Carmelo Ferlito

The relationship between Late Quaternary deformation and volcanism of Mt. Etna (eastern Sicily): new evidence from the sedimentary substratum in the Catania region

Abstract Stratigraphical and structural analyses have been carried out on the Late Quaternary foredeep succession forming the Etnean substratum in the Catania region (eastern Sicily), in order to investigate in detail the chronology of deformation events that have accompanied a significant period of the eruptive activity of Mt. Etna, i.e. from 240 ka to the Present. This episode was characterised, at about 200 ka, by a main change of the petrochemical features of the emitted products from sub-alkaline to alkaline. This can be related to an evolving mantle diapir located beneath the volcano. The new stratigraphical and structural field data, presented in this paper, constrain the development, from 240 to 125 ka, of NW–SE-trending dextral faults associated with minor E–W- to NE–SW-oriented accommodation thrusts and NNW–SSE-trending normal faults that originated in a dominant transpressive regime. Strike-slip tectonics were active during the earlier emissions of sub-alkaline lavas (320–200 ka old) and part of the ancient alkaline products (180–100 ka old), from scattered eruptive systems which developed in local transtensive zones, distributed throughout the whole Etnean region. A major change in the mode of deformation, since 125 ka BP, was related to the propagation of a normal fault belt along the Ionian coast of the Catania region and the eastern sectors of the Etnean edifice. This process was associated with the growth of open folds that deformed the entire foredeep sequence exposed along the southern boundary of the Etnean edifice. During this period, eruptive activity concentrated along the main extensional features where steady and very efficient feeding systems originated. This resulted in a rapid increase in the volume of emitted alkaline products that gave rise to the construction of the modern stratovolcano during the last 80 ka. The collected data emphasise some main aspects of the relationship between tectonic deformation at a regional scale and volcanism in the Etna area. Firstly, the mode of deformation at the onset of Etnean volcanism seems to be inadequate to explain the emplacement at depth of a mantle diapir related to the occurrence of a hotspot, almost independent from the local crustal dynamics. On the other hand, the Late Quaternary structural assemblages recognised on the surface can be interpreted as direct effects of the Europe–Africa convergence, rather than as the products of deformation induced by the emplacement of the mantle diapir. In the different stages of Etnean evolution a direct relationship exists between the mode of deformation and the distribution as well as the capacity of the feeding systems. In particular, the amounts of emitted products in the different stages depend on the intensity of crustal stretching associated with deformation, rather than the volume of available molten material at depth. These conclusions represent a new perspective for the interpretion of the early stages and subsequent evolution of the volcanic activity of Mt. Etna. The proposed model also represents an useful tool in deciphering the relationship between the deformation path, seismicity and volcanic activity of Mt. Etna.

The Morphotectonic map of Mt. Etna

The Morphotectonic Map of Mt. Etna (see attached table) is based on detailed field survey of morphologic and structural features outcropping on the volcanic edifice, supported by detailed analysis of orthophotos, stereo-pair photographs and satellite images. It helps to define more completely and accurately than previously done the structural network of active features that characterizes the volcanic edifice, and the relationships between faulting, fissuring and dyke swarms. Morpho-structural data are drawn on a schematic geological map where the main sedimentary and volcanic units have been reported. Morphotectonic analysis shows that Mt. Etna volcano exhibits active extensional features represented by normal faults and eruptive fissures which are related to shallow low-energy seismicity. These accommodate WNW-ESE striking extension, deduced from structural analysis and seismological data, related to incipient rifting processes at regional scale. The fault segments generally control the present topography and show steep escarpments with very young, mostly Late Pleistocene to Holocene, morphology. The most important structures are located along the eastern base of the volcano (Timpe fault system), where NNW-SSE striking normal faults with dextral-oblique component of motion represent the northern end of the Malta Escarpment system. In the north-eastern slope of the volcano these fault system swings to a NE trend which it keeps northwards along the Ionian Coast to Taormina and as far as Messina Straits. The major fissural eruptions occur along well defined, feeder-dykes and spatter cones belts that cut the upper slopes of the volcano, on the footwall of the Timpe fault system. They form NE trending pure extensional swarms along the NE sector of the volcano and en-echelon systems of N-S to NNE-SSW oriented fractures along NNW-SSE trending oblique-dextral shear-zones in the southern and south-eastern slopes. Such summital eruptive fissuring appears to result from the same ESE-striking regional extensional stress that drives active faulting at the base of the volcano suggesting a common tectonic origin

Lava channel formation during the 2001 eruption on Mount Etna: evidence for mechanical erosion.

We report the direct observation of a peculiar lava channel that was formed near the base of a parasitic cone during the 2001 eruption on Mount Etna. Erosive processes by flowing lava are commonly attributed to thermal erosion. However, field evidence strongly suggests that models of thermal erosion cannot explain the formation of this channel. Here, we put forward the idea that the essential erosion mechanism was abrasive wear. By applying a simple model from tribology we demonstrate that the available data agree favorably with our hypothesis. Consequently, we propose that erosional processes resembling the wear phenomena in glacial erosion are possible in a volcanic environment.

Mechanical erosion by flowing lava

Hot lava is a viscous fluid that, driven by gravity, moves along the Earths surface. Intuitively, one attributes constructional properties to lava–it accumulates in volcanic landforms, compound lava fields and, in the end, entire mountains. On the other hand, there are also examples of the erosive power of lava: on Earth and especially on other planets in the Solar System, there exist channels incised by flowing lava. The origins of these erosive features have long been debated among volcanologists and planetologists. The dominant paradigm is thermal erosion, although it leaves many questions open. After the 2001 eruption on Mount Etna we found a lava channel whose features cannot be explained in the frame of thermal erosion. On the basis of our observations, we have developed a model for mechanical erosion that explains the main field observations, and opens alternative ways to describe erosion by flowing lava.

Amphibole crystallization in the Etnean feeding system: mineral chemistry and trace element partitioning between Mg-hastingsite and alkali basaltic melt

Amphiboles are rather rare in the volcanics of the whole Etnean succession and commonly are represented by kaersutites to titanian pargasites, mostly found in differentiated products. Titanian Mg-hastingsites have been found in lavas and tephra from the 2001 eruption at Mt. Etna. New major (EMPA) and trace element (LAM-ICP/MS) data on amphiboles from this eruption have been compared with reference data for kaersutites from prehistoric eruptions. The two amphibole groups significantly differ from each other in their AlIV, AlVI, K and Mg# values, which are higher in Mg-hastingsite than in kaersutite. Ti and Na are lower in Mg-hastingsite than in kaersutite. REE and trace element patterns for all the analysed Mg-hastingsite crystals are quite homogeneous. Kaersutite patterns generally conform to those of Mg-hastingsite but display higher concentrations for most of the trace elements. The exceptional occurrence, exclusively in tephra, of some crystals under equilibrium conditions with the coexisting residual glass has made it possible to calculate the partition coeffcients between amphibole and melt (Amph/melt D ) for trace elements. A new set of partition coeffcients is then provided, deriving from analyses on five amphibole/melt pairs at equilibrium. These data highlight the effects of amphibole crystallization in controlling some trace element ratios ( e.g. Th (or U)/Ta, Th/Nb, La/Nb) in residual melts of alkali basaltic systems, and suggest new hints for interpreting possible geochemical anomalies of these magmas. In addition, the comparison between the calculated Amph/melt D of Mg-hastingsite and those from literature relative to kaersutite from prehistoric eruptions shows that they are generally lower in the former than in the latter one for most of the trace elements. All of the available data provide constraints on the physical growth conditions for the 2001 Mg-hastingsite. Temperatures around 980 °C and volatile pressures in the range of 200–300 MPa have been estimated by integrating geophysical and petrological data. The highest pressure values are however larger than the lithostatic pressure alone acting on the magma reservoir (~ 6 km b.s.l.), as defined on the grounds of the hypocentres depth of the seismic events associated to the magma rise. This implies that Mg-hastingsite was probably crystallizing in a closed reservoir under overpressure conditions. Finally, microchemical data and trace element partitioning suggest that the differences between Mg-hastingsites and kaersutites in the Etnean products are mainly due to the less differentiated character of the magmas emitted after the 1971, and particularly after the 2001 eruption, compared to the compositions that characterize products of the prehistoric events. Furthermore, also a higher pressure of the system where Mg-hastingsite crystallized would account for its compositional differences respect to prehistoric kaersutite.

Influx of volatiles into shallow reservoirs at Mt. Etna volcano (Italy) responsible for halogen-rich magmas

The study of F-rich mineral phases, namely fluorophlogopite and fluorapatite, occured in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the “Ellittico” eruptive centre; ~15 ka BP) allowed us to define the physical and chemical crystallization conditions of such minerals. Textural evidence suggests a late-stage crystallization of the F-rich minerals, since fluorapatite is exclusively found in the groundmass and fluorophlogopite within lava vesicles. Furthermore, a colourless SiO 2 -rich amorphous phase, characterized by multi-stage deposition, has overgrown the fluorophlogopite crystals. Comparison with simulations of crystal fractionation demonstrates that the benmoreitic lava characterized by the occurrence of F-dominant minerals is anomalously enriched in some major and trace elements ( e.g. , Ti, Fe, K, Ba and, to a minor extent, Rb and REEs). Even the modelling of crustal contamination, possibly caused by assimilation of the sedimentary basement underlying the volcano edifice, is poorly consistent with the geochemical features of the considered benmoreite. Chlorine and fluorine concentrations estimated for this lava sample are 0.20 and 0.34 wt% respectively, which are significantly higher than those of other Etnean prehistoric mugearites and benmoreites. The selective enrichment in major and trace elements, and particularly in halogens, has been therefore related to other rarely recognized differentiation processes acting in the feeding system. Specifically, volatile-induced differentiation, ruled by elemental transfer (as metal-halogen complexes) in a volatile phase, is able to account for the observed geochemical variations. Such a volatile influx might be released by more primitive, deeper and volatile-rich magmas while rising up towards shallower levels of the feeding system. Considering the solubility of fluorine in silicatic systems at low pressure higher than that of chlorine, we suggest that fluorapatite and fluorophlogopite were likely grown during syn- or post-eruption pneumatolytic stages, probably after open-system degassing when a gas phase characterized by a high Cl/F ratio was released. The paramount role played by volatiles is also consistent with the occurrence of SiO 2 -rich amorphous concretions surrounding the fluorophlogopite crystals. Indeed, large amounts of SiF 4 in the gas phase can sublimate under cooling conditions into Si-rich amorphous concretions. On the grounds of our findings, the process here described could have significant implications to explain unexpected eruptive behaviours at Mt. Etna, such as highly explosive dynamics of extrusion or the rather low viscosity of highly evolved lavas.

Comment on “Complex magma dynamics at Mount Etna revealed by seismic, thermal, and volcanological data” by B. Behncke, S. Falsaperla, and E. Pecora

[1] Behncke et al. [2009] deal with three paroxysms which occurred at the Southeast Crater (SEC) of Mount Etna on 16, 19, and 24 November 2006. In particular, the first of these was an exceptional eruptive episode. During this event, a small but significant pyroclastic flow took place at the southeastern base of the SEC. Erroneous assumptions and omissions of evidences affect this paper in particular with regards to the interpretation of the 16 November paroxysm. We believe that, if correctly presented, the assumptions contained in the work would completely reverse the conclusions. [2] In their paper, Behncke et al. integrate volcanological observations and seismic and thermal data to prove that the short (<2 min), violent outburst, which occurred at the southeastern base of the SEC at 1425 UT and gave rise to the small pyroclastic flow, occurred when the lava flow, which has been coming out for several hours from the summit of the SEC, suddenly interacted with water-saturated sediments and interlayered snow. This idea constituted the basis of the recently published papers by Behncke et al. [2008] and Behncke [2009]. This occurrence has posed some intriguing questions concerning the conditions that can give origin to such episodes in basaltic volcanoes, usually characterized by the quiet emission of lava flows and Strombolian activity. As it is often the case when dealing with natural phenomena, other interpretations are possible. In particular, a different interpretation was proposed by Ferlito et al. [2007, also Relationship between the flank sliding of the South East Crater (Mt. Etna, Italy) and the paroxysmal event of November 16, 2006, submitted to Bulletin of Volcanology, 2009], who envisage the outburst as associated with a rapid and superficial fracturing (<250m in depth) occurred at the base (3050m above sea level (asl)) of the SEC due to the overpressure of a small, gas-rich batch of magma. [3] One of the fundamental points emphasized by Behncke et al. [2009] regarding the paroxysm of 16 November is the absolute lack of eruptive fracturing associated with the outburst. The evidence used to support this view is the absence of specific seismic signals associated with fracture opening, even considering that at the moment of the paroxysm the seismic network was integrated with four stations close to the Etnean summit craters (EPLC, ECPN, EPDN, EBEL). In particular, they write (paragraph 54) that ‘‘the lack of any peculiar trace in the seismic record along with the observation that the opening of eruptive fractures at Etna is usually associated with changes in seismic activity [e.g., Patane et al., 2004] leads us to support the hypothesis that the mechanism at the origin of the PDCs was extremely shallow and rootless, as proposed by Behncke et al. [2008].’’ But this sentence is quite dissimilar to that expressed by Patane et al. [2004], reiterated also in the conclusive remarks of Patane et al. [2005, paragraph 12], who conversely stated that ‘‘a repeatedly observed feature at Mt. Etna is that the summit eruptions are not generally preceded by significant variations in the pattern of deformation and volcano-tectonic seismicity [Patane et al., 2004].’’ A conspicuous literature exists with regard to this matter [cf. Cosentino et al., 1989; Gresta and Patane, 1987; Cardaci et al., 1993; Ferrucci and Patane, 1993; Patane et al., 2003]. The lack of specific seismic signals recorded during fracture opening has lately been confirmed at the beginning of the 2004–2005 eruption. In this regard, Di Grazia et al. [2006, paragraph 1] write that ‘‘neither earthquake seismicity heralded or accompanied the opening of the fracture field from which the lava poured out, nor volcanic tremor changed in amplitude and frequency content at the onset of the effusive activity.’’ The eruptive event of 2004–2005, which, according to Di Grazia et al. [2006], did not cause any particular seismicity, was not a <2 min outburst of lava but a 6 month long eruption that emitted 60 10 m of lava [Neri and Acocella, 2006] and had a complex feeding history, involving also the occurrence of shallow magma mixing [Corsaro et al., 2009]. These observations should suffice to consider with more caution the use and interpretation of the seismic signature or its lack as associated to fracture opening at Mount Etna. Furthermore, the volcanic tremor level recorded at all the 4 summit stations at the moment of the outburst was extremely high [see Behncke et al., 2009, Figure 10]. Under these conditions it is unconvincing to affirm the absence of the variation of seismic signal for low magnitude events. Consider that the fracture opened along an E-W direction at 3050 m asl and intersected magma that was just below the surface (the erupting vents at the moment of the outburst JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B12204, doi:10.1029/2009JB006511, 2009 Click Here for Full Article

Dome-like behaviour at Mt. Etna: The case of the 28 December 2014 South East Crater paroxysm

On the 28 December 2014, a violent and short paroxysmal eruption occurred at the South East Crater (SEC) of Mount Etna that led to the formation of huge niches on the SW and NE flanks of the SEC edifice from which a volume of ~3 × 106 m3 of lava was erupted. Two basaltic lava flows discharged at a rate of ~370 m3/s, reaching a maximum distance of ~5 km. The seismicity during the event was scarce and the eruption was not preceded by any notable ground deformation, which instead was dramatic during and immediately after the event. The SO2 flux associated with the eruption was relatively low and even decreased few days before. Observations suggest that the paroxysm was not related to the ascent of volatile-rich fresh magma from a deep reservoir (dyke intrusion), but instead to a collapse of a portion of SEC, similar to what happens on exogenous andesitic domes. The sudden and fast discharge eventually triggered a depressurization in the shallow volcano plumbing system that drew up fresh magma from depth. Integration of data and observations has allowed to formulate a novel interpretation of mechanism leading volcanic activity at Mt. Etna and on basaltic volcanoes worldwide.

Solidification and Turbulence (Non-laminar) during Magma Ascent: Insights from 2D and 3D Analyses of Bubbles and Minerals in an Etnean Dyke

Solidification, emplacement and fluid dynamics of a sub-volcanic rock at Mt Etna have been investigated through two-dimensional (2D) and three-dimensional (3D) textural analyses of the hosted bubbles and minerals. This rock is a 4 3 m thick aphyric dyke (DK) that solidified at a depth of 100–300 m, below the pristine surface level. Seven samples from the dyke rim (DK1) to core (DK7) have been analysed in two dimensions by using a high-resolution scanner, a transmission optical microscope and scanning electron microscopy imaging with back-scattered electrons, and in three dimensions by microfocus X-ray computed tomography. Field observations and mesoscopic polished rock surfaces show bubble-rich, -poor and -free patches even in rock pieces of a few cubic centimetres, with changes in sizes and shapes; even so, their shapes and spatial arrangement can never be attributed to high degrees of strain. In parallel, the amount of bubbles irregularly varies from dyke rim to core, whereas plagioclase (plg), clinopyroxene (cpx), titanomagnetite (timt), and olivine (ol) show only limited variations. The fabric of bubbles retrieved by 3D orientation of their maximum length (i.e. elongation) is invariably random in space for each DK sample. These bubble features have been attributed to transitional to turbulent flows; that is, non-laminar regimes (Reynolds number> 1000), predicted for a long time from numerical models and that occurred before the crystallization of minerals. Water solubility, volume of bubbles, magma density and viscosity models indicate that, at pressure higher than 10 MPa, 1 wt % H2O was dissolved in the original trachybasaltic magma, which, in turn, was close to its liquidus temperature. As the pressure decreased at very shallow levels, the magma significantly degassed and volatile exsolution induced marked crystallization (mostly plg followed by cpx). The viscosity of the system increased, decelerating and halting the magmatic suspension. The textures and fabrics of bubbles were suddenly frozen in, despite crystals continuing to grow under the effect of cooling rate variables from the inner (DK7) to outer (DK1) dyke portions. Fluid-dynamic computations suggest that the DK trachybasaltic magma ascended with a velocity of few metres per second in a transitional to turbulent regime, before the growth of minerals.