las canadas caldera tenerife canary islands a review or the end of a long volcanological controversy abstract the las canadas caldera is one of the best exposed volcanic calderas in the world and one of the few known evolved alkaline volcanoes it truncates the pre teide pico viejo central volcanic edifice the las canadas edifice which started to take shape at the end of the formation of a large basaltic shield that forms the main part of island of tenerife historically the origin of the las canadas caldera has been controversial as it has been interpreted as the result of either multiple vertical collapses or due to a giant sector collapse the available stratigraphical structural volcanological geochronological and geophysical data as well as its comparison to other well known collapse calderas should not offer any doubt as to its direct relationship with a long history of phonolitic explosive volcanism however the existence of large landslide events on tenerife which have significantly modified the flanks of the las canadas volcano have also been used as a potential explanation for the origin of the las canadas depression this contribution reviews the available information on the las canadas caldera the causes of this controversy and rationalises the most plausible explanation for the origin of las canadas caldera based on the current evidence gathered from all previous studies

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

volcanic stratigraphy a review abstract volcanic stratigraphy is a fundamental component of geological mapping in volcanic areas as it yields the basic criteria and essential data for identifying the spatial and temporal relationships between volcanic products and intra inter eruptive processes earth surface tectonic and climatic which in turn provides greater understanding of the geological evolution of a region establishing precise stratigraphic relationships in volcanic successions is not only essential for understanding the past behaviour of volcanoes and for predicting how they might behave in the future but is also critical for establishing guidelines for exploring economic and energy resources associated with volcanic systems or for reconstructing the evolution of sedimentary basins in which volcanism has played a significant role like classical stratigraphy volcanic stratigraphy should also be defined using a systematic methodology that can provide an organised and comprehensive description of the temporal and spatial evolution of volcanic terrain this review explores different methods employed in studies of volcanic stratigraphy examines four case studies that use differing stratigraphic approaches and recommends methods for using systematic volcanic stratigraphy based on the application of the concepts of traditional stratigraphy but adapted to the needs of volcanological environment

conduction model for the thermal influence of lithic clasts in mixtures of hot gases and ejecta the thermal influence of lithic clasts in mixtures of hot gases and ejecta produced during explosive eruptions is an important aspect of the dynamics of eruption columns and pyroclastic flows in order to study this aspect we present a heat conduction model which considers both the temperature distributions of the lithic clasts and the exchange of thermal energy between the lithic clasts and other constituents gases and juvenile pyroclasts the model quantifies the thermal energy of the system by means of a characteristic energy function that describes the energy absorbed by each lithic clast using this energy function and considering some hypothetical arrangements of lithic clasts it is possible to calculate by means of heat balance the temperature variations in mixtures of hot gases and ejecta as a function of time volume fraction of particles to gas and thermal diffusivity initial temperature diameter and volume fraction of lithic clasts theoretical results show that in an eruption a volume fraction of 12 of cool lithic clasts 20 100 c whose size is less than 5 cm will cause the mixture of hot gases and ejecta to cool by 300 c we suggest therefore that the thermal effects of lithic clasts should be considered in numerical models of eruption columns and pyroclastic flows

the holocene secche di lazzaro phreatomagmatic succession stromboli italy evidence of pyroclastic density current origin deduced by facies analysis and ams flow directions the edifice of stromboli volcano gravitationally collapsed several times during its volcanic history 100 ka present the largest holocene event occurred during the final stage of the neostromboli activity 13 5 ka and was accompanied by the emplacement of phreatomagmatic and lahar deposits known as the secche di lazzaro succession a stratigraphic and paleomagnetic study of the secche di lazzaro deposits allows the interpretation of the emplacement and the eruptive processes we identify three main units within the succession that correspond to changing eruption conditions the lower unit ua consists of accretionary lapilli rich thinly bedded parallel to cross stratified ash deposits interpreted to indicate the early stages of the eruption and emplacement of dilute pyroclastic density currents upward the second unit ub of the deposit is more massive and the beds thicker indicating an increase in the sedimentation rate from pyroclastic density currents the upper unit uc caps the succession with thick immediately post eruptive lahars which reworked ash deposited on the volcano s slope flow directions obtained by anisotropy of magnetic susceptibility ams analysis of the basal bed of ua at the type locality suggest a provenance of pyroclastic currents from the sea this is interpreted to be related to the initial base surges associated with water magma interaction that occurred immediately after the lateral collapse which wrapped around the shoulder of the sector collapse scar upward in the stratigraphy upper beds of ua and ub paleoflow directions change and show a provenance from the summit vent probably related to the multiple collapses of a vertical pulsatory eruptive column

rhyolitic phreatomagmatism explored tepexitl tuff ring eastern mexican volcanic belt abstract rhyolitic tepexitl tuff ring in the serdan oriental basin of the eastern trans mexican volcanic belt is a young well preserved circular crater a simple morphology that belies a more complex eruptive history field observations ash studies granulometry componentry and morphology and volatile data loi and xrf allow the broad division of the tepexitl deposits into two sequences a lower sequence characterized by fine grained deposits with abundant bomb and lapilli sags and an upper sequence of coarse grained deposits that show little to no deformation the early eruptions at tepexitl are interpreted to have been dominated by discrete highly efficient phreatomagmatic blasts which caused a progressive deepening of the eruptive center lower sequence followed by a transition to dominantly magmatic behavior in the upper sequence dome growth occurred at the end of the eruption but subsequent retrogressive explosions triggered by external water destroyed all trace of the original dome morphology molten fuel coolant interaction mfci is commonly thought to cause the repetitive water magma interaction in phreatomagmatic eruptions but a process by which magma and viscous felsic magmas can mingle prior to explosive interaction has not been described a viable mechanism for rhyolitic mfci based upon field work at tepexitl and laboratory experiments requires that fluidized sediments intrude marginal fractures in the rhyolite magma creating enough interfacial surface area to initiate phreatomagmatic explosions from within the interior of a rising plug or dome

the caldera forming eruption of volcan ceboruco mexico of magma erupted 95 of which was deposited as fall layers during most of the deposition of p1 eruptive intensity mass flux was almost constant at 4 8 107 kg s 1 producing a plinian column 25 30 km in height size grading at the top of p1 indicates however that mass flux waned dramatically and possibly that there was a brief pause in the eruption during the post p1 phase of the eruption a much smaller volume of magma erupted although mass flux varied by more than an order of magnitude we suggest that caldera collapse began at the end of the p1 phase of the eruption because along with the large differences in mass flux behavior between p1 and post p1 layers there were also dramatic changes in lithic content p1 contains 8 lithics post p1 layers contain 30 60 and magma composition p1 is 98 rhyodacite post p1 layers are 60 90 rhyodacite however the total volume of magma erupted during the jala pumice event is close to that estimated for the caldera these observations appear to conflict with models which envision that after an eruption is initiated by overpressure in the magma chamber caldera collapse begins when the reservoir becomes underpressurized as a result of the removal of magma the conflict arises because firstly the p1 layer makes up too large a proportion 75 of the total volume erupted to correspond to an overpressurized phase and secondly the caldera volume exceeds the post p1 volume of magma by at least a factor of three the mismatches between model and observations could be reconciled if collapse began near the beginning of the eruption but no record of such early collapse is evident in the tephra sequence the apparent inability to place the jala pumice eruptive sequence into existing models of caldera collapse which were constructed to explain the formation of calderas much greater in volume than that at ceboruco may indicate that differences in caldera mechanics exist that depend on size or that a more general model for caldera formation is needed

a multidisciplinary effort to assign realistic source parameters to models of volcanic ash cloud transport and dispersion during eruptions during volcanic eruptions volcanic ash transport and dispersion models vatds are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind those models use input parameters called eruption source parameters such as plume height h mass eruption rate duration d and the mass fraction m63 of erupted debris finer than about 4 or 63 m which can remain in the cloud for many hours or days observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected moreover observed plume height may change during an eruption requiring rapid assignment of new parameters this paper reports on a group effort to improve the accuracy of source parameters used by vatds in the early hours of an eruption we do so by first compiling a list of eruptions for which these parameters are well constrained and then using these data to review and update previously studied parameter relationships we find that the existing scatter in plots of h versus yields an uncertainty within the 50 confidence interval of plus or minus a factor of four in eruption rate for a given plume height this scatter is not clearly attributable to biases in measurement techniques or to well recognized processes such as elutriation from pyroclastic flows sparse data on total grain size distribution suggest that the mass fraction of fine debris m63 could vary by nearly two orders of magnitude between small basaltic eruptions 0 01 and large silicic ones 0 5 we classify eleven eruption types four types each for different sizes of silicic and mafic eruptions submarine eruptions brief or vulcanian eruptions and eruptions that generate co ignimbrite or co pyroclastic flow plumes for each eruption type we assign source parameters we then assign a characteristic eruption type to each of the world s 1500 holocene volcanoes these eruption types and associated parameters can be used for ash cloud modeling in the event of an eruption when no observational constraints on these parameters are available

the rio caliente ignimbrite analysis of a compound intraplinian ignimbrite from a major late quaternary mexican eruption the rio caliente ignimbrite is a multi flow unit orcompound ignimbrite formed during a major late quaternary explosive rhyolitic eruption of la primavera volcano mexico the eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air fall deposits it is therefore anintraplinian ignimbrite air fall layers pyroclastic surge mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units a chaotic near vent facies of the ignimbrite includes co ignimbrite lag breccias segregated from proximal pumice flows the facies locates a central vent but one which could not have been associated with a well defined edifice many of the lithics in the exposed lag breccias and near vent facies of the ignimbrite appear to be fragments of welded rio caliente ignimbrite and indicate considerable vent widening or migration during the eruption nearer vent the ignimbrite is thickest and composed of the largest number of flow units here it is welded and is a simple cooling unit evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded less mobile flows which would be produced by collapse of low eruption columns the analogy of a compound ignimbrite with a compound lava flow is therefore good a compound lava flow forms instead of a simple one when the volumetric discharge rate or intensity is low and in explosive eruptions this predicts lower eruption column heights a corollary is that the ignimbrite has a high aspect ratio

the 0 57 ma plinian eruption of the granadilla member tenerife canary islands an example of complexity in eruption dynamics and evolution the granadilla member is one of the most widely dispersed and largest volume pyroclastic units at tenerife canary islands and represents the culminating eruption to a second cycle of explosive volcanism of the las canadas edifice the member dared at 0 57 ma comprises a plinian fall deposit the granadilla pumice which is overlain by ignimbrite up to 30 m thick the granadilla pumice is up to 9 m thick approximately 10 km from sourer pyle b t value is 5 35 km and is subdivided into four fail units unit 1 is up to 1 2 m thick and is further divisible into another four pumice fall subunits based on bedding and grainsize differences unit 2 is a thin but distinctive ash layer similar to2 cm thick and its wide dispersal 550 km 2 constant thickness planar laminations and ash aggregate textures collectively indicate a phreatoplinian fall origin the lithic rich nature and abundance of unaltered lithic fragments reflect magma interaction with aquifer derived water at depth unit 3 less than or equal to1 8 m thick records a reversal to dry plinian eruptive activity unit 4 the thickest of the fall units up to 6 3 m thick records the maximum dispersal and intensity of the eruption pyle b t and b c values are 5 7 and 6 3 km respectively best illustrated by the presence of large pumice bombs up to 30 cm diameter at distances up to 20 km from vent and reverse grading of lithic and pumice clasts the widespread 500 km 2 nonwelded and pumice rich granadilla ignimbrite unit 5 records the collapse of the plinian eruption column the ignimbrite has a simple sheet like geometry but exhibits a complex internal stratigraphy the base of the ignimbrite locally cuts down through the underlying granadilla pumice removing it entirely indicating up to 9 m of erosion by the pyroclastic flows a coarse vent derived lithic breccia horizon towards the top of the ignimbrite is interpreted to record the onset of caldera collapse late in the eruption minimum volume estimates for the granadilla pumice and ignimbrite are 5 2 and 5 km 3 respectively the dispersal area deposit characteristics and exponential thickness and clast size decay relationships with isopach area 1 2 are consistent with dispersal and fallout from the umbrella region of a moderately high similar to 17 to greater than or equal to 25 km plinian column we propose that the eruption involved two vents probably aligned along a ne sw fissure within the las canadas caldera c 2000 elsevier science b v all rights reserved

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

debris slides rapid earth flows in the carbonate massifs of the campania region southern italy morphological and morphometric data for evaluating triggering susceptibility abstract this paper examines the morphological and morphometric properties of debris slides rapid earth flows which have occurred in pyroclastic deposits of the campania region over the last few centuries the slopes bordering the main carbonate massif of western campania are covered by several metres of pyroclastic deposits from the somma vesuvio volcano that due to the steepness of the slopes are easily eroded by surface waters and periodic debris slides rapid earth flows these flows are sudden and unpredictable and have caused both damage to the environment and loss of life over the last few decades after consulting scientific papers and historical documents the areas most recently affected by debris slides rapid earth flows were analysed through aerial photographs and a geological survey an inventory of 172 landslides was produced and landslides were mapped at a scale of 1 2000 for further geomorphological and morphometric analysis morphological analysis was primarily concerned with predisposing factors it confirmed that 86 of the slope failurecrowns developed in response to man made cuts and tracks 44 or due to the presence of cliffs 42 two different kinds of slopes are affected by debris slides rapid earth flows slopes lacking drainage basins and slopes cut by one or several drainage basins typical triangular shaped earth flows with a landslide mass located at the foot of the slope unchannelled debris slides rapid earth flows were found along the former slopes in contrast channelled debris slides rapid earth flows formed along the latter slopes these originated in the drainage divide area developed along the main water segment and invaded the alluvial plain far from the foot of the slope an intermediate group of flows started as triangular shaped and then became channelled morphometric analysis also investigated the parameters most commonly found in the literature slope angle of the crown and sliding zone crown altitude areal extent of the landslide together with other parameters apical angle of the crown zone and landslide body slope relief energy and potential zone of triggering this analysis identified critical values for the steepness of crown zones and for the steepness of the sliding zone critical values of the apical angle of the crown zone for mixed and unchannelled landslides were also established correlations among some of the investigated parameters were investigated in particular the relationships between slope relief energy and areal extent of landslides or the height of the landslide crown zone all the factors and parameters analysed in this paper can be used to identify areas which need to be stabilized or monitored and could be important for planning defensive measures of debris slides rapid earth flows

vertical and lateral collapses on tenerife canary islands and other volcanic ocean islands recent studies demonstrate that tenerife has undergone large lateral collapses this has led to the suggestion that the las canadas caldera one of the best exposed calderas in the world is the result of lateral collapse we have tested this idea using the available structural stratigraphic volcanological and geochronological data we conclude that the las canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of tenerife our results indicate however that vertical collapses may have played a major role in triggering lateral collapses we propose that the association of vertical and lateral collapse events such as inferred for tenerife can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands

a complex quaternary ignimbrite forming phonolitic eruption the poris member of the diego hernandez formation tenerife canary islands abstract the poris member 0 28 ma of the diego hernandez formation tenerife canary islands is a widespread succession of plinian fall surge and non welded ignimbrite deposits it was erupted from the northeastern sector of the multicyclic las canadas caldera the summit caldera complex of the las canadas edifice we present new stratigraphic data which allow the evolution of the eruption to be divided into six main stages 1 an initial surge producing phreatomagmatic phase 2 generation of a buoyant plinian column culminating in vent wall collapse and temporary vent blockage 3 a resumption of phreatomagmatic activity producing accretionary lapilli bearing ash surges and a phreatomagmatic ignimbrite 4 sustained column collapse resulting in the progressive aggradation of a complex compositionally layered ignimbrite sequence 5 a late plinian phase marked by sharp chemical zonation 6 final vent destruction and the generation of multiple surges and pyroclastic flows volume calculations indicate a total erupted volume in the order of 13 14 km 3 3 4 km 3 dense rock equivalent which together with facies and lithic analysis implies that a small scale caldera collapse occurred contributing to the incremental enlargement of the las canadas caldera the geochemistry and mingling inter relationships of four magmatic endmembers ranging from alkaline mafic to evolved phonolitic liquids indicate that two magma chambers fed stages 4 and 5 of the eruption sequence the eruption was triggered when mafic magma intruded the two chamber system

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

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

generation mechanism of tsunamis from the 1883 krakatau eruption the 1883 eruption of krakatau generated a destructive tsunami which claimed about 34 000 lives we compute the generation and propagation of tsunamis from three models previously proposed they are 1 large scale caldera collapse of the northern part of krakatau island 2 emplacement of pyroclastic flow deposits 3 submarine explosion we numerically solve the non linear shallow water equations on actual bathymetry all three models displace the same volume 11 5 km 3 of water in different ways we compare the results of our computation with the observed tsunami waveforms at batavia and tsunami wave heights along the coasts of java and sumatra the comparison shows that the submarine explosion model of 1 to 5 min duration best explains the generation of the largest tsunami

reconstruction of the ad 1600 huaynaputina eruption based on the correlation of geologic evidence with early spanish chronicles the largest historical eruption vei 6 in the andes began on february 19 and continued until march 6 or 15 ad 1600 at huaynaputina a dacitic stratovolcano located on a high volcanic plateau in south peru tephra falls pyroclastic flows and surges disrupted life in an area of 4900 km2 around the volcano and ash fall was reported 200 500 km away in south peru west bolivia and north chile the aftermath of the large scale eruption was severe and protracted for the people and colonial economy of south peru by linking up the series of events inferred from spanish chronicles with the lithofacies and composition of the tephra bulk volume 11 4 12 1 km3 dense rock equivalent dre 4 6 4 95 km3 we distinguish five eruptive phases 1 during the plinian phase a sustained plinian column 27 35 km high on february 19 20 delivered a dacitic pumice fall of 3 1 km3 dre volume the plinian pumice formed a widespread lobe of 95 000 km2 within the 1 cm isopach strong winds carried fine ash 500 km to the west and west northwest into the pacific ocean the computed volumetric eruption rate was in the range of 5 4 6 6 104 to 1 105 m3 s and the mass eruption rate 1 3 1 6 108 kg s the onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre ad 1600 amphitheater the plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock 1600 m below the vent area 2 during the second phase a dwindling column sent ash falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes 3 during the third ignimbrite forming phase with interspersed hydromagmatic events pyroclastic flows 1 5 2 km3 in volume were channeled into the rio tambo canyon and tributaries the flows with interbedded base surge deposits in proximal tributaries probably produced vigorous columns over high rugged relief around the huaynaputina plateau winds winnowing the columns dispersed a widespread co ignimbrite ash probably mixed with co plinian ash over an area of 265 000 km2 4 during the fourth phase an unusual crystal ash fall was deposited when the residual magma with a crystal content as high as 80 was tapped near the end of the eruption 5 during the fifth phase ash flows produced surge deposits and lag fall breccias near vent small volume ash flow deposits in proximal catchments and a thin ash fall layer in medial to distal areas the proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent the ignimbritic and hydromagmatic phases greatly modified the 400 m diameter plinian vent tapping of the crystal rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes geochemistry and mineralogy of the plinian and post plinian units point to an unusual zoned magma sequence the ignimbrite forming phase tapped a magma batch richer in silica than the less differentiated plinian magma the crystal rich magma of unit 4 was fed by crystal mush in a layered magma reservoir and or from two magma reservoirs at distinct depths the geochemical and mineralogical trend throughout the eruption and preliminary measurements of geobarometers suggest a complex model linking a shallow 6 7 km magma reservoir to a deeper 15 km reservoir the total dre volume 4 6 4 95 km3 of erupted tephra did not lead to caldera collapse ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock this suggests the onset of a funnel type or piecemeal collapse

the initiation and development of a caldera forming plinian eruption 172 ka lower pumice 2 eruption santorini greece the rhyodacitic 172 ka lower pumice 2 lp2 eruption terminated the first magmatic cycle at santorini greece producing a proximal 50 m thick succession of pyroclastic fall deposits diffusely stratified to massive ignimbrites and multiple lithic breccias the eruption commenced with the development of a short lived precursory eruption column depositing a 15 cm blanket of 1 2 cm sized pumice fragments at near vent localities lp2 a1 the precursor deposits are conformably overlain by a 30 m thick sequence of reversely graded ungraded pumice fall deposits that reflect opening and widening of a point source vent increasing mass discharge rates up to 108 kg s 1 and the development of a 36 km high plinian eruption column lp2 a2 a3 the progressive increase in maximum vesicle number density nvf in rhyodacitic pumice from 3 2 109 cm 3 in the basal fall unit of lp2 a2 1 to 9 2 109 cm 3 in lp2 a3 translates to an increase in magma decompression rate from 18 to 29 mpa s 1 over the course of the initial plinian phase this is interpreted to be a consequence of progressive vent widening and a deepening of the fragmentation surface such interpretations are supported by the increase in lithic clast abundance vertically through lp2 a and the occurrence of basement derived deep lithic components in lp2 a3 the increasing lithic clast content and the inability to effectively entrain air into the eruption column due to vent widening resulted in column collapse and the development of pyroclastic density currents pdcs lp2 b a major vent excavation event or the opening of new vents possibly associated with incipient caldera collapse facilitated the ingress of water into the magmatic system the development of widespread pdcs and the deposition of a 20m thick massive phreatomagmatic tuff lp2 c the eruption cumulated in catastrophic caldera collapse the enlargement of a pre existing flooded caldera and the discharge of lithic rich pdcs depositing proximal 9 m thick lithic lag breccias lp2 d

the potential failure of monte nuovo at ischia island southern italy numerical assessment of a likely induced tsunami and its effects on a densely inhabited area ischia is the emergent top of a large volcanic complex that rises more than 1 000 m above the sea floor at the north western end of the gulf of naples caldera resurgence in the central part of the island has resulted in the formation of differentially displaced blocks among which mt epomeo 787 m a s l is the most uplifted deformation and slope instability have been recognised as common features induced by a block resurgence mechanism that causes uplift and favours gravitational loading and flank failure the monte nuovo block a topographic high on the north western flank of mt epomeo has recently been interpreted as a block affected by deep seated gravitational slope deformation this block may undergo a catastrophic failure in the case of renewal of magmatic activity this paper investigates the potential failure of the monte nuovo block as a rockslide debris avalanche the consequent tsunami generation and wave propagation and discusses the catastrophic effects of such an event mobilization prone volume has been estimated at about 160 106 m3 and would move from a maximum elevation of 400 m a s l the landslide itself would sweep away a densely populated territory as large as 3 5 km2 the highest waves generated by the tsunami on which this paper is mainly focussed would hit the northern and western shores of ischia however the high coast would prevent inundation and limit devastation to beaches harbours and surrounding areas most of the tsunami energy would head towards the north east hitting the campania coast severe inundation would affect an area of up to 20 km2 around the mouth of the volturno river including the urban area of castel volturno in contrast less energy would travel towards the south and the gulf of naples would be perturbed by long persisting waves of limited damaging potential

cyclical slope instability and volcanism related to volcano tectonism in resurgent calderas the ischia island italy case study abstract the results of a detailed stratigraphic study carried out in the areas located to the east and south west of mt epomeo at ischia are presented and compared with those of previous geological archaeological and historical investigations to show the relationships among caldera resurgence volcanism and slope instability in the past 5 5 ka resurgence at ischia began at about 30 ka b p and occurred through intermittent uplifting and tectonic quietness phases during the past 5 5 ka reactivation of faults and related volcanic activity was accompanied by emplacement of deposits generated by surface gravitational movements these deposits were generated in four main phases dated between 5 5 and 2 9 ka around 2 9 ka between 2 6 and 2 3 ka and between 2 3 and 1 9 ka respectively deposits formed by gravitational movements preceded and followed the emplacement of volcanic rocks testifying that slope instability was induced by vertical movements which also activated and or reactivated faults and fractures that fed volcanism the results of this study therefore suggest that although slope failure can occur as a consequence of a variety of factors resurgence has to be considered a factor inducing a particularly intense slope instability resurgence is accompanied by activation of faults and renewal of volcanism causing oversteepening of the slopes and generating seismicity that could trigger surface gravitational movements furthermore the availability of large amount of loose material rapidly accumulated along the slopes during eruptions favors landslide generation

causes of complexity in a fallout dominated plinian eruption sequence 312 ka fasnia member diego hernandez formation tenerife spain the 312 ka fasnia eruption from the las canadas caldera on tenerife canary islands spain produced a complex sequence of twenty two intercalated units including 7 pumice fall 7 ignimbrite and 8 ash surge and fall deposits that define two distinct eruption sequences lower and upper fasnia sequences the fallout units themselves are internally complex reflecting waxing and waning of the eruption column while many of the ignimbrites reflect multiple intra plinian partial column collapse events associated with the injection of lithic clasts into the eruption column the lower and upper fasnia eruption phases were each terminated by caldera collapse and complete column collapse events probable blockage of the conduit and vent system during lower fasnia caldera collapse event briefly terminated the eruption resulting in a short lived period of erosion and sedimentation prior to the onset of the upper fasnia phase the transition to the upper fasnia eruption phase coincided with the eruption of more geochemically homogeneous pyroclasts in total 62 km3 of tephra were erupted including 49 km3 of juvenile clasts and n12 km3 of lithic clasts the dre volume of magma erupted was 13 km3 lower fasnia n 5 km3 upper fasnia n 8 km3 two thirds of which 9 10 km3 was deposited purely by fallout the fasnia member is one of the most complex plinian sequences known

vent evolution and lag breccia formation during the cape riva eruption of santorini greece the 18 500 yr b p cape riva cr eruption of santorini vented several km3 or more of magma generating four eruption units a basal plinian fall deposit cr a and three pyroclastic flow deposits cr b to cr d upwards cr b and cr d are welded ignimbrites cr c consists predominantly of up to 25 m thick coarse lithic rich co ignimbrite lag breccias resulting from a climactic phase of the eruption the initial plinian phase occurred from a localized vent in n santorini and subsequent column collapse resulted in emplacement of cr b towards the end of cr b new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias cr c cr d probably records a return to a localized vent as the eruption waned the eruption sampled a zoned magma chamber containing rhyodacite overlying andesite and leaks of these magmas were manifested as the skaros therasia lavas preceding the cr eruption plinian and initial ignimbrite stages occurred while the magma chamber was ove

interaction between volcanic plumes and wind during the 2010 eyjafjallajokull eruption iceland 1 estimates of volcanic source mass flux currently deduced from observations of plume height are crucial for ash dispersion models for aviation and population hazard this study addresses the role of the atmospheric wind in determining the height at which volcanic plumes spread in the atmosphere and the relationship between source mass flux and plume height in a wind field we present a predictive model of volcanic plumes that describes the bending over of the plume trajectory in a crosswind and show that model predictions are in accord with a dataset of historic eruptions if the profile of atmospheric wind shear is described the wind restricts the rise height of volcanic plumes such that obtaining equivalent rise heights for a plume in a windy environment would require an order of magnitude increase in the source mass flux over a plume in a quiescent environment our model calculations are used to calibrate a semi empirical relationship between the plume height and the source mass flux that explicitly includes the atmospheric wind speed we demonstrate that the model can account for the variations in plume height observed during the first explosive phase of the 2010 eyjafjallajokull eruption using independently measured wind speeds and show that changes in the observed plume height are better explained by changing meteorology than abrupt changes in the source mass flux this study shows that unless the wind is properly accounted for estimates of the source mass flux during an explosive eruption are likely to be very significant underpredictions of the volcanic source conditions

the origin of the 1883 krakatau tsunamis abstract hypotheses for the origin of the krakatau tsunamis of 27 august 1883 are reviewed the 151 minute sea wave travel time between krakatau and batavia jakarta first proposed by verbeek cannot be substantiated thus hypotheses for the origin of the tsunamis which rest on very accurate timings of the origins of the air and sea waves such as that of latter 1981 also cannot be substantiated there was one exceptionally large sea wave tsunami on 27 august and many smaller ones most of the smaller waves appear to have been air sea coupled waves rather than true tsunamis of the few sea waves that were probably true tsunamis only one may have originated by emplacement of a pyroclastic flow at a point nearer batavia than krakatau the catastrophic event in the eruption of august 27th was preceded some 40 minutes earlier by a subordinate event which also propagated an air sea coupled wave and a tsunami the largest heard explosion produced an air sea coupled wave but was not associated with a tsunami although the precise time relations between the origins of the air and sea waves cannot be definitely established the most likely scenario for the catastrophic phase of the eruption is broadly similar to that originally proposed by verbeek in which collapse of part of the original volcanic edifice propagated a major volcanic explosion

event stratigraphy of a caldera forming ignimbrite eruption on tenerife the 273 ka poris formation the 273 ka poris formation in the bandas del sur group records a complex compositionally zoned explosive eruption at las canadas caldera on tenerife canary islands the eruption produced widespread pyroclastic density currents that devastated much of the se of tenerife and deposited one of the most extensive ignimbrite sheets on the island the sheet reaches 40 m thick and includes plinian pumice fall layers massive and diffuse stratified pumiceous ignimbrite widespread lithic breccias and co ignimbrite ashfall deposits several facies are fines rich and contain ash pellets and accretionary lapilli eight brief eruptive phases are represented within its lithostratigraphy phase 1 comprised a fluctuating plinian eruption in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island phase 2 emplaced three geographically restricted ignimbrite flow units and associated extensive thin co ignimbrite ashfall layers which contain abundant accretionary lapilli from moist co ignimbrite ash plumes a brief plinian phase phase 3 again dispersing pumice lapilli over southeastern tenerife marked the onset of a large sustained pyroclastic density current phase 4 which then waxed phase 5 covering increasingly larger areas of the island as vents widened and or migrated along opening caldera faults the climax of the poris eruption phase 6 was marked by widespread emplacement of coarse lithic breccias thought to record caldera subsidence this is inferred to have disturbed the magma chamber causing mingling and eruption of tephriphonolite magma and it changed the proximal topography diverting the pyroclastic density current s down the guimar valley phase 7 phase 8 involved post eruption erosion and sedimentary reworking accompanied by minor down slope sliding of ignimbrite this was followed by slope stabilization and pedogenesis the fines rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes they resemble phreatomagmatic deposits but a phreatomagmatic origin is difficult to establish because shards are of bubble wall type and the moisture may have arisen by condensation within ascending thermal co ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast ash pellets formed in co ignimbrite ash clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli

complex variations during a caldera forming plinian eruption including precursor deposits thick pumice fallout co ignimbrite breccias and climactic lag breccias the 184 ka lower pumice 1 eruption sequence santorini greece the 184 ka lower pumice 1 eruption sequence records a complex history of eruption behaviours denoted by two significant eruptive phases 1 a minor precursor lp1 pc and 2 a major plinian phase lp1 a b c the precursor phase produced 13 small volume pyroclastic fallout surge and flow deposits which record the transition from a dominantly magmatic to a phreatomagmatic eruptive style and exhibit a normal dacite to andesitic dacite to reverse andesitic dacite to dacite compositional zonation of juvenile pyroclasts in the stratigraphy incipient bioturbation and variability in unit thickness and lithology reflect multiple time breaks and highlight the episodic nature of volcanism prior to the main plinian eruption phase the plinian magmatic eruption phase is defined by three major stratigraphic divisions including a basal pumice fallout deposit lp1 a an overlying valley confined ignimbrite lp1 b and a compositionally zoned rhyodacite to basaltic andesite lithic rich lag breccia lp1 c which caps the sequence this sequence records the initial development of a buoyant convective eruption column and the transition to eruption column and catastrophic late stage caldera collapse events similarities in pyroclast properties i e chemistry density between the plinian fallout lp1 a and pyroclastic flow lp1 b deposits indicate that changes in magma properties exerted no influence on the dynamics and temporal evolution of the lp1 eruption conversely lithic breccias at the base of the lp1 b ignimbrite suggest that the transition from a buoyant convective column to column collapse was facilitated by mechanical erosion of the conduit system and or the initiation of caldera collapse leading to vent widening an increase in magma discharge rate and the increased incorporation of lithics into the eruption column causing mass overload lithic rich lag breccia deposits lp1 c which cap the eruption sequence record incremental high energy caldera collapse events whereby downfaulting occurred in discrete jumps resulting in variable magma discharge rates and the development of a fissure vent system

the quaternary pyroclastic succession of southeast tenerife canary islands explosive eruptions related caldera subsidence and sector collapse a much revised quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the bandas del sur in southern tenerife new ar 41 ar 39 data obtained for the arico granadilla fasnia poris la caleta and abrigo formations are presented allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments we also provide a minimum age of 287 7 ka for a major sector collapse event at the gaimar valley the bandas del sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics including widespread basal plinian layers predominantly phonolite composition ignimbrites with similar extensive geographic distributions thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns lateral gradations into localized massive facies within palaeo wadis and widespread lithic breccia layers that probably record caldera forming eruptions each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean the succession indicates that las canadas volcano underwent a series of major explosive eruptions each starting with a plinian phase followed by emplacement of ignimbrites and thin ash layers some of coignimbrite origin several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption because passage of each pyroclastic density current was characterized by phases of non deposition and erosion the entire course of each eruption is incompletely recorded at any one location accounting for some previously perceived differences between the units because each current passed into the ocean estimating eruption volumes is virtually impossible nevertheless the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the bandas del sur succession and probably formed a complex nested collapse structure detailed field relationships show that extensive ignimbrite sheets e g the arico poris and la caleta formations relate to previously unrecognized caldera collapse events we envisage that the evolution of the nested las cahadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite related caldera collapse events and large sector collapse events interspersed with edifice building phases

the nature origins and distribution of ash aggregates in a large scale wet eruption deposit oruanui new zealand abstract this study documents the processes and products of volcanic ash aggregation in phreatomagmatic phases of the 25 4 ka oruanui supereruption from taupo volcano new zealand detailed textural and stratigraphic relationships of aggregates are examined in six of the ten erupted units which range from relatively dry styles of eruption and deposition units 2 5 to mixed units 6 7 8 and dominantly wet unit 3 aggregate structures and grain size distributions shift abruptly over vertical scales of cm to dm providing diagnostic features to identify deposits emplaced primarily as vertical fallout or pyroclastic density currents pdcs the six categories of ash aggregates documented here are used to infer distinct volcanic and meteorological interactions in the eruption cloud related to dispersal characteristics and mode of emplacement our field observations support the notion of brown et al 2010 origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife geol soc am bull 122 305 320 that deposits bearing matrix supported accretionary lapilli with concentric internal structure and abundant rim fragments are associated with emplacement of pdcs however on the basis of grain size distributions and field relationships it is inferred that these types of ash aggregates formed their ultrafine ash dominantly 100 km from vent it is inferred that ambient meteorological conditions play a more important role in ash fallout in these cases entrainment of moist air and distal subsidence and melting of ice carried by the plume are both likely to have contributed to the observed features of late stage aggregation in the drier phases of eruption these observations suggest that proximal column influenced aggregation processes which weaken with distance from source are overprinted by secondary later stage aggregation mechanisms farther downwind

contemporaneous convective and collapsing eruptive dynamics the transitional regime of explosive eruptions 1 two contrasting eruptive regimes have been classically postulated to describe the behavior of explosive eruptions convective and collapsing early studies have recognized that many eruptions evolve from the first to the second regime and have assumed the existence of a sharp boundary between them consequently the dynamics of such transition has never been investigated in detail here we present results of integrated deposit analyses and numerical simulations which demonstrate univocally that both dynamics can and often do coexist for hours in a new intermediate transitional regime of explosive eruptions the study elucidates the features of the new regime in terms of mass partitioning pyroclastic current dynamics and interplay among convective and collapsing styles

the las canadas caldera tenerife canary islands an overlapping collapse caldera generated by magma chamber migration abstract the las canadas caldera is one of the most important geological structures of tenerife stratigraphic structural volcanological petrological geochronological and geophysical data suggest that the las canadas caldera resulted from multiple vertical collapse episodes that occurred during the construction of the las canadas edifice upper group three long term 200 ka cycles of phonolitic explosive activity each culminating with a caldera collapse have been identified in the upper group during the construction of the upper group the focus of felsic volcanism migrated from west to east using the results of field observations experimental analogue models and numerical studies we propose that the formation of the overlapping las canadas collapse caldera is related to the migration of the associated magma chamber our model implies that each collapse of this overlapping caldera partly or completely destroyed the feeding magma chamber this destruction led to changes in the local stress field that favoured the formation of a new chamber at one side of the previous one resulting in magma chamber migration the proposed model accounts for the formation of the las canadas caldera in particular it explains the geometrical relationships stratigraphy and chronology of the caldera wall deposits comparison with other overlapping collapse calderas suggests that our model may apply to other overlapping calderas

high magma decompression rates at the peak of a violent caldera forming eruption lower pumice 1 eruption santorini greece we use the deposit sequence resulting from the first catastrophic caldera collapse event recorded at santorini associated with 184 ka lower pumice 1 eruption to study the shallow conduit dynamics at the peak of caldera collapse the main phase of the lower pumice 1 eruption commenced with the development of a sustained buoyant eruption column producing a clast supported framework of rhyodacitic white pumice lp1 a the clasts have densities of 310 740 kg m 3 large coalesced vesicles that define unimodal size distributions and moderate to high vesicle number densities 1 2 109 1 7 109 cm 3 eruption column collapse possibly associated with incipient caldera collapse resulted in the development of pyroclastic flows lp1 b the resulting ignimbrite is characterised by rhyodacitic white pumice with a narrow density range 250 620 kg m 3 and moderate to high vesicle number densities 1 3 109 2 1 109 cm 3 comparable to clasts from lp1 a an absence of deep basement derived lithic clast assemblages together with the occurrence of large vesicles and relatively high vesicle number densities in pumice from the fallout and pyroclastic flow phases suggests shallow fragmentation depths a prolonged period of bubble nucleation and growth and moderate rates of decompression prior to fragmentation 7 11 mpa s 1 evacuation of magma during the pyroclastic flow phase led to under pressurisation of the magma reservoir the propagation of faults associated with the main phase of caldera collapse and the formation of 20 m thick lithic lag breccias lp1 c rhyodacitic pumices from the base of the proximal lithic lag breccias show a broader range of density 330 990 kg m 3 higher vesicle number densities 4 5 109 1 1 1010 cm 3 and higher calculated magma decompression rates of 15 28 mpa s 1 than pyroclasts from the pre collapse eruptive phases in addition the abundance of lithic clasts including deeper basement derived lithic assemblages records the opening of new vents and a deepening of the fragmentation surface these data support numerical simulations which predict rapid increases in magma decompression and mass discharge rates at the onset of caldera collapse

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

determination of the largest clast sizes of tephra deposits for the characterization of explosive eruptions a study of the iavcei commission on tephra hazard modelling the distribution of clasts deposited around a volcano during an explosive eruption typically contoured by isopleth maps provides important insights into the associated plume height wind speed and eruptive style nonetheless a wide range of strategies exists to determine the largest clasts which can lead to very different results with obvious implications for the characterization of eruptive behaviour of active volcanoes the iavcei commission on tephra hazard modelling has carried out a dedicated exercise to assess the influence of various strategies on the determination of the largest clasts suggestions on the selection of sampling area collection strategy choice of clast typologies and clast characterization i e axis measurement and averaging technique are given mostly based on a thorough investigation of two outcrops of a plinian tephra deposit from cotopaxi volcano ecuador located at different distances from the vent these include 1 sampling on a flat paleotopography far from significant slopes to minimize remobilization effects 2 sampling on specified horizontal area sections with the statistically representative sampling area depending on the outcrop grain size and lithic content 3 clast characterization based on the geometric mean of its three orthogonal axes with the approximation of the minimum ellipsoid lithic fragments are better than pumice clasts when present and 4 use of the method of the 50th percentile of a sample of 20 clasts as the best way to assess the largest clasts it is also suggested that all data collected for the construction of isopleth maps be made available to the community through the use of a standardized data collection template to assess the applicability of the new proposed strategy on a large number of deposits and to build a large dataset for the future development and refinement of dispersal models

ascent dynamics of large phreatomagmatic eruption clouds the role of microphysics 1 we examine the impact of abundant surface water interaction on the development of volcanic clouds from large scale 108 kg s 1 magma phreatomagmatic eruptions presenting the first 2 d large eddy simulations of wet volcanic plumes that incorporate the effects of microphysics the active tracer high resolution atmospheric model was forced with field derived inputs from an exceptionally large phreatomagmatic eruption the 27 ka oruanui supereruption taupo volcano new zealand surface water contents were varied from 0 to 40 wt for eruptions with equivalent magma eruption rates of 1 3 108 and 1 1 109 kg s 1 our findings confirm that increased surface water has a pronounced impact on column stability leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents pdcs contrary to the suggestion of previous workers however abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds owing to vigorous microphysics assisted lofting of pdcs the simulated behavior and ash cloud dimensions provide a close match to field evidence from the oruanui case study cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopause while ambient humidity primarily impacts the abundance of airborne hydrometeors particularly ice associated with the volcanic clouds general relationships between eruption style meteorological conditions and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence

estimating the volume of tephra deposits a new simple strategy volume determination of tephra deposits is necessary for the assessment of the dynamics and hazards of explosive volcanoes several methods have been proposed during the past 40 years that include the analysis of crystal concentration of large pumices integrations of various thinning relationships and the inversion of field observations using analytical and computational models regardless of their strong dependence on tephra deposit exposure and distribution of isomass isopach contours empirical integrations of deposit thinning trends still represent the most widely adopted strategy due to their practical and fast application the most recent methods involve the best fitting of thinning data using various exponential segments or a power law curve on semilog plots of thickness or mass area versus square root of isopach area the exponential method is mainly sensitive to the number and the choice of straight segments whereas the power law method can better reproduce the natural thinning of tephra deposits but is strongly sensitive to the proximal or distal extreme of integration we analyze a large data set of tephra deposits and propose a new empirical method for the determination of tephra deposit volumes that is based on the integration of the weibull function the new method shows a better agreement with observed data reconciling the debate on the use of the exponential versus power law method in fact the weibull best fitting only depends on three free parameters can well reproduce the gradual thinning of tephra deposits and does not depend on the choice of arbitrary segments or of arbitrary extremes of integration

lateral variations within coarse co ignimbrite lithic breccias of the kos plateau tuff greece coarse co ignimbrite lithic breccia ebx occurs at the base of ignimbrite e the most voluminous and widespread unit of the kos plateau tuff kpt in greece similar but generally less coarse grained basal lithic breccias dbx are also associated with the ignimbrites in the underlying d unit ebx shows considerable lateral variations in texture geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing f2 10 wt to fines poor f2 0 1 wt lithic clasts are predominantly vent derived andesite although clasts derived locally from the underlying sedimentary formations are also present there are no proximal exposures of kpt there is a highly irregular lower erosional contact at the base of ignimbrite e at the closest exposures to the inferred vent 10 14 km from the centre of the inferred source but no ebx was deposited from 14 to 20 km from source ebx is present over a planar erosional contact at 16 km ebx is a 3 m thick coarse fines poor lithic breccia separated from the overlying fines bearing pumiceous ignimbrite by a sharp contact this grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts pumice and ash or into a thinner one clast thick lithic breccia that grades upward into relatively lithic poor pumiceous ignimbrite distally 27 to 36 km from source ebx is a finer one clast thick lithic breccia that overlies a non erosional base a downcurrent change from strongly erosional to depositional basal contacts of ebx dominantly reflects a depletive pyroclastic density current initially the front of the flow was highly energetic and scoured tens of metres into the underlying deposits once deposition of the lithic clasts began local topography influenced the geometry and distribution of ebx and in some cases ebx was deposited only on topographic crests and slopes on the lee side of ridges the kpt ignimbrites also contain discontinuous lithic rich layers within texturally uniform pumiceous ignimbrite these intra ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non erosional basal contacts the proportion of fine ash within the kpt lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow turbulence induced locally by interaction with topography flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud and to self fluidisation accompanying the settling of coarse dense lithic clasts there are problems in interpreting the kpt lithic breccias as conventional co ignimbrite lithic breccias these problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated non turbulent systems that deposit en masse the kpt coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified waning pyroclastic density currents and from variations in lithic clast supply from source

the structure of a hydrothermal system from an integrated geochemical geophysical and geological approach the ischia island case study the complexity of volcano hosted hydrothermal systems is such that thorough characterization requires extensive and interdisciplinary work we use here an integrated multidisciplinary approach combining geological investigations with hydrogeochemical and soil degassing prospecting and resistivity surveys to provide a comprehensive characterization of the shallow structure of the southwestern ischia s hydrothermal system we show that the investigated area is characterized by a structural setting that although very complex can be schematized in three sectors namely the extra caldera sector ecs caldera floor sector cfs and resurgent caldera sector rcs this contrasted structural setting governs fluid circulation geochemical prospecting shows in fact that the caldera floor sector a structural and topographic low is the area where co2 rich 40 cm3 l hydrothermally mature log mg na ratios 150 g m 2 d 1 is clearly captured by electrical resistivity tomography ert and transient electromagnetic tem surveys as a highly conductive resistivity 10 000 mg l and poorly conductive meteoric derived tds 4 000 mg l waters are observed respectively we finally integrate our observations to build a general model for fluid circulation in the shallowest 0 5 km part of ischia s hydrothermal system

the intensity and magnitude of holocene plinian eruptions from mount st helens volcano dispersal characteristics of the t we wn pu ps ye yn and yb plinian fall deposits of mount st helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities magma discharge based on a theoretical model of tephra dispersal new proximal thickness measurements are combined with an empirical distal extrapolation based on studies of 53 plinian deposits to calculate the magnitude erupted mass of each eruption layer yn 3510 y r b p represents the highest intensity and largest magnitude eruption at mount st helens in post glacial times modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase 26 hours several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of mount st helens peak intensity of the plinian phase was 108 kg s and the total erupted volume was 4 km3 dre of magma small plinian style eruptions are represented by layers such as ps and pu of the pine creek eruptive period 3000 2500 yr b p and have intensities of only 106 kg s when compared with plinian eruptions from other volcanoes the holocene eruptions of mount st helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate sized stratovolcanoes there is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle this relationship may be related to a time dependent process for the accumulation of differentiated and volatile rich magma within the chamber beneath mount st helens

the historic magmatic hydrothermal eruption of the breccia di commenda vulcano italy the historic breccia di commenda bc explosive eruption of vulcano aeolian islands italy opened with a phase that generated a gray fine ash layer dispersed to the northwest phase 1 the eruption continued with a dilute pyroclastic density current pdc that was dispersed to the east followed by the emplacement of radially distributed topographically controlled pdc deposits phase 2 the last phase of the eruption produced a sequence of accretionary lapilli and gray fine ash dispersed toward the southeast phase 3 the most impressive feature of the bc is its high lithic juvenile clast ratio and the yellow color of the deposits of phase 2 lithic fragments are mainly hydrothermally altered rocks in the silicic and advanced argillic facies juvenile fragments ranging from 20 to 40 by volume are mainly confined to the ash component of the deposits and comprise rhyolitic to trachyandesite poorly to non vesicular fragments the fine ash fraction of the deposits is richer in s cu zn pb and as than the bc juvenile lapilli and bombs and also the juvenile components of other la fossa units suggesting that the bc formed in the presence of an anomalously high amount of s and metals sulfur and metals may have been carried as aerosols by chloride and sulfate bearing micro crystals derived from the condensation of magmatic gas in the eruptive cloud the high content of hydrothermally altered lithic clasts in the deposits suggests that explosions involved the fluid saturated hydrothermally altered rocks residing in the conduit zone however the presence of a juvenile component in the deposits supports the idea that this explosion may have been triggered by the ascent of new magma we categorize this eruption as magmatic hydrothermal to emphasize that in this type of phreatomagmatic eruption the external water was an active hydrothermal system rock magnetic temperatures of non altered lava lithic fragments indicate a uniform deposit temperature for the pdc deposits of between 200 and 260 c with a maximum at 280 c these homogeneous relatively low temperatures are consistent with the idea that the phase 2 explosions involved the expansion of abundant steam from the flashing of the hydrothermal system in addition recent paleomagnetic dating of the bc provides an age of between 1000 and 1200 ad younger than that reported in the previously published data suggesting that previous interpretations and the recent history of la fossa and mt pilato require re evaluation

the physical volcanology of the 1600 eruption of huaynaputina southern peru volcan huaynaputina is a group of four vents located at 16 36 s 70 51 w in southern peru that produced one of the largest eruptions of historical times when 11 km3 of magma was erupted during the period 19 february to 6 march 1600 the main eruptive vents are located at 4200 m within an erosion modified amphitheater of a significantly older stratovolcano the eruption proceeded in three stages stage i was an 20 h sustained plinian eruption on 19 20 february that produced an extensive dacite pumice fall deposit magma volume 2 6 km3 throughout medial distal and distal parts of the dispersal area a fine grained plinian ashfall unit overlies the pumice fall deposit this very widespread ash magma volume 6 2 km3 has been recognized in antarctic ice cores a short period of quiescence allowed local erosion of the uppermost stage i deposits and was followed by renewed but intermittent explosive activity between 22 and 26 february stage ii this activity resulted in intercalated pyroclastic flow and pumice fall deposits 1 km3 the flow deposits are valley confined whereas associated co ignimbrite ash fall is found overlying the plinian ash deposit following another period of quiescence vulcanian type explosions of stage iii commenced on 28 february and produced crudely bedded ash lapilli and bombs of dense dacite 1 km3 activity ceased on 6 march compositions erupted are predominantly high k dacites with a phenocryst assemblage of plagioclase hornblende biotite fe ti oxides apatite major elements are broadly similar in all three stages but there are a few important differences stage i pumice has less evolved glass compositions 73 sio2 lower crystal contents 17 20 lower density 1 0 1 3 g cm3 and phase equilibria suggest higher temperature and volatile contents stage ii and stage iii juvenile clasts have more evolved glass 76 sio2 compositions higher crystal contents 25 35 higher densities up to 2 2 g cm3 and lower temperature and volatile contents all juvenile clasts show mineralogical evidence for thermal disequilibrium inflections on a plot of log thickness vs area1 2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system the ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper level winds whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower altitude wind field thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24 32 km and a total column height of 34 46 km the estimated mass discharge rate for the 20 h long stage i eruption is 2 4 108 kg s and the volumetric discharge rate is 3 6 105 m3 s the pumice fall deposit has a dispersal index hildreth and drake 1992 of 4 4 and its index of fragmentation is at least 89 reflecting the dominant volume of fines produced of the 11 km3 total volume of dacite magma erupted in 1600 approximately 85 was evacuated during stage 1 the three main vents range in size from 70 to 400 m alignment of these vents and a late stage dyke parallel to the nnw sse trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre existing weaknesses during stage i this fissure evolved into a large flared vent vent 2 with a diameter of approximately 400 m this vent was active throughout stage ii at the end of which a dome was emplaced within it during stage iii this dome was eviscerated forming the youngest vent in the group vent 3 a minor extra amphitheater vent was produced during the final event of the eruptive sequence recharge may have induced magma to rise away from a deep zone of magma generation and storage subsequently vesiculation in the rising magma batch possibly enhanced by interaction with an ancient hydrothermal system triggered and fueled the sustained plinian eruption of stage i a lower volatile content in the stage ii and stage iii magma led to transitional column behavior and pyroclastic flow generation in stage ii continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage iii no caldera collapse occurred because no shallow magma chamber developed beneath this volcano

complex proximal deposition during the plinian eruptions of 1912 at novarupta alaska proximal 3 km deposits from episodes ii and iii of the 60 h long novarupta 1912 eruption exhibit a very complex stratigraphy the result of at least four transport regimes and diverse depositional mechanisms they contrast with the relatively simple stratigraphy and inferred emplacement mechanisms for the previously documented better known medial distal fall deposits and the valley of ten thousand smokes ignimbrite the proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta and numerous thin complex deposits of pyroclastic density currents pdcs with no regional analogs the locally dispersed component of the fall deposits forms sector confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100 300 m cf several kilometers for the medial distal fall deposits this locally dispersed fall material and many of the associated pdc deposits is rich in andesitic and banded pumices and richer in shallow derived wall rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition there are no marked contrasts in grain size in the near vent deposits however between locally and widely dispersed beds and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial distal samples associated pdc deposits form a spectrum of facies from fines poor avalanched beds through thin bedded landscape mantling beds to channelized lobes of pumice block rich ignimbrite the origins of the novarupta near vent deposits are considered within a spectrum of four transport regimes 1 sustained buoyant plume 2 fountaining with co current flow 3 fountaining with counter current flow and 4 direct lateral ejection the novarupta deposits suggest a model where buoyant stable regime 1 plumes characterized most of episodes ii and iii but were accompanied by transient and variable partitioning of clasts into the other three regimes only one short period of vent blockage and cessation of the plinian plume occurred separating episodes ii and iii which was followed by a single pdc interpreted as an overpressured blast involving direct lateral ejection in contrast regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume which were being strongly affected by short lived instabilities these instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent of the parameters that control explosive eruptive behavior only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products

fall vs flow activity during the 1991 climactic eruption of pinatubo volcano philippines six years after the 1991 mt pinatubo eruption deep erosion incisions into the pyroclastic deposits accumulated around the volcano enabled us to investigate the stratigraphy of the climactic deposits both in valley bottoms and on contiguous ridges stratigraphic relationships between fall flow and surge deposits in the marella drainage system indicate that during the climactic eruption a progressive shift occurred from an early convective regime to a transitional regime feeding both the plinian convective column and mostly dilute density currents to a fully collapsing regime producing mostly dense pyroclastic flows syn plinian dilute density currents surges propagated up to 10 km from the crater both along valley bottoms and on contiguous ridges of the marella valley whereas post plinian pyroclastic flows had greater runout 13 km were confined to valleys and were not associated with significant surges stratigraphic study and grain size analyses allow the identification of three types of intra plinian deposits a lower and often coarse grained surge deposits emplaced during the accumulation of the coarsest portion of the fallout bed at time intervals of 16 24 min b upper fine grained surge deposits interstratified with the fine grained portion of the fall bed and emplaced at shorter time intervals of 3 13 min and c small volume channel confined massive pumiceous flow deposits interbedded with the upper surges in the upper fine grained fall bed maximum clast size isopleths of 1 6 and 0 8 cm for lithics ml and 2 0 and 4 0 cm for pumices mp show almost symmetrical distribution around the vent indicating that the passing of the typhoon yunya during the climactic eruption had little effect on trajectories of high reynold number clasts significant distortion was however observed for the 3 2 cm ml and 6 0 cm mp proximal isopleths whose patterns were probably influenced by the interaction of the clasts falling from column margins with the uprising co ignimbrite ash plumes application of the carey and sparks 1986 model to the undisturbed isopleths generated by the umbrella cloud yields a maximum column height of 42 km in good agreement with satellite measurements systematic stratigraphic and vertical grain size studies of the plinian fall deposit in the marella valley combined with satellite data and eyewitness accounts reveal that the carrying capacity of the convective column and related fallout activity peaked in the early phase of the eruption beginning slightly before 13 41 and gradually declined until its cessation 3 h later most of the pumiceous pyroclastic flow deposits were emplaced after the end of the fallout activity at 16 30 but before the summit caldera collapse at approximately 19 11 only a small volume of pumiceous flow deposits accumulated after the final caldera collapse in contrast to the previous reconstruction of holasek et al 1996 which interpreted the progressive lowering of the column documented by satellite data as due to a decreasing mass eruption rate we suggest that a progressive shift from a plinian column to a large co ignimbrite column could also account for such a variation

the ischia debris avalanche first clear submarine evidence in the mediterranean of a volcanic island prehistorical collapse sector or flank collapse with related debris avalanches is increasingly recognized as a relatively common volcanic behaviour in particular for large hot spot related oceanic islands here we report the case of a catastrophic collapse that occurred at ischia volcanic island in prehistorical times and was driven by the volcano tectonic uplift of mt epomeo the major relief of the island the collapse left a subaerial to submarine horseshoe scar on the southern flank of the island and generated a debris avalanche incorporating thousands of giant blocks dispersed as far as 50 km from the island during the emplacement part of the debris avalanche evolved into a debris flow covering an area of 250 300 km2 this constitutes the first clear evidence of a submarine debris avalanche in the mediterranean sea the major collapse was followed and probably also preceded by recurrent less catastrophic terrestrial and underwater failures two other undersea hummocky deposits are found north and west of the island and might tentatively be correlated to the major southern collapse such volcanic behaviour previously unknown for ischia volcano has likely triggered tsunami waves over the entire bay of naples raising the question of their impact on prehistorical historical communities

an ignimbrite veneer deposit the trail marker of a pyroclastic flow abstract the term ignimbrite veneer deposit ivd is proposed for a new kind of pyroclastic deposit which is found associated with and passes laterally into taupo ignimbrite of valley pond type in new zealand it forms a thin layer mantling the landscape over 15 000 km 2 and is regarded as the deposit from the trailing tail of a pyroclastic flow where a relaxation of shear stress favoured the deposition of the basal part of the flow the ivd differs little in grain size from the associated ignimbrite but it shows a crude internal stratification attributed to the deposition of a succession of layers one after the passage of each pulse of the pyroclastic flow it locally contains laterally discontinuous lenses of coarse pumice lee side lenses on the far vent side of topographic obstacles in nearvent exposures the taupo ivd shows lensoid and cross stratified bed forms even where it stands on a planar surface attributed to deposition from a flow travelling at an exceedingly high velocity an ivd can be distinguished from a poorly sorted pyroclastic fall deposit because the beds in it show more rapid lateral variations in thickness it may show a low angle cross stratification and it contains carbonised wood from trees not in the position of growth from the deposit of a wet base surge because it lacks vesicles and strong antidune like structures and contains carbonised vegetation and from a hot and dry pyroclastic surge deposit because it possesses a high content of pumice and fines the significance of an ivd is that it records the passage of a pyroclastic flow where the flow itself has moved farther on

transition from steady to unsteady plinian eruption column the vei 5 4 6 ka fogo a plinian eruption sao miguel azores abstract the 4 6 ka fogo a trachytic plinian eruption on sao miguel island azores portugal is composed by four main phases 1 initial phreatomagmatic activity which emplaced thin surge deposits with accretionary lapilli 2 fully buoyant steady plinian activity which generated a prominent trachytic basal fall out deposit 3 partial collapsing activity which led to the emplacement of two trachytic trachydacitic intra plinian ignimbrites pink and black ignimbrites and associated surge deposits inter stratified within two fall out deposits 4 vent widening and fully collapsing activity which caused the emplacement of the climactic brown trachydacitic ignimbrite the deposits of these four main phases have been organised into three members fga1 fga2 and fga3 sub divided into 15 stratigraphic or emplacement units the fluctuations from a steady to unsteady plinian eruption column parallel the change in composition of the magma from trachytic to trachydacitic although minimal the change in composition resulted in a marked change in colour of the juvenile clasts from white at the base first phase to pink and light dark grey banded and black second and third phases and to dark brown fourth phase density analysis of juvenile clasts revealed that the change in composition did not significantly affect the density of the juvenile pumice pyroclasts the vesicularity of which increases from 70 to 85 up through the sequence the pink and the black intra plinian ignimbrites were emplaced only within the narrow paleo valleys of the southern flank with maximum thicknesses of 9 m and 5 m respectively transitioning onto adjacent topographic highs where thin veneer surge layers were deposited the fall out deposits inter bedded between the ignimbrites lack basal reverse grading indicating that the eruption column continued to be sustained during the partial collapses generating the intra plinian ignimbrites the climactic dark brown ignimbrite was emplaced radially with a maximum thickness of 40 m the absence of oxidation surfaces or evidence of reworking within the fogo a deposits indicates that there were no time breaks during the different phases of the eruption the on land bulk volume of the three ignimbrites has been estimated around 3 2 km3 while the volume of the intra plinian fall out deposits is 0 003 km3 these bulk volume estimations summed with the basal fall out volume calculated by walker and croasdale 1970 makes a total bulk volume for the fogo a deposits of 4 4 km3 indicating a volcanic explosively index of 5 higher than previously estimated this result is fundamental for the forecasting a possible scenario for a future eruption

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

experimental constraints on pre eruptive conditions of phonolitic magma from the caldera forming el abrigo eruption tenerife canary islands abstract we have performed phase equilibrium experiments to determine the pre eruptive conditions of the largest caldera forming eruption 20 km 3 of dre that has occurred on tenerife canary islands the abrigo ignimbrite is phonolitic and was erupted during the last caldera forming episode ca 190 ka prior to the construction of the teide pico viejo active central complex the abrigo phonolite contains low amounts of phenocrysts ca 7 vol mainly sanidine biotite clinopyroxene and magnetite with minor amounts of titanite apatite and hauyne we have investigated pressures from 50 to 250 mpa temperatures of 700 900 c water contents in the melt from 1 5 to ca 10 wt and oxygen fugacity f o 2 from 1 log unit above the ni nio solid buffer to quartz magnetite fayalite comparison of the natural and experimental phase proportions and compositions indicates that the phonolite at the roof of the abrigo magma reservoir was at 130 50 mpa corresponding to ca 4 5 km below the surface 825 25 c with 3 1 wt dissolved h 2 o and f o 2 at the ni nio buffer 1 log unit this shows that the magma that produced the largest ignimbrite on tenerife was stored at relatively shallow depths but was water undersaturated and its eruption was probably triggered by input of fresh mafic magma comparison of the abrigo phase equilibrium results with those obtained in other phonolitic magmas e g eifel kerguelen somma vesuvius shows that the major element and probably fluid composition h 2 o cl and s may have a strong effect on the presence or absence of phases and their compositions extrapolating phase equilibria results obtained on a specific phonolite to constrain the pre eruptive conditions of other phonolites should be done with care

origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife aggregation of airborne particles is an important way in which the atmosphere is cleansed of fine dust particles such as following explosive eruptions and meteorite impacts we identify successive stages in the growth history of particle aggregates based upon well preserved ash aggregate bearing pyroclastic layers on tenerife the layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper widespread coignimbrite ash fall layer the upper part of each ignimbrite contains whole and fragmented concentric laminated accretionary lapilli whereas the overlying coignimbrite ash fall layer lacks accretionary lapilli and is composed of framework supported smaller and nonlaminated ash pellets sometimes slightly deformed or partly disaggregated the pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer these field relations are repeated numerous times in several different successions and they indicate that ash pellets not accretionary lapilli form within the coignimbrite ash plumes some pellets fell directly to the ground producing coignimbrite ash fall layers but others settled into pyroclastic density currents where they accreted successive concentric laminations of fine ash as they circulated through the variously turbulent levels of the stratified current and heat of the lower part of the current dried and partly lithified them into brittle accretionary lapilli the fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli numerous ignimbrite veneers on tenerife have the form of ash layers a few centimeters thick that drape topography and locally contain matrix supported accretionary lapilli most volcanoes lack laterally continuous field exposure and such accretionary lapilli bearing layers might be mistaken for ash fall deposits we highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits for example during hazard assessments

the late quaternary diego hernandez formation tenerife volcanology of a complex cycle of voluminous explosive phonolitic eruptions abstract the diego hernandez formation dhf 600 ca 180 ka represents the products of the most recent complete cycle of phonolitic explosive volcanism on tenerife canary islands spain we provide a revised and detailed stratigraphy new 40ar 39ar and u th he age determinations for major eruptive units a summary of new chemical data and an overview of the key characteristics of the cycle including volume estimates dispersal patterns eruption styles phreatomagmatic influences and caldera collapse episodes the complex stratigraphy of the dhf is divided into 20 named members each representing a major eruption as well as numerous unnamed members of limited present day exposure the major eruptions are represented by the fortaleza 370 ka roque 347 ka 3 km3 aldea 319 ka 3 km3 fasnia 309 ka 13 km3 poris 268 ka 3 5 km3 arafo 4 km3 caleta 223 ka 3 5 km3 and abrigo between 196 and 171 ka 20 km3 members the aldea fasnia and poris members consist of highly complex successions of plinian fall surge and flow deposits and several of the eruptions produced widespread and internally complex ignimbrite sheets phreatomagmatism occurred most frequently in the opening phase of the eruptions but also recurred repeatedly throughout many of the sequences inferred sources of water include a shallow caldera lake and groundwater and intermittent phreatomagmatic activity was an important influence on eruption style another important factor was conduit and vent instability which frequently loaded the eruption column with dense lithic debris and occasionally triggered column collapse and ignimbrite formation most of the major dhf eruptions were triggered by injection of mafic magma into existing phonolitic magma bodies two phonolitic magma types were available for eruption during the lifetime of the dhf but each was dominant at different times the results presented here support a caldera collapse rather than a landslide model for the origin of the las canadas caldera although the evolution of the caldera is evidently more complex and incremental than first thought

the interaction between regional and local tectonics during resurgent doming the case of the island of ischia italy abstract the volcanic island of ischia is located on the tyrrhenian margin of central italy characterized by plio quaternary nw se and ne sw trending extensional fractures ischia displays a resurgent dome uplifted by at least 800 m in the last 33 ka remote sensing and field data have been collected to study the structural setting of the island the deformation pattern associated with resurgence and the superimposition of the regional and the resurgence induced stress fields nw se and ne sw extensional fracture systems predominate throughout the island and around the resurgent block suggesting a relationship with the regional extensional structures these systems were formed before resurgence and were partly reactivated during resurgence the reactivation of pre existing regional systems during resurgence confined the extent of the uplifted area n s and e w trending systems have been found exclusively at the borders of the dome and are interpreted as being induced by resurgence the topmost resurgent block shows an octagonal shape in map view and is tilted at an angle of 15 around a ne sw trending horizontal axis the block is partly bordered by high angle inward dipping regional faults more than 90 in volume of the volcanic products coeval with resurgence on ischia have been erupted outside the resurgent block area suggesting that the resurgence process locally replaced volcanic activity in the last 33 ka

14c geochronological constraints for the volcanic history of the island of ischia italy over the last 5000 years abstract the chronology of the most significant eruptions at the island of ischia in the last 5000 years has been studied by means of accelerator mass spectrometry to obtain 14 c data the results are in good agreement with stratigraphical and archeological constraints this allows interpretation of the measured dates as ages of the eruptions which produced the deposits overlying or incorporating the sampled paleosols and charcoal fragments respectively these ages define the timing of volcanism in the last 5000 years suggesting that the recent volcanic activity at ischia is characterized by periods of very intense volcanism alternating to periods of quiescence this timing of the volcanism is correlated with the behaviour of the magmatic system and dynamics of resurgence of the mt epomeo block

variability of plinian fall deposits examples from okataina volcanic centre new zealand abstract plinian eruptions are intense sustained open vent volcanic outbursts that produce extensive beds of generally well sorted and generally highly vesicular pumice fallen from eruption plumes up to 55 km high their deposits tend to be almost homogeneous throughout their thickness although inverse grading commonly occurs here we detail departures from this near homogeneity that we observed among the mangaone subgroup a group of 12 plinian pumice fall deposits from okataina volcanic centre one of the dormant rhyolitic volcanoes in the taupo volcanic zone new zealand we propose that plinian eruptions may pass through seven states ranging from inception commencement of activity or renewed activity following a shutdown waxing steady discharge climax and waning leading to column collapse or shutdown decrease to zero intensity some deposits include intercalated water flushed ash rich beds having a bimodal grain size other examples of non homogeneity include phreatoplinian ashes that may alternate with the plinian pumice as though modulated by a delicate balance in water flux and still other eruptions may switch completely from dry to wet conditions when the column wanes towards the latest stages of the eruptions pink pumice occurs in some deposits and is totally absent in others we infer that it oxidized during temporary storage in or around the vent lithics are ubiquitous and concentrations of them may indicate vent wall collapses and explosive ejection of some of the resulting debris some lithic rich horizons neither coincide with apparent variations in the plinian discharge nor interrupt it as is the case with some rain flushed bimodal beds the events that produced these incongruent beds proceeded synchronously with but independent of the main plinian discharge in an isolated portion of the vent system such as a subsidiary vent other variations of density vesicularity and crystallinity among the eruption products record the pre and syn eruptive history of the magma a local inverse relationship between pumice density and crystallinity of one eruptive unit is inferred to reflect a control by crystals on vesiculation and fragmentation

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

resonance and impact of the 2009 samoa tsunami around tutuila american samoa 1 the 2009 samoa tsunami resulted in severe damage and inundation at tutuila american samoa the disparity of the impact and the varying accounts of the tsunami along the coast remain one of the most intriguing aspects of the event we utilize a dispersive wave model to reconstruct the tsunami from the earthquake source for understanding of the wave dynamics around tutuila after validation with water level measurements a fast fourier transform of the computed surface elevation reveals coupled resonance oscillations between 3 and 18 min period over the insular slope and shelf as well as the fringing reefs the resonance which focuses energy according to shelf and embayment configurations provides an explanation of the runup data and eyewitness accounts and identifies the coastal communities prone to tsunami hazards

lithic breccias in intermediate volume phonolitic ignimbrites tenerife canary islands constraints on pyroclastic flow depositional processes abstract lithic occurrences within quaternary phonolitic ignimbrites on tenerife canary islands fall into three broad groupings 1 massive ignimbrite with large floating lithics 2 basal lithic rich bedforms and 3 matrix to clast supported lithic breccias within ignimbrite massive ignimbrite divisions commonly contain large outsized either solitary or scattered lithics which are mostly monomictic and derived locally from substrate erosion by pyroclastic density currents low and high aspect ratio lithic rich bedforms occur at the base of several ignimbrites most bedforms are asymmetric with steeper lee sides and they are buried by stratified and or massive ignimbrite internally most bedforms are massive some however exhibit an upstream inclined stratification indicating upcurrent migration of the bedform which is interpreted to record antidune like flow regimes locally derived basalt clasts are important constituents of some of these bedforms which have also been sourced through substrate erosion matrix to clast supported pebble to boulder sized lithic breccias occur at various heights within massive ignimbrite lithic breccias at or near the top of most ignimbrites contain vent derived lithics and record a late flux of lithic material from the vent during a sustained pyroclastic flow event extensive boulder sized breccias at the top of two ignimbrites are interpreted to record the onset of caldera collapse late in the pyroclastic flow forming eruptions some breccias contain boulders that are oriented with long axes transverse to flow indicating late stage tractional transport thin bedded matrix supported lithic breccias or lithic trains and stratified ignimbrite are commonly developed adjacent to or immediately above topographic highs these facies laterally merge into massive ignimbrite infilling topographic lows massive ignimbrite is interpreted to be deposited from a high particle concentration hindered settling zone inverse grading and long axis parallel to bedding imbrication of lithics within the thin bedded breccia facies are interpreted to record deposition under modified grain flow conditions a variety of depositional processes are thus recorded and the relationship between lithic breccias ignimbrite facies and topography indicates that there were spatial variations in depositional mechanisms at any one instant the vertical sequence of bedform stratified ignimbrite massive ignimbrite characterises the lower part of several ignimbrites and is interpreted to record increasing rates of deposition from a sustained current multilayered lithic breccias at various heights within massive ignimbrite are a form of clast compositional zonation and reflect temporal changes in supply flow competence and capacity