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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 x 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.
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published: 14 June 2016
doi: 10.3389/feart.2016.00067
Frontiers in Earth Science | 1June 2016 | Volume 4 | Article 67
Edited by:
Geoffrey Wadge,
University of Reading, UK
Reviewed by:
Roberto Sulpizio,
Università degli Studi di Bari, Italy
Alessandro Tibaldi,
University of the Studies of Milan
Bicocca, Italy
Valerio Acocella
Specialty section:
This article was submitted to
a section of the journal
Frontiers in Earth Science
Received: 31 March 2016
Accepted: 23 May 2016
Published: 14 June 2016
Acocella V, Neri M, Behncke B,
Bonforte A, Del Negro C and Ganci G
(2016) Why Does a Mature Volcano
Need New Vents? The Case of the
New Southeast Crater at Etna.
Front. Earth Sci. 4:67.
doi: 10.3389/feart.2016.00067
Why Does a Mature Volcano Need
New Vents? The Case of the New
Southeast Crater at Etna
Valerio Acocella 1*, Marco Neri 2, Boris Behncke 2, Alessandro Bonforte 2, Ciro Del Negro 2
and Gaetana Ganci 2
1Dipartimento di Scienze, Università degli Studi Roma Tre, Roma, Italy, 2Istituto Nazionale di Geofisica e Vulcanologia,
Sezione di Catania, Osservatorio Etneo, Catania, Italy
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.
Keywords: eruptive vents, volcano, stress, flank instability, Etna
Mature stratovolcanoes or composite volcanoes usually erupt from a persistent summit crater.
Indeed, most volcanic edifices do not show variations in the location of summit volcanism,
constantly erupting from the same vent, and especially on the short-term (100 of years or less).
This persistency may be found also after major eruptions, and even when these are associated
with important structural variations, as the development of sector collapses, as for example at
Bezymianny in 1956 and at Mount St. Helens in 1980 (e.g., Belousov et al., 2007, and references
therein). Of course, monogenic dike-fed eruptive fissures on the volcano flanks may develop at any
Acocella et al. Growth of the NSEC at Etna
time (Acocella and Neri, 2009, and references therein); however,
these eruptions are usually not accompanied by permanent
variations in the location of summit activity, so that future
eruptions may be expected to occur again from the same summit
Despite this persistency, the geological record of some active
volcanoes, not only with calderas, as Sakurajima and Aso (Japan)
or Okmok (Aleutians), but also including large edifices, as
Etna, show a more complicated eruptive pathway; this displays
multiple permanent (associated with stable polygenetic activity)
craters or cones, suggesting that the location of volcanism within
the edifice may vary. However, on the short term (decades or
less) there is poor direct witnessing of any permanent shift
(creating a new polygenetic cone or crater) in the location of the
eruptive vents within a volcano. The rarity of such occurrence
also underlines the difficulty in detecting and understanding the
possible processes responsible for the variation in the location of
volcanism, which at present remain largely elusive.
FIGURE 1 | (A) Tectonic framework of Mount Etna, black arrows indicate the unstable eastern and southern flanks of the volcano. (B) Eruptive and dry fissures (black
lines) opened at the summit of the volcano during 1998–2007, and lava flows erupted in this time interval (light gray). In the last 100 years Etna summit evolved from
one Central Crater that existed in the early 20th century (C) to 5 summit craters in 2015 (D). VOR, Voragine crater; NEC, Northeast Crater; BN, Bocca Nuova crater;
SEC, Southeast Crater; NSEC, New Southeast Crater. Pit, crater discontinuously active in spring 2007. Original photograph in (C) was scanned from a postcard that
is out of print. The topography in (B) is based on a DEM from Bisson et al. (2016).
Possibly, one of the best-documented shifts in volcanic activity
has recently occurred at Etna. Indeed, one of the most intriguing
features of Etna is the growing number of its summit craters
(Figure 1), which has increased from one (the former Central
Crater, which had existed for many centuries; Guest, 1973) to
four in an interval of just 60 years: the Northeast Crater (1911),
the Voragine (1945), the Bocca Nuova (1968), and the Southeast
Crater (1971) (Del Negro et al., 2013, and references therein).
Since the late-1970s, the Southeast Crater (SEC) has been by
far the most active, and evolved from a large collapse pit into a
>200 m high cone (Behncke et al., 2006), becoming a distinctive
landmark at Etna’s summit from the south and the east. After
an important episode of flank slip in late 2002, associated with
the 2002–2003 eruption (Acocella et al., 2003; Neri et al., 2004),
volcanic activity focused on the SEC and, from 2007, at a new vent
at its SE base (proto-NSEC). This vent, fed by NW-SE striking
dikes propagating from the SEC, formed an independent cone
(the New Southeast Crater, or NSEC), starting from January 2011.
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Acocella et al. Growth of the NSEC at Etna
The shift of the focus of activity from the SEC to the NSEC has
been dealt with only marginally in studies dealing with other
aspects of the activity in that period (Andronico et al., 2008;
Langer et al., 2011; Behncke et al., 2014; Falsaperla et al., 2014; De
Beni et al., 2015; Falsaperla and Neri, 2015), and so far remains
essentially unexplained.
Here, we describe and quantify the main parameters of the
eruptive activity, first at the SEC (spring 2007) and then at the
proto-NSEC (summer-fall 2007), using field and satellite data to
calculate eruption rates and volumes. Then, using field structural
and geodetic (GPS) data, we provide an explanation for the
development of the NSEC. In particular, our study at Etna shows
how flank instability may control the distribution of volcanism,
including the prolonged shift of the summit vent activity at a
mature stratovolcano. Overall, this study provides one of the few
cases of monitored variation in the location of activity on the
summit of a volcanic edifice.
Since the second half of the 20th century, Etna’s eruptive activity
has undergone several notable changes. Firstly, it has intensified
in terms of eruption frequency, long-term average eruption rate,
and explosivity, which is most clearly recently expressed in the
occurrence of two explosive flank eruptions (2001 and 2002–
2003; Acocella and Neri, 2003; Andronico et al., 2005), and an
increasing number of strongly explosive eruptions (“paroxysms”)
at the summit craters (Allard et al., 2006, and references therein).
This activity has been accompanied, in 2001–2010, by accelerated
flank instability in the eastern to southern sectors of the volcano
(Neri et al., 2004; Neri and Acocella, 2006; Bonforte et al., 2007;
Falsaperla et al., 2010). This instability may have been caused
by the pressurization induced by rapid and voluminous magma
accumulation within and below the edifice (Allard et al., 2006),
and may have been the most significant event of this type at
Etna for many decades or more. During the 2001 and 2002–
2003 eruptions and the flank slip episode, the shallow central
conduit system—i.e., the magma pathways leading to the summit
craters—was disrupted; activity ceased at the SEC for 5 years,
whereas the Bocca Nuova remained largely inactive until 2011,
and the Voragine reactivated only in 2013, after more than 13
years of quiescence.
In addition, one of the most peculiar characteristics in the
recent change of Etna’s dynamics is the appearance of numerous,
prolonged series of paroxysmal eruptive episodes at the summit
craters since the mid-1960s. Such events had previously occurred
once or twice per decade (Behncke and Neri, 2003), but at the
turn of the millennium they had become the most characteristic
eruptive manifestation at Etna. In particular, out of a total of
about 150 paroxysmal eruptive episodes at the summit craters
between 1995 and 2001, 105 occurred at the SEC (Behncke et al.,
2006). Many of the paroxysms at the SEC were characterized
by activity from vents not only at its summit, but also from
fissures extending down its north-eastern and southern flanks.
This trend remained remarkably stable during the exceptional
FIGURE 2 | Morpho-structural evolution of the Southeast Crater (SEC)
zone in the last decade, interpreting the aerial view from SE taken from
a helicopter of the Italian Civil Defense (A) and of the Italian Coast
Guard (B). The main eruptive and dry fissures and their age are marked by
white dotted lines. (A) Location of the “pit crater,” formed in the spring of
2007, is the same of the “proto-NSEC” active since autumn 2007. (B) The
proto-NSEC then evolved into the New Southeast Crater (NSEC), whose cone
grew through several tens of paroxysmal eruptions since January 2011.
series of paroxysms in 2000 (Alparone et al., 2003; Behncke et al.,
2006) and another sequence of paroxysms in 2001 (Figure 2A).
In contrast, renewed episodic activity at the SEC in 2006 involved
the opening of new vents and fissures on its SE, WNW, and W
flanks (Neri et al., 2006; Behncke et al., 2008), marking a rotation
by about 90of the main structural trend at the SEC (Figure 2A).
In late-2004, during a long-lived effusive lateral eruption from
vents to the east of the SEC, a collapse pit opened on its eastern
slope (Neri and Acocella, 2006). This pit was filled by renewed
activity from the SEC in the fall of 2006 (Behncke et al., 2008),
but a new collapse pit formed toward the end of this activity, only
to be filled again by renewed SEC activity in the spring of 2007
(Behncke et al., 2014).
Finally, another pit crater formed on the lower east flank
of the SEC cone in mid-May 2007 (Figure 2A), and 3 months
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Acocella et al. Growth of the NSEC at Etna
later this became the focus of renewed eruptive activity, marking
a definitive shift from the SEC to the vent which later would
became known as the New Southeast Crater (NSEC, Figure 2B).
Since this pit crater underwent further changes after the 2007
activity, which is described in detail below, we refer to it as
“proto-NSEC” to distinguish it from the cone (the NSEC) that
started properly growing during the eruptive period initiated in
January 2011 (Figure 2).
Field Observations
Field observations, consisting of volcanological and structural
data, were directly acquired in the field since the 1990s and
have been integrated by the image analysis of the INGV
camera network and by aerial photos taken from helicopters
of the Italian Civil Defense and the Italian Coast Guard.
Field observations and volcanological and structural mapping
consisted of measurements of lava flow fields, eruptive/dry
fractures and faults aided by hand-held GPS (lat/lon precision
up to 2–4 m). The birth and growth of the NSEC have been
documented in detail with the aid of range-finding binoculars
connected to GPS (Behncke et al., 2014), as well as LiDAR
surveys carried out in 2007 and 2010 (Behncke et al., 2016), and
aerophotogrammetry acquired in 2012 and 2014 (De Beni et al.,
Thermal Satellite Data
Satellite data processing techniques have proved well suited to
complement field observations for timely detection of eruptive
events, as well as extraction of parameters allowing lava flow
tracking. Satellite imagery can provide a better understanding
of eruptive activity simply by producing more frequent
observations at a wide variety of wavelengths. In particular, in
the case of short-lived events like lava fountaining episodes,
geostationary satellites, with 15 min sample time, provide a
unique opportunity to follow the fast evolution of the event from
As for the 2007 lava fountains at Mt Etna, data acquired
by MODIS (Moderate Resolution Imaging Spectroradiometer)
aboard EOS polar-orbiting satellites, and SEVIRI (Spinning
Enhanced Visible and InfraRed Imager) aboard the geostationary
satellites MSG, were processed via the HOTSAT multiplatform
system (Ganci et al., 2011b, 2015). HOTSAT can detect thermal
anomalies over volcanic areas and quantify the entity of thermal
activity by means of the radiant heat flux computation. Weather
conditions play an important role in volcanic activity detection
and quantification from satellite, which is the reason why
HOTSAT includes a cloud detection algorithm based on textons
(Ganci et al., 2011a) and, for each processed images, it provides
as output a cloud coverage index. A cloud coverage index equal
to 1 means that all the pixels inside the volcanic area are
flagged as cloudy; whereas an index of 0 means that no cloudy
pixels are detected in the volcanic area. HOTSAT discards those
images showing a cloud index >0.5, while a plot of the cloud
index is provided with the radiant heat flux to visually check
if an attenuated or missing thermal anomaly is due to cloud
partial or total obscuration. The system is currently operational
on Etna and Stromboli and was tested versus ground-based
thermal camera measurements acquired on Etna (Ganci et al.,
2013) and on the Nyiragongo lava lake (Spampinato et al.,
Geodetic Data
Ground deformation measurements collected by GPS surveys
carried out with the periodic network at Etna from 2005
to 2015 have been exploited to analyse the long-term strain
affecting the volcano edifice, with particular focus on its summit.
GPS data collected during the surveys, passing through a N-
S profile crossing the volcano summit (Puglisi et al., 2004;
Puglisi and Bonforte, 2004; Bonforte et al., 2009), allowed strain
calculation. Geodetic measurements on the summit part of
Etna are possible only in the summer, due to the snow cover
during the rest of the year that prevents the access to the
highest GPS points. The ground deformation here analyzed
has been previously described in terms of displacements of the
measurement stations (Bonforte et al., 2008, 2013; Bonaccorso
et al., 2011, 2015). Conversely, here we calculate the horizontal
strain tensor components by the GridStrain routine developed
by Pesci and Teza (2007), already used for investigating the
surface deformation of the eastern flank of Etna by Alparone
et al. (2011). This algorithm allows us to analyse the strain
tensor distribution over an area covered by a geodetic network,
starting from station displacements. In our case, a 1500 m
spaced grid was set up covering the Etna GPS network and
a 2D strain tensor was then calculated at each node of the
grid for every subsequent 1-year period from 2005 to 2015.
From SEC to Proto-NSEC: The 2007
Eruptive Activity
The eruptive activity of the SEC in 2007 marks the important shift
in the location of volcanism from the SEC to the proto-NSEC
and, as such, it is here considered in detail. This activity occurred
in two distinct phases. The first lasted from late March until early
May and consisted of four paroxysmal episodes (29 March, 11
April, 29 April, and 6–7 May; Figures 3,4A–D) from the SEC
and its flanks, including the proto-NSEC. The first of these was
probably the most violent explosive paroxysm at the SEC since
2000, with sustained lava fountaining and a tephra column that
produced widespread ash and lapilli falls to the northeast, but
its main phase lasted <1 h. The next three eruptive episodes
showed a tendency of the activity to become progressively less
explosive and more effusive, lasting longer (up to 13 h, see
Table 1). This phase of activity was followed, in mid-May, by the
formation of the new collapse pit (proto-NSEC) on the lower
ESE flank of the SEC cone. The second phase of activity is
characterized only by the activity from the proto-NSEC, without
any contribution from the SEC. This second phase started with
minor ash emissions from the proto-NSEC in mid-August, and
culminated in two episodes of sustained lava fountaining, tephra
emission and production of lava flows, on 4–5 September and
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Acocella et al. Growth of the NSEC at Etna
FIGURE 3 | (A) Map of Etna’s summit area and upper south and east flanks, showing lava flows emitted during 2007 in different colors and 2006 lavas in Pink.
(B) Inset shows the location of the area in (A).
23–24 November 2007 (Figures 3,4E,F). A third paroxysmal
eruptive episode, which took place on 10 May 2008, marks the
end of the second phase of activity and was followed 3 days later
by the onset of Etna’s latest flank eruption to date (13 May 2008–
6 July 2009). Renewed episodic activity that has been occurring
since January 2011 is considered the onset of the phase that has
morphologically built the cone, now known as the NSEC, from
the proto-NSEC (see Section From Proto-NSEC to NSEC).
Phase 1
29 March Paroxysm
The first of the four eruptive episodes in the spring of 2007
occurred on the morning of 29 March and was probably the
most violent paroxysm at the SEC since 2000. The start of the
activity at 0524 (GMT =local time-2h) was marked by a sharp
increase in the volcanic tremor amplitude. Shortly thereafter, lava
fountaining from the summit vent of the SEC reached heights
of 600–800 m, and a tephra column rose to several kilometers
above the summit, feeding a plume that drifted NE and caused
ash and scoria falls at 6 km from the SEC, and as far as 35
km from Etna’s summit (Andronico and Cristaldi, 2007). Fine
ash was reported even in southern Calabria, more than 80 km
During the climactic activity, lava emission started from two
vents located at 3200 and 3190 m above sea level W and SW
of the SEC, feeding flows partially overlapped that advanced
1.25 km to the plain between Etna’s summit cone complex
and the Torre del Filosofo site (Figures 1,4A). A decrease in
the intensity of Strombolian explosions was evident by 0641,
marking the imminent cessation of activity. Activity at the SEC
summit had diminished to sporadic Strombolian explosions and
emission of an ash column, although lava continued to flow
through a channel carved into the SE flank of the SEC cone,
and then down the steep slope toward the Valle del Bove, that
advanced up to 2.35 km from the vent. Here, the lava flow went
into contact with snow, producing phreatomagmatic explosions,
which in turn produced pyroclastic density currents that sped
down the slope. During the following 2 weeks, the volcanic
tremor curve showed significant periodic increases similar to
the signal accompanying the 29 March paroxysm, but none of
these were accompanied by any visible eruptive activity. These
events were interpreted as “failed” eruptions by Falsaperla et al.
11 April Paroxysm
The first signs of the second paroxysm from SEC in 2007 were
recorded as a thermal anomaly evident in satellite imagery at
0027 GMT on 11 April. Like its predecessor, this paroxysm
was characterized by lava fountaining, lava flow emission, and
generation of a tephra column, which this time was driven to
the SE, causing tephra falls up to the coastline. Lava was at
first emitted from the summit through the channel in the SE
flank, and followed a similar path as the 29 March main flow
(Figure 4B).
Between 0200 and 0300 the activity continued to increase in
vigor, and a shift in the wind direction caused ash falls further
north. At the peak of the activity, between 0257 and 0412, a short
eruptive fissure opened on the lower south flank of the SEC cone,
producing fountains from a number of aligned vents and a lava
flow, which advanced across the snow-covered plain to the south
of the cone (Figure 4B). Explosive interaction between lava, snow
and meltwater occurred while the flow extended southward, until
it stopped at a distance of 0.93 km from its source. The activity at
this new fissure lasted only a few hours, and by early morning
the paroxysm was over, although both the lava flow to the south
and the longer (3.55 km long) flow toward the Valle del Bove
continued to advance for several hours during the morning and
afternoon of 11 April.
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FIGURE 4 | Lava flow maps (A–F) and radiant heat flux curves (A’–F’) for the six paroxysmal eruptive episodes at the Southeast Crater and proto-NSEC
in 2007.
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TABLE 1 | Lava fountain durations and volumes as retrieved from SEVIRI data.
Date Phase 1
Phase 2
Duration phase Phase 3 Duration Phase 3 Lava Tephra Total MOR Lava Lava average
Onset End Onset End Onset End
(GMT) (GMT) 1+2 (hours) (GMT) (hours) volume (m3) volume (m3) volume (m3) (m3/s) area (m3) thickness (m)
29/03 5:24*5:26 0:02 5:26 6:57 1:33 1:35 6:57 14:12 7:15 8.29E +05 3.32E +05 1.16 E +06 203.694 3.83E +05 2.2
11/04 0:27 2:57 2:30 2:57 4:12 1:15 3:45 4:12 18:11 13:59 8.21E +05 3.28E +05 1.15E +06 85.091 4.87E +05 1.7
29/04 9:50*16:56 7:06 16:56 22:42 5:46 12:52 22:42 8:42 10:00 1.03E +06 4.10E +05 1.44 E +06 30.990 5.00E +05 2.1
07/05 23:26 0:27 1:01 0:27 7:56 7:29 8:30 7:56 14:42 6:46 8.35E +05 3.34E +05 1.17E +06 38.186 5.83E +05 1.4
04/09 16:00*17:42 1:42 17:42 2:42 9:00 10:42 2:42 17:12 14:30 1.13E +06 4.41E +05 1.57E +06 40.883 6.42E +05 1.8
23/11 16:00*21:4 2 5:42 21:42 2:26 4:44 10:26 2:26 19:56 17:30 1.56E +06 6.23E +05 2.18 E +06 58.096 7.23E +05 2.2
total 6.20E +06 2.47E +06 8.67E +06
Times with *are obtained from field observations. Tephra volumes were retrieved from literature (Andronico et al., 2008) or computed as 40% of emitted lava; lava flow areas were measured in the field. MORs were computed as total
volumes divided by the eruption durations (Phases 1+2), whereas thickness is obtained from satellite-derived volume (total volumes) divided by lava flow area. MOR, Mean Output Rate.
29 April Paroxysm
The third paroxysm in the spring of 2007 started from the SEC
during the late forenoon of 29 April, from 0950 GMT onward.
The eruptive activity increased much more gradually than the
early stages of the previous two paroxysms, and it was in full
swing by the late afternoon. Explosive activity occurred from
several closely spaced vents at the summit of the SEC cone,
consisting of nearly continuous strong Strombolian bursts, and a
rather dilute tephra column rose a few 100 m above the summit.
Lava was delivered from a vent located immediately below the
summit of the SEC cone, in a notch that lay at the head of the
deep channel carved into the SE flank of the cone during the
2006 activity. The lava flow followed the same path as the main
flows of 29 March and 11 April, splitting into numerous branches
on the slope before reaching the bottom of the Valle del Bove
late that evening, at about 3.6 km from the source (Figure 4C).
Strombolian activity and lava emission continued until 2242,
after which the activity decreased.
7 May Paroxysm
The last of the four paroxysms in the spring of 2007 started
shortly before midnight on 6 May, with the onset of Strombolian
explosions from a cluster of vents aligned along a short (a few tens
of meters long) fissure on the upper SE flank of the SEC cone and
at the summit. The climax phase began at 0027 on 7 May, and a
lava flow issued from the lower end of the fissure, following the
path of the previous lava flows in the direction of the Valle del
Bove, reaching a maximum length of 3.38 km (Figure 4D). The
activity continued at relatively constant levels until 0756, after
which the volcanic activity rapidly decreased.
For several weeks, one vent located at the upper end of the
7 May fissure, just below the notch in the SE rim of the crater,
was seen to be incandescent at night. In mid-May, a new collapse
pit formed on the ESE flank of the SEC cone, in a somewhat
lower position than the preceding collapse pits of 2004–2005 and
fall 2006. This pit-crater marks the onset of formation of the
“proto-NSEC.” Ash emissions from this pit occurred on 20 and
24 May, after which there were no eruptive manifestations until
Phase 2
On the morning of 15 August, the newly formed proto-NSEC, on
the ESE flank of the SEC cone, showed unusual emissions of white
vapor, followed in the afternoon by a few bursts of brownish ash.
From this moment, the eruptive activity focused at this vent only,
without any contribution of the SEC. Similar emissions occurred
during the following days, and by 20 August some incandescence
could be observed in the emissions at night. From then until
the end of August, the explosive activity progressively increased;
from the 26th onward the ash content in the emissions decreased
and the activity eventually became purely Strombolian. During
the first days of September, the explosions produced detonations
audible to tens of kilometers away, and sprays of incandescent
bombs often fell all over the SEC cone and to distances of several
100 m from its base. Finally, a further increase of the eruptive
activity started on the mid-afternoon of 4 September, around
1400 GMT on that day.
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4–5 September Paroxysm
The start of sustained lava fountaining from the proto-NSEC
(“Pit” in Figure 4E) occurred during cloudy weather sometime
around 1600 GMT. Soon thereafter, a dark tephra plume rose
above the weather clouds and drifted eastward, causing ash falls
in areas close to the coastline. When weather clouds finally
cleared away at nightfall, a robust jet of incandescent lava was
shooting to heights of 400-600 m above the vent, and lava spilled
over the rim of the active vent in two places, feeding a flow
that advanced toward the Valle del Bove. This activity continued
without significant changes for the next 9 h, with the lava
fountain remaining remarkably stable, and lava advancing 4.6 km
across the Valle del Bove, mostly on top of October-December
2006 lavas (Figure 4E). At 0242 a decrease in the eruptive
intensity was observed; the lava fountain became discontinuous
before the end of the paroxysm and was marked by a series of
detonations, which launched meter-sized bombs all over the SEC
23–24 November Paroxysm
Following about 2 months of intermittent explosive activity
from the proto-NSEC that had been active in August-September,
Strombolian explosions began to increase on 22 November. At
about 1600 GMT on the following day, a series of explosions from
the Bocca Nuova produced dark ash plumes; these were followed
by an increase in the intensity of Strombolian activity at the SEC.
Between 2030 and 2142, the Strombolian activity passed into
sustained fountaining, and lava flowed over the vent’s rim in three
places, feeding flows toward the ESE and SE (Figure 4F). Overall,
this paroxysm was similar to that of 4–5 September, except for
the following: (a) the fountain was often V-shaped, due to the
presence of two closely-spaced vents, (b) the tephra plume was
driven NNE, (c) the lava traveled less (4 km), largely on top of
the 4–5 September flow, and (d) the duration of the paroxysm
was 5 h. Like its predecessor, the closing stage of the paroxysm at
around 0226 was characterized by bursts of incandescent bombs
resembling fireworks and causing detonations.
From Proto-NSEC to NSEC
A third paroxysmal episode from the proto-NSEC occurred on
the afternoon of 10 May 2008. This event was less well observed
than its two predecessors, because satellite data are sketchy due to
dense cloud cover. This paroxysm erupted 5.73 ×106m3of lava
(Behncke et al., 2016), and was followed 3 days later by the onset
of Etna’s latest flank eruption to date (13 May 2008–6 July 2009;
Bonaccorso et al., 2009).
Following the long-lived flank eruption of 2008–2009, the
proto-NSEC showed occasional signs of reactivation, and its
diameter was significantly widened by collapse and by the
formation of an additional, but smaller, collapse pit on its eastern
rim in November 2009, which soon merged with the proto-NSEC
(Behncke et al., 2014, 2016).
Since January 2011, the pit-crater of the proto-NSEC started to
build-up a relief (a cone) evolving into the NSEC. The NSEC has
been the site of more than 50 eruptive episodes, but since late-
2013 there has been a tendency for such episodes to last longer
and be less explosive (De Beni et al., 2015). As this new cone
began to grow around its eruptive vent, it became evident that the
NSEC had permanently taken over the role of Etna’s most active
summit crater from the SEC.
As of 2016, the NSEC cone, which was built up on sloping
terrain, stood at 250–300 m above the former surface, rivaling
in height its older “sibling,” which it has partly overgrown. Its
volume (50 ×106m3as of October 2014; De Beni et al., 2015) is
somewhat inferior to that of the old SEC (72 ×106m3;Behncke
et al., 2006), but nearly all of it was constructed in <3 years. Much
of its activity involved the opening of eruptive fissures on the
flanks of NSEC growing cone, first seen in August 2011. Between
August 2011 and April 2013, much of this activity focused along
a NW–SE striking fissure, precisely following the structural axis
along which the shift from the SEC to the NSEC had occurred. As
the NSEC grew more mature, eruptive vents and fissures opened
also into other directions, namely to the SW, SSE, E, and NE of
the NSEC.
Erupted Volumes in 2007 from Satellite
The six lava fountaining episodes that occurred at Etna during
2007 were detected and monitored by the HOTSAT system.
Radiant heat flux curves, as well as cloud coverage index are
given for each event in Figures 4A–F. Radiant heat flux curves
generally show peak values around 16 GW during the climax
of each event, except for those recorded on 11 April and 23
November, which are close to 20 GW.
These short-lived explosive events show the same features
in the thermal signal we recorded during the 13 May 2008
and 2011–2013 lava fountaining episodes at Etna (Bonaccorso
et al., 2011; Ganci et al., 2012). Three main phases can be
recognized during each episode in the radiant heat flux curve.
A first phase that includes the onset of thermal activity and
a slow increase of the radiant heat flux; a second phase with
scattered high levels of signal, often accompanied by saturated
pixels in the MIR (medium infrared) images is linked to the
main fountaining phase; and a third phase in which the signal
slowly decreases and that is related to the cooling of the lava
flow. Table 1 shows the timing of the different phases for each
of the six eruptive episodes in 2007. The duration of the three
phases is highly variable, spanning from impulsive events like
the one on 29 March, where the first phase is nearly missing
and the lava fountaining is brief, to longer events like that on
23 November, in which both the preparatory phase and the
fountaining lasted about 5 h, respectively, with a cooling phase of
about 17 h.
Since during this type of short-lived events no steady thermal
state is reached, the simple conversion between lava flow area and
time averaged discharge rate (TADR) cannot be applied (Garel
et al., 2015). Therefore, in order to compute the lava volume,
we applied the approach of Ganci et al. (2012) by modeling the
cooling curve (phase 3) apparent in thermal data acquired by
SEVIRI. We take the measured cooling curve and fit this to a
theoretical cooling curve. Best fit is achieved by adjusting the area
of cooling lava until the measured and theoretical curves match.
This technique was tested using ground-based thermal camera
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Acocella et al. Growth of the NSEC at Etna
images collected during the 12 August 2011 event (Ganci et al.,
2013), and the volumes found for all the 2011–2013 events at
Etna are in good agreement with field measurements (Behncke
et al., 2013; De Beni et al., 2015) and strain meter data processing
results (Bonaccorso and Calvari, 2013). Applying the cooling
curve modeling we retrieve the lava volume for the six events
spanning from 1.15 to 2.18 ×106m3, as given in Table 1.
The Geodetic Frame
The strain tensor evolution around the summit craters and,
especially, at the location of SEC and NSEC, shows that, after
the deflation accompanying the 2004–2005 eruption (Bonaccorso
et al., 2006), an inflation was recorded by the GPS surveys
(Bonforte et al., 2008). This inflation produced a dilatation
of the summit area and, in particular, in the location of the
future NSEC (Figure 5A), where an overall NE–SW dilatation
of approximately 10 ppm is evident. It is remarkable that the
pit crater, which later evolved in the proto-NSEC and finally
in the NSEC, opened during this period. The eruptive activity
that accompanied and followed the opening of this pit-crater
on the eastern flank of SEC in 2007 produced a deflation
of the volcano, but the contraction on the summit area was
minor (around 2 ppm; Figure 5B), especially if compared to
the previous dilatation (more than 10 ppm; Figure 5A). More
significant contraction is visible on the NE flank of the volcano,
across the easternmost segment of the Pernicana Fault System
(Barreca et al., 2013), but this is a recurrent deformation in this
sector of the fault (Bonforte et al., 2007). Later on, from 2007
to 2008, a significant NE–SW stretching of more than 40 ppm
of the entire volcano summit has been recorded (Figure 5C);
this is associated with the intrusion of the NNW-SSE oriented
dike on 13 May 2008 that fed the lateral eruption on the
upper eastern flank until July 2009 (Bonaccorso et al., 2011;
Bonforte et al., 2013). This lateral eruption produced an overall
deflation of the edifice but, again, no significant contraction is
visible at summit (Figure 5D). Once the lateral eruption ended,
the volcano inflated again and a homogeneous dilatation is
visible on the summit and around the NSEC area after 2009,
more intense in 2010 (Figure 5E), but continuing until 2011
(Figure 5F). From 2011, an overall deflation has been measured,
with contraction of the summit area. The contraction of the
summit was stronger (7 ppm) until 2012, and ENE–WSW-
oriented (Figure 5G), but it progressively decreased to about 3–
4 ppm in the following year (Figure 5H) to almost disappear
after 2013 (Figure 5I). Finally, during 2014–2015, dilatation
reappeared, but it affected mostly the southern portion of the
summit area, with a NW-SE orientation (Figure 5J). This may
have prepared the ground for the NE–SW-striking dike intrusion
accompanying the paroxysmal episode at the NSEC on 28
December 2014 (Bonforte and Guglielmino, 2015).
From SEC to NSEC: Changes in the
Eruptive Dynamics
Although it was only with the onset of frequent paroxysmal lava
fountaining in early 2011 that the NSEC cone became definitively
established as a persistent new summit vent replacing the old
SEC, the transition that led to its birth and development had
begun as early as 2004, when a pit-crater opened for the first
time on the upper east flank of the old SEC cone (Neri and
Acocella, 2006). While initially it seemed that this pit resulted
from subtraction of magma from below the SEC during the 2004–
2005 lateral eruption (Allard et al., 2006), hindsight suggests that
it may as well have formed as a result of a more general structural
reorganization in the summit area, which eventually led to the
birth and establishment of the NSEC.
This process went through several stages, marked by the
consecutive formation and filling of a total of four pit-craters
lying progressively further southeast (Behncke et al., 2014), as
well as various eruptive periods, with activity shifting forth and
back between the old SEC and the position of the pit-craters,
located along the axis which would be defined by the future
NSEC. The first activity at the SEC after 5 years of quiescence,
in July 2006, occurred from new vents that lay immediately to
the SE and downslope of the first pit crater formed in late-2004,
but 1 month later the activity switched back to the summit vent
of the SEC. The August-December 2006 eruptive period was
concentrated at the summit vent, but activity also occurred from
new vents and fissures at the SE base of the SEC cone as well as
to the west, in the saddle between the SEC and the Bocca Nuova,
and at the southern base of the Bocca Nuova (Behncke et al., 2008;
Neri et al., 2008; Favalli et al., 2010). The western vents reactivated
during the 29 March 2007 lava fountaining episode, but the later
paroxysms in spring 2007, especially those of 29 April and 6-7
May, produced activity exclusively from a fissure that extended
for a few tens of meters (<100 m) from the summit down the SE
flank of the SEC cone.
Paroxysmal eruptive episodes with lava fountaining, eruption
columns and emission of tephra and lava flows have been the
hallmark of the SEC since the late-1970s, although the crater
also was the site of Strombolian activity accompanied by low-rate
(<0.7 m3/s) lava effusion, as in 1984 and 1996-1998 (Neri et al.,
2011). Some of the paroxysms in 1989–1990 were particularly
violent and voluminous (rate >120 m3/s; Bertagnini et al., 1990;
Carveni et al., 1994; Neri et al., 2011). However, the paroxysms
between 1998 and 2001 were mostly rather short and small-
volume events (Behncke et al., 2006), and the eruptive episodes
of 2006 and March–May 2007 were similar. During all of this
activity, the main vent at the summit of the SEC maintained a
diameter of 50–80 m.
In contrast, the paroxysmal episodes at the NSEC have been
generally more voluminous since the earliest ones (then at the
proto-NSEC) on 4–5 September and 23–24 November 2007 (the
10 May 2008 paroxysm was yet more voluminous). Nearly all
of the paroxysms during the long sequence initiated in January
2011 were more voluminous than the largest episodes of 1998-
2001. Some of the paroxysms, especially in 2013, feature among
the most powerfully explosive events (up to 400 m3/s) at
the summit of Etna of the past few centuries (Behncke et al.,
2016). Finally, the width of the main vent of the NSEC has been
consistently larger (a few tens of meters) than that of the SEC
until 2014, when its activity showed a tendency of becoming less
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Acocella et al. Growth of the NSEC at Etna
FIGURE 5 | Maps of the horizontal strain tensors distribution above Mt. Etna for subsequent 1-year (from June to the successive June) intervals (A–J).
Blue axes are for dilatation, red for contraction. Red shadowed areas indicate the location of NE-Rift and New Southeast Crater NSEC (K) in the frame of the volcanic
edifice (L). Dashed lines (in A–K) mark the Pernicana Fault System (PFS), from Barreca et al. (2013).
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Acocella et al. Growth of the NSEC at Etna
All this points to a significant increase in the efficiency
of magma transport through the conduit—of the SEC and,
moreover, of the NSEC—that may be partly due also to the
dilatation in the summit area that facilitated the shift of the axis
of the conduit.
Overall, the depicted volcanic activity in the last decade points
out to a clear SE shift, of 0.4 km, in the location of the eruptive
vents. This occurred through the development of NW-SE striking
eruptive fissures along the SE flank of the SEC, as well as of pit-
craters at its base (proto-NSEC), which replaced the activity of
the SEC and then evolved in the NSEC. Both the fissures and the
pit-craters suggest the lateral propagation of dikes from the SEC.
Evolution of the Fracture Field and Related
Stress Conditions
Previous studies (Neri and Acocella, 2006) have shown that N–S
oriented fractures formed in the summit crater area since early
1998, from the NE Crater (NEC) to the Voragine Crater (VOR;
Figure 6A). During July–August 1998, this fracture system
enlarged, affecting a larger portion of the summit (Figure 6B).
During nearly all of its paroxysmal eruptive episodes in 2000,
the SEC was affected by N–S eruptive fissures, parallel to the
previously formed fracture system (Figure 6C). In 2001 the N–
S fractures were reactivated and, at the SEC, propagated toward
SE (Figure 6D). The NW–SE fissures developed in 2004–2005
beyond the SEC, induced by the lateral propagation of NW–SE
striking dikes from the SEC, are the easternmost continuation of
this newly-oriented fracture field (Figure 6E). In 2006–2007 this
system was reactivated several times, accompanied by effusive
and explosive eruptions (Figure 6F). The final shift from the
main vent of the SEC cone to the pit crater (proto-NSEC) at its
SE base occurred since September 2007.
The overall evolution of the summit fracture field seems to
result from two main stress conditions. The first, active until a
period between 2001 and 2004, is characterized by an overall E–
W extension on the volcano summit. This condition corresponds
to the presence of the stationary extensional stress field acting
on the longer-term, both at the base and the summit of the
volcano (e.g., McGuire and Pullen, 1989; Lanzafame et al., 1997;
Solaro et al., 2010). The past decades of activity from the SEC
have occurred within such a frame. Although minor shifts in the
location of eruptive vents and the formation of fracture systems at
the SEC have been common ever since its first eruptions in 1978–
1979, the main focus of the activity has remained stable through
early May 2007. It was indeed stable enough to build the large
SEC cone, which is a true miniature stratovolcano (Behncke et al.,
2006), with permanent summit vents and sporadically active,
mostly ephemeral flank vent systems.
The second stress condition started to appear on the
summit since sometime between 2001 and 2004, and has been
characterized by an overall NE–SW trending extension direction
(Figure 6). This extension, well-captured by the strain analysis in
Figure 5A (2005) and, later, in Figure 5C (2007), was responsible
for the development of the NW–SE striking fractures and fissures,
as well as of the related feeder dikes, propagating from the SEC
and responsible for the build-up of the NSEC (Figure 2B). The
FIGURE 6 | (A–F) Evolution of the summit deformation pattern at Etna from
1998 to 2007, involving the development of extensional fractures and normal
faults (modified from Neri and Acocella, 2006).
geodetic data show that the related NE–SW-oriented dilatation
on the volcano summit has been transient, and mostly observed
during periods of inflation (Figure 5).
It is proposed that this transient stress field results from the
enhanced instability of the upper eastern flank of the volcano
during inflation periods. Following the 2002–2003 eruption,
the eastern flank of Etna underwent a major reorganization,
with accelerated eastward slip of the area to the SW of the
Pernicana Fault System (Piano Provenzana area, Figure 7;Neri
et al., 2004, 2005, and references therein). In the following years,
the eastward slip of the medium to lower slope immediately
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Acocella et al. Growth of the NSEC at Etna
FIGURE 7 | Schematic summary of the relationships between flank instability and the development of the recent (post-2003) eruptive fissures on the
summit of Etna, during the repeated periods of inflation of the volcano. Flank instability is summarized by the direction and amount of displacement indicated
by the purple arrows observed between 2003 and 2011 (based on the following sources: Figure 5 of this study; Neri et al., 2004; Solaro et al., 2010; Bonaccorso
et al., 2011; Guglielmino et al., 2011; Alparone et al., 2013; Bonforte et al., 2013). The eruptive fissures refer to the dikes emplaced from the SEC, toward SE, from
2004 to 2015 (SEC–NSEC in Figure), as well as to the dike responsible for the 2008 eruption (Bonforte et al., 2013). The arcuate configuration of the displacement
vectors is responsible for the eastward slip of the NE flank of the volcano, as well as for providing the dilatation accommodating the emplacement of the dikes feeding
the shown eruptive fissures (in the summit area). The topography in (a) is based on a DEM from Bisson et al. (2016).
to the south of the Pernicana continued, promoting during
periods of inflation an overall NE movement of the NE part
of the summit, (Piano Provenzana area, Figure 7). In this way,
this highest part of the edifice responded to the decrease in
buttressing induced by the eastward slip of the medium slope
of the volcano immediately to the south of the Pernicana Fault
System. Therefore, the post 2002–2003 evolution of the volcano
summit during periods of inflation has been characterized by an
overall vortex-like kinematics, with NE trending displacement
vectors from the SEC to the Piano Provenzana area, becoming
E–W to the south of the central portion of the Pernicana Fault
System, and then WNW–ESE in the distal portion of the volcano
south of the Pernicana Fault System (Figure 7;Bonaccorso et al.,
2011; Guglielmino et al., 2011; Alparone et al., 2013; Bonforte
et al., 2013). This clockwise rotation of the slip vectors in
the eastern flank of the volcano marks the relaxation of the
significant acceleration of the instability that took place during
the 2002–2003 eruption (Acocella et al., 2003). This clockwise
rotation in the displacement vectors on the eastern flank of
Etna may result from: (a) the curved geometry of the NE Rift—
Pernicana Fault System, developing along a similar clockwise
trend; (b) the presence of a topographic gradient dominated by
the Etna summit (to the west-southwest) and the sea level (to
the east); (c) the rotation of the displacement vectors to the
side of active strike-slip faults producing earthquakes; these show
motions toward the fault approaching the area epicenter, parallel
to the fault nearby the epicenter and far from the fault passing
the epicenter; a similar rotational pattern has been geodetically
captured during recent earthquakes, as for example at the Hector
Mine event in 1999 (Fialko et al., 2001).
The post-2003 volcanic activity at Etna has been significantly
affected by this major stress variation. The emplacement of
the NW–SE striking dikes feeding the 2004–2005 and the 2006
eruptions, as well as of the long-standing 2008–2009 eruption, is
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Acocella et al. Growth of the NSEC at Etna
related to the development of the NE–SW trending dilatational
stress field at the summit of the volcano (Figure 7;Neri and
Acocella, 2006; Neri et al., 2006; Bonaccorso et al., 2009; Bonforte
et al., 2013). However, it is mostly with the development of
the proto-NSEC, from 2007, that the shift in the axis of Etna’s
summit activity becomes evident. The development of the proto-
NSEC in fact has provided a permanent gauge, or stress marker,
highlighting this new condition of instability of the upper
eastern flank of the volcano. This stress field promoted the
formation of a new NW–SE oriented magmatic system through
the development of multiple dikes, probably propagating from
the conduit of the SEC. It is likely that this new magmatic system
is very shallow, not deeper than 1–2 km, as it is resulting from
flank instability. The increased eruptive frequency of the volcano
since the development of this NW–SE trending system indicates
that the accelerated instability of the volcano flank also somehow
enhanced the shallow rise and extrusion of magma, leading to
a greater efficiency of magma transport through the SEC–NSEC
The growth of the NSEC thus provides an interesting example
of how instability-induced stress variations within a volcano may
induce significant shifts in the locus of volcanic activity. Even
though information is poor, it is likely that also the birth and
development of the other summit craters at Etna (as for example
the SEC) may have been promoted by similar variations in the
summit stress field due to previous episodes of flank instability.
Disruption of the portion of the central conduit system that feeds
the Voragine and Bocca Nuova craters during the major intrusive
and flank slip events of 2001 and 2002–2003 furthermore led to
the deactivation of these craters for a decade; in fact, only in
2011–2015 did this portion of the feeder system (Voragine and
Bocca Nuova) become fully re-established.
Increasing Volcanic Hazard?
The possibility to alter in a permanent way the shallow magmatic
paths following episodes of flank instability remains an important
feature to consider in the summit evolution of an active volcano,
as well as to forecast the opening of new vents (Cappello et al.,
2012, 2013). Indeed, the shift of the location of volcanism from
the SEC to the NSEC is not only significant from a structural
point of view, as it also changes, probably increasing, the context
of volcanic hazards at the volcano. In particular, the growth
of a new cone on the western rim of the Valle del Bove may
promote further instability (as recently observed; Bonforte and
Guglielmino, 2015), both of the cone itself and of the slope upon
which it rests, which by now have merged into a continuum.
Repeated intrusion of magma through the flanks of the cone,
especially in its eastern sector, has in fact led to small to moderate
size collapse events in late-2013 and early 2014; the largest of
these events, on 11 February 2014, entrained hot, active lava,
resulting in a hot, pyroclastic-flow-like avalanche. In addition,
the heightened efficiency and faster speed of magma transport
through the widened conduit has led to an increase in the vigor
of lava fountaining and tephra generation, and many lava flows
generated at the NSEC are significantly longer than the lava flows
emitted by earlier paroxysms at the SEC. For example, the 10 May
2008 lava flow from the proto-NSEC reached a length of >6 km
and stopped only 2 km from the outskirts of the town of Zafferana
Etnea (Vicari et al., 2011; Behncke et al., 2016). Between 2011 and
2013, numerous villages on the south-eastern, eastern and north-
eastern flanks of Etna have experienced repeated heavy tephra
falls, often causing material damage. Higher upslope tourist
facilities and popular hiking areas have repeatedly received rather
intense tephra fallout, including clasts up to 0.5 m in diameter at
more than 5 km distance from the NSEC (De Beni et al., 2015).
The growing number of these paroxysms (Behncke et al.,
2005) inspired the development of a methodology for the near-
real-time forecasting of lava flow hazards (Vicari et al., 2009).
The methodology, based on near-real-time infrared satellite data
to drive numerical simulations of lava flow paths, was tested
at Etna to evaluate the hazard of lava flows emitted during
the 12–13 January 2011 lava fountain (Vicari et al., 2011). By
using SEVIRI satellite thermal data with low-spatial and high
temporal resolution, we obtained a system of early warning
combined with a preliminary estimate of the lava discharge
rates. These satellite-derived discharge rate estimates were used
as input to the MAGFLOW model (Del Negro et al., 2015;
Cappello et al., 2016), allowing us to effectively simulate the
rate of advancing and the maximum length of the lava flow.
In this way, an eruptive scenario has been provided promptly
enough for a response to be effective. Moreover, by simulating
the inundation areas for diverse typologies of possible future
eruptions at the NSEC, we produced a hazard map that may
consider any abrupt change in the eruptive conditions, furnishing
the probable paths of lava flows and the associated inundation
probability. The results obtained from the hazard map suggest
that summit eruptions like at the NSEC should generally pose no
threat to the local population, with the added value that all the
developed procedures required only a short time of intervention
(from few minutes to hours), representing a critical point during
an emergency.
In summary, the eruptive activity at the NSEC, with its
frequent intense and widespread tephra falls and outpouring
of lava flows capable of flowing over long distances enough to
invade vulnerable areas on the flanks of Etna, may represent new
challenges to population and authorities, where the notion of
Etna being a largely effusive and non-explosive volcano is still
widely held. The progressive intensification of summit activity at
the volcano and the increased speed at which new summit vents
are born and grow in locations that are structurally increasingly
unstable, make us suspect that the problems caused by the activity
of the NSEC will remain a constant feature of Etna’s activity in the
near future.
This study has shown how volcanic activity at Etna changed its
location in the last decade. Activity migrated from the SE Crater
to the New SE Crater, at the base of the former cone, in less
than a decade. This shift involved the development of dike fed-
eruptive fissures, pit craters and, finally, the NSEC cone itself.
Geodetic and structural data suggest that the NSEC developed
under the dilatational stress on the volcano summit promoted
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Acocella et al. Growth of the NSEC at Etna
by accelerated flank instability, mainly along the Pernicana Fault
System, during inflation periods. In particular, the NE–SW
oriented dilatation in the NSEC area may result from a vortex-
like displacement of the eastern flank of the volcano immediately
to the south of the Pernicana Fault System. In fact, the direction
of this displacement progressively rotates from NE–SW in the
upper slope of the volcano (NSEC and Piano Provenzana areas)
to E–W in the middle slope and ESE–WNW in the lower slope.
The development of the NSEC is not only important from a
structural point of view, as its formation may also have lead to
an overall increase in the volcanic hazard; this is suggested by the
increased proximity to the upper slope of the Valle del Bove, in
a topographically more unstable area, and increased explosivity
of the eruptions. The case of the NSEC at Etna 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.
MN and VA coordinated the research and mainly wrote the
manuscript. AB provided geodetic data. BB, GG, and CN
provided volcanological field and satellite data. All authors
contributed ideas and input to the research and writing of the
Editors A. Costa, G. Wadge, and R. Sulpizio promoted
the Research Topic. Reviewers R. Sulpizio and A. Tibaldi
provided constructive suggestions. Thanks are due to European
Organisation for the Exploitation of Meteorological Satellites
(EUMETSAT) for SEVIRI data ( as well
as to G. Aiesi, F. Calvagna, S. Consoli e B. Saraceno, and all INGV
colleagues who collaborate to the GPS surveys on Etna.
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
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Frontiers in Earth Science | 16 June 2016 | Volume 4 | Article 67
... Mt. Etna eruptions generally occur from summit craters (i.e., Voragine, Bocca Nuova, North East Crater, Southeast Crater and New Southeast Crater; [33], e.g., during 2011-2015, [34][35][36], as well as at the time of writing, intense paroxysms occurred; [37]), although some voluminous flank eruptions were also recorded in recent years (e.g. [38,39]). ...
... Etna activity. The figure displays the thermal anomalies detected by RASTer on ASTER data of 2000-2020 and three different levels of thermal activity derived from field reports, independent observations and scientific papers [33][34][35][36][37][38][39]49,50]. It should be pointed out that RASTer detections were mostly associated with documented periods of Mt. ...
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The RST (Robust Satellite Techniques) approach is a multi-temporal scheme of satellite data analysis widely used to investigate and monitor thermal volcanic activity from space through high temporal resolution data from sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). In this work, we present the results of the preliminary RST algorithm implementation to thermal infrared (TIR) data, at 90 m spatial resolution, from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Results achieved under the Google Earth Engine (GEE) environment, by analyzing 20 years of satellite observations over three active volcanoes (i.e., Etna, Shishaldin and Shinmoedake) located in different geographic areas, show that the RST-based system, hereafter named RASTer, detected a higher (around 25% more) number of thermal anomalies than the well-established ASTER Volcano Archive (AVA). Despite the availability of a less populated dataset than other sensors, the RST implementation on ASTER data guarantees an efficient identification and mapping of volcanic thermal features even of a low intensity level. To improve the temporal continuity of the active volcanoes monitoring, the possibility of exploiting RASTer is here addressed, in the perspective of an operational multi-satellite observing system. The latter could include mid-high spatial resolution satellite data (e.g., Sentinel-2/MSI, Landsat-8/OLI), as well as those at higher-temporal (lower-spatial) resolution (e.g., EOS/MODIS, Suomi-NPP/VIIRS, Sentinel-3/SLSTR), for which RASTer could provide useful algorithm’s validation and training dataset.
... The impossibility to discriminate the activity of these two cones led to their rename as SEC the whole SEC-NSEC apparatus since November 2020. In this study, however, we will use the name NSEC to avoid confusion with the related literature (Acocella et al., 2016;Cappello et al., 2019). ...
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Low-intensity emission of volcanic ash represents the most frequent eruptive activity worldwide, spanning the whole range of magma compositions, from basalts to rhyolites. The associated ash component is typically characterized by heterogeneous texture and chemical composition, leading to misinterpretation of the role of syn-eruptive processes, such as cooling and degassing during magma ascent or even magma fragmentation. Despite their low intensity, the ash emission eruptions can be continuous for enough time to create problems to health and life networks of the communities all around the volcano. The lack of geophysical and/or geochemical precursor signals makes the petrological monitoring of the emitted ash the only instrument we have to understand the leading mechanisms and their evolution. Formation of low-level plumes related to ash-rich emissions has increasingly become a common eruptive scenario at Mt. Etna (Italy). In January–February 2019, an eruptive cycle of ash-rich emissions started. The onset of this activity was preceded on 24 December 2018 by a powerful Strombolian-like eruption from a fissure opened at the base of the New Southeast Crater. A lava flow from the same fissure and an ash-rich plume, 8–9 km high a.s.l., from the crater Bocca Nuova occurred concurrently. After about 4 weeks of intra-crater strombolian-like activity and strong vent degassing at summit craters, starting from 23 January 2019, at least four episodes of ash-rich emissions were recorded, mainly issued from the Northeast Crater. The episodes were spaced in time every 4–13 days, each lasting about 3–4 days, with the most intense phases of few hours. They formed weak plumes, up to 1 km high above the crater, that were rapidly dispersed toward different directions by dominant winds and recorded up to a distance of 30 km from the vent. By combining observations on the deposits with data on textural and chemical features of the ash components, we were able to discriminate between clasts originated from different crater sources and suggest an interpretive model for syn-eruptive processes and their evolution. Data indicate the occurrence of scarce (<10 vol.%) fresh juvenile material, including at least four groups of clasts with marked differences in microlite content and number density, and matrix glasses and minerals composition. Moreover, a large amount of non-juvenile clasts has been recognized, particularly abundant at the beginning of each episode. We propose that the low amount of juvenile ash results from episodic fast ascent of small magma batches from shallow reservoirs, traveling within a slow rising magma column subjected to cooling, degassing, and crystallization. The large number of non-juvenile clasts deriving from the thick crater infill of variably sealed or thermally altered material at the top of the magma column is suggested to contribute to the ash generation. The presence of a massive, granular crater infilling accumulating in the vent area may contribute to buffer the different geophysical signals associated with the active magma fragmentation process during the low-energy ash eruptions, as already evidenced at other volcanoes.
... Many eruptions occur by magma flowing along pre-existing conduits; however, owing to increasing lithostatic pressure with depth below ~1 km or less, such conduits will rapidly collapse once drained. Moreover, even in open-conduit volcanoes such as Etna or Stromboli, the distribution of active vents changes dramatically over timescales of years or even months [303][304][305] . These considerations and observations demonstrate that the existence of pre-existing pathways and conduits, even if persistently used for degassing, does not guarantee that magma will use them, rather than opening new ones. ...
More than 800 million people live in proximity to active volcanoes and could be directly impacted by potential eruptions. Mitigation of future volcanic hazards requires adequate warning of a pending eruption, which, in turn, requires detailed understanding of the fundamental processes driving volcanic activity. In this Review, we discuss the processes leading up to volcanic eruptions, by following the journey of magma from crustal storage zones to the surface. Magma reservoirs can feed volcanic eruptions if they contain sufficiently hot and mobile magma and are able to supply sufficient energy for the magma to reach the surface. Young volcanic plumbing systems favour volcanic activity, whereas storage becomes more likely in mature volcanic systems with large reservoirs (hundreds of cubic kilometres). Anticipating volcanic activity requires a multidisciplinary approach, as real-time monitoring and geophysical surveys must be combined with petrology and the eruptive history to understand the temporal evolution of volcanic systems over geological timescales. Numerical modelling serves to link different observational timescales, and the inversion of data sets with physics-based statistical approaches is a promising way forward to advance our understanding of the processes controlling recurrence rate and magnitude of volcanic eruptions.
... The Mt. Etna volcano is also one of the most active volcanoes on our planet. It erupts almost continuously, usually from its four summit craters (Figure 2), named Voragine (VOR), Northeast Crater (NEC), Bocca Nuova (BN) and Southeast Crater (SEC), where, since 2011, a new cone has formed and rapidly grown on its eastern slope, named the New Southeastern Crater (NSEC) [1]. Less frequently, magma rises up from lateral fissures and eruptions occur at lower altitudes on the volcano's flanks, producing more hazardous lava flows for those villages and rural areas surrounding the volcano. ...
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We propose a multi-temporal-scale analysis of ground deformation data using both high-rate tilt and GNSS measurements and the DInSAR and daily GNSS solutions in order to investigate a sequence of four paroxysmal episodes of the Voragine crater occurring in December 2015 at Mt. Etna (Italy). The analysis aimed at inferring the magma sources feeding a sequence of very violent eruptions, in order to understand the dynamics and to image the shallow feeding system of the volcano that enabled such a rapid magma accumulation and discharge. The high-rate data allowed us to constrain the sources responsible for the fast and violent dynamics of each paroxysm, while the cumulated deformation measured by DInSAR and daily GNSS solutions, over a period of 12 days encompassing the entire eruptive sequence, also showed the deeper part of the source involved in the considered period, where magma was stored. We defined the dynamics and rates of the magma transfer, with a middle-depth storage of gas-rich magma that charges, more or less continuously, a shallower level where magma stops temporarily, accumulating pressure due to the gas exsolution. This machine-gun-like mechanism could represent a general conceptual model for similar events at Etna and at all volcanoes.
... 1c). After 2007, a new large summit cone grew on the southeast flank of the SEC: the New Southeast Crater (Del Negro et al., 2013;Behncke et al., 2014;Acocella et al., 2016). Eruptions from these summit craters are classified as summit eruptions . ...
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We collected drone data to quantify the kinematics at extensional fractures and normal faults, integrated this information with seismological data to reconstruct the stress field, and critically compared the results with previous fieldwork to assess the best practice. As a key site, we analyzed a sector of the northeast rift of Mt Etna, an area affected by continuous ground deformation linked to gravity sliding of the volcano's eastern flank and dike injections. The studied sector is characterized also by the existence of eruptive craters and fissures and lava flows. This work shows that this rift segment is affected by a series of NNE- to NE-striking, parallel extensional fractures characterized by an opening mode along an average N105.7∘ vector. The stress field is characterized by a σHmin trending northwest–southeast. Normal faults strike parallel to the extensional fractures. The extensional strain obtained by cumulating the net offset at extensional fractures with the fault heave gives a stretching ratio of 1.003 in the northeastern part of the study area and 1.005 in the southwestern part. Given a maximum age of 1614 CE for the offset lavas, we obtained an extension rate of 1.9 cm yr−1 for the last 406 years. This value is consistent with the slip along the Pernicana Fault system, confirming that the NE rift structures accommodate the sliding of the eastern flank of the volcano.
... Lava flows, explosive eruptions with ash plumes, and Strombolian lava fountains commonly occur from one or more of its summit craters named Voragine (VOR), Northeast Crater (NEC), Bocca Nuova (BN), and Southeast Crater (SEC). In the latter, recent activity is located on a new cone formed since 2011 on the volcano's eastern flank, namely the New Southeast Crater (NSEC), [39][40][41][42]. ...
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Between 30 May and 6 June 2019 a series of new flanks eruptions interested the south-east flanks of Mt. Etna, Italy, forming lava flows and explosive activity that was most intense during the first day of the eruption; as a result, volcanic particles were dispersed towards Greece. Lidar measurements performed at the PANhellenic GEophysical observatory of Antikythera (PANGEA) of the National Observatory of Athens (NOA), in Greece, reveal the presence of particles of volcanic origin above the area the days following the eruption. FLEXible PARTicle dispersion model (FLEXPART) simulations and satellite-based SO2 observations from the TROPOspheric Monitoring Instrument onboard the Sentinel-5 Precursor (TROPOMI/S5P), confirm the volcanic plume transport from Etna towards PANGEA and possible mixing with co-existing desert dust particles. Lidar and modeled values are in agreement and the derived sulfate mass concentration is approximately 15 μg/m3. This is the first time that Etna volcanic products are monitored at Antikythera station, in Greece with implications for the investigation of their role in the Mediterranean weather and climate.
The Pleistocene to Holocene Lanín composite volcano, in the Southern Volcanic Zone of the Andes, has summit and flank eruptions of Holocene age. We present here a characterization of flank vents and their deposits produced during the most recent eruptions of the Lanín volcano. Volcanic facies analysis indicates four unreported Holocene eruptive units sourced from flank vents. The associated deposits have basaltic, basaltic andesitic and trachytic compositions, and were formed mainly by low-volume effusive and mainly Strombolian-like explosive eruptions. Pyroclastic deposits were identified at the NE, E and SW volcano flanks, and correspond to concentrated and diluted pyroclastic density current deposits, in addition to non-welded to welded fall deposits. Basaltic lava flows of pahoehoe morphology were produced in a purely effusive episode. An analysis of the distribution of Lanín's flank vents (dyke's orientation and vent alignments) along fault-kinematic analysis in basement rocks were carried out with the aim of assessing the basement anisotropies that may have controlled the location and orientation of dykes, eruptive fissures and vent alignments. Contrary to expectations, the Liquiñe-Ofqui Fault System does not constitute a first-order control on the orientation of dykes and vents alignments in Lanín. The vents arrangement may be partly influenced by pre-Andean crustal scale fault zones, such as the Mocha-Villarica Fault Zone, and local basement structures. The predominance of flank vents over summit eruptions strongly suggests that the load of the volcanic edifice is a controlling factor.
We present measurements of soil CO 2 effluxes combined with soil ( 222 Rn) and ( 220 Rn) from two high‐degassing areas on the lower flanks of Mt. Etna volcano (ZE‐SV on the E flank and PAT on the SW flank). Measurements were conducted periodically from June 2006 to January 2009 in the ZE‐SV area and January 2007 to January 2009 in the PAT area. The results showed significant variations in discharge activity and style. Log values of ( 220 Rn)/( 222 Rn) and CO 2 efflux generally follow a negative correlation, herein parameterized as the Soil Gas Disequilibrium Index (SGDI). Deviations of the SGDI from this negative correlation provide insight into variance of localized and shallow system conditions, namely rock fracturing, residual magma degassing, and near surface interactions between magmatic gases and groundwater. Statistical analysis highlighted signal anomalies, both negative and positive, that were modeled according to the physical properties and the modes of transport for each of the SGDI gas components. The revealed anomalies show correspondence with episodes of magma ascent and eruption, thereby demonstrating the potential of using the SGDI as another instrument for forecasting volcanic activity. An important strength of the SGDI, compared to other magma gas proxies like CO 2 or SO 2 , is that the very short and very different half‐lives of 222 Rn (t 1/2 = 3.85 days) and 220 Rn (t 1/2 = 55 seconds) provide unique information on the timescales of soil gas transport. Coupling the SGDI with other pre‐eruptive proxies enhances the volcanological community's response capabilities, which is critical for effective hazard mitigation.
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We developed the model, and carried out its discussion at the PIRT-2021 conference, within the framework of the research topic “ External Forcing on Volcanoes and Volcanic Processes: Observations, Analysis and Implications ” announced by the journal “Frontiers in Earth Science” in October 2020. Besides other, external processes considered in this Research Topic included astronomical. In this study, in the category “ Hypothesis and Theory ”, we investigate how changes in the position of large bodies of the Solar system can cause natural phenomena, associated with the movement of free masses, such as volcanism, earthquakes and landslides in the lithosphere, as well as various catastrophic events in the atmosphere and hydrosphere. The analysis has shown that the discovered phenomena of celestial bodies’ alignments accompanying manifestations of natural phenomena require going beyond the standard cosmological model and clarify the fundamental mechanism of gravity. We propose the novel Bidirectional Pushing Gravitation model (BPG), which, in addition to application in Earth Sciences, may occur useful in Astrophysics, Cosmology and Gravitation research.
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We developed the model, and carried out its discussion at the PIRT-2021 conference, within the framework of the research topic "External Forcing on Volcanoes and Volcanic Processes: Observations, Analysis and Implications" announced by the journal "Frontiers in Earth Science" in October 2020. Besides other, external processes considered in this Research Topic included astronomical. In this study, in the category "Hypothesis and Theory", we investigate how changes in the position of large bodies of the Solar system can cause natural phenomena, associated with the movement of free masses, such as volcanism, earthquakes and landslides in the lithosphere, as well as various catastrophic events in the atmosphere and hydrosphere. The analysis has shown that the discovered phenomena of celestial bodies' alignments accompanying manifestations of natural phenomena require going beyond the standard cosmological model and clarify the fundamental mechanism of gravity. We propose the novel Bidirectional Pushing Gravitation model (BPG), which, in addition to application in Earth Sciences, may occur useful in Astrophysics, Cosmology and Gravitation research. The Discussion section will be submitted to the journal Frontiers in Earth Science.
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We report a 25year-long data set (1990-2014) of combined continuous tilt and GPS vertical displacement series recorded at Etna volcano. To our knowledge, this is the first time that such a data set on an active volcano has been presented. We show the coherence of the two series, which help draw new insights on how the volcano, characterized by frequent flank eruptions, works in the long-term (tens of years). This data set provides evidence that after the 1992-1993 flank eruption (the biggest in the last three centuries) and the following major recharging phase (1994-2001), all the ensuing eruptions fall within a single long-term reequilibrium phase (2001-2014).
Displacements or velocities obtained by GPS data processing over repeated surveys can provide useful information on tensional states of terrestrial crust, in those areas in which many stations well spatially distributed are present. In particular, the strain (or strain rate) can be computed on the nodes of a regular grid with suitable size to define a high density deformation field. A new method was deployed to generate easily and quickly the deformation pattern from GPS velocities and to evaluate the significance: values, related to an assigned grid point, can be truly considered only if the GPS stations are well distributed around it. The approach validation was performed by means of synthetic data derived from the theoretical displacement field generated by a Mogi model source. A complete analysis on the velocity pattern of the CaGeoNet network (Central Apennine chain, Italy) was performed providing strain rates and showing both extensional and compressive behaviour at the same values, along the Appennine chain axis.
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 >86×106m3, most (~74×106m3) of which is made up by the lava flows of the 2008-2009 flank eruption. The survey also reveals the high lava volume (5.73×106m3) and average eruption rate (~400m3s-1) of the 10 May 2008 paroxysm, whose flow front stopped 6.2km from the vent, not far from the town of Zafferana Etnea.
Satellite remote sensing techniques and lava flow forecasting models have been combined to enable a rapid response during effusive crises at poorly monitored volcanoes. Here, we used the HOTSAT satellite thermal monitoring system and the MAGFLOW lava flow emplacement model to forecast lava flow hazards during the 2014–2015 Fogo eruption. In many ways this was one of the major effusive eruption crises of recent years, since the lava flows actually invaded populated areas. Combining satellite data and modeling allowed mapping of the probable evolution of lava flow-fields while the eruption was ongoing and rapidly gaining as much relevant information as possible. HOTSAT was used to promptly analyze MODIS and SEVIRI data to output hot spot location, lava thermal flux, and effusion rate estimation. This output was used to drive the MAGFLOW simulations of lava flow paths and to continuously update flow simulations. We also show how Landsat-8 OLI and EO-1 ALI images complement the field observations for tracking the flow front position through time, and add considerable data on lava flow advancement to validate the results of numerical simulations. The integration of satellite data and modeling offers great promise in providing a unified and efficient system for global assessment and real time response to effusive eruptions, including (i) the current state of the effusive activity, (ii) the probable evolution of the lava flow field, and (iii) the potential impact of lava flows.
High-resolution digital topography is essential for land management and planning in any type of territory as well as the reproduction of the Earth surface in a geocoded digital format that allows several Digital Earth applications. In a volcanic environment, Digital Elevation Models are a valid reference for multi-temporal analyses aimed to observe frequent changes of a volcano edifice and for the relative detailed morphological and structural analyses. For the first time, a DTM (Digital Terrain Model) and a DSM (Digital Surface Model) covering the entire Mt. Etna volcano (Italy) derived from the same airborne Light Detection and Ranging (LiDAR) are here presented. More than 250 million 3D LiDAR points have been processed to distinguish ground elements from natural and anthropic features. The end product is the highly accurate representation of Mt. Etna landscape (DSM) and ground topography (DTM) dated 2005. Both models have a high spatial resolution of 2 m and cover an area of 620 km2. The DTM has been validated by GPS ground control points. The vertical accuracy has been evaluated, resulting in a root-mean-square-error of ± 0.24 m. The DTM is available as electronic supplement and represents a valid support for various scientific studies.
The integration of satellite data and modeling represents a step toward the next generation of quantitative hazard assessment in response to effusive volcano eruption onset. Satellite-based thermal remote sensing of hotspots related to effusive activity can effectively provide a variety of products suited to timing, locating, and tracking the radiant character of lava flows. Hotspots show the location and occurrence of eruptive events (vents). Discharge rate estimates may indicate the current intensity (effusion rate) and potential magnitude (volume). High-spatial-resolution multispectral satellite data can complement field observations for monitoring the front position (length) and extension of flows (area). Physics-based models driven, or validated, by satellite-derived parameters are now capable of fast and accurate forecast of lava flow inundation scenarios (hazard). Here, we demonstrate the potential of the integrated application of satellite remote-sensing techniques and lava flow models by using a retrospective analysis of the 2004-2005 effusive eruption at Mount Etna in Italy. The lava flow hazard was assessed by using the HOTSAT volcano hotspot detection system, which works with satellite thermal infrared data, and the MAGFLOW lava flow emplacement model, which is able to relate the flow evolution to eruption conditions at the vent. We used HOTSAT to analyze Moderate Resolution Imaging Spectroradiometer (MODIS) and Spinning Enhanced Visible and InfraRed Imager (SEVIRI) data to output hotspot location, lava thermal flux, and effusion rate estimation. This output was used to drive the MAGFLOW simulations of lava flow paths and to continuously update flow simulations. We also show how Land-sat-7 Enhanced Thematic Mapper+ (ETM+) and Earth Observing 1 (EO-1) Advanced Land Imager (ALI) images complement the field observations to track the flow front position in time and add valuable data on lava flow advancement with which to validate the numerical simulations. Such integration at last makes timely forecasts of lava flow hazards during effusive crises possible at the great majority of volcanoes for which no monitoring exists.
The HOTSAT multiplatform system for the analysis of infrared data from satellites provides a framework that allows the detection of volcanic hotspots and an output of their associated radiative power. This multiplatform system can operate on both Moderate Resolution Imaging Spectroradiometer and Spinning Enhanced Visible and Infrared Imager data. The new version of the system is now implemented on graphics processing units and its interface is available on the internet under restricted access conditions. Combining the estimation of time-varying discharge rates using HOTSAT with the MAGFLOW physics-based model to simulate lava flow paths resulted in the first operational system in which satellite observations drive the modelling of lava flow emplacement. This allows the timely definition of the parameters and maps essential for hazard assessment, including the propagation time of lava flows and the maximum run-out distance. The system was first used in an operational context during the paroxysmal episode at Mt Etna on 12-13 January 2011, when we produced real-time predictions of the areas likely to be inundated by lava flows while the eruption was still ongoing. This allowed key at-risk areas to be rapidly and appropriately identified.