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

seismostratigraphy of the western flanks of el hierro and la palma canary islands a record of canary islands volcanism abstract a cruise carried out aboard the spanish r v hesperides using a 255 inch3 airgun allowed to identify three main seismostratigraphic units in the western submarine flanks of the islands of la palma and el hierro canary islands the time structure and thickness maps of each unit together with the interpretation of the seismic facies demonstrate that the evolution of the canary islands is recorded in the marine basin the structure maps show that the older reflectors dip to the nw in contrast to the present day sea floor which dips to the wnw the thickness maps also show that the depocentre moved from a position parallel to the northwest african margin to a position now occupied by the island of la palma despite the high sedimentation rates of hemipelagic background sedimentation in the study area the three seismic units can be interpreted in terms of their relation to the canarian volcanism unit i was deposited before the initiation of volcanism and the arrival of volcaniclastic flows and is thus mainly if not entirely composed of hemipelagic sediments unit ii seems to reflect the shield phases of the easternmost edifices and as such would be constituted by the distal products of erosion and mass wasting of these islands unit iii in contrast results mainly from the shield phase of the two westernmost islands la palma and el hierro and as such reflects more proximal conditions as demonstrated by the presence of lateral collapse deposits from these islands

multiple related flank collapses on volcanic oceanic islands evidence from the debris avalanche deposits in the orotava valley water galleries tenerife canary islands abstract catastrophic flank collapses are common on oceanic islands of volcanic origin and often recognizable from the resulting morphology as large u shaped embayments however post collapse volcanic activity can infill such features thereby obscuring them this study takes advantage of a dense network of long sub horizontal galleries for groundwater extraction in the flank collapse structure of the orotava valley tenerife canary islands this impressive landform is located in the overlapping zone of the ne rift and the canadas edifice of this volcanic island three debris avalanche deposits dads have been identified inside the waterworks bored into the valley s western sector the deeper layer the lower dad l dad was previously described under the local name of mortalon this deposit lies unconformably over older volcanic rocks where a prominent shear zone is developed here interpreted as the detachment plane of a massive rockslide the l dad was therefore produced during the large failure event the orotava landslide ol which carved the depression two younger dads much smaller in volume and not previously described were also identified in the underground strata the intermediate i dad and upper dad u dad both correlate well with the thick breccia deposits cropping out at the base of the marine cliff along the western coast of the valley the i dad and u dad are conformably intercalated between the lava flows and other volcanics infilling the depression and their bases are erosive depositional features without structural deformation zones below them their stratigraphic setting and geometrical reconstruction indicate that both deposits were emplaced after the valley was formed by the ol event in two successive failures here called western orotava landslides wols the younger called wol 2 was dated at 494 22 ka using the 40ar 39ar method these two subaerial failures affected the upper eastern flank of the canadas iii edifice previously destabilized when the ol failure event carved a steep wall at its foot the western lateral escarpment of the orotava valley

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

the off shore continuation of an active basaltic volcano piton de la fournaise reunion island indian ocean structural and geomorphological interpretation from sea beam mapping abstract in 1984 a sea beam survey of the submarine east flank of piton de la fournaise was performed with r v jean charcot three main types of volcanic or volcano tectonic features have been identified 1 1 the subaerial ne and se volcanic rift zones of piton de la fournaise do not extend more than about five kilometers offshore unlike typical hawaiian rift zones which form narrow 2 4 km ridges extending tens of kilometers from the summit the active rift zones of piton de la fournaise widen downslope attaining more than 10 km at their front 2 2 the submarine extension of the grand brule slide is larger than the subaerial portion the entire slide forms a 7 24 km scar bounded by two ramparts to the north and south the slumped material may have moved as a debris flow forming a large talus downslope of the slide however the submarine counterpart of the south area of the grand brule seems to be composed by a slumped block whose structure is apparently not disturbed 3 3 another prominent feature is a conspicuous topographic high that occupies nearly all the center of the surveyed zone this cast flank submarine plateau cannot be associated with any active structure of piton de la fournaise its surface generally dips gently 2 3 and its northern and southern flanks are extensively cut by landslides cones of variable dimensions are observed on the plateau and farther to the east three hypotheses are examined to account for the origin of this morphology a remnant flank of an ancestral fournaise volcano associated with a large buried intrusion found by drilling beneath the grand brule b distinct volcanic massif or c material of a huge and ancient landslide the geophysical data show that the western part of this submarine plateau is reversely magnetized and associated with a moderate positive gravity anomaly this confirms that as suggested by the bathymetric analysis this part of the plateau is relatively coherent conversely the eastern portion of the plateau appears to be poorly magnetized and composed of low density material probably chaotic materials derived from landslides the reversed magnetization of the western part shows that the whole structure is older than 0 7 ma these results show that the volcanic history of piton de la fournaise is more complex than previously thought

caldera or flank collapse in the fogo volcano what age consequences for risk assessment in volcanic islands abstract the target of this study is the fogo island in cape verde where the hazards caldera and or flank collapse and the timing 120 ka or younger are still controversial using high resolution dem field geological and structural analysis and high precision k ar dating we produced an age calibrated volcanic stratigraphy of fogo s summit cha das caldeiras for the last ca 220 ka from this we infer the following evolution and associated processes 1 the fogo volcano formed during seven stages of construction and partial destruction 2 three flank collapses can be recognised the biggest of which occurred between 60 and 43 ka 3 this collapse occurred retrogressively producing at least two distinct collapse blocks 4 the innermost collapse was only partial forming a flat step where a new volcano pico do fogo grew and formed the cha das caldeiras literally flat of the calderas 5 the removal of the buttressing eastern flank by the outermost collapse can be responsible for the seemingly caldera structure 6 the growing load of the young volcano can produce a new flank failure following the innermost fault 7 the young and closely spaced collapse ages here reported indicate a significant risk for the inhabitants of fogo in particular and for the whole cape verde archipelago and atlantic seaboard if a tsunami is produced by a future catastrophic flank collapse

40ar 39ar stratigraphy of pyroclastic units from the canadas volcanic edifice tenerife canary islands and their bearing on the structural evolution many felsic pyroclastic units of various types are exposed in different sectors of tenerife new 40ar 39ar determinations allow them to be placed more precisely in the general volcano stratigraphic succession according to geographic distribution stratigraphic position and isotopic ages four main pyroclastic phases may be identified the first san juan de la rambla phase 2 1 ma is only known in the north of tenerife in the tigaiga massif the second adeje phase 1 8 1 5 ma is most completely developed in the southwest of the island but occasionally occurs in the other sectors the third las americas phase 1 ma is only presently known in the southern region the fourth bandas del sur phase 0 7 0 15 ma is essentially exposed in the southeast sector the results of this work emphasise the complexity of the pre 1 ma eruptive history of tenerife and underline the fact that explosive volcanic activity has taken place for at least the last 2 ma vertical collapse structures have developed as a result of pyroclastic flow activity and these may be as old as 1 6 1 8 ma therefore much older than generally considered the precise location of calderas is difficult to ascertain as a result of the repeated lateral flank collapse during the construction of the canadas volcano

benthic prokaryotic community dynamics along the ardencaple canyon western greenland sea abstract the ardencaple canyon emanating from the eastern greenland continental rise over a distance of about 200 km towards the greenland basin was investigated to determine the effect of enhanced down slope transport mechanisms on deep sea benthic prokaryotic communities the concentration of viable bacterial cells live dead baclight and prokaryotic incorporation rates 3 h thymidine 14 c leucine increased with increasing distance from the continental shelf multidimensional scaling mds results from terminal restriction fragment length polymorphism t rflp analysis indicated a spatial coherence between the benthic bacterial community structure prokaryotic incorporation rates water content protein concentration and the total organic matter in the sediments the community complexity in sediments at 4 5 cm depth was lower in the central parts of the channel compared with the northern and the southern levees while richness in surface sediments of all stations was similar lacking any clear indications for a recent mass sediment transport or funneled shelf drainage flows high similarities between bacterial assemblages in sediments along the canyon course may thus be governed by a combination of an ice edge induced particle flux episodic down slope and canyon guided transport mechanisms

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

flank collapse and large scale landsliding in the cape verde islands off west africa large scale landslides occur on the flanks of many volcanic oceanic islands worldwide none have taken place in historical time but their geohazard potential especially their ability to generate tsunamis is large the cape verde islands are a group of 10 large and several smaller volcanic islands off the coast of west africa between 15 and 17 n a single flank landslide has previously been described from the island of fogo but systematic analysis of the cape verde group has until now been lacking this paper describes and interprets a multibeam bathymetry data set covering the slopes of the western cape verde islands including those of the islands with the most recent volcanic activity fogo in the southwest and santo antao in the northwest all of the larger islands show evidence of large flank landslides although only fogo and the southwest part of santo antao have failed in the last 400 ka tope de coroa the volcano at the southwest end of santo antao has been inactive for the past 170 ka and is judged to have a low landslide potential unless volcanic activity resumes in contrast there would seem to be a high probability of a future east directed landslide on fogo from the area of the highly active pico do fogo volcano although it is impossible to predict a timescale for such an event a tsunami generated by such a landslide could have a catastrophic effect on the adjacent island of santiago and possibly even farther afield on the west african coast

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

features of crustal structure under the canary islands abstract the data and interpretation of a seismic refraction project carried out in the canary islands in 1977 and 1979 are presented the moho is reached at about 15 km under fuerteventura gran canaria and tenerife whereas under lanzarote a value of 11 km has been found the velocity at the surface varies even within one island from 2 8 to 4 4 km s there are variations in the velocity of the main crustal layer under the volcanic deposits a continuous velocity depth function seems to be more appropriate for the island of tenerife contrary to fuerteventura lanzarote and gran canaria which seem to have a layered structure there is evidence for pronounced structural differences between lanzarote and fuerteventura the mantle velocity has a value of about 8 0 km s under the western islands whereas a very low value of 7 4 km s has been found under lanzarote and fuerteventura despite the clear structural differences between the islands our results leave little doubt about the oceanic character of all the islands the differences between the eastern and western group may be due to the islands being situated close to the african continental margin the seismic velocities under the volcanic deposits do not support the idea of a common basement for all the islands they rather suggest independent volcanic edifices finally a comparison of the structure of the crust under the canary islands with that under other volcanic islands reveals remarkable similarities

the origin of certain wide valleys in the canary islands abstract the present work addresses a long term controversy concerning the origin of orotava valley one of the most peculiar valleys in the island of tenerife the canary islands spain this valley is characterized by its great width its flat floors gently inclined towards the sea its parallel and almost vertical lateral slopes and its equally steep but irregular valley head where there are recent volcanic cones the source of the lava flows that have covered the valley floor several different theories were formulated since the last century for the formation of orotava valley examination of other wide volcanic valleys and gorges in tenerife whose origin may be similar to orotava valley indicate an alternation of erosion volcanism analysis of the key features of orotava valley itself leads to formulation of an hypothesis which could be applicable to numerous volcanic areas such valleys are developed as volcanic valleys undergo lateral expansion due to repeated alternations of processes initial erosion of the gorges by streams the burial of the gorges by lava flows and the inversion of the relief by erosion at the base of valley walls the continued occurrence of this mechanism causes the side walls of the valley to retreat progressively giving a configuration similar to that of orotava valley

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

multiple caldera collapses inferred from the shallow electrical resistivity signature of the las canadas caldera tenerife canary islands the las canadas caldera of tenerife lcc is a well exposed caldera depression filled with pyroclastic deposits and lava flows from the active teide pico viejo complex tpvc the caldera s origin is controversial as both the formation by huge lateral flank collapse s and multiple vertical collapses have been proposed although vertical collapses may have facilitated lateral slope failures and thus jointly contribute to the exposed morphology their joint contribution has not been clearly demonstrated using results from 185 audiomagnetotelluric amt soundings carried out between 2004 and 2006 inside the lcc our study provides consistent geophysical constraints in favour of multiple vertical caldera collapse one dimensional modelling reveals a conductive layer at shallow depth 30 1000 m presumably resulting from hydrothermal alteration and weathering underlying the infilling resistive top layer we present the resistivity distribution of both layers resistivity images the topography of the conductive layer across the lcc as well as a cross section in order to highlight the caldera s evolution including the distribution of earlier volcanic edifices the amt phase anisotropy reveals the structural and radial characteristics of the lcc

erosion of the submarine flanks of the canary islands surveying with multibeam echo sounders around old 1 ma volcanic ocean islands reveals that their submarine flanks contain a strong downslope oriented ridge and valley corrugation which modifies the original volcanic morphology of lava terraces and cones by analogy with canyons in other settings this corrugation was probably caused by channel incision by erosive sedimentary mass flows such as turbidity currents and debris flows we adapt a method first used in subaerial geomorphology to isolate the erosion depth exhumation and apply it to the eroded flanks of the 6 8 ma anaga massif of tenerife the channels formed around this massif divert around local topographic highs these highs which are probably original volcanic cones are therefore preferentially preserved during erosion so that their elevations can be used to construct an artificial reference surface terrain depth was calculated by subtracting this reference surface from measured bathymetry comparison of the terrain depth of the old eroded submarine flank of anaga with that of the young mostly unaltered submarine flank of el hierro allows us to infer the mean depth of anaga s submarine erosion which is 100 m volcanic terrains can be dated by radiometric methods so they also provide a way of quantifying long term denudation rates we infer that submarine denudation of anaga has occurred at comparable rates to that of subaerial lowlands and much slower than denudation of highlands illustrated locally by the more extensive erosion of the subaerial anaga edifice

elevated marine deposits in bermuda record a late quaternary megatsunami deposits of coral bearing marine shell conglomerate exposed at elevations higher than 20 m above present day mean sea level msl in bermuda and the bahamas have previously been interpreted as relict intertidal deposits formed during marine isotope stage mis 11 ca 360 420 ka before present on the strength of this evidence a sea level highstand more than 20 m higher than present day msl was inferred for the mis 11 interglacial despite a lack of clear supporting evidence in the oxygen isotope records of deep sea sediment cores we have critically re examined the elevated marine deposits in bermuda and find their geological setting sedimentary relations and microfaunal assemblages to be inconsistent with intertidal deposition over an extended period rather these deposits which comprise a poorly sorted mixture of reef lagoon and shoreline sediments appear to have been carried tens of meters inside karst caves presumably by large waves at some time earlier than ca 310 360 ka before present mis 9 11 we hypothesize that these deposits are the result of a large tsunami during the mid pleistocene in which bermuda was impacted by a wave set that carried sediments from the surrounding reef platform and nearshore waters over the eolianite atoll likely causes for such a megatsunami are the flank collapse of an atlantic island volcano such as the roughly synchronous julan or orotava submarine landslides in the canary islands or a giant submarine landslide on the atlantic continental margin

submarine evidence of a debris avalanche deposit on the eastern slope of santorini volcano greece abstract hummocky seafloor features were discovered on the eastern flank of santorini volcano greece multibeam bathymetric mapping airgun seismic profiling side scan sonar survey and remotely operated vehicle rov dives have been carried out to characterize the nature of the hummocks these hummocks appear to be composed of several tens of blocks that are up to several hundred meters in diameter and are the surface expression of a much larger deposit than is observed in the bathymetry the sidescan and airgun data show that the deposit covers an area of approximately 6 km wide by 20 km long and is up to 75 m thick we estimate the total volume of the deposit to be approximately 4 4 10 9 m 3 sampling of these blocks show they are composed of pyroclastic flow deposits produced during the minoan eruption of santorini ca 3600 bp we propose that the deposit is the result of a multi stage landslide event that was caused by one of the several large earthquakes or volcanic eruptions that have occurred in the vicinity of santorini since the minoan eruption one or more of these events likely triggered the destabilization of a part of the eastern flank of santorini which led to a debris avalanche depositing blocks and forming a hummocky terrain at the base of the island s slope the mass movement later evolved into a turbulent suspension flow that traveled 20 km or more from the presumed initial failure given the size of the landslide deposit it might have a tsunami potentially affecting the islands across the southern aegean sea the understanding of earthquake landslide dynamics has important implications for hazard assessment in this seismically active historical and highly populated region of the world

crustal structure gravity anomalies and flexure of the lithosphere in the vicinity of the canary islands summary simple elastic plate models have been used to calculate the flexure of the lithosphere caused by volcanic loading at the canary islands and sediment loading at the moroccan continental margin by comparing the calculated flexure to observations based on seismic refraction and free air gravity anomaly data constraints have been placed on the long term 106 years elastic thickness of the lithosphere te the best fit between the calculated and observed flexure in the vicinity of the canary islands is for te 20 km this value of te also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the moroccan margin is sufficiently weak te 5 km for sediment loading to contribute little to the island flexure a backstripping study in which gravity data are used to constrain the value of te supports the suggestion that the lithosphere underlying the moroccan margin like its conjugate at the baltimore canyon trough is weak although sediment loading at the moroccan margin appears therefore to have exerted little influence on the structure of the canary islands there is evidence that island flexure may have influenced the stratigraphic development of the moroccan margin especially in the region of the slope anticline a te of 20 km is about 15 km lower than would be expected on the basis of the thermal age of the oceanic lithosphere that underlies the canary islands similar low values have been reported from oceanic islands in the pacific superswell region of french polynesia where they have been attributed to re heating of the lithosphere by one or more hotspots a general e w age progression of the volcanic rocks suggests that the canary islands were also generated by a hotspot they lack however the topographic swell and gravity geoid high which usually accompanies these features one possibility is that the low te values are the result of a pre existing weakness in the oceanic crust a te of 20 km is large enough however for a significant part of the mantle to still be involved in the support of the island loads a more likely explanation is thermal weakening by a hotspot which has been localized enough to reduce te but not to produce a swell or gravity geoid high irrespective of its cause the low te suggests that oceanic lithosphere does not necessarily progressively increase its strength with age so that even 140 ma old lithosphere is vulnerable and can in some regions be significantly weakened by later thermal and mechanical processes

a giant landslide on the north flank of tenerife canary islands the extent of mass wasting along the north flank of tenerife has been mapped using swath bathymetry gloria side scan sonar and 3 5 khz echo sounder data the marine surveys show that north of tenerife a giant landslide is exposed over an area of 5500 km2 of the seafloor more than twice the surface area of the island the landslide truncates an older ridge and valley topography that is associated with the shield building basalts on tenerife we interpret the ridge and valley topography as the result of subaerial erosion the landslide is estimated to have a length of 100 km a width of up to 80 km and a volume of about 1000 km3 it extends onshore into the orotava and icod valleys which have been interpreted as of landslide origin k ar dating of basaltic flows in the steep headwall of orotava suggests an age of formation for the valley is younger than 0 78 ma and may even be younger than 0 27 ma the icod valley is located immediately to the north of the most recent volcano on tenerife las canadas and has been associated with the collapse of its caldera between 1 2 and 0 2 ma a young age for the landslide is supported by the 3 5 khz echo sounder data which show that the landslide is draped by a thin 10 m layer of younger sediment the landslide did not form however during a single catastrophic event but represents the amalgamation of a number of separate landslides the occurrence of the ridge and valley topography in water depths of up to 2 5 km suggests that the shield building basalts have subsided by at least this amount since they formed 3 3 8 0 ma we speculate that this subsidence is caused by some form of stress relaxation that occurs in the underlying lithosphere the giant landslide imaged in our sonar data is associated with the late stages in the development of the most recent volcano on tenerife las canadas which only began at about 1 8 ma thus landsliding may be a particular feature of the time soon after emplacement when because of incomplete isostatic adjustment oceanic volcanoes have their greatest elevations above sea level and therefore are most susceptible to slope failure

submarine geology of the hilina slump and morpho structural evolution of kilauea volcano hawaii abstract marine geophysical data including sea beam bathymetry hawaii mr1 sidescan and seismic reflection profiles along with recent robot submersible observations and samples were acquired over the offshore continuation of the mobile kilauea volcano south flank this slope comprises the three active hot spot volcanoes mauna loa kilauea and loihi seamount and is the locus of the hawaiian hot spot the south flank is the site of frequent low intensity seismicity as well as episodic large magnitude earthquakes its sub aerial portion creeps seaward at a rate of approximately 10 cm year the hilina slump is the only large submarine landslide in the hawaiian archipelago thought to be active and this study is one of the first to more highly resolve submarine slide features there the slump is classified into four distinct zones from nearshore to the island s base estimates of size based on these data indicate a slumped area of 2100 km 2 and a volume of 10 000 12 000 km 3 equivalent to about 10 of the entire island edifice the overall picture gained from these data sets is one of mass wasting of the neovolcanic terrain as it builds upward and seaward though reinforcement by young and pre hawaii seamounts adjacent to the pedestal is apparent extensive lava delta deposits are formed by hyaloclastites and detritus from recent lava flows into the sea these deposits dominate the upper submarine slope offshore of kilauea with pillow breccia revealed at mid depths along the lower flanks massive outcrops of volcanically derived sedimentary rocks were found underlying kilauea thus necessitating a rethinking of previous models of volcanic island development the morphologic and structural evolutionary model for kilauea volcano and the hilina slump proposed here attempts to incorporate this revelation a hazard assessment for the hilina slump is presented where it is suggested that displacement of the south flank to date has been restrained by a still developing northeast lateral submarine boundary when it does fully mature the south flank may be more subject to land slips triggered by large long duration earthquakes and thus kilauea may undergo more frequent episodes of failure with increased displacements

dykes and structures of the ne rift of tenerife canary islands a record of stabilisation and destabilisation of ocean island rift zones many oceanic island rift zones are associated with lateral sector collapses and several models have been proposed to explain this link the north east rift zone nerz of tenerife island spain offers an opportunity to explore this relationship as three successive collapses are located on both sides of the rift we have carried out a systematic and detailed mapping campaign on the rift zone including analysis of about 400 dykes we recorded dyke morphology thickness composition internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes dykes were intruded along the rift but also radiate from several nodes along the rift and form en echelon sets along the walls of collapse scars a striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls this dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events the slump sides would create the necessary strike slip movement to promote en echelon dyke patterns the spreading flank would probably involve a basal decollement lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra volcanic intrusive complexes with continued magma supply spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress however as magmatic intrusion persisted a critical point was reached beyond which further intrusion led to large scale flank failure and sector collapse during the early stages of growth the rift could have been influenced by regional stress strain fields and by pre existing oceanic structures but its later and mature development probably depended largely on the local volcanic and magmatic stress strain fields that are effectively controlled by the rift zone growth the intrusive complex development the flank creep the speed of flank deformation and the associated changes in topography using different approaches a similar rift evolution has been proposed in volcanic oceanic islands elsewhere showing that this model likely reflects a general and widespread process this study however shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple instead a dynamic interplay between external factors e g collapse erosion and internal forces e g intrusions is envisaged this model thus provides a geological framework to understand the evolution of the nerz and may help to predict developments in similar oceanic volcanoes elsewhere

volcanic spreading on mauna loa volcano hawaii evidence from accretion alteration and exhumation of volcaniclastic sediments a transect of submersible dives across the submarine west flank of mauna loa volcano yields compelling evidence for volcanic spreading and associated hydrothermal circulation during volcano growth a frontal bench at the toe of the flank formerly thought to be a downdropped block of mauna loa contains a mix of volcaniclastic lithologies including distally derived siltstone mudstone and hyaloclastite the bench is overlain by bedded gravels and subaerially erupted pillow flows derived from local shoreline crossing lava flows the volcaniclastic strata in the bench were offscraped uplifted and accreted to the edge of the flank as it plowed seaward into the surrounding moat the accreted strata underwent significant diagenesis through deep burial and circulation of hydrothermal fluids expelled from porous sediments beneath the volcano timing constraints for bench growth and breakup suggest that catastrophic failure of the subaerial edifice ca 250 200 ka triggered volcanic spreading by reducing stresses resisting basal sliding and rift zone inflation increased eruptive activity and westward migration of mauna loa9s southwest rift zone gradually rebuilt the massive flank arresting slip prior to detachment of the alika 2 debris avalanche ca 120 ka

crustal and upper mantle seismic structure and lithospheric flexure along the society island hotspot chain summary four wide angle and refraction lines have been shot along the society island hotspot track a 280 km long profile covers oceanic crust to the east of the islands the active hotspot region and the apron of tahiti the other profiles cover the region between moorea and huahine and the western archipelago of huahine raiatea bora bora and adjacent oceanic crust traveltime modelling provided lateral variations in crustal structure along the hotspot chain removal of the contribution of the uppermost low velocity layer which represents the volcanic edifices and debris suggests that the islands and atolls sit on normal oceanic crust and are surrounded by a large volcaniclastic apron seismic profiles running between raiatea and huahine and between raiatea and moorea cover a region where extreme down bending due to volcanic loading is expected a nearly flat crust mantle boundary indicates regional isostasy and clearly supports a stiff plate seismic structural models have been compared with calculated boundaries based on simple 3 d elastic plate models a good fit to the seismic crust mantle boundary was obtained for models with an effective elastic thickness of the lithosphere of 20 25 km these values are in accordance with a model in which the elastic thickness depends on the thermal age of the lithosphere and therefore contradict models that suggest that the region of french polynesia has been thermally rejuvenated amplitude modelling of crustal and upper mantle arrivals suggests that crust on the hotspot trail has been modified with respect to the pre hotspot crust to explain the observations we suggest that during the ongoing late stage volcanic activity the lower crust has been intruded by sills and some melts are trapped in the crust mantle boundary region if late stage volcanism and plutonism continue prolonged crustal growth may form a large underplate or subcrustal plutonic core complex along the hotspot chain

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

giant debris avalanches from the colima volcanic complex mexico implications for long runout landslides 100 km and hazard assessment at least two giant volcanic debris avalanche deposits are associated with the colima volcanic complex located in the western part of the trans mexican volcanic belt one avalanche originated from volcan de colima 3820 m probably 4300 yr ago a much larger avalanche originated from nevado de colima 4240 m 18500 yr ago it traveled more than 120 km from its source and covered an area of at least 2200 km 2 almost twice the area of any previously described avalanche deposit this older debris avalanche is the second largest by volume known has the longest travel distance yet reported and has one of the lowest height length ratios 0 04 the avalanche material was probably hot during emplacement and after initial slope failure may have behaved similarly to a pyroclastic flow such large long runout landslides from volcanic or nonvolcanic constructs present a serious geologic hazard that must be considered in risk assessment

frequent landslides from koolau volcano results from odp hole 1223a giant landslides on the flanks of oceanic islands are considered relatively rare but potentially devastating natural hazards the hawaiian islands are known to produce some of the biggest landslides on earth the nuuanu slide largest of the hawaiian slides is thought to have removed a substantial part of koolau volcano from the island of oahu ocean drilling program odp hole 1223a was drilled to determine the depositional history timing thickness and hazards associated with the nuuanu landslide the only previously known koolau slide site 1223 located 260 km northeast of the island of oahu near the crest of the 500 m high hawaiian arch was drilled to a depth of 41 m eight distinct sandy layers were recovered and more are likely to occur deeper in the section contacts of these sandy layers with pelagic clay are sharp at their base and gradational at the top the layers contain angular fresh glass fragments with compositions that are typical of hawaiian shield volcanoes including the distinctive koolau high sio2 type in seven layers most glasses n90 are degassed b0 03 wt s indicating that they were probably erupted subaerially pleistocene to early eocene radiolaria taxa are present in the hole 1223a cores with mixed ages in some intervals seven of the sand layers are probably associated with koolau landslides and were deposited prior to 1 77 ma among the four thicker sand layers n1 5 m it is unclear which if any are related to the nuuanu slide results from hole 1223a cores demonstrate that hawaiian volcanoes collapse repeatedly and the debris from these many slides can travel great distances n260 km across the ocean floor and over significant bathymetric obstacles 500 m thus landslides from oceanic volcanoes pose a greater risk than previously assumed d 2005 elsevier b v all rights reserved

lateral collapses and active sedimentary processes on the northwestern flank of stromboli volcano italy abstract the marine volcaniclastic system extending to 3 km water depth on the northwest flank of stromboli volcano has been surveyed using seismo acoustic profiling multibeam bathymetry deep towed long range side scan sonar imaging and box core sampling deposits and sea floor topography record repeated flank collapses that have occurred during the last 13 ka and created the present sciara del fuoco scar an extensive 170 km2 debris avalanche deposit characterized by subdued hummocks megablocks is buried by turbidites formed from gravity currents that originated in sciara del fuoco the currents were mainly deflected along the margin of the avalanche deposit but also spilt over onto it the turbidites reflect topographic flow stripping and their thinning towards slightly steeper slopes indicates that some material that settled there drained further downslope as secondary higher concentration flows the estimated volume of the entire fan shaped bulge amounts to 12 2 5 km3 about 3 5 km3 of which is attributed to the post collapse sedimentation the remaining 8 5 2 5 km3 is in the order of the volume computed for the material removed by the multiple sector collapses that affected the nw stromboli flank two units are identified in the distal part of the bulge through the recognition of their fronts and these indicate a layered structure for successive debris avalanche deposits granulometric and particle type characterization of seabed samples shows clast population evolution with sedimentary transport distance from the main entrance point of debris in sciara del fuoco to the confluence with the stromboli canyon and into the marsili basin some 45 km from the island changes in particle populations are due to selective sedimentation and also mixing with a wide range of terrigenous detritus brought by currents that descended the main stromboli canyon from the continental shelf abundant fine sand and silt is produced at the volcano and occurrence of mainly coarse grained material along the channel to the marsili basin indicates that a considerable volume of volcaniclastic material from stromboli lies in the sedimentary succession accumulating on the distal basin floor where a deep sea fan is growing

causal link between quaternary paleoclimatic changes and volcanic islands evolution 1 giant landslides and resulting tsunamis represent the main geologic hazards linked to volcanic island evolution from offshore and onland studies flank failures have been identified around numerous islands in most geodynamic contexts however the triggering conditions are still poorly understood and several causes may act simultaneously to reach a critical threshold here we show that most large volume 10 km3 landslides occur at glacial stages termination and we propose that a causal relationship between flank collapse of volcanic islands and global climatic changes has existed at least since 900 kyr moreover ages reported here favour the hypothesis that major collapses occurred during the onset of glacial to interglacial transitions when sudden influx of melt water from polar ice caps causes rapid sea level rise we propose that rapid sea level rise induces enhanced coastal erosion and sudden changes of pore pressure conditions within basal layers which favour edifice failure

conditions favouring catastrophic landslides on tenerife canary islands catastrophic failures of volcano flanks represent one of the most hazardous geological phenomena these immense mass movements originate either by increasing the destabilizing forces driving forces or by reducing the strength of the materials involved or both the study of large volcanic landslides on tenerife suggests that the presence of weak residual soils palaeosols in combination with the pre existence of deep narrow canyons created by fluvial erosion have played a fundamental role in the initiation of large scale sector collapses of the volcanic edifice regardless of the triggering mechanism considered residual soils strongly reduce the material strength during undrained loading while pre existing canyons control the lateral limits of the landslide the existence of a wet climate in some sectors of the island favours these circumstances

correlation frequency of emplacement and source directions of megaturbidites on the madeira abyssal plain abstract this paper describes the sediments of the whole madeira abyssal plain including the madcap area to the south previous studies have centred on the central sub basin which occupies most of the plain and which has become one of the most studied areas of seafloor the plain is made up of a series of large turbidites separated by thin pelagic layers which can be used to provide a high resolution stratigraphy back to about 750 000 years many of the turbidites were deposited close to oxygen isotope stage boundaries suggesting a strong relationship to sea level climate change although the exact initiation mechanism for the flows is not known data from the northern and southern extensions of the plain shows the effects of different entry points for the four main turbidite groups volcanic rich turbidites enter from the ne and e calcareous turbidites from the w one group of organic rich turbidites enters from the se and one group from the ne and e different turbidite sources operate for limited periods before switching to one of the other source areas it is very rare for two sources to provide turbidites at the same isotope stage boundary

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

rapid formation of the small isles tertiary centre constrained by precise 40ar 39ar and u pb ages the relative chronology of magmatic and tectonic events is key to an understanding of the influence of the iceland plume on the north atlantic in particular the location and duration of magmatism is of fundamental importance initial widespread flood basalt formation occurred in baffin island greenland and britain before complete plate break up at 56 ma after which time magmatism became concentrated in the active rift zone historically the british tertiary igneous province btip has been instrumental in advancing many concepts of igneous petrology however the absolute age and duration of the province remains unresolved here we present new internally consistent 40ar 39ar ages that help to constrain the volcanic activity in the small isles centre to within 2 my this short duration has implications for the onset of magmatism in the larger north atlantic province the rapid unroofing of the rum volcano and more significantly some of the evidence used to propose pulsing of the iceland plume

offshore continuation of volcanic rift zones el hierro canary islands el hierro is the youngest and most southwesterly of the seven canary islands the established view based on subaerial geology is that el hierro is a classic example of an oceanic island with 120 spaced volcanic rift arms vrzs forming a mercedes star however new offshore data do not support this simple interpretation instead of the discrete ridges of vrzs we observe to the nw and ne broad areas of irregular morphology which suggest that rifting activity might not be confined to narrow zones furthermore our data suggest that the anomalously long and steep flanked southern ridge could be part of an older eroded volcanic edifice that predates much of the other submarine flanks of el hierro the southern ridge has a distinctive gullied morphology which strongly contrasts with adjacent flanks there is also a 400 m deep saddle in its longitudinal profile 15 km from the coastline which we interpret as evidence that the southern ridge did not form by continuous dyke intrusion from the el hierro volcanic centre south of the saddle mean flank slopes are 10 steeper 30 with a sharp slope break at 3700 m between the ridge and smoothly sedimented seafloor these steeper slopes and lack of landslide scars to the south of the saddle indicate that the southern ridge is a stable edifice relative to the rest of el hierro surrounding sediments to the southeast appear to onlap the southern ridge a large landslide deposit el julan estimated age 200 ka occurs to the west of the ridge this landslide appears to have been constrained from spreading southeastwards by the southern ridge resulting in an elevation difference of 300 m for the seafloor on either side of the ridge

stratigraphy and k ar ages of the diego hernandez wall and their significance on the las canadas caldera formation tenerife canary islands the las canadas caldera corresponds to a volcanic caldera formed after a long period of explosive activity the structural and lithological variations which can be observed in the caldera wall make clear the complexity of the original las canadas edifice in which different eruptive centres existed the stratigraphic and structural reconstruction of the las canadas caldera indicates that the diego hernandez wall located at the eastern side of the caldera wall comprises the youngest pre caldera deposits determination of their k ar ages has provided the maximum age for the formation of the las canadas caldera the results are internally consistent with stratigraphic relationships and allow two pre caldera volcanic cycles in the diego hernandez wall to be differentiated in accordance with geological evidence the first cycle shows imprecise age limits the second cycle ranges from 0 35 to 0 17 ma this upper pre caldera age suggests that the las canadas caldera and the teide pico viejo were formed more recently than was previously assumed

geological evolution of the volcanic island la gomera canary islands from analysis of its geomorphology abstract erosion and landslide processes played an important role in shaping the geomorphology of the canary islands but for the heavily eroded la gomera island those processes have been unclear we conducted an integrated study of its offshore extension onshore geology and links between them in order to determine the geomorphological evolution of the island relative to the rest of the archipelago the characteristics of the island s barrancos and its interfluves have led us to differentiate four types of morphological areas whose fluvial networks are in different stages of evolution those barrancos offshore are interrupted by la gomera s shelf and its erosion has been much greater than for the rest of the canary islands such erosion is both a function of time and varied swell action la gomera s insular slope is carved by a system of submarine canyons and channels that have morphology indicative of turbidity current erosion many of the barrancos onshore and the canyons offshore have steps or knickpoints some are the result of varied resistance to erosion and others are explained by other mechanisms a ridge whose crest is covered by cone like shape structures is interpreted as of volcanic origin with the particularity that it lacks onshore continuation the opposite of what has been observed elsewhere in the canary islands also in contrast with the other islands of the archipelago la gomera s margin does not seem to have resulted from destructive mass wasting and the only offshore sign of catastrophic events are the presence of two embayments along la gomera s northern shelf edge and a lobe structure on the insular apron at 3000 m depth these features may be the remains of the catastrophic failures that took place on the island 9 4 8 7 ma ago alternatively they could be due to recent catastrophic failures on the upper slope due to sediment steeping since the conclusion of the main volcanic activity 4 0 ma ago la gomera has been intensively eroded by gradual fluvial denudation and secondary failures la gomera s insular margin reveals much regarding its evolution but more comprehensive studies including seismic and coring are needed to understand its whole geological history

a new scenario for the mass transport deposits west canary volcanic province abstract a new temporal history of mass wasting processes for the west of the canary volcanic province is presented its onset has been estimated in the middle upper miocene 13 5 1 2 ma matching with a critical period of construction for this volcanic province seismic profiles show an emplacement longevity from the miocene to quaternary in multiple events defined by stacked lobes of debrites linked to the flank collapses and volcanic avalanches of the volcanic edifices islands and seamounts an evolution of pathways and source areas has been detected from east miocene to west quaternary as well as a migration of the activity to the northwest west of the canary islands e g el hierro and la palma six connected branches i vi three of them described for the first time here of quaternary seismic units of mass transport deposits mtds have been characterized the pleistocene makes up a huge buried mtds system until now unknown pointing a new mass transport sedimentological scenario finally the two southernmost branches v vi up to now unknown are a mainly buried system of stacked and terraced lobes of debrites sourced mainly from the flank collapses of the volcanic seamounts of the canary island seamount province apparently inactive from upper cretaceous

geological and geomechanical models of the pre landslide volcanic edifice of guimar and la orotava mega landslides tenerife abstract the geological and geomechanical characterizations of the volcanic rock mass succession affected by the guimar and la orotava mega landslides tenerife canary island are presented here for the first time the estimated subaerial volume of rocks mobilized by each of these landslides is in the range of 30 50 km 3 one of the largest known in the world field data gallery surveys and borehole drilling have allowed the main types of materials and their structural arrangement to be identified based on these information a geological model of the pre landslide volcanic edifice is proposed a palaeo morphological reconstruction has been produced and possible slope angles and heights of the pre landslide volcanic edifice are presented five main lithological units have been identified in the emerged part of the edifice three of them forming the flanks and two forming the structural axis of the island on the flanks lava flows predominate with different degrees of alteration and proportion of dikes increasing near the structural axis the predominant materials on the structural axis are pyroclastic deposits lava flows and dikes in the submarine edifice four main lithological units have been distinguished formed by hyaloclastites pillow lavas dikes and gravitational deposits slope and basin facies the geomechanical characterization of these materials has been obtained from field data boreholes laboratory tests and literature review the geological and geomechanical models obtained provide the fundamental basis for the explanation of the instability processes that generated the guimar and la orotava mega landslides

timing of a large volcanic flank movement at piton de la fournaise volcano using noise based seismic monitoring and ground deformation measurements in volcanic domains magma transport and pressure build up induce high stress strain perturbations in the surrounding volcanic edifice that may lead to volcanic flank movements and possible instability in this study we focus on the 2007 march april episode of volcanic activity at piton de la fournaise pdf volcano la r eunion island this episode was associated with a large volume of emitted lava 240 106 m3 and a 340 m caldera collapse we present observations of continuous seismic velocity changes measured using cross correlations of ambient seismic noise over 10 yr at pdf overall we observe a large velocity reduction starting a few days prior to the major 2007 april 2 eruption comparison of seismic velocity change measurements with observations of deformation from insar and gps shows that the seismic velocity reduction coincided with a widespread flank movement starting at the time of injection of magma to feed an initial eruption a few days before the 2007 april 2 eruption we emphasize the potential of noise based seismic velocity change measurements together with geodetic observations to detect and monitor possibly hazardous slope instabilities

the crustal structure of el hierro canary islands from 3 d gravity inversion el hierro is the youngest and westernmost island of the canarian archipelago due to its singular and interesting characteristics with extensive landsliding studying its crustal structure can provide very useful information about its origin and evolution with this aim we use the gravity inversion methodology obtaining a crustal model of mass distribution we performed a gravity survey of the entire island completing the gravity anomaly map of this area with marine data from the united states geological survey the calculated bouguer gravity anomaly map was studied using a covariance analysis and the least squares technique to obtain crustal information from these data we used a 3d gravity inversion based on genetic algorithms ga thus our inversion technique aims to determine the geometry of the sources of the observed gravity field upon a prismatic partition of the subsoil volume and adopting a priori density contrast values the results of this gravity study of el hierro island show the correlation of several volcanic structures with the distribution of the gravity field sources the characteristic triple rift system of the island is associated with low density areas and the older volcanic stages with high density structures we also found differences among the structures related to the several landslides which helped the island takes shape

an estimation of the elastic thickness of the lithosphere in the canary archipelago using admittance function a recent compilation of shipborne gravity and bathymetry data has been used to determine the effective elastic thickness eet of the lithosphere beneath the canary islands volcanic complex by means of the admittance technique as a first approach we have computed the admittance which is interpreted in terms of isotropic elastic plate thickness with an eet of 23 km nevertheless the proximity of the west african continental margin wacm together with the lateral variability in lithosphere structure in part caused by the large volcanic history of the canaries require a slightly different approach to minimize the influence of the margin and to check whether or not any significant variation in eet occurs related to the known canary hotspot we have computed the admittance following the isochron lines the results show a higher eet 35 km than that obtained by the first approach but slightly smaller than that predicted by the cooling plate model

prodigious submarine landslides on the hawaiian ridge the extensive area covered by major submarine mass wasting deposits on or near the hawaiian ridge has been delimited by systematic mapping of the hawaiian exclusive economic zone using the side looking sonar system gloria these surveys show that slumps and debris avalanche deposits are exposed over about 100 000 km2 of the ridge and adjacent seafloor from kauai to hawaii covering an area more than 5 times the land area of the islands some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume ranking them among the largest on earth the slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts culminate near the end of subaerial shield building and apparently continue long after dormancy consequently landslide debris is an important element in the internal structure of the volcanoes the dynamic behavior of the volcanoes can be modulated by slope failure and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems the landslides are of two general types slumps and debris avalanches the slumps are slow moving wide up to 110 km and thick about 10 km with transverse blocky ridges and steep toes the debris avalanches are fast moving long up to 230 km compared to width and thinner 0 05 2 km they commonly have a well defined amphitheater at their head and hummocky terrain in the lower part oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high level wave deposits such as the 365 m elevation hulopoe gravel on lanai that are found on several islands most present day submarine canyons were originally carved subaerially in the upper parts of debris avalanches subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters

surveying the flanks of the mid atlantic ridge the atlantis basin north atlantic ocean 36 n new seismic reflection and okean 9 5 khz sidescan sonar data were used to characterise the seismic stratigraphy and structure of the region south of the azores islands north atlantic ocean the new data provide constraints on the tectono sedimentary evolution of deep ocean basins located on the flanks of slow spreading ridges south of the azores thick sedimentary sequences 1 0 s two way travel time correlated with units in the madeira abyssal plain odp sites 950 952 overlay an irregular basement generated by a combination of volcanic and tectonic processes three main cenozoic depositional events are recognised and signed by distinct seismic megasequences the moderate deformation of the sedimentary units and the structural fabric of the underlying oceanic crust shown on seismic and sidescan data suggest the existence of a complex tectonic setting south of the azores illustrating this complex setting is the change from a basement with prominent abyssal hills west of 30 w to a segmented oceanic crust denoting an extensional transtensional tectonic regime east of the latter meridian

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

discrete element simulations of gravitational volcanic deformation 1 deformation structures and geometries 1 we have carried out two dimensional particle dynamics simulations of granular piles subject to frictional coulomb failure criteria to gain a first order understanding of different modes of gravitational deformation within volcanoes under uniform basal and internal strength conditions granular piles grow self similarly developing distinctive stratigraphies morphologies and structures piles constructed upon cohesive substrates exhibit particle avalanching forming outward dipping strata and angle of repose slopes systematic decreases in basal strength lead to progressively deeper and steeper internal detachment faults and slip along a basal decollement landslide forms grade from shallow slumps to deep seated landslide and finally to axial subsidence and outward flank displacements or volcanic spreading surface slopes decrease and develop concave up morphologies with decreasing decollement strength depositional layers tilt progressively inward spatial variations in basal strength cause lateral transitions in pile structure stratigraphy and morphology this approximation of volcanoes as coulomb granular piles reproduces the richness of deformational structures and surface morphologies in many volcanic settings the gentle slopes of hawaiian volcanoes and olympus mons on mars suggest weak basal decollements that enable volcanic spreading high angle normal faults favored above weak decollements are interpreted in both settings and may explain catastrophic sector collapse in hawaii and broad aureole deposits surrounding olympus mons in contrast steeper slopes and shallow detachment faults predominate in the canary islands thought to lack a weak decollement favoring smaller more frequent slope failures than predicted for hawaii the numerical results provide a useful predictive tool for interpreting dynamic behavior and associated geologic hazards of active volcanoes

recurrence of major flank landslides during the last 2 ma history of reunion island new detailed swath bathymetry and backscatter data corroborate the existence of four large bulges on the submarine flanks of reunion island these fan shaped promontories are 20 25 km wide at the coastline and 70 150 km across the seafloor 40 50 km offshore their surfaces are characterized by a speckle sonar pattern indicating the presence of large blocks up to several hundred meters across each bulge results from the superposition of multiple landslide deposits whose older ones are dissected and delimited by erosive channels as much as 200 m deep and 20 km long the submarine flanks of reunion island are thus mostly built by accumulation of debris avalanche fans morphologic and geologic evidence define large subaerial source areas for these mass wasting events in particular inferred headwalls of most landslides having affected the piton des neiges massif generally coincide with the boundaries of its cirques mafate salazie and cilaos whereas recurrent landslides have resulted in the formation of large concentric amphitheatre structures through the piton de la fournaise massif thus about 15 slide events accompanied growth of the reunion island shield since 2 ma

vertical axis rotation of the upper portions of the north east rift of tenerife island inferred from paleomagnetic data abstract paleomagnetic sampling sites were established in 82 dykes along an 8 km long section of the north east rift zone nerz of tenerife canary islands spain of the 70 interpretable sites 16 are of normal polarity and 54 of reversed polarity four normal polarity sites and fifteen reverse polarity sites were excluded from the grand mean calculation for statistical reasons after inverting the reverse polarity sites through the origin the in situ grand mean yields a declination d 023 8 an inclination i 42 3 95 3 2 39 0 n 51 that is discordant to the expected late miocene to pleistocene field direction d 357 6 i 38 8 95 4 7 this discordance can be explained as either a 26 clockwise vertical axis rotation or a 28 wnw side down tilt about an average 009 horizontal tilt axis the sampled section is composed of numerous semi vertical dykes cutting mainly lava flow units that are sub horizontal and cross cut by steeply dipping faults 70 90 field evidence is therefore more compatible with a vertical axis rotation rather than a horizontal axis tilt of the drilled units we argue that this clockwise vertical axis rotation is likely related to strike slip movements that occurred along the edges of the collapse scars and accommodate the emplacement and growth of the underlying intrusive core and associated dykes six new 40ar 39ar age determinations constrain the main interval of dyke emplacement within the nerz between 0 99 ma and 0 56 ma the intrusive activity in the sampled section of the rift appears to have been almost continuous with several intrusion pulses that are probably related to flank destabilisation event s during the mid pleistocene our study thus demonstrates a long lived multi faceted history that shaped the nerz