The geological history of Mount Etna
Mount Etna is a large basaltic composite volcano lying near the eastern coast of Sicily, in a complex geodynamic environment characterized by the collision of the African and Eurasian continental lithospheric plates. This setting contains numerous faults, including the distensive Malta-Iblei escarpment, an important structural boundary between the continental lithosphere of Sicily and the oceanic lithosphere of the Ionian sea floor. Volcanism in the area has been continuing episodically for more than 230 million years, first in the Monti Iblei, in the southeast portion of Sicily, on the foreland of the African plate promontory, and, during the past half million years, in the Etnean area.
The geological history of Etna is subdivided into four main periods (Branca et al., 2004); see also the INGV-Catania web page:
(1) Basal Tholeiites
The start of eruptive activity has been dated at about 500,000 years, during the mid-Pleistocene. At that time the current Etna area was occupied by a broad bay corresponding to the sedimentation base of the foredeep, and the first eruptions took place under the sea. The products of this phase of activity occur in outcrops in the area of Acicastello, Ficarazzi, and Acitrezza on the Ionian coast to the north of Catania, forming intercalations in foredeep sediments represented by grayish-blue clays of the lower to mid Pleistocene. The most famous of these outcrops is the castle rock of Acicastello, which is composed of pillow lavas and associated hyaloclastite (pillow breccias). These originally submarine products have been subjected to isostatic uplift.
(2) The Timpe phase
From no less than 220,000 years ago until about 110,000 years ago, volcanic activity concentrated along the Ionian coast along a fault system known as the "Timpe" (the steps), which represents the northern continuation of the Malta-Iblei escarpment (Azzaro, 1999). The Timpe faults are marked by conspicuous morphological scarps, and terminate to the NNW near Moscarello and Sant'Alfio. 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 Leugatia-Fasano.
(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 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).
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 controled by structures of weakness in the volcanic edifice, where most intrusions occur along a number of main trends.
These are characterized by three main rift zones, the NE, S and W 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. 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.
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Branca S. (2003) Geological and geomorphologic evolution of the Etna volcano NE flank and relationships between lava flow invasions and erosional processes in the Alcantara Valley (Italy). Geomorphology, 53, 247-261.
Branca S., Coltelli M., Groppelli G. (2004) Geological evolution of Etna volcano. In: “Etna Volcano Laboratory” Bonaccorso, Calvari, Coltelli, Del Negro, Falsaperla (Eds), AGU (Geophysical monograph) 143, pp 49-63.
Branca S., Coltelli M., De Beni E., Wijbrans J. (2008) Geological evolution of Mount Etna volcano (Italy) from earliest products until the first central volcanism (between 500 and 100 ka ago) inferred from geochronological and stratigraphic data. Intern. J. Earth Sci., 97:135–152
Calvari, S., Tanner, L.H., Groppelli, G., Norini, G. (2004) A comprehensive model for the opening of the Valle del Bove depression and hazard evaluation for the eastern flank of Etna volcano. In: “Etna Volcano Laboratory” Bonaccorso, Calvari, Coltelli, Del Negro, Falsaperla (Eds), AGU (Geophysical monograph) 143, pp 65-75.
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