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Mt Etna

Prelude: Volcanism on the Hyblean Plateau

Volcanism in eastern Sicily has occurred episodically since the middle Triassic, producing voluminous mafic (that is, silica-poor) lavas and clastic (fragmental) material, much of which was erupted and deposited below the sea level. The sites of volcanic activity were located in the southeastern part of Sicily, on the Hyblean Plateau (Monti Iblei), an area uplifted tectonically during the late Pliocene and Pleistocene. The most recent major cycle of volcanism in that sector lasted from the late Miocene (Tortonian) until the early Pleistocene (~1.4 Ma) and showed a migration of eruptive centers northwards. A notable feature of this volcanism was the brief duration of each eruptive event (maybe a few years to several centuries) and the lack of major volcanic edifices, all activity occurring from regional fissures controlled by the predominant tectonic trends (WSW-ENE). The products of the most recent episodes of Hyblean volcanism lie buried under hundreds of meters of late Pleistocene and recent alluvial sediments, and their precise age is not known, but it appears that there is a more or less continuous series of volcanics becoming progressively younger towards north. On the northern margin of the Catania Plain, an area of tectonic subsidence covered with fluvial deposits between Etna and the northern Hyblean Plateau margin, volcanic rocks outcrop again, but these are considerably younger than those of the Hyblean Plateau and are generally considered the earliest manifestations of volcanism in the Etnean area.

First phase: "pre-Etnean"

Volcanic activity in the Etnean area began around 0.5 Ma (million years) ago. This initial activity, the first of four main phases during the evolution of Etna, was very similar in character to the latest Hyblean volcanism, and its products were mainly submarine lavas and volcaniclastics of tholeiitic composition (for more info of this type of basaltic rock. The famous outcrops of Acicastello, a town some 10 km north of Catania, and nearby locations contain pillow lavas and hyaloclastites produced by this early activity. It is generally believed that this phase of volcanism took place in a vast marine gulf, extending over part of the area occupied by the present Etna, which was subsequently filled by sediments intermixed with volcanics helped by regional tectonic uplift.
About 0.3 Ma ago, more eruptions occurred in various locations in the Etnean area, - mainly on the SW side of Etna - producing transitional tholeiitic and alkalic basalts (pigeonitic tholeiites, alkali basalts and trachybasalts).
This early phase of volcanic activity in the Etnean area is commonly named "Pre-Etnean". The volume of these products is insignificant in comparison of the entire volume of Etna's products, but in terms of time, this phase occupied more than half of the history of the volcano.

	Left: Looking up the cliff of the Acicastello fortress. The cliff consists of tousands of "pillows", the most common type of submarine lava and possibly the most widespread lava type on Earth. Right: On the eastern side of the Acicastello cliff, densely packed pillow lavas are in discordant contact with hyaloclastites (a breccia consisting of volcanic glass and of pillow fragments). The contact whose significance is poorly understood is marked by a broken line in red color.
	Left: The picturesque harbor of the village of Acitrezza, some 2 km north of Acicastello, with the two largest of the islands known as the "Ciclopi" or "Faraglioni", April 1999. The largest island, to the left, is Isola Lachea. Right: Outcrop of columnar basalt on the shore of Acitrezza. The tops of the hexagonal or pentagonal columns form the "mosaic" pattern of the surface. Such columns form during the cooling and contraction of lava (or shallow intrusions). Photo taken in April 1999.
	Left: The smaller, southernmost islands of the "Ciclopi", seen from the harbor of Acitrezza, April 1999. Right: The highest of the "Ciclopi" of Acitrezza (known as "Faraglione grande" or "Faraglione della Madonna"), seen from the harbor of the village in April 1999. The base of the rock consists of magmatic rock, possibly formed by an intrusion which uplifted marine sediments (visible at the summit of the rock). The white patch about half way up the rock is a statue of the Madonna.

Isola Lachea, the largest of the "Ciclopi" of Acitrezza, seen from the harbor of the village in April 1999. The island consists mostly of magmatic rock, probably formed by a shallow intrusion. The island is now a protected area.

Second phase: "Ancient Etna"

The second phase of volcanism at Etna, called "Ancient Etna", began with more localized eruptions (such as the Paternò eruptive center, SSW of Etna, dated at 168 plus/minus 10 ka=thousand years). Several authors (Romano, 1982, Romano et al., 1979) have named these eruptive centers the "Ancient alkali eruptive centers". It is assumed that a first large stratovolcano (the Calanna eruptive center; 100-130 ka) was built at a later stage of this phase. The erupted magmas were alkali basalts and hawaiites.

Third phase: "Trifoglietto II"

The third phase of Etnean volcanism is generally named "Trifoglietto II" and was characterized by the building of several overlapping stratovolcanoes: Trifoglietto II, a large stratovolcano consisting of lavas and pyroclastics, Vavalaci, and Cuvigghiuni being the most important of these volcanoes. Among the products of these volcanoes, most were alkaline differentiated magmas (trachyandesites), leading to a much more explosive volcanism than that of the earlier phases. The growth of the stratovolcanoes was at times interrupted, and in most cases terminated, by caldera collapse. The oldest dated products of this phase are about 80 ka old while another datation higher up in the stratigraphy yielded an age of about 63 ka.

Fourth phase: "Mongibello"

The fourth phase saw the growth of the Mongibello stratovolcano whose evolution is divided into three stages, Ancient, Recent and Modern Mongibello. Its oldest dated products are about 35 ka old. During this phase, Etna produced the most differentiated (trachytic) magmas of its entire history; this occurred about 15-16 ka ago and led to the eruption of ignimbrites (the deposits of pyroclastic flows) which are visible in outcrops near the town of Biancavilla, on Etna's SW flank. Ash from these highly explosive eruptions was distributed over a wide area in the Mediterranean and has recently been identified in lacustrine (related to a lake) deposits in the crater lakes of the Colli Albani near Rome (Calanchi et al., 1996), about 800 km NNW of Etna! This event was probably accompanied by the collapse of a caldera named Ellittico, a depression about 3 km in diameter which has been filled by more recent eruptive products and partially eaten away by more recent caldera collapse. After this period of trachytic magmatism, the compositions of the erupted products became more mafic (that is, poor in silica but with higher Mg and Fe contents), and the activity became predominantly effusive.
Major caldera collapse occurred again several thousand years ago, forming the Valle del Bove, a vast depression some 5 x 7 km across on the eastern side of the mountain. The early stage of caldera formation was characterized by the gravitational failure (sector collapse) of the eastern flank, producing a debris avalanche whose deposits have only recently been identified by Calvari et al. (1998) under a thick succession of fluvial deposits known as the "Chiancone" and outcropping on the lower eastern flank of Etna. The most recent collapse event that created the present shape of the Valle del Bove probably occurred only about 3500 years ago, as revealed by ongoing studies by a British working group.
The growth of the modern summit cone of Etna was interrupted as recently as ~2000 years ago by caldera collapse (Piano caldera), in the context of an unusually explosive (Plinian) eruption which probably occurred in the year 122 B.C. (Coltelli et al., 1998). The outline of the Piano caldera is still well visible as a distinct slope break close to the base of the current summit cone, its maximum diameter being about 2.5 km, with the rim being at about 2900 m elevation. More recently there may have been minor caldera collapse affecting only the area of the present summit cone, such as in 1669 when a large volume of magma was drained from a possible storage area under the volcano during a major eruption from Monti Rossi on the southern flank.
The activity of Etna during the past few thousand years has been characterized by lava emission and Strombolian activity, punctuated at times by more explosive episodes from the summit craters.