Il Potere Della Fantasia: Origin and evolution of Mount Etna

giovedì 10 luglio 2008

Origin and evolution of Mount Etna

Mount Etna is Europe's highest volcano and one of the most important in the world. At present it is about 3,340 m above sea-level, with a 45-km diameter, but its form goes on changing because of its eruptions. The structural complexity of the setting in which it lies, not yet fully understood, is reflected in the large quantity of - often controversial - models proposed for the volcano and its tectonic environment. Over the millennia, Mt. Etna has been formed by a process of construction and destruction, started around 600 thousand years ago. It is believed that in the area covered by the present Etna there was a vast gulf. It was the colossal friction between the Euro-Asiatic plate in the North and the African in the South, corresponding respectively to the Peloritani Mountains and the Hyblean Plateau, that gave origin to the first submarine eruptions of a very fluid lava (tholeiitic basalts) and the first volcanic cones in the middle of the primordial gulf -said pre-Etnean- already cut down to size by the uplifting and emersion of part of the seabed clay. An evidence of this early activity are the pillow lavas in Acitrezza and Acicastello. After the primordial eruptions, and up to about 25 thousand years ago, the many eruptive centres overlapped along an approximately SE—NW axis uplifting and enlarging the volcano form. It is supposed that alkaline magmas were emitted, about 200-100 thousand years ago, to form several new eruptive cones, the main of which, called Mount Calanna, is today incorporated below the volcano. When its activity ceased, about 80 thousand years ago, the lavas became more dangerously fluid, leading to a more explosive volcanism and the construction of a new volcanic cone complex, west to the previous one, named Trifoglietto. The volcanic activity moved farther westward and built an additional crater, named Trifoglietto II. Very likely, it was this the most important volcano formed in the actual Valle del Bove. The system exploded about 64 thousand years ago with a dreadful eruption that emptied the magmatic chamber, giving origin to the huge caldera named Valle del Bove, 1000 m deep and 5000 large. The collapse of the volcano took to a long period (30 thousand years), in which an alternation of basaltic lava effusion and violent explosion phases formed tufa and other pyroclastic products. At the end of this period a new big flank cone farther and farther west started its activity. Mongibello was born: the biggest cone that covers the central area in the actual massif of Etna and includes more than one third of its volume. It is the most recent eruptive centre where the actual summit craters are. The growth of Mongibello can be divided into three stages: Ancient, Recent and Modern Mongibello. The earlier probably includes the eruptive centre called Ellittico, whose activity ended about 14 thousand years ago with the collapse of the volcano top and the formation of the caldera Ellittico, -14 km in diameter- completely filled by the Recent Mongibello products. The Modern Mongibello is characterized by persistent basaltic overflows of lava and poor pyroclastics on the craters. Lavas have turned to a fluid basaltic type again and have formed other cones, the most recent of which are the North-East Crater (1911 ), the New Crater (1968) and the South-East Crater (1981), still active and growing on.