On the volcanology of the Gibeon Kimberlite Field, Namibia

Kimberlite volcanism in the Upper Cretaceous Gibeon Kimberlite Field, southern Namibia, consisting of at least 42 diatremes and a number of associated dykes, is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the Field. The volcanology of the diatremes and dykes as well as their structural setting is reported here. Because of the paleohydrogeological setting, and since juvenile kimberlite occurring in dykes, intrusive plugs, and spherical lapilli is devoid of vesicles, a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes. Karoo dolerite, basalt and sediment xenoliths in the diatremes provide evidence for the former extent of Karoo strata at the time of eruption.     

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.     

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.     

On the growth of maars and diatremes and its relevance to the formation of tuff rings

Small and large maars exist associated with small and large diatremes, respectively, their subsurface feeder structures. The problem of size and growth of maar-diatreme volcanoes is discussed from a phreatomagmatic point of view from field data, some geophysical data, and short-lived historic maar eruptions. A hydrostatic pressure barrier of usually about 20–30 bars is assumed to control the maximum depth level of explosive magma/groundwater interactions. Similar to the situation in submarine and subglacial volcanism, initial maar-forming water vapour explosions are therefore assumed to occur at shallow depth and to produce a small maar with a shallow diatreme. Because of limited availability of groundwater and ejection of groundwater in the form of steam, the confining pressure barrier is displaced downward. Consequently, water vapour explosions can take place at consecutively deeper levels with the result that the diatreme penetrates downward and grows in size. Since maars are collapse craters resulting from ejection of wallrocks fragmented by water vapour explosions at the level of the diatreme root zone, downward penetration of a diatreme not only results in increase in size of a diatreme but also in increase in size of the overlying maar. As availability of groundwater in limited amounts controls formation of diatremes and their downward penetration, lack of groundwater enables magma to rise within a diatreme and to form a scoria cone or lava lake within the maar, as is frequently found in volcanic fields such as the Eifel area in Germany. In contrast, availability of large amounts of water in near surface environments such as shallow marine, lake, water-rich coastal plains, or water-rich fluviatile gravel beds prevents formation of maars and deep diatremes but causes formation of tuff rings.     

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.     

On the last explosions of carbonatite pipe G3b, Gross Brukkaros, Namibia

 Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick. The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume. Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space. The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above. Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin. Received: 11 October 1996 / Accepted: 6 March 1997     

Maar-diatreme phreatomagmatism at Hopi Buttes,Navajo Nation (Arizona), USA

Remnants of an extensive maar-diatreme volcanic field are magnificently exposed at various depths of erosion in the Hopi Buttes volcanic field of northeastern Arizona. Field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that: (1) lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre-eruptive Mio-Pliocene Bidahochi Formation, and the muddy Bidahochi sediment was soft and wet at the time of maar eruptions; (2) beds higher within the rims contain generally increased proportions of sandstone clasts from the Triassic Wingate Formation. In the diatremes, late-emplaced breccia has deeper-seated lithics than more marginal breccia emplaced earlier; and (3) many vents are topped by megacryst-enriched scoria and spatter, and deep-seated xenoliths are known only from upper diatreme and craterfilling tephra. These observations show that: (1) eruptions at Hopi Buttes involved interaction of magma with unconsolidated mudrock at shallow levels, and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment; (2) the locus of explosive activity migrated downward as eruptions progressed; and (3) the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water.     

The Fekete-hegy (Balaton Highland Hungary) “soft-substrate” and “hard-substrate” maar volcanoes in an aligned volcanic complex – Implications for vent geometry,subsurface stratigraphy and the palaeoenvironmental setting

The Fekete-hegy volcanic complex is located in the centre of the Bakony Balaton Highland Volcanic Field, in the Pannonian Basin, which formed from the late Miocene to Pliocene period. The eruption of at least four very closely clustered maar volcanoes into two clearly distinct types of prevolcanic rocks allows the observation and comparison of hard-substrate and soft-substrate maars in one volcanic complex. The analyses of bedding features, determination of the proportion of accidental lithic clasts, granulometry and age determination helped to identify and distinguish the two types of maar volcanoes. Ascending magma interacted with meteoric water in karst aquifers in Mesozoic carbonates, as well as in porous media aquifers in Neogene unconsolidated, wet, siliciclastic sediments. The divided basement setting is reflected by distinct bedding characteristics and morphological features of the individual volcanic edifices as well as a distinct composition of pyroclastic rocks. Country rocks in hard-substrate maars have a steep angle of repose, leading to the formation of steep sided cone-shaped diatremes. Enlargement and filling of these diatreme is mainly a result of shattering material by FCI related shock waves and wall-rock collapse during downward penetration of the explosion locus. Country rocks in soft-substrate maars have much shallower angles of repose, leading to the formation of broad, bowl shaped structures or irregular depressions. Enlargement and filling of these diatremes is mainly the result of substrate collapse, for example due to liquefaction of unconsolidated material by FCI-related shock waves, and its emplacement by gravity flows. The Fekete-hegy is an important example illustrating that the substrate of a volcanic edifice has to be taken into account as an important interface, which can have major control on phreatomagmatic eruptions and the resulting characteristics of the volcanic complex.     

Root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar–diatreme volcanoes

The understanding of processes within the root zone of maar–diatreme volcanoes is important for the interpretation of the geology, volcanology and even hazard assessment of these volcanoes. In the phreatomagmatic model of pipe formation, the irregularly shaped root zone is the site of the phreatomagmatic explosions, and thus functions as the “engine” for pipe formation. In this model the root zone grows over a period of time in a series of many single thermohydraulic, i.e. phreatomagmatic, explosions. The explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels. The ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole-like fashion into the root zone. Many phreatomagmatic eruptions consequently result in the formation of a cone-shaped diatreme. Thus with ongoing eruptions the cone-shaped diatreme has to grow systematically both in depth and diameter. During its growth, processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone. At the surface, the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter.Thermohydraulic explosions, which fragment both magma and the surrounding country rocks, mostly occur within the bottom part of the root zone. Violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme. In larger pipes, via expansion of the mixture of highly pressurized water vapor, juvenile gas phases and explosively produced tephra, the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt. Diatreme-clearing events in large pipes are difficult or impossible to maintain, since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra. Knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes. The combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types.It is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar–diatreme and related volcanoes.     

A model for the formation of vesiculated tuff by the coalescence of accretionary lapilli

Observations on phreatomagmatic ash deposits of Phlegraean Fields and Vesuvius supply evidence for the origin of vesiculated tuff in a cool environment. Early deposition by fallout of a matrix-free bed of damp accretionary lapilli is followed by deposition of cohesive mud or a mud rain. The lapilli bed becomes partly or completely transformed into a vesiculated tuff by mud percolation and eventual coalescence of accretionary lapilli with consequent trapping of air originally contained in the interstices. The proposed mechanism accounts for vesiculated tuff formation in distal deposits beyond limits commonly attained by pyroclastic surges. This same mechanism may, nevertheless, also operate in proximal tuff-ring and cone deposits during fallout of phreatomagmatic ash separating bed sets in surge-dominated successions. The sequence of events in the proposed model fits well with the evolution of a cooling phreatomagmatic ash cloud in which early ash aggregation (accretionary lapilli fallout) is followed closely by steam condensation (mud or muddy rainfall). This new model invoking a cool-temperature origin is intended to be complementary to previously proposed theories. Although difficult to assess because of the often complete obliteration of original lapilli, the process is believed to be relatively common in the generasion of vesiculated tuffs within phreatomagmatic deposits.     

Englacial tephrostratigraphy of Erebus volcano,Antarctica

A tephrostratigraphy for Erebus volcano is presented, including tephra composition, stratigraphy, and eruption mechanism. Tephra from Erebus were collected from glacial ice and firn. Scanning electron microscope images of the ash morphologies help determine their eruption mechanisms The tephra resulted mainly from phreatomagmatic eruptions with fewer from Strombolian eruptions. Tephra having mixed phreatomagmatic–Strombolian origins are common. Two tephra deposited on the East Antarctic ice sheet, ~ 200 km from Erebus, resulted from Plinian and phreatomagmatic eruptions. Glass droplets in some tephra indicate that these shards were produced in both phreatomagmatic and Strombolian eruptions. A budding ash morphology results from small spheres quenched during the process of hydrodynamically splitting off from a parent melt globule. Clustered and rare single xenocrystic analcime crystals, undifferentiated zeolites, and clay are likely accidental clasts entrained from a hydrothermal system present prior to eruption. The phonolite compositions of glass shards confirm Erebus volcano as the eruptive source. The glasses show subtle trends in composition, which correlate with stratigraphic position. Trace element analyses of bulk tephra samples show slight differences that reflect varying feldspar contents.     

Mixed deposits of simultaneous strombolian and phreatomagmatic volcanism: Rothenberg volcano, east Eifel volcanic field

The Pleistocene basanite-tephrite Rothenberg cone complex in the East Eifel was constructed by alternating dominantly Strombolian (S1–3) and dominantly phreatomagmatic (P1–3) phases of volcanism along a NNE-SSW linear vent system. Strombolian eruptions, from the central vent of the S1 scoria cone, and phreatomagmatic eruptions, from a vent on the southern margin of the cone, occurred simultaneously during the second phreatomagmatic phase (P2). The P2 deposits are a complex sequence in which Strombolian fallout ejecta is intimately admixed with phreatomagmatic fallout and pyroclastic surge material. Every bed contains at least trace amounts of ejecta from both sources but, at every site, an alternation of Strombolian-dominant and phreatomagmatic-dominant units is recorded. Each bed also shows marked lateral changes with a progressive northward increase in the proportion of Strombolian material. The two eruptive styles produced morphologically distinct clast populations often with widely separated (5–7 φ) grain size modes. The phreatomagmatic component of the P2 deposits is inferred to be the result of shallow interaction of external water and cool, partially degassed magma which reached the surface at a time when the magma column was retreating from the northern Strombolian central vent.The Rothenberg deposits illustrate the complexity and sensitivity of controls on Strombolian and associated phreatomagmatic volcanism, and the shallow depth of fragmentation during such eruptions. During such shallow eruptions minor, ephemeral and localised variations in the rate of rise and discharge of magma, and vent geometry and hydrology significantly influence the magma:water ratio and hence eruptive style.     

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.     

On the origin of diamantiferous diatremes

Diamantiferous diatremes usually occur in the old platforms and shields where deep fractures are «blind»,i.e., these fractures do not come out to the earth surface. Alkaline-ultrabasic magma ascending along these fractures and encountering an impervious cap of sedimentary and/or volcanic rocks had formed, between the cap and the basemnet rocks, intermediate chambers in which the crystallization of diamonds took place. Under the influence of the increasing pressures in these chambers, the roofs were destroyed and diamantiferous diatremes, dykes and veins of kimberlite have been formed. These diatremes are filled with a typical eruptive breccia in which the fragmental material, formed by the destructive explosion of the magma chamber roof, is cemented by a porphyritic, alkaline-ultrabasic rock known under the name of kimberlite.     

Eruption style and petrology of a new carbonatitic suite from the Mt. Vulture Southern Italy/: The Monticchio Lakes Formation

The Monticchio Lakes Formation MLF is a newly identified carbonatite-melilitite tuff sequence which is exposed in the southwestern sector of the Vulture volcano. It is the youngest example ca. 0.13 m.y. of this type of volcanism in Italy, although other carbonatites of smaller volume, but with similar characteristics, have been discovered recently. This volcanic event occurred in isolation after a 0.35 m.y. period of inactivity at Vulture. The eruption produced two maar-type vents and formed tuff aprons mainly composed of dune beds of lapilli. Depositional features suggest that a dry surge mechanism, possibly triggered by CO₂ expansion, was dominant during tuff emplacement. The MLF event involved a mixture of carbonatite and melilitite liquids which were physically separated before the eruption. Abundant mantle xenoliths are direct evidence of the deep-seated origin of the parental magma and its high velocity of propagation towards the surface. Often, these nodules form the core of lapilli composed of concentric shells of melilitite andror porphyritic carbonatite. Coarse-ash beds alternate with lapilli beds and consist of abundant lumps and spherulae of very fine-grained calcite immersed in a welded, highly compacted carbonatite matrix. Porphyritic carbonatite shells of the lapilli and fine-grained spherulae of calcite in the tuff matrix suggest incipient crystallisation of a carbonatite liquid in subvolcanic conditions and eruption of carbonatite-spray droplets. Dark coloured juvenile fragments mainly consist of melilite, phlogopite, calcite, apatite, perovskite, and häuyne crystals in a carbonatite or melilitite matrix. The rocks have an extremely primitive, ultramafic composition with very high Mga) 85. and Cr and Ni content 1500 ppm-. The calcite contains high SrO, BaO and REE of up to 1.5 wt.%. Similar compositions are typical of primary, magmatic carbonates which are found in both intrusive and extrusive carbonatites. The high modal Sr-Ba-REE-rich calcite, the typical mineralogy, and the high amount of Sr-group elements identify the carbonate component as a carbonatite. The very high Mga, mantle debris and C, O, He isotope ratios in the range of mantle values indicate a near-primary character for the carbonatite which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas.     

Morphologic features of juvenile pyroclasts from magmatic and phreatomagmatic deposits of Vesuvius

Three eruptive sequences of historical and recent activity of Vesuvius were carefully studied using scanning electron microscopy analysis techniques. The aim of this study was to characterize and distinguish deposits from magmatic and phreatomagmatic eruption phases.The sample pretreatment methods from previous authors were reviewed and a washing technique was checked, making it possible to obtain easily examinable samples without modifying their morphologic features.In each stratigraphic sequence the single samples were examined and their shape, type of vesiculation, and state of glass (edge modification by abrasion, alteration, presence of aggregates, secondary minerals and coatings) were analyzed and described.Characteristic features were recognized for deposits from different eruptive phases. Samples from phreatomagmatic deposits show the following features:

• - glass alteration;

• - presence of secondary minerals on the external surfaces or inside the cavities;

• - coatings;

• - presence of aggregate formed by juvenile and lithic particles.

Other features, indicated by many authors as resulting from magma-water interaction, can arise from different mechanisms and cannot discriminate between phreatomagmatic and magmatic nature of deposits.The juvenile clasts from the phreatomagmatic phases of these eruptions of Vesuvius always show primary vesiculation; this observation supports the presence of fragmented magma at the time when interaction occurred.The conclusions from SEM observations are in perfect agreement with the results of granulometric and component analysis on the same eruptive sequences.     

琼北射气岩浆喷发力学机制探讨

琼北第四纪火山区分布着为数众多、大小不一的射气岩浆喷发成因的低平火山口。上升的岩浆遇到地下水会发生射气岩浆喷发,形成地表出露的低平火山口,并伴随出现基浪堆积物。根据低平火山口的半径、爆炸发生时上覆地层的厚度、岩浆与地下水接触面的半径等参数,运用弹性力学基本原理建立了简单的喷发模型,初步模拟爆破冲击力与上述各参数之间的关系和变化规律,并计算出上覆地层中任一点的应力状态,初次探讨了射气岩浆喷发的力学机制     

Reconstructing the eruption magnitude and energy budgets for the pre-historic eruption of the monogenetic ∼5 ka Mt. Gambier Volcanic Complex, south-eastern Australia

Understanding explosive volcanic eruptions, especially phreatomagmatic eruptions, their intensities and energy budgets is of major importance when it comes to risk and hazard studies. With only a few historic occurrences of phreatomagmatic activity, a large amount of our understanding comes from the study of pre-historic volcanic centres, which causes issues when it comes to preservation and vegetation. In this research, we show that using 3D geometrical modelling it is possible to obtain volume estimates for different deposits of a pre-historic, complex, monogenetic centre, the Mt. Gambier Volcanic Complex, south-eastern Australia. Using these volumes, we further explore the energy budgets and the magnitude of this eruption (VEI 4), including dispersal patterns (eruption columns varying between 5 and 10 km, dispersed towards north-east to south), to further our understanding of intraplate, monogenetic eruptions involving phreatomagmatic activity. We also compare which thermodynamic model fits best in the creation of the maar crater of Mt. Gambier: the major-explosion-dominated model or the incremental growth model. In this case, the formation of most of the craters can best be explained by the latter model.     

Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits

The Quaternary Vakinankaratra volcanic field in the central Madagascar highlands consists of scoria cones, lava flows, tuff rings, and maars. These volcanic landforms are the result of processes triggered by intracontinental rifting and overlie Precambrian basement or Neogene volcanic rocks. Infrared-stimulated luminescence (IRSL) dating was applied to 13 samples taken from phreatomagmatic eruption deposits in the Antsirabe–Betafo region with the aim of constraining the chronology of the volcanic activity. Establishing such a chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption deposits and IRSL ages suggest at least five phreatomagmatic eruption events in Late Pleistocene times. In the Lake Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109?±?15 and 90?±?11 ka and is possibly related to an eruption at the Amboniloha volcanic complex to the north. The younger one gives ages between 58?±?4 and 47?±?7 ka and is clearly related to the phreatomagmatic eruption that formed Lake Andraikiba. IRSL ages of a similar eruption deposit directly overlying basement laterite in the vicinity of the Fizinana and Ampasamihaiky volcanic complexes yield coherent ages of 68?±?7 and 65?±?8 ka. These ages provide the upper age limit for the subsequently developed Iavoko, Antsifotra, and Fizinana scoria cones and their associated lava flows. Two phreatomagmatic deposits, identified near Lake Tritrivakely, yield the youngest IRSL ages in the region, with respective ages of 32?±?3 and 19?±?2 ka. The reported K-feldspar IRSL ages are the first recorded numerical ages of phreatomagmatic eruption deposits in Madagascar, and our results confirm the huge potential of this dating approach for reconstructing the volcanic activity of Late Pleistocene to Holocene volcanic provinces.     

Generation and dispersal of fine ash and dust by volcanic eruptions

The volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite-forming, plinian, vulcanian and phreatomagmatic types; these are also the eruptions which produce the widest dispersal of this material, attributed to the superior height attained by their eruptive columns. However, much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water, particularly in phreatomagmatic eruptions. Recent studies made on the dispersal and grain-size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types, have yielded estimates of the quantities of material generated in each grain-size class, besides the extent of their dispersal. Not all of the fine volcanic particles are produced by fragmentation at the eruptive vent; in ignimbrite eruptions, there is good evidence for their large-scale generation in and loss from the moving ash flows.