Debris jets in continental phreatomagmatic volcanoes: A field study of their subterranean deposits in the Coombs Hills vent complex,Antarctica

The Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, some of which are interpreted to fill large vent complexes. Such a complex was re-examined at Coombs Hills to map individual steep-sided cross-cutting bodies in detail, and we found several contrasting types, two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing, non-consolidated debris. These transient conduits were opened because of the propagation of debris jets – upward-moving streams of volcaniclastic debris, steam, magmatic gases +/− liquid water droplets – following explosive magma–aquifer interaction. Some debris jets probably remained wholly subterranean, whereas others made it to the surface, but the studied outcrops do not allow us to differentiate between these cases. The pipes filled with country rock-rich lapilli-tuff or tuff-breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within pre-existing basalt-bearing debris, well away from the complex walls or floor. We infer that once focused jets were formed, they did not incorporate significant amounts of existing debris while travelling through them; instead, incorporation of fragments from the granular host took place near explosion sites. Other basalt-rich tuff-breccia zones, accompanied by domains of in situ peperite and coherent basalt pods, are inferred to have originated by less violent processes.     

The Miocene Costa Giardini diatreme,Iblean Mountains,southern Italy: model for maar-diatreme formation on a submerged carbonate platform

In this paper we present a model for the growth of a maar-diatreme complex in a shallow marine environment. The Miocene-age Costa Giardini diatreme near Sortino, in the region of the Iblei Mountains of southern Sicily, has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions. Vesicular lava clasts, accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas-rich magma. Abundant xenoliths in the upper, late-deposited beds of the ring suggest rapid magma ascent, and deepening of the eruptive vent is shown by the change in slope of the country rock. The interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount. The occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle-lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity. Intrusions of nonvesicular magma, forming plugs and dikes, occur on the western side of the diatreme, and at the margins, close to the contact between breccia deposits and country rock; they indicate involvement of volatile-poor magma, possibly during late stages of activity. We propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment. This allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center. A subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water, at which time the hyaloclastite was deposited. Emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated, locally forming peperites. The sheltered, low-energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts. Ultimately, lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions.     

Two examples of subaqueously welded ash-flow tuffs: the Visean of southern Vosges (France) and the Upper Cretaceous of northern Anatolia (Turkey)

Pyroclastic deposits interpreted as subaqueous ash-flow tuff have been recognized within Archean to Recent marine and lacustrine sequences. Several authors proposed a high-temperature emplacement for some of these tuffs. However, the subaqueous welding of pyroclastic deposits remains controversial.The Visean marine volcaniclastic formations of southern Vosges (France) contain several layers of rhyolitic and rhyodacitic ash-flow tuff. These deposits include, from proximal to distal settings, breccia, lapilli and fine-ash tuff. The breccia and lapilli tuff are partly welded, as indicated by the presence of fiamme, fluidal and axiolitic structures. The lapilli tuff form idealized sections with a lower, coarse and welded unit and an upper, bedded and unwelded fine-ash tuff. Sedimentary structures suggest that the fine-ash tuff units were deposited by turbidity currents. Welded breccias, interbedded in a thick submarine volcanic complex, indicate the close proximity of the volcanic source. The lapilli and fine-ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales. Presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm-wave base.In northern Anatolia, thick massive sequences of rhyodacitic crystal tuff are interbedded with the Upper Cretaceous marine turbidites of the Mudurnu basin. Some of these tuffs are welded. As in southern Vosges, partial welding is attested by the presence of fiamme and fluidal structures. The latter are frequent in the fresh vitric matrix. These tuff units contain a high proportion of vitroclasis, and were emplaced by ash flows. Welded tuff units are associated with non-welded crystal tuff, and contain abundant bioclasts which indicate mixing with water during flowage. At the base, basaltic breccia beds are associated with micritic beds containing a marine fauna. The welded and non-welded tuff sequences are interbedded in an alternation of limestones and marls. These limestones are rich in pelagic microfossils.The evidence above strongly suggest that in both examples, tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment. The sources of the pyroclastic material are unknown in both cases. We propose that the ash flows were produced during submarine fissure eruptions. Such eruptions could produce non-turbulent flows which were insulated by a steam carapace before deposition and welding. The welded ash-flow tuff deposits of southern Vosges and northern Anatolia give strong evidence for existence of subaqueous welding.     

Averno tuff ring in Campi Flegrei (south Italy)

The tuff ring of Averno (3700 years BP) is a wide maar-type, lake-filled volcano which formed during one of the most recent explosive eruptions inside the Campi Flegrei caldera.The eruptive products consist of (a) a basal coarse unit, intercalated ballistic fallout breccia, subplinian pumice deposits and pyroclastic surge bedsets and (b) an upper fine-grained, stratified, pyroclastic surge sequence.During the deposition of the lower unit both purely magmatic (lapilli breccia) and hydromagmatic episodes (wavy and planar bedded, fine ash pyroclastic surge bedsets) coexisted. The hydromagmatic deposits exhibit both erosive and depositional features. The upper unit mostly comprises fine grained, wet pyroclastic surge deposits. The pyroclastic surges were controlled by a highly irregular pre-existing topography, produced by volcano-tectonic dislocation of older tuff rings and cones.Both the upper and lower units show decreasing depletion of fines with increasing distance from the vent. The ballistic fallout layers, however, exhibit only a weak increase in fines with distance from the vent, in spite of marked fining of the lapilli and blocks. The deposits consist dominantly of moderately to highly vesicular juvenile material, generated by primary magmatic volatile driven fragmentation followed by episodes of near-surface magma-water interaction.The evolution of the eruption toward increased fragmentation and a more hydromagmatic character may reflect that the progressive depletion in magmatic volatiles and a decrease in conduit pressure during the last stage of the eruption, possibly associated with a widening of the vent at sea level.     

Deep-bedded ultramafic diatremes in the Missouri River Breaks volcanic field,Montana, USA: 1 km of syn-eruptive subsidence

The ultramafic Eocene Missouri River Breaks volcanic field (MRBVF, Montana, USA) includes over 50 diatremes emplaced in a mostly soft substrate. The current erosion level is 1.3–1.5 km below the pre-eruptive surface, exposing the deep part of the diatreme structures and some dikes. Five representative diatremes are described here; they are 200-375 m across and have sub-vertical walls. Their infill consists mostly of 55-90 % bedded pyroclastic rocks (fine tuffs to coarse lapilli tuffs) with concave-upward bedding, and 45–10 % non-bedded pyroclastic rocks (medium lapilli tuffs to tuff breccias). The latter zones form steep columns 15–135 m in horizontal dimension, which cross-cut the bedded pyroclastic rocks. Megablocks of the host sedimentary formations are also present in the diatremes, some being found 1 km or more below their sources. The diatreme infill contains abundant lithic clasts and ash-sized particles, indicating efficient fragmentation of magma and country rocks. The spherical to sub-spherical juvenile clasts are non-vesicular. They are accompanied by minor accretionary lapilli and armored lapilli. The deposits of dilute pyroclastic density currents are locally observed. Our main interpretations are as follows: (1) the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation; (2) the bedded pyroclastic rocks were deposited on the crater floor, and subsided by 1.0–1.3 km to their current location, with subsidence taking place mostly during the eruption; (3) the observed non-bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material; the debris jets did not empty the mature diatreme, occupying only a fraction of its width, and some debris jets probably did not reach the crater floor; (4) the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth – there was never a 1.3–1.5-km-deep empty hole with sub-vertical walls, otherwise the soft substrate would have collapsed inward, which it only did near the surface, to create the megablocks. We infer that syn-eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0.8–1.1 km or more, after which limited post-eruptive subsidence occurred. This makes the MRBVF diatremes an extreme end-member case of syn-eruptive subsidence in the spectrum of possibilities for maar-diatreme volcanoes worldwide.     

Eruption processes and deposit characteristics at the monogenetic Mt. Gambier Volcanic Complex, SE Australia: implications for alternating magmatic and phreatomagmatic activity

The ～5 ka Mt. Gambier Volcanic Complex in the Newer Volcanics Province, Australia is an extremely complex monogenetic, volcanic system that preserves at least 14 eruption points aligned along a fissure system. The complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner. The facies are (1) coherent to vesicular fragmental alkali basalt (effusive/Hawaiian spatter and lava flows); (2) massive scoriaceous fine lapilli with coarse ash (Strombolian fallout); (3) bedded scoriaceous fine lapilli tuff (violent Strombolian fallout); (4) thin–medium bedded, undulating very fine lapilli in coarse ash (dry phreatomagmatic surge-modified fallout); (5) palagonite-altered, cross-bedded, medium lapilli to fine ash (wet phreatomagmatic base surges); and (6) massive, palagonite-altered, very poorly sorted tuff breccia and lapilli tuff (phreato-Vulcanian pyroclastic flows). Since most deposits are lithified, to quantify the grain size distributions (GSDs), image analysis was performed. The facies are distinct based on their GSDs and the fine ash to coarse+fine ash ratios. These provide insights into the fragmentation intensities and water–magma interaction efficiencies for each facies. The eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles, except for a “magmatic horizon” of effusive activity occurring at both ends of the volcanic complex simultaneously. The eruption foci are located along NW–SE trending lineaments, indicating that the complex was fed by multiple dykes following the subsurface structures related to the Tartwaup Fault System. Possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates, viscosity, crystallinity, degassing and magma discharge rate, as well as hydrological parameters.     

Primary and redeposited facies from a large-magnitude, rhyolitic, phreatomagmatic eruption: Cana Creek Tuff, Late Carboniferous, Australia

The Cana Creek Tuff is one of four rhyolitic ignimbrite members of the Late Carboniferous Currabubula Formation, a volcanogenic conglomeratic braidplain sequence exposed along the western margin of the New England Orogen in northeastern New South Wales. The source is not exposed but was probably located tens of kilometres to the west of existing outcrops. The medial to distal parts of the tuff average about 70 m in thickness, are widespread (minimum present area 1400 km²), and comprise a primary pyroclastic facies (ignimbrite, ash-fall tuff) and a redeposited volcaniclastic facies (sandstone, conglomerate). Both facies are composed of differing proportions of crystal fragments (quartz, plagioclase, K-feldspar), pumiceous clasts (pumice, shards, fine ash), and accidental lithics. The eruption responsible for this unit was explosive and of large magnitude (dense rock equivalent volume about 100 km³). That it was also phreatomagmatic in character is proposed on the basis of: the intimate association of primary and redeposited facies; the presence of accretionary lapilli both in ignimbrite and in ash-fall tuff; the fine grain size of juvenile pyroclasts; the low grade of the ignimbrite; and the close similarity in facies, composition and magnitude to the deposits from the 20,000y. B.P. phreatomagmatic eruption at Taupo, New Zealand (the Wairakei and parts of the Hinuera Formations). The eruption began and ended from a vent with excess water available, possibly submersed in a caldera lake, and generated volcaniclastic sheet floods and debris flows. The emplacement of the primary pyroclastic facies is correlated with an intervening stage when the water:magma mass ratio was lower. The deposits from a large-magnitude, phreatomagmatic eruption are predicted to show systematic lateral variations in facies. Primary pyroclastic facies predominate near the source although the preserved stratigraphy is an incomplete record because of widespread contemporaneous erosion. Volcaniclastic facies, redeposited from proximal sites by floods, dominate at medial and distal locations. In areas hundreds of kilometres from the source, the eruption is registered by thin layers of fine-grained airfall ash.     

Block‐and‐ash flow deposit of the Narcondam Volcano: Product of dacite–andesite dome collapse in the Burma–Java subduction complex

Narcondam Island in the Andaman Sea represents a dacite–andesite dome volcano in the volcanic chain of the Burma–Java subduction complex. The pyroclasts of andesitic composition are restricted to the periphery of the dome predominantly in the form of block‐and‐ash deposits and minor base surge deposits. Besides pyroclastic deposits, andesitic lava occurs dominantly at the basal part of the dome whereas dacitic lava occupies the central part of the dome. The pyroclasts are represented by non‐vesiculated to poorly vesiculated blocks of andesite, lapilli, and ash. The hot debris derived from dome collapse was deposited initially as massive to reversely‐graded beds with the grain support at the lower part and matrix support at the upper part. This sequence is overlain by repetitive beds of lapilli breccia to tuff breccia. These deposits are recognized as a basal avalanche rather than lahar deposit. This basal avalanche was punctuated by an ash‐cloud surge deposit representing a sequence of thinly bedded units of normal graded unit to parallel laminated beds.     

The San Venanzo maar and tuff ring,Umbria, Italy: eruptive behaviour of a carbonatite-melilitite volcano

The late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x10⁶ m³ of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x10⁶ m³ of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO₂ which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions.     

Debris flow deposits within the Palaeogene lava fields of NW Scotland: evidence for mass wasting of the volcanic landscape during emplacement of the Ardnamurchan Central Complex

Coarse fragmental rocks, previously interpreted as primary pyroclastic accumulations infilling flared vents (Richey JE 1938) “The rhythmic eruptions of Ben Hiant, Ardnamurchan, a tertiary volcano. Bull Volcanol” 2(3):1–21), are re-interpreted as predominantly debris flow deposits, with minor hyperconcentrated and stream-flow deposits, temporally and spatially associated with the Palaeogene Ardnamurchan Central Complex (ACC), NW Scotland. These volcaniclastic rocks are conglomerates and breccias, interbedded with siltstones and sandstones, which formed by surface processes on a dissected landscape, developed in response to shallow emplacement of the ACC. Clast-matrix and photo-statistical analyses allow the palaeo-topography and drainage system to be reconstructed and the development of a palaeo-geographic model for the volcanic landscape. Slabs of basalt, dolerite and sandstone were transported as megablocks during catastrophic, gravity-driven events. Lower energy intervals during volcanic hiatuses are marked by lacustrine-fluvial volcaniclastic siltstones and sandstones preserving palynomorph assemblages. We suggest that shallow intrusion is a plausible initiation mechanism for mass wasting in other large igneous provinces. Historically, deposits of the type described here may have been misidentified as vent facies pyroclastic materials.     

Reconstruction of a kimberlite eruption,using an integrated volcanological,geochemical and numerical approach: A case study of the Fox Kimberlite,NWT, Canada

An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. > 40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al₂O₃ and Na₂O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~ 30% loss of fines whereas the column collapse deposit exhibits ~ 50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.     

Capelinhos 1957–1958, Faial, Azores: deposits formed by an emergent surtseyan eruption

The 1957–1958 eruption of Capelinhos, Faial island, Azores, involved three periods of surtseyan, hydromagmatic activity: two in 1957 and one in 1958. Deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally >1 km from the vent. Five lithofacies are identified: lithofacies I is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout. Lithofacies II consists of beds with internally discontinuous lenses, and has sand-wave structures that increase in abundance toward the outer margins of the tuff cone. This lithofacies is interpreted as having been deposited from pyroclastic surges. Lithofacies III is composed of mantle-bedded deposits with laterally discontinuous internal stratigraphy. This lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges. In the outer flanks of the tuff cone, lithofacies III grades laterally into fallout beds of lithofacies I. Lithofacies IV consists of alternating beds of coarse ash aggregates and non-aggregated fine ash, and is particularly well developed in distal regions. Some of this facies was formed by fallout. Alternating beds also occur plastered against obstacles up to 2 km from the vent, indicating an origin from wet pyroclastic surges. The orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated. The distinction between surge and fallout in distal regions is uncertain because wind-drifted fallout and decelerating surge clouds can generate similar deposits. Lithofacies V consists of scoria lapilli beds interpreted to be fallout from hawaiian-style fire-fountaining in the later stages of the eruption. Juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular (25–60% of clasts <30% vesicles). However, on both micro- and macroscopic scales, there is a wide range in clast vesicularity (up to 70% vesicles) indicating that, although fragmentation was predominantly hydromagmatic, vesiculation and magmatic-volatile-driven fragmentation operated simultaneously.     

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.     

Mafic alkalic magmatism in central Kachchh,India: a monogenetic volcanic field in the northwestern Deccan Traps

Magmatism in Kachchh, in the northwestern Deccan continental flood basalt province, is represented not only by typical tholeiitic flows and dikes, but also plug-like bodies, in Mesozoic sandstone, of alkali basalt, basanite, melanephelinite and nephelinite, containing mantle nodules. They form the base of the local Deccan stratigraphy and their volcanological context was poorly understood. Based on new and published field, petrographic and geochemical data, we identify this suite as an eroded monogenetic volcanic field. The plugs are shallow-level intrusions (necks, sills, dikes, sheets, laccoliths); one of them is known to have fed a lava flow. We have found local peperites reflecting mingling between magmas and soft sediment, and the remains of a pyroclastic vent composed of non-bedded lapilli tuff breccia, injected by mafic alkalic dikes. The lapilli tuff matrix contains basaltic fragments, glass shards, and detrital quartz and microcline, with secondary zeolites, and there are abundant lithic blocks of mafic alkalic rocks. We interpret this deposit as a maar-diatreme, formed due to phreatomagmatic explosions and associated wall rock fragmentation and collapse. This is one of few known hydrovolcanic vents in the Deccan Traps. The central Kachchh monogenetic volcanic field has >30 individual structures exposed over an area of ∼1,800 km² and possibly many more if compositionally identical igneous intrusions in northern Kachchh are proven by future dating work to be contemporaneous. The central Kachchh monogenetic volcanic field implies low-degree mantle melting and limited, periodic magma supply. Regional directed extension was absent or at best insignificant during its formation, in contrast to the contemporaneous significant directed extension and vigorous mantle melting under the main area of the Deccan flood basalts. The central Kachchh field demonstrates regional-scale volcanological, compositional, and tectonic variability within flood basalt provinces, and adds the Deccan Traps to the list of such provinces containing monogenetic- and/or hydrovolcanism, namely the Karoo-Ferrar and Emeishan flood basalts, and plateau basalts in Saudi Arabia, Libya, and Patagonia.     

Stratigraphic framework of Holocene volcaniclastic deposits, Akutan Volcano, east-central Aleutian Islands, Alaska

 Akutan Volcano is one of the most active volcanoes in the Aleutian arc, but until recently little was known about its history and eruptive character. Following a brief but sustained period of intense seismic activity in March 1996, the Alaska Volcano Observatory began investigating the geology of the volcano and evaluating potential volcanic hazards that could affect residents of Akutan Island. During these studies new information was obtained about the Holocene eruptive history of the volcano on the basis of stratigraphic studies of volcaniclastic deposits and radiocarbon dating of associated buried soils and peat. A black, scoria-bearing, lapilli tephra, informally named the "Akutan tephra," is up to 2 m thick and is found over most of the island, primarily east of the volcano summit. Six radiocarbon ages on the humic fraction of soil A-horizons beneath the tephra indicate that the Akutan tephra was erupted approximately 1611 years B.P. At several locations the Akutan tephra is within a conformable stratigraphic sequence of pyroclastic-flow and lahar deposits that are all part of the same eruptive sequence. The thickness, widespread distribution, and conformable stratigraphic association with overlying pyroclastic-flow and lahar deposits indicate that the Akutan tephra likely records a major eruption of Akutan Volcano that may have formed the present summit caldera. Noncohesive lahar and pyroclastic-flow deposits that predate the Akutan tephra occur in the major valleys that head on the volcano and are evidence for six to eight earlier Holocene eruptions. These eruptions were strombolian to subplinian events that generated limited amounts of tephra and small pyroclastic flows that extended only a few kilometers from the vent. The pyroclastic flows melted snow and ice on the volcano flanks and formed lahars that traveled several kilometers down broad, formerly glaciated valleys, reaching the coast as thin, watery, hyperconcentrated flows or water floods. Slightly cohesive lahars in Hot Springs valley and Long valley could have formed from minor flank collapses of hydrothermally altered volcanic bedrock. These lahars may be unrelated to eruptive activity. Received: 31 August 1998 / Accepted: 30 January 1999     

Eruptive conditions and depositional processes of Narbona Pass Maar volcano,Navajo volcanic field,Navajo Nation,New Mexico (USA)

Phreatomagmatic deposits at Narbona Pass, a mid-Tertiary maar in the Navajo volcanic field (NVF), New Mexico (USA), were characterized in order to reconstruct the evolution and dynamic conditions of the eruption. Our findings shed light on the temporal evolution of the eruption, dominant depositional mechanisms, influence of liquid water on deposit characteristics, geometry and evolution of the vent, efficiency of fragmentation, and the relative importance of magmatic and external volatiles. The basal deposits form a thick (5–20 m), massive lapilli tuff to tuff-breccia deposit. This is overlain by alternating bedded sequences of symmetrical to antidune cross-stratified tuff and lapilli tuff; and diffusely-stratified, clast-supported, reversely-graded lapilli tuffs that pinch and swell laterally. This sequence is interpreted to reflect an initial vent-clearing phase that produced concentrated pyroclastic density currents, followed by a pulsating eruption that produced multiple density currents with varying particle concentrations and flow conditions to yield the well-stratified deposits. Only minor localized soft-sediment deformation was observed, no accretionary lapilli were found, and grain accretion occurs on the lee side of dunes. This suggests that little to no liquid water existed in the density currents during deposition. Juvenile material is dominantly present as blocky fine ash and finely vesiculated fine to coarse lapilli pumice. This indicates that phreatomagmatic fragmentation was predominant, but also that the magma was volatile-rich and vesiculating at the time of eruption. This is the first study to document a significant magmatic volatile component in an NVF maar-diatreme eruption. The top of the phreatomagmatic sequence abruptly contacts the overlying minette lava flows, indicating no gradual drying-out period between the explosive and effusive phases. The lithology of the accidental clasts is consistent throughout the vertical pyroclastic stratigraphy, suggesting that the diatreme eruption did not penetrate below the base of the uppermost country rock unit, a sandstone aquifer ∼360 m thick. By comparison, other NVF diatremes several tens of kilometers away were excavated to depths of ∼1,000 m beneath the paleosurface (e.g., Delaney PT. Ship Rock, New Mexico: the vent of a violent volcanic eruption. In: Beus SS (ed) Geological society of America Centennial Field Guide, Rocky Mountain Section 2:411–415 (1987)). This can be accounted for by structurally controlled variations in aquifer thickness beneath different regions of the volcanic field. Variations in accidental clast composition and bedding style around the edifice are indicative of a laterally migrating or widening vent that encountered lateral variations in subsurface geology. We offer reasonable evidence that this subsurface lithology controlled the availability of external water to the magma, which in turn controlled characteristics of deposits and their distribution around the vent. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Characteristics of submarine pumice-rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline: field and experimental assessment

This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia, medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish the products of subaerial versus subaqueous eruptions.     

The Sarikavak Tephra, Galatea, north central Turkey: a case study of a Miocene complex plinian eruption deposit

The Sarikavak Tephra from the central Galatean Volcanic Province (Turkey) represents the deposit of a complex multiple phase plinian eruption of Miocene age. The eruptive sequence is subdivided into the Lower-, Middle-, and Upper Sarikavak Tephra (LSKT, MSKT, USKT) which differ in type of deposits, lithology and eruptive mechanisms.The Lower Sarikavak Tephra is characterised by pumice fall deposits with minor interbedded fine-grained ash beds in the lower LSKT-A. Deposits are well stratified and enriched in lithic fragments up to >50 wt% in some layers. The upper LSKT-B is mainly reversely graded pumice fall with minor amounts of lithics. It represents the main plinian phase of the eruption. The LSKT-A and B units are separated from each other by a fine-grained ash fall deposit. The Middle Sarikavak Tephra is predominantly composed of cross-bedded ash-and-pumice surge deposits with minor pumice fall deposits in the lower MSKT-A and major pyroclastic flow deposits in the upper MSKT-B unit. The Upper Sarikavak Tephra shows subaerial laminated surge deposits in USKT-A and subaqueous tephra beds in USKT-B.Isopach maps of the LSKT pumice fall deposits as well as the fine ash at the LSKT-A/B boundary indicate NNE–SSW extending depositional fans with the source area in the western part of the Ovaçik caldera. The MSKT pyroclastic flow and surge deposits form a SW-extending main lobe related to paleotopography where the deposits are thickest.Internal bedding and lithic distribution of the LSKT-A result from intermittent activity due to significant vent wall instabilities. Reductions in eruption power from (partial) plugging of the vent produced fine ash deposits in near-vent locations and subsequent explosive expulsion of wall rock debris was responsible for the high lithic contents of the lapilli fall deposits. A period of vent closure promoted fine ash fall deposition at the end of LSKT-A. The subsequent main plinian phase of the LSKT-B evolved from stable vent conditions after some initial gravitational column collapses during the early ascent of the re-established eruption plume. The ash-and-pumice surges of the MSKT-A are interpreted as deposits from phreatomagmatic activity prior to the main pyroclastic flow formation of the MSKT-B.     

Pre-emergent construction of a lacustrine basaltic volcano,Pahvant Butte,Utah (USA)

 The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1 lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and (b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation, entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition of the mound lithofacies. Received: 10 October 1995 / Accepted: 18 April 1996     

Volcanology of the Aries micaceous kimberlite,central Kimberley Basin,Western Australia

The Neoproterozoic (815.4 ± 4.3 Ma) Aries kimberlite intrudes the King Leopold Sandstone and the Carson Volcanics in the central Kimberley Basin, northern Western Australia. Aries is comprised of a N–NNE-trending series of three diatremes and associated hypabyssal kimberlite dykes and plugs. The diatremes are volumetrically dominated by massive, clast-supported, accidental lithic-rich kimberlite breccias that were intruded by hypabyssal macrocrystic phlogopite kimberlite dykes and plugs with variably uniform- to globular segregationary-textured groundmasses. Lower-diatreme facies, accidental lithic-rich breccias probably formed through fall-back of debris into the vent with a major contribution from the collapse of the vent walls. These massive breccias are overlain by a sequence of bedded volcaniclastic breccias in the upper part of the north lobe diatreme. Abundant, poorly vesicular to nonvesicular, juvenile kimberlite ash and lapilli, with morphologies that are indicative of phreatomagmatic fragmentation processes, occur in a reversely graded volcaniclastic kimberlite breccia unit at the base of this sequence. This unit and overlying bedded accidental lithic-rich breccias are interpreted to be sediment gravity-flow deposits (including possible debris flows) derived from the collapse of the crater walls and/or tephra ring deposits that surrounded the crater. Diatreme-forming eruptions may have been initiated by magma–water interactions along fracture and joint-controlled aquifers within the King Leopold Sandstone. The current level of exposure of the diatremes probably extends from the lower-diatreme facies up into the base of a bedded upper-diatreme sequence.