Phreatomagmatic Research Articles

Holocene phreatomagmatic eruptions alongside the densely populated northern shoreline of Lake Kivu, East African Rift: timing and hazard implications

The Virunga Volcanic Province (VVP) represents the most active zone of volcanism in the western branch of the East African Rift System. While the VVP’s two historically active volcanoes, Nyamulagira and Nyiragongo, have built scoria cones and lava flows in the adjacent lava fields, several small phreatomagmatic eruptive centers lie along Lake Kivu’s northern shoreline, highlighting the potential for explosive magma-water interaction. Their presence in the densely urbanized Sake-Goma-Gisenyi area necessitates an assessment of their eruptive mechanisms and chronology. Some of these eruptive centers possess multiple vents, and depositional contacts suggest distinct eruptive phases within a single structure. Depositional facies range from polymict tuff breccia to tuff and loose lapilli, often impacted by blocks and volcanic bombs. Along with the presence of dilute pyroclastic density current (PDC) deposits, indicators of magma-water interaction include the presence of fine palagonitized ash, ash aggregates, cross-bedding, and ballistic impact sags. We estimate that at least 15 phreatomagmatic eruptions occurred in the Holocene, during which Lake Kivu rose to its current water level. Radiocarbon dates of five paleosols in the top of volcanic tuff deposits range between ∼2500 and ∼150 cal. year bp and suggest centennial- to millennial-scale recurrence of phreatomagmatic activity. A vast part of the currently urbanized zone on the northern shoreline of Lake Kivu was most likely impacted by products from phreatomagmatic activity, including PDC events, during the Late Holocene, highlighting the need to consider explosive magma-water interaction as a potential scenario in future risk assessments.

Phreatomagmatic maar-diatreme volcanoes and their incremental growth: a model

Abstract We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.

Rheological controls on the eruption potential and style of an andesite volcano: A case study from Mt. Ruapehu, New Zealand

The evolving magma rheology of three recent Ruapehu eruptions (1969, 1977, and 1995) is estimated using a combination of thermodynamic models and rheological calculations, supported by textural observations of the erupted scoria. We use a well-established thermodynamic model to determine the composition of these representative Ruapehu magmas from 300MPa to ∼30MPa. The outputs of the model provide the changing crystal and bubble content in a closed system (assuming no gas loss), as well as the fractionating melt compositions. We calculate the melt viscosity, and the effect of bubbles and crystals, to quantify the rheology of the magma during ascent (under assumed equilibrium conditions). The moderately high phenocryst content of Ruapehu scoria (∼30%) means that only a small amount of additional microlite crystallisation (∼5%) would result in a yield strength, which may lead the magma to stall. However, if the strain rates are high enough, more crystallisation would be possible without developing a yield stress. This suggests that microlite-rich magmas are almost certain to stall unless they encounter significant fluid addition from a source such as a hydrothermal system, groundwater, or surface water (i.e., Ruapehu's Crater Lake).Ruapehu magmas are initially H2O-undersaturated and as a consequence, crystallisation and bubble growth were suppressed until the magma achieved saturation, at ∼100 to 50MPa. From this analysis, we suggest that Ruapehu magmas are more likely to erupt compared to magmas of a similar composition that are H2O-saturated. This partly explains the regular, albeit small-volume eruptions at Ruapehu and the propensity for phreatomagmatic eruptions when the magma:water ratio is low.

Phreatomagmatic and water-influenced Strombolian eruptions of a small-volume parasitic cone complex on the southern ringplain of Mt. Ruapehu, New Zealand: Facies architecture and eruption mechanisms of the Ohakune Volcanic Complex controlled by an unstable fissure eruption

The Ohakune Volcanic Complex is a late Pleistocene tuff ring – scoria/spatter cone complex located south of Ruapehu volcano. This small-volume volcano consists of an outer E-W elongated compound tuff ring edifice, three inner scoria-spatter cones and further volcanic depressions, located on the Ohakune Fault. We quantified accurately the variations of the eruptive styles and processes through time by systematic sampling of key stratigraphic marker beds at proximal and distal locations, and the determination of grain size distribution, componentry, density and vesicularity. Using a Digital Terrain Model coupled with stratigraphic data, we also determined the spatial distribution and volume of each identified unit and individual edifices within the Ohakune Volcanic Complex. Activity began with a shallow phreatomagmatic phase characterized by an almost continuous generation of a low eruptive column, accompanied by wet pyroclastic density currents, together with the ejection of juvenile fragments and accidental lithics from the surrounding country rocks. Subsequent activity was dominated by a variety of Strombolian eruptions exhibiting differing intensities that were at times disrupted by phreatic blasts or phreatomagmatic explosions due to the interaction with external water and/or sudden changes in magma discharge rate. At least three major vent-shifting events occurred during the eruption, which is demonstrated by the truncation of the initial tuff ring and the infilling of the truncated area by several coarse grained surge units. Our study indicates that approx. 12×106m3 DRE magma erupted within maximum 2.5 to 5months through multiple vents. The erupted magma ascended from a depth of 16–18km, and reached the surface within approximately 50h. Alternating eruption styles, frequent vent-shifting and a variety of emplacement mechanisms inferred from the deposits of the Ohakune Volcanic Complex demonstrate the unpredictable nature of small-volume volcanism, contributing to our understanding of hazards attributed to this type of activity. Future small-volume eruptions within the Taupo Volcanic Zone may be of similar nature.

Relationship between eruptive style and vesicularity of juvenile clasts during eruptive episode A of Towada Volcano, Northeast Japan

It has been reported that juvenile pumice lapilli found in plinian eruptions have high vesicularity, while those found in phreatoplinian eruptions have low vesicularity. However, juvenile glass shards from phreatoplinian eruptions consist of large, expanded bubbles such as bubble wall-type glass. These glass shards seem to possess high vesicularity, unlike the pumice lapilli. This study examines the factors causing this difference, especially focusing on the temporal variations in the vesicularity of the juvenile pyroclasts from eruptive episode A of Towada Volcano, Northeast Japan. This examination was conducted through four analyses: density measurements of pumice lapilli, thin section texture classification of pumice lapilli, classification of glass shards, and surface texture classification of pumice lapilli. Further, pumice lapilli from plinian eruptions have a low density, and those from phreatoplinian eruptions are characterized by high density. The density of the pumice lapilli depends on the eruption style and is hence determined after the eruption. A progressive increase in the amount of large bubbles is observed in glass shards ejected during an eruptive magmatic to phreatomagmatic sequence. Because it does not hinge on the eruptive style, it is assumed that the vesicularity of the glass shards is kept from the conduit before contact with water, especially on fragmentation by magma vesiculation in the conduit. The surfaces of the pumice lapilli show a similar increase in vesicularity with time as glass shards. However, this increase is not successive throughout, but decreases temporarily at the phreatomagmatic stage of the eruption, as in the case of density. This indicates that the successive bubble growth continues within the pumice, and additional vesiculation is superposed when the magmatic eruption comes into contact with water. Because of this, different juvenile clasts exhibit different vesicularities upon cooling. Interestingly, magma vesicularity increases before coming into contact with water, i.e., before transitioning to the phreatomagmatic stage. In fact, the magma–water ratio aside, this might be an important factor causing phreatomagmatic eruptions.

Subaqueous cryptodome eruption, hydrothermal activity and related seafloor morphologies on the andesitic North Su volcano

North Su is a double-peaked active andesite submarine volcano located in the eastern Manus Basin of the Bismarck Sea that reaches a depth of 1154m. It hosts a vigorous and varied hydrothermal system with black and white smoker vents along with several areas of diffuse venting and deposits of native sulfur. Geologic mapping based on ROV observations from 2006 and 2011 combined with morphologic features identified from repeated bathymetric surveys in 2002 and 2011 documents the emplacement of a volcanic cryptodome between 2006 and 2011. We use our observations and rock analyses to interpret an eruption scenario where highly viscous, crystal-rich andesitic magma erupted slowly into the water-saturated, gravel-dominated slope of North Su. An intense fragmentation process produced abundant blocky clasts of a heterogeneous magma (olivine crystals within a rhyolitic groundmass) that only rarely breached through the clastic cover onto the seafloor. Phreatic and phreatomagmatic explosions beneath the seafloor cause mixing of juvenile and pre-existing lithic clasts and produce a volcaniclastic deposit. This volcaniclastic deposit consists of blocky, non-altered clasts next, variably (1–100%) altered clasts, hydrothermal precipitates and crystal fragments. The usually applied parameters to identify juvenile subaqueous lava fragments, i.e. fluidal shape or chilled margin, were not applicable to distinguish between pre-existing non-altered clasts and juvenile clasts. This deposit is updomed during further injection of magma and mechanical disruption. Gas-propelled turbulent clast-recycling causes clasts to develop variably rounded shapes. An abundance of blocky clasts and the lack of clasts typical for the contact of liquid lava with water is interpreted to be the result of a cooled, high-viscosity, crystal-rich magma that failed as a brittle solid upon stress. The high viscosity allows the lava to form blocky and short lobes. The pervasive volcaniclastic cover on North Su is partly cemented by hydrothermal precipitates. These hydrothermally-cemented breccias, crusts and single pillars show that hydrothermal circulation through a thick layer of volcaniclastic deposits can temporarily increase slope stability through precipitation and cementation.

Application of tephra volume models to ejecta volumes from subsurface explosion experiments

Deposit volume is a critical factor for reconstructing an explosive eruption. Volume estimate models typically used for large Plinian deposits have been adapted and improved repeatedly over the last few decades. Less work has been done to refine a method for estimating the volume from smaller deposits produced by discrete phreatic and phreatomagmatic explosions. The characterization of the volume and distribution of deposits is required to quantify the physical hazards presented by different explosion types and develop appropriate models of future eruptions. Six classic tephra volume models were assessed using a dataset from subsurface explosion experiments. The models typically did a poor job modelling the volume of proximal deposits as a component of total deposit volume of discrete explosion deposits. Models reproduced medial and distal deposit volumes with greater success, particularly the Exponential model and a more recent Linear Regression model. It is therefore recommended, when possible, to use digital elevation models produced from GPS or laser-based methods to characterize proximal deposits separately and to use tephra volume estimates for medial and distal deposits. Additionally, this dataset enabled the comparison of ejecta volumes with crater diameters and highlighted that this relationship only holds for simple crater scenarios without any lateral vent migration, collapse or erosion of the crater under study. The assessment and improvement of these methods are required to ensure accurate deposit volumes as they serve as one of the most important inputs to hazard assessments and numerical models.

Geochemistry of fluid discharges from Peteroa volcano (Argentina-Chile) in 2010–2015: Insights into compositional changes related to the fluid source region(s)

This study presents the first geochemical data of fluid discharges collected from February 2010 to March 2015 from the Planchón–Peteroa–Azufre Volcanic Complex (PPAVC), located in the Transitional Southern Volcanic Zone (TSVZ) at the border between Argentina and Chile. During the study period, from January 2010 to July 2011, Peteroa volcano experienced phreatic to phreatomagmatic eruption possibly related to the devastating Maule earthquake occurred on February 27, 2010. The compositional dataset includes low temperature (from 43.2 to 102°C) gas discharges from (i) the summit of Peteroa volcano and (ii) the SE flank of Azufre volcano, both marked by a significant magmatic fluid contribution, as well as bubbling gases located at the foothill of the Peteroa volcanic edifice, which showed a chemical signature typical of hydrothermal fluids. In 2012, strong compositional changes affected the Peteroa gases from the summit area: the acidic gas species, especially SO2, increased, suggesting an input of fluids from magma degassing. Nevertheless, the R/Ra and δ13C–CO2 values decreased, which would imply an enhanced contribution from a meteoric-hydrothermal source. In 2014–2015, the chemical and isotopic compositions of the 2010–2011 gases were partially restored. The anomalous decoupling between the chemical and the isotopic parameters was tentatively interpreted as produced by degassing activity from a small batch of dacitic magma that in 2012 masked the compositional signature of the magmatic fluids released from a basaltic magma that dominated the gas chemistry in 2010–2011. This explanation reliably justifies the observed geochemical data, although the mechanisms leading to the change in time of the dominating magmatic fluid source are not clear. At this regard, a geophysical survey able to provide information on the location of the two magma batches could be useful to clarify the possible relationships between the compositional changes that affected the Peteroa fluid discharges and the 2010–2011 eruptive activity.

Volcanic and sedimentary facies of the Huka Group arc-basin sequence, Wairakei–Tauhara Geothermal Field, New Zealand

ABSTRACTDetailed facies analyses of intra-arc basin stratigraphy provide spatial and temporal insights into the nature and timing of past volcanic, sedimentary and tectonic events. Rift and caldera basins hosting lakes in the central Taupo Volcanic Zone (TVZ) continental arc preserve a sequence of locally and regionally sourced volcanic deposits and interbedded lacustrine sediments. A series of new continuous core samples from Wairakei–Tauhara Geothermal Fields in the TVZ intersecting thick Huka Group stratigraphy has provided a unique opportunity to study c. 300 ka of local arc volcanism, sedimentary processes and ancient surface environments. Facies analysis is utilised on 11 cored wells from across Wairakei–Tauhara, including a newly defined Huka Group Reference Section well (WKM15). Interpreted facies identify that the Huka Group commenced with a large ignimbrite (Waiora Ignimbrite Member) emplaced onto faulted relief covered by a pre-existing ancient lake, Lake Huka. Graded volcaniclastic facies, lavas and sediments comprising the Waiora Volcaniclastics Member then accumulated in the basins over the next c. 100 ka. A spatially variable switch from volcaniclastic (Waiora Volcaniclastics Member) to lacustrine siltstones and sandstones (Huka Falls Formation) deposition then followed in Lake Huka. Excluding an intra-lake phreatomagmatic eruption (Middle Huka Falls Formation), these lacustrine sediments reflect a c. 200 ka local pyroclastic hiatus. Late Huka Group deposition became gradually more fluvial as rivers prograded marginal areas of the lake until its destruction by the 25.4 ka Oruanui eruption. The drill core samples provide new complete and detailed insights into processes and environments associated with the Huka Group. These enhance our understanding of the geological history at Wairakei–Tauhara Geothermal Fields and contribute to our knowledge of the ancient processes that occurred during evolution of the TVZ.

Multiple ash layers in late Quaternary sediments from the Central Indian Basin

We have investigated three sediment cores collected from water depths >5000m along the transect 76°30′E in close proximity to a fracture zone in the Central Indian Basin (CIB). The cores yielded five volcanic horizons of which four have visual and dispersed shards. Rhyolitic glass shards of bubble wall, platy, angular and blocky types were retrieved from various stratigraphic horizons in the cores. The abundance of glass shards, composition of bulk sediments, and 230Thexcess ages of the host sediments were used to distinguish the volcanic horizons. Of the four volcanic horizons, three are now newly reported and correspond to ages of ~85, 107–109 and 142–146ka while the fourth horizon is of 70–75ka. By using trace element ratios and Cr and Nb-based normative calculations, cryptotephra has been identified for the first time from the CIB sediment. The cryptotephra forms the fifth ash horizon and is of ~34ka.A comparison with the published data on volcanic tephra in and around the Indian Ocean indicate the shard rich horizon (SRH) of 70–75ka to resemble the Younger Toba Tuffs (YTT), while the other volcanic horizons that were deposited during different time periods do not correlate with any known marine or terrestrial records. These tephra layers have produced a tephrostratigraphic framework across the tectonically and volcanically complex regions of the CIB. Due to the lack of terrestrial equivalents of these tephra, it is hypothesized that the newly found volcanic horizons may have been derived from submarine volcanic eruptions. Multiple layers of submarine volcaniclastic deposits found at water depths as great as 5300m reaffirm the growing belief that submarine phreatomagmatic eruptions are much more common in the intraplate region of the Indian Ocean than previously reported.

The latest explosive eruptions of Ciomadul (Csomád) volcano, East Carpathians — A tephrostratigraphic approach for the 51–29 ka BP time interval

The most recent, mainly explosive eruptions of Ciomadul, the youngest volcano in the Carpatho-Pannonian Region, have been constrained by detailed field volcanological studies, major element pumice glass geochemistry, luminescence and radiocarbon dating, and a critical evaluation of available geochronological data. These investigations were complemented by the first tephrostratigraphic studies of the lacustrine infill of Ciomadul's twin craters (St. Ana and Mohoş) that received tephra deposition during the last eruptions of the volcano. Our analysis shows that significant explosive activity, collectively called EPPA (Early Phreatomagmatic and Plinian Activity), started at Ciomadul in or around the present-day Mohoş, the older crater, at ≥51ka BP. These eruptions resulted in a thick succession of pyroclastic-fall deposits found in both proximal and medial/distal localities around the volcano, characterized by highly silicic (rhyolitic) glass chemical compositions (ca. 75.2–79.8wt.% SiO2). The EPPA stage was terminated by a subplinian/plinian eruption at ≥43ka BP, producing pumiceous pyroclastic-fall and -flow deposits of similar glass composition, probably from a “Proto-St. Ana” vent located at or around the younger crater hosting the present-day Lake St. Ana. After a quiescent period with a proposed lava dome growth in the St. Ana crater, a new explosive stage began, defined as MPA (Middle Plinian Activity). In particular, a significant two-phase eruption occurred at ~31.5ka BP, producing pyroclastic flows from vulcanian explosions disrupting the preexisting lava dome of Sf. Ana, and followed by pumiceous fallout from a plinian eruption column. Related pyroclastic deposits show a characteristic, less evolved rhyolitic glass composition (ca. 70.2–74.5wt.% SiO2) and occur both in proximal and medial/distal localities up to 21km from source. The MPA eruptions, that may have pre-shaped a crater similar to, but possibly smaller than, the present-daySt. Ana crater, was followed by a so far unknown, but likewise violent last eruptive stage from the same vent, creating the final morphology of the crater. This stage, referred to as LSPA (Latest St. Ana Phreatomagmatic Activity), produced pyroclastic-fall deposits of more evolved rhyolitic glass composition (ca. 72.8–78.8wt.% SiO2) compared to that of the previous MPA stage. According to radiocarbon age constraints on bulk sediment, charcoal and organic matter from lacustrine sediments recovered from both craters, the last of these phreatomagmatic eruptions – that draped the landscape toward the east and southeast of the volcano – occurred at ~29.6ka BP, some 2000years later than the previously suggested last eruption of Ciomadul.

Archean Xenocrysts in Modern Volcanic Rocks from Kamchatka: Insight into the Basement and Paleodrainage

AbstractWe report U-Pb zircon ages and in situ δ18O values for crystals of zircon, rutile, and corundum erupted in the course of a March 2009 phreatomagmatic explosion of Koryaksky volcano, Eastern Volcanic Front, Kamchatka, Russia. Zircon crystals display a wide spectrum of crystallization ages ranging from Cenozoic to Archean, including single grains and grain-age clusters, ca. 1.7, 11, 37–56, 89–99, 152, 1461, 1946, 2584–2734, and 3314 Ma. The older end of this spectrum represents the oldest known zircon ages from Kamchatka. Oxygen isotopic values span from normal to low δ18O in zircon (3.3‰ to 5.5‰), corundum (4.3‰ to 5.5‰), and rutile (2‰ to −2.8‰). As Koryaksky volcano is built over Cretaceous-Eocene and younger crust, the presence of older zircon crystals requires that these accessory minerals with their diverse ages and δ18O values were derived from heterogeneous detrital components in sediment that contaminated the magma shortly before or during the phreatomagmatic eruption. We also present a com...

Origin and K-Ar age of the phreatomagmatic breccia at the Trepča Pb-Zn-Ag skarn deposit, Kosovo: Implications for ore-forming processes

The Trepca Pb-Zn-Ag skarn deposit in Kosovo is spatially and temporarily related to the phreatomagmatic breccia of Oligocene age (~23 Ma). The deposit shows features typical for skarn deposits worldwide, including a stage of isochemical metamorphism, a prograde stage of an anhydrous, low oxygen and low sulfur fugacity character and a retrograde stage characterized by an increase in the water activity as well as by an increase in oxygen and sulfur fugacities. The mineralization is hosted by the recrystallized Upper Triassic limestone. The prograde mineralization consists mainly of Ca-Fe-Mn±Mg pyroxenes. The host recrystallized limestone at the contact with the prograde (skarn) mineralization have the increased content of Fe, Mn, Mo, As, Au, Cs, Ga, REE and Y suggesting their transport by infiltrating magmatic fluids. The decreased d13C and d18O values reflect a contribution of magmatic CO2. The retrograde mineral assemblage comprises ilvaite, magnetite, arsenopyrite, pyrrhotite, marcasite, pyrite, Ca-Fe-Mn±Mg carbonates and quartz. Hydrothermal ore minerals, mostly galena, sphalerite and pyrite, were deposited contemporaneously with the retrograde stage of the skarn development. Syn-ore and post-ore carbonates reflect a diminishing influence of magmatic CO2. Syn-ore carbonates are enriched in Fe, Mg, Mn, many chalcophile elements, including Ag, As, Bi, Cd, Cu, Pb, Sb and Zn, as well as in Au, Y and REE. The post-ore stage accompanied the precipitation of significant amount of Ca-rich carbonates including the travertine deposits at the deposit surface. The phreatomagmatic breccia was developed along a NW dipping contact between the ore bearing recrystallized limestone and the overlying schist. It has an inverted cone shape with the vertical extension up to 800 m and diameter up to 150-m. The upper part of the diatreme (an underground segment of the phreatomagmatic breccia) is characterized by the presence of a hydrothermally altered rootless quartz-latite dike surrounded by an unsorted polymict breccia mantle. Despite the alteration processes, the dike has a preserved porphyritic texture. Partly preserved sanidine, accompanied with the mixture of muscovite and quartz, reflects a near-neutral to weakly acidic environment. The clasts of country rocks and skarn mineralization underwent intense milling and mixing due to repeated magmatic penetrations. Sericitization of the breccia matrix, locally accompanied with minor kaolinitization, point to an increased water activity under near-neutral to weakly acidic conditions. Large fragments originally composed of anhydrous skarn minerals (pyroxenes) are usually completely altered to a mixture of fibroradial magnetite, quartz and various amount of carbonates suggesting an increase in oxygen fugacity. Their pyrite rims reflect that the increase in oxygen fugacity was followed by an increase in sulfur fugacity. The clast predominantly composed of Fe-sulfides and minor Bi-sulfides point that the increase in sulfur fugacity was locally sufficient to complete sulfidation of hedenbergite to pyrrhotite and/or pyrite. Although the phreatomagmatic breccia at the Trepca Pb-Zn-Ag skarn deposit does not carry significant amounts of the ore mineralization, its formation was crucial for the ore deposition. Phreatomagmatic explosions and formation of the breccia turned the system from the lithostatic to hydrostatic regime and triggered the retrograde stage increasing the water activity and oxygen fugacity in the system. In addition, cooling and decompression of the system contributed to more effective degassing of magmatic sulfur increased the sulfur fugacity.

Reconstruction of the geology and structure of Lake Rotomahana and its hydrothermal systems from high-resolution multibeam mapping and seismic surveys: Effects of the 1886 Tarawera Rift eruption

Present-day Lake Rotomahana is one of the two focal points of the most destructive eruption in New Zealand's historical record, i.e., that of Mt. Tarawera on 10 June 1886, with devastating loss of life and presumed destruction of the iconic Pink and White Terraces that adorned the margins of the lake. Basaltic dikes are considered to have ascended near surface in the area, intruding into hydrothermally altered and water-saturated ground beneath the existing lake. The consequential hydrothermal and phreatomagmatic eruptions ejected 0.5325km3 of material from the lakefloor and below, plastering the nearby landscape for several kilometers with mud and other debris. The eruption buried the natural outlet of the lake, with the bottom of the craters becoming filled by water within months and completely concealed from view within years; today Lake Rotomahana has depths up to 118m.High-resolution (0.5m) bathymetric mapping, when combined with a 2-D seismic reflection survey, enables us to ‘see’ details of the maar craters on the lakefloor, including those parts subsequently buried by sediment. The large Rotomahana Crater described by workers immediately after the eruption measures ~2.5km in diameter near its southwestern end, and excavated ground to 155m below present-day lake level. The vent system, as revealed by the present study, forms an array of right-stepping (dextral) craters, with the main crater being host to two sub-craters Rotomahana West Crater and Rotomahana East Crater today buried beneath the lakefloor, and which are in-filled by 36 and 37m of sediment, respectively. Subordinate craters along the same 057° Tarawera Rift trace include Hochstetter Crater (11m of infill), Waingongongongo Crater (14m) and Rotomakariri Crater (26m). These craters host a total 0.0268km3 of sediment. Other features highlighted by the bathymetric data include; craters not filled by sediment, sediment fan deltas, volcanic ridges and dikes, submerged wave-cut terraces formed during times of lower lake levels and gas pockmarks, all either related to the 1886 eruptive episode or post-eruption hydrothermal and erosional processes.Application of results from bubble plume, CO2 flux, magnetic and heat flux surveys of Lake Rotomahana to this study, when combined with regional earthquake relocation analysis and broadband magnetotelluric data, suggest an explanation in terms of a magmatic heat source located south of Waimangu and a corresponding convective water/heat transport system extending thence to beneath the western end of the lake. A holistic approach has provided a coherent context for the eruption and its effect on the historical Pink and White Terraces hydrothermal system that appears to have been the eastern-most extension of a larger system that lay beneath the Waimangu area before the eruption. The newly named Pink Terraces hydrothermal system (~1.5km2) is a continuation of the historical hydrothermal activity that was concentrated on the western shores of the old lake, and together with the formation of the new, post-1886 Patiti hydrothermal system (~1km2) located SW of Patiti Island, mark the two distinct areas of hydrothermal activity in the lake today.

Phreatomagmatic eruptions of 2014 and 2015 in Kuchinoerabujima Volcano triggered by a shallow intrusion of magma

Phreatomagmatic eruptions of 2014 and 2015 in Kuchinoerabujima Volcano triggered by a shallow intrusion of magma

The ELSA tephra stack: Volcanic activity in the Eifel during the last 500,000 years

Tephra layers of individual volcanic eruptions are traced in several cores from Eifel maar lakes, drilled between 1998 and 2014 by the Eifel Laminated Sediment Archive (ELSA). All sediment cores are dated by 14C and tuned to the Greenland interstadial succession. Tephra layers were characterized by the petrographic composition of basement rock fragments, glass shards and characteristic volcanic minerals. 10 marker tephra, including the well-established Laacher See Tephra and Dümpelmaar Tephra can be identified in the cores spanning the last glacial cycle. Older cores down to the beginning of the Elsterian, show numerous tephra sourced from Strombolian and phreatomagmatic eruptions, including the 40Ar/39Ar dated differentiated tephra from Glees and Hüttenberg. In total, at least 91 individual tephra can be identified since the onset of the Eifel volcanic activity at about 500,000 b2k, which marks the end of the ELSA tephra stack with 35 Strombolian, 48 phreatomagmatic and 8 tephra layers of evolved magma composition. Many eruptions cluster near timings of the global climate transitions at 140,000, 110,000 and 60,000 b2k. In total, the eruptions show a pattern, which resembles timing of phases of global sea level and continental ice sheet changes, indicating a relation between endogenic and exogenic processes.

Geology of the Mid-Miocene Rooster Comb Caldera and Lake Owyhee Volcanic Field, eastern Oregon: Silicic volcanism associated with Grande Ronde flood basalt

The Lake Owyhee Volcanic Field (LOVF) of eastern Oregon consists of rhyolitic caldera centers and lava fields contemporaneous with and spatially related to Mid-Miocene Columbia River flood basalt volcanism. Previous studies delineated two calderas in the southeastern part of LOVF near Owyhee Reservoir, the result of eruptions of two ignimbrites, the Tuff of Leslie Gulch and the Tuff of Spring Creek. Our new interpretation is that these two map units are differentially altered parts of a single ignimbrite produced in a major phreatomagmatic eruption at ~15.8Ma. Areas previously mapped as Tuff of Spring Creek are locations where the ignimbrite contains abundant clinoptilolite±mordenite, which made it susceptible to erosion. The resistant intracaldera Tuff of Leslie Gulch has an alteration assemblage of albite±quartz, indicative of low-temperature hydrothermal alteration. Our new mapping of caldera lake sediments and pre- and post-caldera rhyolitic lavas and intrusions that are chemically similar to intracaldera Tuff of Leslie Gulch point to a single ~20×25km caldera, which we name the Rooster Comb Caldera. Erosion of the resurgently uplifted southern half of the caldera created dramatic exposures of intracaldera Tuff of Leslie Gulch cut by post-caldera rhyolite dikes and intrusions that are the deeper-level equivalents of lava domes and flows that erupted into the caldera lake preserved in exposures to the northeast.The Rooster Comb Caldera has features in common with more southerly Mid-Miocene calderas of the McDermitt Volcanic Field and High Rock Caldera Complex, including formation in a basinal setting shortly after flood basalt eruptions ceased in the region, and forming on eruption of peralkaline ignimbrite. The volcanism at Rooster Comb Caldera postdates the main activity at McDermitt and High Rock, but, like it, begins ~300ky after flood basalt volcanism begins in the area, and while flood basalts don't erupt through the silicic focus, are contemporaneous with the latest stages of eruptions nearby. High Rock and McDermitt rhyolites are associated with propagation of Steens Basalt dikes to the south, and LOVF rhyolites with later propagation of Grande Ronde Basalt dikes to the north and north-northwest.

Tephra ring interpretation in light of evolving maar–diatreme concepts: Stracciacappa maar (central Italy)

Maar–diatreme volcanoes in their simplest form are thought to result primarily from discrete, subsurface phreatomagmatic explosions. Tephra ring deposits around maar craters are commonly interpreted in terms of a framework wherein explosions begin at shallow levels and migrate downward, ejecting progressively deeper-seated lithic clasts that contribute to the tephra rings. Additionally, variations in grain size of the deposits are sometimes inferred to record different degrees of fragmentation, which are in turn related to variations in magma–water ratios at the explosion sites. Recent detailed studies of diatremes and of maar tephra rings suggest a different conceptual model wherein explosions can happen at various vertical and lateral locations within a diatreme during its eruptive lifetime, rather than being limited to simple downward migration. Experiments, numerical modeling, and field data indicate that most explosions deeper than about 200–250m will not vent to the surface, but instead contribute to mixing within the diatremes. Arrival of deep-seated lithics at the surface is related to this mixing process. Experiments also indicate that explosion phenomena can result in a range of emplacement mechanisms and grain sizes in tephra rings, even in the absence of variations in explosion mechanisms (fragmentation efficiency). Here we document the tephra ring of Stracciacappa maar (central Italy), and interpret its eruption and emplacement history in terms of evolving conceptual models along with a comparison to more traditional interpretations. Because most elements of both conceptual models are viable, the work points to an important need to develop criteria to distinguish between the various inferred mechanisms.

Hydrothermal fluid flow disruptions evidenced by subsurface changes in heat transfer modality: The La Fossa cone of Vulcano (Italy) case study

Detecting volcanic unrest is of primary importance for eruption forecasting, especially on volcanoes characterized by highly dangerous , and often seemingly unpredictable, phreatic or phreatomagmatic eruptions. We present a simple and innovative analysis of shallow vertical temperature profiles to depths of 70 cm. These data were recorded at La Fossa cone of Vulcano (Aeolian Islands, Italy), during an episode of increased hydrothermal and seismic activities that occurred between September and December 2009. This work involves the use of the coefficient of determination (R 2) on vertical temperature profiles in order to identify changes in conductive versus convec-tive heat transfer modality. The increase in convective heat transfer can be related to the disruption of the hydrothermal system due to its pressurization and/or variation of ground permeability between the hydrothermal system and the surface. While raw temperature data do not evidence any significant variation during the period investigated and the classic temperature gradient is highly influenced by seasonal variations, the fluctuation of R 2 displayed striking spikes that coincided with the seismic swarm inside the volcanic edifice. Such a low-cost device associated with easy real-time data processing could constitute a very promising, yet deceptively simple, technique to monitor hydrothermal systems, in order to assess the hazard posed by high-energy eruptions for populations living close to active volcanoes.

Shallow-seated explosions in the construction of the Motukorea tuff ring (Auckland, New Zealand): Evidence from lithic and sedimentary characteristics

At least 52 eruption centres are scattered within the ~360km2 Auckland Volcanic Field (AVF). Motukorea, now an island in the Waitemata Harbour, is one of 39 AVF volcanoes that experienced a phreatomagmatic explosive phase, before a magmatic phase. The volcano erupted through a ~200–300m-thick, consolidated, mudstone/sandstone sequence of the Miocene Waitemata Group, which overlies the Waipapa Terrane greywacke basement. Detailed field descriptions of the sedimentary characteristics of the early phreatomagmatic deposits were carried out, along with examination of lithics. The ejecta ring deposit comprises 55 to 60vol.% lithics, of which Waitemata Group fragments constitute approximately 90vol.%, whereas <10vol.% are Waipapa fragments, suggesting a dominance of shallow fragmentation. The sedimentary characteristics of the stratigraphic sequence at Motukorea suggest a dominance of wet surges at the beginning of the eruption with progression into drier sequences upwards. This is reflected in increasing inter-bedded juvenile-pyroclast-dominated fall deposits up-sequence. These characteristics are attributed to the changing hydrogeological conditions within the diatreme and the host rocks. These findings shed light on the eruption dynamics of phreatomagmatic eruptions through consolidated rocks in the AVF and enable the depiction of a scenario of future eruptions within the field in similar substrates.