Vortex rings can easily be generated in the laboratory or with homemade devices, but they have also been observed on volcanoes, since the eighteenth century. However, the physical conditions under ...which volcanic vortex rings form are still unknown. In order to better understand this phenomenon and provide clues on the dynamics of the volcanic vortex rings, we performed a series of finite element simulations to investigate which model configuration leads to the rings formation that best matches the field observations. Results show that the formation of volcanic vortex rings requires a combination of fast gas release from gas bubbles (slugs) at the top of the magma conduit and regularity in the shape of the emitting vent. Our findings offer important insights into the geometry of the uppermost portion of vortex-forming volcanic conduits. Volcanic vortex ring studies may form the basis for a cross-disciplinary assessment of the upper conduit dynamics of volcanic vents.
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through ...back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An–FeO variation: 1) Low Amplitude–High Frequency (LAHF) and 2) High Amplitude–Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution–regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H
2O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An–FeO increase, result from mixing with more primitive and volatile-rich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H
2O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An–FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn- or post-eruptive conditions.
Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H
2O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
It is usually believed that volcanoes explicate their activity erupting magma, in which volatiles (mostly H2O) are dissolved in modest quantities. At Mount Etna, the maximum H2O found in olivine melt ...inclusions is 3.5wt%, which would correspond to a moles H2O/mol basalt ratio of 0.14. A reappraisal of published data is proposed here, and a comparison is made between the gas flux and the volume of erupted lavas. The results are surprising: the moles H2O/mol basalt ratio is 1.41, which means that Mount Etna erupts 10 times the maximum H2O that could be dissolved in magma and 40% more moles of gas (H2O, CO2 and S) than moles of basalt. By calculating the molar volume of the basaltic melt components (silicate tetrahedra and metallic cations) and of the gas phase at a pressure of 250MPa, it is possible to envisage the magma within the deep plumbing system as a solution made of ~70% continuum gas phase (mostly H2O) at a supercritical state (density 360kg/m3) and 30% basaltic melt components. The transition from this low-density (1140kg/m3) water melt solution (WMS) to the high-density (2800kg/m3) basalt, usually erupted (defined as a continuous melt phase, CMP), occurs in the last 2km and marks the boundary between a deep and a shallow plumbing system. The depth of this boundary varies with time, being driven by the rate at which the gas escapes the WMS to feed the persistent gas plume at the summit craters, leaving the CMP, which accumulates within the shallow plumbing system, until erupted. The overpressure of the gas phase in the WMS, acting like a piston cylinder, is fundamental in driving the eruption. In this work, the thermal contribution provided to the CMP by the large gas flux has also been considered, proving that it can supply the heat necessary to maintain the CMP. The volcano is here considered a dynamic system in which the eruptive activity is ruled by discontinuities in the flux of gas and heat. Negative fluctuations in the gas flux would decrease the heat supply, promote viscosity and trigger eruptions. Moreover, this view of the volcanic system, subverting the common paradigm in which the gas emitted is associated with an equivalent amount of degassing magma, explains the phenomenon, known as the ‘excess degassing problem’, which affects volcanoes of basaltic and andesitic nature worldwide.
The enrichment in potassium shown by the basic lavas erupted at Mount Etna volcano after 1971 (K
2O max
~
2.2
wt.%) has been considered by previous researchers to be too high to be related to simple ...crystal fractionation and instead linked this high K
2O content to either crustal assimilation or changes in the magma source. Unfortunately all existing models for the post-1971 K
2O enrichment fail to explain the phenomenon satisfactorily leaving the question still open.
We present a critical re-examination of published data for major elements (633 whole rock analyses), trace elements (376 whole rock analyses) and isotopic ratios (136
87Sr/
86Sr analyses), for historical and pre-historical lavas. Potassium enrichment is not limited to the products of the last 35
years. A comparable increase in potassium is noticed in lavas erupted during the pre-historic phase of the recent Mongibello (K
2O max
~
2.5
wt.%) and in lavas related to the early phase of the ancient Mongibello (K
2O max
~
3
wt.%). Moreover, data from melt inclusions in olivines from the 2001 and 2002 eruptions, reveal that potassium contents remain constant for melts with entrapment pressure between 490 to 100
MPa and increase significantly in melts entrapped at pressures below
~
100
MPa.
We propose that supercritical fluids coming from deeper magmas and carrying alkali Cl-complexes migrate through basic to intermediate magmas residing in the shallow feeding system. As chlorine exsolves and leaves the system alkalis are released contributing to the observed potassium enrichment of the shallow magma. Fluctuations of the volatiles influx throughout time are likely related to the magma supply rate. Considering that the amount of magma entering a plumbing system is determined by the rate of regional extension, the flux of alkali Cl-complexes entering the melt might be related to an extensional regime acting in the Etnean area.
►K increment unrelated to Crystal fractionation found in Etnean lavas of last 60
ka. ►The volatile-induced differentiation provides a good explanation of K increment. ►K-enriched lavas indicate high magma influx associated to extensional tectonics.
Seven rock samples were systematically collected from innermost to the outermost portion of a dike outcropping at Mt. Etna volcano. Results show that, from dike core-to-rim, plagioclase, ...clinopyroxene and titanomagnetite show compositional variations due to increasing cooling rate. Plagioclase is progressively enriched in An from innermost to the outermost part of the dike. Similarly, clinopyroxene components En+CaTs+CaFeTs increase, whereas Di+Hd decrease. The Usp content in titanomagnetite also systematically decrease from dike core-to-rim. Partition coefficients and thermometers based on the crystal-liquid exchange reaction indicate that, due to rapid cooling rates at the dike outer portions, early-formed crystal nuclei do not re-equilibrate with the melt. The chemistry of minerals progressively deviates from that of equilibrium; consequently, from dike core-to-rim, mineral compositions resemble those of high-temperature formation. The chemical variations of clinopyroxene and plagioclase in dike samples mirror those obtained from cooling experiments carried out on alkaline basalts. Accordingly, we used an experimental equation based on clinopyroxene compositional variation as a function of cooling rate to determine the cooling conditions experienced by the crystals during dike emplacement. The estimated cooling rates are comparable to those predicted by thermal modeling based on an explicit finite-difference scheme.
► Crystal textures and compositions change as a function of cooling rate from dike core-to-rim. ► Crystals do not re-equilibrate with the melt at the dike rim. ► The composition of crystals experiencing rapid cooling rates suggests high-temperature formation.
Offshore data in the western Ionian Sea indicate that the NW–SE-trending dextral shear zone of the Alfeo-Etna Fault System turns to the N–S direction near the Ionian coastline, where the extensional ...Timpe Fault System is located. Morpho-structural data show that NW–SE-trending right-lateral strike-slip faults connect the Timpe Fault System with the upper slope of the volcano, where the eruptive activity mainly occurs along the N–S to NE–SW-trending fissures. Fault systems are related to the ~E–W-trending extension and they are seismically active having given rise to shallow and low-moderate magnitude earthquakes in the last 150 years. As a whole, morpho-structural, geodetic and seismological data, seismic profiles and bathymetric maps suggest that similar geometric and kinematic features characterize the shear zone both on the eastern flank of the volcano and in the Ionian offshore. The Alfeo-Etna Fault System probably represents a major kinematic boundary in the western Ionian Sea associated with the Africa–Europe plate convergence since it accommodates, by right-lateral kinematics, the differential motion of adjacent western Ionian compartments. Along this major tectonic alignment, crustal structures such as releasing bends, pull-apart basins and extensional horsetails occur both offshore and on-land, where they probably represent the pathway for magma uprising from depth.
In July and August 2019, Stromboli volcano underwent two dangerous paroxysms previously considered “unexpected” because of the absence of significant changes in usually monitored parameters. We ...applied a multidisciplinary approach to search for signals able to indicate the possibility of larger explosive activity and to devise a model to explain the observed variations. We analysed geodetic data, satellite thermal data, images from remote cameras and seismic data in a timespan crossing the eruptive period of 2019 to identify precursors of the two paroxysms on a medium-term time span (months) and to perform an in-depth analysis of the signals recorded on a short time scale (hours, minutes) before the paroxysm. We developed a model that explains the observations. We call the model “push and go” where the uppermost feeding system of Stromboli is made up of a lower section occupied by a low viscosity, low density magma that is largely composed of gases and a shallower section occupied by the accumulated melt. We hypothesize that the paroxysms are triggered when an overpressure in the lower section is built up; the explosion will occur at the very moment such overpressure overcomes the confining pressure of the highly viscous magma above it.
Piton de la Fournaise is an active shield volcano located in the eastern area of the Réunion Island (Indian Ocean) whose activity is characterized by effusive and explosive episodes with the emission ...of scarcely differentiated magmas with mostly tholeiitic affinity. The presently active edifice has grown within the Enclos Fouqué caldera, a polylobate plain bounded on its western side by the 80–200 m high Bellecombe vertical cliffs. This escarpment exposes a vertical sequence of 12 lava flows cut by a dike with an age > 5.5 kyrs. In this work, the Bellecombe products were investigated by X-ray fluorescence, Inductively Coupled Plasma Mass Spectroscopy, a Scanning Electron Microscope and X-ray computed microtomography in order to characterize the evolution over time of the magmatic system feeding the eruptive activity prior to the Enclos Fouqué caldera collapse. The results indicate that lava flows share a geochemical affinity with the two main series documented at Piton de la Fournaise, namely, Steady State Basalts (SSB) at the bottom and top of the sequence and Abnormal basalt Group (AbG) with different degrees of differentiation in the central part. The emission of these two different products in both a restricted area and timespan testifies to the dynamic activity of the plumbing system, capable of shifting rapidly from central to eccentric activity in the recent past.
The growth of Mount Etna volcano reflects the superimposition of various eruptive centers, the most voluminous of which is the Ellittico, whose stratigraphic sequence is well exposed on the steep ...walls of Valle del Bove. The uppermost levels of the sequence have been sampled and investigated through a new set of geochemical data on mineral phases and bulk rock. Sampled rocks display a marked bimodality with aphyric banded trachyandesites, which are some of the most evolved and rare products of the entire Etnean succession (SiO2 58–60 wt.%), intercalated in plagioclase rich porphyritic mugearites (SiO2 49–50 wt.%, P.I. 35–40). In this paper, we provide a detailed textural, mineralogical, and chemical characterization of these products, providing a new interpretative model for their genesis and significance in the context of the Etnean system. Our approach discusses, in a critical way, the “classic” fractional crystallization model of magmas, not supported by field evidence, and proposes a novel hypothesis in which the aphyric-banded trachyandesites represent be the primary products of a gas-induced partial melting of hypabyssal sills and dykes. This hypothesis represents a step towards a comprehensive description of igneous systems that takes into account not exclusively the evolution of basaltic melts, but also the role of volatile contributions in governing volcanic behavior.
This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical ...conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 °C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 °C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H2O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to ΔNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H2O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface.
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