morphometry of terrestrial shield volcanoes abstract shield volcanoes are described as low angle edifices built primarily by the accumulation of successive lava flows this generic view of shield volcano morphology is based on a limited number of monogenetic shields from iceland and mexico and a small set of large oceanic islands hawaii galapagos here the morphometry of 158 monogenetic and polygenetic shield volcanoes is analyzed quantitatively from 90 meter resolution srtm dems using the morvolc algorithm an additional set of 24 lava dominated shield like volcanoes considered so far as stratovolcanoes are documented for comparison results show that there is a large variation in shield size volumes from 0 1 to 1000 km3 profile shape height basal width h wb ratios mostly from 0 01 to 0 1 flank slope gradients average slopes mostly from 1 to 15 elongation and summit truncation although there is no clear cut morphometric difference between shield volcanoes and stratovolcanoes an approximate threshold can be drawn at 12 average slope and 0 10 h wb ratio principal component analysis of the obtained database enables to identify four key morphometric descriptors size steepness plan shape and truncation hierarchical cluster analysis of these descriptors results in 12 end member shield types with intermediate cases defining a continuum of morphologies the shield types can be linked in terms of growth stages and shape evolution related to 1 magma composition and rheology effusion rate and lava pyroclast ratio which will condition edifice steepness 2 spatial distribution of vents in turn related to the magmatic feeding system and the tectonic framework which will control edifice plan shape and 3 caldera formation which will condition edifice truncation

calderas a planetary perspective caldera bearing volcanoes are the largest volcanic edifices on a planet and are expressions of long lived near surface magma chambers on earth only three morphological classes of calderas are common shield calderas e g mauna loa hawaii stratocone calderas e g crater lake oregon and ash flow calderas e g valles caldera new mexico each of these classes has a characteristic caldera size magma type and tectonic location no calderas have been recognized on the moon or mercury but mars has large shield calderas as well as the unique highland patera calderas the latter may represent an explosive ultramafic style of volcanism unlike any volcanic activity in modern earth history radar images suggest that venus has at least one large caldera bearing shield volcano and possible calderas of other types jupiter s satellite io has abundant large shieldlike calderas but no shield massifs the number and variety of calderas on a planet may be proportional to planet mass consistent with a previous suggestion that volcanic complexity increases with planet size io is an exception but heating in io is generated tidally rather than by the decay of radioactive elements so that its calderas are not related to its size the occurrence of shield volcanoes on four solar system bodies suggests that thermal and tectonic mechanisms i e hot spots giving rise to construction of shield mountains and associated collapsed summits are basic processes in planetary geology hot spot volcanism is expected to be most important on planets lacking plate tectonics on earth large shields have lifetimes of only a few million years but on mars large shields may have been active for 1 5 109 years shield calderas on earth are smaller than 20 km in diameter but known or suspected shield calderas on mars venus and io have considerably larger diameters assuming that caldera diameters are related to magma chamber diameters and depths magma chambers on other planets are significantly larger and or deeper than their counterparts on earth terrestrial magma chambers are small because terrestrial shield volcanoes are small in comparison to shields on mars and venus and terrestrial shields are small because the earth s mobile plates move growing shields away from their deep sources thus distributing magma along a narrow chain rather than piling it up around a central vent finally the long lava flows of mars are also explained if the volume of erupted magma has any simple relation to magma chamber volume

the pattern of circumferential and radial eruptive fissures on the volcanoes of fernandina and isabela islands galapagos maps of the eruptive vents on the active shield volcanoes of fernandina and isabela islands galapagos made from aerial photographs display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks on some volcano flanks either circumferential or radial eruptions have been dominant in recent time the location of circumferential vents outside the calderas is independent of caldera related normal faults the eruptive fissures are the surface expression of dike emplacement and the dike orientations are interpreted to be controlled by the state of stress in the volcano very few subaerial volcanoes display a pattern of fissures similar to that of the galapagos volcanoes some seamounts and shield volcanoes on mars morphologically resemble the galapagos volcanoes but more specific evidence is needed to determine if they also share common structure and eruptive style

the caldera of volcan fernandina a remote sensing study of its structure and recent activity air photographs taken in 1946 1960 and 1982 together with spot hvr 1 images obtained in april and october of 1988 are used to characterize recent activity in and around the caldera of fernandina volcano west galapagos islands the eruptive and collapse events during this time span appear to be distributed in a nw se band across the summit and caldera on the flanks of the volcano subtle topographic ridges indicate that this is a long term preferred orientation of extra caldera activity as well although radial and arcuate fissures are found on all sectors the caldera is formed from the coalescence of multiple collapse features that are also distributed along a nw se direction and these give the caldera its elongate and scalloped outline the nw and se benches consist of lavas that ponded in once separated depressions that have been incorporated into the caldera by more recent collapse the volume of individual eruptions within the caldera over the observed 42 years appears to be small 4x106 m3 in comparison to the volumes of individual flows exposed in the caldera walls 120 150x106 m3 field observations in 1989 of lavas exposed in the caldera walls and their cross cutting relationships show that there have been at least three generations of calderas and that at times each was completely filled an interplay between a varying supply rate to the volcano and a regional stress regime is suggested to be the cause of long term spatial and volumetric variations in activity when supply is high the caldera is filled in relative to collapse and dikes tend to propagate in all directions through the edifice at other times such as the present supply is relatively low eruptions are small the caldera is far from being filled in and dike propagation is influenced by an extra volcano stress regime

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

linear volcanic features at alba patera mars probable spatter ridges a number of extensive linear constructional features occur on the north and east flanks of alba patera they are found in a 150 km wide zone and comprise aligned ridge elements that are locally associated with short volcanic flows they are among the youngest volcanic features in the region and are believed to be martian analogs of terrestrial spatter ridges on the basis of measurements made on viking images it appears that active volcanic fissures on mars were larger than their terrestrial counterparts

poleward flows in the southern california current system glider observations and numerical simulation 1 three years of continuous spray glider observations in the southern california current system ccs are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern ccs gliders provide upper ocean observations with good across shore and temporal resolution along two across shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state persistent poleward flows are observed in three areas within 100 km of the coast at point conception within the southern california bight scb and offshore of the scb and the santa rosa ridge srr poleward transport by the flows within the scb and offshore of the srr exceeds the poleward transport off point conception suggesting that the poleward flows are not continuous over the 225 km between observation lines the numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching point conception the poleward current offshore of the srr is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast this westward propagation is tied to westward propagating density anomalies originating in the scb during the spring summer upwelling season when wind stress curl is most strongly positive the across shore wave number frequency and phase speed of the westward propagation and the lack of across shore transport of salinity along isopycnals are consistent with first mode baroclinic rossby dynamics

volcanism on mars more than 15 years of planetary exploration of mars have given insight into the geologic processes that have shaped its surface the newly acquired viking data have shown that volcanism is one of the most important geologic processes operating on mars throughout its history in situ chemical analyses of martian soil by the viking lander spacecraft indicate mafic to ultramafic source rocks this is consistent both with available remote sensing data which indicate the presence of mafic minerals such as pyroxene and olivine and with petrologic modeling based on available geophysical data which suggest that martian lavas are probably iron rich and ultramafic these data strongly suggest that basaltic volcanism is widespread on mars and much of the photogeological data may be studied in this context photogeological analysis of the martian surface has shown two main types of volcanic morphologies the first type is central volcanoes which are volcanic landforms developed by continued and prolonged eruption from a point source vent this category includes 1 shields the classic low profile volcanic mountains of which olympus mons is the most spectacular example 2 domes steep sided constructs such as tharsis tholus that may represent lower rates of eruption than the shields or possibly more silicic lava compositions 3 highland patera radially textured low profile volcanoes that occur in the cratered terrain and are interpreted as ash shields 4 alba patera an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree and 5 various small features such as cinder cones the second major category is volcanic plains which are units recognized by several criteria of which the presence of mare ridges and flow lobes are the most useful volcanic plains are subdivided into four main groups 1 simple flows broad smooth to rolling plains that contain numerous mare type ridges but no flow lobes interpreted as being composed of thick single cooling units 2 complex flows displaying multiple overlapping flow lobes interpreted to be indicative of thin multiple cooling units 3 undifierentiated flows plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers and 4 questionable plains volcanic units heavily modified by other processes erosion tectonism etc so that their origins are uncertain when these categories of volcanic morphologies are combined with relative age data provided by crater statistics a volcanic history for mars can be derived as follows early heavy bombardment of mars was accompanied and followed by small scale fluvial channeling extensive flood volcanism the plateau plains and ash shield volcanism in the cratered terrain shortly after this time less extensive flood volcanism continued to resurface the planet during formation of the northern southern hemisphere dichotomy central volcanism became more prominent with the development of the alba patera center as well as the older shields and domes of the northern hemisphere early tharsis and elysium regions the development of the tharsis and elysium uplifts may have triggered the release of large scale catastrophic floods producing large channels continued uplift and lithospheric thinning concentrated volcanic activity in the tharsis region producing large shield volcanoes and extensive lava plains both central vent and plains volcanism have been active throughout martian history but the volumes of extrusion have gradually decreased with time this is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time gradually turning off the volcanism although many questions remain regarding martian volcanism the viking data have provided a remarkable detailed overview of the probable nature of the volcanic history of mars

subaerial submarine evidence of structures feeding magma to stromboli volcano italy and relations with edifice flank failure and creep abstract structural field data including parameters of 109 sheets strike mostly n10 55 e average dip angle mostly 70 90 average thickness two peaks at 1 2 m and 4 2 m and of 14 eruptive fissures and vents as well as bathymetric and numerical data suggest that stromboli experienced the development of a volcanic rift zone across the cone summit with a tendency to widen towards the lower submarine flanks during the last 100 ka inside this rift zone intrusive sheets mostly strike ne and become more abundant more steeply dipping and thicker on approaching the rift axis this rift persistence reflects the edifice s state of stress with a nw se horizontal least principal stress on the magma intrusion paths after 13 ka ago sheets mainly intruded with a horseshoe shaped geometry in plan view along the boundaries of the northwestern unstable volcano flank where four main lateral collapses occurred during the last 13 ka they also intruded with a ne strike across the volcano summit and the northeastern volcano flank the horseshoe shaped geometry is influenced by the lateral creep of the volcano flank which provides space for intrusion along the transtensional boundaries of the unstable sector where thinner sheets were emplaced when the collapse depression was filled up by new deposits ne striking thicker dykes emplaced across the northwestern volcano flank indicating a rapid re establishment of regional tectonic forces across the cone unbuttressing due to the present sciara del fuoco depression and ongoing flank creeping can account for intrusions also inside the sciara del fuoco the latter in 2007

high resolution studies of the summit of axial volcano sea beam bathymetry sea marc i side scan 30 khz towed camera and submersible data from axial volcano juan de fuca ridge jdfr have been integrated to produce detailed maps of the volcano s summit region in order to elucidate its volcanic and tectonic evolution the proximity of axial volcano the youngest edifice of the cobb eickelberg seamount chain to the jdfr superposes a hotspot derived radial stress pattern onto the linear stress field of a mid ocean ridge spreading center the morphology of the summit of axial volcano reflects the differing eruptive styles produced from this superposition long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south axial volcano has a distinct summit plateau similar to those found on many other submarine volcanos the process by which summit plateaus are constructed is illustrated by the lava flows originating from the south rift zone which have partially filled the southern half of the caldera and have smoothed the flanks the summit plateau contains penetrative ne sw fracturing that is approximately parallel to the jdfr the unusual rectangular caldera is 3 8 km in size and its trend is about 160 three possible mechanisms are discussed for the caldera s formation 1 canary island type trapdoor formation 2 overlapping spreading center interaction and 3 magma source migration although none of these models is fully compatible with the present data base the overlapping spreading center model is preferred postcaldera volcanism probably began with the formation of the central caldera eruptive complex which was characterized by relatively low volume high viscosity flows the most recent volcanism has apparently taken place along the northern and southern rift zones which have erupted more fluid lavas resulting in areally extensive flows covering up to 60 of the caldera floor extensive low temperature venting and areas of high temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of axial volcano

extension and uplift at alba patera mars insights from mola observations and loading models high resolution mars orbiter laser altimeter mola profiles of the large shield volcano alba patera call for a critical reexamination of models for the growth and evolution of this volcanic edifice an extensive system of graben cuts across alba patera extending from ceraunius fossae to the south to tantalus fossae to the northeast in the vicinity of the edifice the otherwise generally north south orientations of these graben are deflected to circumferential directions mola topography reveals that the circumferential graben are located well up the flanks of alba patera both the type extensional and location midflank of faulting on alba patera are inconsistent with the state of stress predicted for purely surface loaded flexure models to constrain the conditions governing the evolution of alba patera we employ finite element models for the volcano and lithospheric stress field under a combination of loading mechanisms buoyant sublithospheric loads representing underplated magma low density mantle residuum or dynamic support from mantle convection and intralithospheric displacements representing sills trapped by horizontal compressive stresses in the upper lithosphere can generate the observed midflank slope breaks and circumferential extensional fault zones at radial distances comparable to that of the subsurface load s characteristic radial extent both mechanisms moderate stresses in the upper lithosphere in favor of continued magma ascent however bottom loading requires a disk like load geometry and an elastic lithosphere thickness te sufficiently small to allow the load s shape to be apparent in the surface deformation te 32 km such low te is inconsistent with mars global surveyor mgs gravity and topography data in contrast intralithospheric sill complexes can produce the observed topography and faulting at te values consistent with those inferred from gravity and topography 50 te 100 km for these conditions principal stress orientations on the lower flanks are also consistent with the orientations of lower flank graben provided that faulting was induced by dike intrusion or tensile failure rather than shear failure

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

the may 2005 eruption of fernandina volcano galapagos the first circumferential dike intrusion observed by gps and insar the may 2005 eruption of fernandina volcano galapagos occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for several weeks this was the first circumferential dike intrusion ever observed by both insar and gps measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on galapagos volcanoes but are rare elsewhere pre and post eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera a shallow sill at 1 km depth and a deeper point source at 5 km depth and we infer that this system also existed at the time of the 2005 eruption the co eruption deformation is dominated by uplift near the 2005 eruptive fissures superimposed on a broad subsidence centered on the caldera modeling of the co eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion we found that a single planar dike could not match both the insar and gps data our best fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping 45 60 toward the caldera at the surface and more gently dipping 12 14 at depth where it connects to the horizontal sub caldera sill the shallow sill is underlain by the deep point source the geometry of this modeled magmatic system is consistent with the petrology of fernandina lavas which suggest that circumferential eruptions tap the shallowest parts of the system whereas radial eruptions are fed from deeper levels the recent history of eruptions at fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress feedback relationship and alternate in time with one another

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

geology and petrology of floreana island galapagos archipelago ecuador abstract floreana is the sixth largest of the galapagos islands and lies at the extreme southern margin of the galapagos platform in sharp contrast to the other major islands floreana lacks a well developed volcanic center but instead is dominated by pyroclastic vents and derivative ash and cinder deposits the majority of exposed lavas are xenolith bearing alkali olivine basalts that constitute a coherent suite referred to as the main series the flank series comprises a group of isolated vents on the southern flank of the island which erupted xenolith poor basaltic lavas that are generally poorer in incompatible trace elements and richer in plagioclase paleomagnetic measurements supported by k ar age determinations record a relatively limited timespan for subaerial volcanism shield development reached maximum dimensions by 1 0 ma with the final phases of pyroclastic activity at an end by 0 3 ma main series rocks are phenocryst poor and typically contain clusters of xenocrysts in an extremely fine grained granular matrix petrogenetic modelling indicates they represent products of partial melting of a lree enriched spinel lherzolite source little modified by subsequent fractional crystallization porphyritic flank series lavas experienced rather different ascent histories and may have resided in shallow level magma chambers prior to eruption isotopic geochemical and petrographic data demonstrate that flank series lavas are not cogenetic in any simple way with main series flows floreana lavas contain a diverse suite of ultramafic xenoliths al augite group megacrysts are likely to be cognate and comagmatic with host alkali olivine basalts transition metal contents of megacrysts and host lavas however preclude clinopyroxene fractionation as a mechanism of magmatic differentiation it is proposed that megacrysts were products of passive crystallization and were never effectively separated from host magmas cr diopside group nodules may have crystallized at very high temperatures from advanced partial melts and are less likely to bear any direct genetic connection to host basalts

regional fracture patterns around volcanoes possible evidence for volcanic spreading on venus abstract magellan data show that the surface of venus is dominated by volcanic landforms including large flow fields and a wide range of volcanic edifices that occur in different magmatic and tectonic environments this study presents the results from a comprehensive survey of volcano rift interaction in the bat region and its surroundings we carried out structural mapping of examples where interaction between volcanoes and regional fractures results in a deflection of the fractures around the volcanic features and discuss the nature of the local volcano related stress fields that might be responsible for the observed variations of the regional fracture systems we propose that the deflection of the regional fractures around these venusian volcanoes might be related to volcanic spreading a process recognized as of great importance in the tectonic evolution of volcanoes on earth and mars but not previously described on venus

tectonics of nisyros island greece by field and offshore data and analogue modelling abstract we use new on land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at nisyros island greece which has an active caldera that shows a complicated network of faults fractures and volcano tectonic structures we measured 157 faults that show dominant dip slip normal motions along planes mainly striking ne sw nne ssw and nnw sse inside the caldera dykes necks and morphometric parameters of volcanic domes explosion craters and fumarole pits indicate the control by ne striking discontinuities on magma and gas paths the nnw and ne striking faults bound a major block that underwent repeated downthrow and uplift movements during the late pleistocene holocene experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass shaped fault pattern as seen in plan view with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank all these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses can account for the overall deformation field

growth and persistence of hawaiian volcanic rift zones hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms the intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike trapping stress field within the prism possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis faulting and gravitational sagging of the topography in extreme models with very steep slopes and high poisson s ratio corresponding to the gelatin models of rift zones by fiske and jackson 1972 results of finite element calculations indicate that gravity induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by fiske and jackson however the mechanism does not work for gently sloping flanks or a more acceptable poisson s ratio of about 0 25 additionally trapping stresses in the gravity loading and density stratification models will not persist after a few dike injection episodes therefore in mature hawaiian rift zones with possibly thousands of dikes additional processes must act to control the stresses that permit continued dike intrusion and rift persistence it is proposed that accommodation to dike emplacement occurs by slip on deep faults possibly of the type proposed for the 1975 kalapana hawaii earthquake as suggested by others for this earthquake the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis other possible faulting mechanisms such as shallow gravity slides and normal faulting of the flanks do not appear to favor rift zone persistence in this model the horizontal stress generated by a standing column of magma at the time of dike emplacement the stresses in the ridges and the fault strength are coupled this results in a feedback between the maximum height that magma can rise along the rift fault friction and fault width through this feedback the slope of the volcano flank is controlled by the fault friction when applied to kilauea volcano the model yields an estimate for the coefficient of fault friction as high as 0 39 assuming normal hydrostatic pore fluid pressure an implication of this model supported by other studies is that rift intrusion and lateral spreading could be major contributors to volcano growth

a new model for the growth of basaltic shields based on deformation of fernandina volcano galapagos islands article history space geodetic measurements of surface deformation produced by the most recent eruptions at fernandina the most frequently erupting volcano in the galapagos archipelago reveal that all have initiated with the intrusion of subhorizontal sills from a shallow magma reservoir this includes eruptions from fissures that are oriented both radially and circumferentially with respect to the summit caldera a synthetic aperture radar sar image acquired 1 2 h before the start of a radial fissure eruption in 2009 captures one of these sills in the midst of its propagation toward the surface galapagos eruptive fissures of all orientations have previously been presumed to be fed by vertical dikes and this assumption has guided models of the origin of the eruptive fissure geometry and overall development of the volcanoes our findings allow us to reinterpret the internal structure and evolution of galapagos volcanoes and of similar basaltic shields furthermore we note that stress changes generated by the emplacement of subhorizontal sills feeding one type of eruption may control the geometry of subsequent eruptive fissures specifically circumferential fissures tend to open within areas uplifted by sill intrusions that initiated previous radial fissure eruptions this mechanism provides a possible explanation for the pattern of eruptive fissures that characterizes all the western galapagos volcanoes as well as the alternation between radial and circumferential fissure eruptions at fernandina the same model suggests that the next eruption of fernandina will be from a circumferential fissure in the area uplifted by the 2009 sill intrusion just southwest of the caldera rim 2013 elsevier b v all rights reserved

a model for radial dike emplacement in composite cones based on observations from summer coon volcano colorado usa we mapped the geometry of 13 silicic dikes at summer coon an eroded oligocene stratovolcano in southern colorado to investigate various characteristics of radial dike emplacement in composite volcanoes exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries surprisingly most dikes at summer coon increase in thickness with distance from the center of the volcano magma pressure in a dike is expected to lessen away from the pressurized source region which would encourage a blade like dike to decrease in thickness with distance from the center of the volcano we attribute the observed thickness pattern as evidence of a driving pressure gradient which is caused by decreasing host rock shear modulus and horizontal stress both due to decreasing emplacement depths beneath the sloping flanks of the volcano based on data from summer coon we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface near the interface between volcanic cone and basement which may represent a neutral buoyancy surface or stress barrier magma is transported subhorizontally and radially away from the center of the volcano in blade like dikes the dikes thicken with increasing radial distance and offset segments and fingers form along the upper peripheries of the intrusions eruptions may occur anywhere along the length of the dikes but the erupted volume will generally be greater for dike fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source observed eruptive volumes vent locations and vent area intrusions from inferred post glacial dike fed eruptions at mount adams washington usa support the proposed model hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano

geologic evidence for a magma chamber beneath newberry volcano oregon at newberry volcano central oregon more than 0 5 m y of magmatic activity including caldera collapse and renewed caldera filling volcanism has created a structural and thermal chimney that channels magma ascent holocene rhyolitic eruptions 1 have been confined mainly within the caldera in an area 5 km in diameter 2 have been very similar in chemical composition phenocryst mineralogy and eruptive style and 3 have occurred as recently as 1300 years ago with repose periods of 2000 3000 years between eruptions holocene basaltic andesite eruptions are widespread on the flanks but are excluded from the area of rhyolitic volcanism basaltic andesite in fissures at the edge of the rhyolite area has silicic inclusions and shows mixed basalt rhyolite magma relations these geologic relations and the high geothermal gradient that characterizes the lower part of a drill hole in the caldera u s geological survey newberry 2 indicate that a rhyolitic magma chamber has existed beneath the caldera throughout the holocene its longevity probably is a result of intermittent underplating by basaltic magma

a shallow dipping dike fed the 1995 flank eruption at fernandina volcano galapagos observed by satellite radar interferometry satellite radar interferometry data reveal strong localized uplift in a semi circular pattern on the southwest flank of fernandina volcano galapagos where an eruption took place in january to april 1995 the observations show a maximum decrease in radar range of 0 75 m and they are consistent with a model of a shallow dipping dike intrusion feeding this fissure eruption we solve for the best fit single rectangular dislocation dike source using non linear inver sion techniques where simulated annealing searching algo rithm is used to avoid local minima the best fit dike is 3 8 km long 2 3 km high 0 86 m thick and with a 34 dip the total dike volume 7 5 x 10 3 km 3 is 40 of the vol ume of extrusive materials estimated to have been produced during the eruption the data do not permit a sub vertical dike implying that the least principal stress direction is not horizontal as is usually assumed

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

laccoliths and small volcanoes on the flank of the east pacific rise clusters of low circular domes and discs each a few hundred metres high and a few kilometres across have been mapped but not sampled during a seabeam traverse of old 9 18 m y oceanic crust at at 19 20 s in the south pacific where they replace the typical lineated fault block terrain for several hundred square kilometres the landforms are probably produced by laccolith intrusion beneath a sediment cover in an off axis volcanic province their overall shapes are distinctly different from the small volcanic shields that also dot the rise flank and from small satellite cones that are abundant on archipelagic aprons though some of the domes do bear small cratered shields and cones on their peripheries and even shallow summit depressions

imaging rapidly deforming ocean island volcanoes in the western galapagos archipelago ecuador using local body wave arrival time tomography methods to determine 3 d seismic velocity structure we imaged the plumbing system of sierra negra volcano galapagos this hot spot volcanic chain includes some of the fastest deforming volcanoes in the world making this an ideal location to study shield volcano plumbing systems we inverted p and s wave arrivals recorded on a 15 station temporary array between july 2009 and june 2011 using an a priori 1 d velocity model constrained by offshore refraction studies with local seismicity from nearby volcanoes as well as the ring fault system the model resolution is good between depths of 3 and 15 5 km the propagation of s waves throughout this volume argues against any large high melt accumulations although a shallow melt sill may exist above 5 km we image a broad low velocity region 25 km laterally below sierra negra at depths 8 15 km no large regional velocity increase is found within the limits of good resolution suggesting that crust is thicker than 15 km beneath the western galapagos archipelago our results are consistent with crustal accretion of mafic cumulates from a large volume magma chamber that may span the boundary between preplume and accreted crust the similarity between our results and those of hawaii leave open the possibility that the crust has also been thickened by under plating

the geology of hotei regio titan correlation of cassini vims and radar abstract joint cassini vims and radar sar data of 700 km wide hotei regio reveal a rich collection of geological features that correlate between the two sets of images the degree of correlation is greater than anywhere else seen on titan central to hotei regio is a basin filled with cryovolcanic flows that are anomalously bright in vims data in particular at 5 m and quite variable in roughness in sar the edges of the flows are dark in sar data and appear to overrun a vims bright substrate sar stereo topography shows the flows to be viscous 100 200 m thick on its southern edge the basin is ringed by higher 1 km mountainous terrain the mountains show mixed texture in sar data some regions are extremely rough exhibit low and spectrally neutral albedo in vims data and may be partly coated with darker hydrocarbons around the southern margin of hotei regio the sar image shows several large dendritic radar bright channels that flow down from the mountainous terrain and terminate in dark blue patches seen in vims images whose infrared color is consistent with enrichment in water ice the patches are in depressions that we interpret to be filled with fluvial deposits eroded and transported by liquid methane in the channels in the vims images the dark blue patches are encased in a latticework of lighter bands that we suggest to demark a set of circumferential and radial fault systems bounding structural depressions conceivably the circular features are tectonic structures that are remnant from an ancient impact structure we suggest that impact generated structures may have simply served as zones of weakness no direct causal connection such as impact induced volcanism is implied we also speculate that two large dark features lying on the northern margin of hotei regio could be calderas in summary the preservation of such a broad suite of vims infrared color variations and the detailed correlation with features in the sar image and sar topography evidence a complex set of geological processes pluvial fluvial tectonic cryovolcanic impact that have likely remained active up to very recent geological time

space geodetic evidence for multiple magma reservoirs and subvolcanic lateral intrusions at fernandina volcano galapagos islands 1 using interferometric synthetic aperture radar insar measurements of the surface deformation at fernandina volcano galapagos ecuador acquired between january 2003 and september 2010 we study the structure and the dynamics of the shallow magmatic system of the volcano through the analysis of spatial and temporal variations of the measured line of sight displacement we identify multiple sources of deformation beneath the summit and the southern flank at least two sources are considered to represent permanent zones of magma storage given their persistent or recurrent activity elastic deformation models indicate the presence of a flat topped magma reservoir at 1 1 km below sea level and an oblate spheroid cavity at 4 9 km b s l the two reservoirs are hydraulically connected this inferred structure of the shallow storage system is in agreement with previous geodetic studies and previous petrological analysis of both subaerial and submarine lavas the almost eight year long observation interval provides for the first time geodetic evidence for two subvolcanic lateral intrusions from the central storage system in december 2006 and august 2007 subvolcanic lateral intrusions could provide the explanation for enigmatic volcanic events at fernandina such as the rapid uplift at punta espinoza in 1927 and the 1968 caldera collapse without significant eruption

dike indicators in the hadriaca patera promethei terra region mars abstract the eastern hellas basin rim has been highly modified by a multitude of both volcanic and fluvial processes it is covered by a large part of the circum hellas volcanic province consisting of the volcanic plains of hesperia planum and western promethei terra as well as the two central volcanoes hadriaca and tyrrhena patera and their associated lava flows large outflow channels of dao niger harmakhis reull and teviot valles dissect the wrinkle ridged plains postdating the major volcanic events the origin of these outflow channels is still ambiguous as the mobilization mechanism for the underground volatile reservoirs has yet to be revealed due to the proximity of the volcanic structures many previous studies have suggested via models that the released ice or ground water was mobilized due to endogenic heating from magmatic activity in the region to find evidence of this we conducted a systematic search for any magmatic intrusions at 50 m pixel resolution this paper provides detailed mapping of several areas where we found features which we interpret to be dikes and shows the overall distribution of such structures in the eastern hellas rim region the hadriaca patera volcano flanks exhibit a concentric pattern of linear ridges and fractures indicative of a vast ring dike system the dikes occupy mostly the area near the summit and the hellas facing flank extending there for over 400 km away from the caldera a hadriaca radial dike swarm pattern is observed extending southwards from the volcano composed of isolated parallel linear ridge group outcrops and cut by the valles canyons an additional isolated dike ridge group exists south of lower reull valles possibly controlled by the tyrrhena patera volcano some 1300 km away all dikes within the study region are roughly late hesperian in age either predating the valles formation or being roughly concurrent with them in total 500 dike segments were identified with a combined length of 2500 km since 90 of the dikes are ridges they suggest that the entire region has undergone large scale erosion of at least a few tens of meters to expose the dikes the dike orientations were most probably controlled by the volcanic centers and possibly partly by a hellas concentric fracture pattern

linesmanship and the practice of linear geo art the theory of linesmanship and the practice of linear geo art are explained as critiques of methods observed in commonly published classic to recent papers devoted to fracture analysis and related forms of self delusion

submarine central volcanoes on the nazca plate high resolution sonar observations abstract this paper describes observations of 214 submarine central volcanoes made with long range side scan sonar and narrow beam echosounder between peru and the east pacific rise they are all circular in plan with basal diameters ranging from less than 1 to over 10 km profiles of four volcanoes were obtained with the narrow beam echosounder and those of a further nine were inferred from their acoustic shadows all the profiles show steep mostly concave sides a distinct summit region could be identified on about two thirds of the volcanoes in just under half of these the volcano culminates in a simple crater but the rest have broad slightly convex summit plateaus with or without craters almost half of all the volcanoes have one or more craters summit plateaus are relatively more common on the larger volcanoes so that for central volcanoes with a basal diameter greater than 4 km the commonest morphology is a steep sided truncated cone with a slightly domed summit plateau in most cases this form must represent the mode of formation not the result of subsequent wave erosion it is suggested that this form is the result of relatively fluid lavas travelling outward on low slopes from a central vent but terminating in steeply dipping hyaloclastites and volcanic breccias the spatial distribution of volcanoes is very non random in the area studied the east pacific rise out to 6 5 ma has less than one volcano per 10 4 km 2 whereas a field hundreds of kilometers across with a peak density of over 60 per 10 4 km 2 was seen north of mendana fracture zone on 6 5 26 ma old crust

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

structural controls on the morphology of galapagos shields abstract although galapagos shield volcanoes exhibit a variety of morphologies their steep slopes and deep wide calderas distinguish them from shields of other provinces we suggest that the differences in these morphologies may reflect differences in the geometry of subjacent magma chambers and loading conditions on the margins of those chambers we present the results of calculations using an elastic solution for stresses and displacements along a free surface interacting with a magma chamber at depth for relatively shallow magma chambers 2 to 4 km deep calculations that employ a ratio of approximately 10 1 for elastic shear modulus to magma pressure produce steep surface slopes comparable to those found on volcan wolf and darwin broad gentle slopes similar to those found on sierra negra and alcedo are developed for elastic shear modulus to magma pressure ratios of 20 1 to 50 1 and for magma chamber depths greater than 3 km

1998 eruption at volcan cerro azul galapagos islands i syn eruptive petrogenesis the 1998 eruption of volcn cerro azul in the galpagos islands produced two intra caldera vents and a flank vent that erupted more than 1 0 10 8 m 3 of lava lava compositions changed notably during the 5 week eruption and contemporaneous eruptions in the caldera and on the flank produced different compositions lavas erupted from the flank vent range from 6 3 to 14 1 mgo nearly the entire range of mgo contents previously reported from the volcano on site monitoring of eruptive activity is linked with petrogenetic processes such that geochemical variations are evaluated in a temporal con text lavas from the 1998 eruption record two petroge netic stages characterized by progressively more mafic lavas as the eruption proceeded crystal compositions whole rock major and trace element compositions and isotope ratios indicate that early lavas are the product of mixing between 1998 magma and remnant magma of the 1979 eruption intra caldera lavas and later lavas have no 1979 signature but were produced by the 1998 mag ma incorporating olivine and clinopyroxene xenocrysts thus early magma petrogenesis is characterized by mix ing with the 1979 magma followed by the magma pro gressively entraining wehrlite cumulate mush

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

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

on trapdoor faulting at sierra negra volcano galapagos abstract rapid uplift observed during 1992 1997 at sierra negra volcano western galapagos was followed by deformation that we have previously interpreted as trapdoor faulting on an intra caldera fracture system at that time no direct evidence for the trapdoor faulting was available to support this interpretation here we report field evidence of recent faulting at sierra negra and provide direct measurements of the faulting using pixel offsets between satellite radar amplitude backscatter images our field observations carried out in 2001 indicate that recent faulting occurred along the southern part of the intra caldera fault system with maximum offset of at least 0 5 1 m differences between two satellite radar amplitude images show 1 4 0 2 m range change across this fault system this suggests that 1 5 0 2 m of faulting occurred assuming vertical ground displacements consistent with our previous modeling results results of numerical modeling indicate that growing sills can cause faulting of the intrusion roof on outward dipping or nearly vertical faults this occurs at high excess magma pressure when the lateral extent half length of the sill becomes greater than its depth the derived 2 km deep sierra negra sill exhibits trapdoor faulting 3 km south of the center of the sill therefore these modeling results provide a plausible mechanical explanation for the trapdoor fault at sierra negra

a giant circumferential dyke swarm associated with the high arctic large igneous province halip abstract in this study we identify a giant circumferential mafic dyke swarm associated with the 135 75 ma high arctic large igneous province halip previously a halip giant radiating mafic dyke swarm with portions scattered across the canadian high arctic islands northern greenland svalbard and franz josef land was recognized in a pre drift plate tectonic reconstruction of the arctic region the radiating swarm has been interpreted to focus above a mantle plume responsible for halip magmatism the newly recognized halip giant circumferential swarm has a centre that is near the focus of the halip radiating system and hence is likely related to the halip plume elements of the circumferential swarm are located in each of the four regions where the radiating system is found the circumferential swarm has a quasi circular or slightly elliptical geometry an outer diameter of 1600 km and an arc of 220 it is one of the largest giant circumferential dyke swarms recognized on earth and could be linked to the outer edge of the flattening plume head it is also the first such swarm to have been identified by means of a plate tectonic reconstruction although giant circumferential dyke swarms appear to be relatively rare on earth possible analogues are common on venus and are also found on mars on venus giant circular or elliptical tectono magmatic features termed coronae are characterized by an annulus of graben or fissures and prominent topography some coronae include a radiating graben fissure system both radiating and circumferential graben may be underlain by dykes if so coronae could be analogues for terrestrial giant circumferential dyke swarms such as observed in the case of the halip

the geology and geochemistry of isla marchena galapagos archipelago an ocean island adjacent to a mid ocean ridge abstract isla marchena is the subaerial exposure of a shield volcano located between the galapagos platform and the galapagos spreading center the geologic evolution of the island can be divided into two lava series separated by formation of a cadera and a period of explosive activity both lava series contain evolved alkali olivine and tholeiitic basalts average mg 55 2 and are virtually aphyric with the exception of a plagioclase rich horizon in the pre caldera section although the major element compositions of the two series are similar the abundances of incompatible elements and ratios of highly moderately incompatible elements increase systematically with decreasing stratigraphic age these compositional differences together with textural and mineral chemical evidence for disequilibrium are not easily explained by any single petrologic mechanism the combined effects of a periodically replenished magma chamber assimilation fractionation and in situ crystallization of a solidification zone are possible mechanisms that may be responsible for the temporal changes in lava composition morb like abundances of trace elements in addition to unusually depleted 87 sr 86 sr and 143 nd 144 nd ratios for ocean island basalts suggest that magmas associated with the galapagos plume interacted with those of the galapagos spreading center

results from new gps and gravity monitoring networks at fernandina and sierra negra volcanoes galapagos 2000 2002 gps and gravity networks were established on sierra negra and fernandina volcanoes in 2000 and remeasured in 2001 and 2002 after a decade in which the caldera inflated by over 2 5 m sierra negra s caldera began to subside between 2000 and 2002 at a rate of about 9 cm a neither inflation nor deflation was accompanied by eruption simple elastic modeling indicates that the deformation at sierra negra can be attributed to volumetric contraction of a 2 1 km deep sill beneath the caldera caused by either movement of magma out of the sill or loss of bubbles through degassing precise gravity measurements support but do not prove the former mechanism fernandina volcano is in a phase of slow refilling of the summit reservoir after the 1995 eruption causing lateral expansion of the caldera rim by about 3 cm a our estimate for the depth of the shallowest part of the fernandina magma chamber is 1 to 2 km neither volcano shows evidence of flank spreading

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

the volcanic history of volcan alcedo galapagos archipelago a case study of rhyolitic oceanic volcanism volcan alcedo is one of the seven western galapagos shields and is the only active galapagos volcano known to have erupted rhyolite as well as basalt the volcano stands 4 km above the sea floor and has a subaerial volume of 200 km3 nearly all of which is basalt as volcan alcedo grew it built an elongate domal shield which was partly truncated during repeated caldera collapse and partial filling episodes an outward dipping sequence of basalt flows at least 250 m thick forms the steepest to 33 flanks of the volcano and is not tilted thus a constructional origin for the steep upper flanks is favored about 1 km3 of rhyolite erupted late in the volcano s history from at least three vents and in 2 5 episodes the most explosive of these produced a tephra blanket that covers the eastern half of the volcano homogeneous rhyolitic pumice is overlain by dacite rhyolite commingled pumice with no stratigraphic break the tephra is notable for its low density and coarse grain size the calculated height of the eruption plume is 23 30 km and the intensity is estimated to have been 1 2x108 kg s rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase the age of the rhyolitic eruptions is 120 ka on the basis of k ar ages between ten and 20 basaltic lava flows are younger than the rhyolites recent faulting resulted in a moat around part of the caldera floor alcedo most resently erupted sometime between 1946 and 1960 from its southern flank alcedo maintains an active transient hydrothermal system acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional scale earthquake