the different building rates of the two island groups for example the lack of evidence for high level magma chambers in the canaries associated with their lower outputs implies that there is less possibility for degassing of magma below the summit before lateral intrusion down rift zones hence cones rather than lava terraces are more commonly observed the apparent lack of slumping or volcano spreading could also reflect a lack of driving pressure from extensive high level magma chambers in the canaries

characterization of debris avalanche deposits in japan abstract seventy one debris avalanche deposits are identified from 52 japanese quaternary volcanoes the structures of these volcanoes are mostly stratovolcanoes and lava domes no avalanche deposit is found in calderas pyroclastic cones or maar volcanoes debris avalanche deposits are found in 18 of all quaternary volcanoes or 25 of quaternary stratovolcanoes and lava domes the ratio rises to 49 when considering only active stratovolcanoes and lava domes at least five debris avalanche deposits have formed since 9th century the maximum height difference of sliding h for each debris avalanche ranges from 200 to 2400 meters and the maximum runout distance of sliding l ranges from 1 6 to 32 km the ratio h l ranges from 0 2 to 0 06 and becomes smaller in larger debris avalanches they are more mobile than landslides in non volcanic areas volume is within a range of 0 03 and 9 km 3 there is no relation between the direction of sliding and the regional horizontal compressional stress axis at the site of a volcano two dimensional computer simulations of avalanches using a simplified physical model are made the maximum velocity is mainly controlled by the length of steep slope

stress fields associated with the growth of a large shield volcano la palma canary islands 1 measurements of dyke orientations and fault slip data have been taken in 39 stations located in the northern part of the island of la palma canary archipelago these structures affect the materials belonging to the submarine growth stage and to the lower units of the subaerial shield volcano stage four sets of dykes have been distinguished groups i ii iii 1 and iii 2 the two younger dyke groups iii 1 and iii 2 with highly variable strikes are clearly related to the subaerial growth stage most of the measured faults are normal field relationships and the use of stress inversion methods allowed us to separate three different stress tensors tensor 2 is associated with the dykes of group iii 1 given the spatial distribution of the stations with faults of tensor 2 and dykes of its coeval group iii 1 it has been possible to obtain a map of trajectories of the maximum sh and minimum sh compressive stress axes in the horizontal comparison between this map and the results of theoretical models suggests that the stress field active during at least the early stages of growth of the shield volcano in la palma from near 1 7 myr to at least 1 2 myr ago includes a compressive radial volcanic component plus a regional tensile stress of a tectonic origin this second component might be the imprint in the atlantic oceanic lithosphere of the cenozoic convergence between the african and eurasian plates

three armed rifts or masked radial pattern of eruptive fissures the intriguing case of el hierro volcano canary islands abstract using new surface structural data as well as subsurface structural data obtained from seventeen water galleries we provide a comprehensive model of the volcano tectonic evolution of el hierro canary islands we have identified measured and analysed more than 1700 volcano structural elements including vents eruptive fissures dykes and faults the new data provide important information on the main structural patterns of the island and on its stress and strain fields all of which are crucial for reliable hazard assessments we conducted temporal and spatial analyses of the main structural elements focusing on their relative age and association with the three main cycles in the construction of the island the tinor edifice the el golfo las playas edifice and the rift volcanism a radial strike distribution which can be related to constructive episodes is observed in the on land structures a similar strike distribution is seen in the submarine eruptive fissures which are radial with respect to the centre of the island however the volcano structural elements identified onshore and reflecting the entire volcano tectonic evolution of the island also show a predominant ne sw strike which coincides with the main regional trend of the canary archipelago as a whole two other dominant directions of structural elements n s and wnw ese are evident from the establishment of the el golfo las playas edifice during the second constructive cycle we suggest that the radial striking structures reflect comparatively uniform stress fields during the constructive episodes mainly conditioned by the combination of overburden pressure gravitational spreading and magma induced stresses in each of the volcanic edifices by contrast in the shallower parts of the edifice the ne sw n s and wnw ese striking structures reflect local stress fields related to the formation of mega landslides and masking the general and regional radial patterns