Relationships between volcanic patterns and neotectonics in Eastern Anatolia from analysis of satellite images and DEM

The late Neogene to Quaternary volcanism in Eastern Anatolia is related to the Arabia–Eurasia convergence but a clear deformation pattern has not yet been established in this region. We have used the distribution and shape of volcanoes and fault geometry as indicators of the tectonic regime. Volcanic edifices and related faults were analyzed in vertical view using SAR–ERS, Spot images and a Digital Elevation Model (DEM). In several places, adjacent volcanoes that form linear clusters or elongated volcanoes are clearly rooted on vertical tension fractures. These are compatible with horizontal σ₃ striking 90°N, associated with σ₁ horizontal (strike-slip regime) or vertical (extensional regime). We mapped the recent faults that are directly associated to volcanoes. Volcanic vents are related to tail-crack, horsetail or releasing bend structures. In this work, it has been possible to define the ESE-striking, 270-km-long Tutak–Hamur–Çaldiran fault that forms a releasing bend testifying to right-lateral motion. Extension is well documented for few places but no recent fold has been observed. Since 8 Ma, the tectonic system is principally strike-slip. Most of the tension fractures being 2 to 10 km in length, so we infer that they affect only part of the crust. Most strike-slip fault zones are of several tens to a few hundred kilometers long and thus not of lithospheric scale. Therefore, the channels used by the magma to reach the surface are crustal structures.     

Volcanic facies architecture of an intra-arc strike-slip basin, Santa Rita Mountains, Southern Arizona

The three-dimensional arrangement of volcanic deposits in strike-slip basins is not only the product of volcanic processes, but also of tectonic processes. We use a strike-slip basin within the Jurassic arc of southern Arizona (Santa Rita Glance Conglomerate) to construct a facies model for a strike-slip basin dominated by volcanism. This model is applicable to releasing-bend strike-slip basins, bounded on one side by a curved and dipping strike-slip fault, and on the other by curved normal faults. Numerous, very deep unconformities are formed during localized uplift in the basin as it passes through smaller restraining bends along the strike-slip fault. In our facies model, the basin fill thins and volcanism decreases markedly away from the master strike-slip fault (“deep” end), where subsidence is greatest, toward the basin-bounding normal faults (“shallow” end). Talus cone-alluvial fan deposits are largely restricted to the master fault-proximal (deep) end of the basin. Volcanic centers are sited along the master fault and along splays of it within the master fault-proximal (deep) end of the basin. To a lesser degree, volcanic centers also form along the curved faults that form structural highs between sub-basins and those that bound the distal ends of the basin. Abundant volcanism along the master fault and its splays kept the deep (master fault-proximal) end of the basin overfilled, so that it could not provide accommodation for reworked tuffs and extrabasinally-sourced ignimbrites that dominate the shallow (underfilled) end of the basin. This pattern of basin fill contrasts markedly with that of nonvolcanic strike-slip basins on transform margins, where clastic sedimentation commonly cannot keep pace with subsidence in the master fault-proximal end. Volcanic and subvolcanic rocks in the strike-slip basin largely record polygenetic (explosive and effusive) small-volume eruptions from many vents in the complexly faulted basin, referred to here as multi-vent complexes. Multi-vent complexes like these reflect proximity to a continuously active fault zone, where numerous strands of the fault frequently plumb small batches of magma to the surface. Releasing-bend extension promotes small, multivent styles of volcanism in preference to caldera collapse, which is more likely to form at releasing step-overs along a strike-slip fault. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The 10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand: Part 2. Petrological insights into magma storage and transport during regional extension

The six eruption episodes of the 10 ka Pahoka–Mangamate (PM) sequence (see companion paper) occurred over a ?200–400-year period from a 15-km-long zone of multiple vents within the Tongariro Volcanic Centre (TgVC), located at the southern end of the Taupo Volcanic Zone (TVZ). Most TgVC eruptives are plagioclase-dominant pyroxene andesites and dacites, with strongly porphyritic textures indicating their derivation from magmas that ascended slowly and stagnated at shallow depths. In contrast, the PM pyroclastic eruptives show petrographic features (presence of phenocrystic and groundmass hornblende, and the coexistence of olivine and augite without plagioclase during crystallisation of phenocrysts and microphenocrysts) which suggest that their crystallisation occurred at depth. Depths exceeding 8 km are indicated for the dacitic magmas, and >20 km for the andesitic and basaltic andesitic magmas. Other petrographic features (aphyric nature, lack of reaction rims around hornblende, and the common occurrence of skeletal microphenocrystic to groundmass olivine in the andesites and basaltic andesites) suggest the PM magmas ascended rapidly immediately prior to their eruption, without any significant stagnation at shallow depths in the crust. The PM eruptives show three distinct linear trends in many oxide–oxide diagrams, suggesting geochemical division of the six episodes into three chronologically-sequential groups, early, middle and late. Disequilibrium features on a variety of scales (banded pumice, heterogeneous glassy matrix and presence of reversely zoned phenocrysts) suggest that each group contains the mixing products of two end-member magmas. Both of these end-member magmas are clearly different in each of the three groups, showing that the PM magma system was completely renewed at least three times during the eruption sequence. Minor compositional diversity within the eruptives of each group also allows the PM magmas to be distinguished in terms of their source vents. Because petrography suggests that the PM magmas did not stagnate at shallow levels during their ascent, the minor diversity in magmas from different vents indicates that magmas ascended from depth through separate conduits/dikes to erupt at different vents either simultaneously or sequentially. These unique modes of magma transport and eruption support the inferred simultaneous or sequential tapping of small separate magma bodies by regional rifting in the southern Taupo Volcanic Zone during the PM eruption sequence (see companion paper).     

The petrology and geochemistry of volcanic rocks from the Northern Harşit river area, Pontid Volcanic Province, Northeast Turkey

Two volcanic cycles can be distinguished, in the Pontid magmatic arc. They comprise an Upper Cretaceous, Lower Volcanic Cycle of which only the waning stage contains abundant pyroclastic rocks. The latter show spatial association to the fault pattern and are closely related to mineralisation. The overlying, early Tertiary, Upper Volcanic Cycle shows evidence of explosive vulcanicity in the Upper Basic Series. Dacites and rhyodacites are only locally developed and again show spatial association with the faulting.Comparison of the major and minor element chemistries of the two cycles demonstrates the clear separation into a lower tholeiitic and an upper calc-alkaline cycle. The rocks show similar chemistry to volcanic suites from island arcs in other areas.The origin of the tholeiitic magma is ascribed to melting of “dry” amphibolite during early subduction of Tethyan ocean floor beneath “Pontian Land”. This resulted in low K abundances and K/Rb ratios, and some Fe enrichment in the tholeiitic basalts.The calc-alkaline magma is thought to be derived from a later stage in the subduction process when melting of amphibole was joined by melting of biotite or phlogopite. The Upper Volcanic Cycle is thus enriched in K and shows no Fe enrichment due to a probable higher water content. The higher Cr and Ni contents of the Upper Volcanic Cycle, together with K, may imply some melting of lherzolite overlying the subducted slab.     

Pyroclastic density currents resulting from the interaction of basaltic magma with hydrothermally altered rock: an example from the 2006 summit eruptions of Mount Etna,Italy

After 16 months of quiescence, Mount Etna began to erupt again in mid-July 2006. The activity was concentrated at and around the Southeast Crater (SEC), one of the four craters on the summit of Etna, and eruptive activity continued intermittently for 5 months. During this period, numerous vents displayed a wide range of eruptive styles at different times. Virtually all explosive activities took place at vents at the summit of the SEC and on its flanks. Eruptive episodes, which lasted from 1 day to 2 weeks, became shorter and more violent with time. Volcanic activity at these vents was often accompanied by dramatic mass-wasting processes such as collapse of parts of the cone, highly unusual flowage processes involving both old rocks and fresh magmatic material, and magma–water interaction. The most dramatic events took place on 16 November, when numerous rockfalls and pyroclastic density currents (PDCs) were generated during the opening of a large fracture on the SE flank of the SEC cone. The largest PDCs were clearly triggered explosively, and there is evidence that much of the energy was generated during the interaction of intruding magma with wet rocks on the cone’s flanks. The most mobile PDCs traveled up to 1 km from their source. This previously unknown process on Etna may not be unique on this volcano and is likely to have taken place on other volcanoes. It represents a newly recognized hazard to those who visit and work in the vicinity of the summit of Etna.     

Quantitative petrogenetic constraints on the Pliocene alkali basaltic volcanism of the SE Spain Volcanic Province

Alkali basalts of Pliocene age are the last episode of volcanism in the SE Spain Volcanic Province, postdating a complex series of Miocene calc-alkaline to ultrapotassic rocks. This volcanism is represented by small outcrops and vents NW of Cartagena that has been interpreted as a volcanic episode similar to the contemporaneous monogenetic alkaline basaltic volcanism of the Iberian Peninsula and Western/Central Europe. However, their geochemical signature is characterised by relatively higher ⁸⁷Sr/⁸⁶Sr ratios as well as distinct trace element anomalies which, at different scale, are only found in the spatially related calc-alkaline to ultrapotassic volcanism. Quantitative modelling of these data demonstrate that the geochemical signature of the Pliocene alkali basalts of Cartagena can be explained by the interaction between primitive melts generated from a sublithospheric mantle source similar to that identified for other volcanic regions of Spain, and liquids derived from the overlying lithospheric mantle. This interaction implies that the alkali basalts show some geochemical features only observed in mantle lithosphere-derived melts (e.g. Sr isotope enrichment and Th–U–Pb positive anomalies), while retaining an overall geochemical signature similar to other Iberian basalts (e.g. Rb–K negative anomalies). This model also implies that beneath the SEVP, enriched (metasomatized) portions were still present within the lithospheric mantle after the Miocene magmatic episodes.     

The volcanic and magmatic evolution of Volcán Ollagüe, a high-K, late quaternary stratovolcano in the Andean Central Volcanic Zone

Volcán Ollagüe is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulée sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagüe collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km² and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulée of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagüe, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagüe predates the last glacial episode ( 11.000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome.Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.     

Elliptical boundary of an intraplate volcanic field, Auckland, New Zealand

The outer vents in the Auckland Volcanic Field lie within 19 to 559 m (mean 224.75 m) of a ‘best-fit' ellipse with a 28.9-km-long major axis trending almost north–south, and a minor axis 16.5 km long. The ellipse has formed the outer boundary of the field since the inception of volcanism 140,000 years ago. We present the following testable hypotheses as an explanation of this pattern: The boundary is the expression of a corresponding elliptical source area at depth in the lithospheric mantle (possibly asthenosphere material trapped at this level). The ellipse may represent a depth contour on a very small upper mantle dome or of a lens intruding into a neck of an extensional structure. Alternatively it could be the boundary of a flat elliptical area where tensional stresses allow decompressional melting. The elliptical tensional region may either have developed in a releasing bend during strike-slip faulting along a fundamental lithosphere structure inherited from Mesozoic tectonics, associated with the NNW-trending Dun Mountain ophiolite belt, or may represent the tip of a fracture along which the Auckland Volcanic Province is propagating northward.     

Shallow magma transport for the 2002–3 Mt. Etna eruption inferred from thermal infrared surveys

The 26 October 2002–28 January 2003 eruption of Mt. Etna volcano was characterised by lava effusion and by an uncommon explosivity along a 1 km-long-eruptive fissure on the southern, upper flank of the volcano. The intense activity promoted rapid growth of cinder cones and several effusive vents. Analysis of thermal images, recorded throughout the eruption, allowed investigation of the distribution of vents along the eruptive fissure, and of the nature of explosive activity. The spatial and temporal distribution of active vents revealed phases of dike intrusion, expansion, geometric stabilization and drainage. These phases were characterised by different styles of explosive activity, with a gradual transition from fire fountaining through transitional phases to mild strombolian activity, and ending with non-explosive lava effusion. Here we interpret the mechanisms of the dike emplacement and the eruptive dynamics, according to changes in the eruptive style, vent morphology and apparent temperature variations at vents, detected through thermal imaging. This is the first time that dike emplacement and eruptive activity have been tracked using a handheld thermal camera and we believe that its use was crucial to gain a detailed understanding of the eruptive event.     

Multiphase flow above explosion sites in debris-filled volcanic vents: Insights from analogue experiments

Discrete explosive bursts are known from many volcanic eruptions. In maar–diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by “debris jets” driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analogue experiments. Specific volumes of compressed (0.5–2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5–1 s. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the “intrusions” that formed them. An additional conclusion is that complex structures can form quickly from discrete events.     

Infrastructure and evolution of ocean-ridge discontinuities in Iceland

There are two main ocean-ridge discontinuities in Iceland: the Tjörnes Fracture Zone (TFZ) and the South Iceland Seismic Zone (SISZ). The TFZ is a 120-km-long and as much as 70-km-wide WNW-trending zone of high seismicity. It has three main seismic lineaments: the Husavik-Flatey Fault (HFF), the Dalvik lineament, and the Grimsey lineament. The HFF, a dextral strike-slip fault and active as a transform fault for about 9 Ma, has a cumulative transform-parallel displacement of some 60 km. Offshore, the HFF is marked by a transform (fracture-zone) valley, 5–10 km wide and 3–4 km deep. Onshore the Flateyjarskagi Peninsula the HFF is marked by a 3–5-km-wide zone of intense crustal deformation with numerous strike-slip and normal faults, transform-parallel dykes, dense sets of mineral veins, and subzones of completely crushed rocks, that is, fault cores. Where the HFF comes on land on Tjörnes there is a similar, but much thinner, zone of crushed rocks. The seismic lineaments are located a few tens of kilometres south (Dalvik) and north (Grimsey) of, and run subparallel with, the HFF. Both lineaments are composed of sets of NNW-trending sinistral faults arranged en echelon.The SISZ is a 70-km-long and 10–20-km wide zone of almost continuous seismicity located between the overlapping West and East Volcanic Zones. It produces the largest earthquakes in Iceland, some of which exceed M7, during which the N–S width of the zone may be as great as 50–60 km. The SISZ is partly covered with Holocene lava flows where the seismogenic faults occur as dextral NNE-trending and sinistral ENE-trending conjugate arrays with push-ups between their nearby ends. The same fault-segment trends occur in the Pleistocene pile north of the Holocene lava flows.The HFF is neither perpendicular to the nearby ridge segments nor parallel with the spreading vector. As a consequence, the North Volcanic Zone has propagated to the north and the Kolbeinsey Ridge to the south during the past 1 Ma, resulting in the development of the Grimsey and Dalvik lineaments. Similarly, the tip of the East Volcanic Zone has been propagating rapidly to the southwest during the past 3 Ma. The tip has been at its present location for no more than several hundred thousand years, thus making the SISZ less stable than the HFF. If the propagation of the tip of the East Volcanic Zone continues, it will eventually reach the Reykjanes Ridge, whereby either the West or the East Volcanic Zone becomes extinct. Then the SISZ dies out as a major seismic zone.     

The Luingo caldera: The south-easternmost collapse caldera in the Altiplano–Puna plateau,NW Argentina

This article identifies the Pucarilla–Cerro Tipillas Volcanic Complex and its major eruptive source, the Luingo caldera (26° 10′S–66° 40′W). Detailed geological mapping, stratigraphic sections, facies analysis and correlations, including the identification of typical caldera components, allow us to infer the position of a collapse caldera, elongated at N65° and with a diameter of 19 km × 13 km, which is responsible for an eruption of 135 km³ (DRE) of magma. The high-crystal contents of the associated ignimbrites, combined with its tectonic setting, indicate that regional and local tectonic structures played a crucial role in the formation of the caldera.     

Late Pleistocene and Holocene activity of the Atacazo–Ninahuilca Volcanic Complex (Ecuador)

The Atacazo–Ninahuilca Volcanic Complex (ANVC) is located in the Western Cordillera of Ecuador, 10 km southwest of Quito. At least six periods of Pleistocene to Holocene activity (N1 to N6) have been preserved in the geologic record as tephra fallouts and pyroclastic flow deposits. New field data, including petrographic and whole-rock geochemical analyses of over forty soil and tephra sections, 100 pumice and lithic samples, and 10 new ¹⁴C ages allow us to constrain: (1) the tephra fall isopachs and detailed characteristics of the last two events (N5–N6) including volume estimates of the tephra and pyroclastic flow deposits and the corresponding volcanic explosivity index (VEI); (2) the petrographical and geochemical correlations between domes, tephras, and pyroclastic flow deposits; and, (3) the timing of the last 4 eruptive events and a period of quiescence that endured a few thousand years (1000–4000).     

Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California,Mexico

Extensive CO₂ vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO₂ leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74–207 m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO₂ vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of tuff vents in the Campi Flegrei caldera (southern Italy): toward a new chronostratigraphic reconstruction of the Holocene volcanic activity

The Campi Flegrei hosts numerous monogenetic vents inferred to be younger than the 15 ka Neapolitan Yellow Tuff. Sanidine crystals from the three young Campi Flegrei vents of Fondi di Baia, Bacoli and Nisida were dated using ⁴⁰Ar/³⁹Ar geochronology. These vents, together with several other young edifices, occur roughly along the inner border of the Campi Flegrei caldera, suggesting that the volcanic conduits are controlled by caldera-bounding faults. Plateau ages of ∼9.6 ka (Fondi di Baia), ∼8.6 ka (Bacoli) and ∼3.9 ka (Nisida) indicate eruptive activity during intervals previously interpreted as quiescent. A critical revision, involving calendar age correction of literature ¹⁴C data and available ⁴⁰Ar/³⁹Ar age data, is presented. A new reference chronostratigraphic framework for Holocene Phlegrean activity, which significantly differs from the previously adopted ones, is proposed. This has important implications for understanding the Campi Flegrei eruptive history and, ultimately, for the evaluation of related volcanic risk and hazard, for which the inferred history of its recent activity is generally taken into account.     

A new method for sampling and analyzing volcanic sublimates — application to Merapi volcano, Java

A new method for the sampling of sublimates from high-temperature volcanic gases has been used at Merapi volcano, Java, in 1978. The sublimates were collected on the inner walls of silica tubes introduced into fumarolic vents. Volcanic gases were allowed to move freely through the tubes and as they cooled, a fraction of the volatile components condensed on the inner walls of the tubes along the temperature gradient. The sublimates were then analyzed by a combination of light microscopy, scanning electron microscopy, electron microprobe and X-ray diffraction.Six successive zones of different compositions and mineralogical associations have been identified along the covered range of temperatures (900° to around 400°C). From the high to the low temperatures, these zones are composed of: (1) cristobalite, magnetite, hercynite; (2) molybdenite; (3) acmite; (4) halite, sylvite; (5) sphalerite, pyrite; and (6) galena. Equilibrium calculations show that these crystalline phases are stable for pS₂, pC₁, and pO₂, values typical of magma-buffered gases that have not been contaminated by atmospheric oxygen.The deposits observed in the tubes may be useful in aiding the understanding of the mechanisms acting during the cooling of the gaseous phase on its way to the surface and before its emission into the atmosphere.     

Subsurface structure across the axis of the Tongariro Volcanic Centre,New Zealand

The relationship between structure and volcanism in the Tongariro Volcanic Centre, New Zealand, is largely masked by a mantle of young volcanic deposits. Here we report the results of an integrated geophysical investigation (using gravity, multi-level aeromagnetic and magnetotelluric methods) of subsurface deposits and structures in the upper 1–2 km across the axis of the Tongariro Volcanic Centre. Modelling of these data across the Tama Lakes saddle shows that the outcropping volcanic deposits are up to 800 m thick, underlain by Tertiary sediments (of a few 10's to a few 100 m in thickness) and in turn lying above a basement of probable Mesozoic greywacke. Basement faulting is shown to be concentrated in the centre of the rift, which is 18 km wide at this location, but no vertical offset is resolved at the rift axis. Vertical displacements on basement faults of 250–300 m are modelled giving a minimum total basement subsidence of 650 m. A 5 km-wide, deep low resistivity zone occurs at the axis of the rift which is interpreted as either resulting from extensive fracturing and/or hydrothermal alteration within the basement. Steep-sided volcanic bodies with a high proportion of lavas/dykes coincide with the Waihi fault and the rift axis. Coincidence with the Waihi Fault suggests that this fault system may have provided magma pathways to the surface and a focus for dyke emplacement, which could have contributed to rift extension. The lack of offset at the rift axis may reflect the juvenile nature of faulting at this location, which is consistent with the notion of a migration of faulting towards the centre of the graben, alternatively, rifting may have been entirely accommodated by dyke emplacement.     

Geochemistry and magmatic properties of eruption episodes from Haroharo linear vent zone, Okataina Volcanic Centre, New Zealand during the last 10 kyr

Post-10 ka rhyolitic eruptions from the Haroharo linear vent zone, Okataina Volcanic Centre, have occurred from several simultaneously active vents spread over 12 km. Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous magma batches. Three magmas totalling ~8 km³ were erupted during the 9.5 ka Rotoma episode. The most evolved Rotoma magma (SiO₂=76.5–77.9 wt%, Sr=96–112 ppm) erupted from a southeastern vent, and is characterised by a cummingtonite-dominant mineralogy, a temperature of 739±14°C, and fO₂ of NNO+0.52±0.11. The least evolved (SiO₂=75.0–76.4 wt%, Sr=128–138 ppm, orthopyroxene+ hornblende-dominant) Rotoma magma erupted from several vents, and was hotter (764±18°C) and more reduced (NNO+0.40±0.13). The ~11 km³ Whakatane episode occurred at 5.6 ka and also erupted three magmas, each from a separate vent. The most evolved (SiO₂=73.3–76.2 wt%, Sr=88–100 ppm) Whakatane magma erupted from the southwestern (Makatiti) vent and is cummingtonite-dominant, cool (745±11°C), and reduced (NNO+0.34±0.08). The least evolved (SiO₂=72.8–74.1 wt%, Sr=132–134 ppm) magma was erupted from the northeastern (Pararoa) vent and is characterised by an orthopyroxene+ hornblende-dominant mineralogy, temperature of 764±18°C, and fO₂ of NNO+0.40±0.13. Compositionally intermediate magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids. A single ~13 km³ magma erupted during the intervening 8.1 ka Mamaku episode was relatively homogeneous in composition (SiO₂=76.1–76.8 wt%, Sr=104–112 ppm), temperature (736±18°C), and oxygen fugacity (NNO+0.19±0.12). Some of the vents tapped a single magma while others tapped several. Deposit stratigraphy suggests that the eruptions alternated between magmas, which were often simultaneously erupted from separate vents. Both effusive and explosive activity alternated, but was predominantly effusive (>75% erupted as lava domes and flows). The plumbing systems which fed the vents are inferred to be complex, with magma experiencing different conditions in the conduits. As the eruption of several magmas was essentially concurrent, the episodes were likely triggered by a common event such as magmatic intrusion or seismic disturbance.     

Hg bioaccumulation in marine copepods around hydrothermal vents and the adjacent marine environment in northeastern Taiwan

The Hg concentration in seawater and copepod samples collected from the area around hydrothermal vents at Kueishan Island and the adjacent marine environment in northeastern Taiwan were analyzed to study Hg bioaccumulation in copepods living in polluted and clean marine environments. The seawater collected from the hydrothermal vent area had an extremely high concentration of dissolved Hg, 50.6–256 ng l⁻¹. There was slightly higher Hg content in the copepods, 0.08–0.88 μg g⁻¹. The dissolved Hg concentration in the hydrothermal vent seawater was two to three orders of magnitude higher than that in the adjacent environment. The bioconcentration factor of the studied copepods ranged within 10³–10⁶, and showed higher dissolved concentration as the bioconcentration factor was lower. A substantial abundance, but with less copepod diversity was recorded in the seawater around the hydrothermal vent area. Temora turbinata was the species of opportunity under the hydrothermal vent influence.     

Miocene hydrovolcanism in NW Colorado,USA, fuelled by explosive mixing of basic magma and wet unconsolidated sediment

The Yampa and Elkhead Mountains volcanic fields were erupted into sediment-filled fault basins during Miocene crustal extension in NW Colorado. Post-Miocene uplift and erosion has exposed alkali basalt lavas, pyroclastic deposits, volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths. The occurrence of these different phenomena was related to the degree of lithification of the rocks through which the magmas rose. Hydrovolcanic interactions only occurred where rising basaltic magma encountered wet, porous, non-lithified sediments of the 600 m thick Miocene Brown's Park Formation. The interactions were fuelled by groundwater in these sediments: there was probably no standing surface water. Dykes intruded into the sediments have pillowed sides, and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water. Volcanic necks in the sediments consist of basaltic tuff, sediment blocks and separated grains derived from the sediments, lithic blocks (mostly derived from a conglomerate forming the local base of the Brown's Park Formation), and dykes composed of disaggregated sediment. The necks are cut by contemporaneous basalt dykes. Hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air-fall, pyroclastic surge, and massive, poorly sorted deposits (MPSDs). All these contain sub-equal volumes of basaltic tuff and disaggregated sediment grains from the Brown's Park Formation. Possible explosive and effusive modes of formation for the MPSDs are discussed. Contemporaneous strombolian scoria deposits overlie lithified Cretaceous sedimentary rocks or thick basalt lavas. Volcanic necks intruded into the Cretaceous rocks consist of basalt clasts (some with spindle-shape), lithic clasts, and megacrysts derived from the magma, and are cut by basalt dykes. Rarely, strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits, recording changes in eruption behaviour during one eruption. The hydrovolcanic eruptions occurred by interaction of magma with groundwater in the Brown's Park sediments. The explosive interactions disaggregated the sediment. Such direct digestion of sediment by the magma in the vents would probably not have released enough water to maintain a water/magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones. Probably, additional water (perhaps 76% of the total) was derived by flow through the permeable sediments (especially the basal conglomerate to the formation), and into the vents.