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Persistently active volcanoes such as Etna, in southern Italy, are subject to frequent morphological and structural changes, especially at the summit. In recent decades, in particular, Etna has shown an evident increase in both summit and flank eruptive activity. This caused a striking transformation of the morphologies of its summit craters, which increased in number and size, also causing the formation of new small eruptive vents, fumarolic fields, fractures and crater collapses. Sometimes these morpho-structural modifications of the top of the volcano have been so rapid that they have not been all recorded accurately, or they have occurred in sequences so rapid as to overlap the effects of the eruptions, making some transitional events between an eruption and the next one. Eruptive activity during the period considered occurred mostly at the summit craters of Etna (May 2016: Voragine; February-April 2017: New Southeast Crater and fissures on its slopes; August and November-December 2018: New Southeast Crater). This was interrupted by the brief fissure eruption on the upper southeast flank of the volcano on 24-27 December 2018; renewed eruptions occurred at the New Southeast Crater and fissures on its flanks in May-July 2019. Finally, in September 2019, eruptive activity shifted to the Northeast Crater and Voragine, the latter feeding intermittent lava flows into the adjacent Bocca Nuova crater. In cases like this, satellite observations can complete terrestrial monitoring systems, providing a useful contribution of knowledge and detail of the eruptive activity and morpho-structural transformations of greater significance. In this study, we analysed the Mt. Etna activity using data from the Multispectral Instrument (MSI) and the Operational Land Imager (OLI), respectively onboard Sentinel-2 and Landsat 8 satellites, processed by means of the recently proposed NHI (Normalized Hotspot Indices) algorithm. The latter allowed us to identify thermal anomalies associated to main effusive and explosive activities as well as to the smaller eruptive events, revealing in some cases thermal phenomena several days in advance that can be interpreted as potential precursors. In addition, NHI also showed a fair sensitivity in grasping the incipient fracturing of the Etna summit area, an important phenomenon in the life of this volcano due to its close correlation with the slow lateral collapses that characterize its flanks, and which in turn can trigger lateral eruptions that are potentially dangerous for the Etnean populations.

On 11 February 2014, a considerable volume (0.82 to 1.29 × 106 m3) of unstable and hot rocks detached from the lower–eastern flank of the New Southeast Crater (NSEC) at Mt. Etna, producing a pyroclastic density current (PDC). This event was by far the most extensive ever recorded at Mt. Etna since 1999 and has attracted the attention of the scientific community and civil protection to this type of volcanic phenomena, usually occurring without any clear volcanological precursor and especially toward the mechanisms which led to the crater collapse, the PDC flow dynamics and the related volcanic hazard. We present here the results of the investigation carried out on the 11 February 2014 collapse and PDC events; data were obtained through a multidisciplinary approach which includes the analysis of photograph, images from visible and thermal surveillance cameras, and the detailed stratigraphic, textural and petrographic investigations of the PDC deposits. Results suggest that the collapse and consequent PDC was the result of a progressive thermal and mechanical weakening of the cone by repeated surges of magma passing through it during the eruptive activity prior to the 11 February 2014 events, as well as pervasive heating and corrosion by volcanic gas. The collapse of the lower portion of the NSEC was followed by the formation of a relatively hot (up to 750 °C) dense flow which travelled about 2.3 km from the source, stopping shortly after the break of the slope and emplacing the main body of the deposit which ranges between 0.39 and 0.92 × 106 m3. This flow was accompanied a relatively hot cloud of fine ash that dispersed over a wider area. The results presented may contribute to the understanding of this very complex type of volcanic phenomena at Mt. Etna and in similar volcanic settings of the world. In addition, results will lay the basis for the modeling of crater collapse and relative PDC events and consequently for the planning of hazard assessment strategies aimed at reducing the potential risks to scientists and tens of thousands of tourists visiting Etna's summit areas every year.

A lengthy period of eruptive activity from the summit craters of Mt. Etna started in January 2011. It culminated in early December 2015 with a spectacular sequence of intense eruptive events involving all four summit craters (Voragine, Bocca Nuova, New Southeast Crater, and Northeast Crater). The activity consisted of high eruption columns, Strombolian explosions, lava flows and widespread ash falls that repeatedly interfered with air traffic. The most powerful episode occurred on 3 December 2015 from the Voragine. After three further potent episodes from the Voragine, activity shifted to the New Southeast Crater on 6 December 2015, where Strombolian activity and lava flow emission lasted for two days and were fed by the most primitive magma of the study period. Activity once more shifted to the Northeast Crater, where ash emission and weak Strombolian activity took place for several days. Sporadic ash emissions from all craters continued until 18 December, when all activity ceased. Although resembling the summit eruptions of 1998–1999, which also involved all four summit craters, this multifaceted eruptive sequence occurred in an exceptionally short time window of less than three days, unprecedented in the recent activity of Mt. Etna. It also produced important morphostructural changes of the summit area with the coalescence of Voragine and Bocca Nuova in a single large crater, the “Central Crater”, reproducing the morphological setting of the summit cone before the formation of Bocca Nuova in 1968. The December 2015 volcanic crisis was followed closely by the staff of the Etna Observatory to monitor the on-going activity and forecast its evolution, in accordance with protocols agreed with the Italian Civil Protection Department.

The quantification of eruptive activity represents one major challenge in volcanology. Digital comparison of lidar-based elevation models of Etna (Italy) was made to quantify the volumes of volcanics emitted in 2007–2010. During this period, Etna produced several summit paroxysms followed by a flank eruption. We integrated the total volume difference resulting from the subtraction of the 2007 and 2010 digital elevation models with volumes of eruptive products based on field and aerial surveys to attribute volumes with hitherto unrealized precision to poorly constrained eruptions. The total erupted volume of 2007–2010 is >86 × 106 m3, most (~74 × 106 m3) of which is made up by the lava flows of the 2008–2009 flank eruption. The survey also reveals the high lava volume (5.73 × 106 m3) and average eruption rate (~400 m3 s−1) of the 10 May 2008 paroxysm, whose flow front stopped 6.2 km from the vent, not far from the town of Zafferana Etnea.

Determining the volume of the various types of products of a highly frequent active volcano can be very difficult, especially if most of them are deposited on a growing volcanic cone. The New South-East Crater (NSEC) of Mt Etna, Italy, may be considered one of the best case studies because of tens of paroxysmal episodes which it produced in the last few years. On 25-26 October 2013, a lava fountain at the NSEC produced magma jets up to 500 m high, a maximum ~8 km high column, a multilobate lava flow field 1.3–1.5 km long, and almost 30 m of growth in height of the NSEC cone. Mapping of explosive and effusive deposits allowed us to calculate the total volume of erupted products, including lava flows, proximal, and distal tephra fallout, and the amount of coarse pyroclastics on the cone. The estimation of the latter products was also confirmed subtracting digital elevation models (DEMs) obtained at different stages of the NSEC growth. Results show that the volume of tephra fallout away from the cone was only

During the years 2013–2014, the New Southeast Crater (NSEC) at the summit of Mount Etna produced frequent episodes of lava fountaining (paroxysms), and its cone continued to grow at unprecedented rates. Many of the episodes were of rather brief duration and violently explosive, producing mostly pyroclastic material and minor volumes of lava. Other episodes, especially those since mid-December 2013, were characterized by violent Strombolian activity without producing sustained lava fountains and significant amounts of tephra, but emitting more voluminous lava flows. One episode of intense Strombolian and effusive activity that was possibly fed from the NSEC conduit occurred from vents located approximately 1 km north of the crater, on the east flank of the Northeast Crater, in July–August 2014. The evolution of the NSEC cone between 2012 and 2014 was documented by repeated GPS surveys carried out both from a distance and on the cone itself, by the acquisition of comparison photographs, and by two aerophotogrammetric surveys. From these surveys the highest point of the NSEC results to have grown from 190 m (May 2012) to 215 m (October 2014) above the pre-cone surface reaching an elevation of 3290 m, and its volume more than doubled to 50.0 ± 6.5 × 10 6 m 3 , representing the 40% of the total (bulk) volume of the volcanic products including pyroclastic fallout erupted in 2011–2014, which is 147.2 × 10 6 m 3 (101.3 × 10 6 m 3 dense-rock equivalent). The whole of the 2011–2014 NSEC activity marks an unusually high frequency of rather explosive, tephra-rich eruptive episodes compared to Etna's activity in past decades and centuries, although the average magma production rate in this interval is close to the supposed long-term output rate of the volcano. The latest eruptive episodes show a tendency of the NSEC coalescing with the old Southeast Crater cone, which therefore represents a miniature example of a growing compound volcano at the summit of Etna.

Between January 2011 and April 2012, the Southeast Crater (SEC) on Mount Etna was the site of 25 episodes of lava fountaining, which led to the construction of a new pyroclastic cone on the eastern flank of the SEC. During these episodes lava overflows reached 4.3 km in length with an area of 3.19 km 2 and a volume of 28 × 10 6 m 3 . The new cone, informally called New Southeast Crater (NSEC), grew over a pre-existing subsidence depression (pit crater), which had been formed in 2007–2009. The evolution of the NSEC cone was documented from its start by repeated GPS surveys carried out both from a distance and on the cone itself, and by the acquisition of comparison photographs. These surveys reveal that after the cessation of the lava fountains in April 2012, the highest point of the NSEC stood 190 m above the pre-cone surface, while the cone volume was about 19 × 10 6 m 3 , representing 38% of the total (bulk) volume of the volcanic products including pyroclastic fallout erupted in 2011–2012, which is 50 × 10 6 m 3 (about 33 × 10 6 m 3 dense-rock equivalent). Growth of the new cone took place exclusively during the paroxysmal phases of the lava fountaining episodes, which were nearly always rather brief (on the average 2 h). Overall, the paroxysmal phases of all 25 episodes represent 51 h of lava fountaining activity — the time needed to build the cone. This is the fastest documented growth of a newborn volcanic cone both in terms of volume and height. Mean effusion rates during the lava fountaining episodes on 20 August 2011 (E11), as well as 12 and 24 April 2012 (E24 and E25) exceeded 500 m 3 /s (with maximum rates of 980 m 3 /s during E11) and thus they are among the highest effusion rates ever recorded at Etna. The composition of the erupted products varies in time, reflecting different rates of magma supply into the shallow feeding system, but without notable effects on the eruptive phenomenology. This implies that the dynamics leading to the episodic lava fountaining was largely, though not entirely, controlled by the repeated formation and collapse of a foam layer in the uppermost portion of the magmatic reservoir of the NSEC.

During the spring of 2007, paroxysmal activity occurred at the Southeast Crater of Mt. Etna, always associated with sharp rises in the amplitude of the volcanic tremor. Activity ranged from strong Strombolian explosions to lava fountains coupled with copious emission of lava flows and tephra. During inter-eruptive periods, recurrent seismic unrest episodes were observed in the form of temporary enhancements of the volcanic tremor amplitude, but they did not culminate in eruptive activity. Here, we present the results of an analysis of these inter-eruptive periods by integrating seismic volcanic tremor, in-soil radon, plume SO2 flux, and thermal data. SO2 flux and thermal radiation are envisaged as the “smoking gun,” and certifying that changes in seismic or radon data can be considered as volcanogenic. Short-term changes were investigated by pattern classification based on Kohonen maps and fuzzy clustering on volcanic tremor, radon, and ambient parameters (pressure and temperature). Our results unveil “failed” eruptions between February and April 2007 that are explained as ascending magma batches, which triggered repeated episodes of gas pulses and rock fracturing, but that failed to reach the surface.

[1] Since the second half of the 1990s, the eruptive activity of Mount Etna has provided evidence that both explosive and effusive eruptions display periodic variations in discharge and eruption style. In this work, a multiparametric approach, consisting of comparing volcanological, geophysical, and geochemical data, was applied to explore the volcano's dynamics during 2009–2011. In particular, temporal and/or spatial variations of seismicity (volcano-tectonic earthquakes, volcanic tremor, and long-period and very long period events), ground deformation (GPS and tiltmeter data), and geochemistry (SO2 flux, CO2 flux, CO2/SO2 ratio) were studied to understand the volcanic activity, as well as to investigate magma movement in both deep and shallow portions of the plumbing system, feeding the 2011 eruptive period. After the volcano deflation, accompanying the onset of the 2008–2009 eruption, a new recharging phase began in August 2008. This new volcanic cycle evolved from an initial recharge phase of the intermediate-shallower plumbing system and inflation, followed by (i) accelerated displacement in the volcano's eastern flank since April 2009 and (ii) renewal of summit volcanic activity during the second half of 2010, culminating in 2011 in a cyclic eruptive behavior with 18 lava fountains from New Southeast Crater (NSEC). Furthermore, supported by the geochemical data, the inversion of ground deformation GPS data and the locations of the tremor sources are used here to constrain both the area and the depth range of magma degassing, allowing reconstructing the intermediate and shallow storage zones feeding the 2011 cyclic fountaining NSEC activity.

Mature volcanoes usually erupt from a persistent summit crater. Permanent shifts in vent location are expected to occur after significant structural variations and are seldom documented. Here, we provide such an example that recently occurred at Etna. Eruptive activity at Mount Etna during 2007 focused at the Southeast Crater (SEC), the youngest (formed in 1971) and most active of the four summit craters, and consisted of six paroxysmal episodes. The related erupted volumes, determined by field-based measurements and radiant heat flux curves measured by satellite, totalled 8.67 × 106 m3. The first four episodes occurred, between late-March and early-May, from the summit of the SEC and short fissures on its flanks. The last two episodes occurred, in September and November, from a new vent (“pit crater” or “proto-NSEC”) at the SE base of the SEC cone; this marked the definitive demise of the old SEC and the shift to the new vent. The latter, fed by NW-SE striking dikes propagating from the SEC conduit, formed since early 2011 an independent cone (the New Southeast Crater, or “NSEC”) at the base of the SEC. Detailed geodetic reconstruction and structural field observations allow defining the surface deformation pattern of Mount Etna in the last decade. These suggest that the NSEC developed under the NE–SW trending tensile stresses on the volcano summit promoted by accelerated instability of the NE flank of the volcano during inflation periods. The development of the NSEC is not only important from a structural point of view, as its formation may also lead to an increase in volcanic hazard. The case of the NSEC at Etna here reported shows how flank instability may control the distribution and impact of volcanism, including the prolonged shift of the summit vent activity in a mature volcano.

Volcano monitoring aims at the recognition of changes in instrumentally observable parameters before hazardous activity in order to alert governmental authorities. Among these parameters seismic data in general and volcanic tremor in particular play a key role. Recent major explosive eruptions such as Okmok (Aleutians) and Chaiten (Chile) in 2008 and numerous smaller events at Mt Etna (Italy), have shown that the period of premonitory seismic activity can be short (only a few hours), which entails the necessity of effective automatic data processing near on-line. Here we present a synoptic pattern classification analysis based on Self Organizing Maps and Fuzzy Cluster Analysis which is applied to volcanic tremor data recorded during a series of paroxysmal eruptive episodes and a flank eruption at Etna in 2007–2008. In total, eight episodes were analyzed; in six of these significant changes in the dynamic regime of the volcano were detected up to 9 h prior to the onset of eruptive activity, and long before changes in volcanic tremor amplitude and spectral content became evident in classical analysis. In two cases, the state transition was

article i nfo We conducted geophysical-geochemical measurements on a ∼ 2k m N-S profile cutting across the Pernicana Fault, one of the most active tectonic features on the NE flank of Mt. Etna. The profile passes from the unstable E flank of the volcano (to the south) to the stable N flank and significant fluctuations in electrical resistivity, self-potential, and soil gas emissions (CO2, Rn and Th) are found. The detailed multidisciplinary analysis reveals a complex interplay between the structural setting, uprising hydrothermal fluids, meteoric fluids percolating downwards, ground permeability, and surface topography. In particular, the recovered fluid circulation model highlights that the southern sector is heavily fractured and faulted, allowing the formation of convective hydrothermal cells. Although the existence of a hydrothermal system in a volcanic area does not surprise, these results have great implications in terms of flank dynamics at Mt. Etna. Indeed, the hydrothermal activity, interacting with the Pernicana Fault activity, could enhance the flank instability. Our approach should be further extended along the full extent of the boundary between the stable and unstable sectors of Etna for a better evaluation of the geohazard in this active tectonic area.

One of the best-studied volcanoes of the world, Mt. Etna in Sicily, repeatedly exhibits eruptive scenarios that depart from the behavior commonly considered typical for this volcano. Episodes of intense explosive activity, pyroclastic flows, dome growth and cone collapse pose a variety of previously underestimated threats to human lives in the summit area of the volcano. However, retrospective analysis of these events shows that they were likely caused by the same very sets of premises and starting conditions as “normal” eruptions, yet combined in an unexpected, probably unique, way. To cope with such unexpected consequences, we involve an approach of artificial intelligence developed specially for needs of the geosciences, the event bush. Scenarios inferred from the event bush fit the observed ones and allow to foresee other low-probability events that may occur at the volcano. Application of the event bush provides a more impartial vision of volcanic phenomena and may serve as an intermediary between expert knowledge and numerical assessment, e.g., by means of Bayesian Belief Networks.

Despite the recent recognition of Mount Etna as a periodically violently explosive volcano, the hazards from various types of pyroclastic density currents (PDCs) have until now received virtually no attention at this volcano. Large-scale pyroclastic flows last occurred during the caldera-forming Ellittico eruptions, 15–16 ka ago, and the risk of them occurring in the near future is negligible. However, minor PDCs can affect much of the summit area and portions of the upper flanks of the volcano. During the past ~ 20 years, small pyroclastic flows or base-surge-like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of Etna. Four different mechanisms of PDC generation have been identified during these events: (1) collapse of pyroclastic fountains (as in 2000 and possibly in 1986); (2) phreatomagmatic explosions resulting from mixing of lava with wet rock (2006); (3) phreatomagmatic explosions resulting from mixing of lava with thick snow (2007); (4) disintegration of the unstable flanks of a lava dome-like structure growing over the rim of one of the summit craters (1999). All of these recent PDCs were of a rather minor extent (maximum runout lengths were about 1.5 km in November 2006 and March 2007) and thus they represented no threat for populated areas and human property around the volcano. Yet, events of this type pose a significant threat to the lives of people visiting the summit area of Etna, and areas in a radius of 2 km from the summit craters should be off-limits anytime an event capable of producing similar PDCs occurs. The most likely source of further PDCs in the near future is the Southeast Crater, the youngest, most active and most unstable of the four summit craters of Etna, where 6 of the 8 documented recent PDCs originated. It is likely that similar hazards exist in a number of volcanic settings elsewhere, especially at snow- or glacier-covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration.

After 16 months of quiescence, Mount Etna began to erupt again in mid-July 2006. The activity was concentrated at and around the Southeast Crater (SEC), one of the four craters on the summit of Etna, and eruptive activity continued intermittently for 5 months. During this period, numerous vents displayed a wide range of eruptive styles at different times. Virtually all explosive activities took place at vents at the summit of the SEC and on its flanks. Eruptive episodes, which lasted from 1 day to 2 weeks, became shorter and more violent with time. Volcanic activity at these vents was often accompanied by dramatic mass-wasting processes such as collapse of parts of the cone, highly unusual flowage processes involving both old rocks and fresh magmatic material, and magma–water interaction. The most dramatic events took place on 16 November, when numerous rockfalls and pyroclastic density currents (PDCs) were generated during the opening of a large fracture on the SE flank of the SEC cone. The largest PDCs were clearly triggered explosively, and there is evidence that much of the energy was generated during the interaction of intruding magma with wet rocks on the cone’s flanks. The most mobile PDCs traveled up to 1 km from their source. This previously unknown process on Etna may not be unique on this volcano and is likely to have taken place on other volcanoes. It represents a newly recognized hazard to those who visit and work in the vicinity of the summit of Etna.

Volcanism at Mount Etna (Italy) has been observed for millennia and inspired ancient mythologies as well as scientific thought through countless generations. Yet much of our understanding of the way this volcano works stems from studies of the past 20 years, and in particular from strengthened monitoring since the late 1980s. In addition, the eruptive activity of Etna has undergone significant changes during the past 13 years, and these have led to an improved understanding of the relationship between the plumbing system of the volcano and instability of its eastern to southern flanks. Following the end of the 1991–1993 eruption, a new eruptive cycle began, which so far has produced about 0.23 km 3 of lavas and pyroclastics (dense-rock equivalent). The cycle evolved from initial recharging of the plumbing system and inflation, followed by powerful summit eruptions and slow spreading of the eastern to southern flanks, to a sequence of flank eruptions accompanied by accelerated flank displacement. Structurally, the volcanic system has become increasingly unstable during this period. Volcanological, geophysical and geochemical data allow the cause–effect and feedback relationships between magma accumulation below the volcano, flank instability, and the shift from continuous summit activity to episodic flank eruptions to be investigated. In this scenario, the growth of magma storage areas at a depth of 3–5 km below sea level exerts pressure against those flank sectors prone to displacement, causing them to detach from the stable portions of the volcanic edifice. Geochemical data indicate that magma remains stored below the volcano, even during phases of intense eruptive activity, thus causing a net volumetric increase that is accommodated by flank displacement. Instability can be enhanced by the forceful uprise of magma through the flanks, as in 2001, when the first flank eruption of the current eruptive cycle took place. Subsequent flank eruptions in 2002–2003 and 2004–2004, on the other hand, were, at least in part, facilitated by the opening of fractures at the head of moving flank sector, although the eruptions were significantly dissimilar from one another. Renewed inflation of the volcano after the 2004–2005 eruption, continued displacement of the unstable flank sector, and gradual resumption of summit activity in late-2005, demonstrate that the same feedback mechanisms continue to be active, and the Etna system remains highly unstable. The evolution of earlier eruptive cycles shows that a return to a state of relative stability is only possible once a voluminous flank eruption effectively drains the magmatic plumbing system.

Between 1971 and 2001, the Southeast Crater was the most productive of the four summit craters of Mount Etna, with activity that can be compared, on a global scale, to the opening phases of the Pu‘u ‘Ō‘ō-Kūpaianaha eruption of Kīlauea volcano, Hawai‘i. The period of highest eruptive rate was between 1996 and 2001, when near-continuous activity occurred in five phases. These were characterized by a wide range of eruptive styles and intensities from quiet, non-explosive lava emission to brief, violent lava-fountaining episodes. Much of the cone growth occurred during these fountaining episodes, totaling 105 events. Many showed complex dynamics such as different eruptive styles at multiple vents, and resulted in the growth of minor edifices on the flanks of the Southeast Crater cone. Small pyroclastic flows were produced during some of the eruptive episodes, when oblique tephra jets showered the steep flanks of the cone with hot bombs and scoriae. Fluctuations in the eruptive style and eruption rates were controlled by a complex interplay between changes in the conduit geometry (including the growth of a shallow magma reservoir under the Southeast Crater), magma supply rates, and flank instability. During this period, volume calculations were made with the aid of GIS and image analysis of video footage obtained by a monitoring telecamera. Between 1996 and 2001, the bulk volume of the cone increased by ~36×106 m3, giving a total (1971–2001) volume of ~72×106 m3. At the same time, the cone gained ~105 m in height, reaching an elevation of about 3,300 m. The total DRE volume of the 1996–2001 products was ~90×106m3. This mostly comprised lava flows (72×106 m3) erupted at the summit and onto the flanks of the cone. These values indicate that the productivity of the Southeast Crater increased fourfold during 1996–2001 with respect to the previous 25 years, coinciding with a general increase in the eruptive output rates and eruption intensity at Etna. This phase of intense summit activity has been followed, since the summer of 2001, by a period of increased structural instability of the volcano, marked by a series of important flank eruptions.

The island of Pantelleria is an active volcano located in the Sicily Channel (Southern Italy), in the middle of a continental rift system. Since the 1980s the island was periodically surveyed by using geodetic techniques (EDM, levelling, GPS and high precise gravimetry) to monitor the regional and local volcanic dynamics. Gravity data, collected between 1990 and 1998, show short and long wavelength changes due to the combined effect of shallow and deep sources. They reflect, to some degree, the structural setting of the island as delineated by the Bouguer anomaly field, which indicates that the island is broken up into two main basement blocks. The latter are bordered by two lineaments, probably regional faults related to the global geodynamics of the Sicily Channel Rift Zone. Moreover, the inverse correlation between the gravity and altimetric variations suggests that: i) Pantelleria is kinematically divided in two blocks; ii) the observed behaviour is strongly influenced by the geodynamics of the Sicily Channel. A new interpretation of the fully reprocessed data sets is presented, focusing on the spatial–temporal features of the horizontal ground deformation and gravity changes compared to the Bouguer anomaly and altimetric data. This leads to conclude that volcanism on the island has been probably strongly influenced by the global geodynamics of the Sicily Channel, and future eruptions are most likely to occur at the structural boundary separating the two blocks.

Large-scale flank instability on Mount Etna is associated with a distinct set of faults radiating generally from the summit area and restricted to the volcanic edifice itself. New observations and mapping of very recent and continuing deformation along these faults and related structures have been analysed in combination with published information, including recent seismic and eruption data, enabling the faults to be placed in three groups. Two of these, the Pernicana fault system (PFS) and the Ragalna fault system (RFS) bound, respectively, the northern and south-western margins of instability. Their activity responds to cycles of magma pressure associated with flank eruptions, together with subsequent deflation as gravity dominates. These cycles may operate at different depths, with the RFS bordering deep-seated instability. Their positions appear governed by the contact, in the substrate of the volcano, between relatively weak early Quaternary clays and stronger rocks of the Apennine–Maghrebian Chain that rise towards the north and west in the subsurface, buttressing the edifice in these directions. The unstable mass to the un-buttressed south and east is thus defined by its weak substrate and displays structures similar to those produced in model experiments. The third fault group, the Mascaluci–-Trecastagni fault system, borders a rather faster-moving zone of instability in the eastern part of the large unstable mass, outlining one element in a nested pattern in map view. Low-angle detachments below the unstable zones are thought to occur at different levels above a deep and laterally extensive detachment associated with the RFS, producing a nested pattern in section as well. This is illustrated by the PFS where the long-recognised western half of the fault borders a fast moving zone of instability riding above a detachment that daylights as a thrusted deformation front marked by recurring landsliding at an approximate mid-slope position on the volcano. Downslope, the newly recognised eastern extension of the PFS, exhibiting slip-rates an-order-of-magnitude lower than the western segment, is thought to border a deeper slow-moving detachment that daylights offshore. Windows of deformed sub-Etnean clays at anomalously high altitudes may indicate where similar detachments, no longer mechanically favoured and now inactive, have daylighted. As a result, the edifice can be considered, overall, as consisting of multiple unstable areas, nested in plan view and with basal detachments occurring at different levels in section. This model of edifice behaviour is regarded as an evolving one, with detachments waxing and waning in their activity as flank movement progresses.

While the eruptive record of Mount Etna is reasonably complete for the past 400 years, the activity of the early and late 1960s, which took place at the summit, is poorly documented in the scientific literature. From 1955 to 1971, the Central and Northeast Craters were the sites of long-lived mild Strombolian and effusive activity, and numerous brief episodes of vigorous eruptive activity, which led to repeated overflows of lava onto the external flanks of the volcano. A reconstruction of the sequence of the more important of these events based on research in largely obscure and nearly inaccessible sources permits a better understanding of the eruption dynamics and rough estimates of erupted volumes and of the changes to the morphology of the summit area. During the first half of 1964, the activity culminated in a series of highly dynamic events at the Central Crater including the opening of a fissure on the E flank of the central summit cone, lava fountains, voluminous tephra emission, prolonged strong activity with continuous lava overflows, and growth of large pyroclastic intracrater cones. Among the most notable processes during this eruption was the breaching of a section of the crater wall, which was caused by lateral pressure of lava ponding within the crater. Comparison with the apparently similar summit activity of 1999 allows us to state that (a) lava overflows from large pit craters at the summit are often accompanied by breaching of the crater walls, which represents a significant hazard to nearby observers, and that (b) eruptive activity in 1999 was much more complex and voluminous than in 1964. For 1960s standards however, the 1964 activity was the most important summit eruption in terms of intensity and output rates for about 100 years, causing profound changes to the summit morphology and obliterating definitively the former Central Crater.

The July–August 2001 eruption of Mt. Etna stimulated widespread public and media interest, caused significant damage to tourist facilities, and for several days threatened the town of Nicolosi on the S flank of the volcano. Seven eruptive fissures were active, five on the S flank between 3,050 and 2,100 m altitude, and two on the NE flank between 3,080 and 2,600 m elevation. All produced lava flows over various periods during the eruption, the most voluminous of which reached a length of 6.9 km. Mineralogically, the 2001 lavas fall into two distinct groups, indicating that magma was supplied through two different and largely independent pathways, one extending laterally from the central conduit system through radial fissures, the other being a vertically ascending eccentric dike. Furthermore, one of the eccentric vents, at 2,570 m elevation, was the site of vigorous phreatomagmatic activity as the dike cut through a shallow aquifer, during both the initial and closing stages of the eruption. For 6 days the magma column feeding this vent was more or less effectively sealed from the aquifer, permitting powerful explosive and effusive magmatic activity. While the eruption was characterized by a highly dynamic evolution, complex interactions between some of the eruptive fissures, and changing eruptive styles, its total volume (~25×106 m3 of lava and 5–10×106 m3 of pyroclastics) was relatively small in comparison with other recent eruptions of Etna. Effusion rates were calculated on a daily basis and reached peaks of 14–16 m3 s-1, while the average effusion rate at all fissures was about 11 m3 s-1, which is not exceptionally high. The eruption showed a number of peculiar features, but none of these (except the contemporaneous lateral and eccentric activity) represented a significant deviation from Etna's eruptive behavior in the long term. However, the 2001 eruption could be but the first in a series of flank eruptions, some of which might be more voluminous and hazardous. Placed in a long-term context, the eruption confirms a distinct trend, initiated during the past 50 years, toward higher production rates and more frequent eruptions, which might bring Etna back to similar levels of activity as during the early to mid seventeenth century.

Mount Etna is among the few volcanoes on Earth that erupt nearly continuously, but its activity (in terms of output rate and flank eruption frequency) undergoes significant fluctuations in time. Such fluctuations do not occur randomly, but represent various stages of cycles on a scale of decades and centuries. Recurrent patterns are particularly evident since 1865, with four complete cycles and a fifth one initiated in 1993. Each cycle consists of three phases, beginning with low-level activity followed by nearly continuous summit activity and culminating with a series of flank eruptions, the last commonly being the most voluminous. A distinct increase in the output rate of Etna, along with more frequent and voluminous summit and flank eruptions since 1950, may be interpreted as part of a longer cycle that began after a large eruption in 1669 and has not yet reached its culminating phase. If that trend continues, the activity of Etna might become similar to that of the 17th century, when flank eruptions were more voluminous than they have been since; however, it is difficult to assess when this will take place.

During an eruption at the Bocca Nuova, one of the summit craters of Mt. Etna, in October–November 1999 a part of the crater floor near its WNW rim was uplifted to form a dome-shaped feature that consisted of older lava and pyroclastics filling the crater. This endogenous dome grew rapidly over the crater rim, thus being perched precariously over the steep outer slope of the Bocca Nuova, and near-continuous collapse of its steep flanks generated swiftly moving pyroclastic avalanches over a period of several hours. These avalanches advanced at speeds of 10–20 m/s and extended up to 0.7 km from their source on top of lavas emplaced immediately before. Their deposits were subsequently covered by lava flows that issued from vents below the front of the dome and from the Bocca Nuova itself. Growth of the dome was caused by the vertical intrusion of magma in the marginal western part of the crater, which deformed and uplifted previously emplaced, still hot and plastically deformable eruptive products filling the crater. The resulting avalanches had all the characteristics of pyroclastic flows spawned by collapse of unstable flanks of lava domes, but in this case the magma involved was of mafic (hawaiitic) composition and would, under normal circumstances, have produced fluid lava flows. The formation of the dome and the generation of the pyroclastic avalanches owe their occurrence to the rheological properties of the eruptive products filling the crater, which were transformed into the dome, and to the morphological configuration of the Bocca Nuova and its surroundings. The density contrast between successive erupted products may also have played a role. Although events of this type are to be considered exceptional at Etna, their recurrence might represent a serious hazard to visitors to the summit area.

During 2013 Mount Etna volcano experienced intense eruptive activity at the summit craters, foremost at the New Southeast Crater and to a minor degree at the Voragine and Bocca Nuova (BN), which took place in two cycles, February–April and September–December. In this work, we mainly focus on the period between these cycles, applying a multiparametric approach. The period from the end of April to 5 September showed a gradual increase in the amplitude of long-period (LP) events and volcanic tremor, a slight inflation testified by both tilt and GPS data, and a CO2 flux increase. Such variations were interpreted as due to a gradual pressurization of the plumbing system, from the shallowest part, where LP and volcanic tremor are located, down to about 3–9 km below sea level, pressure source depths obtained by both geodetic and CO2 data. On 5 September, at the same time as a large explosion at BN, we observed an instantaneous variation of the aforementioned signals (decrease in amplitude of LP events and volcanic tremor, slight deflation, and CO2 flux decrease) and the activation of a new infrasonic source located at BN. In the light of it, the BN explosion probably caused the instantaneous end of the pressurization, and the opening of a new vent at BN, that has become a new steady source of infrasonic events. This apparently slight change in the plumbing system also led to the gradual resumption of activity at the New Southeast Crater, culminating with the second lava fountain cycle of 2013.

Stromboli is known for its mild, persistent explosive activity from the vents located within the summit crater depression at the uppermost part of the Sciara del Fuoco (SdF) depression. Effusive activity (lava flows) at this volcano normally occurs every 5–15 years, involving often the opening of eruptive fissures along the SdF, and more rarely overflows from the summit crater. Between the end of the 2007 effusive eruption and December 2012, the number of lava flows inside and outside the crater depression has increased significantly, reaching a total of 28, with an average of 4.8 episodes per year. An open question is why this activity has become so frequent during the last 6 years and was quite rare before. In this paper, we describe this exceptional activity and propose an interpretation based on the structural state of the volcano, changed after the 2002–2003 and even more after the 2007 flank effusive eruption. We use images from the Stromboli fixed cameras network, as well as ground photos, plume SO2 and CO2 fluxes released by the summit crater, and continuous fumarole temperature recording, to unravel the interplay between magma supply, structural and morphology changes, and lava flow output. Our results might help forecast the future behaviour and hazard at Stromboli and might be applicable to other open-conduit volcanoes.

Mount Etna produces frequent eruptions from its summit craters and from fissures on its flanks. The flank fissures trend approximately radially to the summit, and are mainly concentrated in three rift zones that are located on the NE, S and W flanks. Many flank eruptions result from lateral magma transfer from the central conduit into fractures intersecting the flanks, although some eruptions are fed through newly formed conduits that are not directly linked to the central conduit. We analyzed the structural features of eruptions from 1900 to the present, one of the most active periods in the documented eruptive history of Etna, which comprised 35 summit and 33 flank events. Except for a small eruption on the W flank in 1974, all of the flank eruptions in this interval occurred on or near the NE and S rifts. Eruptions in the NE sector were generally shorter, but their fissure systems developed more rapidly and were longer than those in the S sector. In contrast, summit eruptions had longer mean durations, but generally lower effusion rates (excluding paroxysmal events characterized by very high effusion rates that lasted only a few hours). This database was examined considering the main parameters (frequency and strike) of the eruptive fissures that were active over the last ~2 ka. The distribution in time and space of summit and flank eruptions appears to be closely linked to the dynamics of the unstable E to S flank sector of Etna, which is undergoing periodic displacements induced by subvolcanic magma accumulation and gravitational pull. In this framework, magma accumulation below Etna exerts pressure against the unbuttressed E and S flanks, which have moved away from the rest of the volcano. This has caused an extension to the detachment zones, and has facilitated magma transfer from the central conduit into the flanks.

The individuation of areas that are more likely to be affected by new events in volcanic regions is of fundamental relevance for the mitigation of the possible consequences, both in terms of loss of human life and material properties. Here, we describe a methodology for defining flexible high-detail lava-hazard maps and a technique for the validation of the results obtained. The methodology relies on: (i) an accurate analysis of the past behavior of the volcano; (ii) a new version of the SCIARA model for lava-flow simulation (based on the macroscopic cellular automata paradigm); and (iii) high-performance parallel computing for increasing computational efficiency. The new release of the SCIARA model introduces a Bingham-like rheology as part of the minimization algorithm of the differences for the determination of outflows from a generic cell, and an improved approach to lava cooling. The method is here applied to Mount Etna, the most active volcano in Europe, and applications to landuse planning and hazard mitigation are presented. © 2011 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

Active volcanoes are spread all over the world and are located in specific areas correlated to geologic structures. In the last 10,000 years more than 1300 volcanoes have erupted, but only half of the eruptions have been reported in historical texts. It has been estimated that every year 50 volcanic eruption may occur threating about 10 % of the worldwide population. Considering the constant increase in human population and that many major cities are placed in the proximity of active volcanoes, the number of people subjected to the risks caused by volcanic eruptions is also increasing. Stated the problem dimension, it shall be defined the method to counter act the crisis scenario. A volcanic eruption can be faced under different point of view, on one side it can be deeply studied in order to improve the knowledge of geophysical mechanism which drives the eruption itself, on the other hand it is important to set up a service based on the integration of different dataset and aimed to deliver added values products toward whose are in charge to manage the risk associated. One response to last sentence is the Sistema Rischio Vulcanico (ASI-SRV) project funded by the Italian Space Agency (ASI) in the frame of the National Space Plan 2003-2005 under the Earth Observations section for natural risks management. The ASI-SRV Project is coordinated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) which is responsible, in Iyaly, at national level for the volcanic monitoring. The main goal of the ASISRV project is to define, develop and demonstrate tools and products, based on the integration Earth Observation data (EO) with data coming from the ground monitoring network developed at national and local level, to support the risk management decision procedures. The project philosophy is to implement, by incremental versions, specific modules which allow to process, store and visualize through a dedicated Web-GIS tools, added value information derived by EO and No EO data.

We are grateful to EUMETSAT for SEVIRI data, to NASA for MODIS data, and toNOAAfor AVHRR data. The authors thank one anonymous reviewer and V. Acocella for their helpful and constructive comments. This study was performed with the financial support from the V3‐LAVA project (INGV‐DPC 2007‐2009 contract).

We describe a new type of secondary rootless phreatomagmatic explosions observed at active lava flows at volcanoes Klyuchevskoy (Russia) and Etna (Italy). The explosions occurred at considerable (up to 5 km) distances from primary volcanic vents, generally at steep (15–35°) slopes, and in places where incandescent basaltic or basaltic-andesitic lava propagated over ice/water-saturated substrate. The explosions produced high (up to 7 km) vertical ash/steam-laden clouds as well as pyroclastic flows that traveled up to 2 km downslope. Individual lobes of the pyroclastic flow deposits were up to 2 m thick, had steep lateral margins, and were composed of angular to subrounded bomb-size clasts in a poorly sorted ash–lapilli matrix. Character of the juvenile rock clasts in the pyroclastic flows (poorly vesiculated with chilled and fractured cauliflower outer surfaces) indicated their origin by explosive fragmentation of lava due to contact with external water. Non-juvenile rocks derived from the substrate of the lava flows comprised up to 75% in some of the pyroclastic flow deposits. We suggest a model where gradual heating of a water-saturated substrate under the advancing lava flow elevates pore pressure and thus reduces basal friction (in the case of frozen substrate water is initially formed by thawing of the substrate along the contact with lava). On steep slope this leads to gravitational instability and sliding of a part of the active lava flow and water-saturated substrate. The sliding lava and substrate disintegrate and intermix, triggering explosive “fuel–coolant” type interaction that produces large volume of fine-grained clastic material. Relatively cold steam-laden cloud of the phreatomagmatic explosion has limited capacity to transport upward the produced clastic material, thus part of it descends downslope in the form of pyroclastic flow. Similar explosive events were described for active lava flows of Llaima (Chile), Pavlof (Alaska), and Hekla (Iceland) indicating that this type of explosions and related hazard is common at snow/ice-clad volcanoes and sometimes happens also on fluid-saturated hydrothermally altered slopes.

[1] Fracture reactivation is a widespread process in nature even though evidence of magma-induced reactivation is less documented. Here we provide evidence of the reactivation of a fracture system on the upper flank of the Mt. Etna volcano and consider its possible implications in understanding the recent volcanic and tectonic activity. A NNW–SSE trending fracture, partly accompanied by magma emplacement in the form of a laterally propagating dike, formed in 1989 on the upper SE flank of Etna. Lava effusions in 1991–1993, 2001, and 2006 were associated with volcano-tectonic (VT) seismicity and ground deformations on the upper part of the volcano, which document the seismogenetic involvement of the 1989 fractures, although without eruptive phenomena along the discontinuity. In addition to the aforementioned episodes of VT seismicity, differences in the characteristics of the background seismic radiation (volcanic tremor) were measured at stations close to these fractures during the eruptive activity on 24 November 2006, for which more detailed volcanological and seismological time histories are available. Moving on from these findings, we analyze volcanic tremor data recorded close to the summit and along the S flank of the volcano to highlight the interactions between seismic radiation and the 1989 fracture system. Centroid location of volcanic tremor and wave field characteristics at stations of the permanent local seismic network of Etna highlight the guidance role played by the 1989 fractures during the eruptive activity on 24 November 2006. In addition, the collected data shed light on hitherto unknown structural features, which appear to connect the volcano summit to the lower SE slope and also play an important role in controlling the instability of the E flank. More generally, this study shows how (1) using an integrated approach, it is possible to link apparently different features to a common structure, showing uniform and distinct dynamics relevant at the volcano scale, and (2) fracture reactivation can also occur by means of magma intrusion, playing an important role in the transfer of magma within a volcanic edifice.

This work was sponsored by the Italian Ministry for Education, University and Research, FIRB project n° RBAU01RMZ4 “Lava flow simulations by Cellular Automata”, and by the National Civil Defence Department and INGV (National Institute of Geophysics and Volcanology), project V3_6/09 “V3_6 – Etna”.

The 2001 and 2002–2003 flank eruptions on Mount Etna (Italy) were characterized by intense explosive activity which led to the formation of two large monogenetic scoria cones (one from each eruption) on the upper southern flank of the volcano. Continuous monitoring of Etna, especially during flank eruptions, has provided detailed information on the growth of these cones. They differ in genesis, shape, and size. A set of high resolution (1 m) digital elevation models (DEMs) derived from light detection and ranging (LIDAR) data collected during four different surveys (2004, 2005, 2006, and 2007) has been used to map morphology and to extract the morphometric parameters of the scoria cones. By comparing LIDAR-derived DEMs with a pre-eruption (1998) 10 m DEM, the volume of the two scoria cones was calculated for the first time. Comparison of the LIDAR-derived DEMs revealed in unprecedented detail morphological changes during scoria cone degradation. In particular, the morphologically more exposed and structurally weaker 2002–2003 cone was eroded rapidly during the first few years after its emplacement mainly due to gravitational instability of slopes and wind erosion.

[1] The eruptive episode of Mount Etna’s Southeast Crater (SEC) on 16 November 2006, which culminated with phreatomagmatic explosions and a peculiar volcaniclastic flowage event, is the subject of different interpretations. Behncke [2009] and Behncke et al. [2008, 2009] interpret the explosions as resulting from mixing of flowing lava with fluid-saturated, hydrothermally altered rock and describe the resulting flow as a low-temperature (but potentially deadly) pyroclastic density current (PDC). Norini et al. [2009] speak of gravity-induced flank collapse affecting the SEC cone, leading to the emplacement of a landslide (or debris avalanche) deposit. Finally, Ferlito et al. [2009], commenting our recent work [Behncke et al., 2009], repropose their earlier [Ferlito et al., 2007] scenario of a shallow intrusion from the SEC conduit, caused by unloading and decompression when a part of the SEC cone flank was removed (‘‘sector collapse’’), leading to the explosive opening of an eruptive fissure, which discharged a pyroclastic flow. An outstanding feature of this event is that it was not accompanied by any significant change in the seismic signal, which led us [Behncke et al., 2009] to exclude the opening of an eruptive fissure. However, Ferlito et al. point out that seismic evidence alone does not rule out their scenario and cite the lack of seismic signals accompanying the start of the (rather voluminous, in terms of lava discharge, but purely effusive) 2004–2005 Etna eruption as support for their hypothesis. Finally, they describe what they interpret as the source fissure for the phreatomagmatic explosions and PDCs, which was the site of minor lava extrusion toward the end of the 16 November 2006 event. [2] On their Web site, Ferlito et al. host a short (

[1] Three eruptive episodes during the 2006 summit eruptions of Mount Etna were exceptionally well documented by visual, seismic, and thermal monitoring. The first (16 November) was strongly explosive, with vigorous Strombolian activity and ash emission from multiple vents, lava emission, and phreatomagmatic explosions generating pyroclastic density currents (PDCs). The second episode (19 November) had a rather weakly explosive component, with mild Strombolian activity but more voluminous lava emission. The third (24 November) was a moderately explosive paroxysm, with intermittent lava fountaining and generation of a tephra column as well as lava emission and PDCs. Data recorded by a thermal monitoring camera clearly document the different phases of each paroxysm, weather clouds occasionally hampering thermal monitoring. The images show a rapid onset of the volcanic activity, which during each of the paroxysms reached a peak in eruptive and thermal intensity and then decreased gradually. The stronger phreatomagmatic explosions and PDCs on 16 and 24 November did not yield any seismic signature linked to the opening of new vents nor were they associated with peculiar characteristics of the seismic signal. Nevertheless, eruptive styles (Strombolian activity, lava emission) and different levels in the intensity of explosive activity were generally well reflected in the amplitude and frequency content of the seismic signal and in the source location of the volcanic tremor centroid throughout the three eruptive episodes. This multidisciplinary study, therefore, not only provides a key to distinguish between endogenous and exogenous origins of the phenomena observed but also documents the complex magma dynamics within the volcano.

1986–2007 period amounts to 112 ± 12 10 6 m 3 ,a t a mean annual rate of 5.3 10 6 m 3 . The comparison of the various surveys furthermore emphasizes the levels of accuracy and resolution of the different techniques applied. The Lidar technology used in 2007 allows production of high-precision maps in near-real-time, facilitating work concerning environmental hazards such as numerical simulations of, e.g., lava flows. Citation: Neri, M., F. Mazzarini, S. Tarquini, M. Bisson, I. Isola, B. Behncke, and M. T. Pareschi (2008), The changing face of Mount Etna’s summit area documented with Lidar technology, Geophys. Res. Lett., 35, L09305, doi:10.1029/2008GL033740.

[1] Continuous soil radon monitoring was carried out near the Southeast Crater (SEC) of Mt. Etna during the 10-day July 2006 Strombolian-effusive eruption. This signal was compared with simultaneously acquired volcanic tremor and thermal radiance data. The onset of explosive activity and a lava fountaining episode were preceded by some hours with increases in radon soil emission by 4–5 orders of magnitude, which we interpret as precursors. Minor changes in eruptive behavior did not produce significant variations in the monitored parameters. The remarkably high radon concentrations we observed are unprecedented in the literature. We interpret peaks in radon activity as due primarily to microfracturing of uranium-bearing rock. These observations suggest that radon measurements in the summit area of Etna are strongly controlled by the state of stress within the volcano and demonstrate the usefulness of radon data acquisition before and during eruptions. Citation: Neri, M., B. Behncke, M. Burton, G. Galli, S. Giammanco, E. Pecora, E. Privitera, and D. Reitano (2006), Continuous soil radon monitoring during the July 2006 Etna eruption, Geophys. Res. Lett., 33, L24316, doi:10.1029/ 2006GL028394.

Mount Etna produced two significant eruptions in 2001 and 2002–2003, which we have analysed using geological, seismic and deformation data. These eruptions showed some similarities, such as the activating of two magmatic plumbing systems (central–lateral and eccentric), but they differed in their triggering mechanisms. While the 2001 eruption was largely the result of the emplacement of a N–S eccentric dike (independent from the central conduits) consistent with E–W regional extension, the 2002–2003 eruption occurred in response to a major flank slip on the eastern and southeastern sides of the volcano. This is demonstrated by the spatial and temporal distribution of seismicity and deformation preceding and accompanying the two eruptions. During the months prior to the 2001 eruption, most epicenters were concentrated on the southern flank, at depths of 5–15 km below sea level. During the 4 days before the eruption, earthquake hypocenters migrated to shallower levels (from 5 km bsl. upward) indicating the emplacement of the eccentric dike. This is confirmed by the patterns of ground fracturing observed in the field and deformation documented by electronic distance measurements (EDM). In contrast, the months before the 2002–2003 eruption were characterised by shallower seismicity, mainly concentrated along the active faults bordering the slipping flank sector. Flank slip accelerated in September 2002 and a second, more vigorous acceleration of flank slip occurred on 26–27 October 2002, accompanying the opening of eruptive vents. The very short (2 h) seismic crisis preceding the onset of eruptive activity stands in neat contrast with the 4 days of intense seismicity before the 2001 eruption. Subsequently, flank slip-deformation extended all over the eastern and southeastern flanks of the volcano, causing serious damage in this sector. The events of 2001–2003 can be seen as a continuous chain of intimately interacting processes including regional tectonics, magma accumulation and eruption, and flank instability. In this scenario the 2001 eruption led to increased flank instability that subsequently accelerated and culminated with the massive flank slip, which in turn facilitated the 2002–2003 eruption. This sequence of events points to a long-term feedback mechanism between magmatism and flank instability at Etna.

[1] Soil radon emissions have been proved as a useful tool for predicting earthquakes and volcanic eruptions and furthermore aided in determining the location of active faults. Continuous radon monitoring was carried out near Southeast Crater of Mt. Etna in September–November 1998, during a period of frequent eruptive episodes at that crater. Radon anomalies were detected when eruptive episodes and the accompanying volcanic tremor became increasingly intense: no anomalies in radon activity were observed during the first five, and weaker, eruptive episodes, whereas significant spikes in radon activity preceded the latter five episodes by ≥46 hours. This probably reflects increased gas leakage through fractures intersecting the shallow plumbing system, as gas pressure in the Southeast Crater conduit became higher with time. Radon monitoring thus might serve to better understand eruptive mechanisms and possible precursors, making further studies in this field a promising perspective.

[1] Since ∼200 Ma, the Hyblean Mountains (southeastern Sicily) have been the site of recurrent, mainly submarine, mafic volcanism. In the upper Pliocene, large volumes of tholeiitic products were erupted in a shallow marine environment, leading to the growth of volcanic islands. The deposits of this tholeiitic episode can be grouped into two main facies associations: (1) pillow lavas and hyaloclastite tuffs erupted and deposited entirely under water, and (2) submarine pillow breccias, fed from subaerial lava flows entering the ocean and subaerial lavas, which are distinguished into a coastal lava platform facies and an inland facies. Lava flows into the ocean led to the growth of lava deltas consisting of a basal flow foot breccia and an overlying sequence of pāhoehoe flow units, the latter rarely exceeding 0.5 m in thickness. In outcrop, this facies association shows evidence for significant instability of the lava deltas during growth, indicating very similar processes to those observed during the current eruption of Kīlauea volcano, Hawaii. Away from the coast of the growing island, the inland facies is characterized by thicker (up to several meters) pāhoehoe flow units showing evidence for sheet flow inflation and limited outcrops of scoria and/or bomb deposits. Evidence for lava transport through tubes exists in the form of tumuli and multiple vesicle layers; drained tubes are absent. All tholeiitic products were emitted in a single, instantaneous, voluminous event (≥120 km3), consistent with very high effusion rates similar to those of flood basalts or the 1783 Skaftar Fires (Iceland) eruption.

Flank instability and collapse are observed at many volcanoes. Among these, Mt. Etna is characterized by the spreading of its eastern and southern flanks. The eastern spreading area is bordered to the north by the E–W-trending Pernicana Fault System (PFS). During the 2002–2003 Etna eruption, ground fracturing along the PFS migrated eastward from the NE Rift, to as far as the 18 km distant coastline. The deformation consisted of dextral en-echelon segments, with sinistral and normal kinematics. Both of these components of displacement were one order of magnitude larger (~1 m) in the western, previously known, portion of the PFS with respect to the newly surveyed (~9 km long) eastern section (~0.1 m). This eastern section is located along a pre-existing, but previously unknown, fault, where displaced man-made structures give overall slip rates (1–1.9 cm/year), only slightly lower than those calculated for the western portion (1.4–2.3 cm/year). After an initial rapid motion during the first days of the 2002–2003 eruption, movement of the western portion of the PFS decreased dramatically, while parts of the eastern portion continued to move. These data suggest a model of spreading of the eastern flank of Etna along the PFS, characterized by eruptions along the NE Rift, instantaneous, short-lived, meter-scale displacements along the western PFS and more long-lived centimeter-scale displacements along the eastern PFS. The surface deformation then migrated southwards, reactivating, one after the other, the NNW–SSE-trending Timpe and Trecastagni faults, with displacements of ~0.1 and ~0.04 m, respectively. These structures, along with the PFS, mark the boundaries of two adjacent blocks, moving at different times and rates. The new extent of the PFS and previous activity over its full length indicate that the sliding eastern flank extends well below the Ionian Sea. The clustering of seismic activity above 4 km b.s.l. during the eruption suggests a deep decollement for the moving mass. The collected data thus suggests a significant movement (volume >1,100 km3) of the eastern flank of Etna, both on-shore and off-shore.