Posted by Etna Week Part 1
Mount Etna – Brief Anatomy of an Exceptional Volcano
By guest blogger Dr. Boris Behncke.
Italy truly deserves to be called “the Cradle of Volcanology” – not only because it hosts virtually all existing types of volcanoes and volcanic rock compositions, and seven of its volcanoes have had confirmed eruptions during the historical period (i.e. the past approximately 2700 years), but also because the earliest surviving eyewitness account of an eruption was written in Italy, the first volcano observatory and the first geothermal power plant were built in Italy, and three volcanological terms denoting styles of eruptive activity – Strombolian, Plinian, and Vulcanian – have their origin in this country. The word “volcano” itself has its origin from the southernmost of the Aeolian Islands, in the Tyrrhenian Sea north of Sicily, Vulcano. Vesuvius, uncomfortably close to Naples and its very densely populated suburbs, is possibly still the most famous volcano worldwide, and certainly one of the most dangerous volcanoes on Earth.
Snow-covered Etna seen from the village of Trecastagni, on the southeast flank of the volcano, in January 2008, taken by Boris Behncke.
In this exceptionally varied volcanic setting, Mount Etna on the island of Sicily is a volcano of superlatives. It is the most active volcano of Europe and – after Kīlauea on Hawai’i – possibly the second most active volcano on Earth, in terms of eruption frequency and long-term average magma output rate. It has the longest record of documented eruptions of all volcanoes worldwide, and can be said to be virtually continuously active, with significant eruptive events occurring almost every year. Its summit stands at 3330 m elevation as of 2010 (Neri et al., 2008), making it the tallest mountain in the Mediterranean basin, and the highest summit in Italy to the south of the Alps. Besides the four nearly continuously active craters at its summit, Etna has approximately 350 craters and minor vents on its flanks, each of which erupts only once, and many of which form sizable cones, like miniature volcanoes, on the flanks of the mountain.
But what makes Etna really unique is its incredible versatility in terms of eruptive styles, eruption magnitudes, and eruption locations. During the historical period, it has produced numerous effusive, Hawaiian to Strombolian style eruptions from vents both at its summit and on its flanks, sometimes purely effusive activity going on for years, countless short-lived episodes of violent Strombolian to sub-Plinian fire fountaining accompanied by voluminous lava and tephra emission, Vulcanian and phreatomagmatic explosions and persistent ash emission sometimes lasting for months (Branca and Del Carlo, 2005). This record is punctuated by a Plinian eruption in 122 B.C. (Coltelli et al., 1998), which caused devastation and hardship to the population of Catania, a city that had been founded more than 600 years earlier by the Greek at the southern base of the volcano. Pyroclastic flows, the most lethal and destructive of all volcanic phenomena, have been observed on a number of occasions in recent years, but luckily affected only the remote summit area (Behncke, 2009).
In spite of its frequent and sometimes violent and hazardous activity, Etna has claimed a surprisingly small number of human victims – less than 80 deaths can be with certainty attributed to the activity of the volcano in the past 2700 years. This figure might be higher because the historical record contains several gaps up to a few hundred years long, like the Arab domination from the 9th to 11th centuries A.D. (all Arab records were lost during the Christian recolonization); yet it is clear that Etna is not a killer volcano, and this is one of the reasons why the people living on its slopes call it “the friendly volcano”.
Geological setting and evolution of Etna
Like all things in Italy, the geodynamic setting of Etna is a bit complicated. For this reason, the origin of Etna has been ascribed by various workers to subduction, rifting, and a mantle plume, and more recently, some more exotic factors.
Sicily lies on the boundary between two converging (or colliding) lithospheric plates, the African plate to the south and the Eurasian plate to the north. This convergent margin runs across much of the Mediterranean along a general east-west trend, but shows a marked bend in Italy, where it turns NNW up to the Alps before taking on a SE trend in the Balkans toward Greece. Differently from many convergent plate margins, where one plate consists of oceanic and the other of continental lithosphere (like in the Pacific Northwest of the U.S. where the oceanic Pacific plate runs into, and dives underneath the continental North American plate), the colliding plate margins in Italy are heterogeneous, with bits of oceanic lithosphere alternating with continental lithosphere. The character of convergence therefore changes over short distances from subduction, as in the Calabrian and Aegean arcs, to mountain building as in northern Sicily and along the Apennines and the Alps.
The volcanoes of the Aeolian Islands are believed to be, at least in part, due to the subduction of oceanic lithosphere of the Ionian Sea under the Calabrian arc. However, rather than consistently erupting calc-alkaline magmas as subduction-related volcanoes commonly do, the Aeolian volcanoes also produce more sodium and potassium-rich magmas, which some scientists attribute to magma melting at different depths along a very steeply dipping subducting lithospheric slab (Tommasini et al., 1997).
While out in the Ionian Sea, to the east of Sicily, the northern margin of the African plate consists of oceanic lithosphere subducting underneath the Calabrian arc, on the island itself it is constituted by rather thick continental lithosphere, which makes up the southeast corner of Sicily. Rather than subducting, it bites and pushes into the continental lithosphere of the southern margin of the Eurasian plate. The result is mountain building – much like in the Himalayas or in the Rocky Mountains – going on in the Peloritani, Nebrodi, and Madonie mountain belts, which together constitute the northern backbone of Sicily. Etna lies just north of the plate boundary and away from the Calabrian arc subduction zone, in a rather uncommon place for a volcano to occur, plate tectonically speaking. For this reason, some researchers have invoked a hot spot origin of Etna, and of the older volcanic area of the Monti Iblei to the south, where volcanism has occurred over more than 200 million years (Schmincke et al., 1997; Tanguy et al., 1997; Behncke, 1999). Schiano et al. (2001) present arguments for a transition from a hot spot origin to a more and more pronounced subduction component in Etna’s magmas. However, the hot spot model is not plausible because volcanism has shown a northward shift from the Monti Iblei to Etna, which would require movement of the African plate to the south, whereas in reality the contrary is the case (the African plate is moving northward).
Interpretative sketch of the geodynamic setting of Mount Etna, based on Gvirtzman and Nur (1999). From Armienti et al. (2004)
A series of recent publications (Gvirtzman and Nur, 1999; Doglioni et al., 2001; Schellart, 2010) places Etna into a context of slab rollback related to the subduction of Ionian oceanic lithosphere below the Calabrian arc. Slab rollback means that the bend where an oceanic plate starts descending into subduction moves gradually away from the subduction zone due to the weight of the subducting plate, and consequently it sort of tears the subduction zone and the overriding plate into the direction of the subducting plate. In the case of the Ionian plate subduction this would mean that the subduction zone migrates southeast, which is well illustrated here at Highly Allochthonous. This leads to the tearing open of a gap between the subduction setting of the Ionian oceanic lithosphere and the Calabrian arc to the east, and the continental collisional setting of Sicily to the west, which in turn causes decompression and the formation of magma in the upper mantle below this opening gap, or “window”. The magma rises along the intersection of a number of major regional fault systems and feeds the activity of Etna.
Whatever the geodynamic cause of Etna, it appears to be highly efficient. Throughout its roughly half-million-years-long history, Etnean volcanism has become more and more vigorous and increasingly focused on a large volcanic edifice, eventually leading to the building of the large mountain that dominates Sicily today. The geological evolution of Etna is subdivided into four main phases: (1) the Basal Tholeiitic phase, (2) the Timpe phase, (3) the Valle del Bove centers phase, and (4) the stratovolcano phase.
Geological map of Mount Etna, from the INGV-Catania web site (courtesy of Stefano Branca). Key: (1) Recent alluvial deposits; (2) Mongibello (past 15,000 years) eruptive products (2a) “Chiancone” volcaniclastic debris deposit; (3) Ellittico eruptive products; (4) Valle del Bove centers eruptive products; (5) Timpe phase eruptive products; (6) Basal Tholeiites; (7) Sedimentary basement; “Faglia” = fault, “Orlo della Valle del Bove” = Valle del Bove rim; “Crateri Sommitali” = Summit craters
Distribution of eruptive products of the four main phases of volcanism in the Etna area: (a) Basal Tholeiites; (b) Timpe phase; (c) Valle del Bove eruptive centers; (d) Stratovolcano phase. From Branca et al. (2004)
(1) Basal Tholeiites. The first phase of volcanic activity in the Etnean area occurred about 500,000 years ago, in an area then occupied by a broad bay – referred to as the pre-Etnean gulf – leading to the emission of submarine lava (pillow lava) and associated debris, which is known as hyaloclastite. Some eruptive events lasted long enough to build small volcanic islands, much the same way as the island of Surtsey formed in 1963-1967 off the south coast of Iceland. Outcrops of these earliest products of Etnean volcanism occur along the coast of the Ionian sea immediately to the north of Catania, in the area of the fishing villages of Acicastello and Acitrezza. The castle rock of Acicastello is a world-class geological site (unfortunately not placed under protection as it would certainly deserve) where pillow lavas can be seen next to a typical breccia of small glassy fragments (hyaloclastite) and debris of shattered pillows (pillow breccia or flow-foot breccia).
The spectacular outcrop of Acicastello, on the coast of the Ionian Sea at the southeast base of Etna, where the earliest eruptive products of the volcano (Basal Tholeiites) are exposed. The left (western) part of the outcrop consists of densely packed pillow lavas, whereas breccias of shattered pillows and hyaloclastite (fragments of volcanic glass altered into yellowish-brown palagonite) make up the right portion of the section. Photos by Boris Behncke.
The products of this earliest phase of volcanism in the Etna area are tholeiitic basalts – virtually the same magma that is currently being erupted from Kīlauea (Hawai’i), which is the reason that this phase in the evolution of Etna is referred to as “Basal Tholeiites”. Besides the outcrops of Acicastello and nearby locations such as Acitrezza and Ficarazzi, products of this phase occur also further to the west, near the town of Adrano at the southwest base of the volcano.
(2) Timpe phase. The second main phase of Etnean volcanism occurred between >220,000 years ago and about 110,000 years ago in a narrow belt along the Ionian coast along a fault system known as the “Timpe” (the steps). The Timpe faults are marked by conspicuous morphological scarps, and terminate to the NNW near Moscarello and Sant’Alfio on the east flank of Etna. During this phase, numerous fissure eruptions occurred in this relatively restricted elongate belt along the Ionian coast, and led to the growth of a NNW-SSE elongated shield volcano about 15 km long. The internal structure of this shield volcano is today exposed in the Timpe fault scarps between Acireale and Moscarello. During this eruptive period, sporadic volcanism also occurred along the valley of the Simeto river, constructing, amongst others, the large scoria cone that constitutes the hill of Paternò and a number of thin, strongly eroded, lava flows like those cropping out in the northern periphery of Catania at Leucatia-Fasano. The products of this phase showed a shift from tholeiitic to alkali basaltic compositions.
(3) Valle del Bove eruptive centers. About 110,000 years ago, the focus of volcanism shifted from the Ionian coast into the area now occupied by the Valle del Bove. In this period, the character of Etna’s activity underwent a profound change, from sporadic fissure eruptions as during the first two phases, to a more centralized activity of both effusive and explosive character. This activity led to the construction of the first composite volcanic edifices in the Etna region, the Rocche and Tarderia volcanoes. The products of these eruptive centers crop out along the base of the southern flank of the Valle del Bove at Tarderia and Monte Cicirello. Subsequently, the activity concentrated in the southeastern sector of the Valle del Bove, at Piano del Trifoglietto, where the main eruptive center of this phase was built up, Trifoglietto volcano, which reached a maximum elevation of about 2400 m. Three minor eruptive centers formed subsequently on the flanks of Trifoglietto, which are named Giannicola, Salifizio and Cuvigghiuni; their activity continued until about 60,000 years ago. This phase marks the formation of a stratovolcano structure in the Etna edifice and the superposition of different eruptive centers.
(4) Stratovolcano phase. About 60,000 years ago, a further shift in the focus of eruptive activity toward northwest marks the end of the Valle del Bove centers, and the start of the building of the largest eruptive center of Etna, now named Ellittico (the elliptical), which constitutes the main structure of the volcano. The Ellittico volcano produced intense effusive and explosive activity, constructing a large edifice, whose summit may have reached a height of 3600-3800 m. Numerous flank eruptions generated lava flows that reached the Simeto river valley to the west of Etna. About 25,000 years ago, the Alcantara river was deviated from its former valley closer to Etna (in correspondence with the towns of Linguaglossa and Piedimonte Etneo) into the present-day Alcantara valley (Branca, 2003). Much of the Ellittico lavas and pyroclastics are present in outcrops in the northern wall of the Valle del Bove.
The tan-colored clastic rocks in these photos are the pyroclastic-flow deposits emplaced during the climactic explosive eruptions at the end of the Ellittico stage of Mount Etna, about 15,000 years ago. These deposits occur in outcrops near the town of Biancavilla on the lower southwest flank of the volcano. Photos taken in August 2001 by Boris Behncke
The Ellittico stage ended about 15,000 years ago with a series of powerful explosive (Plinian) eruptions (Coltelli et al., 2000), which destroyed the summit of the volcano leaving a caldera about 4 km in diameter. Intense eruptive activity continued during the past 15,000 years, largely filling the Ellittico caldera, and building up a new summit cone. This current summit edifice is called Mongibello. About 9000 years ago, a portion of the upper east flank of Etna underwent gravitational collapse, generating a catastrophic landslide (the Milo debris avalanche), and forming the huge collapse depression of the Valle del Bove, which still today bites deeply into the eastern sector of the volcano (Calvari et al., 2004).
Aerial view of the Valle del Bove, a huge collapse depression formed by a massive sector collapse of Etna’s eastern flank about 9000 years ago. Much of the depression has been filled by more recent lava flows; the original depth must have been significantly greater. This view is from the southeast, showing the summit craters in the upper center. Photo taken in August 2007 by Boris Behncke
Following the Valle del Bove sector collapse, remobilization of the debris avalanche deposit by alluvial processes led to the generation of a detritic-alluvional deposit, known as Chiancone, which crops out between Pozzillo and Riposto along the Ionian coast. This huge collapse of the eastern flank of the Mongibello edifice has exposed a large portion of the internal structure of both the Valle del Bove eruptive centers and of the Ellittico volcano, which crop out in the walls of the depression. The eruptive activity of the Mongibello is strongly controlled by structures of weakness in the volcanic edifice, where most intrusions occur along a number of main trends.
These predominant trends are characterized by three main rift zones, the Northeast, South and West rift zones. Although much of the activity of the Mongibello volcano is effusive, numerous strongly explosive events are known as well, mostly from the summit craters (Coltelli et al, 2000). The most powerful eruption of this eruptive phase occurred in historical time, in 122 B.C. (Coltelli et al., 1998). This eruption, which occurred from the summit of the volcano, produced a large volume of pyroclastics (ash and lapilli), which fell in a sector on the southeast flank of the volcano, causing heavy damage in the city of Catania.
(Part 2 to follow later this week.)
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