Causes of Explosive Basaltic Volcanism in the 2001 Eruption of Mount Etna, Sicily Student No. 339860
Causes of Explosive Basaltic
Volcanism in the 2001
Eruption of Mount Etna,
Sicily
Student No. 339860
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Student No. 339860
Contents
1.0 Abstract ................................................................................................................................ 2
2.0 Introduction .......................................................................................................................... 3
3.0 Background .......................................................................................................................... 7
3.1 Structural Setting ............................................................................................................. 7
3.2 Local Setting .................................................................................................................... 8
3.3 Volcanic Evolution ........................................................................................................ 10
4.0 Overview of the 2001 Eruption.......................................................................................... 12
4.1 Geochemical Results for Erupted Products ................................................................... 14
5.0 Discussion .......................................................................................................................... 15
5.1.First Stage (19-24 July) .............................................................................................. 15
5.2 Second Stage (24-25 July) ......................................................................................... 16
5.3 Third Stage (25th
July – 1st August) ........................................................................... 17
5.4 Fourth Stage (1st-6
th August) ...................................................................................... 17
6.0 Conclusions ........................................................................................................................ 19
References ................................................................................................................................ 19
1.0 Abstract
This paper attempts to better understand the chronology, processes and products involved in
the explosive ash-forming events of the July-August 2001 eccentric eruption at Mount Etna,
Sicily. A literature review, coupled with analysis of geochemical results published in recent
journal articles, has been combined to answer these questions. The findings are therefore used
to elucidate the stance on the events involved, with four stages defined to represent the
aforementioned objectives. Stage 1 is considered to involve phreatomagmatic activity; Stage
2 represents a transitional stage towards Strombolian events; Stage 3 wholly involves
Strombolian activity; whilst Stage 4 is considered indicative of Vulcanian activity.
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2.0 Introduction
Situated on the eastern coast of Sicily (Fig. 1), Mount Etna is a stratovolcano, rising to a
height of some 3330m a.s.l and dominating the lives of more than 2 million people (Kilburn
& McGuire, 2001, p. 115), including Catania, Sicily’s second largest city (Fig. 2).
Etna’s activity has long been thought of as mild, typified by lava flows, fire fountaining and
mild Strombolian activity from both summit craters and parasitic vents. Chester et al. (1985,
p. 17) noted that in historical times (up to 1985) violent activity had been rare, with the
exception of the 1979 eruption, which generated enough ash to disrupt population centres to
the east and southeast, including Catania’s Fontanarossa airport.
The 2001 eruption (Fig. 3) represents one of the most explosive events in living memory,
which included the release of juvenile ash, predominantly of 0.4 to 1mm in diameter, with a
plume height of 5km a.s.l. (Taddeucci, Pompilio, & Scarlato, 2004, p. 33). Accompanying
ash falls trended in an E and SE direction (Figure 9) and were recorded as far away as
Cefalonia, Greece, 500km from the eruption site, reaching the island just 24 hours after the
eruption (Dellino & Kyriakopoulos, 2003, p. 341). Involving both lateral and eccentric
eruptions simultaneously (Andronico, Cristaldi, Del Carlo, & Taddeucci, 2009; Behncke &
Neri, 2003; Coltelli, Miraglia, & Scollo, 2008; Corsaro, Miraglia, & Pompilio, 2007; Dellino
& Kyriakopoulos, 2003; Lanzafame, Neri, Acocella, Billi, Funiciello, & Giordano, 2003;
Metrich, Allard, Spilliaert, Andronico, & Burton, 2004; Neri, Acocella, Behncke, Maiolino,
Ursino, & Velardita, 2005; Scollo, Del Carlo, & Coltelli, 2007; Taddeucci, Pompilio, &
Scarlato, 2004; Viccaro, Ferlito, Cortesogno, Cristofolini, & Gaggero, 2006) it may be
considered that this could pave the way for future violent eruptions such as that of 2002-03.
Much has been written and documented specifically on both the surface and subsurface
processes and the eruptive products of the July 2001 event. However, it is not considered that
a great degree of agreement has been forthcoming between authors on the processes involved
in the eccentric eruption, with different hypotheses on the processes involved in the
production of tephra during this eruption type. This could represent a number of serious flaws
in the accuracy of the overall understanding of the hazards represented by this eruption,
potentially leading to incorrect analysis of any similar future eruptions (such as 2002-03), and
the hazards posed to the Mount Etna region. It is therefore the aim of this report to: (1)
outline the timescale of violent basaltic events of the 2001 Etnean eruption, as considered by
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Behncke and Neri (2003); (2) identify the products involved; and (3) discuss the processes
involved in the formation of these products, by means of a literature review.
Figure 1: Location of Mount Etna in relation to other volcanic centres in Italy. Inset: location of the
main population centres on Etna’s flanks. Modified from Chester, Duncan, & Dibben (2008, p. 218).
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Figure 2: Populations of the main towns and cities of the Etna region in 1985. (After Chester et al
1985, p.60).
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Figure 3: Map of the eruptive fissures and lava flows during the 2001 July-August eruption of Etna.
(Scollo, Del Carlo, & Coltelli, 2007, p. 148)
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3.0 Background
3.1 Structural Setting
The tectonic history of Eastern Sicily is greatly complex (Figure 4), chiefly governed by the
collision of the Eurasian and African plates, comprising the opening and closing of the Ionian
Sea, and the subduction, bending and possible compression of the Aeolian-Calabrian arc
(Scarth & Tanguy, 2001, p. 55; Patane, Agostino, La Delfa, & Leonardi, 2009, p. 307). As
subduction neared an end, thrusting of the Eurasian plate over the African plate occurred,
forming the Appeninic-Maghrebian mountain chain situated to the west of the volcano
(Barberi, Civetta, Gasparini, Innocenti, Scandone, & Villari, 1974, p. 123; Catalano, Torrisi,
& Ferlito, 2004, p. 315). As a result of this, the Tyrrehnian part of Sicily is under contraction,
whilst the Ionian side is under extension (Lanzafame, Neri, Acocella, Billi, Funiciello, &
Giordano, 2003).
Figure 4: The main regional tectonic features of Mount Etna identifying the compressive domain to
the west, considered to be a result of Africa-Europe plate collision, and the tensional domain of the
Calabrian Arc Appeninic to the east (Barberi, Cocina, Neri, Privitera, & Spampinato, 2000, p. 319).
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3.2 Local Setting
The collision of the African and Eurasian plates, as well as more complex regionalised
structural patterns, has resulted in distinct rift zones (Figure 4) which has allowed magma to
move laterally and radially away from the volcano’s more centrally-located vertical to sub-
vertical network of dikes (Rust, Behncke, Neri, & Ciocanel, 2005, p. 141), producing flank
eruptions at distances greater than 10km from the central craters.
Several hypotheses have been put forward for Etna’s volcanic formation. Tanguy,
Condomines, & Kieffer (1997, p. 239) have proposed a hot-spot theory, which has been
backed up by the 3He/
4He ratio of olivine phenocrysts in lavas having remained steady
throughout the volcano’s history, and suggests the sub-continental lithosphere as the single
source feeding Etnean magmas (Clocchiatti, Schiano, Ottolini, & Bottazzi, 1998, p. 399).
This theory involves the presence of a mantle diapir resulting from a localised high strain in
the lithosphere, which acts as a heat flow channel (Tanguy, Condomines, & Kieffer, 1997, p.
239).
A ‘slab-window’ theory has been suggested by (Doglioni, Innocenti, & Mariotti, 2001, p. 25).
This considers a vertical slab window, causeed by the rollback of the Ionian lithosphere,
which allowed for alkali magmatism beneath Etna. The area of rollback is thought to occur
along the Malta escarpment, between Sicily and the Ionian Sea (Patane, La Delfa, & Tanguy,
2006, p. 832), with this feavily faulted zone of the volcano allowing the percolation of
magma through the host rock, to the surface.
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Figure 4: Map identifying the main North and South - and, to a lesser extent – West rift zones of the
volcano. The main eruptive fractures and parasitic cones are also denoted (after- Crisci, Iovine, Di
Gregorio, & Lupiano, 2008). Key: 1. Eruptive fracture; 2. Parasitic cone; 3. Volcanics; 4.
Sedimentary basement.
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Figure 5: A simplified cartoon of the tectonic evolution of the eastern Sicily-Ionian Sea transition,
and related emplacement of Mount Etna along theactive right-lateral transtensional fault, resulting
from the theory of differential rollback in the footwall of teh accretionary wedge. Modified from
Doglioni, Innocenti, & Mariotti, (2001).
3.3 Volcanic Evolution
Reasonably accurate documentation over the last 400 years of Etna’s eruptive activity has
been used to better understand the pattern of volcanic behaviour from historic eruptions
(Tanguy, Kieffer, & Patane, 1996, p. 259). Indications from this data are that effusive activity
is dominant, with lava flows and Strombolian activity from active centres. However, more
recent eruptions – particularly in 2001 – indicates a further stage of evolution with more
explosive eruptions and the emission of, at times, two compositionally distinct magmas (Neri,
Acocella, Behncke, Maiolino, Ursino, & Velardita, 2005, p. 253), and voluminous ash
emissions. Consequently, Tanguy, Condomines & Kieffer (1997, p. 247) indicate the activity
of eruptions previous to 1997 were a result of “subtle interactions” between mantle diapirism,
the development of a permanent deep magma reservoir at a depth of 20-30km, and temporary
shallow chambers allowing crystal fractination (Figure 6).
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Figure 6: A schematic representation of the magmatic and structural evolution of Mount Etna,
modified from Tanguy, Condomines, & Kieffer (1997, p. 245). Arrows inducate the movement in the
mantle and resulting tensional stresses in the crust. Note the change in scale from early magma
evolution in the asthenosphere, to present day. Porphyritic trachybasalts have formed where magma
ponding occurs at shallower depths, whilst aphyric trachybasalts are erupted directly from a deep
chamber.
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4.0 Overview of the 2001 Eruption
The eruption was characterised by an initial paroxysmal fire fountaining event located at the
Southeast Crater on the 13th
July 2001 (Viccaro, Ferlito, Cortesogno, Cristofolini, &
Gaggero, 2006, p. 141), followed, days later, by seven separate lava flows – two originating
from vents on the Southeast Crater, and five erupting from vents on five separate newly-
formed fissure systems on the southern flank (Behncke & Neri, 2003, p. 465). Whilst lava
flows were ongoing explosive activity in the form of two end-member, predominantly ash-
forming events, and a transitional Strombolian and fire fountaining (Metrich, Allard,
Spilliaert, Andronico, & Burton, 2004, p. 3) phase took place. These explosive products were
almost exclusively formed at one fissure system locted at an elevation of 2570m in an area
known as the “Piano del Lago” (Behncke & Neri, 2003, p. 466; Corsaro, Miraglia, &
Pompilio, 2007, p. 402; Viccaro, Ferlito, Cortesogno, Cristofolini, & Gaggero, 2006, p. 141).
Lava flows ceased on the 9th
August 2001 (Behncke & Neri, 2003, p. 466), marking the end
of the eruption.
A brief chronology of the explosive eruptive activity and products produced from the Piano
del Lago fissure system is outlined in Table 1, with an isopach map of the first ash event in
Figure 7. The geochemical composition of lavas and ash are defined in Table 2 and Figure 8.
Stage Start Date End Date Products Notes
1 19/07/20011 24/07/2001
1 Ash & Lapilli
1
200-300m high pyroclastic
column1,4
2 24/07/20011 25/07/2001
1
Lava fountaining2,3,4
Ash,
Lapilli & Scoria
Up to 500m high4
3 25/07/20011 01/08/2001
1
Lava fountaining3,4
and
Scoria2,3
100-120m high scoria cone
formed1,3,4
Lava flowed
from vent at base of cone
from 26th to 1st August1,3
4 01/08/20011,3
06/08/20013
Ash & dense non-vesicular
blocks3
Pulsing ash explosions up to
5km high1,3
Progressively
diminishing until 6th
August1
Table 1: An overview of the explosive activity observed at the Piano del Lago fissure system during
the 2001 eruption, compiled from data available from 1: Behncke & Neri (2003); 2: Metrich et al
(2004); 3: Taddeucci, Pompilio & Scaroato (2004); 4: Viccaro et al (2006).
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Figure 7: Isomass map of the tephra deposits formed between 21-24 July 2001. Curves are given in
kg m-2
, with coordinates in UTM-Datum ED50. From Scollo, Del Carlo, & Coltelli (2007, p. 152).
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4.1 Geochemical Results for Erupted Products
Eruption
Type
Plagioclase
(%) Clinopyroxene (%) P.I Index (%)
Lateral 16-23% 10-19% 33-40%
Eccentric 5-7% 8-13% 18-22%
Table 2: Comparison of plagioclase, clinopyroxene and Porphyricity Index (P.I.) of Lateral and
Eccentric lavas from the 2001 eruption using data gathered by Corsaro, Miraglia, & Pompilio (2007,
p. 403).
Figure 8: Daily componentry of the airborne ash collected during the 2001 eruption, modified from
Taddeucci, Pompilio, & Scarlato (2004, p. 40). The numbers to the right indicate 1: Ash and lapilli
stage; 2: Lava fountaining and scoria stage; 3: Ash and block stage.
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5.0 Discussion
For the purpose of this section each of the four events, as described in Table 1, will be
considered separately.
5.1 First Stage (19-24 July)
As mentioned in Table 1 the explosive products of this phase were predominantly ash and
Lapilli, as attested by the proximal section in Fig. 7. Taddeucci et al (2004, p. 40) indicated
that these ash particles were sideromelane rich, although it should also be noted that a large
proportion of lithics were incorporated during this stage relative to the other stages (Fig. 8).
The same authors also suggest that, throughout the whole period of the eruptive events
(Stages 1-4) emission rates were at an intermediate level during this stage.
It is considered by Behncke and Neri (2003, p. 465) that visual evidence of continuous
“uprush” and “tephra-finger” activity of the pyroclastic column points to phreatomagmatic
activity. This is backed up by evidence of a high lithic content during this stage (Table 1),
which is considered to represent a high energy event, such as phreatomagmatic activity
(Houghton & Smith, 1993, p. 414).
It was previously considered by Chester et al. (1985, p. 139) that such activity was unlikely at
Mount Etna, and that any activity would be “limited”. However, evidence of previous
phreatomagmatic eruptive activity at Etna is considered in the 18.7 ka flank eruption at Etna
(Andronico, Branca, & Del Carlo, 2001, p. 235). Whilst this historic eruption was at an
elevation of 800m a.s.l. where impermeable Pleistocene clays are located (Andronico et al.
2001, p. 235), no such deposits are considered to exist at the level of the 2001 eruption
(2570m). In this instance, where only volcanic products exist, the presence of a shallow
impermeable ash layer (of unknown depth) beneath the Piano Del Lago, inferred by Behncke
and Neri (2003, p. 472) seems the most plausible location for groundwater to pond, especially
as the same authors consider meltwater to collect at this location following each winter.
It is widely accepted by authors (Andronico, Cristaldi, Del Carlo, & Taddeucci, 2009;
Behncke & Neri, 2003; Corsaro, Miraglia, & Pompilio, 2007; Dellino & Kyriakopoulos,
2003; Guest, Cole, Duncan, & Chester, 2003, p. 199; Lanzafame, Neri, Acocella, Billi,
Funiciello, & Giordano, 2003; Metrich, Allard, Spilliaert, Andronico, & Burton, 2004; Neri,
Acocella, Behncke, Maiolino, Ursino, & Velardita, 2005; Patane, La Delfa, & Tanguy, 2006;
Taddeucci, Pompilio, & Scarlato, 2004; Viccaro, Ferlito, Cortesogno, Cristofolini, &
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Gaggero, 2006.) that an eccentric dike cut through this shallow aquifer, allowing magma-
water interaction. This has also occurred at a number of other basaltic volcanoes, most
notably Surtsey, but also including (but not limited to) Taupo Volcanic Centre (Wilson &
Smith, 1985, p. 329), Crater Hill, New Zealand (Houghton, Wilson, & Smith, 1999, p. 119),
Croscat, Garrotxa Volcanic Field (Di Traglia, Cimarelli, de Rita, & Gimeno Torrente, 2009,
p. 89), Fontana, Nicaragua (Costantini, Houghton, & Bonadonna, In Press) the Canary
Islands (Clarke, Troll, & Carracedo, 2009, p. 226) and the Prebble and Mawson Formations,
Antarctica (Elliot & Hanson, 2001, p. 183). In such magma-water interactions, it is not
considered that the chemistry of the magma is an important factor in the explosivity of the
eruption compared other variables such as the depth of the interaction and the amount of
water made available.
5.2 Second Stage (24-25 July)
Characterised by a mixture of ash, lapilli, fire fountaining and scoria production (Table 1) this
activity was marked by decreasing lithic ejecta and increasing Sideromelane (Figure 8). It is
considered that this stage is characteristic of an transitional phase between the first
(phreatomagmatic stage) and the third stage, indicated by the waning of phreatomagmatic
activity (Behncke & Neri, 2003, p. 472) and a change to gradually increasing lava
fountaining and scoria production (Taddeucci, Pompilio, & Scarlato, 2004, p. 35; Viccaro,
Ferlito, Cortesogno, Cristofolini, & Gaggero, 2006, p. 142).
In order for a change in events it is considered that magma-water interaction must have been
reduced in one of three ways: 1) All of the groundwater was consumed; 2) the fragmentation
level of the magma rose above the watertable (Behncke & Neri, 2003, p. 472); 3) An increase
in emission rates of the magma allowed the conduit to seal itself, preventing water interaction
(Taddeucci, Pompilio, & Scarlato, 2004, p. 52).
Behncke & Neri (2003, p. 473) consider the first point to be unlikely, because of their
hypothesis that phreatomagmatic activity occurred at Stage 4, and therefore consider the
second option. They noted that when ejection angles were low (with magma high in conduit)
activity moved from phreatomagatic to Strombolian and vica verca. However, more recent
evidence suggests that the underlying eccentrically located magma was volatile rich and
microlite (crystal) poor ((Table 2) (Clocchiatti, Condomines, Guenot, & Tanguy, 2004, p.
397; Taddeucci, Pompilio, & Scarlato, 2004, p. 37)). This increase in volatiles, whilst being
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microlite poor and therefore of a lower viscosity, would suggest a more rapid ascent,
contributable to the theory of an increased emission rate. It can be hypothesised that whilst
this magma was more volatile rich, the low crystal content of the magma allowed a
significant portion of the volatiles to readily exsolve from the magma, resulting in
Strombolian activity.
5.3 Third Stage (25th
July – 1st August)
It is considered that the third stage is marked by the complete transition of activity to dry
magmatic eruptions, with a reduction of lithic fragments within ash samples and an increase
in sideromelane content (Fig. 8), leading to predominantly lava fountaining and scoria cone
production over ash-forming events (Behncke & Neri, 2003, p. 465). Over the coming days it
is noted that a more tachylite-rich ash with a greater crystal content was erupted on the 30th
July to the 1st August, at the end of this third stage (Fig. 8).
The trend towards more tachylite-rich erupted products suggests the presence of magma
mixing at depth (Taddeucci, Pompilio, & Scarlato, 2004, p. 52). A greater crystal content of
the magma suggests that it had a residence time greater than the previously erupted magma,
allowing crystallisation and loss of volatiles either through this process, or by slower
migration due to the higher viscosity attributable to crystallisation. This hypothesis agrees
with that of Clocchiatti et al. (2004, p. 411), Taddeucci, Pompili & Scarlato (2004, p. 53) and
Viccaro et. al (2006, p. 156).
5.4 Fourth Stage (1st-6
th August)
This stage was marked by the eruption of block and ash deposits (Table 1) and coincides with
the almost complete replacement of sideromelane ash with tachylite ash (Figure 8). It has
been considered by Behncke & Neri (2003, p.466) that this activity marks a final
phreatomagmatic phase. However, this stage was marked by a much higher eruption column
of 5km a.s.l. (Taddeucci, Pompilio, & Scarlato, 2004, p. 33), with Dellino and Kyriakopoulos
(2003, p. 341) noting that ash falls occurred as far away (500km) as Cefalonia, Greece. These
observations suggest that different mechanisms may have been involved.
Combined with evidence from the ash componentry (deficient in lithics), it is not considered
that this phase involved water-magma interaction, or that, at the very least this was not the
main cause of the eruption style. However, Dellino and Kyriakopoulos (2003 p. 342), in
assessing ash fall at large distances, identified stepped features, considered to relate to melt-
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fragmentation in a brittle mode, known to occur during magma-water interaction. The same
authors noted that the glass particles showed blocky textures with few isolated vesicles of
spherical form (2003, p. 342), suggesting that exsolution and expansion did not play a major
role in the eruptive dynamics.
It is hypothesised that the final stage, dominated by ash pulsations (Taddeucci, Pompilio, &
Scarlato, 2004, p. 33), is related to the ingress of this new tachylite- and crystal-rich magma
into the conduit system, which, owing to its higher viscosity, is assumed to have ascended at
a slower rate relative to the previous magma. Slow ascent would allow the magma to degas,
reducing the fragmentation potential. This contradicts the hypothesis posed by Clocchiatti et.
al. (2004, p. 411) who consider the high explosivity to be a result of this magma moving from
a minimum depth of 30km below the volcano to mix with the eccentrically erupting magma
within the space of a few months, as inferred from 210
Pb-226
Ra disequilibria, and hence a high
emission rate. However, a high emission rate was not observed during this final stage
according to Taddeucci, Pompilio & Scarlato, 2004, p.52).
It is considered that a combination of these aforementioned events led to the voluminous ash
production event. With low effusion rates of this final phase magma it is considered that
voluminous and pulsating ash production could not have occurred. Instead, it is proposed that
a deep magma ascended relatively rapidly, possibly through a network of dikes, to mix with
the eccentrically erupting magma. Ascent was slowed at depth where mixing of the two
magmas occurred due to the presence of the original, now more viscous (higher microlite
content) magma, resulting in degassing of this new magma body. It is then considered that
the hypothesis of Taddeucci, Pompilio & Scarlato (2004, p. 53) fits well, whereby a plug of
this combined, microlite-rich magma – with a high yield strength and high liquid and bulk
viscosities – prevents the upward migration of magma. Ash pulsations are therefore produced
when the build up of volatiles from the degassing magma, collecting beneath this plug,
exceed the plug’s strength, causing Vulcanian behaviour.
This is not considered to represent the first time an Etnean ash-forming event has occurred in
this way, with the Plinian eruption of 122BC thought to have involved increased viscosity
caused by microlites (Sable, Houghton, Del Carlo, & Coltelli, 2006, p. 351). Other basaltic
volcanoes, such as Fontana, Nicaragua (Costantini, Houghton, & Bonadonna, In Press) and
Tarawera in 1886 (Houghton, Wilson, Del Carlo, Coltelli, Sable, & Carey, 2004, p. 12) have
acted similarly in the past.
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6.0 Conclusions
The 2001 eccentric explosive basaltic eruption is considered to be representative of the
following stages:
1) Initial phreatomagmatic activity (19th
-24th
July, 2001), caused by the intrusion of
magma into a shallow aquifer, creating a 200-300m high eruption column which
dispersed predominantly ash and Lapilli with a high lithic content.
2) Transition from phreatomagmatic to Strombolian activity (24th
-25th
July, 2001),
resulting from an increase in the emissivity of a volatile- and sideromelane-rich, but
microlite poor magma.
3) Strombolian activity (25th
July to 1st August, 2001) through the continued ascent of
the magma involved in stage 2.
4) Vulcanian activity (1st-6
th August, 2001) resulting in a 5km a.s.l. tephra column,
forming block and ash deposits. It is considered that this activity was caused by a
more tachylite- and mircrolite-rich magma essentially plugging the conduit. Pulsing
ash and block events are therefore produced when the strength of the magma plug is
exceeded by the strength of degassing volatiles from the underlying magma.
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