Origins and energetics of maar volcanoes: examples from the ultrapotassic Sabatini Volcanic District (Roman Province,Central Italy)

Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water. Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions, eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from either monogenetic or polygenetic maars ranged 0.004–0.07 km³ during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 10¹⁵–4 × 10¹⁷ J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation.     

A new model for the fracture energy budget of phreatomagmatic explosions

The fragmentation of magma and of the hosting country rocks is a major process in explosive eruptions. It is important to quantify the mechanical energy needed for fragmentation in order to assess the physical processes of this volcanic phenomenon. This paper presents a method to calculate the fragmentation energy of country rock using granulometry data of a typical phreatomagmatic Eifel maar volcano explosion. The total fracture area of country rock fragments in one tephra layer was quantified and related to the critical fragmentation energy of these country rocks. The rock parameters critical shear stress and critical fragmentation energy were determined experimentally, whereas the pre-volcanic crack inventory was measured in the field. The paper concludes with the calculation of the energy balance (i.e. partitioning of thermal energy into kinetical energy and mechanical energy of the fragmentation) of one Eifel maar volcanic explosion.     

The La Breña — El Jagüey Maar Complex,Durango, México: I. Geological evolution

The La Breña — El Jagüey Maar Complex, of probable Holocene age, is one of the youngest eruptive centers in the Durango Volcanic Field (DVF), a Quaternary lava plain that covers 2100 km² and includes about 100 cinder and lava cones. The volcanic complex consists of two intersecting maars — La Breña and El Jagüey — at least two pre-maar scoria cones and associated lavas, and a series of nested post-maar lava and scoria cones that erupted within La Breña Maar and flooded its floor with lava to form one or more lava lakes. We believe that El Jagüey Maar formed first, but pyroclastic deposits associated with its formation are exposed at only a few places in the lower maar walls. A perennial lake in the bottom of El Jagüey marks the top of an aquifer about 60 m below the lava plain. Interaction of the rising basanitic magmas with this aquifer was probably responsible for the hydromagmatic eruptions at the maar complex. In the southeastern quadrant of La Breña and in most parts of El Jagüey, the upper maar walls expose a thick pyroclastic sequence of tuffs, tuff breccias, and breccias that is dominated by thinly layered sandwave and plane-parallel surge beds and contains minor interlayered scoria-fall horizons. We conclude that these deposits in the upper walls of both maars erupted during the formation of La Breña, based on: (1) thickness variations in a prominent scoria-fall marker bed interlayered with the surge deposits; (2) inferred transport directions for ballistic clasts, channels, and dune-like bedforms; and (3) lateral facies changes in the surge deposits. Some of the surge clouds from La Breña apparently travelled down the inner southwestern wall of El Jagüey, fanned out across its floor, and climbed up the opposite walls before emerging onto the surrounding lava plain. These clouds deposited steep, inward-dipping surge deposits along the lower walls of El Jagüey. Following this hydromagmatic phase, which was responsible for the formation of the maars, a series of strombolian eruptions took place from vents within La Breña. At many places along the maar rims these eruptions completely buried the surge beds under a thick sequence of post-maar scoriae and ashes. The outer flanks of the maar complex and the surrounding lava plain are also blanketed by post-maar ashes. The final phase of activity involved effusive eruptions of post-maar lavas from vents on the floor of La Breña. The evolutionary sequence from hydromagmatic eruptions during formation of the maars, through strombolian eruptions of the post-maar scoriae and ashes, and finally to the post-maar lavas appears to reflect the declining influence of magma-groundwater interactions with time. Basanitic magmas from all eruptive stages carried spinel-lherzolite and feldspathic-granulite xenoliths to the surface. The La Breña — El Jagüey Maar Complex contains the only known hydromagmatic vents in the DVF and the largest spinel-lherzolite xenoliths, which range up to 30 cm diameter. These two observations indicate an unusually rapid ascent rate for these basanitic magmas compared to those from other DVF vents.     

琼北地区晚更新世射气岩浆喷发初步研究

琼北地区晚更新世射气岩浆喷发形成众多的低平火山口 ,出露典型的基浪堆积物 ,在火口垣露头上可清晰地观察到大型低角度交错层理、板状层理和波状层理以及远源相的球粒状增生火山砾。玄武质岩浆在上升过程中遇水爆炸形成低平火山口及基浪堆积 ,为认识琼北地区新生代以来的火山活动规律和琼北 -雷南地区的构造环境 ,以及未来火山灾害预测提供了重要的依据     

Stratigraphy and eruptive dynamics of a pulsating Plinian eruption of Somma-Vesuvius: the Pomici di Mercato (8900 years B.P.)

New volcanological studies allow reconstruction of the eruption dynamics of the Pomici di Mercato eruption (ca 8,900 cal. yr B.P.) of Somma-Vesuvius. Three main Eruptive Phases are distinguished based on two distinct erosion surfaces that interrupt stratigraphic continuity of the deposits, indicating that time breaks occurred during the eruption. Absence of reworked volcaniclastic deposits on top of the erosion surfaces suggests that quiescent periods between eruptive phases were short perhaps lasting only days to weeks. Each of the Eruptive Phases was characterised by deposition of alternating fall and pyroclastic density current (PDC) deposits. The fallout deposits blanketed a wide area toward the east, while the more restricted PDC deposits inundated the volcano slopes. Eruptive dynamics were driven by brittle magmatic fragmentation of a phonolitic magma, which, because of its mechanical fragility, produced a significant amount of fine ash. External water did not significantly contribute either to fragmentation dynamics or to mechanical energy release during the eruption. Column heights were between 18 and 22 km, corresponding to mass discharge rates between 1.4 and 6 × 10⁷ kg s⁻¹. The estimated on land volume of fall deposits ranges from a minimum of 2.3 km³ to a maximum of 7.4 km³. Calculation of physical parameters of the dilute pyroclastic density currents indicates speeds of a few tens of m s⁻¹ and densities of a few kg m⁻³ (average of the lowermost 10 m of the currents), resulting in dynamic pressures lower than 3 kPa. These data suggest that the potential impact of pyroclastic density currents of the Pomici di Mercato eruption was smaller than those of other Plinian and sub-Plinian eruptions of Somma-Vesuvius, especially those of 1631 AD and 472 AD (4–14 kPa), which represent reference values for the Vesuvian emergency plan. The pulsating and long-lasting behaviour of the Pomici di Mercato eruption is unique in the history of large explosive eruptions of Somma-Vesuvius. We suggest an eruptive scheme in which discrete magma batches rose from the magma chamber through a network of fractures. The injection and rise of the different magma batches was controlled by the interplay between magma chamber overpressure and local stress. The intermittent discharge of magma during a large explosive eruption is unusual for Somma-Vesuvius, as well as for other volcanoes worldwide, and yields new insights for improving our knowledge of the dynamics of explosive eruptions.     

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.     

The structural position of earthquakes relative to elements of deep-seated fault tectonics and their interpretation for the Magadan shelf of the Sea of Okhotsk

This paper presents a quantitative analysis of the relationship between earthquakes and crustal tectonic fragmentation based on a correlation analysis of fault density and discordance measure with parameters of seismic activity (the specific number and specific energy of earthquakes) for the Magadan shelf of the Sea of Okhotsk. These materials revealed essential differences in the structural position of earthquakes on land and in sea. The Magadan shelf of the Sea of Okhotsk will most likely generate earthquakes of energy class K ≥ 12 in areas with lower density (0.04 < τ ≤ 0.06 km⁻¹) and lower discordance measure (2 < ‖D‖ ≤ 4) for the faults identified from gravity data. One cause of this structural and geodynamic feature in the spatial position of earthquake epicenters is, in these authors’ opinion, thermal isostasy, that is, the cooling of the lithosphere and asthenosphere as heat is released into the space around the Earth (the heat was entering the upper layers of the Earth from the mantle during the Mesozoic/Cenozoic phase of its development), resulting in seafloor subsidence. Seafloor subsidence and continental uplift produce rotational tangential forces that affect the stress buildup in the Pacific seismic belt. The annual releases of rotational energy and earthquake energy have the same order of magnitude, 10¹⁸ J/yr.     

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption.During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity.The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport.The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.     

Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma

Sub-Plinian to Plinian eruptions of basic magma present a challenge to modeling volcanic behavior because many models rely on magma becoming viscous enough during ascent to behave brittlely and cause fragmentation. Such models are unable, however, to strain low viscosity magma fast enough for it to behave brittlely. That assumes that such magmas actually have low viscosities, but the rare Plinian eruptions of basic magma may in fact result from them being anomalously viscous. Here, we examine two such eruptions, the 122 B.C. eruption of hawaiitic basalt from Mt. Etna and the late Pleistocene eruption of basaltic andesite from Masaya Caldera, to test whether they were anomalously viscous. We carried out hydrothermal experiments on both magmas and analyzed glass inclusions in plagioclase phenocrysts from each to determine their most likely pre-eruptive temperatures and water contents. We find that the hawaiite was last stored at 1,000–1,020°C, whereas the basaltic andesite was last stored at 1,010–1,060°C, and that both were water saturated with ∼3.0 wt.% water dissolved in them. Such water contents are not high enough to trigger Plinian explosive behavior, as much more hydrous basic magmas erupt less violently. In addition, despite being relatively cool, the viscosities of both magmas would range from ∼10^2.2–2.5 Pa s before erupting to ∼10⁴ Pa s when essentially degassed, all of which are too fluid to cause brittle disruption. Without invoking special external forces to explain all such eruptions, one of the more plausible explanations is that when the bubble content reaches some critical value the fragile foam-like magma disrupts. The rarity of Plinian eruptions of basic magma may be because such magmas must ascend fast enough to retain their bubbles.     

Deep explosive focal depths during maar forming magmatic-hydrothermal eruption: Baccano Crater,Central Italy

We describe the eruptive activity of the Pleistocene composite Baccano maar crater in the Sabatini Volcanic Complex (Central Italy) combining stratigraphy, grain size/componentry and rare earth element and Yttrium (REY) composition of its eruptive products with the stratigraphy and geothermal data derived from deep wells drilled on the Baccano structural high. The main lithological characteristics of the basal Baccano maar pyroclastic deposit, composed of more than 60% wt of non-thermometamorphosed lithic clasts from the sedimentary basement, show that the first eruption was magmatic-hydrothermal in nature. The lithology of the sedimentary lithic clasts indicates that the fragmentation level was at a depth of −1,000 to −1,200 m, with fragment depth verified by deep well stratigraphy. The 15% wt juvenile non-vesicular glass components suggest that magma played a minor role in powering the eruption. Assuming that the high-salinity hot hydrothermal fluids (365<T<410°C and P∼25 MPa), hosted in the highly permeable and confined aquifer below the Baccano maar are representative of those at the time of the eruption, we propose that hydrofracturing would have triggered the eruption caused by overpressure at the top of the geothermal aquifer. REY analysis performed on pyroclastic fragments and basement rocks suggest that partial dissolution of the deeper limestones (>−1,400 m) by the aggressive hydrothermal fluids enriched in acid components (HF, HCl, and H2SO4) may have contributed to increased CO₂ partial pressure that helped to drive the hydrofracturing. This could have caused rapid vapour separation and pressure drop, allowing the almost simultaneous breaking of the aquifer cover and brecciation of the calcareous units down to −1,000 to −1,200 m depth. The relative abundance of calcareous lithics in the basal part of the first Baccano eruptive unit, representing about the upper 200 m of stratigraphy below the top of the Baccano structural high, reveals the descent of the piezometric surface during the eruption. Combining deep well information and maar product stratigraphy, using also REY data from maar pyroclastic fragments and the basement rocks we draw an interpretative model for the Baccano maar-forming eruption, concluding that a) magmatic-hydrothermal eruptions may originate deeper than previously thought, and b) hydrothermal fluids circulating in limestone aquifers may play an important role in triggering such eruptions.     

Evaluation methods of heat discharge and their applications to the major active volcanoes in Japan

The rates at which thermal energy is released by non-eruptive mechanisms associated with active volcanoes in Japan have been estimated from surface temperature distributions or from shapes of the fumarolic plume rise. Values of 5.3 x 10⁷ W and 1.2 x 10⁸ W have been calculated for non-eruptive release rates in 100-km segments of the Northeast and the Southwest Japan Arc, respectively. It appears that non-eruptive heat discharge is of the same order as that caused by eruptions.     

Influence de la Nature des Magmas sur l’Activité Phréatomagmatique: Approche Volcanologique et Thermodynamique

Many volcanic forms resulting from phreatomagmatic eruptions of differentiated magmas have been studied in the Massif Central (France), in the Phlegrean Fields (Italy), and on Saõ Miguel island (Azores). They show a continuous series between explosion crater maar type — and the hyaoloclastic tuff-cone. An essential feature of this morphological series is the preponderance of tuff-rings resulting from subaerial eruptions. Subaerial tuff-rings of basic compositions are less common than maars. A thermodynamic approach shows that the quantity of heat supplied by the different kinds of magmas and the water / magma ratio are the essential parameters controlling the activity, and the resulting morohology of these volcanoes.     

Plume dynamics, thermal energy and long-distance transport of vulcanian eruption clouds from Augustine Volcano, Alaska

Augustine, an island volcano in Lower Cook Inlet, southern Alaska, erupted in January, 1976, after 12 years of dormancy. By April, when the eruptions ended, a new lava dome had been extruded into the summit crater and about 0.1 km³ of pyroclastics had been deposited on the island, mainly as pyroclastic debris avalanches and pumice flows. The ventclearing phase in January was highly explosive and we have been able to document 13 major vulcanian eruptions.The timing, thermal energy, mass loading of fine particles and the horizontal dispersion of these eruption clouds were determined from radar measurements of cloud height, reports of pilots flying in plumes, satellite photography, seismic records and infrasonic detection of air waves. A lower estimate of the mass of fine (r < 68 μm) particles injected into the troposphere from the 13 main eruptions in January is 5.5–18 × 10¹² g. The corresponding mass loading of fine particles within individual eruption clouds is 0.3–1 g m⁻³. We calculated thermal energies of 4 × 10¹⁴ to 35 × 10¹⁴ J for individual eruptions by applying convective plume rise theory to observed cloud heights and seismically determined eruption durations. This energy range compares favorably with the 4–16 × 10¹⁴ J of thermal energy, calculated from the cooling of juvenile material contained in a typical eruption cloud.The vulcanian eruption clouds stayed intact for at least 700 km downwind. Satellite images in both visible and infrared wavebands, showing the Gulf of Alaska just after sunrise on January 23, reveal a series of puffs strung out downwind from the volcano, 20–30 km in diameter and with their tops at altitudes of about 8 km, overlying a continuous plume at altitude 4 km. Each puff corresponded to a seismically and infrasonically timed eruption. A substantial portion of the material injected into the atmosphere between January 22 and 25 was rapidly transported by the subpolar jet stream through southwestern Canada and the western United States, then northeast across the States into the Atlantic. The clouds were observed passing over Tucson, Arizona, on January 25 at an elevation of 7 km.Several of the eruptions penetrated into the stratosphere. Sun photometer measurements, taken at Mauna Loa, Hawaii, six weeks after the eruption, showed an increased stratospheric optical thickness of 0.01 (wavelength 0.5 μm), which decayed in about 5 months. The maximum column mass loading of the veil was 4–10 × 10⁻⁷ g cm⁻². The mass of the veil, spread-ever a fourth of the earth's surface, is 10 to 100 times larger than can be accounted for by assuming that injected ash and converted sulfate particles from the 13 main Augustine eruptions are the only components contributing to the stratospheric turbidity observed at Mauna Loa.     

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.     

Evolution of the Quaternary melitite-nephelinite Herchenberg volcano (East Eifel)

The Quaternary Herchenberg composite tephra cone (East Eifel, FR Germany) with an original bulk volume of 1.17·10⁷ m³ (DRE of 8.2·10⁶ m³) and dimensions of ca. 900·600·90 m (length·width·height) erupted in three main stages: (a) Initial eruptions along a NW-trending, 500-m-long fissure were dominantly Vulcanian in the northwest and Strombolian in the southeast. Removal of the unstable, underlying 20-m-thick Tertiary clays resulted in major collapse and repeated lateral caving of the crater. The northwestern Lower Cone 1 (LC1) was constructed by alternating Vulcanian and Strombolian eruptions. (b) Cone-building, mainly Strombolian eruptions resulted in two major scoria cones beginning initially in the northwest (Cone 1) and terminating in the southeast (Cones 2 and 3) following a period of simultaneous activity of cones 1 and 2. Lapilli deposits are subdivided by thin phreatomagmatic marker beds rich in Tertiary clays in the early stages and Devonian clasts in the later stages. Three dikes intruded radially into the flanks of cone 1. (c) The eruption and deposition of fine-grained uppermost layers (phreatomagmatic tuffs, accretionary lapilli, and Strombolian fallout lapilli) presumably from the northwestern center (cone 1) terminated the activity of Herchenberg volcano. The Herchenberg volcano is distinguished from most Strombolian scoria cones in the Eifel by (1) small volume of agglutinates in central craters, (2) scarcity of scoria bomb breccias, (3) well-bedded tephra deposits even in the proximal facies, (4) moderate fragmentation of tephra (small proportions of both ash and coarse lapilli/bomb-size fraction), (5) abundance of dense ellipsoidal juvenile lapilli, and (6) characteristic depositional cycles in the early eruptive stages beginning with laterally emplaced, fine-grained, xenolith-rich tephra and ending with fallout scoria lapilli. Herchenberg tephra is distinguished from maar deposits by (1) paucity of xenoliths, (2) higher depositional temperatures, (3) coarser grain size and thicker bedding, (4) absence of glassy quenched clasts except in the initial stages and late phreatomagmatic marker beds, and (5) predominance of Strombolian, cone-building activity. The characteristics of Herchenberg deposits are interpreted as due to a high proportion of magmatic volatiles (dominantly CO₂) relative to low-viscosity magma during most of the eruptive activity.     

Sub-surface dynamics and eruptive styles of maars in the Colli Albani Volcanic District, Central Italy

Eruptive scenarios associated with the possible reactivation of maar-forming events in the Quaternary, ultrapotassic Colli Albani Volcanic District (CAVD) provides implications for volcanic hazard assessment in the densely populated area near Rome. Based on detailed stratigraphy, grain size, componentry, ash morphoscopy and petro-chemical analyses of maar eruption products, along with textural analysis of cored juvenile clasts, we attempt to reconstruct the eruptive dynamics of the Prata Porci and Albano maars, as related to pre- and syn-eruptive interactions between trachybasaltic to K-foiditic feeder magmas and carbonate–silicoclastic and subvolcanic country rocks. Magma volumes in the order of 0.5–3.1 × 10⁸ m³ were erupted during the monogenetic Prata Porci maar activity and the three eruptive cycles of the Albano multiple maar, originating loose to strongly lithified, wet and dry pyroclastic surge deposits, Strombolian scoria fall horizons and lithic-rich explosion breccias. These deposits contain a wide range of accessory and accidental lithic clasts, with significant vertical stratigraphic variations in the lithic types and abundances. The two maar study cases hold a record of repeated transitions between magmatic (i.e, Strombolian fallout) and hydromagmatic (wet and dry pyroclastic surges) activity styles. Evidence of phreatic explosions, a common precursor of explosive volcanic activity, is only found at the base of the Prata Porci eruptive succession. The quantitative evaluation of the proportions of the different eruptive styles in the stratigraphic record of the two maars, based on magma vs. lithic volume estimates, reveals a prevailing magmatic character in terms of erupted magma volumes despite the hydromagmatic footprint. Different degrees of explosive magma–water interaction were apparently controlled by the different hydrogeological and geological–structural settings. In the Prata Porci case, shifts in the depth of magma fragmentation are proposed to have accompanied eruption style changes. In the Albano case, a deeply dissected geothermal aquifer in peri-caldera setting and variable mass eruption rates were the main controlling factors of repeated shifts in the eruptive style. Finally, textural evidence from cored juvenile clasts and analytical modeling of melt–solid heat transfer indicate that the interacting substrate in the Prata Porci case was at low, uniform temperature (~ 100 °C) as compared to the highly variable temperatures (up to 700–800 °C) inferred for the geothermal system beneath Albano.     

Eruption dynamics of Hawaiian-style fountains: the case study of episode 1 of the Kīlauea Iki 1959 eruption

Hawaiian eruptions are characterized by fountains of gas and ejecta, sustained for hours to days that reach tens to hundreds of meters in height. Quantitative analysis of the pyroclastic products from the 1959 eruption of Kīlauea Iki, Kīlauea volcano, Hawai‘i, provides insights into the processes occurring during typical Hawaiian fountaining activity. This short-lived but powerful eruption contained 17 fountaining episodes and produced a cone and tephra blanket as well as a lava lake that interacted with the vent and fountain during all but the first episode of the eruption, the focus of this paper. Microtextural analysis of Hawaiian fountaining products from this opening episode is used to infer vesiculation processes within the fountain and shallow conduit. Vesicle number densities for all clasts are high (10⁶–10⁷ cm⁻³). Post-fragmentation expansion of bubbles within the thermally-insulated fountain overprints the pre-fragmentation bubble populations, leading to a reduction in vesicle number density and increase in mean vesicle size. However, early quenched rims of some clasts, with vesicle number densities approaching 10⁷ cm⁻³, are probably a valid approximation to magma conditions near fragmentation. The extent of clast evolution from low vesicle-to-melt ratio and corresponding high vesicle number density to higher vesicle-to-melt ratio and lower vesicle-number density corresponds to the length of residence time within the fountain.     

Gravitational energy of core evolution: implications for thermal history and geodynamo power

Using density–pressure relationships for mantle silicate and core alloy closely matching PREM we have constructed six models of the Earth in different evolutionary states. Gravitational energies and elastic strain energies are calculated for models with homogeneous composition, separated mantle and liquid core, separated inner and outer cores with the inner core either liquid or solid and models with increased densities, representing cooling of either the mantle or core. In this way we have isolated the gravitational energy released by each of several evolutionary processes and subtracted the consequent increase in strain energy to obtain the net energy released as heat or geodynamo power. Radiogenic heat (∼7.8×10³⁰ J) is found to contribute only about 25% of the total heat budget, the balance originating as residual gravitational energy from the original accretion and from core separation (14×10³⁰ J). The total energy of compositional convection, driven by inner core formation, is 3.68×10²⁸ J and this is the most important (or even the only) energy source for the dynamo for the most recent 2 billion years. It appears unlikely that the inner core existed much before that time. The total net (gravitational minus strain) energy released in the core by the process of inner core formation, 11.92×10²⁸ J, is not much less than the thermal energy released in this process, 15.1×10²⁸ J. In the mantle the net (gravitational minus strain) energy released by thermal contraction is about 20% of the heat release. All of the numerical results are presented in a manner that allows simple rescaling to any revised density estimates.     

A new method for the determination of the specific kinetic energy (SKE) released to pyroclastic particles at magmatic fragmentation: theory and first experimental results

Brittle magmatic fragmentation plays a crucial role in explosive eruptions. It represents the starting point of hazardous explosive events that can affect large areas surrounding erupting volcanoes. Knowing the initial energy released during this fragmentation process is fundamental for the understanding of the subsequent dynamics of the eruptive gas-particle mixture and consequently for the forecasting of the erupting column’s behavior. The specific kinetic energy (SKE) of the particles quantifies the initial velocity shortly after the fragmentation and is therefore a necessary variable to model the gas-particle conduit flow and eruptive column regime. In this paper, we present a new method for its determination based on fragmentation experiments and identification of the timings of energy release. The results obtained on compositions representative for basaltic and phonolitic melts show a direct dependence on magma material properties: poorly vesiculated basaltic melts from Stromboli show the highest SKE values ranging from 7.3 to 11.8 kJ/kg, while experiments with highly vesiculated samples from Stromboli and Vesuvius result in lower SKE values (3.1 to 3.8 kJ/kg). The described methodology presents a useful tool for quantitative estimation of the kinetic energy release of magmatic fragmentation processes, which can contribute to the improvement of hazard assessment.     

Syn- and posteruptive hazards of maar–diatreme volcanoes

Maar–diatreme volcanoes represent the second most common volcano type on continents and islands. This study presents a first review of syn- and posteruptive volcanic and related hazards and intends to stimulate future research in this field. Maar–diatreme volcanoes are phreatomagmatic monogenetic volcanoes. They may erupt explosively for days to 15 years. Above the preeruptive surface a relatively flat tephra ring forms. Below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone-shaped diatreme and its root zone. During activity both the maar-crater and the diatreme grow in depth and diameter. Inside the diatreme, which may penetrate downwards for up to 2.5 km, fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits. Ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside, thus the diatreme represents a kind of growing sinkhole. Due to the subsidence of the diatreme underneath, the maar-crater is a subsidence crater and also grows in depth and diameter with ongoing activity. As long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle.Syneruptive hazards of maar–diatreme volcanoes are earthquakes, eruption clouds, tephra fall, base surges, ballistic blocks and bombs, lahars, volcanic gases, cutting of the growing maar crater into the preeruptive ground, formation of a tephra ring, fragmentation of country rocks, thus destruction of area and ground, changes in groundwater table, and potential renewal of eruptions. The main hazards mostly affect an area 3 to possibly 5 km in radius. Distal effects are comparable to those of small eruption clouds from polygenetic volcanoes. Syneruptive effects on infrastructure, people, animals, vegetation, agricultural land, and drainage are pointed out. Posteruptive hazards concern erosion and formation of lahars. Inside the crater a lake usually forms and diverse types of sediments accumulate in the crater. Volcanic gases may be released in the crater. Compaction and other diagenetic processes within the diatreme fill result in its subsidence. This posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time. It may last millions of years. Various studies and monitoring are suggested for syn- and posteruptive activities of maar–diatreme volcanoes erupting in the future. The recently formed maar–diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic, limnic, and biologic evolution, and potential posteruptive hazards. For future maar–diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km. Areas that are potentially endangered by maar–diatreme eruptions in the future are pointed out.